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Diesel Engine Basics
A diesel engine is an internal combustion engine that uses compression ignition to ignite the fuel as it is injected into the engine.

DIESEL ENGINES VS. GASOLINE ENGINES

It is helpful to an understanding of how diesel engines work to compare the differences between a diesel engine and a gasoline engine. The main differences between a gasoline engine and a diesel engine are:

A gasoline engine takes a mixture of gas and air, compresses it, and ignites the mixture with a spark. A diesel engine takes air, compresses it, and then injects fuel into the compressed air. The heat of the compressed air ignites the fuel spontaneously. A diesel engine does not contain a spark plug.

A gasoline engine compresses at a ratio of 8:1 to 12:1, while a diesel engine compresses at a ratio of 14:1 to as high as 25:1. The higher compression ratio of the diesel engine leads to better efficiency.

Gasoline engines generally use either carburetion, in which the air and fuel are mixed long before the air enters the cylinder, or port fuel injection, in which the fuel is injected just prior to the intake stroke (outside the cylinder). In a gasoline engine, therefore, all of the fuel is loaded into the cylinder during the intake stroke and then compressed. The compression of the fuel/air mixture limits the compression ratio of the engine – if it compresses the air too much, the fuel/air mixture spontaneously ignites and causes knocking. Diesel engines use direct fuel injection i.e. diesel fuel is injected directly into the cylinder. A diesel engine compresses only air, so the compression ratio can be much higher. The higher the compression ratio, the more power generated.

Diesel fuel injectors, unlike gasoline injectors, must be able to withstand the temperature and pressure inside the cylinder and still deliver the fuel in a fine mist. To ensure that the mist is evenly distributed throughout the cylinder, some diesel engines are equipped with special induction valves or pre-combustion chambers. Newer diesel engines are equipped with high-pressure common rail fuel systems. See Diesel Fuel System Basics for more information on this type of fuel system.

Diesel engines may be equipped with a glow plug. When a diesel engine is cold, the compression process may not raise the air temperature high enough to ignite the fuel. The glow plug is an electrically heated wire that facilitates fuel ignition when the engine is cold. Glow plugs are typically found on small diesel engines. Gasoline engines do not require glow plugs as they do not rely on spontaneous combustion.
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