55 Vehicle Architectures and Design The advantages of a parallel hybrid architecture are as follows: • It only needs two propulsion components: IC engine and motor/generator. In parallel HEV, motor can be used as generator and vice versa. • A smaller engine and a smaller motor can be used to get the same performance, until bat- teries are depleted. For short-trip missions, both can be rated at half the maximum power to provide the total power, assuming that the batteries are never depleted. For long-distance trips, engine may be rated for the maximum power, while the motor/generator may still be rated to half the maximum power or even smaller.
The disadvantages of a parallel hybrid architecture are as follows: • The control complexity increases significantly, since power flow has to be regulated and blended from two parallel sources. • The power blending from the IC engine and the motor necessitates a complex mechanical device. A simpler architecture with reduced components capable of delivering good acceleration perfor- mance is the ‘through-the-road’ parallel architecture. This architecture has one engine powering the front wheels and one electric machine powering the rear wheels.
The engine is the primary propulsion unit of the vehicle, while the electric machine is used for load leveling supplying the pro- pulsion power during acceleration and capturing vehicle kinetic energy through regeneration during deceleration. When the charge in the energy storage device falls below a set level, the engine is com- manded to deliver torque in excess of what is required to meet the driver demand. The additional energy supplied by the engine is used to charge the energy storage device by operating the rear elec- tric machine as a generator. Power is transferred from the engine to the electric machine through the road, and hence the name through-the-road parallel.
This charge-sustaining mechanism may not be that efficient, but the architedture offers great simplicity without the need for a coupling device to blend the torque from the engine and the electric motor. The vehicle can deliver good accelera- tion performance using the parallel operation of two propulsion units; the fuel efficiency can also be improved by about 20%–30% compared to a similarly sized IC engine vehicle. Additionally, the architecture gives four-wheel drive capability.
3.2.1.2 Series–Parallel Hybrid The parallel hybrid architecture is better suited to passenger vehicles where the electric motor and IC engine can be operated in parallel to deliver high performance when higher power is required [1]. On the other hand, the series hybrid architecture is equipped with a small output engine that can always be operated in its most efficient operating region to generate electrical power. The out- put electric power can be used directly to drive the propulsion motor or charge the energy storage system.
Series hybrid vehicles tend to be heavy and typically have difficulty meeting acceleration requirements since the powertrain components need to be sized for the maximum continuous output power for charge-sustaining operation. The advanced hybrids combine the benefits of series and parallel architectures into a series– parallel hybrid architecture with charge-sustaining capability [1,2]. In these combination hybrids, the IC engine is also used to charge the battery. The architecture is relatively more complicated, involving additional mechanical links and controls compared to the series hybrid, and an additional generator compared to the parallel hybrid.
The vehicle is primarily a parallel HEV but with a small series element added to the architecture. The small series element ensures that the battery charge is sustained in prolonged wait periods such as in traffic lights or in a traffic jam. The controller for the series–parallel architectures effectively utilizes the IC engine and electric motors to deliver up to their maximum capabilities through flexible adaptation with driving conditions.