Energy Efficiency of Vehicles  Printable view
Fig. 1   In Hong Kong, buses, light buses and trucks usually use diesel engines.

Most vehicles on the road are powered by internal combustion engines. The most common internal combustion engines for vehicles are fuelled with petrol or diesel. In Hong Kong, private cars are mostly powered by petrol internal combustion engines, while commercial vehicles e.g. buses and trucks are driven by diesel internal combustion engines. Although both engines have similar structures, they have significant differences in terms of operation, energy efficiency, and the amounts and types of emission they produce.

Petrol and diesel engines

Fig. 2   A petrol engine under the hood of a private car

There are two main differences between the design of a petrol engine and a diesel engine. First, petrol engines use spark ignition while diesel engines use compression ignition. Inside an engine, fuel is burned to release and convert the chemical energy to kinetic energy. In a petrol engine (Fig. 3), a spark plug produces sparks to ignite the mixture of petrol and air. In a diesel engine (Fig. 4), air is compressed to a high temperature so that when a fine mist of diesel is pumped into the compressed air, the intense heat causes the diesel to ignite. This is why diesel engine is called a compression ignition engine.

Fig. 3   The structure of a petrol engine Fig. 4   The structure of a diesel engine
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The second difference is the range of compression ratios of the engines (see the animations below to understand the meaning of "compression ratio"). Diesel engines are designed to operate at higher compression ratios, usually between 15 to 20, while petrol engines have lower compression ratios usually between 8 to 10.Since the higher the compression ratio of an engine the higher the thermal efficiency, so diesel engines generally have a better fuel efficiency than petrol engines. To power a similar sized vehicle, a diesel engine will be about 15% - 25 % better on fuel efficiency than a petrol engine [1].

But diesel engines also have disadvantages. Although diesel engines are more energy efficient, they produce significantly more particulates in the exhaust gas than petrol engines. Diesel engines are also slightly noisier and less refined. To withstand the high compression ratio, diesel engines are more expensive to produce. These factors make diesel engines more suitable for commercial vehicles, because these vehicles usually cover longer distances each day, making diesel engines' fuel efficiency advantage important.

Take a look at the following two animations on a petrol engine and a diesel engine to understand more about their operations.

Flash animation: Inside a petrol engine

Flash animation: Inside a diesel engine