Have you ever thought about why airplane engines perform better in the cold winter months? The reason is air density, and its effect on how an engine works.

How Air Density Affects Engines

As you can see in the diagram below, cold air molecules move slower and collide with less energy than hot molecules, causing cold air to become more dense. As temperature drops, more air molecules enter an engine, and as temperature rises, less air molecules enter an engine.

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Understanding Engine Air-Fuel Ratio

Engines are most efficient when they operate near the air-to-fuel ratio that completely burns all air and all fuel in the cylinder. This is called the Stoichiometric ratio. For 100LL fuel, the Stoichiometric ratio is approximately 15:1, or 15 parts air to every 1 part of fuel.

If an engine runs rich of the Stoichiometric ratio, unburned fuel will exit the engine's exhaust. If an engine runs lean of the ratio, unburned air will exit the engine's exhaust. We'll discuss the advantages of running rich and lean in a later post, but for now, we'll stick with the 15:1 ratio.

Why Lower Density Altitude Generates More Power

How does this affect aircraft engines in the winter? To maintain the proper air-fuel ratio at low temperatures, aircraft engines compensate and add more fuel. With more air and more fuel in the cylinder, the engine outputs more power. The opposite is true with hot temperatures, where less dense air, combined with less fuel, results in less power output.

The next time you're rolling down the runway on a cold winter day, you'll know why your engine is roaring with horsepower.