Continuously Variable Transmission Modifications and Control for a Diesel Hybrid Electric Powertrain

The Center for Transportation Research (CTR) Vehicle Systems team modified a Nissan CK-2 Continuously Variable Transmission (CVT) for a diesel hybrid powertrain application. Mechanical and electrical modifications were made to the CVT, both internal and external to the transmission. The goal of this experiment was to investigate and demonstrate the potential of CVT for diesel engines hybrid electric vehicles (HEVs) in fuel economy and emissions. The test set-up consisted of a diesel engine coupled to an electric motor driving a Continuously Variable Transmission (CVT). This hybrid drive is connected to a dynamometer and a DC electrical power source creating a vehicle context by combining advanced computer models and emulation techniques. The experiment focuses on the impact particular transmission control strategies have on measured fuel economy and emissions specifically, nitrogen oxides (NOx) and particulate matter (PM). The same hardware and test procedure were used throughout the entire experiment to assess the impact of different control approaches. INTRODUCTION The CVT parallel hybrid configuration used in this experiment provides tremendous flexibility in the choice of both engine torque and speed operation. The electric motor can replace, assist, or absorb the engine torque independently from driver expectations. In addition, the CVT allows decoupling between engine and wheel speeds. This paper compares different transmission control approaches and evaluates their impact on fuel economy and emissions. But first, in order to obtain a fuel economy and emissions reference, we first operated the CVT in an emulated manual transmission mode. Therefore, the electric motor was disabled, and the CVT acted as a manual transmission. Then, we controlled the hybrid CVT to keep the diesel engine on its best efficiency curve. The best efficiency curve describes the optimal engine operating point for each power demand from an energy perspective. Therefore, the engine torque and the CVT ratio were both controlled to operate the engine at the most efficient point while satisfying the power demand. However, when the engine operates on its best efficiency curve, it produces excessive NOx emissions. We used simulation to design a trade-off between fuel economy and NOx emissions. For each engine power demand, we interpreted NOx emissions and fuel consumption data to define the best trade-off curve. The engine torque and the CVT ratio were controlled to operate the engine on this curve while satisfying engine power demand. Fuel economy and emissions results obtained from the experiments are compared and described in this paper. The mechanical and electrical modifications made to the CVT to improve efficiency and to control the transmission are also discussed. CVT MODIFICATIONS FOR HEV APPLICATION The transmission was a modified Nissan CK-2 CVT, which uses a Van Doorne push-type belt that is commercially available in several Japanese production vehicles. Mechanical and electrical modifications were made to the CVT, both internal and external to the transmission. In stock trim, an off-board transmission control unit that controls torque converter lock-up, CVT ratio, and hydraulic pressure accompanies the CVT. In its original design, the mechanical hydraulic pump is connected to the engine through the torque converter. REMOVAL OF THE TORQUE CONVERTER Hybridization requires disconnecting the engine from the transmission by using a clutch, temporarily allowing the electrical motor alone to propel the vehicle. The torque converter was replaced by a clutch. The clutch benefits system efficiency because clutch efficiency is better than torque converter efficiency. The reverse planetary gear was removed because HEVs with electric-only capabilities can use the electric motor for reverse. CONTROL OF THE CVT RATIO All CVT control is done with the ANL-developed PSATPRO control software; additional hardware has been added to support this approach [1]. The stock electronic control unit for the transmission was not used. A new driver for the stock ratio control stepper motor in the hydraulic control circuit was installed to control CVT ratio according to PSAT-PRO hybrid control computer. Figure 1: Stepper motor for ratio control REMOVAL OF THE INTERNAL PUMP The main modification to the CVT was the removal of the internal high-pressure hydraulic pump. We replaced it with an off-board pump. The pump is a key component of the CVT because it provides adequate belt/pulleys clamping. By using an external pump, much higher power transmission efficiencies can be achieved because an electrically driven pump allows the optimal control of hydraulic clamping pressure by decoupling the pump from the transmission input shaft (see Figures 2 and 3). Figure 2: Removal of the CVT oil pump Figure 3: Modified Nissan CVT CONTROL OF THE CLAMPING PRESSURE Supply clamping pressure is a key parameter for efficient use of this type of transmission. Ideally, the pressure should always be minimized to allow efficient torque transmission while avoiding belt slipping (if pressure is too low) or overheating and abnormal wear (if pressure is too high). For this reason, a dynamic control algorithm has been integrated into the powertrain controller to allow optimal CVT operation. This algorithm uses instantaneous CVT input shaft torque and ratio measurement to calculate optimum supply pressure and accordingly command hydraulic pump. Figure 4 shows in parallel the CVT pressure, the input torque used to dynamically control the hydraulic pump, and the vehicle wheel speed during a test. Figure 4: CVT pressure control The test data show a substantial improvement in the efficiency of the transmission. CVT INSTRUMENTATION The bell-housing of the CVT transmission was modified to mount a HBM T-10F flat torque sensor for measured CVT input torque. Flanges that mate to the splined shaft and the engine output were also machined. Figure 5: CVT torque sensor The hydraulic pump was instrumented to evaluate its energy use. This parameter needs to be taken into account to validate the actual gain in efficiency of the dynamic pressure control. Therefore, the power consumed by the auxiliary pump was taken into account. The electric power consumed by the pump was drawn from an emulated battery to reflect its impact on fuel economy (see Figure 6).