Abstract Exhaust gas process in an internal combustion engine for the purpose of removing combustion products from the cylinder has always been considered. At the same time for reducing NOx as poisonous pollutant especially in diesel engines, use of EGR is widely employed as an effective way. In this investigation the influence of application of EGR in an IDI diesel engine on residual gas mass fraction as an indicator of exhaust performance which affects the engine combustion through its influence on charge mass temperature and dilution was evaluated via an analytical procedure based on experimental data. It was observed that use of EGR contributes decreasing backflow mass and increasing trapped mass which both the former and latter are consisting parts of overall cylinder residual mass. The out coming result was that the cylinder residual gas mass fraction totally reduces as the EGR ratio goes further.

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