In the design of any aero engine, weight and reliability are all important. Every component not used, is one which can not fail. Eliminating the need for a complete (water cooling) system which is critical for the correct operation of some aero engines, and keeping component count as low as possible helps us obtain the reliability goal. Reducing the number of components not only gets rid of weight but reduces ownership costs, both in the original purchase price and with each service in the years to come.
Provided the aircraft engine cooling installation has been designed properly, very few water- or air-cooled engines which have been expressly designed for aircraft use will overheat. It is true that CHT's in air-cooled engines will vary widely depending on the power setting, but these engines have been designed for this type of operation and this should not be a problem in the long term. The lack of "shock cooling" is a much heard argument used in favour of a water-cooled aero engine. However we believe that this phenomenon is rarely an issue in ultralight and light sport aircraft because the outside air temperatures at the low altitudes at which these aircraft generally fly are not nearly as cold as what can be encountered by high flying GA aircraft.
In a small volume, high revving engine such as the Rotax 9 series, it would seem that cylinder heads could not be designed with enough cooling fin area to cool the engine adequately when operating at high power levels. (We presume they would have otherwise done so). This meant having a dual cooling system, air to cool the cylinder barrels and water to cool the cylinder heads. Extra complexity means more things to go wrong.
Water cooled engines usually use a mixture of water and a special coolant - sometimes the coolant may not be diluted with water. If a leak has lowered the coolant level, many people do not hesitate to fill up the coolant reservoir with plain water. However if the correct proportion of the active ingredients of the coolant are not present in the cooling system, then this can cause the coolant to boil before the maximum allowable CHT's have been reached. This means that if water is added to the cooling system, the maximum CHT's must be decreased which can result in lower continuous power being safely available. The water cooling system often operates under pressure to allow higher coolant temperatures. If a leak occurs and pressure is lost, the coolant will boil at a lower temperature. Depending on the engine power setting, boiling coolant will usually result in a rupture of the coolant lines meaning the engine can no longer operate and will need to be shut down quickly.
The ULPower engine has a very large cooling fin area. The front and back cylinder heads are mirrored to allow a large fin area over and around the exhaust port to improve cooling in a critical volume on both heads. Providing the engine is properly installed, air alone will keep CHT's to acceptable levels even under conditions of continuous high power loading and high outside air temperatures.
It is normally easy to check that air-cooled engines have their full cooling capacity by looking down or through the cooling fins with the aid of a flashlight. A thin stiff wire or compressed air is all that is usually needed to clear a blocked passage. A water-cooled engine's radiator can also be easily checked on its' outside surface, but the removal of built up dust and the straightening the thin and fragile cooling fins caused by insect/stone strikes is not easily performed. The use of compressed air to clean the delicate fins on water radiators can sometimes do more damage than good. If one or more of the internal passages of the water cooler have been blocked, this is difficult to detect and/or repair.
Air cooled engines have the undeniable safety advantage that they can still continue to function even if one (or more) cylinder head seals are leaking, admittedly with reduced power production but the pilot will not be forced to make an emergency landing when the problem first develops. In fact many air-cooled engine aircraft owners might be flying for days without realising they have a leaking cylinder head seal. On the other hand, a water-cooled engine with even a small leak in a cylinder head seal will quickly become inoperable as the water cooling system will be subjected to high internal pressures caused by combustions gases escaping into the water cooling circuit. This abnormal pressure will rupture the conduits or radiator or force a connection to separate. Without the all important water cooling this type engine cannot function even at low power levels and it must be shut down very quickly to avoid serious damage. A defective cylinder head seal, or leak in the water cooling circuit of an aircraft with a water-cooled engine means: engine off and going down!