Effect of Boost Pressure on Performance, Combustion and Exhaust Emissions in Direct Injection Compressed Natural Gas(CNG) Engine

There is an increasing interest in supercharging S.I. engines operating on natural gas mainly due to its superior knock resisting properties in comparison to traditional liquid fuels. With direct injection of gaseous fuel there is still some penalty in volumetric efficiency when injecting the fuel at early and partial direct injection. The present work reports the effect of low boost pressures on engine performance, combustion and exhaust emissions of CNG-DI engine. The experimental test was carried out in 4-stroke spark ignition CNG-DI engine with compression ratio of 14. The investigation was done at full load and homogenous piston for stoichiometric air-fuel ratio in order to assess the maximum output of the engine. The test includes variation of engine speed and boost pressure. Injection parameters are found to have significant influence on engine performance. Simulated port injection (Early injection) when inlet valves are still open (300BTDC) and partial direct injection in which part of the injection occurs after the inlet valves are closed (180BTDC) were varied at each operating speeds with variation of boost pressure from 2.5 to 10 kPa. Injection pressure was kept constant at 18 bars for all test points. Narrow angle injector (NAI) was used to get faster mixing rate at operating engine speeds of 2000 rpm to 5000 rpm. Ignition timing was set to get maximum brake toque (MBT). In order to examine effects of boost pressure experimental work was done for naturally aspirated engine and the results were compared to supercharging system.
For boost pressure above 7.5 kPa, it is observed that there is better performance at all operating engine speeds compared to naturally aspirated engine. With increasing supercharging pressure there is better heat release and mass fraction burned resulting in better combustion efficiency. In-cylinder pressure and temperature is high with increasing boost pressure giving high NOx emission. Supercharging system is found to reduce the penalty of volumetric efficiency at port and partial direct injections. With supercharging, partial direct injection gives better performance for speeds 2000 to 4000 rpm compared to early injection timings, whereas early injection timing gives better performance for engine speeds 4000 to 5000 rpm. At high engine speed, early injection timing gives enough time for mixture preparation with penalty for volumetric efficiency loss reduced by supercharging.