Catalytic converters are designed to work best when the exhaust gases passing through them are the result of nearly perfect combustion.Ī perfectly stoichiometric mixture burns very hot and can damage engine components if the engine is placed under high load at this fuel–air mixture.

This is the time that elapses from the spark plug firing until 90% of the fuel–air mix is combusted, typically some 80 degrees of crankshaft rotation later. For a four-stroke engine would mean 5 milliseconds for each piston stroke, and 20 milliseconds to complete one four stroke, 720 degree cycle (the Otto cycle).

(100 revolutions per second 10 milliseconds per revolution of the crankshaft. Most of the combustion process is completed in approximately 2 milliseconds at an engine speed of 6,000 revolutions per minute. In practice, this is never quite achieved, due primarily to the very short time available in an internal combustion engine for each combustion cycle. In theory, a stoichiometric mixture has just enough air to completely burn the available fuel. For precise air–fuel ratio calculations, the oxygen content of combustion air should be specified because of different air density due to different altitude or intake air temperature, possible dilution by ambient water vapor, or enrichment by oxygen additions. Some engines are designed with features to allow lean-burn. Lean mixtures are more efficient but may cause higher temperatures, which can lead to the formation of nitrogen oxides. Ratios higher than stoichiometric (where the air is in excess) are considered 'lean'. Rich mixtures are less efficient, but may produce more power and burn cooler. Ratios lower than stoichiometric (where the fuel is in excess) are considered 'rich'.

If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture, often abbreviated to stoich. In an internal combustion engine or industrial furnace, the air–fuel ratio is an important measure for anti-pollution and performance-tuning reasons. These are known as the lower and upper explosive limits. Typically a range of fuel to air ratios exists, outside of which ignition will not occur. The air–fuel ratio determines whether a mixture is combustible at all, how much energy is being released, and how much unwanted pollutants are produced in the reaction. The combustion may take place in a controlled manner such as in an internal combustion engine or industrial furnace, or may result in an explosion (e.g., a dust explosion, gas or vapor explosion or in a thermobaric weapon). Air–fuel ratio ( AFR) is the mass ratio of air to a solid, liquid, or gaseous fuel present in a combustion process.

4 cycles of a combustion engine