Fuel injection involves spraying or injecting fuel direCtly ...
GASOLINE FUEL INJECTION Gasoline Fuel injection as a concept has been around for many years the first documented systems were experimented with in the late 1800s. The first mainstream use of gasoline injection came in the 1950s from various manufacturers but large scale use did not begin until the 1960s with a Bosch system that was used on many European vehicles. All of these early systems were mechanical injection systems. By far the most popular method of introducing fuel to a gasoline engine was the carburetor. The carburetor used the venturi principle to draw fuel into the intake air stream. As the air flowed through the throat of the carburetor it passed through a narrowed section called a venturi. This caused the air to speed up to pass through which dropped its pressure. This is known as Bernoulli’s theorem and is the same principle that allows aircraft wings to create the lift necessary to fly a plane.
Located at this narrow section is a series of jets or pipes connected to a fuel bowl that has atmospheric pressure acting on it the fuel is then pushed up the pipe into the venturi. This method worked very well during normal operation, as the airflow increased the amount of fuel pushed or drawn out of the fuel bowl increased proportionately. However during special operating modes, idle, full power, acceleration and deceleration, it was necessary to have additional ways to adjust the fuelling amount.
This need to have additional ways to adjust the air fuel ratio, such as the idle circuit at left, made carburetors quite complex in their makeup and the control of the air fuel ratio was minimal at best.
The introduction of the catalytic converter in 1975 spelled the beginning of the end for the carburetor. Strict emission control requirements in 1980 caused the introduction of the three way catalytic converter.
This converter utilizes a reduction catalyst, Rhodium to reduce NOx gas emissions and an oxidizing catalyst, Platinum and or Palladium to reduce Hydrocarbon and carbon monoxide emissions. This converter works best at reducing all three emissions in a very narrow operational band near the stoichiometric air fuel ratio of 14.7:1 so in order for this converter to work properly precise control over the air fuel mixture became essential.
Electronic fuel injection became the way to gain this control. Fuel injection involves spraying or injecting fuel directly into the engine's intake manifold. Fuel injection, especially when it is electronically controlled, has several major advantages over carbureted systems.
These include improved driveability under all conditions, improved fuel control and economy, decreased exhaust emissions, and an increase in engine efficiency and power. In the 1980s manufacturers began to replace carburetors with electronic fuel Injection (EFI) systems.
Many of the early EFI systems were throttle body injection (TBI) systems in which the fuel was injected above the throttle plates similar to carburetors.
Engines equipped with TBI have gradually become equipped with port fuel injection (PFI), which has injectors located in the intake ports of the cylinders. Since the 1995 model year, all new cars in North America are equipped with EFI systems.
Diesel engines, for quite some time, have been equipped with fuel injection systems. The two basic differences between gasoline injection and diesel injection are, diesel fuel is injected directly into the cylinders at very high pressures, from a low of around 6,000 PSI up to 34,000 PSI, and diesel fuel injection systems are operated mechanically rather than electronically. Although late-model diesel systems use electronic fuel controls, most fuel injection systems are mechanically actuated systems.
Gasoline fuel injection involves injecting the fuel into the incoming airflow at relatively low pressure 30 PSI to 80 PSI, before it reaches the engine cylinder, through a throttle body or more commonly into the intake manifold just before the intake valve.
Some manufacturers are starting to introduce direct gasoline injection systems operating at higher pressures of approximately 800 to 3,000 PSI. These systems at this time however are extremely rare and are usually limited to high performance vehicles.
MERCEDES BENZ GDI ENGINE PACKAGE
Electronic fuel injection systems only inject fuel during part of the engine's cycle usually during the intake stroke. The engine fuel needs are measured by intake airflow past a sensor, mass airflow or volume system, or by intake manifold pressure (vacuum) and engine speed, pressure density system. The airflow or manifold vacuum sensor converts its reading to an electrical signal and sends it to the engine control computer. The computer processes this signal (and others) and calculates the fuel needs of the engine. The computer then grounds the fuel injector’s power circuit completing it and opening the fuel injector/s. The computer determines the amount of time the injector stays open and sprays fuel. This “on time” interval is known as the injector pulse width and is measured in milliseconds, ms, or thousandths of a second. Throttle body injection systems have a throttle body assembly mounted on the intake manifold in the position usually occupied by a carburetor. The throttle body assembly usually contains one or two injectors. Throttle body injection or TBI On port fuel injection systems, fuel injectors are mounted near each intake valve and will have an injector for every cylinder. Aside from the differences in injector location and number of injectors, operation of throttle body and port systems is quite similar with regard to fuel and air metering, sensors, and computer operation. Port-type continuous injection systems (CIS) have been used in the past on many European import vehicles. These systems deliver a steady stream of pressurized fuel into the intake manifold. The injectors do not pulse on and off as in port and throttle body systems.
In CIS, the amount of fuel delivered is controlled by the rate of airflow entering the engine. The airflow sensor is the large metal flap that is placed in the air induction system so that air entering must move the flap to get by, this can be seen in the K-Jetronic picture on the previous page as item 8. This sensor also controls movement of a plunger in a fuel distributor, item 7 that alters fuel flow to the injectors. When introduced, CIS was a mechanically controlled system. However, oxygen sensor feedback circuits and other electronic controls were added to the system. CIS systems that have electronic controls are commonly referred to as CIS-E systems. These systems have given way to conventional EFI systems. ELECTRONIC FUEL INJECTION SYSTEM Electronic fuel injection (EFI) has proven to be the most precise, reliable, and cost effective method of delivering fuel to the combustion chambers of today’s vehicles. EFI systems must provide the correct air/fuel ratio for all engine loads, speeds, and temperature conditions. To accomplish this, an EFI system uses a fuel delivery system, air induction system, input sensors, control computer, fuel injectors, and some sort of idle speed control. In a typical EFI fuel delivery system, fuel is drawn from the fuel tank by an in-tank or chassismounted electric fuel pump. Before it reaches the injectors, the fuel passes through a filter that removes dirt and impurities. A fuel line pressure regulator controls the fuel pressure between 28 and approximately 80 psi depending on the system. This fuel pressure generates the spraying force needed to inject the fuel. Excess fuel not required by the engine returns to the fuel tank through a fuel return line. THROTTLE BODY FUEL INJECTION For some auto manufacturers, TBI served as a stepping stone from carburetors to more advanced port fuel injection systems. TBI units were used on many engines during the 1980s but have been phased out in favour of port injection systems. The throttle body unit is similar in size and shape to a carburetor and, like a carburetor, mounts on the intake manifold. The injector(s) spray fuel down into a throttle body chamber leading to the intake manifold. The intake manifold feeds the air/fuel mixture to all cylinders.
TBI Operation The basic TBI assembly consists of two major castings: a throttle body with a throttle valve to control airflow and a fuel body to supply the required fuel. A fuel pressure regulator and fuel injector are integral parts of the fuel body. Also included as part of the assembly is a device to control idle speed and one to provide throttle valve positioning data. The throttle body casting has ports that can be located above, below, or at the throttle valve depending on the manufacturer's design. These ports generate vacuum signals for different sensors and for devices in the emission control system, such as the EGR valve, the charcoal canister purge system, and so on. TBI Pressure regulator The fuel pressure regulator used on the throttle body assembly is similar to a diaphragm-operated relief valve. Fuel pressure is on one side of the diaphragm and atmospheric pressure is on the other side. The regulator is designed to provide a constant pressure drop across the tip of the injectors on the fuel injector throughout the range of engine loads and speeds. As the engine air cleaner becomes clogged during operation the vacuum level in the air horn where the injector is located can increase. If not corrected for this could cause more fuel to be drawn through the injector for a given pulse width or on time. The pressure regulator allows the fuel pressure to be corrected because the air side of the regulator is exposed to the same vacuum conditions as the injector tip so when exposed to lower pressure, (vacuum), the regulator valve unloads at a lower pressure so the pressure across the injector tip is constant and therefore a given injector on time will always delivers the same amount of fuel. If the regulator pressure fails to too high a pressure, a strong fuel odour is usually detected and the engine runs too rich. On the other hand, regulator pressure that is too low results in poor engine performance or detonation can take place, due to the lean mixture. TBI Fuel injectors The fuel injector/s is/are usually bottom fed that is their fuel supply enters near the bottom of the injector. They are solenoid operated and pulsed on and off by the vehicle engine control computer. Surrounding the injector inlet is a fine screen filter where the incoming fuel is filtered. When the injector's solenoid is energized, a normally closed valve is lifted. Fuel under pressure is then allowed to flow through a calibrated orifice which atomizes the fuel into a fine mist and spray it into the airflow of the throttle body bore just above the throttle plate.
TBI Advantages Throttle body systems provide immensely improved fuel metering when compared to carburetors and they were a relatively cheap way to get some of the advantages of EFI. A lot of manufacturers initially offered these systems as a stepping stone to full blown port injection systems. Their main advantage when compared to port injection systems is that they are less expensive to produce and simpler to service. They also don't have injector balance problems to the extent that port injection systems do when the injectors begin to clog. However, throttle body units are not as efficient as port systems. The disadvantages are primarily manifold related. Like a carburetor system, fuel is still not distributed equally to all cylinders because of the distance the air fuel mixture has to travel in the air induction system; more remote cylinders are starved while cylinders closer to the throttle body tend to receive a slightly enriched mixture. A cold manifold may also cause fuel to condense and puddle in the manifold adding to distribution and vaporization problems. TBI systems will require EFE, (early fuel evaporation), systems such as intake air heaters, (electric or manifold mounted), or coolant or exhaust circulation through the intake manifold. Like a carburetor, throttle body injection systems must be mounted above the combustion chamber level, which combined with exhaust or coolant circulation eliminates the possibility of tuning the manifold design for more efficient air distribution. PORT FUEL INJECTION OR MPI, (Multi point injection). PFI systems use one injector at each cylinder They are mounted in the intake manifold near the cylinder head where they can inject a fine, atomized fuel mist as close as possible to the intake valve. The fuel is delivered to each cylinder’s injector from a fuel manifold usually referred to as a fuel rail. The fuel rail assembly on a PFI system of V-6 and V-8 engines usually consists of a left and right-hand rail assembly. The two rails can be connected either by crossover and return fuel tubes or by a mechanical bracket arrangement. Since each cylinder has its own injector, fuel distribution is exactly equal. With no fuel coming from the throttle plate to wet the manifold walls, there is no need for manifold heating or any early fuel evaporation system as in carburetor and throttle body injection systems. Fuel does not collect in puddles at the base of the manifold.
Because the fuel delivery is equal the air induction system can be modified to give optimum efficiency. This means the intake manifold passages can be tuned or specifically designed for high efficiency giving better low-speed power availability and better high speed breathing characteristics. The port-type systems provide a more accurate and efficient delivery of fuel. Some engines are now equipped with variable induction intake manifolds that have separate runners for low and high speeds. This induction tuning technology is only possible with port injection.
FUEL INJECTOR The fuel injector is an electromechanical device that meters and atomizes fuel so it can be sprayed into the intake manifold. O-rings are used to seal the injector at the intake manifold, throttle body, and fuel rail mounting positions. These O-rings provide thermal insulation to prevent the formation of vapour bubbles and promote good hot start, characteristics. They also dampen potentially damaging vibration. When the injector is electrically energized, a solenoid-operated valve opens, and a fine mist of fuel sprays from the injector tip. Two different valve designs are commonly used. The first consists of a valve body and a nozzle or needle valve that has a specially ground pintle. A movable armature is attached to the nozzle valve, which is pressed against the nozzle body sealing seat by a helical spring. The solenoid winding is located at the back of the valve body. When the solenoid winding is energized, it creates a magnetic field that draws the armature back and pulls the nozzle valve from its seat. When the solenoid is de-energized, the magnetic field collapses and the helical spring forces the nozzle valve back on its seat. The second popular valve design uses a ball valve and valve seat. In this case, the magnetic field created by the solenoid coil pulls a plunger upward lifting the ball valve from its seat. Once again, a spring is used to return the valve to its seated or closed position. There have been some problems with deposits on injector tips. Since small quantities of gum are present in gasoline, injector deposits usually occur when this gum bakes onto the injector tips after a hot engine is shut off. Most oil companies have added a detergent to their gasoline to help prevent injector tip deposits. Car manufacturers and auto parts stores sell detergents to place in the fuel tank to clean injector tips. Some auto manufacturers do not want you to add any type of injector cleaner to the gasoline. It seems that the chemicals in the cleaner may damage the coating on the coil windings in the injector. Some manufacturers and auto parts suppliers have designed deposit-resistant injectors. These injectors have several different pintle tip and orifice designs to help prevent deposits. On one type of deposit-resistant injector, the pintle seat opens outward away from the injector body and more clearance is provided between the pintle and the body.
Throttle Body The throttle body in a port fuel injection system controls the amount of air that enters the engine as well as the amount of vacuum in the manifold. The throttle body is a cast aluminium housing with a single throttle plate attached to the throttle shaft. The throttle position sensor is attached to the throttle shaft. The TPS enables the ECM to know where the throttle is positioned at all times.
Idle Air Control, AIC The Idle Air Control or IAC valve/motor may also be attached to the housing although sometimes the IAC is remotely mounted. The IAC is a stepper motor lets the computer precisely control the amount of air entering the engine during idle operation usually by allowing air to bypass the throttle, this allows the ECM control idle speed. The throttle shaft position is controlled by the accelerator pedal usually although on some designs the IAC motor will control throttle position rather than bypassing air around the throttle but the result is the same in that the ECM has control over air flow and idle speed. The throttle shaft extends the full length of the housing. The throttle bore controls the amount of incoming air that enters the air induction system. A small amount of coolant may also be routed through a passage in the throttle body to prevent icing during cold weather. Pressure regulator. The pressure regulator in port injection systems is similar to the regulator used in TBI systems. A diaphragm and valve assembly is positioned on the center of the regulator, and a diaphragm spring seats the valve on the fuel outlet. When fuel pressure reaches the setting of the regulator, the diaphragm moves against the spring tension, and the valve opens. This action allows fuel to flow through the return line to the fuel tank. Port systems present a problem for fuel regulation because the injectors have their tips located in the intake manifold below the throttle plate where constant changes in vacuum would affect the amount of fuel injected (at a given pulse width increased vacuum would allow more fuel to be pulled from the injector tip).
To compensate for these fluctuations port injection systems are equipped with fuel pressure regulators that have manifold vacuum acting on the regulator diaphragm. This vacuum works with the fuel pressure to move the diaphragm and open the valve. When the engine is running at idle speed, high manifold vacuum is supplied to the regulator. Under this condition, relatively low fuel pressure can open the regulator valve. When the engine is running under heavy load and/or wideopen throttle, a very low vacuum is supplied to the regulator. During these times, the vacuum does not help open the regulator valve and a relatively high fuel pressure is required to open the valve. Therefore when engine vacuum is high fuel pressure is low and when engine vacuum is low fuel pressure is high. This change in fuel pressure allows the computer to control fuelling precisely based on pulse width or on time of the injector alone, in other words the regulator strives to maintain a constant pressure drop across the injector tips so that a 10 millisecond on time would deliver the same amount of fuel regardless of the pressure/vacuum present in the manifold.
PORT FIRING CONTROL. While all port injection systems operate using an injector at each cylinder, they do not fire the injectors in the same manner. This one statement best defines the difference between typical multiport injection systems (MPI) and sequential fuel injection systems (SFI). SFI systems control each injector individually so it is opened just before the intake valve opens. This means the mixture is never static in the intake manifold and adjustments to the mixture can be made almost instantaneously between the firing of one injector and the next. Sequential firing is the most accurate and desirable method of regulating port injection. In MPI systems there are several main firing methods for the injectors, the most common being grouped single fire and batch or simultaneous double fire. In group fire systems the injectors are grouped together and each group of the injectors are turned on at the same time and the groups are fired alternately, with one group firing each engine revolution. When they fire they deliver the entire fuel requirement at once. With this method only two injectors can be fired relatively close to the time when the corresponding intake valve is about to open, so the fuel charge for the remaining cylinders must stand in the intake manifold for varying periods of time. Simultaneous or batch fire systems fire all of the injectors at the same time once each engine revolution delivering half the required fuel load at a time. Batch fire systems also have the fuel waiting in the intake for varying periods of time before the intake valve opens. Both of these systems are at a disadvantage because of this waiting even though the period of wait time is extremely short, equal fuel distribution can be compromised by a cylinder with an open intake valve “poaching” fuel from a cylinder that has fuel waiting. However these systems are a huge improvement from throttle body systems in this regard. The primary advantage of SFI is the ability to make instantaneous changes to the mixture and to provide perfect fuel distribution but in order to do so it must have a control circuit and power transistor for each injector where as batch fire system require only one control circuit, (it fires all the injectors at once), and group fire systems require only two or three control circuits. These control circuits add to the cost and complexity of the engine management system.
In SFI systems, each injector is connected individually into the computer, and the computer completes the ground for each injector, one at a time and allows cycle to cycle changes in fuelling and although its programming is more complex it allows more detailed fuel maps to be used and provides superior power and fuel economy. In group fire MPI systems, the injectors are grouped and all injectors within the group share the same common ground wire, this system is the next best thing to sequential and allows relatively fast fuelling adjustments and easier programming. Batch fire injection systems fire all of the injectors at the same time for every engine revolution all of the injectors share the same ground and power circuit. This type of system offers easy programming and relatively fast adjustments to the air/fuel mixture. The injectors are connected in parallel so the ECM sends out just one signal for all injectors. They all open and close at the same time. It simplifies the electronics with only a slight loss of injection efficiency.
SYSTEM SENSORS The ability of the fuel injection system to control the air/fuel ratio depends on its ability to properly time the injector pulses with the compression stroke of each cylinder and its ability to vary the injector "on" time, according to changing engine demands. Both tasks require the use of sensors that monitor the operating conditions of the engine.
AIRFLOW SENSOR. EFI systems try to operate at a stoichiometric ratio most of the time although this ratio is altered during cold start, Idle, full power/acceleration and deceleration conditions. This ratio is 14.7:1 air to fuel by weight for gasoline engines. To accurately control the proportion of fuel to air in the air/fuel charge, the fuel system must be able to measure the amount of air entering the engine. Any air that enters the engine without being measured will consequently throw off the air fuel ratio. This “leakage air” through vacuum or induction hose leakage is a common ailment with fully electronically controlled engines today, most times this causes an increase in engine idle speed and rough running. Several sensors have been developed to measure and or calculate air intake. MANIFOLD ABSOLUTE PRESSURE SENSOR Originally most EFI systems used absolute pressure (MAP) sensors to calculate airflow. The MAP sensor measures changes in the intake manifold pressure that result from changes in engine load and speed. The pressure measured by the MAP sensor is the difference between barometric pressure (outside air) and manifold pressure (vacuum). This type of system is known as a speed density system in that the computer looks at the manifold absolute pressure, (density), and the engine speed and calculates the air volume being ingested by comparing these two variables to known engine volumetric efficiency data. At closed throttle, the engine produces high vacuum value which creates a low voltage signal from the MAP. Wide-open throttle produces a low vacuum signal which in turn produces a high voltage value from the MAP sensor. The use of this sensor also allows the control computer to adjust automatically for different altitudes. The sensor consists of a silicone wafer that flexes with changes in the manifold vacuum this flexing changes the chips resistance. The control computer sends a voltage reference signal to the MAP sensor as the sensor resistance changes in reaction to manifold pressure/vacuum differences the output voltage also changes. The control computer can determine the difference between manifold pressure and atmospheric pressure by monitoring this output voltage. A wide open throttle position, higher pressure, little or no vacuum (high voltage) requires more fuel. A closed throttle position, lower pressure, high vacuum (low voltage) requires less fuel. A MAP sensor relies on an air temperature sensor, sometimes included in the MAP sensor, to adjust its base pulse signal to match incoming air temperature and therefore density. In EFI systems with a MAP sensor, the computer program is designed to calculate the amount of air entering the engine from the MAP and engine RPM input signals. Most EFI systems will use Mass air flow sensors, (MAF), these systems do not require MAP sensors however there are a few engines with both of these sensors. In these cases, the MAP is used mainly as a backup if the MAF fails.
AIR TEMPERATURE SENSOR Cold air is denser (weighs more) than warm air. Cold, dense air can burn more fuel than the same volume of warm air because it contains more oxygen molecules. Engine management computers relying on speed density systems to calculate airflow and system that rely on air flow sensors only must also have air temperature information in order to correctly calculate the air fuel mixture. Airflow measurement sensors only measure air volume and must have their readings adjusted to account for differences in air temperature. Most systems do this by using an air temperature sensor mounted in the throttle body of the induction system. The air sensor measures air temperature and sends an electronic signal to the control computer. The computer uses this input along with the air volume or density input to determine the amount of oxygen entering the engine. In some early EFI systems, the incoming air is heated to a set temperature. In these systems an air temperature sensor is used to ensure that this predetermined operating temperature is maintained. MASS AIRFLOW SENSORS A mass airflow sensor (MAF), does the job of a volume airflow sensor and an air temperature sensor. It measures air mass. The mass of a given amount of air is calculated by multiplying its volume by its density. As explained previously, the denser the air, the more oxygen it contains. Monitoring the oxygen in a given volume of air is important, since oxygen is the prime catalyst in the combustion process. From a measurement of mass, the electronic control module adjusts the fuel delivery for the oxygen content in a given volume of air. The accuracy of air/fuel ratios is greatly enhanced when matching fuel to air mass as compared to either volume sensors or speed density systems. There are two basic types of mass airflow sensor, the hot wire type which converts air flowing past a heated sensing element into a voltage signal. The strength of this signal is determined by the energy needed to keep the element at a constant temperature above the incoming ambient air temperature. As the volume and density (mass) of airflow across the heated element changes, the temperature of the element is affected and the current flow to the element is adjusted to maintain the desired temperature of the heating element. The varying current flow parallels the particular characteristics of the incoming air (hot, dry, cold, or humid). The electronic control module monitors the changes in current to determine air mass and to calculate precise fuel requirements. The element temperature is set at 100° to 200°C above incoming air temperature. Each time the ignition switch is turned to the off position, the wire is heated to approximately 1,000°C for 1 second to burn off any accumulated dust and contaminants The second type is the hot film or thin film type, which uses an upstream and a downstream sensor. There is a heater between the two sensors and like the hot wire type the ECM tries to maintain a set temperature. The ECM compares the temperature difference between the upstream and downstream resistors and interprets this as the airflow.
Karman Vortex Sensor Another older design of airflow sensor used mostly on imports is the Karman Vortex sensor; it works on a different operating principle. Air entering the airflow sensor assembly passes through vanes arranged around the inside of a tube. As the air flows through the vanes, it begins to swirl. The outer part of the swirling air exerts high pressure against the outside of the housing. There is a low pressure area in the center. The low-pressure area moves in a circular motion as the air swirls through the intake tube. Two pressure-sensing tubes near the end of the tube sense the low-pressure area as it moves around. An electronic sensor counts how many times the low-pressure area is sensed. The faster the airflow, the more times the low pressure area is sensed. This is translated into a signal that indicates to the combustion control computer how much air is flowing into the intake manifold.
OTHER EFI SYSTEM SENSORS In addition to airflow, air mass, or manifold absolute pressure readings, the control computer relies on input from a number of other system sensors. This input further adjusts the injector pulse width to match engine operating conditions. Operating conditions are communicated to the control computer by the following types of sensors. COOLANT TEMPERATURE. This sensor is one of the most important signals to the engine management system in terms of fuel efficiency. The coolant temperature sensor signals the electronic control module when the engine needs cold fuel enrichment, as it does during warm-up. This adds to the injector base pulse width, but its influence on fuelling decreases to zero as the engine warms up. This sensor also has a large effect on engine timing and plays a huge role in emission control. There are two types of these sensor NTC, or negative temperature coefficient, where the resistance decreases as temperature increases, and PTC, or positive temperature coefficient, where the resistance increases as temperature rises. NTCs are by far the most popular. By looking at the ECT circuit at left you can see that an NTC will have a high voltage signal at low temperature and this voltage will decrease as the engine warms up.
THROTTLE POSITION. The throttle position sensor provides the engine management computer with precise throttle position information. The TPS is a variable resistance potentiometer in most applications although Caterpillar uses a pulse width generating sensor for what it claims to be a more accurate and reliable signal. Some EFI systems rely on the result of throttle management by the driver to determine power intent, (do I want to go faster or slower), however most trucks and newer systems are use drive by wire technology where there is no physical connection between the drivers foot and the throttle plate. Usually the TPS will also have one or two contact switches integral to the sensor. The switches in the sensor signal the electronic control module for idle enrichment when the throttle is closed. These same throttle switches signal the electronic control module when the throttle is near the wide-open throttle position to provide full load enrichment. ENGINE SPEED. The engine speed sensor is usually of the variable reluctance type although sometimes Hall Effect sensors are used for this purpose. The variable reluctance sensor requires no reference voltage to operate and has only two wires. It consists of a permanent magnet and a wire coil with an iron core. As the teeth on a reluctor or toothed wheel pass the sensor core a magnetic field is first built up and then collapsed inducing a small AC current in the coil. The frequency and intensity of this current will change with engine speed. The ECM reads the frequency to determine engine speed. This sensor is also known as induction pulse generator. This sensor is sometimes used with a tone ring with different tooth configurations to signal the computer as to engine position. OXYGEN SENSOR The oxygen sensor is arguably the most important sensor in any EFI system. The O2 sensor allows the engine control computer to monitor engine exhaust conditions and adjust engine fuelling to compensate for rich or lean burn conditions. This setup gives the computer closed loop control of engine fuelling. Closed loop means that the computer can select a fuelling level from its fuelling maps then monitor the exhaust for a rich, (not enough oxygen in the exhaust), or lean, (too much oxygen in the exhaust), condition and then fine tune its fuelling to ensure a stoichiometric burn condition to optimize engine power, fuel consumption and emissions.
Most new systems allow the engine management system to “learn” from past conditions and automatically adjust fuelling maps to compensate for ageing components in the engine by monitoring the exhaust output. This ability to “learn” has greatly improved emission controls throughout the years. Oxygen sensors typically have a life of approximately 75,000 to 100,000 Kilometres and more than pay for their replacement cost with fuel economy savings. There are two main types of oxygen sensors in use today the Zirconium Dioxide type and the Titanium Oxide type. The zirconium dioxide oxygen sensor is a galvanic voltage generator. This produces a voltage as much as 1.0-volt when a exhaust stream from a gasoline engine runs rich and little voltage when it operates lean (excess air) A normal oxygen content for an engine operating at stoichiometric ratio 14.7:1 is 2%. A rich exhaust (excessive fuel) has almost no oxygen. When there is a large difference in the amount of oxygen touching the outside and inside surfaces, there is more conduction of oxygen ions across the element, and the sensor puts out a voltage signal above 600mV. With lean exhaust (excessive oxygen) there is about two percent oxygen in the exhaust. This is a smaller difference in oxygen from the outside surfaces that results in less conduction and a voltage signal below 300 mV. The voltages are monitored and used by the ECM to "fine tune" the air/fuel ratio to achieve the ideal mixture desired. Titanium Oxide Oxygen sensors unlike Zirconium dioxide oxygen sensors indicate exhaust gas oxygen content using resistance. The sensor resistance changes dramatically at the stoichiometric ratio which provides feedback to the engine management system. All O2 sensors create a waveform that constantly switches from lean to rich and the computer adjusts the injector pulse width to compensate. This switching usually occurs several times a second and the most common problem with O2 sensors is that the switching speed slows down over time and the computer can’t adjust the fuelling quickly enough. O2 sensors also need to be at approximately 600 degrees Fahrenheit 310 Degrees Celsius in order to work so a lot of the sensors will have a heating element to speed the warming process. When the O2 sensor is sending signals to the computer it creates a feed back loop in that the computer reads to O2 sensor information and then sets the fuelling and that will change the exhaust gas oxygen content then the Computer reads the O2 again and changes the fuelling based on its input. This operation known as “closed loop” operation, while the engine and therefore the O2 sensor is warming up the engine operates in “open loop”. The latest type of O2 sensor is known as the Planar or Wide range oxygen sensor. Oxygen sensors up till now have read only rich lean conditions and are known as switching oxygen sensors because they switch back and forth between rich and lean several times a second and the ECM responds by leaning out or enriching the air fuel ratio. These sensors are not as valuable in GDI or direct gasoline injection systems which are capable of running a stratified air fuel ratio that may be very lean or in “lean burn” diesel applications. The Planar or wide range O2 sensor reads exhaust O2 levels over a much wider range and will be essential for newer technologies.
ADDITIONAL INPUT INFORMATION SENSORS. Additional sensors are also used to provide the following information on engine conditions. NOTE: This list does not attempt to cover all of the sensors that are used by all manufacturers. It contains the most common. Detonation, or knock sensor, usually a piezoresistive sensor. Crankshaft position sensor either a hall effect or an electromagnetic pulse generator. Camshaft position sensor either a hall effect or an electromagnetic pulse generator. Air conditioner operation input to the ECM Gearshift lever position input to the ECM Battery voltage Input to the ECM ELECTRONIC CONTROL MODULE The heart of the fuel injection system is the control computer or electronic control module (ECM). The ECM is a small computer that is usually mounted within the passenger compartment to keep it away from the heat and vibration of the engine. The ECM includes solid state devices, including integrated circuits and a microprocessor.
The ECM receives signals from all the system sensors, processes them, and transmits programmed electrical pulses to the fuel injectors. Both incoming and outgoing signals are sent through a wiring harness and a multiple-pin connector. Electronic feedback in the ECM means the unit is selfregulating and is controlling the injectors on the basis of operating performance or parameters rather than on preprogrammed instructions. An ECM feedback loop, for example, reads signals from the oxygen sensor, varies the pulse width of the injectors, and again reads the signals from the oxygen sensor. This is repeated until the injectors are pulsed for just the amount of time needed to get the proper amount of oxygen into the exhaust stream. While this interaction is occurring, the system is operating in closed loop. When conditions, such as starting or wide-open throttle, demand that the signals from the oxygen sensor be ignored, the system operates in open loop. During open loop, injector pulse length is controlled by set parameters contained in the ECM memory banks. The ECM stores complex fuelling, timing and other operating condition “maps” than are constantly compared to actual operating conditions. The ECM tries to get actual operating conditions based on sensor input to match the “maps” stored in memory.
Gasoline Direct Injection or GDI GDI systems were first introduced in 1955 by Mercedes however cost and complexity led to its demise rather quickly. Mitsubishi reintroduced the direct injection concept in 1996 and has since produced over 400,000 engines with this technology however the expected fuel economies have not yet been realized. After relatively good sales in Europe, Mitsubishi basically shelved the technology in 2001. General Motors has introduced GDI on only two models the Opel Vectra in Europe and the 2005 Soltice in North America. Because GDI did not immediately lead to fuel economy improvements sufficient to offset the increased cost most manufacturers are marketing the technology mostly on their higher end performance machines.
In direct gasoline-injection engines, the A/F mixture is formed directly in the combustion chamber. There are three basic ways, when under no load or light load the system can operate in stratified or lean burn mode when the fuel is injected near the end of the compression stroke at relatively high pressure, (800 to 3000 psi), through the injectors. This would normally create a problem for gasoline engines as a lean mixture is notoriously hard to ignite.
To overcome this problem two methods are employed wall guided and spray guided mixing. In wall guided the top of the piston has a sharp wall like area formed into it and as the fuel is sprayed in near the top of piston travel on the compression stroke the swirling air and the small amount of fuel mix to form a relatively stoichiometric mixture in the small area bounded by the “wall” which the is fairly easy to light. In the spray guided system the spray in pressure is quite high and the injector is specially designed to concentrate the spray in a small area then the mixture is immediately ignited., this is only available with high speed piezoelectric injectors, see below. Under moderate load the system operates at a stoichiometric ratio and fuel can be injected during the intake stroke and the ignition takes place as in a normal gasoline engine. In heavy load or fuel power operation I relatively rich mixture is injected, usually on the intake stroke. In newer systems with piezoelectric injectors the fuel injection can be broken into up to seven events providing a combination of these strategies including post ignition injection which makes the gas engine take on the high torque characteristics of a Diesel engine. These systems are being introduced by Mercedes, Audi and BMW. The precise metering, preparation and distribution of the intake air and the injected fuel for every combustion stroke leads to exceptional power performance, low fuel-consumption figures and low emission levels.
The high-pressure circuit of the direct gasoline injection is fed by a high-pressure pump compressing the fuel to the high pressure level required in the fuel rail. The injectors attached to the fuel rail meter and atomize the fuel extremely fast and under high pressure in order to achieve the best possible mixture formation directly in the combustion chamber.
Piezo electric injectors The injectors work via the piezoelectric effect, as the name might hint. Piezoelectricity is the ability of crystals to generate a voltage in response to applied mechanical stress. Piezoelectricity was discovered by a man called Pierre Curie and the word is derived from the Greek piezein, which means to squeeze or press. The piezoelectric effect is reversible in that piezoelectric crystals, when subjected to an externally applied voltage, can change shape by a small amount. Piezoelectric injectors use small discs of quartzlike crystalline material that deforms when subjected to a high-voltage low-current source, which provides the injector opening and closing action. Piezo-crystals have been used for years in pressure measurement devices because when they are pressurized they produce an electrical current. This method simply reverses the process and deforms the crystal by applying a current. With conventional solenoid type injectors coil saturation time led to relatively slow operation. Piezoelectric injector have no coil saturation time to worry about and with cycle times as low as 0.2 milliseconds, such injectors are several times faster than conventional solenoids, this in turn allows up to seven injector events during each compression stroke.
The precision of each injector is also increased, with injector-to-injector variation of only a few percent.
Advancements in engine management systems are occurring all the time, this one from Delphi, a former subsidiary of General Motors allows the ECM to detect combustion quality by comparing the reverse ion flow through the cylinder gases after a combustion event.
Delphi’s ionization current sensing, (ion sense), ignition subsystem consists of one ignition coil per cylinder and high temperature resistant electronics. Eliminating moving parts and high-voltage leads helps provide maximum energy supply to the spark plug. The spark plug is not only used as an actuator to ignite the air/fuel mixture, but also as an in-cylinder sensor to monitor the combustion process. Its signal contains misfire and knock information and may also be used for engine control features requiring knowledge of the actual combustion characteristics. Ionized current sensing is a technology based on the principle that electrical current flow in an ionized gas, (as during combustion), is proportional to the flame electrical conductivity. By placing a direct current bias on the spark plug electrodes, the conductivity can be measured. In the configuration shown in figure 1, the spark current is used to create this bias voltage, eliminating the need for any additional voltage source.
Located at this narrow section is a series of jets or pipes connected to a fuel bowl that has atmospheric pressure acting on it the fuel is then pushed up the pipe into the venturi. This method worked very well during normal operation, as the airflow increased the amount of fuel pushed or drawn out of the fuel bowl increased proportionately. However during special operating modes, idle, full power, acceleration and deceleration, it was necessary to have additional ways to adjust the fuelling amount.
This need to have additional ways to adjust the air fuel ratio, such as the idle circuit at left, made carburetors quite complex in their makeup and the control of the air fuel ratio was minimal at best.
The introduction of the catalytic converter in 1975 spelled the beginning of the end for the carburetor. Strict emission control requirements in 1980 caused the introduction of the three way catalytic converter.
This converter utilizes a reduction catalyst, Rhodium to reduce NOx gas emissions and an oxidizing catalyst, Platinum and or Palladium to reduce Hydrocarbon and carbon monoxide emissions. This converter works best at reducing all three emissions in a very narrow operational band near the stoichiometric air fuel ratio of 14.7:1 so in order for this converter to work properly precise control over the air fuel mixture became essential.
Electronic fuel injection became the way to gain this control. Fuel injection involves spraying or injecting fuel directly into the engine's intake manifold. Fuel injection, especially when it is electronically controlled, has several major advantages over carbureted systems.
These include improved driveability under all conditions, improved fuel control and economy, decreased exhaust emissions, and an increase in engine efficiency and power. In the 1980s manufacturers began to replace carburetors with electronic fuel Injection (EFI) systems.
Many of the early EFI systems were throttle body injection (TBI) systems in which the fuel was injected above the throttle plates similar to carburetors.
Engines equipped with TBI have gradually become equipped with port fuel injection (PFI), which has injectors located in the intake ports of the cylinders. Since the 1995 model year, all new cars in North America are equipped with EFI systems.
Diesel engines, for quite some time, have been equipped with fuel injection systems. The two basic differences between gasoline injection and diesel injection are, diesel fuel is injected directly into the cylinders at very high pressures, from a low of around 6,000 PSI up to 34,000 PSI, and diesel fuel injection systems are operated mechanically rather than electronically. Although late-model diesel systems use electronic fuel controls, most fuel injection systems are mechanically actuated systems.
Gasoline fuel injection involves injecting the fuel into the incoming airflow at relatively low pressure 30 PSI to 80 PSI, before it reaches the engine cylinder, through a throttle body or more commonly into the intake manifold just before the intake valve.
Some manufacturers are starting to introduce direct gasoline injection systems operating at higher pressures of approximately 800 to 3,000 PSI. These systems at this time however are extremely rare and are usually limited to high performance vehicles.
MERCEDES BENZ GDI ENGINE PACKAGE
Electronic fuel injection systems only inject fuel during part of the engine's cycle usually during the intake stroke. The engine fuel needs are measured by intake airflow past a sensor, mass airflow or volume system, or by intake manifold pressure (vacuum) and engine speed, pressure density system. The airflow or manifold vacuum sensor converts its reading to an electrical signal and sends it to the engine control computer. The computer processes this signal (and others) and calculates the fuel needs of the engine. The computer then grounds the fuel injector’s power circuit completing it and opening the fuel injector/s. The computer determines the amount of time the injector stays open and sprays fuel. This “on time” interval is known as the injector pulse width and is measured in milliseconds, ms, or thousandths of a second. Throttle body injection systems have a throttle body assembly mounted on the intake manifold in the position usually occupied by a carburetor. The throttle body assembly usually contains one or two injectors. Throttle body injection or TBI On port fuel injection systems, fuel injectors are mounted near each intake valve and will have an injector for every cylinder. Aside from the differences in injector location and number of injectors, operation of throttle body and port systems is quite similar with regard to fuel and air metering, sensors, and computer operation. Port-type continuous injection systems (CIS) have been used in the past on many European import vehicles. These systems deliver a steady stream of pressurized fuel into the intake manifold. The injectors do not pulse on and off as in port and throttle body systems.
In CIS, the amount of fuel delivered is controlled by the rate of airflow entering the engine. The airflow sensor is the large metal flap that is placed in the air induction system so that air entering must move the flap to get by, this can be seen in the K-Jetronic picture on the previous page as item 8. This sensor also controls movement of a plunger in a fuel distributor, item 7 that alters fuel flow to the injectors. When introduced, CIS was a mechanically controlled system. However, oxygen sensor feedback circuits and other electronic controls were added to the system. CIS systems that have electronic controls are commonly referred to as CIS-E systems. These systems have given way to conventional EFI systems. ELECTRONIC FUEL INJECTION SYSTEM Electronic fuel injection (EFI) has proven to be the most precise, reliable, and cost effective method of delivering fuel to the combustion chambers of today’s vehicles. EFI systems must provide the correct air/fuel ratio for all engine loads, speeds, and temperature conditions. To accomplish this, an EFI system uses a fuel delivery system, air induction system, input sensors, control computer, fuel injectors, and some sort of idle speed control. In a typical EFI fuel delivery system, fuel is drawn from the fuel tank by an in-tank or chassismounted electric fuel pump. Before it reaches the injectors, the fuel passes through a filter that removes dirt and impurities. A fuel line pressure regulator controls the fuel pressure between 28 and approximately 80 psi depending on the system. This fuel pressure generates the spraying force needed to inject the fuel. Excess fuel not required by the engine returns to the fuel tank through a fuel return line. THROTTLE BODY FUEL INJECTION For some auto manufacturers, TBI served as a stepping stone from carburetors to more advanced port fuel injection systems. TBI units were used on many engines during the 1980s but have been phased out in favour of port injection systems. The throttle body unit is similar in size and shape to a carburetor and, like a carburetor, mounts on the intake manifold. The injector(s) spray fuel down into a throttle body chamber leading to the intake manifold. The intake manifold feeds the air/fuel mixture to all cylinders.
TBI Operation The basic TBI assembly consists of two major castings: a throttle body with a throttle valve to control airflow and a fuel body to supply the required fuel. A fuel pressure regulator and fuel injector are integral parts of the fuel body. Also included as part of the assembly is a device to control idle speed and one to provide throttle valve positioning data. The throttle body casting has ports that can be located above, below, or at the throttle valve depending on the manufacturer's design. These ports generate vacuum signals for different sensors and for devices in the emission control system, such as the EGR valve, the charcoal canister purge system, and so on. TBI Pressure regulator The fuel pressure regulator used on the throttle body assembly is similar to a diaphragm-operated relief valve. Fuel pressure is on one side of the diaphragm and atmospheric pressure is on the other side. The regulator is designed to provide a constant pressure drop across the tip of the injectors on the fuel injector throughout the range of engine loads and speeds. As the engine air cleaner becomes clogged during operation the vacuum level in the air horn where the injector is located can increase. If not corrected for this could cause more fuel to be drawn through the injector for a given pulse width or on time. The pressure regulator allows the fuel pressure to be corrected because the air side of the regulator is exposed to the same vacuum conditions as the injector tip so when exposed to lower pressure, (vacuum), the regulator valve unloads at a lower pressure so the pressure across the injector tip is constant and therefore a given injector on time will always delivers the same amount of fuel. If the regulator pressure fails to too high a pressure, a strong fuel odour is usually detected and the engine runs too rich. On the other hand, regulator pressure that is too low results in poor engine performance or detonation can take place, due to the lean mixture. TBI Fuel injectors The fuel injector/s is/are usually bottom fed that is their fuel supply enters near the bottom of the injector. They are solenoid operated and pulsed on and off by the vehicle engine control computer. Surrounding the injector inlet is a fine screen filter where the incoming fuel is filtered. When the injector's solenoid is energized, a normally closed valve is lifted. Fuel under pressure is then allowed to flow through a calibrated orifice which atomizes the fuel into a fine mist and spray it into the airflow of the throttle body bore just above the throttle plate.
TBI Advantages Throttle body systems provide immensely improved fuel metering when compared to carburetors and they were a relatively cheap way to get some of the advantages of EFI. A lot of manufacturers initially offered these systems as a stepping stone to full blown port injection systems. Their main advantage when compared to port injection systems is that they are less expensive to produce and simpler to service. They also don't have injector balance problems to the extent that port injection systems do when the injectors begin to clog. However, throttle body units are not as efficient as port systems. The disadvantages are primarily manifold related. Like a carburetor system, fuel is still not distributed equally to all cylinders because of the distance the air fuel mixture has to travel in the air induction system; more remote cylinders are starved while cylinders closer to the throttle body tend to receive a slightly enriched mixture. A cold manifold may also cause fuel to condense and puddle in the manifold adding to distribution and vaporization problems. TBI systems will require EFE, (early fuel evaporation), systems such as intake air heaters, (electric or manifold mounted), or coolant or exhaust circulation through the intake manifold. Like a carburetor, throttle body injection systems must be mounted above the combustion chamber level, which combined with exhaust or coolant circulation eliminates the possibility of tuning the manifold design for more efficient air distribution. PORT FUEL INJECTION OR MPI, (Multi point injection). PFI systems use one injector at each cylinder They are mounted in the intake manifold near the cylinder head where they can inject a fine, atomized fuel mist as close as possible to the intake valve. The fuel is delivered to each cylinder’s injector from a fuel manifold usually referred to as a fuel rail. The fuel rail assembly on a PFI system of V-6 and V-8 engines usually consists of a left and right-hand rail assembly. The two rails can be connected either by crossover and return fuel tubes or by a mechanical bracket arrangement. Since each cylinder has its own injector, fuel distribution is exactly equal. With no fuel coming from the throttle plate to wet the manifold walls, there is no need for manifold heating or any early fuel evaporation system as in carburetor and throttle body injection systems. Fuel does not collect in puddles at the base of the manifold.
Because the fuel delivery is equal the air induction system can be modified to give optimum efficiency. This means the intake manifold passages can be tuned or specifically designed for high efficiency giving better low-speed power availability and better high speed breathing characteristics. The port-type systems provide a more accurate and efficient delivery of fuel. Some engines are now equipped with variable induction intake manifolds that have separate runners for low and high speeds. This induction tuning technology is only possible with port injection.
FUEL INJECTOR The fuel injector is an electromechanical device that meters and atomizes fuel so it can be sprayed into the intake manifold. O-rings are used to seal the injector at the intake manifold, throttle body, and fuel rail mounting positions. These O-rings provide thermal insulation to prevent the formation of vapour bubbles and promote good hot start, characteristics. They also dampen potentially damaging vibration. When the injector is electrically energized, a solenoid-operated valve opens, and a fine mist of fuel sprays from the injector tip. Two different valve designs are commonly used. The first consists of a valve body and a nozzle or needle valve that has a specially ground pintle. A movable armature is attached to the nozzle valve, which is pressed against the nozzle body sealing seat by a helical spring. The solenoid winding is located at the back of the valve body. When the solenoid winding is energized, it creates a magnetic field that draws the armature back and pulls the nozzle valve from its seat. When the solenoid is de-energized, the magnetic field collapses and the helical spring forces the nozzle valve back on its seat. The second popular valve design uses a ball valve and valve seat. In this case, the magnetic field created by the solenoid coil pulls a plunger upward lifting the ball valve from its seat. Once again, a spring is used to return the valve to its seated or closed position. There have been some problems with deposits on injector tips. Since small quantities of gum are present in gasoline, injector deposits usually occur when this gum bakes onto the injector tips after a hot engine is shut off. Most oil companies have added a detergent to their gasoline to help prevent injector tip deposits. Car manufacturers and auto parts stores sell detergents to place in the fuel tank to clean injector tips. Some auto manufacturers do not want you to add any type of injector cleaner to the gasoline. It seems that the chemicals in the cleaner may damage the coating on the coil windings in the injector. Some manufacturers and auto parts suppliers have designed deposit-resistant injectors. These injectors have several different pintle tip and orifice designs to help prevent deposits. On one type of deposit-resistant injector, the pintle seat opens outward away from the injector body and more clearance is provided between the pintle and the body.
Throttle Body The throttle body in a port fuel injection system controls the amount of air that enters the engine as well as the amount of vacuum in the manifold. The throttle body is a cast aluminium housing with a single throttle plate attached to the throttle shaft. The throttle position sensor is attached to the throttle shaft. The TPS enables the ECM to know where the throttle is positioned at all times.
Idle Air Control, AIC The Idle Air Control or IAC valve/motor may also be attached to the housing although sometimes the IAC is remotely mounted. The IAC is a stepper motor lets the computer precisely control the amount of air entering the engine during idle operation usually by allowing air to bypass the throttle, this allows the ECM control idle speed. The throttle shaft position is controlled by the accelerator pedal usually although on some designs the IAC motor will control throttle position rather than bypassing air around the throttle but the result is the same in that the ECM has control over air flow and idle speed. The throttle shaft extends the full length of the housing. The throttle bore controls the amount of incoming air that enters the air induction system. A small amount of coolant may also be routed through a passage in the throttle body to prevent icing during cold weather. Pressure regulator. The pressure regulator in port injection systems is similar to the regulator used in TBI systems. A diaphragm and valve assembly is positioned on the center of the regulator, and a diaphragm spring seats the valve on the fuel outlet. When fuel pressure reaches the setting of the regulator, the diaphragm moves against the spring tension, and the valve opens. This action allows fuel to flow through the return line to the fuel tank. Port systems present a problem for fuel regulation because the injectors have their tips located in the intake manifold below the throttle plate where constant changes in vacuum would affect the amount of fuel injected (at a given pulse width increased vacuum would allow more fuel to be pulled from the injector tip).
To compensate for these fluctuations port injection systems are equipped with fuel pressure regulators that have manifold vacuum acting on the regulator diaphragm. This vacuum works with the fuel pressure to move the diaphragm and open the valve. When the engine is running at idle speed, high manifold vacuum is supplied to the regulator. Under this condition, relatively low fuel pressure can open the regulator valve. When the engine is running under heavy load and/or wideopen throttle, a very low vacuum is supplied to the regulator. During these times, the vacuum does not help open the regulator valve and a relatively high fuel pressure is required to open the valve. Therefore when engine vacuum is high fuel pressure is low and when engine vacuum is low fuel pressure is high. This change in fuel pressure allows the computer to control fuelling precisely based on pulse width or on time of the injector alone, in other words the regulator strives to maintain a constant pressure drop across the injector tips so that a 10 millisecond on time would deliver the same amount of fuel regardless of the pressure/vacuum present in the manifold.
PORT FIRING CONTROL. While all port injection systems operate using an injector at each cylinder, they do not fire the injectors in the same manner. This one statement best defines the difference between typical multiport injection systems (MPI) and sequential fuel injection systems (SFI). SFI systems control each injector individually so it is opened just before the intake valve opens. This means the mixture is never static in the intake manifold and adjustments to the mixture can be made almost instantaneously between the firing of one injector and the next. Sequential firing is the most accurate and desirable method of regulating port injection. In MPI systems there are several main firing methods for the injectors, the most common being grouped single fire and batch or simultaneous double fire. In group fire systems the injectors are grouped together and each group of the injectors are turned on at the same time and the groups are fired alternately, with one group firing each engine revolution. When they fire they deliver the entire fuel requirement at once. With this method only two injectors can be fired relatively close to the time when the corresponding intake valve is about to open, so the fuel charge for the remaining cylinders must stand in the intake manifold for varying periods of time. Simultaneous or batch fire systems fire all of the injectors at the same time once each engine revolution delivering half the required fuel load at a time. Batch fire systems also have the fuel waiting in the intake for varying periods of time before the intake valve opens. Both of these systems are at a disadvantage because of this waiting even though the period of wait time is extremely short, equal fuel distribution can be compromised by a cylinder with an open intake valve “poaching” fuel from a cylinder that has fuel waiting. However these systems are a huge improvement from throttle body systems in this regard. The primary advantage of SFI is the ability to make instantaneous changes to the mixture and to provide perfect fuel distribution but in order to do so it must have a control circuit and power transistor for each injector where as batch fire system require only one control circuit, (it fires all the injectors at once), and group fire systems require only two or three control circuits. These control circuits add to the cost and complexity of the engine management system.
In SFI systems, each injector is connected individually into the computer, and the computer completes the ground for each injector, one at a time and allows cycle to cycle changes in fuelling and although its programming is more complex it allows more detailed fuel maps to be used and provides superior power and fuel economy. In group fire MPI systems, the injectors are grouped and all injectors within the group share the same common ground wire, this system is the next best thing to sequential and allows relatively fast fuelling adjustments and easier programming. Batch fire injection systems fire all of the injectors at the same time for every engine revolution all of the injectors share the same ground and power circuit. This type of system offers easy programming and relatively fast adjustments to the air/fuel mixture. The injectors are connected in parallel so the ECM sends out just one signal for all injectors. They all open and close at the same time. It simplifies the electronics with only a slight loss of injection efficiency.
SYSTEM SENSORS The ability of the fuel injection system to control the air/fuel ratio depends on its ability to properly time the injector pulses with the compression stroke of each cylinder and its ability to vary the injector "on" time, according to changing engine demands. Both tasks require the use of sensors that monitor the operating conditions of the engine.
AIRFLOW SENSOR. EFI systems try to operate at a stoichiometric ratio most of the time although this ratio is altered during cold start, Idle, full power/acceleration and deceleration conditions. This ratio is 14.7:1 air to fuel by weight for gasoline engines. To accurately control the proportion of fuel to air in the air/fuel charge, the fuel system must be able to measure the amount of air entering the engine. Any air that enters the engine without being measured will consequently throw off the air fuel ratio. This “leakage air” through vacuum or induction hose leakage is a common ailment with fully electronically controlled engines today, most times this causes an increase in engine idle speed and rough running. Several sensors have been developed to measure and or calculate air intake. MANIFOLD ABSOLUTE PRESSURE SENSOR Originally most EFI systems used absolute pressure (MAP) sensors to calculate airflow. The MAP sensor measures changes in the intake manifold pressure that result from changes in engine load and speed. The pressure measured by the MAP sensor is the difference between barometric pressure (outside air) and manifold pressure (vacuum). This type of system is known as a speed density system in that the computer looks at the manifold absolute pressure, (density), and the engine speed and calculates the air volume being ingested by comparing these two variables to known engine volumetric efficiency data. At closed throttle, the engine produces high vacuum value which creates a low voltage signal from the MAP. Wide-open throttle produces a low vacuum signal which in turn produces a high voltage value from the MAP sensor. The use of this sensor also allows the control computer to adjust automatically for different altitudes. The sensor consists of a silicone wafer that flexes with changes in the manifold vacuum this flexing changes the chips resistance. The control computer sends a voltage reference signal to the MAP sensor as the sensor resistance changes in reaction to manifold pressure/vacuum differences the output voltage also changes. The control computer can determine the difference between manifold pressure and atmospheric pressure by monitoring this output voltage. A wide open throttle position, higher pressure, little or no vacuum (high voltage) requires more fuel. A closed throttle position, lower pressure, high vacuum (low voltage) requires less fuel. A MAP sensor relies on an air temperature sensor, sometimes included in the MAP sensor, to adjust its base pulse signal to match incoming air temperature and therefore density. In EFI systems with a MAP sensor, the computer program is designed to calculate the amount of air entering the engine from the MAP and engine RPM input signals. Most EFI systems will use Mass air flow sensors, (MAF), these systems do not require MAP sensors however there are a few engines with both of these sensors. In these cases, the MAP is used mainly as a backup if the MAF fails.
AIR TEMPERATURE SENSOR Cold air is denser (weighs more) than warm air. Cold, dense air can burn more fuel than the same volume of warm air because it contains more oxygen molecules. Engine management computers relying on speed density systems to calculate airflow and system that rely on air flow sensors only must also have air temperature information in order to correctly calculate the air fuel mixture. Airflow measurement sensors only measure air volume and must have their readings adjusted to account for differences in air temperature. Most systems do this by using an air temperature sensor mounted in the throttle body of the induction system. The air sensor measures air temperature and sends an electronic signal to the control computer. The computer uses this input along with the air volume or density input to determine the amount of oxygen entering the engine. In some early EFI systems, the incoming air is heated to a set temperature. In these systems an air temperature sensor is used to ensure that this predetermined operating temperature is maintained. MASS AIRFLOW SENSORS A mass airflow sensor (MAF), does the job of a volume airflow sensor and an air temperature sensor. It measures air mass. The mass of a given amount of air is calculated by multiplying its volume by its density. As explained previously, the denser the air, the more oxygen it contains. Monitoring the oxygen in a given volume of air is important, since oxygen is the prime catalyst in the combustion process. From a measurement of mass, the electronic control module adjusts the fuel delivery for the oxygen content in a given volume of air. The accuracy of air/fuel ratios is greatly enhanced when matching fuel to air mass as compared to either volume sensors or speed density systems. There are two basic types of mass airflow sensor, the hot wire type which converts air flowing past a heated sensing element into a voltage signal. The strength of this signal is determined by the energy needed to keep the element at a constant temperature above the incoming ambient air temperature. As the volume and density (mass) of airflow across the heated element changes, the temperature of the element is affected and the current flow to the element is adjusted to maintain the desired temperature of the heating element. The varying current flow parallels the particular characteristics of the incoming air (hot, dry, cold, or humid). The electronic control module monitors the changes in current to determine air mass and to calculate precise fuel requirements. The element temperature is set at 100° to 200°C above incoming air temperature. Each time the ignition switch is turned to the off position, the wire is heated to approximately 1,000°C for 1 second to burn off any accumulated dust and contaminants The second type is the hot film or thin film type, which uses an upstream and a downstream sensor. There is a heater between the two sensors and like the hot wire type the ECM tries to maintain a set temperature. The ECM compares the temperature difference between the upstream and downstream resistors and interprets this as the airflow.
Karman Vortex Sensor Another older design of airflow sensor used mostly on imports is the Karman Vortex sensor; it works on a different operating principle. Air entering the airflow sensor assembly passes through vanes arranged around the inside of a tube. As the air flows through the vanes, it begins to swirl. The outer part of the swirling air exerts high pressure against the outside of the housing. There is a low pressure area in the center. The low-pressure area moves in a circular motion as the air swirls through the intake tube. Two pressure-sensing tubes near the end of the tube sense the low-pressure area as it moves around. An electronic sensor counts how many times the low-pressure area is sensed. The faster the airflow, the more times the low pressure area is sensed. This is translated into a signal that indicates to the combustion control computer how much air is flowing into the intake manifold.
OTHER EFI SYSTEM SENSORS In addition to airflow, air mass, or manifold absolute pressure readings, the control computer relies on input from a number of other system sensors. This input further adjusts the injector pulse width to match engine operating conditions. Operating conditions are communicated to the control computer by the following types of sensors. COOLANT TEMPERATURE. This sensor is one of the most important signals to the engine management system in terms of fuel efficiency. The coolant temperature sensor signals the electronic control module when the engine needs cold fuel enrichment, as it does during warm-up. This adds to the injector base pulse width, but its influence on fuelling decreases to zero as the engine warms up. This sensor also has a large effect on engine timing and plays a huge role in emission control. There are two types of these sensor NTC, or negative temperature coefficient, where the resistance decreases as temperature increases, and PTC, or positive temperature coefficient, where the resistance increases as temperature rises. NTCs are by far the most popular. By looking at the ECT circuit at left you can see that an NTC will have a high voltage signal at low temperature and this voltage will decrease as the engine warms up.
THROTTLE POSITION. The throttle position sensor provides the engine management computer with precise throttle position information. The TPS is a variable resistance potentiometer in most applications although Caterpillar uses a pulse width generating sensor for what it claims to be a more accurate and reliable signal. Some EFI systems rely on the result of throttle management by the driver to determine power intent, (do I want to go faster or slower), however most trucks and newer systems are use drive by wire technology where there is no physical connection between the drivers foot and the throttle plate. Usually the TPS will also have one or two contact switches integral to the sensor. The switches in the sensor signal the electronic control module for idle enrichment when the throttle is closed. These same throttle switches signal the electronic control module when the throttle is near the wide-open throttle position to provide full load enrichment. ENGINE SPEED. The engine speed sensor is usually of the variable reluctance type although sometimes Hall Effect sensors are used for this purpose. The variable reluctance sensor requires no reference voltage to operate and has only two wires. It consists of a permanent magnet and a wire coil with an iron core. As the teeth on a reluctor or toothed wheel pass the sensor core a magnetic field is first built up and then collapsed inducing a small AC current in the coil. The frequency and intensity of this current will change with engine speed. The ECM reads the frequency to determine engine speed. This sensor is also known as induction pulse generator. This sensor is sometimes used with a tone ring with different tooth configurations to signal the computer as to engine position. OXYGEN SENSOR The oxygen sensor is arguably the most important sensor in any EFI system. The O2 sensor allows the engine control computer to monitor engine exhaust conditions and adjust engine fuelling to compensate for rich or lean burn conditions. This setup gives the computer closed loop control of engine fuelling. Closed loop means that the computer can select a fuelling level from its fuelling maps then monitor the exhaust for a rich, (not enough oxygen in the exhaust), or lean, (too much oxygen in the exhaust), condition and then fine tune its fuelling to ensure a stoichiometric burn condition to optimize engine power, fuel consumption and emissions.
Most new systems allow the engine management system to “learn” from past conditions and automatically adjust fuelling maps to compensate for ageing components in the engine by monitoring the exhaust output. This ability to “learn” has greatly improved emission controls throughout the years. Oxygen sensors typically have a life of approximately 75,000 to 100,000 Kilometres and more than pay for their replacement cost with fuel economy savings. There are two main types of oxygen sensors in use today the Zirconium Dioxide type and the Titanium Oxide type. The zirconium dioxide oxygen sensor is a galvanic voltage generator. This produces a voltage as much as 1.0-volt when a exhaust stream from a gasoline engine runs rich and little voltage when it operates lean (excess air) A normal oxygen content for an engine operating at stoichiometric ratio 14.7:1 is 2%. A rich exhaust (excessive fuel) has almost no oxygen. When there is a large difference in the amount of oxygen touching the outside and inside surfaces, there is more conduction of oxygen ions across the element, and the sensor puts out a voltage signal above 600mV. With lean exhaust (excessive oxygen) there is about two percent oxygen in the exhaust. This is a smaller difference in oxygen from the outside surfaces that results in less conduction and a voltage signal below 300 mV. The voltages are monitored and used by the ECM to "fine tune" the air/fuel ratio to achieve the ideal mixture desired. Titanium Oxide Oxygen sensors unlike Zirconium dioxide oxygen sensors indicate exhaust gas oxygen content using resistance. The sensor resistance changes dramatically at the stoichiometric ratio which provides feedback to the engine management system. All O2 sensors create a waveform that constantly switches from lean to rich and the computer adjusts the injector pulse width to compensate. This switching usually occurs several times a second and the most common problem with O2 sensors is that the switching speed slows down over time and the computer can’t adjust the fuelling quickly enough. O2 sensors also need to be at approximately 600 degrees Fahrenheit 310 Degrees Celsius in order to work so a lot of the sensors will have a heating element to speed the warming process. When the O2 sensor is sending signals to the computer it creates a feed back loop in that the computer reads to O2 sensor information and then sets the fuelling and that will change the exhaust gas oxygen content then the Computer reads the O2 again and changes the fuelling based on its input. This operation known as “closed loop” operation, while the engine and therefore the O2 sensor is warming up the engine operates in “open loop”. The latest type of O2 sensor is known as the Planar or Wide range oxygen sensor. Oxygen sensors up till now have read only rich lean conditions and are known as switching oxygen sensors because they switch back and forth between rich and lean several times a second and the ECM responds by leaning out or enriching the air fuel ratio. These sensors are not as valuable in GDI or direct gasoline injection systems which are capable of running a stratified air fuel ratio that may be very lean or in “lean burn” diesel applications. The Planar or wide range O2 sensor reads exhaust O2 levels over a much wider range and will be essential for newer technologies.
ADDITIONAL INPUT INFORMATION SENSORS. Additional sensors are also used to provide the following information on engine conditions. NOTE: This list does not attempt to cover all of the sensors that are used by all manufacturers. It contains the most common. Detonation, or knock sensor, usually a piezoresistive sensor. Crankshaft position sensor either a hall effect or an electromagnetic pulse generator. Camshaft position sensor either a hall effect or an electromagnetic pulse generator. Air conditioner operation input to the ECM Gearshift lever position input to the ECM Battery voltage Input to the ECM ELECTRONIC CONTROL MODULE The heart of the fuel injection system is the control computer or electronic control module (ECM). The ECM is a small computer that is usually mounted within the passenger compartment to keep it away from the heat and vibration of the engine. The ECM includes solid state devices, including integrated circuits and a microprocessor.
The ECM receives signals from all the system sensors, processes them, and transmits programmed electrical pulses to the fuel injectors. Both incoming and outgoing signals are sent through a wiring harness and a multiple-pin connector. Electronic feedback in the ECM means the unit is selfregulating and is controlling the injectors on the basis of operating performance or parameters rather than on preprogrammed instructions. An ECM feedback loop, for example, reads signals from the oxygen sensor, varies the pulse width of the injectors, and again reads the signals from the oxygen sensor. This is repeated until the injectors are pulsed for just the amount of time needed to get the proper amount of oxygen into the exhaust stream. While this interaction is occurring, the system is operating in closed loop. When conditions, such as starting or wide-open throttle, demand that the signals from the oxygen sensor be ignored, the system operates in open loop. During open loop, injector pulse length is controlled by set parameters contained in the ECM memory banks. The ECM stores complex fuelling, timing and other operating condition “maps” than are constantly compared to actual operating conditions. The ECM tries to get actual operating conditions based on sensor input to match the “maps” stored in memory.
Gasoline Direct Injection or GDI GDI systems were first introduced in 1955 by Mercedes however cost and complexity led to its demise rather quickly. Mitsubishi reintroduced the direct injection concept in 1996 and has since produced over 400,000 engines with this technology however the expected fuel economies have not yet been realized. After relatively good sales in Europe, Mitsubishi basically shelved the technology in 2001. General Motors has introduced GDI on only two models the Opel Vectra in Europe and the 2005 Soltice in North America. Because GDI did not immediately lead to fuel economy improvements sufficient to offset the increased cost most manufacturers are marketing the technology mostly on their higher end performance machines.
In direct gasoline-injection engines, the A/F mixture is formed directly in the combustion chamber. There are three basic ways, when under no load or light load the system can operate in stratified or lean burn mode when the fuel is injected near the end of the compression stroke at relatively high pressure, (800 to 3000 psi), through the injectors. This would normally create a problem for gasoline engines as a lean mixture is notoriously hard to ignite.
To overcome this problem two methods are employed wall guided and spray guided mixing. In wall guided the top of the piston has a sharp wall like area formed into it and as the fuel is sprayed in near the top of piston travel on the compression stroke the swirling air and the small amount of fuel mix to form a relatively stoichiometric mixture in the small area bounded by the “wall” which the is fairly easy to light. In the spray guided system the spray in pressure is quite high and the injector is specially designed to concentrate the spray in a small area then the mixture is immediately ignited., this is only available with high speed piezoelectric injectors, see below. Under moderate load the system operates at a stoichiometric ratio and fuel can be injected during the intake stroke and the ignition takes place as in a normal gasoline engine. In heavy load or fuel power operation I relatively rich mixture is injected, usually on the intake stroke. In newer systems with piezoelectric injectors the fuel injection can be broken into up to seven events providing a combination of these strategies including post ignition injection which makes the gas engine take on the high torque characteristics of a Diesel engine. These systems are being introduced by Mercedes, Audi and BMW. The precise metering, preparation and distribution of the intake air and the injected fuel for every combustion stroke leads to exceptional power performance, low fuel-consumption figures and low emission levels.
The high-pressure circuit of the direct gasoline injection is fed by a high-pressure pump compressing the fuel to the high pressure level required in the fuel rail. The injectors attached to the fuel rail meter and atomize the fuel extremely fast and under high pressure in order to achieve the best possible mixture formation directly in the combustion chamber.
Piezo electric injectors The injectors work via the piezoelectric effect, as the name might hint. Piezoelectricity is the ability of crystals to generate a voltage in response to applied mechanical stress. Piezoelectricity was discovered by a man called Pierre Curie and the word is derived from the Greek piezein, which means to squeeze or press. The piezoelectric effect is reversible in that piezoelectric crystals, when subjected to an externally applied voltage, can change shape by a small amount. Piezoelectric injectors use small discs of quartzlike crystalline material that deforms when subjected to a high-voltage low-current source, which provides the injector opening and closing action. Piezo-crystals have been used for years in pressure measurement devices because when they are pressurized they produce an electrical current. This method simply reverses the process and deforms the crystal by applying a current. With conventional solenoid type injectors coil saturation time led to relatively slow operation. Piezoelectric injector have no coil saturation time to worry about and with cycle times as low as 0.2 milliseconds, such injectors are several times faster than conventional solenoids, this in turn allows up to seven injector events during each compression stroke.
The precision of each injector is also increased, with injector-to-injector variation of only a few percent.
Advancements in engine management systems are occurring all the time, this one from Delphi, a former subsidiary of General Motors allows the ECM to detect combustion quality by comparing the reverse ion flow through the cylinder gases after a combustion event.
Delphi’s ionization current sensing, (ion sense), ignition subsystem consists of one ignition coil per cylinder and high temperature resistant electronics. Eliminating moving parts and high-voltage leads helps provide maximum energy supply to the spark plug. The spark plug is not only used as an actuator to ignite the air/fuel mixture, but also as an in-cylinder sensor to monitor the combustion process. Its signal contains misfire and knock information and may also be used for engine control features requiring knowledge of the actual combustion characteristics. Ionized current sensing is a technology based on the principle that electrical current flow in an ionized gas, (as during combustion), is proportional to the flame electrical conductivity. By placing a direct current bias on the spark plug electrodes, the conductivity can be measured. In the configuration shown in figure 1, the spark current is used to create this bias voltage, eliminating the need for any additional voltage source.
