COMBUSTION ANALYSIS OF A CNG DIRECT INJECTION SPARK IGNITION ENGINE

An experimental study was carried out on a dedicated compressed natural gas direct injection (CNG-DI) engine with a compression ratio (CR) of 14 and a central injection system. Several injection timing parameters from early injection timing (300 degrees BTDC) to partial direct injection (180 degrees BTDC) to full direct injection (120 degrees BTDC) were investigated. The 300 degrees BTDC injection timing experiment was carried out to simulate the performance of a port injection engine and the result is used as a benchmark for engine performance. The full DI resulted in a 20% higher performance than the early injection timing for low engine speeds up to 2750 rpm. 180 degrees BTDC injection timing shows the highest performance over an extensive range of engine speed because it has a similar volumetric efficiency to full DI. However, the earlier injection timing allowed for a better air-fuel mixing and gives superior performance for engine speeds above 4500 rpm. The engine performance could be explained by analysis of the heat release rate that shows that at low and intermediate engine speeds of 2000 and 3000, the full DI and partial DI resulted in the fastest heat release rate whereas at a high engine speed of 5000 rpm, the simulated port injection operation resulted in the fastest heat release rate.