Air-cooled engine

Air-cooled enginesrely on the circulation of air directly overheat dissipation finsor hot areas of theengineto cool them in order to keep the engine within operating temperatures. Air-cooled designs are far simpler than their liquid-cooled counterparts, which require a separateradiator,coolant reservoir, piping and pumps.

Air-cooled engines are widely seen in applications where weight or simplicity is the primary goal. Their simplicity makes them suited for uses in small applications likechainsawsandlawn mowers,as well as smallgeneratorsand similar roles. These qualities also make them highly suitable for aviation use, where they are widely used ingeneral aviation aircraftand asauxiliary power unitson larger aircraft. Their simplicity, in particular, also makes them common onmotorcycles.

A cylinder from an air-cooled aviation engine, aContinental C85.Notice the rows of fins on both thesteelcylinder barrel and thealuminumcylinder head. The fins provide additional surface area for air to pass over the cylinder and absorb heat.

Introduction[edit]

Most moderninternal combustion enginesare cooled by a closed circuit carrying liquidcoolantthrough channels in the engine block and cylinder head. A fluid in these channels absorbs heat and then flows to aheat exchangerorradiatorwhere the coolant releases heat into the air (orraw water,in the case ofmarine engines). Thus, while they are notultimatelycooled by the liquid, as the heat is exchanged with some other fluid like air, because of the liquid-coolant circuit they are known asliquid-cooled.

In contrast, heat generated by an air-cooled engine is released directly into the air. Typically this is facilitated with metalfinscovering the outside of the Cylinder Head andcylinderswhich increase the surface area that air can act on. Air may be force fed with the use of a fan and shroud to achieve efficient cooling with high volumes of air or simply by natural air flow with well designed and angled fins.

In all combustion engines, a great percentage of the heat generated, around 44%, escapes through the exhaust. Another 8% or so ends up in theoil,which itself has to be cooled in anoil cooler.This means less than half of the heat has to be removed through other systems. In an air-cooled engine, only about 12% of the heat flows out through the metal fins.^[1]Air cooled engines usually run noisier, however it provides more simplicity which gives benefits when it comes to servicing and part replacement and is usually cheaper to be maintained.^[2]

Applications[edit]

Road vehicles[edit]

Manymotorcyclesuse air cooling for the sake of reducing weight and complexity. Few current production automobiles have air-cooled engines (such asTatra 815), but historically it was common for many high-volume vehicles. The orientation of the engine cylinders is commonly found in either single-cylinder or coupled in groups of two, and cylinders are commonly oriented in a horizontal fashion as aFlat engine,while verticalStraight-four enginehave been used. Examples of past air-cooled road vehicles, in roughly chronological order, include:

Franklin(1902-1934)
New Way(1905) - limited production run out from the "CLARKMOBILE"
Chevrolet Series M Copper-Cooled(1921-1923) (very few built)^[3]
Tatraall-wheel-drive military trucks
Tatra 11(1923-1927) and subsequent models
Tatra T77(1934-1938)
Tatra T87(1936-1950)
Tatra T97(1936-1939)
Tatra T600 Tatraplan(1946-1952)
Tatra T603(1955-1975)
Tatra T613(1974-1996)
Tatra T700(1996-1999)
Crosley(1939-1945)
The East GermanTrabant(1957-1991)
Trabant 500(1957-1962)
Trabant 600(1962-1965)
Trabant 601(1964-1989)
ZAZ Zaporozhets(1958-1994)
Fiat 500(1957-1975)
Fiat 126(1972-1987)
Porsche 356(1948-1965)
Porsche 911(1964-1998)
Porsche 912(1965-1969, 1976)
VW-Porsche 914(1969-1976)
TheVolkswagen Beetle,Type 2,SP2,Karmann Ghia,andType 3all utilized thesame air-cooled engine(1938-2013) with various displacements
Volkswagen Type 2 (T3)(1979–1982)
Volkswagen Type 4(1968-1974)
Volkswagen Gol (G1)(1980-1986)
Toyota U engine(1961-1976)
Chevrolet Corvair(1960-1969)
Citroën 2CV(1948-1990) (Featured a high pressure oil cooling system, and used a fan bolted to the crankshaft end)
Citroën GSandGSA
Honda 1300(1969-1973)
NSU Prinz
Royal Enfield Motorcycles (India):The 350cc and 500cc Twinspark motorcycle engines are air-cooled
Oltcit Club(1981–1995)T13/653, G11/631 and VO36/630
Demak Dzm 200 2015

Aviation[edit]

During the 1920s and 30s there was a great debate in the aviation industry about the merits of air-cooled vs. liquid-cooled designs. At the beginning of this period, the liquid used for cooling was water at ambient pressure. The amount of heat carried away by a fluid is a function of its capacity and the difference in input and output temperatures. As theboiling pointof water is reduced with lower pressure, and the water could not be efficiently pumped as steam, radiators had to have enough cooling power to account for the loss in cooling power as the aircraft climbed. The resulting radiators were quite large and caused a significant amount ofaerodynamic drag.^[4]

This placed the two designs roughly equal in terms of power to drag, but the air-cooled designs were almost always lighter and simpler. In 1921, theUS Navy,largely due to the efforts of CommanderBruce G. Leighton,decided that the simplicity of the air-cooled design would result in less maintenance workload, which was paramount given the limited working area ofaircraft carriers.Leighton's efforts led to the Navy underwriting air-cooled engine development atPratt & WhitneyandWright Aeronautical.^[4]

Most other groups, especially in Europe where aircraft performance was rapidly improving, were more concerned with the issue of drag. While air-cooled designs were common on light aircraft and trainers, as well as sometransport aircraftandbombers,liquid-cooled designs remained much more common forfightersand high-performance bombers. The drag issue was upset by the 1929 introduction of theNACA cowl,which greatly reduced the drag of air-cooled engines in spite of their larger frontal area, and the drag related to cooling was at this point largely even.^[4]

In the late 1920s into the 1930s, a number of European companies introduced cooling system that kept the water under pressure allowed it to reach much higher temperatures without boiling, carrying away more heat and thus reducing the volume of water required and the size of the radiator by as much as 30%. They could also eliminate the radiator entirely usingevaporative cooling,allowing it to turn to steam and running the steam through tubes located just under the skin of the wings and fuselage, where the fast moving outside air condensed it back to water. While this concept was used on a number of record-setting aircraft in the late 1930s, it always proved impractical for production aircraft for a wide variety of reasons.^[5]

In 1929,Curtissbegan experiments replacing water withethylene glycolin aCurtiss D-12engine. Glycol could run up to 250 C and reduced the radiator size by 50% compared to water cooled designs. The experiments were extremely successful and by 1932 the company had switched all future designs to this coolant. At the time,Union Carbideheld a monopoly on the industrial process to make glycol, so it was initially used only in the US, withAllison Enginespicking it up soon after. It was not until the mid-1930s thatRolls-Royceadopted it as supplies improved, converting all of their engines to glycol. With the much smaller radiators and less fluid in the system, the weight and drag of these designs was well below contemporary air-cooled designs. On a weight basis, these liquid-cooled designs offered as much as 30% better performance.^[6]

In the late- and post-war era, the high-performance field quickly moved tojet engines.This took away the primary market for late-model liquid-cooled engines. Those roles that remained with piston power were mostly slower designs and civilian aircraft. In these roles, the simplicity and reduction in servicing needs is far more important that drag, and from the end of the war on almost all piston aviation engines have been air-cooled, with few exceptions.^[6]

As of 2020^[update],most of the engines manufactured byLycomingandContinentalare used by major manufacturers of light aircraftCirrus,Cessnaand so on. Other engine manufactures using air-cooled engine technology areULPowerandJabiru,more active in the Light-Sport Aircraft (LSA) andultralight aircraftmarket.Rotaxuses a combination of air-cooled cylinders and liquid-cooled cylinder heads.

Diesel engines[edit]

Some small diesel engines, e.g. those made byDeutz AGandLister Petterare air-cooled. Probably the only big Euro 5 truck air-cooled engine (V8 320 kW power 2100 N·m torque one) is being produced byTatra.

Stationary or portable engines[edit]

Stationary or portable engines were commercially introduced early in the 1900s. The first commercial production was by the New Way Motor Company of Lansing, Michigan, US. The company produced air-cooled engines in single and twin cylinders in both horizontal and vertical cylinder format. Subsequent to their initial production which was exported worldwide, other companies took up the advantages of this cooling method, especially in small portable engines. Applications include mowers, generators, outboard motors, pump sets, saw benches and auxiliary power plants and more.

References[edit]

^Thomas, Kas (1997-02-19)."Shock Cooling: Myth or Reality?".AVWeb.Aviation Publishing Group. Archived fromthe originalon 2008-12-02.
^YouMotorcycle (2015-12-19)."Air Cooled vs. Liquid Cooled Motorcycle Engines".YouMotorcycle.Retrieved2020-04-23.
^Sloan 1964,pp. 71–94, Chapter 5, "The copper-cooled engine"
^^a ^b ^cTaylor 1971,p. 53.
^Taylor 1971,p. 55.
^^a ^bTaylor 1971,p. 56.

Bibliography[edit]

Cited sources[edit]

Sloan, Alfred P.(1964), McDonald, John (ed.),My Years with General Motors,Garden City, NY, US: Doubleday,LCCN 64011306,OCLC 802024.Republished in 1990 with a new introduction byPeter Drucker(ISBN 978-0385042352).
Taylor, C. Fayette (1971).Aircraft Propulsion.Smithsonian Institution.