Motorbikes, small aircraft and gasoline-powered lawnmowers often still have air-cooled combustion engines to this day. Even legendary car models such as the VW Beetle or the Citroën 2CV, which were produced in large quantities until the 1970s, still had air-cooled engines under the hood. Sports-car drivers, in particular, appreciated this drive, not least because of its powerful sound. That’s why some Porsche enthusiasts were sorry when the Swabian sports-car maker – as the last of Germany’s car manufacturers – said farewell to air cooling in 1998 with its new 911 model.
Nowadays, the combustion engines of almost all car models are liquid-cooled. The coolant – a mixture of water and glycol – circulates in a closed loop, and is cooled in the radiator. Modern combustion engines reach an efficiency of around 30 to 45 percent. Roughly estimated, some three-fifths of the heat created at temperatures of up to 2,500 °C remains unused – it’s waste heat. Around half of the waste heat is discharged through the tailpipe with the exhaust gases; the remainder raises the temperature of the engine, especially the cylinder block, the cylinder head and the pistons. To prevent the engine from overheating, the cylinder block and cylinder head are cooled with coolant.
During operation, the cooling circuit has a gauge pressure of up to 1.5 bar. This increases the coolant’s boiling point. At a gauge pressure of 1.5 bar, the water-glycol-based coolant only boils once it reaches around 125 °C and not at 110 °C, its boiling point under atmospheric pressure. This creates a larger temperature gradient in the radiator, which can be used to transfer heat from the hot coolant to the ambient air. The operating state of the engine dictates the coolant temperature; if no exceptionally high engine output is required, it averages at approximately 90 °C.