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At its base, a car engine is an air pump. Air goes in, fuel is used to ignite the air, and the waste exits as exhaust gas. The amount of air going into the engine can be changed, but the amount of fuel used to ignite the air is very important. An engine under heavy loads will require more fuel to keep the engine going, while an engine at idle won't need very much fuel at all.

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In order to optimize gas mileage and meet emissions, an Air/Fuel ratio of 14.7 to 1 is stoichiometric. The '14.7' denotes the air going into the engine and the '1' denotes the fuel used to ignite the air.

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A car engine running lean means it has an AFR (air/fuel ratio) higher than 14.7:1 I.e. 16:1. This means it is injecting proportionally less fuel into the engine. In most scenarios, when an engine runs lean it can lead to detonation, which can destroy the insides of a car engine. However, say a car is experiencing deceleration. Less fuel is needed, as the amount of power the engine needs to produce is theoretically non-existent. Thought the engine cannot run of zero fuel, the injectors can ease off and use less fuel to increase gas mileage, but still add enough fuel to keep the engine running.

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Most engines, however, run air/fuel ratios lower than 14.7:1. This would look like 13:1. This does not mean the engine uses less air, it proportionally denotes that the amount of air compared to fuel has decreased. I.e. the injectors squirt more fuel into the engine. Why would an engine need to run rich at certain times? The average car engine under load, or acceleration, will be taking in more air as engine RPM increase and air flow increases, and to accommodate for the minor increase in air, more fuel is added to the mix to produce a more powerful reaction inside the engine. This allows the engine to output more power, which is slightly inefficient, but necessary to accelerate

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Because an engine is an air pump, certain measures can be taken to increase air flow. Supercharging/Turbocharging are very common methods of squeezing more air into the engine. In order to maximize power potential and to prevent detonation, the engine is tuned to squirt more fuel in to account for the extra air. The AFR of an turbocharged engine, to produce the best results, should be around 11:1 or 12:1 under load.

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Edgar, Julian. “Tuning Air/Fuel Ratios.” AutoSpeed Articles RSS, AutoSpeed, 12 Nov. 2002, www.autospeed.com/cms/article.html?&title=Tuning-AirFuel-Ratios&A=1595.

“Air/Fuel Ratio Tuning Rich vs Lean.” ECU Tuning | Turbobygarrett, www.turbobygarrett.com/turbobygarrett/airfuel_ratio_tuning_rich_vs_lean.