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Direct injection vs indirect injection

In an indirect injection diesel engine fuel is injected into a small prechamber which is connected to the cylinder by a narrow opening. The initial combustion takes place in this prechamber. This has the effect of slowing the rate of combustion, which tends to reduce audible noise.

In a direct injection diesel engine, fuel is injected directly into the cylinder. The piston incorporates a depression (often toroidal) which is where initial combustion takes place. Direct injection diesel engines are generally more efficient than indirect injection engines, but have a tendency to produce greater amounts of audible noise.

In either case the location where combustion takes place, called a swirl chamber, is carefully designed to ensure adequate mixing of the atomized fuel with the compression-heated air.