Abstract
Alternative transportation fuels have been receiving increasing attention as likely solutions to the problems of urban air quality, global warming and strategically important excessive dependence on imported oil. Natural gas as a motor fuel, has advantages such as a very good ability for easy mixture formation, both outside and inside the engine cylinder, a wide range of air-fuel ratios (leading to lean or extremely lean mixtures), high heating value and antiknock resistance. These properties enable this SI engine to work with relative high compression ratios and therefore a higher total efficiency.
Exhaust emissions are directly related to the fuel consumption by vehicles, which in turn, are related to the number of vehicles. The fuel consumption by transportation sector in Delhi city was estimated to be about 250,000 tonnes of petrol and about 50,000 tonnes of high speed diesel in the year 1989 by the Central Pollution Control Board and the trend is rising exponentially with the increase in number of vehicles and the present date is witnessing a steep rise in the number of usage of alternately fueled vehicles to curtail emissions.
The performances of alternate fueled engines are low compared to the base engines with a major thrust given to retrofit systems. Dedicated engines desire major design changes to complete the conventional fuels and in the attempt to develop a dedicated compressed natural gas engine technology, the initial attempt was to experimentally evaluate the significance of the piston crown geometry on an existing OEM engine to optimize the crown geometry. Three piston geometries were identified and experimented on a 100 cc single cylinder 4-Stroke S.I engine. The performance curves were compared at different load conditions under both gasoline and compressed natural gas ( CNG ) mode and the optimal piston was evaluated.
The results show a remarkable change in power and performance aspects due to the change in the piston crown geometries. The engine when run in CNG mode was able to deliver adequate power to compete the conventional gasoline engines. The engine under the gasoline mode ran close to stoichiometric conditions whereas in CNG mode the engine ran extremely lean.