Compressed Natural Gas Direct Injection (Spark Plug Fuel Injector)

The increasing concerns over energy security and the emission of pollutant gases have triggered greater efforts towards developing alternatives to conventional fuels for road vehicles. In the presence of these concerns, automotive engine technology is challenged by the increasing divergence between higher power output, better fuel economy and lower pollutant emission requirements (Stan, 2002). Several alternatives to gasoline and diesel fuels have been studied on current internal combustion (IC) engines. These include natural gas (NG), which is predominantly methane, liquefied petroleum gas (LPG), hydrogen, as well as ethanol and methanol. They are used either as supplement or replacement to gasoline in spark ignition (SI) engines. For compression ignition (CI) engines, dual fuel operation with diesel fuel providing pilot ignition source has been successful for heavy-duty applications. CI engines have also benefited from the use of various alternative fuels of vegetable origins as diesel replacement. LPG is a promising alternative fuel mainly due to its relatively high energy density, high octane rating and low pollutant emissions. It can be stored as liquid at moderate pressure, which gives it major advantage over most other alternative fuels. Methanol on the other hand has a very high octane rating but low heating value and stoichiometric air fuel ratio (AFR). Thus it leads to higher volumetric fuel consumption when compared to gasoline. Hydrogen fuel for electrically driven fuel cell cars, seen as the future replacement to IC engine technology, is undergoing relatively slower research and development and is expected to be in large scale production at some distance of time. IC engines is therefore will remain the key power source in the 21st century until fuel cell vehicles become widespread (Morita, 2003) Natural gas use has various advantages over conventional fuels mainly due to its potential for higher thermal efficiency (due to higher octane value that allows the use of higher compression ratios), and lower CO2 emission (due to lower carbon-to-hydrogen ratio) (Shiga et. al. 2002). From the supply point of view, natural gas has the advantage of energy diversification and the total reserves have been estimated in the same order as petroleum but with only 60% of its production rate (Vuorenkoski, 2004). According to the statistics by the International Association for Natural Gas Vehicles (IANGV, 2009), there are approximately 11.2 million NGVs in operation worldwide with 13