Engine Optimization Concepts for CVT-Hybrid Systems to Obtain the Best Performance and Fuel Efficiency. Professor Andrew A. Frank Univ.

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1 Engine Optimization Concepts for CVT-Hybrid Systems to Obtain the Best Performance and Fuel Efficiency Professor Andrew A. Frank Univ. of CA-Davis Abstract: The objective of the advanced transmission system concepts such as the Continuously Variable Transmission (CVT) and Hybrid Electric Drives is to improve fuel efficiency, lower emissions and reduce powertrain part count while not impacting cost. The control of the system, however, can greatly affect the final fuel consumption, performance and emissions for any of the possible configurations. This paper describes an engine control philosophy for a hybrid electric CVT powertrain concept with the fewest number of mechanical parts but with many modes of operation such as: 1. All electric operation 2. Regenerative braking to maintain the battery charge at a desired level. 3. Engine charge for maintaining the battery state of charge 4. Highway cruise efficiency. 5. Power enhancement by use of the electrical energy for passing and highway maneuvers. 6. Trailer towing and high load applications The paper describes the optimization theory based upon the unique multidimensional characteristics of the CVT and illustrates the results of three alternative operating strategies with simulation and vehicle test data. The best of the tried control schemes is described and compared with the others. The reasons for the differences are described. Figure 1. The optimum regions for best overall drivetrain efficiency when using gasoline to maintain charge in the battery. The dark blue represents the electric only operation and the dark blue negative torque part represents regenerative braking by the electric motor.

2 Note: Gasoline is not be used to charge the battery unless absolutely necessary. Battery charge should be done by regenerative braking or by using the engine at a high efficiency point where it would normally be in a very low efficiency point of operation. For example, when the power required to drive the car is very low due to a slow deceleration in a normal car the engine efficiency is less than ½ the maximum possible than it would be more effective to run the engine at the best efficiency and put enough charge into the batteries from the electric motor so that the power is the amount desired by the driver.. The conventional vehicle with a CVT. The conventional CVT vehicles being currently manufactured, has shown that it is possible to improve fuel economy over a Conventional multi-speed Transmission (CT). The transmission however, does not allow a fuel economy as good as can be achieved by a manual transmission (MT) in the hands of a knowledgeable driver. This is well known for many years and has been well documented. The purpose of this paper is to show that the fuel economy of a conventional car with a CVT (CV-CVT) is necessarily limite. This is due to a compromise between fuel economy and performance and drivability. But the hybrid electric CVT car can restore the fuel economy possibility to the vehicle to achieve the best possible efficiency. We will show that it is possible to more than double the fuel efficiency of a CV using the hybrid electric-cvt powertrain concept. The reason a CV-CVT car is caught with the dilemma of choosing fuel economy or performance is because of the dynamics of the CVT. In any CVT powertrain system, the dynamics governing the acceleration of the vehicle is: α [ R Ie ωe + Te Re Tlosses Tdrag ] [ I R + I ] driveshaft = 2 e driveshaft Where: α drshaft = Acceleration of the driveshaft proportional to vehicle accel. R = CVT ratio or engine speed/driveshaft speed R dot = CVT ratio rate I drshaft = vehicle inertia reflected to the driveshaft or CVT output I e = Engine or CVT input inertia T e = Engine input torque to the CVT T losses = CVT losses transferred to the output shaft ω e = Engine speed This equation governs the CVT powertrain system. The equation can describe the acceleration of the driveshaft or the torque at the driveshaft. It says the with any CVT system the output torque is equal to the input torque times the ratio of the transmission but plus a second term that is proportional to the rate of change of ratio R dot.

3 It is this second term that make the CVT controller balance vehicle response and fuel economy. This term R dot times the angular momentum of the input is a torque in the negative direction of R. In a vehicle, when the driver demands more power, the transmission generally wishes to (downshift) or increase the gear ratio. With a CVT this means a positive R dot. But according to the equation this will result in a negative term causing the vehicle to decelerate or not accelerate as much. In addition, this term is proportional to the momentum of the input inertia meaning the faster the engine is running the worse this effect. To achieve good fuel economy for an internal combustion engine, it is normally desired to operate at near maximum torque at a given speed determined by the power required at a given instant. See figure 1 below. The yellow line represents the best available fuel efficiency for a given power. If the engine is connected to a CVT and the torque is near maximum at a given speed achieving good fuel economy, then when more power is required the CVT ratio must increase or R dot must be positive. The trouble is the engine is already near maximum torque so we cannot increase R much without incurring a negative torque or a deceleration and lower engine efficiency. In the CV-CVT cars the engine steady state operating point then must be lowered in order to leave torque head room to compensate for the R dot as in the red line in the figure. The consequence of this is that the engine efficiency is lowered to leave the torque head room for acceleration capability. This is shown below: Figure 2. Engine efficiency and best operating line for a given power (yellow),and CV-CVT engine operating line (red)

4 The more performance desired from a CV-CVT car the lower the red CVT operating line needs to be and the lower the fuel economy. Due to the need for good response and performance in vehicles today, then how can we obtain both super fuel efficiency and high performance in operation?? The problem can be solved by examining the dynamic equation of motion above. Some characteristics to notice is that the two terms that affect output torque are independent of each other. The reason is fundamental to the CVT mechanism. The engine torque time the ratio term is the ordinary transmission term. The Ratio rate term has to do with how fast the contact elements are steered to change ratio. These control mechanisms are clearly independent of each other. This means that they can be controlled independently. This opens the door to the possibility of optimal control. One way would be to use one parameter to provide the best possible fuel efficiency, and the other parameter to provide the level of response desired. This strategy means there must be two independent energy sources to make the gasoline engine operate at it s best efficiency for any power and for the vehicle to have excellent acceleration response. The Hybrid Electric Vehicle Concept The hybrid electric vehicle has two energy sources to be controlled at the same time. These energy sources have different paths for efficiency depending on the kind of energy that is being used. If the vehicle is a battery sustaining hybrid, then the source of energy is only one. All hybrids today in production are charge sustaining. If the vehicle is charge depleting then there are two independent energy sources, usually electricity from the wall and Gasoline or Diesel. The Hybrid electric concept can be illustrated as shown in the following figure. In either case, instantaneously there are two sources of energy which can solve the dual energy source problem. The following figure shows a powertrain configuration that allows the engine in a CVT hybrid to operate at it s best efficiency for any power and the vehicle have high performance. Here the electric motor torque is added to the gasoline engine torque. This is physically accomplished simply by having the gasoline engine and electric motor on the same shaft. When operating a cruise speed the engine is operating on the IOL at pt. A if the driver depresses the accelerator pedal demanding more power, then the electric motor instantly provides torque and power to match the demand to pt. B on the Figure. The vehicle begins to accelerate and the engine stays on the IOL. The motor torque will drop as the powertrain accelerates because the accelerator pedal power remains the same. The motor torque goes to zero and the engine is now operating at the commanded power on the IOL at pt. C. Thus the engine stays on the IOL and the CVT R dot term is compensated by the electric motor. The result is a smooth acceleration to a higher vehicle speed. Only the battery supplies the energy to for this acceleration. Now if the accelerator pedal is returned to the original position, the torque needs to drop to point D. Now we can leave the engine on the IOL as the engine is slowed down to produce the desired power. The

5 excess energy then is regenerated back to the batteries by the electric motor acting as a generator. The engine remains on the IOL but now we are taking energy through a generator-battery-motor path that will be less than 50% efficient back to point A The solution is not to keep the engine at the IOL but rather throttle the engine torque until the loss of efficiency is below going through the generator-battery-motor system. Thus, we find that a better efficiency will occur if the engine is throttled to reduce torque. this is shown in Figure below in what we call the Ideal Operating Band. This is shown in Figure below. The system is very simple due to the use of a smaller engine with fewer cylinders. In addition this configuration can eliminate many conventional components such as an engine starter motor, a generator, water and oil pumps for the engine, torque converter, reverse gear, etc. Electric motor energy source Min. eng speed Gasoline engine Energy operation Figure 3. The engine plus electric motor torque curve and the operation in a CVT Hybrid vehicle. The configuration has been constructed into many vehicles at the Hybrid Electric Vehicle Center at UCDavis. The latest one is a Ford Explorer with much better performance than it s CV. We will be constructing a hybrid CVT that essentially doubles the performance and fuel economy at the same time. Of course, not all the performance and fuel economy can be attributed to the CVT and control strategy. Some of it will also come from a more efficient CVT with less than 1/3

6 the losses of the conventional CVT, and some of it will come from regeneration of braking energy. Our objective in this paper is to explain the way to optimize the CVT/electric motor system. Notice in the hybrid design that we do not have a separate generator, rather we use only one traction motor/generator for the system. The objective is to greatly simplify the mechanical system so that it reduces the number of moving parts in the powertrain to a small percentage (like 10%) of the number of parts of a CV. By parts reduction and higher fuel efficiency the initial incremental cost of the electric and battery components can be justified until the production volume comes up. At some time in the future it is expected that cost of such a Hybrid/CVT vehicle would be less than a comparable CV. Figure 4. A Hybrid Electric powertrain with two energy sources There are two kinds of hybrids to be considered, the classic ones now being manufactured called Charge Sustaining (CS) hybrids, and the kind we have been researching called Charge Depleting (CD) hybrids. The Charge Depleting hybrids can use energy from the wall plug as well as gasoline. These vehicles are designed to perform the same regardless of whether the batteries are charged or not. The only motivations for the driver to charge is economics and green action. The cost of electricity to travel the same distance is about ¼ the cost of gasoline so there is great motivation to charge the car s battery from house electricity. Of course, the guilty conscience of driving a car using electricity is also gone. We have constructed vehicles that can go 100 km without using any gasoline in the city before we need to start the

7 gasoline engine. This mode of operation is Zero emission or ZEV. We set the speed threshold for this mode at 100 kph.so that it is ZEV for the first 100 km and below 100 kph. This means that the vehicle in the city is ZEV. An average person driving 20,000 km a year would use only 1/10 th the amount of gasoline that a comparable conventional car uses. Annual Gasoline Use for Various Vehicle Types Annual Gasoline Use (gallons per vehicle) CV HEV 0 HEV 20 HEV 60 Figure 5. Annual Energy Consumption of various hybrid vehicles with different all electric range compared with the conventional vehicle CV all driving 20,000 km/year It should be noted that the amount of annual gasoline consumed is inversely proportional to battery size. Where HEV (i) represents vehicles that have (i) all electric range before the gasoline engine needs to come on to make the vehicle charge sustaining. HEV 0 is the hybrids manufactured today by Toyota and Honda and Ford. Above 100 kph the charge depleting vehicle becomes charge sustaining like the standard CS hybrid. Thus the charge Depleting feature is primarily for low speed city driving. This is where zero emissions is needed most. It should also be pointed out that once the engine does come on, it is still at SULEV standards or below. In the Charge sustaining mode for these hybrid vehicles the objective is to keep the batteries charged using as much waste energy as possible. The waste energy needs to be categorized. 1. Inefficient power to slow the vehicle down, such as closed throttle operation. 2. Energy for low levels of power required to drive the vehicle where the normal engine efficiency of operation is poor with gasoline or Diesel, such as when the drive power is less than 10% of the rated engine, and 3. energy to stop the vehicle; normally done by the mechanical brakes.. These three energy situations need to consider energy use as efficiently as possible and reuse recovered energy. But energy recovery from these sources is not without further losses. Thus, from an efficiency point of view, the losses in the path of energy recovery

8 must be efficient enough to make the effort worth while. For example, if the electric motor/generator is 80% efficient and the batteries are 80 % efficient on charge and 80% efficient on discharge, and if you have a motor that is 80% efficient, then the efficiency of storage and retrieval of energy from shaft to shaft is now about 32% efficient. This level of efficiency then makes it difficult to justify the additional cost of the components. Fortunately, there are other benefits to the two energy source systems. The first is that the engine can be downsized, making it operate at higher more efficient loads and thus improving the efficiency of the engine. The second is that the primary engine low efficiency operational modes can be eliminated. These are such modes like engine idle and other closed throttle operation, and low efficiency low load operation of the engine such as low speed cruise or slight down grades. To get the best from the hybrid concept, all components must be the highest efficiency possible and the control strategy must account for all the energy losses in providing energy to the wheels. Thus, the control strategy must keep in mind some fundamental principals: 1. Don t use the electrical path unless it is more efficient than throttling the engine. 2. Down size the engine as much as possible. The engine must be able to maintain desired average cruise speed. 3. Use a CVT or as many gears as possible in the drivetrain to allow a wide range for power optimization. 4. All energy sources must be coordinated on an instant by instant basis. The downsizing of the gasoline or Diesel engine is the next important feature. Since the energy to drive a vehicle is the same on a given driving cycle, then the engine could be down sized to the average power required for the driving cycle. For a 1300 kg car, driving on the US city cycle, it is about 7 kw. The engine thus could theoretically be sized down this far. Most conventional cars of this size have engines from 100 kw to 200 kw. The reason that these engines are needed is for vehicle performance and other more stringent driving cycles such as mountain driving and trailer towing. Thus downsizing is limited by the most stringent driving cycle expected for the vehicle. Thus the benefits of engine downsizing is limited by the expected vehicle use. Our calculations show that for the USA, an engine that is about 25% of normal could be possible. We have also found that, the smaller the engine, the higher the load and thus the higher the fuel efficiency. The down side then is that this little engine is now required to have a higher duty factor and must be more durable than a conventional engine. Thus for efficiency the engine should be small as possible and for maintaining steady speed on a level road, then performance in a hybrid is accomplished by the electric motor. This element and the consequent the battery, can be sized for the acceleration performance desired. Our vehicles use a gasoline engine about 40 kw and a 75 kw electric motor plus the CVT. Philosophically, the engine is used for efficiency in a hybrid electric and the electric motor is used for acceleration and performance.

9 Engine operating concepts for parallel hybrid powertrains. To operate the engine during normal driving, it is necessary to select the operating policy for the engine controlled by the CVT. The first concept is to operate the engine at a constant speed using the CVT to regulate the engine speed as the vehicle drives. This is shown in red, in the figure below. A second concept of engine optimization is to operate the engine along the ideal operating line as shown in blue on the same figure. These two concepts of operation was implemented in our Engine/Motor/CVT parallel hybrid vehicle. The result for these two different control policies is measured. The results show different fuel economies while driving the city cycle. This is shown below in Figure. Figure 6. Normal operating mode [red] and IOL mode [blue] engine operation in dynamometer EPA City driving. The engine operating points during city sysle driving are shown. The resulting fuel consumption was lower than the conventional car by a substantial amount but we found that the traction batteries in vehicle were cycled so much that they would begin to rise in temperature. This clearly indicates that these policies are not as fuel efficient as they cold be. As mentioned above, during charge sustaining operation, we should be using the engine in areas where it is more efficient than circulating energy through the electric motor and generator. Or we need to minimize the circulation of energy through the electrical system and batteries. Of course, what ever energy we take out of the batteries for normal driving must be replaced. So these two requirements need to be met in an independent way. We start the vehicle electrically since we have enough electrical power and torque

10 to provide equivalent or better acceleration than a CV up to about 80 kph using the electric motor and CVT. This then allows us to couple the gasoline engine at a speed that will maintain the battery charge over the driving cycle. Thus we have two independent techniques to optimize for vehicle operation. The engine turn on speed and the best efficiency operating mode. We find the best engine operating area is bounded by the IOL for maximum torque and a Lower Ideal Operating line set by the engine efficiency and the turn around efficiency of the electric motor/generator and battery system. We call this the Ideal Operating Band (IOB). The use of the IOB in the US EPA city driving cycle is shown in Figure below. Figure 7. Ideal operating band [IOB] for best fuel economy for city driving This figure shows the engine operating in the Ideal Operating Band, IOB, while driving the EPA dynamometer city cycle. The Results and Conclusions These control strategies and energy management concepts for a Hybrid CVT car show that any strategy can provide benefits over a conventional car. But it shown that the results can be better by using an operating strategy that is optimized at every point in time by choosing the best efficiency between throttling the engine or circulating energy through the electric motor/generator and battery. The table below summarizes a matrix of results with the control modes and the engine turn on policy.

11 Table 1 Results of Dynamometer test on three control policies and three engine start speeds Normal Mode City Highway Turn On Speed (mph) Electrical Usage (Wh) Gasoline Usage (gal) Fuel Economy Electrical Usage (Wh) Gasoline Usage (gal) Fuel Economy IOL Mode City Highway Turn On Speed (mph) Electrical Usage (Wh) Gasoline Usage (gal) Fuel Economy Electrical Usage (Wh) Gasoline Usage (gal) Fuel Economy IOB Mode City Highway Turn On Speed (mph) Electrical Usage (Wh) Gasoline Usage (gal) Fuel Economy Electrical Usage (Wh) Gasoline Usage (gal) Fuel Economy From these results it is clear that more optimization can be done. But the results show that sustaining the batteries can be done by using engine the turn on speed, and that fuel economy is affected by the choice of operating strategy. The choice of the best strategy needs to be done considering the battery characteristics and the operating temperature and many other factors. And example of further operating optimization in a parallel Hybrid Chevrolet Suburban SUV is shown in the following figure. Figure 8. Optimization of charge regions for a Chevrolet Suburban showing regions for sustaining charge, depleting charge, and motor only operation.

12 This figure shows that the best place to replace the charge in the batteries is along the very low torque operating area where the engine efficiency is clearly lower than cycling energy through the electric system. Of course, any negative torque can go through the electric motor/generator to the batteries as shown in the blue. Acknowledgments We wish to thank the students of UCDavis whose master s thesis provided the results for this paper. Mr. Rob Shurhoff and Vern Francisco. These vehicles were constructed by the student teams of UCDavis for the Future Car and later the Future Truck contest run by the USDOE. It is these contests that provide the funds to do this research.