Effect of Exhaust Gas Recirculation on Combustion and Odorous Emissions in Direct Injection Diesel Engines

This study investigated the effect of exhaust gas recirculation (EGR) on combustion and odorous emissions in a direct injection (DI) diesel engine. EGR up to 60% was examined in the engine start-up conditions as well as once the engine was stabilized. In emissions, exhaust odor, irritation, formaldehyde (HCHO) and total hydrocarbon (THC) are compared without or with different EGR rates. After engine warm-up, reduced exhaust odor and eye irritation are obtained at 30% and 60% EGR rates than no EGR in the outdoor assessment environment, although HCHO emission at 60% EGR rate is 50% higher than at 30% EGR rate or non EGR. Early in the engine start-up time, 60% EGR rate shows very high emissions of HCHO and THC. Combustion analysis is performed by taking cylinder pressures during combustion and analyzing heat release rates. Cylinder pressures at 1, 2, 4, 8, 16 and 32 minutes after engine start for different EGR rates are taken. There is no change in ignition delays between non EGR and 30% EGR rate. The ignition delay with 60% EGR rate early in the engine start-up time is 1-1.5°CA longer than non EGR or 30% EGR rate and main combustion takes place after top dead center (TDC). 60% EGR rate also showed lower combustion temperature. Longer ignition delay with lower combustion temperature and occurrence of main combustion after TDC are thought to be responsible for higher HCHO and THC emissions at 60% EGR rate.