Figure 3 shows a typical vehicle drive cycle in Bangalore city. It is the vehicle velocity versus time. In a high density city traffic, this velocity versus time profile is fairly independent of the type and power of the vehicle. The vehicle velocity is governed by the city traffiic and not by the vehicle torquecapability. The average speed is less than 15 kmph, but it will vary with trafic condictions. However, it is evident that a major portion of the city drive cycle consists of acceleration and deceleration. In a conventional ICE vehicle, the energy during deceleration is lost as heat while braking, as most braking mechanisms are based on friction principle. Whereas, in EV or HEV, during deceleration, the kinetic energy is not wasted but is instead put back into the storage device (like battey). This is known as regenerative braking mechanism. In comparison to the internal combustion engine, an electric motor is relatively a simpler and far more efficient machine. Moving parts consist primarily of the motor shaft and bearings. The motoring efficiency is typically in excess of 85%. In addition, the electric motor torque characteristics are more compatible with the load demand than the torque characteristic of the ICE. A vehicle needs high torque at low speeds for acceleration and demands less torque during cruising. An electric motor develops high torque even at low rpm, and the torque decreases with speed, in step with the vehicle’s load demand. In contrast, an ICE develops very little torque at low speed and must accelerate through nearly 80% of its maximum speed before it can deliver maximum torque. A multi-ratio geared transmission is therefore necessary in order to correctly match ICE output characteristics to the vehicle load demand curve. As a consequence an EV drive train usually needs just one fixed gear ratio eliminating the need for changing the gear ratios while running. Further, a reverse gear is unnecessary because the rotational direction of the motor itself can be reversed simply by electrical polarity reversal. These advantages lead to a far less complex and more efficient propulsion system.
Figure 3: City drive cycle
The main and possibly only significant disadvantage of the EV over ICE vehicles is the very low energy density of its storage systems (battery, flywheel, ultracap). This reflects in limited range of the vehicle. This along with long re-fuel or re-charge time form one of the major challenges toward EV transition. In this context, the HEVs can be considered as technological solution that will provide the bridge between ICE vehicles and the EVs.