The main function of a cooling system is to guarantee that the engine runs at its optimum operating temperature, and most modern internal combustion engines achieve this with a closed circuit that carries liquid coolant through passages in the engine block and cylinder head. A water pump pushes the mixture of fresh water and antifreeze through these machined channels, where it absorbs heat before flowing onward to a radiator or heat exchanger. At the radiator, the hot coolant releases its heat to the surrounding air, or, in marine applications, to raw water; having been cooled, it returns once more to the engine and the cycle repeats continuously. An alternative method, used mostly in small utility power-plants, is air cooling: rather than circulating liquid, the engine relies on fast-moving air directed over heat-dissipation fins cast into the block and cylinder head, maintaining temperatures within safe operating limits.
What is a cooler engine?
A cooler engine is an air-cooled engine which relies on the circulation of air directly over heat dissipation fins or hot areas to cool them, and it keeps the engine within operating temperatures.
A cooler engine is an engine whose temperature has been lowered by a cooling system that removes unwanted heat. The oil cooler lowers the overall temperature of the engine by extracting heat from the oil before the oil returns to lubricate components. Lower oil temperature lowers the overall temperature of the engine, guaranteeing stable lubrication, reliable efficiency, and protection against heat-induced breakdowns. Water-cooled engines use coolant pushed through the cylinder block by a water pump; cooler water absorbs heat from the engine and is then cooled in the radiator, which acts as a heat exchanger. Air-cooled engines rely on the circulation of air over cooling fins to pull heat away, and they often use oil cooling to maintain safe temperatures. Whether water, air, or oil is used, the goal is the same: the engine must run at its optimum operating temperature to prevent wear that occurs when the engine is cold and to avoid shock cooling that causes engine failure.
In jet propulsion, a pre-cooled jet engine enables jet engines with turbomachinery to be used at high speeds by avoiding the need for an air condenser. The precooled jet engine lowers the temperature of the incoming air before it enters the compressor, allowing the engine to operate efficiently at high Mach numbers. This cooling approach is analogous to the way an oil cooler extracts heat from hot oil or a radiator dissipates heat to the surrounding air: both systems remove heat so that the engine withstands steep grades, long hours under heavy load, and extreme operating conditions without breakdowns that cost thousands in repairs and downtime.
How is an airplane engine cooling system designed?
An airplane engine cooling system is designed with two primary goals: keep the engine cool enough to operate safely and reliably, and minimize the drag penalty of the cooling airflow. The system must therefore cool effectively over the entire flight envelope while adding as little drag and weight as possible.
The first step in the design process is to calculate the pressure coefficient over the entire aircraft so that the best location for cooling air inlets and outlets is determined. Inlet area is sized to be just large enough for extended full-power ground runs, and the pressure rise required equals the sum of the pressure drop across engine cylinder baffles and the oil cooler. The engine manufacturer directly specifies cooling air quantity and baffle pressure drop. Engine baffles use aluminium guides and rubber seals to guide the flow, and cowl flaps hinged on the underside of the cowling can be opened from the cockpit when extra cooling is required.
Air-cooled engines are designed with cooling fins: small, thin pieces of metal that protrude from the cylinder heads and barrels, absorb heat from the engines, and transfer it to the exterior airflow. Variable-geometry features let the designer size the system for optimum flow in cruise or at maximum speed, and cowl flaps or similar devices regulate airflow so it is always linked to engine power setting.
Oil attracts a lot of heat as it passes through the engine, so an air-cooled oil-cooler forms part of the oil system and cooling air must flow through this cooler before it leaves the airplane. To restrict cooling power required and cooler weight to minimum values, the highest possible oil temperature is utilized, and the oil cooler design’s air pressure-drop value depends upon the type of cooling system installation. The ideal path is for cooling air to flow into the airplane, pass through the fins and cooler, and leave flowing directly downstream at as close to free-stream airspeed as possible. Exhaust augmenters or other devices will be used to change the rules normally applied to a pressure-cowl system.
During a program aiding with the repair of a tiny piston-engine aircraft, this theoretical information assisted me. I helped set up fresh baffles and examined the schematic diagram in detail.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
How does an aircraft engine cooling system work?
An aircraft engine cooling system works by moving excess heat from hot internal parts to the outside air. Cylinders have fins and these fins increase surface area so heat travels from the engine to the airplane exterior. Cool ambient air enters the engine compartment through two openings located one on each side of the propeller spinner, absorbs the heat and then expels it through aft openings. A pressure chamber sealed off on the top side of the engine with baffles forces the air to flow tightly around the fins. Flexible baffle seal material mates the baffles to the cowling to prevent leaks. The hottest parts are primarily the cylinders, and cylinder head temperatures rise quickly. Without temperature controls the engine overheats on takeoff.
The most common means of controlling cooling is the use of cowl flaps. These flaps regulate air circulation over the engine; when opened they let more air flow, and when closed they keep the engine warm at high altitude. Oil serves as an effective means to transfer heat out of internal components. Oil picks up part of the soaked-in heat and dissipates it to the atmosphere through the oil cooler, which forms part of the oil system. Excessive heat warps engine surfaces, vaporizes oil, and causes other forms of severe damage, so the continuous transfer of heat to the airstream is vital for safe operation.
What is an air cooled engine in aviation?
An air-cooled engine in aviation is a powerplant that relies on air, not liquid coolant, to keep cylinder temperatures within limits. Cooling fins machined into the heads and barrels increase surface area so that cool slip-stream air forced through the cowling carries heat away. Because the aircraft already makes its own exhaust in flight, no water pump, radiator, hoses, or header tanks are required, eliminating coolant leakage, freezing, and evaporation problems while holding part count and possible failing parts to a minimum. The engine is mounted with the cylinders in a horizontal position, giving a short, stiff crankcase and letting baffles duct air to the hottest zones. Horizontally opposed Lycoming and Continental flat-fours and flat-sixes, Rotax and ULPower direct-drive units, and the big radial Wright R-1820 Cyclone all share this finned, air-cooled architecture. The same design choice gives lower weight forward of the firewall, simpler installation, faster warm-up, and reliable operation from polar strips to desert strips, which is why the vast majority of personal airplanes - Cessnas, Pipers, Beechcraft, Lancairs, light-sport, and ultralights - are powered by air-cooled engines.
What is a liquid cooled engine in aviation?
A liquid cooled engine in aviation transfers heat from the cylinders to the coolant, which is cooled within a radiator placed in the airstream, allowing for higher performance. The coolant radiator must be large enough to cool the liquid efficiently, and it is generally mounted with the cylinders in a horizontal position.
A liquid-cooled engine in aviation is a piston engine that circulates a liquid - usually water mixed with antifreeze - through internal passages to carry away the heat generated during combustion. Heat is transferred from the cylinders and cylinder heads to the coolant, which is then sent through tubing to a radiator placed in the airstream. The heated liquid, typically at 135°C (275°F), is cooled by 20°C (68°F) air flowing through the radiator. The system requires one or more pumps, fluid reservoirs, hoses, clamps, temperature sensors, a thermostat, and a fan to regulate the temperature and pressure of the liquid.
Liquid-cooled engines are less common in aviation than air-cooled types, yet they have been part of aviation history from the start and gained notable success in World War II in the Merlin-powered P-51 Mustang, Supermarine Spitfire, and P-38 Lightning. Today the Extra 400 is the only airplane currently in production powered by a liquid-cooled engine, and RAM Aircraft markets a liquid-cooled retrofit for Cessna 414A twin-engine airplanes. Teledyne Continental Motors has developed a liquid-cooled version of its big-bore 550 series, and specialty firms like Liquid Cooled Air Power, Inc. and ASAP can convert a 320 ci Lycoming into a 200-hp liquid-cooled 344 ci engine or a 360 ci core into a 260-hp 390 ci engine, each with a certified time-between-overhaul of 5,000 hours.
The design offers several advantages: it keeps all parts at suitably low temperatures, reduces thermal stress, provides more stable and uniform working temperatures, and allows far less temperature differential across the cylinder head during operation. These qualities refine wear characteristics, permit more precise tolerances, and increase power output while maintaining compact, rigid engine blocks that can be cast in monolith banks rather than cantilevering out from the crankcase. A liquid-cooled engine can supply cabin heat with a heat exchanger that extracts heat from the engine coolant, eliminating any risk of carbon monoxide poisoning and shock cooling or heating problems.
Drawbacks include added weight, complexity, and cost. Liquid-cooled ICEs are heavier, more intricate, and more expensive than air-cooled ICEs. They require more maintenance and care, introduce higher noise levels, and show more sensitivity to ambient temperature and altitude. The biggest challenge in adapting an aircraft for liquid cooling is finding a place for the radiator, and the radiator and pumping system add weight, cost, and plumbing complexity that are not practical for every light-aircraft installation.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
