6.2.2 Engine Cooling
Within the cylinders the fuel/air mixture that is ignited, combusts – generating immense heat. Most of this is directed and expelled through the exhaust. This accounts for only about 40% of the dissipated heat. The remaining heat is expelled through 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 ultimately permanent damage to the engine components. Monitoring the engine temperature instruments and keeping indications within its limits, or ‘in the green,’ helps in avoiding excessive operating temperatures.
Oil Cooling is another means of dissipating the heat by reducing friction about the engines moving parts is how oil cooling is achieved. This will be discussed in detail in chapter 6.2.3 – Engine Lubrication. Today, however, most small aircraft are primarily air cooled, and in this section we will explore the different ways in which air is used for cooling the engine’s external surface.
The entire Cowling is designed around the concept of streamlining the mounted engine and allowing for maximum air cooling. Therefore in front of the engine Cowling, just behind the propeller are cut-out openings. This forces fast moving free air to flow through the spaces in the engine compartment. From here, Baffles Plates are placed in front of this fast moving airflow so to deflect it over the hottest parts of the engine – the cylinders. Small finned grooves called Cooling Fins are etched into the engine cylinders to increase the surface area and thereby allowing for optimum cooling. As the cool air introduced into the cowling absorbs all of the latent heat, it reduces engine temperatures, but still needs to be removed from the system, just like the exhaust gasses.
Cowl flaps (adjustable hinged flaps that fit over the lower part of the cowling), that can be adjusted as required, are used to expel or retain this hot engine air. When the engine temperatures are seen to be low, the cowl flaps can be closed, thereby limiting the expulsion of hot engine compartment air and increasing engine temperature. When engine temperatures are high, the cowl flaps can be opened as to allow for a greater flow of cool air through the system, thereby reducing engine temperatures. This simple works effectively within the single (normally aspirated) engine.
An important factor when considering the effectiveness of the air cooled system is airspeed. Engine cooling will be less effective at lower speeds such as taxiing, compared to straight and level flight. This is due to a reduced airflow travelling through the engine compartment. More airflow means more cooling. Conversely, high-speed descents at low R.P.M settings can shock cool the engine, subjecting it to abrupt temperature fluctuations.
Cylinder Head Temperature Gauge
Most modern aircraft are equipped with a Cylinder-Head Temperature (CHT) gauge that indicates an immediate and accurate cylinder temperature reading. This colour coded instrument is marked with a green arc to indicate the normal operating range. Whereas a red line indicates the maximum allowable cylinder head temperature. Any indication beyond the red line should be amended for immediately by increasing forward speed (lowering nose attitude), reducing power, enriching the mixture and using the cowl flaps to help reduce and normalise temperatures.