In an internal combustion engine, this refers to (here's a surprise) the mixture of fuel (Gasoline or Diesel Fuel, usually) and air that enters the combustion chamber and is detonated by the spark from the spark plug. (Or, as The_Custodian points out, in the case of a Diesel engine, the pressure itself generates the heat necessary to ignite the fuel.)

You want to have as much air and fuel as you can manage (if you're going for performance), but no extra of either. Unburned fuel is wasteful and can adversely affect the interior surfaces of the combustion chamber over time, and too much air ain't much better.

So you want to oxidize as much fuel as possible. This is what the carburetor (or in some engines, fuel injector) is for. They achieve the same end (the vaporization of fuel) in different ways, but do the same job - to effectively mix fuel and air prior to detonation.

So that's what is generally meant by the term "mixture" in this context.

Fuel-air mixture is one of the most important principles of internal combustion engine operation. For gasoline, the stoichiometric fuel-air ratio is 14.7:1. That is, 1 unit of fuel mass is consumed for every 14.7 units of air mass that are drawn into the engine. The stoichiometric is neither most fuel efficient nor delivers the most power; it is a compromise. The Stoichiometric ratio usually is the least polluting, because the catalytic converter can most easily remove pollutants at such a ratio. This mode is used during cruising and light acceleration.

For optimum power, a 12.7:1 (slightly fuel-rich) fuel-air ratio should be used. The latent heat of vaporization of the extra fuel cools down the combustion chamber, making the air more dense. Since this dense air is heavier than normal air, more fuel can be drawn in, increasing power. This cooling effect also protects engine parts from melting (if you are running very high temperatures), and lowers the octane requirement. However, it is not very fuel efficient, it can foul spark plugs, and is polluting (the catalytic converter is outside its optimum range, and VERY polluting unburned hydrocarbons are released). Modern cars usually only use this mode (called fuel enrichment mode) under hard acceleration.

For maximum fuel economy, a 16:1 (fuel-lean) fuel-air ratio should be used. However, the lack of extra fuel to cool the engine results in hot,less dense intake air, reducing power. Also, the extra heat puts stress on engine parts, and increases octane requirement. Although less polluting than fuel-rich, fuel-lean produces large amount of nitrogen oxides, as well as putting the catalytic converter out of its range. Lean mixtures are generally not used on modern, pollution-controlled vehicles.

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