piston engines: cooling

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