Experimental investigation of the effect of variable valve lift on combustion stability and exhaust emissions in a diesel/methane CRDI engine

Diesel/methane combustion still encounters challenges in terms of lower combustion stability and elevated levels of HC and CO emissions. The valve lift (VL) technique is a method that contributes to increased in-cylinder volumetric efficiency, thereby improving engine performance and reducing emissions. Hence, the application of VL in diesel/methane operations would be highly significant. Therefore, an experimental study on the VL for reducing HC and CO emissions and enhancing combustion stability in a diesel/methane CRDI engine was performed at varying methane fractions and torques. In the study, three different cam profiles were used with VL extrapolated using Fourier series and designed: the original VL of 4.46 mm, as well as two different VLs of 4.0 mm and 4.9 mm. The methane in the total fuel energy was initially increased to 20 % and later further increased to 40 %. The experimental findings demonstrated that the augmented VL mitigated the prolonged ignition delay caused by the increase of methane fraction, thereby ensuring enhanced combustion stability across all torques. On the other hand, in diesel/methane operation, increasing the VL resulted in maximum reductions of approximately 17 % in HC, 6 % in CO emissions, and 75 % in smoke opacity.