A Practical Continuously Variable Accessory ... - SLIDEBLAST.COM
© 2008 Fallbrook Technologies Inc.
NuVinci® Development Briefing: A Continuously Variable Accessory Drive for Alternators and Other Engine-Driven Accessories Scott McBroom Program Manager 512-279-6219
Rob Smithson CTO, VP - Business Development 512-279-6201 Fallbrook Technologies Inc.
This Development Briefing describes the demonstration of a NuVinci Continuously Variable Accessory Drive (CVAD) that delivers increased accessory power at idle while meeting the power, packaging and thermal requirements to operate in severe under-the-hood environments. Application: Context:
Problem:
Effect:
Army and Marine Corps Tactical Wheeled Vehicle (TWV) fleet As the TWV’s mission has evolved, so too has their need for additional electric power to run their Command, Control, Communications, Computers, Intelligence, and Recognition (C4IR) systems. Insufficient alternator power at idle. TWV alternators do not make their rated electric power when the engine is idling. This is because; at idle, the alternator speed falls below its rated speed. Simply sizing the pulley ratio to spin the alternator faster (past the rated speed) at idle will cause the alternator to over-speed and fail. A shortage of electric power at idle could cause a disruption in mission and/or effectiveness as well as cause more frequent battery replacements.
Workaround:
In response to this issue, the Army’s has specified higher power alternators; such that, during idle, a minimum power threshold is met for some, but not all situations. More often than not, these newer alternators do not meet the field power requirement.
The Solution:
A low cost continuously variable transmission (CVT) used with the stock alternators can meet or exceed all power requirements. This is a considerably more economical alternative to more expensive alternators that the Army is now buying. Equipped with such a CVT, the alternators can run continuously at their rated speed, regardless of the engine speed, producing up to 75% more power at idle than without the CVT.
MARKET DRIVERS FOR THE MILITARY – The primary issue is “available power at idle.” Increasing electric power demands in the battlefield strains the standard TWV alternator and battery system; comments from military officers include “On-going and future vehicle power programs do not address October 9, 2009
© 2008 Fallbrook Technologies Inc. urgent/immediate need for legacy fleet”1 and “Inadequate power with existing equipment at curb idle.”1 Typical usage of the TWV fleet inherently involves a high proportion of low engine speed operation, whether due to slow speeds required by off-road maneuvers, convoy protocols, operation in heavily trafficked urban environments, or just waiting. To address the need for more power at idle, several solutions being fielded.1 One is to simply increase capacity – replace the stock alternator with a higher output alternator. Note that these high power alternators often cost between $2,800 to $4,300, and in some cases upgrading means doubling the alternator cost. This may also require upgrading to higher capacity cabling. Another solution is operating at “fast idle”. At first, fast idle appears to be an easier and less costly solution; however, significant logistical implications arise due to the increased fuel use from fast idle. Fuel and range affect combat power. In addition, fast idle raises noise signature and emissions levels. Many applications may require an auxiliary power unit (APU). Depending on the power levels these provide, the APU could be driven at constant speed from the transmission’s power take-off (PTO) or be configured as a stand alone APU mounted on the truck. APU systems are typically used to deliver AC power, or they may also be used to supplement the vehicle’s 28V DC power buss. The cost to produce auxiliary power using these systems is high compared to alternators. They also consume payload space and increase vehicle curb-weight, which both impact logistics. For these reasons, an APU is not the best choice for supplementing DC power. FOR THE PRIVATE SECTOR – The main driver is fuel economy savings, with additional benefits possible for emissions and performance. Belt-driven air conditioning systems are a major source of losses, and it is likely that air conditioning systems will require more power to deliver the same cooling performance in the future due to regulatory requirements to move to environmentally friendly refrigerants. Accessory drives are a little-recognized contributor to fuel consumption because previous technology offered no practical alternatives for producing an accessory drive whose speed was continuously optimized for all conditions. While CVT-equipped accessory drives have been proposed2,3,4, the technology used to implement the drives was impractical due to packaging and/or cost limitations. Indeed the authors are unaware of any CVT accessory drives in production today. The belt driven accessories for a typical U.S. passenger car or truck usually consist of a power steering pump, alternator, water pump, AC compressor, and, in some cases, a hydraulic brake booster. Some may also include emissions control pumps, an oil pump, and a cooling fan. Annually in the U.S., automotive AC units alone consume nearly 26 billion liters of gas (7 billion gallons) and generate 62 billion kg CO2. That’s equivalent to 5.5% of the national fuel usage annually5. Automotive Air Conditioning [portion of total fuel usage]
Fuel Usage [liters annually]
CO2 Emissions [kg annually]
United States
5.5 %
26.0 B
62 B
European Union
3.2 %
6.9 B
16 B
Japan
3.5 %
1.7 B
4B
India
10-20 %
0.5 B
1.1 B
In addition to consumer pressures, there are regulatory pressures as well. In the latest version of the U.S.’ Corporate Average Fuel Economy (CAFE) regulation, fuel economy ratings must improve in the U.S. by 25% by 2015, and 40% by 2020. (Market and political pressures may require greater improvements sooner.)
October 9, 2009
© 2008 Fallbrook Technologies Inc.
New E.U. regulations for air conditioning refrigerants are also coming – cleaner refrigerants are less efficient. In general, the most recent results for alternative refrigerants published recently at the 2008 SAE Mobile Air Conditioning Conference show reduced performance when compared to current R-134a systems, particularly at idle. Such systems are also expected to be implemented in the US and other major auto markets6. The broad industry consensus is that consumers want better fuel economy but they don’t want to sacrifice AC performance in the next generation system.
SOLUTION DECOUPLING THE ACCESSORY SPEED FROM THE ENGINE SPEED If a device could be made that decouples engine speed from accessory speed, accessories would be free to operate at the speed that most efficiently met the required accessory load. There are several ways to accomplish this goal. Most recently, the electrification of engine accessories has generated interest. Electrification of engine accessories has demonstrated up to 15% improvement in fuel economy in a research environment.8 While it may yield improved accessory efficiency and performance, it does so at a much higher cost than the incumbent system. Additionally, it still places high demand on the engine driven alternator. The end result is that alternator power at idle becomes an issue for this approach as well. Another method involves the use of a two-speed gearbox and clutch. When the engine speed hits a set point, the gearbox shifts from high to low. While this system offers an improvement of the current arrangement, it clearly sub-optimizes efficiency improvement when compared to the potential of a constant-speed drive and it has NVH and belt life issues stemming from the gear changes. Under normal transient driving conditions, a two speed drive will shift multiple times with every acceleration event, creating noticeable step change differences in NVH and accessory performance that the consumer will find objectionable. In this paper, the authors will detail the demonstration of another option, a Continuously Variable Accessory Drive (CVAD). The results will show that a CVAD offers improved alternator power at engine idle and improved fuel economy when replacing the crankshaft pulley.
A CONTINUOUSLY VARIABLE ACCESSORY DRIVE (CVAD) FOR THE MILTARY Working in conjunction with the U.S. Army National Automotive Center, the Army's Family of Medium Tactical Vehicles (FMTV) was selected for a demonstration of the continuously variable planetary (CVP) traction drive transmission integrated with the alternator. The best solution to any problem is one that delivers maximum results with minimal complexity, easy installation, and low cost. Figure 1 - Stock FMTV Alternator Packaging PROJECT GOALS – This proof-of-concept project had to demonstrate the following: 1. Package on the existing FMTV beltline with little or no modification to the vehicle. 2. Output maximum available alternator current at any engine speed (~200A at 28VDC). This meant that the CVAD had to shift fast enough to mitigate rapid engine transients. 3. Demonstrate thermal stability at engine compartment temperature levels of 93º C (200º F) . October 9, 2009
© 2008 Fallbrook Technologies Inc.
BASELINE - The FMTV, for this demonstration project, consists of: •
A Caterpillar C7 engine (6-Cylinder, 7.2 L, electronic controlled, fuel-injected, turbocharged and after-cooled, as well as EPA certified). The engine produces 330 hp (246 kW) @ 2200 rpm and 860 lb-ft (1,166 Nm) @ 1440 rpm.
•
A C.E. Niehoff N1224-3 alternator. The N1224-3 is a dual voltage (14 V and 28 V) alternator producing 200A (peak) at 28V. The N1224 will be the standard equipment on the FTMV A1R. Figure 1 is a picture of the packaging space available for the CVP. (Note: The image is taken from a variant of FMTV not using the N1224.)
CONTINUOUSLY VARIABLE ACCESSORY DRIVE (CVAD) CVAD DEVELOPMENT – The first goal of the project was to determine if a CVP of sufficient power capacity could be packaged in the available space of the engine compartment. Working with BAE Systems, the FMTV manufacturer, a 3-D CAD model of the engine compartment was obtained. With that model, engineers were able to make packaging trade-off studies using four of the numerous primary power-paths available in the CVP6. A “U” drive configuration was selected, Figure 2. Power comes in from the belt, transfers it to one traction ring, through the planet balls, and brings the power out of the other traction ring. In such a configuration, where the carrier is not allowed to rotate, the variator is capable of 4:1 ratio range. For this particular demonstration only 3.75:1 was needed, since this diesel engine idles at 700 rpm and has a maximum speed of 2,600 rpm.
Engine Power from Belt
Pump and Heat Exchanger Alternator CVP Diesel Engine
Shift Torque
Alternator Shaft
Engine Speed
Alternator Speed
Speed Reference Control System
Figure 2 - Power Path Through the CVP
(2400 rpm)
Figure 3 - CVAD Schematic
Figure 3 depicts a high-level schematic of the NuVinci CVAD system. The system includes a CVP, an actuator (to change speed ratio), a controller, and a small pump (1/2 gpm) and heat exchanger (15 cm x 23 cm). In this particular demonstrator, a stepper motor was selected for shift actuation for simplicity and speed of implementation; however, hydraulic, electro-hydraulic, and flyweight governor are other shift actuation means that could be used. The requirement of packaging on the existing FMTV beltline with minimal modification to the vehicle was met. Figure 4 shows the CVAD installed on the FMTV. FAST SHIFTING - The second goal of the project was to demonstrate that the CVAD could output maximum available alternator current at any engine speed. The Niehoff N1224-3 alternator current falls off at an engine speed of 1,140 rpm. Working back to the alternator, October 9, 2009
© 2008 Fallbrook Technologies Inc.
Alternator CVAD
Figure 4 - CVAD Installation on the FMTV knowing there is a fixed pulley ratio of 2.41 between the engine’s harmonic balancer and the alternator, it follows that the fall-off speed of the alternator is 2,000 rpm. This means that to keep the alternator at maximum current, CVAD output speed should not fall below 2,000 rpm, the reference speed. Discussion with the engine manufacturer (CAT) revealed that the fastest engine acceleration the CVAD would have to mitigate is 2,600 rpm/sec. This was a worst case scenario that might occur if the vehicle were in park, with no loads on the engine, and someone were to push the accelerator pedal to the floor. The fastest engine deceleration would occur during transmission shifts. In this case the engine might decelerate as fast as -1,200 rpm/sec. A test stand was built to develop the control system required to manage these worst case engine transients. Shown in Figure 5, the CVAD was put on a test stand designed to replicate the CAT C7 belt line, with an electric motor capable of reproducing the desired engine transients. The C.E. Niehoff N1224 alternator was mounted via shaft, behind the headstand, and was connected
Drive Motor to Simulate Engine
Replicated Belt-line of a CAT C7
Alternator
CVAD
Figure 5 – CVAD on the Test Stand October 9, 2009
© 2008 Fallbrook Technologies Inc.
to a variable resistance load-bank. The results of high speed engine transient testing are shown in Figure 6. At the 874 second mark, the engine (simulated by the electric motor) accelerates from idle to 2,300 rpm at 2,621 rpm/sec. The alternator speed (labeled CVP Output Speed) goes from 2,400 rpm to 3,825 rpm before returning back to the set point 2.2 seconds later. Similarly, when the engine quickly decelerates from 2,300 rpm to idle at -1,217 rpm/sec the alternator goes from the 2,400 rpm set point to 1,600 rpm and the back to 2,400 rpm in 3.8 seconds. This response time is not due to any limitation in the CVP. The limitation is due to the command protocol in the stepper motor firmware.
4500
4000
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1424 rpm
Speed (rpm)
3000
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2.2 sec
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-1217 (rpm/sec)
1000
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CVP Input Speed 0 872
877
CVP Output Speed 882
Simulated Engine Speed 887
892
Time (sec)
Figure 6 - CVAD Control System Response Other actuation means such as a solenoid driven hydraulic spool valve and mechanical flyweight are among methods being considered that can offer a faster control response. The net result is that this shift response is adequate for maintaining the alternator above its rated speed during engine transients. UNDERHOOD SURVIVAL – The third goal of the project was to determine if the CVAD could maintain thermal equilibrium while operating at full load and at an ambient under hood temperature of 95º C (203 ºF). 1 A hot box was constructed for the test stand that enclosed CVAD and the belt line. A heating element and a circulation fan were used to maintain the ambient temperature in the box near the 95º C (203 ºF) set point. The circulation fan roughly replicates the continuous mass air 1
It should be noted that typical under hood temperature is 85º C (185 ºF). The higher value represents a more realistic number for a vehicle idling in the desert. Even higher temps will be tested as a part of product development. October 9, 2009
© 2008 Fallbrook Technologies Inc.
Engine Speed
CVAD Input
CVAD Case Outlet Fluid Hot Box Inlet Fluid
Figure 7. Thermal Stability Testing
flow generated by the engine’s cooling fan and the volume of the box is smaller than the volume of air under hood in the FMTV, thereby making this a conservative simulation. A sample of the typical thermal stability testing results is shown in Figure 7. For the sake of readability, only the first 80 minutes of the 160 minute second test are shown. Simulated transient engine operation with the alternator running at full load was used to produce the worst case scenario for heat rejection required from the CVP. At about 15 minutes into the test the CVAD case temperature stabilizes at 110ºC (230º F) and traction fluid exit temperature from the CVP into the heat exchanger, stabilizes at 105º C (221ºF). This is below the 130ºC (266ºF) maximum continuous operating temperature recommended by the maker of the traction fluid, the Valvoline division of Ashland Chemical. DISCUSSION – In Figure 8, the performance curves of the stock alternator are presented as a function of engine speed. As is typical, the manufacturer provides two curves, one for ambient temperature of 25ºC and the other curve represents performance at under-hood temperatures. With a CVAD controlling the belt speed input, the alternator system has increased accessory power capacity (shaded region). Figure 9 shows alternator current produced with CVAD as the engine speed fluctuates during typical vehicle operation (in this graph, the EPA Heavy Duty Urban Dynamometer Driving Schedule 2 ). With CVAD installed, output stays near maximum. Without CVAD, current production falls significantly. CVAD increases alternator power output at idle by more than 75%
2
The EPA Heavy Duty Urban Dynamometer Driving Schedule was selected because the FMTV Operational Requirements Document (ORD) only specifies a percentage of time in urban, cross country, and trail driving conditions. No vehicle speed driving cycle is given, therefore the authors inferred that the possible speeds of urban and cross country driving could be approximated by the HD-UDDS. October 9, 2009
© 2008 Fallbrook Technologies Inc.
Current (A)
Enables increased Increased electrical Electrical CVAD enables CVAD enables increased electrical Power at low Lowengine Enginespeeds Speeds power power at low engine speeds
250
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Alternator Current (w/ CVAD)
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– Cold (Stock Pulley)
Alternator Current
– Hot (Stock Pulley)
0 600
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2600
Engine Speed (rpm)
Figure 8 - CVAD Provides More Alternator Power Output at Low Engine Speeds Over the course of this 1,200 second driving schedule, that translates to a 34% increase in total energy produced, as shown in Figure 10. Over the course of a full day of vehicle operation, this gap in total energy available would continue to increase. In addition to the immediately available increase in electrical power, this has obvious implications for reduction of battery deep cycling for both military and civilian use (transit buses and emergency vehicles are two clear examples) with a corresponding reduction in battery maintenance. 7
2500
75% More Power at Idle 2300 6 2100
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1300 1100
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Electrical Power (Stock)
Electrical Power (CVAD)
Figure 9 - CVAD-Equipped Alternator Produces 75% more Current as Engine Speed Fluctuates (HD-UDDS cycle)
October 9, 2009
© 2008 Fallbrook Technologies Inc.
2500
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34% More Energy
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Figure 10 - Comparison of Alternator Energy Production over the EPA UDDS Figure 11 compares the power requirements of CVAD equipped alternator to the stock alternator as a function of maximum current for a given engine speed. In the base system, the alternator is unable to produce the rated current at engine speeds below 920rpm (see the line between A and B). As base alternator speed is increased (as engine speed increases) the input power increases to overcome extra mechanical losses related to increased speed of alternator. The CVAD input power required to keep alternator at 2400rpm at rated current is also plotted. Observing Figure 11 we can see that below 920 rpm engine speed, the CVAD equipped system can maintain the rated current while the base system cannot. Assuming the base system is operated at point C (high idle) to achieve the rated current, the stock alternator input power is slightly less than the CVAD input power. For the CVAD system, the rated current can be achieved at 700 rpm (point D). Thus, the CVAD system could be more fuel efficient than the base system if the difference in fuel requirement between high idle of 920rpm and engine idle of 700rpm is less than the small difference in energy to run the CVAD. At engine speeds above 920 rpm, keeping the alternator at rated speed with a CVAD comes with a minor penalty, (E). It is a small fraction of the engine’s power and should not significantly affect the engine’s fuel economy over any given drive cycle. The CVAD also enables possible improvements in the alternator efficiency by reduction of the required operating speed range of the alternator at same or less cost than conventional designs. To improve fuel economy over any given drive cycle, the CVAD alternator need only improve its current system efficiency by approximately 5%. This can be done with incremental improvements in the CVAD/alternator design. When compounded with possible improvement in alternator peak efficiency via design optimization and operation in its most efficient operational regimes, this is a very achievable target.
October 9, 2009
© 2008 Fallbrook Technologies Inc.
MORE THAN POWER AT IDLE, FUEL ECONOMY ON THE CRANKSHAFT -
Rated Current
Determining the fuel economy effect of separating accessory speed from engine Am speed pswas beyond the scope of the demonstration project; however, a parallel effort was undertaken to D quantify the effects of replacing the engine crankshaft pulley with a CVAD running all belt driven accessories at a constant speed. B E
A test fixture was constructed to measure torque losses as a function speed and load for the water pump, power steering pump and alternator of a Ford 5.4L gasoline engine. AC compressor Power data was not taken. Using commercially available, dynamic vehicle simulation software, the torque loss tables for the accessories were entered into the software. C The Ford Expedition vehicle parameters, engine map, transmission map and shift logic were also entered to create a vehicle model.
Change in % Change Fuel in Drive Economy Fuel Alternator Input Power Stock Cycle with Economy Alternator + CVT Input Power CVAD with (mpg) CVAD
Baseline results were calculated by simulating Stock Alt Amp the vehicle driving the U.S. EPA’s Federal City + 0.5 + 3.6% Urban Driving Schedule A (FUDS) and the Alternator Amp with CVT Highway Fuel Economy Test (HWFET). For Highway + 1.8 + 10% the baseline simulation, the accessory speeds 700 proportional 900 1100 1500 and 1700 1900power 2100over2300 2500cycle was were directly to the 1300 engine speed accessory each drive calculated. Then the same set of simulations was conducted with the CVAD inserted, including Engine Speed (rpm) its losses as taken from the test stand. In this run, the speeds of the accessories were independent of engine speed. For11. this simulation they wereand set Power to run at fixed speed (equivalent Figure Comparison of Amp to engine idle speed). The results of the simulations are shown below. 9 These results are consistent with results recently published by Gates Corporation and Hyundai Motor Company showing 0.3% (low load city driving) to 8.6% (high load highway driving) improvement in fuel economy with a two speed accessory belt drive system on a Ford 5.4L F150. Since the two speed system will clearly be sub-optimized for most driving conditions, the improvement predicted for the CVAD system is expected.
CVAD on the crank-shaft
Figure 12 - CVAD Mounted on the Crankshaft of a 5.4L Expedition To begin validating the simulation results, a duplicate CVAD has been installed on the crank shaft of a 5.4L Ford Expedition, see Figure 12 and is to be tested at an independent, not-for-profit, research and development company, in their EPA certified test facility.
October 9, 2009
© 2008 Fallbrook Technologies Inc.
After discussions with several accessory device manufacturers, it is expected that additional fuel economy benefits can be derived by sizing each accessory’s pulley for constant speed operation. In this scenario, the pulley for each accessory is sized so that all accessories are running at their most efficient condition. The next logical step is to redesign the accessories themselves for constant speed operation. Current accessory design is a compromise of performance over a wide speed range versus efficiency at its most frequent operating condition. OTHER ACCESSORY APPLICATIONS, MILITARY AND COMMERCIAL In addition to CVAD, the CVP has numerous other potential uses. PTO electric generators and hydraulic pumps can be controlled to a desired output, regardless of the engine speed, potentially eliminating expensive power electronics in the generator case. In the case of hydraulic pumps, variable displacement pumps can be replaced with fixed displacement pumps and a CVP. For stand alone power generation (gensets), a CVP between a diesel engine and the generator can be used to mitigate wet stacking.
CONCLUSION The CVP has demonstrated itself to be practical option for controlling accessory speed independent of engine speed. Specifically, with respect to demonstrating power at idle, all three of the projects goals were successful. ; Maintain thermal stability at engine compartment temperature levels of 93º C (200º) ; Package on the existing FMTV beltline with little or no modification to the vehicle. ; Output maximum available alternator current at any engine speed.
Additionally, it has been shown that a fuel economy advantage is possible, when the CVP is mounted on the crank shaft pulley and all the accessories are run at a constant speed.
REFERENCES 1. Cross, Jim, Shaffer, Ed, “On Board Vehicle Power Briefing & Way Forward,” Presented at Joint Service Power Expo, April 2007 2. Masashi, U., et al, “Continuously Variable Transmission for Accessory Drive System”, 1999 Proceedings. JSAE Annual Congress. 3. Van der Iieijaen, A.C., “Continuously Variable Accessory Drive and other methods to reduce additional fuel consumption caused by engine accessories” ; Technische Universiteit Eindhoven Department Mechanical Engineering; Eindhoven, July, 2004 4. “An Evaluation of the Morse Constant Speed Accessory Drive”; Technology Assessment & Evaluation Branch, Office of Mobile Source Air Pollution Control, US EPA; June 1976 5. http://www.sae.org/altrefrigerant/presentations/presr-rugh.pdf 6. Atkinson, Ward, “SAE I-MAC Study Cross Country A/C Comfort Evaluation”, Presented at Mobile Air Conditioning Leadership Summit, June 2008 7. Pohl, Brad; Simister, Matthew; SAE Paper 04CVT-9, “Configuration Analysis of a Spherical Traction Drive CVT/IVT, © 2004 SAE International. 8. Bishop, John; SAE Paper 2006-01-0215 “Accessory Electrification in Class 8 Tractors”, 2005 SAE World Congress, © 2006 SAE International. 9. Ali, Imtiaz et al; SAE Paper 2008-01-1521, “E3 System – A Two Speed Accessory Belt Drive System for Reduced Fuel Consumption”, 2008 SAE Fuels and Lubricants Congress, June 2008.
October 9, 2009
© 2008 Fallbrook Technologies Inc.
DEFINITIONS, ACRONYMS, ABBREVIATIONS A
- Amps or Amperes, a unit of electric current
C4
- Command, Control, Communications, and Computers
CVAD
- Continuously Variable Accessory Drive
CVP
- Continuously Variable Planetary Transmission
EPA
- Environmental Protection Agency
FMTV - Family of Medium Tactical Vehicles kW
- Kilowatt, an SI unit of power
kJ
- Kilojoules hours, an SI unit of energy
PTO
- Power Take Off
V
- Volts, a unit of electric potential
October 9, 2009
NuVinci® Development Briefing: A Continuously Variable Accessory Drive for Alternators and Other Engine-Driven Accessories Scott McBroom Program Manager 512-279-6219
Rob Smithson CTO, VP - Business Development 512-279-6201 Fallbrook Technologies Inc.
This Development Briefing describes the demonstration of a NuVinci Continuously Variable Accessory Drive (CVAD) that delivers increased accessory power at idle while meeting the power, packaging and thermal requirements to operate in severe under-the-hood environments. Application: Context:
Problem:
Effect:
Army and Marine Corps Tactical Wheeled Vehicle (TWV) fleet As the TWV’s mission has evolved, so too has their need for additional electric power to run their Command, Control, Communications, Computers, Intelligence, and Recognition (C4IR) systems. Insufficient alternator power at idle. TWV alternators do not make their rated electric power when the engine is idling. This is because; at idle, the alternator speed falls below its rated speed. Simply sizing the pulley ratio to spin the alternator faster (past the rated speed) at idle will cause the alternator to over-speed and fail. A shortage of electric power at idle could cause a disruption in mission and/or effectiveness as well as cause more frequent battery replacements.
Workaround:
In response to this issue, the Army’s has specified higher power alternators; such that, during idle, a minimum power threshold is met for some, but not all situations. More often than not, these newer alternators do not meet the field power requirement.
The Solution:
A low cost continuously variable transmission (CVT) used with the stock alternators can meet or exceed all power requirements. This is a considerably more economical alternative to more expensive alternators that the Army is now buying. Equipped with such a CVT, the alternators can run continuously at their rated speed, regardless of the engine speed, producing up to 75% more power at idle than without the CVT.
MARKET DRIVERS FOR THE MILITARY – The primary issue is “available power at idle.” Increasing electric power demands in the battlefield strains the standard TWV alternator and battery system; comments from military officers include “On-going and future vehicle power programs do not address October 9, 2009
© 2008 Fallbrook Technologies Inc. urgent/immediate need for legacy fleet”1 and “Inadequate power with existing equipment at curb idle.”1 Typical usage of the TWV fleet inherently involves a high proportion of low engine speed operation, whether due to slow speeds required by off-road maneuvers, convoy protocols, operation in heavily trafficked urban environments, or just waiting. To address the need for more power at idle, several solutions being fielded.1 One is to simply increase capacity – replace the stock alternator with a higher output alternator. Note that these high power alternators often cost between $2,800 to $4,300, and in some cases upgrading means doubling the alternator cost. This may also require upgrading to higher capacity cabling. Another solution is operating at “fast idle”. At first, fast idle appears to be an easier and less costly solution; however, significant logistical implications arise due to the increased fuel use from fast idle. Fuel and range affect combat power. In addition, fast idle raises noise signature and emissions levels. Many applications may require an auxiliary power unit (APU). Depending on the power levels these provide, the APU could be driven at constant speed from the transmission’s power take-off (PTO) or be configured as a stand alone APU mounted on the truck. APU systems are typically used to deliver AC power, or they may also be used to supplement the vehicle’s 28V DC power buss. The cost to produce auxiliary power using these systems is high compared to alternators. They also consume payload space and increase vehicle curb-weight, which both impact logistics. For these reasons, an APU is not the best choice for supplementing DC power. FOR THE PRIVATE SECTOR – The main driver is fuel economy savings, with additional benefits possible for emissions and performance. Belt-driven air conditioning systems are a major source of losses, and it is likely that air conditioning systems will require more power to deliver the same cooling performance in the future due to regulatory requirements to move to environmentally friendly refrigerants. Accessory drives are a little-recognized contributor to fuel consumption because previous technology offered no practical alternatives for producing an accessory drive whose speed was continuously optimized for all conditions. While CVT-equipped accessory drives have been proposed2,3,4, the technology used to implement the drives was impractical due to packaging and/or cost limitations. Indeed the authors are unaware of any CVT accessory drives in production today. The belt driven accessories for a typical U.S. passenger car or truck usually consist of a power steering pump, alternator, water pump, AC compressor, and, in some cases, a hydraulic brake booster. Some may also include emissions control pumps, an oil pump, and a cooling fan. Annually in the U.S., automotive AC units alone consume nearly 26 billion liters of gas (7 billion gallons) and generate 62 billion kg CO2. That’s equivalent to 5.5% of the national fuel usage annually5. Automotive Air Conditioning [portion of total fuel usage]
Fuel Usage [liters annually]
CO2 Emissions [kg annually]
United States
5.5 %
26.0 B
62 B
European Union
3.2 %
6.9 B
16 B
Japan
3.5 %
1.7 B
4B
India
10-20 %
0.5 B
1.1 B
In addition to consumer pressures, there are regulatory pressures as well. In the latest version of the U.S.’ Corporate Average Fuel Economy (CAFE) regulation, fuel economy ratings must improve in the U.S. by 25% by 2015, and 40% by 2020. (Market and political pressures may require greater improvements sooner.)
October 9, 2009
© 2008 Fallbrook Technologies Inc.
New E.U. regulations for air conditioning refrigerants are also coming – cleaner refrigerants are less efficient. In general, the most recent results for alternative refrigerants published recently at the 2008 SAE Mobile Air Conditioning Conference show reduced performance when compared to current R-134a systems, particularly at idle. Such systems are also expected to be implemented in the US and other major auto markets6. The broad industry consensus is that consumers want better fuel economy but they don’t want to sacrifice AC performance in the next generation system.
SOLUTION DECOUPLING THE ACCESSORY SPEED FROM THE ENGINE SPEED If a device could be made that decouples engine speed from accessory speed, accessories would be free to operate at the speed that most efficiently met the required accessory load. There are several ways to accomplish this goal. Most recently, the electrification of engine accessories has generated interest. Electrification of engine accessories has demonstrated up to 15% improvement in fuel economy in a research environment.8 While it may yield improved accessory efficiency and performance, it does so at a much higher cost than the incumbent system. Additionally, it still places high demand on the engine driven alternator. The end result is that alternator power at idle becomes an issue for this approach as well. Another method involves the use of a two-speed gearbox and clutch. When the engine speed hits a set point, the gearbox shifts from high to low. While this system offers an improvement of the current arrangement, it clearly sub-optimizes efficiency improvement when compared to the potential of a constant-speed drive and it has NVH and belt life issues stemming from the gear changes. Under normal transient driving conditions, a two speed drive will shift multiple times with every acceleration event, creating noticeable step change differences in NVH and accessory performance that the consumer will find objectionable. In this paper, the authors will detail the demonstration of another option, a Continuously Variable Accessory Drive (CVAD). The results will show that a CVAD offers improved alternator power at engine idle and improved fuel economy when replacing the crankshaft pulley.
A CONTINUOUSLY VARIABLE ACCESSORY DRIVE (CVAD) FOR THE MILTARY Working in conjunction with the U.S. Army National Automotive Center, the Army's Family of Medium Tactical Vehicles (FMTV) was selected for a demonstration of the continuously variable planetary (CVP) traction drive transmission integrated with the alternator. The best solution to any problem is one that delivers maximum results with minimal complexity, easy installation, and low cost. Figure 1 - Stock FMTV Alternator Packaging PROJECT GOALS – This proof-of-concept project had to demonstrate the following: 1. Package on the existing FMTV beltline with little or no modification to the vehicle. 2. Output maximum available alternator current at any engine speed (~200A at 28VDC). This meant that the CVAD had to shift fast enough to mitigate rapid engine transients. 3. Demonstrate thermal stability at engine compartment temperature levels of 93º C (200º F) . October 9, 2009
© 2008 Fallbrook Technologies Inc.
BASELINE - The FMTV, for this demonstration project, consists of: •
A Caterpillar C7 engine (6-Cylinder, 7.2 L, electronic controlled, fuel-injected, turbocharged and after-cooled, as well as EPA certified). The engine produces 330 hp (246 kW) @ 2200 rpm and 860 lb-ft (1,166 Nm) @ 1440 rpm.
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A C.E. Niehoff N1224-3 alternator. The N1224-3 is a dual voltage (14 V and 28 V) alternator producing 200A (peak) at 28V. The N1224 will be the standard equipment on the FTMV A1R. Figure 1 is a picture of the packaging space available for the CVP. (Note: The image is taken from a variant of FMTV not using the N1224.)
CONTINUOUSLY VARIABLE ACCESSORY DRIVE (CVAD) CVAD DEVELOPMENT – The first goal of the project was to determine if a CVP of sufficient power capacity could be packaged in the available space of the engine compartment. Working with BAE Systems, the FMTV manufacturer, a 3-D CAD model of the engine compartment was obtained. With that model, engineers were able to make packaging trade-off studies using four of the numerous primary power-paths available in the CVP6. A “U” drive configuration was selected, Figure 2. Power comes in from the belt, transfers it to one traction ring, through the planet balls, and brings the power out of the other traction ring. In such a configuration, where the carrier is not allowed to rotate, the variator is capable of 4:1 ratio range. For this particular demonstration only 3.75:1 was needed, since this diesel engine idles at 700 rpm and has a maximum speed of 2,600 rpm.
Engine Power from Belt
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Figure 2 - Power Path Through the CVP
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Figure 3 - CVAD Schematic
Figure 3 depicts a high-level schematic of the NuVinci CVAD system. The system includes a CVP, an actuator (to change speed ratio), a controller, and a small pump (1/2 gpm) and heat exchanger (15 cm x 23 cm). In this particular demonstrator, a stepper motor was selected for shift actuation for simplicity and speed of implementation; however, hydraulic, electro-hydraulic, and flyweight governor are other shift actuation means that could be used. The requirement of packaging on the existing FMTV beltline with minimal modification to the vehicle was met. Figure 4 shows the CVAD installed on the FMTV. FAST SHIFTING - The second goal of the project was to demonstrate that the CVAD could output maximum available alternator current at any engine speed. The Niehoff N1224-3 alternator current falls off at an engine speed of 1,140 rpm. Working back to the alternator, October 9, 2009
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Alternator CVAD
Figure 4 - CVAD Installation on the FMTV knowing there is a fixed pulley ratio of 2.41 between the engine’s harmonic balancer and the alternator, it follows that the fall-off speed of the alternator is 2,000 rpm. This means that to keep the alternator at maximum current, CVAD output speed should not fall below 2,000 rpm, the reference speed. Discussion with the engine manufacturer (CAT) revealed that the fastest engine acceleration the CVAD would have to mitigate is 2,600 rpm/sec. This was a worst case scenario that might occur if the vehicle were in park, with no loads on the engine, and someone were to push the accelerator pedal to the floor. The fastest engine deceleration would occur during transmission shifts. In this case the engine might decelerate as fast as -1,200 rpm/sec. A test stand was built to develop the control system required to manage these worst case engine transients. Shown in Figure 5, the CVAD was put on a test stand designed to replicate the CAT C7 belt line, with an electric motor capable of reproducing the desired engine transients. The C.E. Niehoff N1224 alternator was mounted via shaft, behind the headstand, and was connected
Drive Motor to Simulate Engine
Replicated Belt-line of a CAT C7
Alternator
CVAD
Figure 5 – CVAD on the Test Stand October 9, 2009
© 2008 Fallbrook Technologies Inc.
to a variable resistance load-bank. The results of high speed engine transient testing are shown in Figure 6. At the 874 second mark, the engine (simulated by the electric motor) accelerates from idle to 2,300 rpm at 2,621 rpm/sec. The alternator speed (labeled CVP Output Speed) goes from 2,400 rpm to 3,825 rpm before returning back to the set point 2.2 seconds later. Similarly, when the engine quickly decelerates from 2,300 rpm to idle at -1,217 rpm/sec the alternator goes from the 2,400 rpm set point to 1,600 rpm and the back to 2,400 rpm in 3.8 seconds. This response time is not due to any limitation in the CVP. The limitation is due to the command protocol in the stepper motor firmware.
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Figure 6 - CVAD Control System Response Other actuation means such as a solenoid driven hydraulic spool valve and mechanical flyweight are among methods being considered that can offer a faster control response. The net result is that this shift response is adequate for maintaining the alternator above its rated speed during engine transients. UNDERHOOD SURVIVAL – The third goal of the project was to determine if the CVAD could maintain thermal equilibrium while operating at full load and at an ambient under hood temperature of 95º C (203 ºF). 1 A hot box was constructed for the test stand that enclosed CVAD and the belt line. A heating element and a circulation fan were used to maintain the ambient temperature in the box near the 95º C (203 ºF) set point. The circulation fan roughly replicates the continuous mass air 1
It should be noted that typical under hood temperature is 85º C (185 ºF). The higher value represents a more realistic number for a vehicle idling in the desert. Even higher temps will be tested as a part of product development. October 9, 2009
© 2008 Fallbrook Technologies Inc.
Engine Speed
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Figure 7. Thermal Stability Testing
flow generated by the engine’s cooling fan and the volume of the box is smaller than the volume of air under hood in the FMTV, thereby making this a conservative simulation. A sample of the typical thermal stability testing results is shown in Figure 7. For the sake of readability, only the first 80 minutes of the 160 minute second test are shown. Simulated transient engine operation with the alternator running at full load was used to produce the worst case scenario for heat rejection required from the CVP. At about 15 minutes into the test the CVAD case temperature stabilizes at 110ºC (230º F) and traction fluid exit temperature from the CVP into the heat exchanger, stabilizes at 105º C (221ºF). This is below the 130ºC (266ºF) maximum continuous operating temperature recommended by the maker of the traction fluid, the Valvoline division of Ashland Chemical. DISCUSSION – In Figure 8, the performance curves of the stock alternator are presented as a function of engine speed. As is typical, the manufacturer provides two curves, one for ambient temperature of 25ºC and the other curve represents performance at under-hood temperatures. With a CVAD controlling the belt speed input, the alternator system has increased accessory power capacity (shaded region). Figure 9 shows alternator current produced with CVAD as the engine speed fluctuates during typical vehicle operation (in this graph, the EPA Heavy Duty Urban Dynamometer Driving Schedule 2 ). With CVAD installed, output stays near maximum. Without CVAD, current production falls significantly. CVAD increases alternator power output at idle by more than 75%
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The EPA Heavy Duty Urban Dynamometer Driving Schedule was selected because the FMTV Operational Requirements Document (ORD) only specifies a percentage of time in urban, cross country, and trail driving conditions. No vehicle speed driving cycle is given, therefore the authors inferred that the possible speeds of urban and cross country driving could be approximated by the HD-UDDS. October 9, 2009
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Current (A)
Enables increased Increased electrical Electrical CVAD enables CVAD enables increased electrical Power at low Lowengine Enginespeeds Speeds power power at low engine speeds
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Figure 8 - CVAD Provides More Alternator Power Output at Low Engine Speeds Over the course of this 1,200 second driving schedule, that translates to a 34% increase in total energy produced, as shown in Figure 10. Over the course of a full day of vehicle operation, this gap in total energy available would continue to increase. In addition to the immediately available increase in electrical power, this has obvious implications for reduction of battery deep cycling for both military and civilian use (transit buses and emergency vehicles are two clear examples) with a corresponding reduction in battery maintenance. 7
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Figure 9 - CVAD-Equipped Alternator Produces 75% more Current as Engine Speed Fluctuates (HD-UDDS cycle)
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Figure 10 - Comparison of Alternator Energy Production over the EPA UDDS Figure 11 compares the power requirements of CVAD equipped alternator to the stock alternator as a function of maximum current for a given engine speed. In the base system, the alternator is unable to produce the rated current at engine speeds below 920rpm (see the line between A and B). As base alternator speed is increased (as engine speed increases) the input power increases to overcome extra mechanical losses related to increased speed of alternator. The CVAD input power required to keep alternator at 2400rpm at rated current is also plotted. Observing Figure 11 we can see that below 920 rpm engine speed, the CVAD equipped system can maintain the rated current while the base system cannot. Assuming the base system is operated at point C (high idle) to achieve the rated current, the stock alternator input power is slightly less than the CVAD input power. For the CVAD system, the rated current can be achieved at 700 rpm (point D). Thus, the CVAD system could be more fuel efficient than the base system if the difference in fuel requirement between high idle of 920rpm and engine idle of 700rpm is less than the small difference in energy to run the CVAD. At engine speeds above 920 rpm, keeping the alternator at rated speed with a CVAD comes with a minor penalty, (E). It is a small fraction of the engine’s power and should not significantly affect the engine’s fuel economy over any given drive cycle. The CVAD also enables possible improvements in the alternator efficiency by reduction of the required operating speed range of the alternator at same or less cost than conventional designs. To improve fuel economy over any given drive cycle, the CVAD alternator need only improve its current system efficiency by approximately 5%. This can be done with incremental improvements in the CVAD/alternator design. When compounded with possible improvement in alternator peak efficiency via design optimization and operation in its most efficient operational regimes, this is a very achievable target.
October 9, 2009
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MORE THAN POWER AT IDLE, FUEL ECONOMY ON THE CRANKSHAFT -
Rated Current
Determining the fuel economy effect of separating accessory speed from engine Am speed pswas beyond the scope of the demonstration project; however, a parallel effort was undertaken to D quantify the effects of replacing the engine crankshaft pulley with a CVAD running all belt driven accessories at a constant speed. B E
A test fixture was constructed to measure torque losses as a function speed and load for the water pump, power steering pump and alternator of a Ford 5.4L gasoline engine. AC compressor Power data was not taken. Using commercially available, dynamic vehicle simulation software, the torque loss tables for the accessories were entered into the software. C The Ford Expedition vehicle parameters, engine map, transmission map and shift logic were also entered to create a vehicle model.
Change in % Change Fuel in Drive Economy Fuel Alternator Input Power Stock Cycle with Economy Alternator + CVT Input Power CVAD with (mpg) CVAD
Baseline results were calculated by simulating Stock Alt Amp the vehicle driving the U.S. EPA’s Federal City + 0.5 + 3.6% Urban Driving Schedule A (FUDS) and the Alternator Amp with CVT Highway Fuel Economy Test (HWFET). For Highway + 1.8 + 10% the baseline simulation, the accessory speeds 700 proportional 900 1100 1500 and 1700 1900power 2100over2300 2500cycle was were directly to the 1300 engine speed accessory each drive calculated. Then the same set of simulations was conducted with the CVAD inserted, including Engine Speed (rpm) its losses as taken from the test stand. In this run, the speeds of the accessories were independent of engine speed. For11. this simulation they wereand set Power to run at fixed speed (equivalent Figure Comparison of Amp to engine idle speed). The results of the simulations are shown below. 9 These results are consistent with results recently published by Gates Corporation and Hyundai Motor Company showing 0.3% (low load city driving) to 8.6% (high load highway driving) improvement in fuel economy with a two speed accessory belt drive system on a Ford 5.4L F150. Since the two speed system will clearly be sub-optimized for most driving conditions, the improvement predicted for the CVAD system is expected.
CVAD on the crank-shaft
Figure 12 - CVAD Mounted on the Crankshaft of a 5.4L Expedition To begin validating the simulation results, a duplicate CVAD has been installed on the crank shaft of a 5.4L Ford Expedition, see Figure 12 and is to be tested at an independent, not-for-profit, research and development company, in their EPA certified test facility.
October 9, 2009
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After discussions with several accessory device manufacturers, it is expected that additional fuel economy benefits can be derived by sizing each accessory’s pulley for constant speed operation. In this scenario, the pulley for each accessory is sized so that all accessories are running at their most efficient condition. The next logical step is to redesign the accessories themselves for constant speed operation. Current accessory design is a compromise of performance over a wide speed range versus efficiency at its most frequent operating condition. OTHER ACCESSORY APPLICATIONS, MILITARY AND COMMERCIAL In addition to CVAD, the CVP has numerous other potential uses. PTO electric generators and hydraulic pumps can be controlled to a desired output, regardless of the engine speed, potentially eliminating expensive power electronics in the generator case. In the case of hydraulic pumps, variable displacement pumps can be replaced with fixed displacement pumps and a CVP. For stand alone power generation (gensets), a CVP between a diesel engine and the generator can be used to mitigate wet stacking.
CONCLUSION The CVP has demonstrated itself to be practical option for controlling accessory speed independent of engine speed. Specifically, with respect to demonstrating power at idle, all three of the projects goals were successful. ; Maintain thermal stability at engine compartment temperature levels of 93º C (200º) ; Package on the existing FMTV beltline with little or no modification to the vehicle. ; Output maximum available alternator current at any engine speed.
Additionally, it has been shown that a fuel economy advantage is possible, when the CVP is mounted on the crank shaft pulley and all the accessories are run at a constant speed.
REFERENCES 1. Cross, Jim, Shaffer, Ed, “On Board Vehicle Power Briefing & Way Forward,” Presented at Joint Service Power Expo, April 2007 2. Masashi, U., et al, “Continuously Variable Transmission for Accessory Drive System”, 1999 Proceedings. JSAE Annual Congress. 3. Van der Iieijaen, A.C., “Continuously Variable Accessory Drive and other methods to reduce additional fuel consumption caused by engine accessories” ; Technische Universiteit Eindhoven Department Mechanical Engineering; Eindhoven, July, 2004 4. “An Evaluation of the Morse Constant Speed Accessory Drive”; Technology Assessment & Evaluation Branch, Office of Mobile Source Air Pollution Control, US EPA; June 1976 5. http://www.sae.org/altrefrigerant/presentations/presr-rugh.pdf 6. Atkinson, Ward, “SAE I-MAC Study Cross Country A/C Comfort Evaluation”, Presented at Mobile Air Conditioning Leadership Summit, June 2008 7. Pohl, Brad; Simister, Matthew; SAE Paper 04CVT-9, “Configuration Analysis of a Spherical Traction Drive CVT/IVT, © 2004 SAE International. 8. Bishop, John; SAE Paper 2006-01-0215 “Accessory Electrification in Class 8 Tractors”, 2005 SAE World Congress, © 2006 SAE International. 9. Ali, Imtiaz et al; SAE Paper 2008-01-1521, “E3 System – A Two Speed Accessory Belt Drive System for Reduced Fuel Consumption”, 2008 SAE Fuels and Lubricants Congress, June 2008.
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DEFINITIONS, ACRONYMS, ABBREVIATIONS A
- Amps or Amperes, a unit of electric current
C4
- Command, Control, Communications, and Computers
CVAD
- Continuously Variable Accessory Drive
CVP
- Continuously Variable Planetary Transmission
EPA
- Environmental Protection Agency
FMTV - Family of Medium Tactical Vehicles kW
- Kilowatt, an SI unit of power
kJ
- Kilojoules hours, an SI unit of energy
PTO
- Power Take Off
V
- Volts, a unit of electric potential
October 9, 2009
