Modelling of an Off-Road Hybrid P2 Power Unit with Engine and Electric Motor connected by means of a One-Way Clutch

Due to increasingly stringent regulations on Internal Combustion Engine emissions, companies are heavily investing on new strategies regarding hybrid and electric vehicles. The goal of this thesis, indeed, is to investigate if an Off-road Hybrid P2 Power Unit can be realized with a one-way clutch, used as coupling device between ICE and electric motor. First of all, considering the working principle of one-way clutch, what are advantages and disadvantages of using this device instead of a friction clutch, more common in this kind of applications, have been evaluated. One-way clutch transmits torque from outer sleeve, engaged with the engine in this application, to inner sleeve, engaged with the electric motor, if sleeve speeds are equal. While, if inner sleeve speed is higher than the outer one, as it should occur when vehicle is in “pure electric mode”, clutch is in overrunning mode and the engine does not receive any external load. It never happens, instead, that outer sleeve speed is higher than the inner one, since at least, the inner sleeve is forced to rotate with the speed imposed by the outer. Understanding the behaviour of one-way clutch under different working conditions is necessary since continuous engagement and disengagement of the clutch can cause wearing of one-way clutch inner bodies, leading to the failure of component. To investigate in a more precise way the behaviour of the system, models on Simulink have been created. In the first model, it has been considered engine, one-way clutch, and e-motor. Results have shown that when engine is turned on, each time engine instantaneous torque becomes negative, one-way clutch disengages engine and motor, since a negative torque can not be transmitted from engine to electric motor. Moreover, high positive torque picks can reduce one-way clutch life. In the second model, to solve these problems, an elastic joint between engine flywheel and one-way clutch has been added. Elastic joint reduces engine torque oscillations, leading instantaneous torque picks to be lower. Furthermore, it has been noticed that instantaneous engine torque is always positive starting from a certain speed on, avoiding unnecessary disengagement. In the end, a first attempt control strategy has been defined. This has led to further consideration about possible disengagement between ICE and e-motor. During transient, indeed, if both engine and e-motor generate maximum torque, it could happen that e-motor acceleration is higher than engine one, hence one-way clutch is in overrunning mode and so, engine and e-motor are decoupled. Next step of the project will be testing the system on test bench, to validate the model. In particular, it is important to investigate how engine torque and engagement-disengagement frequency influences clutch life. Once it will be shown how dangerous they are, control strategy must be further modified to find the right compromise between performance and life of component.