Air-fuel ratio - Kuunenpi

In an internal combustion engine, the piston receives the expansion force caused by the combustion of a mixture of air and fuel (mixture gas), which is then converted into rotational force by the crank. The ratio of the mixture of air and fuel is called the air-fuel ratio. It is usually expressed as the weight ratio of the two. In a gasoline engine, the most flammable ratio is about 1 part gasoline to 14 parts air, which is called the theoretical air-fuel ratio. If the mixture is leaner than 1:16, the flame will go out and misfire will occur, the power will drop rapidly, and the amount of hydrocarbons and carbon monoxide in the exhaust gas will increase rapidly. If the mixture is too rich, the amount of hydrocarbons in the exhaust will increase. Therefore, the carburetor and fuel injection device should be precisely controlled to keep the mixture at the theoretical air-fuel ratio, but unfortunately, the generation of nitrogen oxides is at its highest near the theoretical air-fuel ratio. For this reason, it is impossible to simultaneously reduce the three harmful components in the exhaust (carbon monoxide, hydrocarbons, and nitrogen monoxide) by controlling the air-fuel ratio alone, and various methods are used to reduce them, such as stratified intake, slow combustion, secondary air injection, exhaust gas recirculation, and catalysts.

[Takashima Shizuo]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

内燃機関は、空気と燃料の混合気（ミクスチャーガスmixture gas）の燃焼による膨張力をピストンで受け、クランクで回転力に変えて取り出すものであるが、この空気と燃料との混合の比率を空燃比という。通常は双方の重量の比で表す。ガソリンエンジンではガソリン1に対して空気14前後がもっとも燃焼しやすく、これを理論空燃比という。1対16より混合気が薄いと火炎が消えて失火し、急速に出力が低下するとともに、排出ガス中の炭化水素、一酸化炭素が急増する。また混合気が濃すぎても排気中の炭化水素は増える。したがって混合気を理論空燃比に保つよう気化器や燃料噴射装置を精密にコントロールすべきであるが、不都合なことに窒素酸化物の生成は理論空燃比付近で最大となる。このため排気中の三つの有害成分（一酸化炭素、炭化水素、一酸化窒素）を空燃比のコントロールだけで同時に減少させることは不可能で、層状吸気、遅速燃焼、二次空気噴射、排気再循環、触媒などさまざまな減少方法がとられている。

［高島鎮雄］

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