In this method of cooling, the heat is dissipated directly to the air after being conducted through the cylinder walls. Fins and flanges on the outer surfaces of the cylinders and heads serve to increase the area exposed to the cooling air and so raise the rate of cooling. The basic principle involved in this method is to have a current of air flowing continuously over the heated surface of the engine from where the heat is to be removed. The amount of heat dissipated depends on the following factors:
- The surface area of metal in contact with air.
- Rate of air flow.
- The temperature difference between the heated surface and the air.
- The conductivity of the metal.
For effective air cooling, the surface area of the metal which comes in contact with air is increased by providing fins over the cylinder barrels. More the surface area in contact with air, more the heat is dissipated. Higher the rate of air flow, higher the heat is dissipated. Similarly, higher the temperature difference between the heated surface and the air, higher will be the heat dissipation. A metal having higher conductivity dissipates more amount of heat.
|Fins On Engine Cylinder|
ADVANTAGES OF AIR COOLED ENGINES
- Lighter in weight due to the absence of radiator, cooling jackets, and coolant.
- No topping up the cooling system.
- No leaks to guard against.
- Anti-freeze not required.
- The engine warms up faster than with water cooled designs.
- Can be operated in cold climates where water may freeze.
- Can be used in areas where there is a scarcity of cooling water.
- Less efficient cooling, because the coefficient of heat transfer for air is less than that for water.
- Not easy to maintain even cooling all around the cylinder, distortion of the cylinder may take place.
- More noisy operation.
- Limited use in motorcycles, and scooters where the cylinders are exposed to the air stream.
Fins On Engine Cylinder
The surface area over the cylinder is increased by means of fins. These fins are either cast as an integral part of the cylinder or separate finned barrels are inserted over the cylinder barrel. Sometimes, particularly in aero engines, the fins are machined from the forged cylinder blanks.
As a rule, the fins are usually made of about the cylinder wall thickness at their roots, tapering down to about one-half the root thickness. The length of the fins varies from one-quarter to one-third of the cylinder diameter. The distance between the two fin centers is about one-quarter to one-third of their length. The total length of the finned cylinder barrel is from 1 to 1.5 times the cylinder bore. Another rule based upon experimental considerations is to allow 1400 to 2400 cm of cooling fins area per IP. This gives about the correct cylinder tempera at 50 to 70 km/h airspeed.
Fan cooling is used in larger air-cooled engines, particularly on cars. A fan, having two or four blades, is driven either at the engine or twice the engine speed; and the air-flow is directed on the cylinder heads. The cooling depends chiefly upon the engine speed and not upon the forward speed of the car. The fan usually absorbs about 1 IP for every 15 to 20 BP.
In the case of small single cylinder engines, an excellent arrangement of fan cooling is that a fan of about flywheel diameter is mounted on the main shaft and enclosed in a metal casing so arranged that the air is drawn in at the center and expelled peripherally through a belt mounted duct directing it on to the exhaust side of the cylinder.
In the small, air-cooled engine the blower type fan works quite satisfactorily if suitable guides and ducts are provided for the air streams. The system is also used for larger engines. Each facing the cooling system on the suction side of the fan, more satisfactory cooling effects obtained. Sometimes, the flywheel itself is designed to function as a cooling fan; and the air is discharged backward through it, after having been drawn past the cylinder barrels.
In Fiat and Corvair engines, the quality of cooling air is regulated thermostatically. When the temperature of the air discharged from the cylinder increases above the normal value, the thermostat actuates a large valve, vane or disc in the air outlet duct to allow a greater quantity of air to flow.
MODERN AIR COOLED ENGINES
At present, air cooling is used on engines in scooters, motorcycles, airplanes, combat tanks, small stationary installations and in one model of an American rear-engine car. In Germany, air cooling is used in some petrol and C.I engines including 2,4 and 8 cylinder models.
A good example of the modern air-cooled type is the Krupp four-cylinder opposed compression ignition engine. This has a cooling fan fitted at the front end and is driven by the engine. It forces the cooling air through a casing around the front end of the crankcase and hence to the horizontal cylinder barrels which are ribbed and enclosed in rectangular casings.
Another more recent example is the Krupp eight-cylinder V-type petrol engine, which has a very similar cooling arrangement.
The Volkswagon, Dutch D.A.F Citroen two-cylinder opposed, Chevrolet Corvair six-cylinder horizontally opposed, Fiat 500 D, two-cylinder in-line and N.S.U. two-cylinder are examples of modern air-cooled engines.