The normal operating range of air/fuel ratio for a CI engine with diesel fuel is

Concept:

• In SI engine charge consists of air and fuel, and ignition takes place with the help of a spark.
• When the mixture is lean the fuel will not burn by a spark.
• However, in the case of the CI engine, the fuel is supplied in fine particles at the end of the compression stroke, compressed air having high pressure and temperature and on coming in contact with this air it burns.
• Hence there is no problem of the combustion of fuel even with the lean mixtures.

• A mixture that contains just enough air for complete combustion of all the fuel in the mixture is called a chemically correct or stoichiometric fuel-air ratio.
• A mixture having more fuel than that in a chemically correct mixture is termed as a rich mixture and a mixture that contains less fuel (or excess air) is called a lean mixture.
• For most of the hydrocarbon fuels, the stoichiometric air-fuel ratio is 15 : 1.
• SI engine operates around this ratio during normal operation.
• The air-fuel ratio for CI engines varies from 18 : 1 to 80 : 1 from full load to no load.