Cooling (combustion engine)

The cooling system in an internal combustion engine mainly dissipates the heat that is given off by the hot gas to the walls of the combustion chamber and cylinder because the cycle is not ideal. Compared to electric motors , the efficiency is low and requires a correspondingly high cooling capacity. Temperatures that are too high damage the engine (the lubricating film tears off ( piston seizure ), the valves burn ). With Stirling engines , the process heat to be dissipated is given off via the cooling system.

Air and water cooling are used as the primary cooling system . With an oil cooler , the lubricant is often used for cooling - mostly with air cooling.

Air cooling

Air duct in the VW Beetle
(blue: cold air, red: warm air)

Air-cooled engine, cut open

The metal lamellae around the cylinder and the cylinder head are characteristic. The significantly higher thermal conductivity in relation to the air dissipates the heat, the large surfaces facilitate the transition to the surrounding air.

use

In general, the air-cooled internal combustion engine has been largely replaced by liquid-cooled versions. In motor vehicles it is still used on some motorcycles , manufacturers such as Harley-Davidson , Buell , Ducati , Moto Guzzi or BMW have or had numerous models with air cooling in their product range. In the course of the EURO4 standard, many manufacturers have switched to water cooling. On the other hand, propellers - airplanes and RC cars with combustion engines are equipped with air-cooled motors, as well as small motors for lawnmowers or chainsaws . These engines often (like some motorcycle engines) do without a fan.

Air-cooled cars were always equipped with a fan . Such cars were recently produced in the form of the Porsche 911 with a 6-cylinder boxer engine (until 1998) and the Tatra 700 with an 8-cylinder V-engine (until 1999). Other historic passenger cars with air-cooled engines include VW Beetles , Citroën 2CV ("duck"), Honda 1300 and Trabant (except type 1.1). A well-known air-cooled van was the VW bus (until 1982). Some trucks were also equipped with air-cooled ( diesel ) engines in the past , including Magirus-Deutz . Robur manufactured air-cooled trucks with both diesel and gasoline engines. Tatra still produces trucks with air-cooled engines at the customer's request.

advantages

Simple and inexpensive construction due to the small number of components involved
Fast reaching of the operating temperature (with the presence of appropriate regulating devices)
Climatically independent: no freezing of coolant at low outside temperatures, no water requirement in hot and / or arid regions
Reliable operation, largely maintenance-free
In military aircraft lower sensitivity shelling

disadvantage

Large noise emissions due to the lack of a sound-absorbing water jacket ( but this can be a desired effect in sports cars )

The heat transfer coefficient between air and a solid is about a factor of 50 to 100 lower than the heat transfer coefficient between water and a solid. Therefore strong ribbing of the cylinder surface and possibly a powerful fan is necessary.

Distribution of the cooling air not optimally possible, which limits the engine output. Cooling fans, if present and active, therefore work with a very high output (up to 8% of the engine output) and can be several times larger than that of a water pump .

Larger fluctuations in the operating temperature depending on the outside temperature

Larger piston clearances are required and therefore more prone to piston tilting

limited effective and uneven heating of the passenger compartment

Further disadvantages of the air-cooled four-stroke engine : additional oil cooler with thermostat required; Components such as ducts, valves (valve angles), spark plugs and injection nozzles cannot be arranged in any structural way that would be required for optimal exhaust gas behavior within the engine, which makes compliance with modern exhaust gas regulations difficult. Likewise, thermally highly stressed zones such as the area between the exhaust valve seats and the spark plug can be better controlled in multi-valve arrangements with liquid cooling, which allows a higher compression ratio without the risk of knocking in favor of exhaust gas and consumption values . However, air-cooled two-stroke engines are not an alternative, as they show poor exhaust gas behavior for other reasons.

Components

Cooling fan

Many motorcycle engines get by without a fan (airflow cooling). In other areas of application, a cooling fan accelerates the heat exchange. Cooling fans are widespread in stationary engines, for example in power or compressed air units, in vehicles with low speeds or other engines that are subject to high thermal loads. The fan wheel is usually located directly on the crankshaft. If it is driven by a V-belt , the cooling fails immediately if it breaks and the operation of the motor must be stopped immediately.

Cooling air duct

The air is blown directly onto the cylinders and cylinder heads - usually guided by baffles . They have cooling fins to enlarge the surface that emits heat.

Cooling air regulation

In order to quickly reach the operating temperature even when the outside temperature is low, some air-cooled engines have thermostat-controlled flaps, for example the corresponding VW engines.

oil cooler

In many types, the engine oil is also cooled. In older Deutz engines, it is pumped through a spiral pipe system in the “wind tunnel” of the cooling fan.

gallery

Air-cooled motor under the "alligator hood" of a Magirus-Deutz round hood
Air-cooled V10 engine of the Tatra 815
Prototype of a Tatra W18 air-cooled engine (1943)
Air-cooled engine from VW in cross section
Air-cooled Porsche engine in the 550 Spyder
Air-cooled four-cylinder in- line engine from a Yamaha XJR 1300
Air-cooled V2 engine from a Buell XB model
A heavily ribbed cylinder head of a traveling wind cooled moped , here: NSU Quickly

Liquid cooling

Various components of a liquid cooling system: heat exchangers, electric coolant pumps, shut-off valve, hoses, lines

With a few exceptions, modern four-stroke engines are liquid-cooled, with a mixture of water and an anti-freeze and anti-corrosion agent used as the coolant . Therefore, the term water cooling has established itself in common parlance .