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EV – Challenges In Going Electric

II ELECTRIC VEHICLE

 
Electric vehicles in general have an electric motor as one of the prime movers that propel the wheels. It may be a single motor EV wherein the motor shaft is coupled to the wheels through a differential at the front or rear axle. Multi motor configurations are also possible wherein the motor is embedded within the wheel. Such motors are called in-wheel motors. The vehicle may be driven by both electric propulsion system and the ICE propulsion system. They are called parallel hybrid vehicles. The electric vehicles may be classified as (i) Zero Emission Vehicles (ZEV) or just Electric Vehicles (EVs) and (ii) low pollution vehicles or Hybrid Electric Vehicles (HEVs) [4]. Most electric vehicles use rechargeable batteries for energy storage. The batteries are the on-board storage devices for the EVs. On the other hand, a hybrid electric vehicle combines multiple energy storage sources. The hybrid electric vehicle can be broadly classified into the series hybrid and the parallel hybrid. In the series hybrid structure, the electric motor acts as the only torque source. A single motor may transmit the power to the wheels or several motors may be used. The in-wheel motors are generally less efficient due to higher torque requirements. The energy to drive the motors is obtained from multiple energy sources of different types. Figure 1 shows the general hybrid electric engine block schematic.

Figure 1: Generic hybrid electric engine

This is a general topology that will fit conventional ICE based propulsion, electric propulsion and hybrid electric propulsion systems. Referring to Figure 1, it can be seen that the conventional ICE vehicles use petrol or diesel as fuel input and the power from the ICE is transmitted to the transmission system comprising differential and wheels, through the gear box (GB). Only these parts of the block schematic comes into effect for conventional ICE vehicles. For electric vehciles (EV), battery bank, multiport power port, inverter, induction motor part of the block schematic is effective. The multi-power port converter is an important block which performs instantaneous power routing in EVs. When the vehicle is in motion, the power to drive the wheels flows from the stored energy in the battery bank through the multi-power port and then onto the inverter, followed by induction motor and transmission. When the EV which is in motion is required to brake, then the kinetic energy of the whole vehicle by virtue of the EV being in motion is decreased by passing the energy at the wheels to the battery through the induction motor, inverter and multipower port. It should be noted here that the induction motor will act as induction generator during this regenerative braking mode. The inverter and multi-power port should be capable of bi-directional power flow. When the EV is parked and put incharging mode, the power flow is from the electric source to the battery through the multi-power port.

In the case of hybrid electric vehicles, both petrol/diesel fuel and batteries are used as energy sources. In the case of series hybrid vehicles, the ICE is coupled to an electric generator or alternator. The output of the alternator is rectified and the power drawn from ICE and that drawn from battery are together used to drive the induction motor through the inverter. In parallel hybrid electric vehicles, the petrol, ICE, gear box (GB) parts of the system are used to drive one set of wheels and the battery, multi-power port, inverter and induction motor are used to drive another set of wheels. Here the electric generator or alternator and rectifier are not used. If even the electric generator and rectifier are used in a parallel hybrid topology, then it is called a combination hybrid. Here, during standstill condition when the vehicle is not in motion as in waiting for traffic signals, the ICE may be used to charge up the batteries. When the vehicle is parked, then the batteries can be charged from the electric grid source (wall outlet). If the EV or HEV can be charged from external power source, then such a vehicle is called plug-in EV (PEV) or plug-in HEV (PHEV) [5].

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