The fuel efficiency and exhaust gas emissions of a low heat rejection free-piston diesel engine

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Abstract
This article investigates the in-cylinder heat transfer losses of a free-piston diesel engine and compares the results with those of a conventional engine, using a multidimensional simulation model. In-cylinder heat transfer, fuel efficiency, and nitrogen oxides (NO) emissions formation are studied, along with the suitability of the free-piston engine to use a low heat rejection combustion chamber design. It is found that the high piston acceleration around top dead centre and the fast power stroke expansion in the free-piston engine lead to reduced heat transfer losses and reduced NO emissions formation, the latter in the order of 17 per cent compared with conventional engines. Even for highly insulated combustion chambers, the free-piston engine is predicted to have lower NO emissions than the conventional engine in the original configuration. The use of a low heat rejection combustion chamber is found to benefit engine fuel efficiency in both free-piston and conventional engines, with a 30 per cent heat loss reduction improving the indicated efficiency by approximately 3 percentage points. With a simpler implementation of combustion chamber insulation due to the low side forces on the piston, it is argued that the free-piston engine should be well suited for such an operation.

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