The increasingly tight emissions regulations that new motorcycles have to meet has seen the gradual phasing out of traditional air cooled engines, but just what is meant by the term ‘air cooled’ and how do they differ from the more common water cooled motors?
These days every internal combustion engined car on sale, and the majority of motorcycles, have water cooled engines. These bikes can be distinguished by their radiator, usually situated behind the front wheel, and the lack of fins on the engine block, and they are usually quieter too.
Air cooled engines dissipate the heat through heavy fins on the cylinders and have a much wider operating temperature. With nothing but fresh air to keep them cool, air cooled motors can get pretty hot when ridden slowly on a warm summer’s day. Previously, higher performance air cooled engines sometimes featured an oil cooler, which would circulate engine oil through a small radiator/oil cooler and around the cylinder heads. This was also a popular trick employed by BMW and Triumph on some of their retro models, as it helped to get the bike through Euro3 emissions regulations, while allowing them to retain the traditional air cooling fins that are loved by so many motorcyclists.
The term water cooled is actually something of a misnomer. The liquid itself usually contains a number of chemicals including antifreeze and corrosion inhibitors. Liquid cooling works by pumping the fluid through the thin water jacket that encases the engine block, cooling it through the radiator and keeping the fluid at a consistent temperature. The pipes, radiator, water pump and heat exchanger all add weight to the engine, and for many riders it can all look somewhat ugly, especially on a naked bike. That said, manufacturers are becoming increasingly clever when it comes to packaging their water cooled engines and making them aesthetically pleasing. Triumph’s latest 900cc and 1200cc parallel twins from the Bonneville range are a perfect case in point. Visually, they look just like old air cooled engines but underneath are ultra-modern liquid cooled motors.
By controlling the temperature and eliminating potential hot spots in the motor, water cooling helps make the engine more efficient and allows for tighter tolerances, helping engineers extract more power from the engine.
This is also the reason why liquid cooled engines are generally cleaner than their air cooled counterparts. The cooler and controlled temperature in the cylinder head allows for better detonation of the fuel and gives the engineers an opportunity to optimise the exhaust emissions, particularly on V-twin configuration engines where the temperature of the rear cylinder can get especially hot. When temperatures get too hot in the combustion chamber, the fuel and air mixture cannot be controlled optimally, which is why air cooled engines usually run lower compression ratios and have wider tolerances on components such as valve clearances. This can lead to an air cooled bike burning more oil.
Faced with getting bikes through exhaust emissions tests, manufacturers increasingly ‘lean off’ the fuel mixture, reducing the amount of petrol in the fuel/air mixture to reduce the gases emitted. This works fine on liquid cooled bikes, where the temperature is controlled, but is less effective on old fashioned air cooled units. Indeed, because a rich mixture causes the bike to run cooler, air cooled motors tend to be tuned to run rich in the past – and that isn’t good for the environment.