Move Research  

05/01/10

ELECTRIC PILL N.1

The electric car is a system composed basically of batteries, inverter, electric motor, transmission, wheels.

In such a system the efficiency is the ratio between the energy exploited and the one requested by wheel transmission.

Internal combustion engines have a maximum 30% efficiency rate, but rarely in real everyday use the global efficiency is more than 10%, because of factors like engine size, way of driving, environmental conditions.

This reduction is so remarkable because other mechanical transmission elements are responsible for a further energy loss in the whole system. It's quite clear that the energy balance between input and output is low.

An electric car has a definitely higher global efficiency deriving from the single efficiency of: battery charging and discharging, inverter, electric motor.

If the battery discharging efficiency is about 90%, the efficiency of the inverter is 95%, 90% the one of the electric motor, we can gain a wheel transmission efficiency close to 78%.

Accounting slight mechanical losses (mainly because there is no gear), wheel efficiency is 70-75%. The global energy efficiency of an electic car can be improved thanks to regenerative braking or wheel motors.

Wheel motors are basically electric motors mounted directly onto the wheels, thus avoiding mechanical transmission and its energy waste. The problems to face are the motor's own weight and its consequent inactivity at high speed.

The presence of the batteries and the electric motor (that can also work as a generator) gives a chance to regenerative braking: the vehicle's kinetic energy must be discarded and rather than being dissipated as heat resulting from braking friction, it can be partly reused to produce electric energy through the generator, that will be stored in the battery and used if needed. This system enables a 85% electric car energy efficiency.

The advantages highlighted in this energy analysis as well as the overcoming of past problems like cost, weight, battery size and charging times, show that the electric car has a key role in the sustainable mobility challenge.

Author: CIRPS Sapienza University of Rome Automotive Research Group

 

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