Circulation of Cooling Air:
With air cooling, the engine structure is directly cooled by inducing air to flow over its high-temperature surfaces. These are finned to present a greater cooling surface area to the air (Figure 1), which in non-motor-cycle applications is forced to circulate over them by means of a powerful fan (Figures 2 a and b). The car engine structure is almost entirely enclosed by sheet metal ducting, which incorporates a system of baffles. A similar arrangement is used with the diesel engine (Figure 3).
Fig.1 An air-cooled cylinder and head
These baffles ensure that the through flow of air is properly directed over the cooling surfaces of the cylinders and cylinder heads. To maintain uniform temperatures, the air is forced to circulate around the entire circumference of each cylinder and its cylinder head, the direction of flow being along the cooling fins.
These are greatest in number, consistent with providing a sufficient area of flow, on the high-temperature surfaces of the cylinder head in the region of the exhaust valve. The complete system forms what is known as a plenum chamber in which the internal air pressure is higher than that of the atmosphere. Finally, the heated air is discharged from the plenum chamber to the atmosphere, or redirected to heat the car interior.
Fig.2 Air-cooling systems with (a) vertical (b) horizontal bearing axis for impeller
Air-cooling fan assembly:
The forced circulation of air around the engine is generally provided by a centrifugal fan or impeller, which rotates in a spiral shaped housing. This type of fan is capable of overcoming the appreciable resistance offered to the air as it flows around the ducted and finned cylinders and cylinder heads. The fan is driven by a V-belt and pulley system from the engine.
During operation the air enters at the eye of the impeller via an inlet housing, and flows between each pair of blades. Centrifugal force thus acts upon the enclosed air, which is then discharged under pressure in a radial direction (Figure 4). To increase fan efficiency, the impeller blades may be curved backwards instead of being straight. The fan housing is also fitted with an inlet ring to minimize any recirculation of air at the point of entry.
Fig.3 Cut-away view of modern air-cooled diesel engine
Air-cooling throttle valve:
This regulates the quantity of air entering the cooling fan, in accordance with engine cooling requirements. It usually takes the form of a movable throttle ring, which acts as a baffle surface at the entrance to the inlet housing (Refer Figure 4).
Fig.4 Air-cooling fan and throttle value.
At normal running temperatures, the throttle ring is retracted from the eye of the impeller to allow free access for the incoming air. When the engine is cold, the throttle ring is advanced towards the eye of the impeller, thus reducing the quantity of air circulated around the engine.
Automatic operation of the cooling fan throttle valve is effected by a thermostat, which therefore serves to regulate the rate of engine cooling. It is linked mechanically through a simple leverage system to the throttle valve, so that adequate movement is transmitted to the ring within the available working stroke of the unit. The thermostat is generally installed in the hot air duct leading from the lower part of the plenum chamber (Figure 5).
Fig.5 Air-cooling thermostat.