Low-speed diesel engines (as used in ships and other applications where overall engine weight is relatively unimportant) can reach effective efficiencies of up to 55%. A small efficiency loss is also avoided compared with non-direct-injection gasoline engines since unburned fuel is not present during valve overlap and therefore no fuel goes directly from the intake/injection to the exhaust. The diesel engine has the highest thermal efficiency ( engine efficiency) of any practical internal or external combustion engine due to its very high expansion ratio and inherent lean burn which enables heat dissipation by the excess air. The torque a diesel engine produces is controlled by manipulating the air-fuel ratio (λ) instead of throttling the intake air, the diesel engine relies on altering the amount of fuel that is injected, and the air-fuel ratio is usually high. With the fuel being injected into the air just before combustion, the dispersion of the fuel is uneven this is called a heterogeneous air-fuel mixture. This increases the air temperature inside the cylinder so that atomised diesel fuel injected into the combustion chamber ignites. Air is inducted into the chamber during the intake stroke, and compressed during the compression stroke. Diesel engines work by compressing only air, or air plus residual combustion gases from the exhaust (known as exhaust gas recirculation, "EGR").
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