AIR-FUEL MIXTURE AND STOICHIOMETRIC RATIO
CHEMICAL COMBUSTION OF FUEL
Subhankar Karmakar
Assistant Professor; SGIT
Jindal Nagar; Ghaziabad
Chemical Combustion is basically a rapid oxidation process of hydro-carbon fuel inside thekjm combustion chamber in the presence of air. The oxidation of fuel is basically a Exothermic or heat liberating chemical process.
In SI engines generally we use volatile hydrocarbon as fuel. The intermixing of fuel with air takes place outside the engine and the device that prepares air-fuel mixture of required mixture strength is called CARBURETION and the device is known as CARBURETTOR.
Estimation of air quantity needed for complete combustion of a given fuel
We know that any chemical reaction can be represented by the corresponding chemical equation like
CH4 + 2O2 = CO2 + 2H2O
here molecular weight of CH4
μCH4 = 12 + 4x1 = 16
μ2O2 = 2x16x2 = 64
μCO2 = 12 + 2x16 = 44
μ2H2O
= 2x(2+16) = 36
For complete combustion of CH4
16 kg CH4 needs 64 kg O2 therefore,
1 kg of CH4 needs (64/16) = 4 kg of O2
For 23 kg of O2 air needed is 100 kg
hence, for 4 kg of O2 air needed is (100/23)x4 = 17.39 kg of air.
Air-fuel ratio will be 17.39 : 1
The ratio of air fuel mixture, needed for the complete combustion of the fuel or the chemically correct ratio of air fuel mixture required for complete combustion of the fuel is called " Stoichiometric Air fuel mixture. "
If the amount of air in the air-fuel mixture is less than the chemically correct amount of air, then the mixture is called rich mixture, where as if the quantity of air is more than the chemically corrected amount of air it is called lean mixture.
The strength of air-fuel mixture has a profound influences on the process of combustion. The required mixture strength for different operation conditions are different.
| Different Operating Conditions |
Required air-fuel Mixture Strength |
| For Max. Efficiency | 17 : 1, 16.4% weak |
| For Max. Power | 12 : 1 17.8% rich |
| For Starting, Idling, & Low load running | 11 : 1 ~ 16 : 1 very rich mixture |
| For accelerated motion | 13 : 1 rich mixture |
| For Part Load running Cruising Range | 17 : 1 Lean Mixture Strength |
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