Increasing Engine Efficiency by Aluminium Spraying
Page 49
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ANEW combustion-head spraying process for obtaining higher thermal and economic efficiency has just been released after considerable investigation and development by Mr. H. Weslake, of Twickenham. He has now completed arrangements with Metal Sprayers, Ltd., Air Ducts Buildings, Great West Road, Brentford, to install the necessary plant for the treatment of combustion heads by this method. The cost is trifling, considering the marked benefits it confers in respect of power, smoothness and especially economy.
Broadly speaking, the process consists of spraying a thin layer of an aluminum-base alloy on to the prepared internal surface of the combustion head.
When the benefits to be derived from this form of spraying were first discovered, the impression gained was that the action was thermally identical with that of an aluminium bead. This last-named, in certain circumstances, can be very beneficial to power, through dispersing and distributing the heat.
Whereas, however, solid aluminium heads show an efficiency gain only under conditions of high loading or in engines which are prone to detonation, and, if used where light loads are the average, frequently cause a loss through too rapid heat dispersal, the same metal sprayed on to a cast-iron surface produces a marked improvement at all ordinary loads, compressions, and temperatures.
It was, therefore, evident that the increased efficiency was not explainable on a heat-conductivity or dispersal basis. Furthermore, experiment showed that results were equally as good with a thin as with a thick layer of metal.
Defining Waste Heat.
The problems of what to do with waste heat, and exactly what constitutes waste heat have long been among the principal ones with which those in research circles have had to deal. Until now it has probably been correct to describe heat as " waste " directly it penetrates the surface of the containing chamber, always provided that this surface has already arrived at the critical temperature at which any lower degree would withdraw heat unnecessarily from the burning charge and anything higher would promote detonation.
This critical temperature is one that is very difficult to arrive at. It is undoubtedly above the boiling point of water, but below the combustion-head temperature generally reached by aircoolIt'g. Furthermore, it is not a fixed quantity, but varies according to the distribution of heat in the head. The point to note, however, is that any heat which penetrates below the surface tends to become cumulative and thus leads to detonation. It must, therefore, be led away.
AU forms of cooling have for their objective the removal or spreading of only waste heat, and are not intended to reduce the combustion temperature of the charge itself. The most useful combustion is always one which shows the greatest temperature difference between the degree of heat registered immediately prior to the passage of the E park and that attained after ignition.
Again, as driving effort is a direct measure of temporary heat expansion, the greater this is, up to a high limit, the more dynamically effective is the combustion, if without detonation.
The main points of thermo-dynamic necessity, therefore, are that, whilst every means shall be taken to keep the surrounds down to the required "critical," as defined above, it is desirable that the temperature of the combustion itself shall, on the contrary, be as high as possible, consistent with the maintenance of propellant burning.
Direct removal and dispersal of waste heat has been the onlY recognized method of coping with it. Conservation of explosion heat has received no attention at all, Other than by Making the interior smooth and shaping the head for the minimum exposure of absorbing surface.
Whilst aluminium, by virtue of its high degree of conductivity, conforms well with conventional methods of dispersing waste heat, it has two more properties, which are defeated by this conductive capacity when the metal is used as a solid casting, but which become active when the metal forms only a thin coating on the less-conductive cast-iron.
Energy Loss Minimized.
These properties are beat reflectivity and heat radiation, and they fit in admirably with the local requirements in that they keep the exposed surfaces cool. Instead of being conducted to the nearest water-way and finally lost, the heat is partly reflected back into the charge again, where it is of service, whilst any of it that passes through the thin layer into the mass of iron below, has still an avenue of return by radiation.
To what relative extent these qualities give the sprayed layer its curious potency is a matter receiving much attention. That they both enter into it seems tolerably certain, for the first impression received from the running of an engine with a newly coated head is one of extremely rough vigour, even when quite cool. As carbon collects, however, part, at least, of the reflectivity is interfered with, and the aggressive roughness gives place to smooth power. This is evidently due to the increase of radiation which is less affected by the carbon coating and which tends to replace reflectivity, as the decrease of the latter allows more heat to penetrate.
Further developments are being watched with great interest, but the efficiency. improvements obtained already are so marked, especially in consumption improvement, and the process so cheaply and quickly carried out, that their immediate availability to the public is warranted, and it is understood that Metal Sprayers, Ltd., is equipped to undertake the treatment.