6.2.2 Engine Cooling
The burning fuel within the cylinders produces intense heat, most of which is expelled through the exhaust system. Much of the remaining heat, however, must be removed, or at least dissipated, to prevent the engine from overheating. Otherwise, the extremely high engine temperatures can lead to loss of power, excessive oil consumption, detonation, and serious engine damage.
Vital to the internal cooling of the engine is the oil system. This will be discussed in the following chapter (Engine Lubrication). Most small aircraft are air cooled, and in this section we will explore the various methods air is used for cooling the engine’s external surface.
Openings in front of the engine cowling allow for air to flow freely through the engine compartment. Baffles direct this air over fins attached to the engine cylinders, and other parts of the engine, where the air absorbs the engine heat. Expulsion of the hot air takes place through one or more openings in the lower, aft portion of the engine cowling. This circulation, or movement of excessive heat constantly dissipated by the hot running engine is how air cooling works. Simple, yet effective. Operating the engine in an excessive temperature range can cause loss of power, excessive oil consumption, and detonation. It could also lead to serious permanent damage, such as scoring the cylinder walls, damaging the pistons and rings, and burning and warping the valves. Monitoring the engine temperature instruments and keeping indications within limits, or ‘in the green,’ helps avoid high operating temperatures.
To summarise, the outside air enters the engine compartment through an inlet behind the propeller hub. Baffles direct this flow to the hottest parts of the engine, typically the cylinders, which have fins as to increase the area exposed to the airflow.
It is important to understand that the air cooling system is less effective at lower airspeeds, such as ground operations, take-offs, go-arounds. Conversely, high-speed descents can shock cool the engine, subjecting it to abrupt temperature fluctuations.
Cowl flaps are hinged covers that fit over the opening through which the hot air is expelled. If the engine temperature is low, the cowl flaps can be closed, thereby restricting the flow of expelled hot air and increasing engine temperature. If the engine temperature is high, the cowl flaps can be opened to permit a greater flow of air through the system, thereby decreasing the engine temperature.
Most aircraft are equipped with a cylinder-head temperature (CHT) gauge that indicates a direct and immediate cylinder temperature change. This instrument is calibrated in degrees Celsius or Fahrenheit and is usually colour coded with a green arc to indicate the normal operating range. A red line on the instrument indicates maximum allowable cylinder head temperature.
179747052274500To avoid excessive CHT indications, the pilot should immediately increase airspeed, enrich the fuel-air mixture, and/or reduce power. On aircraft equipped with cowl flaps, use the cowl flap positions to help reduce temperatures.