6.2.2 Engine Cooling
The combusting fuel within the cylinders generated immense heat, most of which is directed towards the exhaust system. This accounts for only about 40% of the dissipated heat. The remaining heat is dissipated by the cooling system and lubricating systems. If cooling is not accounted for, the extremely high engine temperatures can lead to loss of power, excessive oil consumption, poor lubrication, detonation, and permanent damage to the engine components. Monitoring the engine temperature instruments and keeping indications within limits, or ‘in the green,’ helps avoid high operating temperatures.
Cooling if achieved in the following ways:
Oil Cooling: This will be discussed in the following chapter (Engine Lubrication). A means of dissipating the heat by reducing friction about the engines moving parts. Most small aircraft, however, are primarily air cooled, and in this section we will explore the various methods air is used for cooling the engine’s external surface.
The front of the engine Cowling has openings just behind the propeller. This forces free air flow through the engine compartment. Baffles Plates ensure the airflow is directed over the Cooling Fins. These small grooves attached to the engine cylinders increases the surface area and allows for optimum cooling. Cowl flaps or, the hinged covers that fit over the opening aft the engine on the lower part of the cowling, can be adjusted as required to expel or retain hot engine air. 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 allowing a greater flow of air through the system, thereby cooling engine temperatures. Simple, yet effective.
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, the cylinders, which have fins as to increase the area exposed to the airflow. This hot air is then expelled through the cowl flaps. Air speed is an important factor when considering the air cooled system. Cooling will be less effective at low taxi speeds compared to straight and level flight. More airflow means more cooling. Conversely, high-speed descents can shock cool the engine, subjecting it to abrupt temperature fluctuations.
Most aircraft are equipped with a Cylinder-Head Temperature (CHT) gauge that indicates a direct and immediate cylinder temperature change. This instrument is colour coded with a green arc to indicate the normal operating range. A red line on the instrument indicates maximum allowable cylinder head temperature. Anything beyond the red line is cause for concern and can be amended for by increasing forward speed, reducing power, adjusting the mixture and use the cowl flaps to help reduce temperatures.