Aircraft Engine Cooling System

• Aircraft engines convert heat energy into mechanical energy.
• Only approximately one-third of heat produced is converted, with the remaining two-thirds wasted.
• Cooling systems remove this unused heat for peak engine efficiency.

Cooling System - Reciprocating Engine

• Internal combustion engines convert about 30% of heat to useful work, and expel 40-45% through exhaust.
• The remaining 25-30% of heat is absorbed by the oil and metal mass, and is removed by the cooling system.
• Engine performance suffers without heat removal, due to decreased volumetric efficiency and adverse effects on the fuel/air mixture.
• Excessive heat reduces oil's ability to lubricate and shortens the life of engine parts.
• The most common cooling methods are direct air cooling and liquid cooling.

Cooling System - Reciprocating Engine - Air Cooling

• Almost all modern aircraft engines are air cooled.
• Effective air cooling requires a large surface area for heat dissipation.
• Air-cooled engines use cooling fins, cast or machined onto cylinder barrels and heads.
• Fins provide a large surface area for heat transfer to the surrounding airflow.
• Fins cast into piston undersides can provide additional cooling and transfer heat to the engine oil.
• Older engine cylinder fins were thick and shallow whereas later designs are deeper and thinner.
• Modern aircraft engines use steel cylinder barrels with fins machined onto the surface.
• Aluminum cylinder heads with fins are screwed into the barrels.
• The exhaust valve has more fin area due to being the hottest part of the cylinder.
• The intake has fewer cooling fins because the fuel/air mixture cools the area sufficiently.

Cooling System- Reciprocating Engine- Air Cooling - Cowlings

• A side effect of air cooling is increased drag.
• Early aircraft cruised at speeds where drag was of little concern.
• Drag became unacceptable once aircraft development achieved airspeeds over 120 mph.

Cooling System- Reciprocating Engine- Air Cooling - Cowling - Radial Engine Cowling

• The Townend or speed ring was developed to help reduce drag on aircraft with radial engines.
• A Townend ring is an airfoil-shaped ring installed around the circumference of a radial engine.
• Airfoil shape smooths airflow around the engine and enhances air flow uniformity to each cylinder.
• Properly installed Townend rings can reduce drag by 11% on some aircraft.
• As aircraft and engine designs advanced to produce higher cruising speeds, increased need for a more efficient cooling system that provided less drag developed.
• In the 1930s, the NACA cowling became developed.
• This streamlined cowling completely covers all portions of a radial engine and extends all the way back to the fuselage.
• All NACA cowlings have an airfoil shape that converts incoming air into a solid jet blast to produce thrust.

Cooling System- Reciprocating Engine- Air Cooling - Cowling - Opposed Engine Cowling

• Cylinders on early horizontally opposed engines protruded out into the airstream for cooling.
• Because the cylinders were directly behind each other, a thin sheet metal hood on each side of the engine was used to force air down between the cylinder fins.
• Modern reciprocating engines use cowlings that enclose the entire engine.
• Cooling air enters through two forward-facing openings and exits through one or more openings at the bottom rear of the cowl.
• Rubberized strips seal the cowling to prevent excessive air leakage.
• Cooling air enters the cowling at a pressure above ambient due to forward motion and propwash, resulting in pressure cooling.
• The outlet on lower cowls is flared to facilitate pressure cooling by creating low pressure when outside air flows past the opening.
• The low-pressure area pulls air down through the cylinders, into the lower cowl, and out the cowling.

Cooling System- Reciprocating Engine- Air Cooling - Baffles and Deflectors

• Only 15-30% of total ram airflow approaches an airborne engine cowling and enters the cowling for cooling.
• Baffles and deflectors can be installed to maximize the effectiveness of airflow.
• Baffles and deflectors are sheet metal panels that block and redirect airflow for effective cooling.
• Baffles and deflectors are installed between the cowling and engine, and also between engine cylinders.
• Baffles divide the cowling to force air around the cylinders and into the lower cowl.
• Baffles installed between cylinders force cooling air into contact with all parts of a cylinder, and are called inter-cylinder baffles or pressure baffles.

Cooling System- Reciprocating Engine- Air Cooling - Baffles and Deflectors - Cowl Flaps

• The amount of cooling air flowing into the cowling is controlled by the use of cowl flaps on some aircraft.
• Cowl flaps are hinged doors installed at the bottom rear of the cowling where cooling air exits.
• Open cowl flaps create a stronger low-pressure area that pulls more air through the cylinders.
• Closed cowl flaps weaken the low-pressure area, reducing air drawn between the cylinders.
• Cowl flap position is controlled from the cockpit, and they are typically operated manually, electrically, or hydraulically.
• Cowl flaps are usually fully open during ground operations.
• Airflow through the cowling is greatly reduced when operating on the ground.
• Once the aircraft is in level flight, more air is forced into the cowling.
• Cowls flaps can then be closed so the drag produced can be elminiated.

Cooling System- Reciprocating Engine- Air Cooling - Augmentor Systems

• Augmenter tubes can augment airflow through the cylinders on some aircraft.
• Like cowl flaps, augmenter tubes create a low-pressure area at the lower rear of the cowling that increase the airflow through the cylinder cooling fins.
• Reciprocating engines use augmenter tubes to improve engine cooling.
• Exhaust gases from exhaust collectors flow into an augmenter tube, creating low pressure.
• The low pressure created draws additional cooling air over the engine cylinders.
• Exhaust gases from the engine are routed into a collector and discharged into the inlet of a stainless steel augmenter tube, in an augmenter system.
• A low pressure area created draws air from above the engine through the cylinder fins, due to High-velocity exhaust gases.
• Exhaust gases and cooling air exit at the rear of the augmenter tube.

This covers aircraft engine cooling systems, focusing on their purpose, efficiency, and design features. It addresses overheating consequences and materials used.