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Piston Engine Propulsion
Cooling
• Almost all modern aircraft engines are air cooled. However, to be effective, an engine must have a great deal of surface area that gives up heat.
• To accomplish this, all air-cooled engines utilize cooling fins that are either cast or machined into the exterior surfaces of the cylinder barrels and heads.
• The fins provide a very large surface area for transferring heat to the surrounding airflow.
There are two types of cooling engines.• Air cooling, and• Liquid cooling
Air Cooling
• The cylinders on early opposed engines were also stuck out into the air stream to provide cooling.
• However since the cylinders were directly behind each other, a thin sheet metal hood had to be installed on each side of the engine to force air down between the cylinder fins.
• Now all opposed engines also have cowlings that surround the entire engine.
• With this type of cowling, cooling air enters through forward facing openings and exits out one or more openings in the bottom rear of the cowling.
• The rest of the cowling is sealed with rubber strips to prevent excessive air leakage.
• Because of the ram effect produced by the forward motion and prop wash, cooling air enters a cowling at a pressure above ambient.
• This produces what is known as pressure
cooling.
• To facilitate the pressure cooling process, the outlet on lower cowls is flared so that when the outside air flows past the opening, an area of low pressure is created in the bottom of the cowling.
• This low-pressure area draws air down through the cylinders and into the lower cowl where it can exit the cowling.
COOLING SYSTEMS – Air Cooling
Cross section of typical light aircraft
Modern flat-four air cooled engine
Air Cooling Flows
Cylinders
Engine and MountsProp and
Spinner
Cowl Outline
Air Intakes
Air Outlet
Ra
m a
ir c
oo
lin
g f
low
Liquid Cooling• Liquid cooled aircraft engines are constructed with a metal
water jacket that surrounds the cylinders.
• As coolant circulates in the water jacket, heat passes from the cylinder walls and heads to the coolant.
• A coolant pump circulates the coolant in a pressurized loop from the water jacket to the radiator, where heat is transferred from the coolant to the air.
• To allow for higher engine temperatures and a smaller radiator, many liquid-cooled systems are pressurized.
COOLING SYSTEMS – Liquid Cooling
Water Passages
Radiator
Engine Assembly
Thermostatic Valve
Water Pump
Top Hose
Bottom Hose
Pressure Relief Cap
Cooling Fan
Over Flow
Thermostatic Valve CLOSED
Coolant circulates around engine block only
Coolant/engine COLD:-
COOLING SYSTEMS – Liquid Cooling
Thermostatic Valve OPEN
Coolant circulates around engine block and radiator
Coolant/engine HOT:-
COOLING SYSTEMS – Liquid Cooling
Typical Radiator
Top Hose Connection
Bottom Hose Connection
End Chamber
COOLING SYSTEMS – Radiator
Cool Air Flow In
Warm Air Flow Out
Expansion Chamber
ConnectionHot Coolant
In
Cooler Coolant Out
Cooled Coolant
CLOSED
Thermostatic Valve
OPEN
To
Radiator
No Flow Flow
Rubber Diaphragm
Wax pellet
‘Jiggle’ pin
Typical Thermostatic
Valve
COOLING SYSTEMS – Liquid Cooling
Feet
Other features:-
Screen Demist
Facia
Expansion Tank
Heat control valve
Fan
Heating Unit
Control valves
Heater matrix
Air intake duct
COOLING SYSTEMS – Liquid Cooling