Aircraft Engine Cooling

Now that we are familiar with the crankshaft, connecting rods, and other internal engine parts, it is pretty obvious there are a bunch of gizmos whirling around inside an engine. All that motion generates a surprising amount of heat due to friction. Combine this with the heat radiating through the engine from fuel burning in the combustion chambers, and it is easy to see an engine needs some way to both shed that heat or reduce the internal friction generating it in the first place.

In a typical liquid-cooled automotive engine, the water-based cooling system is clearly how most excess heat is transferred to the atmosphere. The water is forced through the inside of the engine by a water pump. The now-hot water … more properly coolant, as it is mixed with antifreeze … passes through the radiator, where the heat is passed to the atmosphere by the passing air.

An air-cooled engine has fins on its cylinders and cylinder heads that pass heat directly to the atmosphere.

That is about half of it, actually. In reality, cooling duties in an air-cooled engine are shared between the cooling fins on the cylinders and cylinder heads, plus the oil in the lubrication system.

This is an important point: Air-cooled aircraft piston engines are, in reality, both air- and oil-cooled. As a pilot, you will find oil and cylinder head temperatures are related.

The cooling fins part of an air-cooled engine are dead simple. The fins natively absorb internal engine heat and pass it directly to the passing atmosphere. In typical aviation practice, there is no fan assist; the passing air provides the necessary air motion. This super-simple arrangement is air cooling’s big advantage. It is inexpensive, lightweight, and nearly maintenance- and surely leak-free, so it is stone reliable.

One thing often overlooked regarding air cooling is the hotter the engine gets, the more efficient the transfer of heat from cooling fins to the atmosphere. That is because the difference in temperature between the hot engine fins and the cool atmosphere gets greater and greater as engine temperature rises, speeding heat transfer. A water-cooled engine keeps the coolant somewhere in the low 200° F range, so on a 100° F day, there is maybe only 100° F difference between the cooling medium (water/antifreeze) and the air. In an air-cooled engine, the cylinder heads can be 450° F, so even on a 100° F day, the cooling air is 350° F cooler than the engine, speeding heat transfer.

Likewise, when the engine is idling on the ground, there is little airflow over the engine, but relatively little engine heat is generated, so except for the hottest days with extended ground running, cooling is not negatively affected. But as a pilot, expect to see definite variations in both cylinder head and oil temperature; it is normal for an air-cooled engine. Liquid-cooled engines, such as in your car, run at a more consistent temperature because the water is denser than air and takes longer to absorb … or shed … heat. Thus, the water-cooled engine is more thermally stable. But the air-cooled engine is lighter and simpler.

The other takeaway is an air-cooled engine expands and contracts with heat like a balloon. Liquid-cooled engines are far more thermally stable, so their internal clearances … such as the piston-to-cylinder wall dimension … are run much tighter than in an air-cooled engine. This means the pilot must warm up his engine before blasting it with full power and avoid sudden, power-off descents that shock-cool the engine. And no matter what the pilot does, an air-cooled engine’s loose clearances promote oil consumption (burning) and dirty the oil rapidly.

A quick final thought: All modern Rotax engines use air-cooled cylinders and water-cooled cylinder heads. As the easy majority of heat is found in the cylinder head, a pilot treats such Rotaxes as water-cooled.