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Differences in efficiency between Diesel and Gasoline engines

Why is a diesel engine more efficient than a gasoline engine?

The compression ratio of an engine refers to the extent to which gasses can be compressed in the engine cylinder. A high engine compression ratio is desirable as it creates greater heat from compression and also mixes and evaporates air and fuel molecules better by forcing them into a tighter space. An engine with a high compression ratio allows for more power when fuel combustion occurs and can generate more force from that power.

Compression of air

In a diesel engine only air is compressed after which fuel is injected. This allows diesel engines to reach very high compression ratios of between 14:1 and 23:1.

In a gasoline engine, both air and fuel are sucked into the engine at the same time and this limits compression in the engine cylinder. Gasoline also has a lower combustion temperature. Compression ratios in cars usually range between 7:1 to 10:1. High compression ratios can make gasoline engines knock or detonate if lower octane rated fuel is used. This can reduce efficiency or cause engine damage.

Engine speed control

Efficiency of an engine is also determined by how engine speed is controlled. Speed is directly related to how much fuel is burned.

In gasoline engines, the amount of air entering the engine is controlled by the butterfly valve in the carburetor. In the carburetor, the amount of fuel that can be mixed with air is determined by the rate of air flow. Reducing air entering the engine also reduces the amount of fuel entering the engine. Increasing fuel intake does not help as engine speed does not increase beyond the point where the fuel burns all of the available oxygen.

In diesel engines on the other hand there is no restriction on the amount of air entering the engine. Therefore there is always enough oxygen available to be burned. Engine speed therefore is restricted only by the amount of fuel injected into the engine cylinder. As more fuel is added, the engine will try to attain a greater speed to keep up with the higher fuel injection rate. In fact diesel engines need a governor that controls the amount of fuel injected in order to limit speed.

Spark Plugs

Gasoline engines need spark plugs to ignite the petroleum which is not required in diesel engines as the high compression ratio allows air to be heated to very high temperatures.

Fuel composition

The composition of diesel and gasoline fuel also make a difference in the efficiency of these engines. Diesel is heavier than gasoline. Since the chains of hydrogen and carbon atoms that make up these fuels are longer and heavier in diesel, it has 17% more of these atoms in comparison to gasoline thereby allowing diesel engines to be more fuel efficient.


It should be noted that diesel engines are heavier and take longer to accelerate than gasoline engines. A diesel engine takes longer to be warmed up and can be difficult to start during winter. As a solution, many diesel engines come with glow plugs that help to heat up the engine in conjunction with an engine heating system.

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