EGR Response in a Turbo-charged and After-cooled DI Diesel Engine and Its Effects on Smoke Opacity

Three thermo-wires with amplifying circuits have been developed to measure the time-resolved concentration of the exhaust gas recirculated into the intake manifold by a rotary valve-based exhaust gas recirculation (EGR) system of a diesel engine. Good agreement was found between the EGR rates measured by the temperature based system and a conventional CO2 tracing system. The developed EGR measuring system was used to investigate the EGR transient response in a turbo-charged and after-cooled diesel engine with a real-time measure and control system. The EGR response under EGR valve step change and engine transient operating conditions are discussed.

At first, the engine was running under a certain steady condition with zero recirculated exhaust gas, then the rotary valve opened to maximum within 0.1s to demonstrate the EGR step change behavior. EGR rate and air intake stabilized in 0.5s. The combustion process characterized by the maximal in-cylinder pressure came to stabilization in 1.9s. Because of the in-cylinder thermal lag and combustion fluctuations the transient smoke opacities were much different from those at constant EGR rate operation and increased gradually to stabilization in approximately 5s. So, when EGR system is adopted to realize HCCI combustion this performance lag must to be taken into consideration.

The engine typical transient operations named constant speed and increasing torque (CSIT) operations, were realized by evenly boosting the accelerator so as to make the engine increase torque from 100N·m to 500N·m at constant 1600 rpm. Test results showed that the sudden decrease in air intake (resulted by the turbo-lag) brought a dramatic increase of the pressure drop across the EGR valve and resulted in greater EGR rate which deteriorated the smoke opacity under transient operations. The peak smoke opacity under 16CSIT5 operation (constant 1600 rpm from initial 100N·m to final 500N·m within 5s) with the EGR system controlled by the steady-operation strategies was 3 times that of an engine without EGR. In order to obtain better transient performance of the diesel engine equipped with the EGR, its control strategy must be optimized.