In precombustion chamber Diesels engines, the fuel is injected into the pre-chamber and ignites there. The fuel that comes out of the nozzel into the combustion cylinder on the top of the piston is already burning. It is a rich mixture and completes its burning with the air in the cylinder. There is a small recess in the piston to clear the nozzle because the nozzle projects below the surface of the cylinder head.

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