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Diesel engine noise is classified into mechanical noise, flow dynamic noise and combustion noise. Among these, combustion noise level is higher than the others due to the high compression ratio of diesel combustion and auto ignition. The injected fuel is mixed with air in the ignition delay process, followed by simultaneous ignition of the premixed mixture. This process results in a rapid pressure rise, which is the main source of combustion noise. The amount of fuel burned during premixed combustion is mainly affected by the ignition delay. The exhaust gas recirculation (EGR) rate has an impact on ignition delay, and thus, it influences the combustion noise characteristics. Therefore, during the transient state, the combustion noise characteristics change as the EGR rate deviates from the target value. In this study, the effect of the EGR rate deviation during the transient state of the combustion noise is examined. A 1.6 liter diesel engine with a VGT was used for the experiment.

More emissions are produced when Diesel engines operate in the transient state than in the steady state. This discrepancy is due to mismatching between the air-charging system and the fueling system. Moreover, the difference in the response time between the intake pressure and the exhaust pressure caused by turbo-lag leads to an excess supply of EGR. In this study, a model that can calculate the EGR rate of the intake gas was developed. In the model, temperatures of the air, the EGR gas and the mixture gas were measured with thermocouples which have a fast response. The EGR rate was calculated through the energy balance equation considering heat transfer. Moreover, the estimated EGR rate was applied to a closed-loop control system that receives feedback from 50 % of the mass fraction burned (MFB50) by a 2.2 L Diesel engine. When there is a difference between the target EGR rate and the estimated EGR rate, the target MFB50 can be modified.