Air-cooled engines rely on the circulation of air directly over hot parts of the engine to cool them.
A cylinder from an air-cooled aviation engine, a Continental C85. Notice the rows of fins on both the steel cylinder barrel and the aluminum cylinder head. The fins provide additional surface area for air to pass over the cylinder and absorb heat.
1 Introduction 2 Applications
2.1 Road vehicles 2.2 Aviation 2.3 Diesel engines 2.4 Stationary or portable engines
3 References 4 Bibliography
4.1 Cited sources 4.2 Further reading
Introduction 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 (or raw water, in the case of marine engines). Thus, while they are not ultimately cooled by the liquid, 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. (Direct Cooled Engine) Typically this is facilitated with metal fins covering the outside of the Cylinder Head and cylinders which 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, 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.  Applications 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:
New Way (1905) - limited production run out from the "CLARKMOBILE"
GM "copper-cooled" models of Chevrolet, Olds, and Oakland (1921-1923)
(very few built)
Tatra all-wheel-drive military trucks.
Most aviation piston engines are air-cooled. Air cooled engines are
lighter than their major alternative - water cooled engines, because
the lower weight of air cooled engines in aircraft design being a more
important factor than the lower drag of water cooled engines. While
water cooled engines were widely used from the early days of flight,
air cooled engines were the dominant choice in aircraft. Following the
Second World War, turbojets and jet turbine powered aircraft have come
to dominate flight regimes where water cooled piston engines offered a
drag advantage. Thus today, piston engines are mostly used in slower
general aviation aircraft where low weight is an advantage. Therefore,
most aero engines produced today are of the air cooled variety. Today,
most of the engines currently (2005) manufactured by Lycoming and
Continental and used by major manufacturers of light aircraft Cirrus,
^ Sloan 1964, pp. 71–94, Chapter 5, "The copper-cooled engine".
Bibliography Cited sources
Sloan, Alfred P. (1964), McDonald, John, ed., 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).
Biermann, A. E. (1941). "The design of fins for air-cooled cylinders" (pdf). Report Nº 726. NACA. [permanent dead link]
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.
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Aircraft piston engine components, systems and terminology
Alternator Capacitor discharge ignition Dual ignition Electronic fuel injection Generator Ignition system Magneto Spark plug Starter
Propeller governor Propeller speed reduction unit Spinner
Autofeather Blade pitch Constant-speed Contra-rotating Counter-rotating Scimitar Single-blade Variable-pitch
Annunciator panel EFIS EICAS Flight data recorder Glass cockpit Hobbs meter Tachometer
Fuel and induction system
Avgas Carburetor Fuel injection Gascolator Inlet manifold Intercooler Pressure carburetor Supercharger Turbocharger Updraft carburetor
Auxiliary power unit Coffman starter Hydraulic system Ice protection s