Plenty of myths surround the workings of internal-combustion engines. One is that an engine running lean—meaning too much air is going into the cylinders—tend to run hot. Jason Fenske of Engineering Explained busts that myth in this video.
The ideal ratio of gasoline to air for combustion is 14.7:1, meaning 14.7 parts air to one part gas by mass. A lean mixture contains more air than that, more than can actually be used in combustion. The opposite of a lean ratio is a rich ratio, which has less than 14.7 parts air and thus too much fuel.
Temperatures actually tend to peak with that ideal ratio, decreasing both when an engine is running lean and when it's running rich, Fenske said.
High temperatures also correspond to high levels of nitrogen-oxide (NOx) emissions, one of the main pollutants created by internal-combustion engines, Fenkse noted. The chemical reaction that creates NOx occurs at high temperatures, he said.
Why does running lean or rich cause temperatures to drop? Because of the leftovers. Excess fuel in a rich mixture transitions from liquid to gas, cooling the cylinder as it does. Excess air from a lean mixture won't be burned in combustion, so its presence lowers cylinder temperature. This is the basic principle behind the exhaust-gas recirculation systems some automakers use to reduce NOx emissions, Fenkse noted.
So where did the myth that lean engines run hot come from? Fenske believes people may be confusing engines that run "less rich" with those that run genuinely lean.
When tuning for power, engines are typically set up to run rich, Fenkse said. Again, "rich" is anything with less than a 14.7:1 air:fuel ratio. As the mixture gets closer to that ideal ratio, temperatures tend to increase, along with the chance of knock. The higher risk of knock closer to the ideal ratio, compared to richer mixtures that are further away from that ratio, may be the source of the confusion, Fenske said. Generally, the closer the mixture is to 14.7:1, the hotter the combustion temperatures.