6.2.2 Engine Cooling
Combustion takes place within the cylinders when the combined mixture of fuel/air is ignited by the spark plugs – forcing the pistons to move by generating excessive energy and heat. Most of these burnt gasses are expelled out of the cylinder through the exhaust system via the exhaust valve. This barely accounts for half of the heat that needs to be removed. Cooling and lubricating systems are used to get rid of the latent heat that remains within the engine after exhaust gasses are expelled. If such cooling were not accounted for, the extremely high engine temperatures would be detrimental. Resulting in loss of power, excessive oil consumption, poor lubrication, detonation, and ultimately if left unattended would result in permanent damage to the engine components. For this reason, monitoring the engine temperature instruments and ensuring indications are kept within the limits at all times, or ‘in the green,’ helps in avoiding excessive operating temperatures and the accompanying catastrophic consequences.
Since cool air is readily available at altitude where most small aircraft operate, Air Cooling is the primary method used to cool the piston engine’ external surface, and in this section we will explore the different ways in which the relative air flow is used to achieve this important task. Oil Cooling is another means of dissipating the heat by reducing friction about the engines moving. This will be discussed in detail in chapter 6.2.3 – Engine Lubrication.
Close inspection of any aircraft, it becomes clear that the entire Cowling is designed around the concept of streamlining the mounted engine and provisions are made, allowing for maximum air cooling. For instance, in front of the engine Cowling, just behind the propeller are cut-out openings. This gap behind the propeller allows fast moving free air to be forced through the gaps in the engine compartment. Looking under the cowling, one can see Baffles Plates that are placed in front of this fast moving airflow so to deflect the cool air over the hottest parts of the engine – the cylinders. Small finned grooves called Cooling Fins are etched into the engine cylinders. This increases the surface area and thereby allows for optimum cooling. Engine temperatures begin to reduce as the cool air introduced into the cowling absorbs all of the latent heat, however, this hot air 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 as required. On a cold day when the engine temperatures are seen to be low, the cowl flaps can be closed, thereby limiting the expulsion of hot engine compartment air. This helps warm the engine after start-up. Conversely, 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 design works effectively within the single (normally aspirated) engine aircraft.
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. Basically, more airflow means better cooling. On the other hand, high-speed descents at low R.P.M settings can shock cool the engine, subjecting it to abrupt temperature fluctuations. This should be avoided wherever possible.
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. The C.H.T instrument is colour-coded, 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 bring temperatures back to within the normal operating range.