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Stoichiometric, or chemically correct, air fuel ratio A stioichiometric mixture of air and fuel is one that contains just sufficient oxygen for the complete combustion of the fuel. A mixture which has an excess of air is termed a weak mixture, and one which has a deficiency of air is termed a rich mixture. The percentage of excess air is given by the following, Percentage excess air (Acutal A/F ratio - Stoichiometric A/F ratio)/(Stoichiometric A/F ratio) (where A = Air and F = Fuel) For gaseious fuels the ratios are expressed by Volume and for Solid and Liquid Fuels the Ratio are expressed by Mass. The equation above gives a positive result when the mixture is weak, and a negative result when the mixture is rich. For boiler plant the mixture is usually greater than 20% weak; for gas turbines it can be as much as 30% weak. Petrol engines have to meet various conditions of load and speed, and operate over a wide range mixture strengths.

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