56 Electric and Hybrid Vehicles Engine Engine Wheel Wheel Generator Generator Power- split Power- split Transmission Transmission Driveshaft Dif. Driveshaft Dif. Battery Motor Battery Motor Inverter Inverter Wheel Wheel FIGURE 3.4 Series–parallel combination HEV. The series–parallel architecture is the one that has been used in the first ever commercially avail- able hybrid vehicle, the Toyota Prius. The vehicle architecture uses a mechanical power-split device that was developed by the Japanese researchers from Equos Research [3,4].
The compact transaxle design in the powertrain integrates two electric motors to simplify the manufacture of both con- ventional vehicles and hybrid vehicles in the same production plant without significant alteration of the same assembly line. Most of the series–parallel passenger hybrid vehicles available in the market today have three power plants (the engine and two electric machines) mounted in the front transaxle. The power-split device divides the output from the engine into mechanical and electrical transmission paths using a planetary gearset. The components’ arrangement in the series–parallel architecture based on the Toyota Prius hybrid design is shown in Figure 3.4.
The power-split device allocates power from the IC engine to the front wheels through the driveshaft and the electric gen- erator depending on the driving condition. The power through the generator is used to recharge the batteries. The electric motor can also deliver power to the front wheels in parallel to the IC engine. The inverter is bidirectional and is used to either charge the batteries from the generator or to condi- tion the power for the electric motor. For short bursts of acceleration, power can be delivered to the driveshaft from both the internal combustion engine and the electric motor.
A central control unit regulates the power flow for the system using multiple feedback signals from the various sensors. The series–parallel hybrids are capable of providing continuous high output power compared to either a series or a parallel hybrid for similarly sized powertrain components. The series–parallel vehicle can operate in all three modes of series, parallel and power-split mode. There is significantly greater flexibility in the control strategy design for a series–parallel hybrid vehicle that leads to better fuel economy and lower emissions.
3.2.1.3 Series–Parallel 2 Ч 2 Hybrid A series–parallel 2 Ч 2 vehicle architecture with two electric machines, one IC engine and a battery energy storage system is shown in Figure 3.5. The architecture offers the same attractive features of series–parallel architecture described in the previous section but with an inherent four-wheel drive capability. The engine is coupled to one set of wheels, typically the front wheels, through a transmission. One electric machine is coupled to the engine mechanically while the other electric machine is coupled to the other set of wheels, typically the rear wheels. The front-mounted electric machine or generator is mostly used for generation and starting.
The rear-mounted motor is used for regenerative braking and traction. The engine is used for traction and also for supplying power to the generator. During acceleration, the engine powers the front wheels while the rear-mounted elec- tric machine powers the rear wheels. However, during peak acceleration demand, the front electric machine can be operated as a motor to add torque at the front axle. The torque blending between the front axle and rear axles is through electronic controls and does not require any mechanical power- split device like that in the Prius series–parallel architecture.
The downside to the series–p arallel 2 Ч 2 is the complexity of the control algorithm and the mounting requirements of powertrain components in both the front and rear axles.