In the petrol engine cylinder, the air fuel mixture is not at rest, but is in highly turbulent condition. The turbulence breaks the filament of a flame into a ragged front, thus presenting a far grater area of surface from which hear is being radiated; hence its advance is speeded up enormously. The combustion process may be developed in two stages. One the growth and development of a self-propagating nucleus flame and the other the spread of the flame throughout the combustion chamber. The former is a chemical process depending upon the nature of the fuel, temperature and pressure, the proportion of the exhaust gas and also upon the temperature coefficient of the fuel. During the combustion, there is a rise of temperature and pressure due to the combustion of the fuel ignited. Both temperature and pressure combine to accelerate the velocity of the flame front in compressing the unburnt portion of the charge in the knocking zone. Ultimately the temperature in this zone reaches such a high value feat chemical reaction proceed, at for grater rather than that at which the flame is advancing. Hence we have combustion accompanied by flame, producing a very high rate of pressure rise resulted in detonation. There is much interest in employing gaseous fuels to power spark ignition engines whether for stationary or mobile automotive applications because of the many positive economic, environmental and technical features associated with their usage. However, the incidence of knock remains a significant barrier to achieving their optimum performance potential. Experimental results are presented of the knocking behavior of a number of common gaseous fuels that include methane, hydrogen, propane and carbon monoxide and their mixtures. Comparison with the corresponding performance with liquid fuels is also made. Guidelines for achieving extended knock free operation with these fuels are to be outlined.
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