Effects of Numerical Models on Prediction of Cylinder Pressure Ringing in a DI Diesel Engine

Pressure ringing phenomena in internal combustion engine are often observed in cylinder pressure measurement, which may be due to combustion dynamics, pressure oscillation inside the combustion chamber and/or inside a drilled probe hole for cylinder pressure sensor installation. In the present study, combustion process in a production DI diesel engine instrumented with pressure sensors in the cylinder head was analyzed using 3D combustion CFD simulation. Three combustion models (the CTC model with the Shell autoignition model, the Sage model with detailed chemistry, and the ECFM-3Z model) and three reaction mechanisms (the Shell autoignition model, the Chalmers reduced n-heptane mechanism, and the IFP PRF mechanism) were employed to validate their capability in capturing pressure ringing phenomena. Grid size within the drilled hole and speed of sound CFL number were varied to evaluate the effects on pressure ringing prediction. Both the Sage and ECFM-3Z models coupled with detailed reaction mechanisms reproduced the pressure ringing due to the NTC related reactions included in these reaction mechanisms, but the Shell/CTC model could not. The PRF mechanism showed relatively more evident NTC behavior and predicted much higher oscillation magnitude in the simulation than the Chalmers mechanism, which resulted in higher relative intensity in spectrum. With the same turbulent combustion model and the CFL number, both the Chalmers and PRF mechanisms produced the same resonance frequency, since observed pressure ringing is mainly due to open-end cavity mode inside the drilled hole and does not depend on the chemistry. Parametric studies were conducted to investigate the effect of the drilled hole length on pressure measurement. Both measured and simulation results showed that the flush mounted pressure sensor does not have the resonance frequency of open-end cavity mode.