Effect of Boost Pressure on Engine Performance and Exhaust Emissions in Direct-Injection Compressed Natural Gas (CNG-DI) Spark Ignition Engine

摘要

Natural gas vehicle gives 11-22% lower power output compared to equivalent liquid-fuelled vehicles. With direct injection of gaseous fuel there has been a problem of reduction in volumetric efficiency when injecting the fuel at early and partial direct injection. This is due to some of the air that will be displaced by the fuel as these injection timings have inlet valve open during injection. Recently, there is an increasing interest in supercharging S.I. engines operating on natural gas mainly to its superior knock resisting properties in comparison to traditional liquid fuels. The present work reports the effect of boost pressure on engine performance and exhaust emissions. The experimental study was conducted on a 4-stroke direct injection compressed natural gas spark ignition engine with compression ratio of 14. Simulated port injection (early injection), 300° BTDC, and partial direct injection, 180° BTDC, were varied at each operating speed with variation of supercharging pressure. For boost pressure above 7.5 kPa it is proven that there is better performance at all operating engine speeds compared to without supercharging system. In-cylinder pressure and temperature would be high with increasing boost pressure that outcome high NO_x emission. Boost pressure has no significant influence on HC emission while it makes CO emission level lower due to improvement of brake specific fuel consumption. With supercharging system, partial direct injection gives better performance for speeds 2000 to 4000 RPM compared to early injection timings. Whereas early injection timing with boost pressure gives better performance for engine speeds 4000 to 5000 RPM compared to partial direct injection due to enough time available for mixture preparation.