There are many factors that affect the air-fuel ratio in an engine. The most common and important factor is the air-to-fuel ratio of the fuel being used. This is because the air-fuel ratio controls how much oxygen is available to burn the fuel. If there is too much oxygen, the fuel will not burn completely and will create emissions. If there is too little oxygen, the engine will run lean and may damage itself. Other factors that can affect the air-fuel ratio are intake air temperature, exhaust gas recirculation, and camshaft timing.