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Air-cooled engines rely on the circulation of air directly over hot parts of the engine to cool them.
Most modern internal combustion engines are cooled by a closed circuit carrying liquid coolant through channels in the engine block and cylinder head, where the coolant absorbs heat, to a heat exchanger or radiator where the coolant releases heat into the air. Thus, while they are ultimately cooled by air, because of the liquid-coolant circuit they are known as water-cooled. In contrast, heat generated by an air-cooled engine is released directly into the air. Typically this is facilitated with metal fins covering the outside of the cylinders which increase the surface area that air can act on.
In all combustion engines, a great percentage of the heat generated (around 44%) escapes through the exhaust, not through either a liquid cooling system nor through the metal fins of an air-cooled engine (12%). About 8% of the heat energy finds its way into the oil, which although primarily meant for lubrication, also plays a role in heat dissipation via a cooler. 
 Road vehicles
Many motorcycles use air cooling for the sake of reducing weight and complexity. Few current production automobiles have air-cooled engines (such as Tatra 815), but historically it was common for many high-volume vehicles. Examples of past air-cooled road vehicles, in roughly chronological order, include:
- Franklin (1902-1934)
- GM "copper-cooled" models of Chevrolet, Olds, and Oakland (1921-1923) (very few built)
- 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)
- ZAZ Zaporozhets (1958-1994)
- Fiat 126 (1972-2000)
- Porsche 356 (1948-1965)
- VW-Porsche 914 (1969-1976)
- Porsche 911 (1964-1998)
- The Volkswagen Beetle, Type 2, SP2, Karmann Ghia, and Type 3 all utilized the same air-cooled engine (1938-2003) with various displacements.Volkswagen Type 2 (T3) (1979–1982).
- Volkswagen Type 4 (1968-1974)
- 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 GS and GSA
- Honda 1300 (1969-1973)
- The East German Trabant (1957-1991)
- NSU Prinz
- Tatra all wheel drive military trucks.
- Royal Enfield Motorcycles (India): The 350cc and 500cc Twinspark motorcycle engines are air cooled
Most aviation piston engines are air-cooled, including most of the engines currently (2005) manufactured by Lycoming and Continental and used by major manufacturers of light aircraft Cirrus, Cessna and so on. Notable exceptions have included the Allison V-1710 and Rolls-Royce series of (most well known, the Merlin V-1650) liquid-cooled V12 engines which powered P-51 Mustangs, Avro Lancasters, Hawker Hurricanes and Spitfires.
Other engine manufactures using air-cooled engine technology are ULPower and Jabiru, more active in the Light-Sport Aircraft (LSA) and ultralight aircraft market. Rotax uses a combination of air-cooled cylinders and liquid-cooled cylinder heads.
 Diesel engines
Some small diesel engines, e.g. those made by Deutz AG and Lister Petter are air-cooled. Probably the only big Euro 5 truck air-cooled engine (V8 320 kW power 2100 Nm torque one) is being produced by Tatra.
- Sloan 1964, pp. 71–94, Chapter 5, "The copper-cooled engine".
 Cited sources
- Sloan, Alfred P. (1964), in McDonald, John, My Years with General Motors, Garden City, NY, USA: Doubleday, LCCN 64011306, OCLC 802024. Republished in 1990 with a new introduction by Peter Drucker (ISBN 978-0385042352).
 Further reading
- P V Lamarque, "The design of cooling fins for Motor-Cycle Engines". Report of the Automobile Research Committee, Institution of Automobile Engineers Magazine, March 1943 issue, and also in "The Institution of Automobile Engineers. Proceedings XXXVII, Session 1942-1943, pp 99-134 and 309-312.
- Julius Mackerle, "Air-cooled Automotive Engines", Charles Griffin & Company Ltd., London 1972.