Validation of a Polytropic-Based Blow by Model and Predicting Crevice Flow Cyclic Variations in a CNG SI Engine

Due to the required clearance between the cylinder liner and piston of a reciprocating IC engine, a few crevices appear among compression rings and cylinder-piston. Although the gas leakage into the crankcase can be controlled substantially via the use of two or three compression rings, it is impossible to remove the existing crevices and the ring orices of the cylinder-piston-ring pack. Escape of unburned mixture into the crevices can result in a serious impact on the work done per cycle, fuel conversion eciency, and pollutant emission, which are all important in the ring cycle. In the present work, rst a Polytropic-Based Blowby Model (PBBM) was validated using a Thermodynamic Simulation Code (TSC) equipped with a blowby sub-model. Then, using the experimental results extracted from a single-cylinder SI research engine in skip-re and NG fuel mode, the P 􀀀 of an Ensemble Average Cycle (EAC) was calculated from the P􀀀s of 200 experimental cycles. The crevice ow rates of the EAC were estimated using TSC and PBBM. The results showed a good agreement between them. The study of blowby in 200 experimental cycles via the use of PBBM revealed that the existing cyclic variations of the cylinder pressure caused corresponding cyclic changes in Net Cylinder Mass Lost (NCML) and Net Mass Passed (NMP) through the ring orices and the pressure inside the crevices. A linear correlation was observed between maximum NCML and cylinder peak pressure while an inverse correlation was detected between cylinder peak pressure and NCML at the EVO position.