A common rail heavy duty diesel engine with exhaust gas recirculation(EGR) cooler and variable geometry turbocharger was used to study the influence of EGR cooled-gas temperature on engine performance and exhaust emissions by bench test. The results show that the fuel consumption

NOx and smoke decrease when the EGR cooled-gas temperature declines. With every one degree decrease in EGR cooled-gas temperature

the reductions of fuel consumption

NOx and smoke at different operating conditions are quite different. At medium speed with low load

the fuel consumption reduces most; at high speed with low load

the NOx and smoke decreases most. Taking the energy consumption by EGR cooler into consideration

the actual fuel consumption can be calculated. In the principle of as low actual fuel consumption and low emissions as possible

the optimized EGR cooled-gas temperatures at which the actual fuel consumptions are the lowest can be determined at different operating conditions.