An air-cooled engine is simpler to design then the water cooled engine because the heat is transferred from the cylinder and head directly to the air. There is no need for water jackets, hoses, water pump, radiator, etc. But, the engine design uses many fins on the cylinder and head for effective heat transfer to the surrounding air.
The amount of heat transfer that occurs between two mediums (in this case the metal of the engine and air) depends on three factors : (i) the temperature difference (AT) between them, («) the heat transfer area in contact between them, and (Hi) the heat transfer coefficient. The heat transfer coefficient between metal to water is about 100 times better than metal to air. Therefore, either the metal to air surface area or the temperature difference (AT) or a combination of the two should be 100 times larger in an air-cooled engine to obtain the same amount of heat transfer. The fins increase the contact area between the two mediums.
Additionally, the cylinder and heat temperature in an air-cooled engine is about twice that of a comparable water-cooled engine. Therefore, more and larger fins are provided in the critical areas of the combustion chamber and exhaust valve/port areas to prevent occurrence of abnormal combustion. Because of the higher upper-cylinder temperatures, more heat goes through the piston and cylinder to oil. All automotive air-cooled engines use oil coolers to assist removal of excess heat. Most air-cooled engines use a higher viscosity oil to compensate for these higher temperatures.
The high power demand is the weak point of an air-cooled engine. If this causes too high a temperature in the combustion chamber, pre-ignition and/or detonation occur, with possible damage of the engine in a short time period. Combustion chamber temperature in an air-cooled
engine is normally controlled by maintaining the compression pressure low, the ignition timing slightly retarded, and/or the air-fuel ratio rich relative to a water-cooled engine. Nowadays nearly all of the high power motorcycle engines have changed over to water cooling. This has provided better combustion chamber temperature control, and has allowed an increase in the power output and reduction in exhaust emissions. In addition, the more even cylinder temperatures have reduced localised hot spots so that piston and cylinder destruction and scuffing have been minimised.
Air-cooling System Components
When the vehicle is normally operated at a speed sufficient to provide enough air movement to transfer heat, the entire cooling system is merely the fins on the air-cooled engine. Many motor cycles are examples of this. But, improved streamlining usually requires a smaller frontal area which demands narrower engines. In this case cylinders have to be placed behind each other, and this requires duct work (cowling) to insure enough air flow to the rear cylinders of the engine. The shape of the cowling (Fig. 12.43) guides the forced convection current around all the cylinders and provides a direct exist after the air has extracted and absorbed the heat from the engine.
Some engine configurations, such as flat four-cylinder engines, employ baffles to improve
Fig. 12.43 Air-cooling system for an in-line four-cylinder engine.
the air distribution between cylinders and to direct additional air to critical components such as the oil-cooler (Fig. 12.44).
A fan or blower is installed in air-cooled engines, with little or slow movement of air, to sufficiently improve air flow past the fins. Most engines that use blower fans also use shrouds or ducts to insure the flow of cooling air past the fins. Two classes of fan used with air-cooled engines are the radial flow type (Fig. 12.44C), where the air is flung outwards by centrifugal force, and the axial flow type (Fig. 12.44D), where the air is pushed along parallel to the axis of the fan spindle. The radial-flow fan is more compact for a given output but tends to be noisy. The axial-flow fan is more consistent, reliable and delivers large quantities of air. Hence, the former is used with small engines, while the latter is preferred for heavy-duty high output engines.
The amount of blower air circulating between the cowling and the cylinders may be regulated by a throttle ring located on the inlet side of the fan. The function of this ring is to vary the effective inlet-passage exposed area of the fan to suit the operating conditions of the
Fig. 12.44. Air-cooling system for a horizontally opposed four-cylinder engine.
engine (Fig. 12.45). This can be automatically achieved by incorporating a thermostat in a hot working region of the engine so that it senses the temperature change. When the engine is cold,
Fig. 12.45. Air-cooled engine with fan discharge control. A. Thermostat opens throttle ring. B. Thermostat closes throttle ring.
the thermostat actuates either a linkage or a hydraulic servo connected to the throttle ring to restrict the air-flow to the fan. When the engine is hot, the restriction is removed, thus permitting more air to circulate. The thermostat provides the same benefits as those used on water-cooled engines and should always be in good working condition.
Advantages of Air Cooling
The major advantages of air cooling are simplicity, light weight and relatively low cost. There is no requirement for water jackets, water pump, hoses, radiator, or coolant. Cylinder head and cylinder castings are less complicated as there is no water jacketing. Since there is no liquid coolant, the problems caused by boiling, freezing, or corrosion are absent. In most cases the only service and maintenance required is to remove airborne debris from the fins, and to keep the blower drive belt, thermostat and damper doors in working condition.
Many air-cooled engines offer better cylinder bore and ring life. Piston, rings and cylinder warm-up occurs much faster because there is less material and liquid mass to warm up. Since the fins do not start transferring heat until they get hot and there is a large AT available, the parts and the oil that is next to them come into normal operating temperature fairly fast.