ALTERNATIVE PROPULSION SYSTEMS

Following factors provided motivation for the development of alternative propulsion systems for road vehicles;

• Global warming -  Reduction desired in the emission of greenhouse gas, carbon dioxide
• Control of urban air pollution
• Higher energy efficiency - to prolong availability of petroleum fuels as the crude reserves are diminishing
• Energy security – to be independent of import of energy from other countries

The design of power unit of vehicles is governed by several factors e.g., type of available fuel/energy, economics of energy availability and environmental considerations. The following vehicle power plants have been under detailed investigations and some of them are already introduced in the market.

• Hybrid - electric propulsion
• Fuel cells
• Gas turbines
• Stirling engine
• Batteries for electric vehicle

The hybrid electric and fuel cell vehicles hold a greater promise of practical application.  Hybrid electric vehicles are built around the existing reciprocating IC engines and some vehicle models are already in market. The fuel cell vehicle is a zero emission vehicle and all the major auto- companies are pursuing its development as a future power plant.  Hence, only these two are discussed here.

HYBRID ELECTRIC VEHICLES (HEV)

 Motivating factors for HEV development are;

• Power required by vehicle to operate within cities may be around 4 to 7 kW although the rated engine power ranges from 25 to over 100kW. The engine thus, operates in the city under very low load conditions giving high fuel consumption and emissions.
• Small engine can be employed and operated at constant load and speed at the point of its maximum efficiency, and another propulsion system can take care of the transient operation. High vehicle fuel efficiencies are thus, obtained.
• Engine  can be tuned to its lowest emissions at the operating load and speed point
• Emission control and exhaust after-treatment at steady engine load and speed operation is more efficient.
• Hybrid electric vehicle (HEV) allows achieving precisely this objective.

The hybrid electric vehicle employs two different energy storage and two different propulsion systems:

• A conventional propulsion system like IC engine, and
• An on-board rechargeable electric energy storage system coupled with electric motor(s)