Fig. 127.-Simple Air-Cooled Engine With Radiating Flanges Cast Integral With Cylinders and Air Blast for Cooling From Friction Driven Fans.

Q. Will air absorb heat as readily as water?

A. The heat absorbing power of air is very much less than that of a similar volume of liquid such as water or oil.

Q. What is necessary in order to cool successfully by air? A In order to compensate for the lessened power of heat absorption possessed by air it is necessary to provide more radiating surface from which the heat can be taken and also to use rapidly moving currents of air passing over the augmented surface.

Q. Can an engine cylinder be cooled too much?

A. It is possible to cool a gas engine cylinder to such a point that the efficiency of the motor will be greatly reduced and cause it to lose power.

Q. What is the permissible temperature of an air-cooled cylinder?

A. Theoretically a cylinder may become heated to the point where the cylinder oil is not affected unduly by the heat, but in actual practice owing to the facility with which metals may become distorted by expansion due to heat it is not recommended that temperatures exceeding 325 to 350 degrees F. be exceeded.

Q. What is the temperature of a water cooled cylinder?

A. The temperature of a water cooled cylinder must a ways remain below 212 degrees F. or the boiling point of water because if the cylinder is allowed to become hotter than this it will produce boiling and rapid evaporation of the cooling liquid.

Q. What percentage of heat is lost through the cooling system?

A. The amount of heat lost through the jacket water of the average internal combustion cylinder is given a different value by various authorities, this ranging from 40 to 55% of the total heat generated by the combustion of the gas in the cylinders.

Q. Is it desirable to reduce this heat loss?

A. If it is possible to reduce the loss of heat units one will obtain greater efficiency from a given amount of fuel because more heat is turned into power. As a rule the loss with an air cooling system is considerably less, and air-cooled engines are more efficient than the water-cooled forms.

Q. How many methods of air cooling are used?

A. There are two methods of air cooling, one depending on the radiation of heat from a largely increased surface alone, the other by convection and radiation, this involving passing a current of air at high velocity over the radiating surfaces.

Q. Describe simple radiating systems.

A. A simple radiation system is used only in some forms of stationary engines of small power. To cool by radiation it is merely

necessary to increase the external area of the cylinder by any suitable means. The air immediately surrounding the cylinder is heated by contact with it and as it is lighter than cool air it rises and its place is taken by cooler, heavier air which in turn becomes heated and makes way for more air at a lower temperature than that surrounding the cylinder. As should be expected cooling by this method is too sluggish for general use, so the combined radiation

and convection systems are generally employed in motor vehicle service.

Q. Describe air cooling by radiation and convection.

A. A simple single cylinder air cooling engine in which the heat absorption is by radiation and convection is outlined at Fig. 127. In this power plant the cylinder is provided with radiating flanges, which are swept by air blasts set up by cooling fans placed at each side of the cylinder and driven by frictional contact with the

flywheel rim. The more rapid passage of the air currents produces a better cooling effect than would be obtained by the simple radiation method. On some types of fast moving vehicles where the engine is exposed, as in motorcycle practice, it is not necessary to provide a fan for producing the air blast because these are automatically produced by the rapid movement of the cylinder through the air.

Q. What are the usual methods of increasing the effective radiating surface?

A. The cylinder is generally provided with flanges which may be

cast integral or which may be made of some metal that is a better conductor of heat than cast iron and forced in place around the cylinder by shrinking them against the smoothly machined exterior. Sometimes the cylinder is provided with inserted pins or with projecting spines cast integrally. Other forms have been made in which metal combs have been fitted to the cylinder either by peening these in grooves milled longitudinally on the cylinder walls or by inserting

them in the mould and pouring in the molten metal when the cylinder is cast.

Q. Which is the most efficient method of flange application? A. Owing to the loss of heat at joints the integrally cast flange is generally preferred to applied flanges.

Q. How are air currents produced for cooling cylinders? A. Air currents may be directed against the cooling flanges by means of simple fans, as shown at Fig. 127, or by flywheel suction as shown at Figs. 128 and 129. In some forms of aeroplane power plants, notably the Gnome motor, which has revolving cylinders and which is illustrated at Fig. 130, the cylinders are cooled by rapid rotation through the air.