AIR COOLING FOR COMMERCIAL VEHICLES?
Page 25
Page 26
If you've noticed an error in this article please click here to report it so we can fix it.
Important Potential Advantages and Possibilities Discussed. New Lines of Progress. Interesting Developments.
IT IS NOT so very many Years ago that the idea of dispensing with the water-cooling systemon an internal-combustion engine of the type that is normally fitted to commercial vehicles would have been laughed to scorn, for it is only recently that the, real possibilities of direct air cooling on large units have been given serious consideration.
Before discussing the probabilities of any developrne,nt being effected •in this direction it is well to consider what advantages a change from the con • ventional system may hold-out. In the • • first place it may be stated that all internal-combustion engines ar.) eventually air-cooled ; that is to say,
• the heat of combustion in the cylinder heads is first of all transmitted to the water in the cylinder 'jacketssurrounding them, and as then transported, not by conduction, but by motion of the water 'itself, 'into the radiator. . Then 'that water, passing through the tubes of the radiator, dissipates. the heat into the current of air which is flowing around the outer surfaces 'of the tubes.
Water in the cooling system,. there fore, is only a m m ediu for the convey ance of heat, as it were, and when one conies to consider the question this machine is in itself responsible fox a great deal of complication.
In the first place, we have a radiator which is expensive to make, delicate, and usually situated in a position where it is vulnerable ; there is the weight of the water carried—although this is not very important where commercial vehicles are concerned—usually there is some form of pump, with its attendant glands, which require periodical attention, .and last, but not least, we have; the water itself, which is liable to freeze in winter ; incidentally, it is the only part of a modern automobile 'rwhich susceptible to cold—and, on the other hand, if the engine is slightly off trine, it may boil away, thus necessitating stops for replenishment and consequent waste of time. The ideal commercial vehicle is one that requires a minimum amount of attention and never calls a halt at the roadside.
Therefore, if an air-cooled engine can be made to give the same performance as a water-cooled unit, and if its productibn cost is the same, there is a great deal to be said for the use of air-cooled engines on commercial vehicles.
In the main, the criticism that has been directed against air cooling is that it is very liable to cause "pinking" and necessitate frequent decarbonization. Its use on comparatively, slow-moving vehicles would not be contemplated for one moment if the old sys tem of relying on the artificial draught induced by the passage of a vehicle through the air were relied upon to dissipate the heat generated in the combustion heads, but this system has long since been.superseded, and modern progress. in medium-sized automobile engines suggests that there are distinct possibilities of air cooling.being used on engines of up to 40 h.p. or 60 h.p.
Two chief difficulties in design have had to be tackled ; one of these is that uneven distribution of 'metal in the cylinder head on any air-cooled engine produces hot-spots that are liable to cause warping, with resultant compression leaks and the possibility of pre-ignition occurring. .A better knowledge of the desiderata to be obtained in combustion,head design has orvercome this difficulty to a very
great extent. The next question was of quickly conducting the heat away from the cylinder walls. Cast-iron, which is .almost ideal for the bearing surface of a reciprocating piston, has a very low factor of conductivity. Cylinders ,made -of aluminium are expensive and fragile but.a new process has recently been evolved -whereby a cast-iron cylinder barrel can have copper fins electrically amalgamated to it, and, as cooper has a very high factor of conductivity indeed, this at once enables the heat to be taken away with absolute success. 'When we say that an air-cooled engine running on a comptes.sion ratio of 7 to 1. has been tested to maintaie its full load on full throttle for four hours on end, it shows what has been achieved in this direction.
Further, as regards getting rid of the heat horn the cylinder walls, the air-blast system used on the air-cooled Chevrolet—the latest production of
.0\\*
Section of a conventional presentday air-cooled cylinder. Note even distribution of metal.
General Motors, Lta., of America — is Interesting. At the front of the engine is what is known as a SifOCCO blower, which, in plain English, is a highspeed exhauster fan, which sucks the air from the top of a shroud placed over t h e tour cylinders. It exhausts the heated air into the atmosphere, t h e stream being directed un de rneath the floorboards of the car. By this method it is claimed t hat more effective cooling is obtained than would be the case were a fan made to direct an air stream so that ia impinges on to the engine. This last-named system is apt to produce localized cooling, and is by no means so satisfactory as the exhauster method.
A feature that must not be overlooked in connection with air-cooled engines for commercial vehicles is the advance that recently has been made in connection with fuels. Fuel dopes, such as lead tetratyl alcohol, and even benzole, etc., etc., are not only useful on racing 'engines, but, by reason of the fact that they slaw down the speed of flame propagation, they are extremely useful on low-speed commercial vehicle power units in which the familiar sound of conking is more often than not induced by what is known as residual detonation, due to the fact of the cylinder, head being large. It is more than likely that these " dopes " or anti-detonators will be common the world over in the near future ; in. fact, one 'well-known brand of netrol which is being sold in this country already contains a large proportion of anti-de-tenant, which, incidentally, does no increase its price or render it unsuitable commercially.
When discussing this project of air-cooled commercial vehicles, the question of the effect of a large number of lorries or motor coaches in traffic was raised, it being thought that the hot air given off by the engines would be excessive, and that the odours
that usedbe associated with aircooled engnes would render them unsuitable for use in city streets.
The total amount-of heat that has to ledissipated from an air-cooled engine, however, •is precisely the same as that which has to be given out by the radiator of a water-cooled unit, and, as regards odours, the types of aircooled engines that are now being produced are 'certainly not offenders in this respect.
One big advantage that an aircooled unit possesses is that there is no water to boil in the summer, or leads to be cracked by frost in -winter. A radiator that is boiling in summer does not necessarily mean that the engine itself is overheating; it may merely mean that the cooling system is unable to dissipate its heat sufficiently quickly
owing to the difference between the temperature of the radiator tubes and the surrounding air having been diminished by increased climatic heat. -This rise in temperature would not affect an air-cooled unit so greatly, owing to the fact that the variation would be less in proportion to the differences in the tem peratures. To take an extreme case, no water-cool2d engine could possibly work if the atmosphere were at a temperature of 100 degrees C., whereas an aircooled engine could continue to function owing to the fact that its cylinders would still be radiating heat into air at that temperature, their own being considerably higher. Thus, it will be seen that those manufacturers who are out to cater for world trade would do well to consider the possibilities of the aircooled commercial vehicle. There certainly are difficulties in the way, but it would seem that the potential advantages outweigh the probable disadvantages, for which reason we have called attention to the subject. Air-cooling is receiving considerable attention from engineers interested in the production of touring automobiles, and striking advances have been made. If only by reason of the fact that an air-cooled engine
can be expected to reduce fuel costs, its potential possibilities are worthy of investigation, while if it can also tend to success in overseas trade so much the better. Viewed from that aspect, there is no sound reason why air-cooled ,commercial vehicles should not be successful.