Effect of injection timing on combustion and IMEP variation of a bi‐fuel compressed natural gas SI engine

Abstract

AbstractCompressed natural gas (CNG) is a gaseous fuel stored under high pressure in a cylindrical tank. When the CNG is injected into the intake manifold, it expands in the manifold and displaces the air which reduces the volumetric efficiency of the engine. Thus, the corresponding overall energy inside the combustion chamber is also reduced, resulting in reduced power output. CNG injection timing with respect to intake valve opening and closing can be optimized to minimize the losses in volumetric efficiency loss and hence, the power output of the engine. In this study, the combustion instability of the CNG fueled engine has been studied under low throttle condition and compared with gasoline. A comparative investigation of the effects of the start of injection (SOI) timing on combustion stability was conducted on a single‐cylinder spark‐ignition (SI) engine fuelled with gasoline and CNG at 25% throttle opening condition. The ignition timing was kept fixed at 35°, and 40° CA bTDC for gasoline and CNG, respectively, and the SOI timing was varied from 240° CA bTDC to 100° CA aTDC of the intake stroke. The results indicated that SOI timing significantly affects in‐cylinder air‐fuel mixture formation and combustion stability for gasoline and CNG engine. SOI timing corresponds to the closed valve period, shows the higher fluctuation in combustion for gasoline compared to CNG. CNG shows lower fluctuation throughout SOI timing due to homogeneous air‐fuel mixture formation. Early SOI timing forms fuel stratification in the intake port and forms a rich air‐fuel mixture, which reduces the net power output of the CNG engine. CNG requires optimum SOI timing for improved combustion compared to gasoline. It reduced the combustion instability and fluctuation in indicated mean effective pressure. The results will benefit the development of a closed‐loop engine control system for a CNG system.