6.2.3 Engine Lubrication
The primary function of the engine oil system is to reduce friction between moving parts which would otherwise generate heat if not sufficiently lubricated. Other functions include:
Cushioning effect to engine parts subject to shock-loading
Aids as an effective cooling agent (along with air cooling)
Removing heat from the cylinders
Providing a seal between the cylinder walls and pistons
Carrying away contaminants
Operation of the propeller Constant Speed Unit (C.S.U)
Viscosity describes the resistance of an oil to flow and is primarily affected by temperature. Low temperatures increase viscosity (stickiness), creating a dragging effect, hindering its ability to circulate and perform as it should. At high temperatures, viscosity decreases and the oil becomes so thin that it begins to break down, resulting in rapid wear of moving parts. Because reciprocating engines have high operating temperatures and pressures, we require high viscosity oil. Other qualities of suitable lubricating oil include:
High flash point (temperature at which flammable vapors are released)
High anti-friction characteristics
Maximum fluidity at low temperatures
Maximum anti cooling ability
Maximum resistance to oxidation
Be non-corrosive
Lubrication Systems
Reciprocating engines use either a wet-sump or a dry-sump oil system. In a wet-sump system, the oil is located in a sump that is an integral part of the engine. Whereas a dry-sump system makes use of a separate, self-contained oil tank and engine driven pumps to achieve circulation.
The main component of a wet-sump system is the gear-type oil pump, which draws oil from the sump and routes it to the engine. Located before the oil pump is the by-pass valve which allows unfiltered oil to enter the system in case of any blockage. Similarly, an oil pressure relief valve ensures pressure is neither too high as to allow leaks, nor too low so to ensure adequate lubrication. After the oil passes through the engine, it drains back into to the sump, completing the cycle. In some engines, additional lubrication is supplied by the rotating crankshaft, which splashes oil onto portions of the engine.
An oil pump also supplies oil pressure in a dry-sump system, but the source of the oil is located in a separate oil tank. After oil is routed through the engine, it is pumped from the various locations in the engine back to the oil tank by scavenge pumps. Since changes in temperature significantly affects the viscosity of our oil and therefore its effectiveness, an oil cooler which is placed in the airflow (similar to a radiator) and allows for oil temperature regulation. Dry-sump systems allow for a greater volume of oil to be supplied to the engine, as well as inverted flight, which makes them more suitable for aerobatic and turbine aircraft.
The oil pressure gauge provides a direct indication of the oil system operation. It measures the pressure in pounds per square inch (psi) of the oil supplied to the engine. There should be an indication of oil pressure during engine start. Oil pressure should be kept within the limits. Refer to the Pilots Operating Handbook (P.O.H) for manufacturer limitations.
the oil temperature gauge measures the temperature of oil. A green area shows the normal operating range, and the red line indicates the maximum allowable temperature. Unlike oil pressure, changes in oil temperature occur gradually. This is particularly noticeable after starting a cold engine, when it may take several minutes or longer for the gauge to show any increase in oil temperature.
It is important to periodically check the oil temperature during flight, especially when operating in high or low ambient air temperature:

High oil temperature indications may signal a plugged oil line, a low oil quantity, a blocked oil cooler, or a defective temperature gauge.
Low oil temperature indications may signal improper oil viscosity during cold weather operations.

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The oil filler cap and dipstick (for measuring the oil quantity) are usually accessible through a panel in the engine cowling. If the quantity does not meet the manufacturer’s recommended operating levels, oil should be added. The POH or placards near the access panel provide information about the correct oil type and weight, as well as the minimum and maximum oil quantity. Within the filler neck is an oil filter to prevent foreign particles entering the engine compartments. At the bottom of the sump is a quick drain valve to manually remove water or sludge. Checking oil quantity is part of your pre-flight checks and should be done prior to every flight.

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 removed 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.
Air Cooling
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. Most small aircraft, however, 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.

Cowling Design
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 free air flow through the spaces in the engine compartment. Baffles Plates are placed in this fast moving airflow to deflect it over the hottest parts of the engine, the cylinders. Cooling Fins are small finned grooves attached to the engine cylinders to increase the surface area and allows 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. Simple, yet effective.

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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 GaugeMost 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.

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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.

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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.

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