The fan club

Thursday, May 16, 2013

Tags :  coolant-system

Most engineers these days might consider a vehicle fan as that placed either in front or behind the radiator, pushing or pulling air through it and therefore cooling a heat transfer fluid and indirectly cooling the engine. While this may be ubiquitous now, 50 years or so ago the situation was different, when air-cooled engines devoid of any engine coolant were always a possible option.

Of course though, the heat transfer characteristics of air are not as efficient as those of the primary constituent of a traditional engine coolant – water. For instance, the specific heat of air at normal atmospheric conditions is about 1.0 kJ/kg deg K, while that of water is about 4.2. However, the density of water is something like 840 times that of air, so even if we ignore the effects of the heat transfer coefficients using air it will take at least 840 times the volumetric flow rate of traditional engine coolant to absorb the equivalent amount of heat energy. To ensure that the air traverses all the parts of the engine, a fan of some description generally has to be used in, around and between the cylinders.  

Perhaps the most famous manufacturer using air-cooled engines for many years, including its high-performance and racecars, is Porsche. Famed for its reliability if not perhaps for its ultimate out-and-out performance, such was the technology surrounding the head gasket design of liquid-cooled engines that designer Ferdinand Porsche’s would say that an engine of his had never lost all its air. Even so, the compromises in engine design would be considered too great today since the space between bores needs to be considerably larger, and cylinder configurations are limited. Porsche in particular was famous at one time for its six-cylinder flat or ‘boxer’ engine for roadcars, and flat eight, 12 and even 16 cylinders (on one occasion) for its race machines.

For the roadcars, fans were mounted horizontally and belt-driven off the nose of the crankshaft, whereas for racing, drive would come from the crankshaft via a bevel gear. The fan would therefore invariably be positioned centrally and directly above the engine. The 1.5 litre eight-cylinder Formula One engine of 1962 had a 250 mm diameter axial flow fan using 17 blades, giving an airflow of around 890 m3 per minute using only about 6.5 kW of crankshaft power. Axial flow fans are very good at shifting large amounts of air at low pressure differentials, and by ensuring that there were no restrictions to flow made them ideal for the application. By the end of the 1970s the 4.5 litre flat 12 engine, producing around 440 kW at 8400 rpm, used a slightly smaller version, this time with only six blades.

Critical to the whole design is the internal ducting and cowling necessary to ensure that the engine is cooled uniformly along its length, and that the least internal stress is built up due to any temperature variations. Likewise, the decision to ‘blow’ down into the engine rather than ‘suck’ up through it was essential to the cooling of critical areas – in particular the cylinder barrels and the high-temperature zones between.

In cooling the engine using only air, more stress is inevitably placed on the oil circuit and the additional necessary cooling. Air-cooled engines therefore run slightly hotter and can be more marginal on lubrication. To ensure better reliability, air-cooled engines tend to be fuelled more on the ‘rich’ side to keep piston crown temperatures down. Air-cooled engines therefore tend to be less fuel efficient, which is partly why we don’t see them in modern vehicles.

Popular among many classic car owners, you could say these are the fans of the fan cars.

Fig. 1 - Air-cooled Porsche racer engine

Fig. 2 - Air-cooled Porsche 911 engine bay

Written by John Coxon

Leave your comment