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AS AN ESSENTIAL NEED DESPITE the great progress that has been made by the automobile industry and, in particular, by the commercialvehicle section, It cannot be overlooked that the efficiency of combustion in internal-combustion engines, whether of the carburetter type or of the injection type, is far from being so satisfactory a proposition as it ought to be.
There are two major considerations which demand an improvement In this connection. First, there is the economic aspect. Transport users to-day are expending a considerable sum of money without any return—i.e., fuel is being burned to waste. Second, there is the question of the pollution of the atmosphere. The method which will solve one of these problems will also solve the other. Naturally, the first point is the more important one, particularly at the present time, when financial considerations are uppermost in everyone's mind.
Dealing with the pollution of the atmosphere in the first case, fuels consisting of carbon and hydrogen do not at once split up into their final products—viz., carbon-dioxide and steam—during combustion in the cylinder ; the process is one involving the formation of a number of intermediate substances which are capable of retarding the entire process. It is during this action that the dangerous carbon-monoxide is evolved. It is colourless and odourless, hence two of the factors which endow it with such potency —its effects are felt before either the senses of sight or smell can warn those in danger.
Carbon Monoxide Dangers.
It is interesting to note the results of research work carried out by the Prussian State Institute for the Hygiene of Water, Soil and Air. The maximum permissible percentage of carbon-monoxide in the atmosphere is .02. If air be breathed which contains a percentage of .05 there will be injury to health, whilst respiration in an atmosphere containing .037 per cent, of carbonmonoxide over a period of two hours usually involves death.
Investigations by the same B24 authority show the carbon-monoxide content of exhaust gases to average 4.3 per cent.; in addition, there are other combustible components present in the exhaust. If it be assumed that the average motor vehicle in London consumes 1 gallon of petrol per day, and that there are 170,000 vehicles in service, the yearly volume of poisonous gas generated in this way is computed to he 2,980,000,000 cubic feet.
Turning now to the economic side of the question, there is, on the average, a waste of something like 50 per cent, of the calorific value of the fuel. Working on the foregoing basis of the number of vehicles in service in the Metropolis, and assuming petrol to cost is. 4id. per gallon, the yearly cost of the inefficiency of present-day standards of combustion is at least £3,200,000.
These figures, which are essentially of an approximate nature, show in a definite manner that, in the interests of industrial progress, further intensive research must be pursued in connection with combustion, whether in petrol engines or in oil engines; the latter, of course, have a higher thermal efficiency than the former, but the need for improvement is but little less marked.
For some time such research work has been proceeding in Germany, and, of course, in this and other countries. We have received details of an interesting German method of investigation of the mixing process of combustible oil and air. Its use enables a photographic record to be obtained. In the form in which it is illustrated in these pages it is suitable for investigations in connection with compression-ignition engines, but in a somewhat modified form it can be employed for carburetter motors.
Reference to the illustration inquestion and to the following explanation shows the method of operation. A cast-steel casing (a) replaces the cylinder of the engine. Each side wall consists of a stout piece of optical glass. An injection valve (b) feeds the combustible fuel into the apparatus, whilst there is a safety valve (c) to prevent an excessive rise of pressure. A spark gap (d) produces short-period lighting for the photographic exposure. This light is collected by a parabolic mirror (e) and passed through the transparent walls of the test cham her, finally reaching the electrically driven cinema camera (f).
Any opaque or semi-transparent object within the test chamber will cast its shadow upon the sensitized surface of the film and so produce a photographic record. The entire apparatus works in a dark room, as it is not possible to provide a mechanical shutter to deal with the required frequency of exposure.
Valuable Data Obtained.
Use of this equipment has enabled detailed investigation to be made of the degree of atomization and the penetration power of fuel jets in compressed air.
By use of what is described as a "glycerine cup," it is possible to catch part of the injected fuel and thus to ascertain the size of the drops. We reproduce four photographs showing the variation which occurs in the size of the drops when injection takes place at different pressures.
In the case of the four examples photographed the pressures used were 2.22 tons, 1.78 tons, 1.4 tons and .953 ton per sq. in. In all cases the reflective force in the chamber amounted to .064 ton per sq. in. It is deduced from these results that the quality of combustion absciutely depends upon the size of the drops. It must be remembered that there is available only a very short period in which to subject to the chemical reaction process all the molecules of a drop ; consequently, the finer the degree of atomization the greater will bra the amount of heat extracted from the fuel present and the more work clone per unit of fuel.
A section of the film showing the process of injection can, by a suit-able calibration, be employed to demonstrate the comparative effects as regards different degrees of penetration. The use of such apparatus, coupled with an experimental engine in which the results with the photographic set can be put to practical test, reveals the definite connection between injection experiments and power outputs. By a suitable modification the injection apparatus may be replaced by appropriate connections for carburetted mixture.
We understand that this method, involving the use of the cinema film, was introduced by Dr. Sass. It has enabled important advances to be made, but, of course, the possibilities are by no means exhausted.