From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Article from 'THE AUTOCAR' discussing and comparing various self-starting systems, with a focus on the acetylene or explosion starter.
| Identifier | ExFiles\Box 61b\3\ scan0008 | |
| Date | 7th September 1912 | |
| R.R. 235a (150 H) (S.A. 801. 19-2-15) G.{Mr Griffiths - Chief Accountant / Mr Gnapp} 0950. THE AUTOCAR, September 7th, 1912. 425 Self-Starters. With particular reference to the Acetylene or Explosion Starter. By J Dalrymple Bell. THE self-starting systems at present on the market may be tabulated alphabetically as follows : Acetylene, air, electric, and mechanical, and on these recent criticisms and discussions have appeared, but they have been mainly confined to the compressed air and electric systems. Except for the purpose of comparison, this article deals in the main with the acetylene or explosion starter, and at the outset let me emphasise the fact, for the information of those who are ignorant on the subject, or who, for some reason of their own, assume ignorance on the subject, and impart it to others, that acetylene in its dissolved form is perfectly safe and infinitely safer than when it is produced by many generators. Again, as the process of dissolving and compressing is carried into effect in accordance with Home Office regulations, there is no question about the legality of its use. For the purpose of self-starting crude acetylene such as is obtained from the ordinary generator is not suitable. It contains sulphuretted and phosphorated hydrogen; the former burns to sulphuric acid, with consequent corrosive action, and the latter renders acetylene very inflammable, with the result that the explosion is too rapid. As dissolved acetylene is pure, and as it is under pressure, rendered low and constant by a reducing valve, the advantages of its use are obvious. In the use of acetylene for starting there are two principal problems involved. One is to get it into the proper cylinder or cylinders and the other is to ensure its proper mixture with air. The explosive range of air and acetylene is very wide compared to that of petrol vapour and air. The explosive range of acetylene and air is said to be from 3% to 50%; that is to say, a mixture consisting of 2 1/2 parts of acetylene and 97 1/2 parts of air would explode, and a mixture of fifty parts of acetylene and fifty parts of air would also explode, and, of course, any mixture between these limits; but it is by no means the case that any mixture between these limits will do for starting. If the mixture is in the neighbourhood of the upper or lower limits the explosion is not sufficiently powerful. The explosive mixture which gives the most powerful effect is a 12% one, and to obtain successful results a mixture somewhere about this proportion must be used. In American practice neat acetylene is allowed to enter the engine cylinders, and is caused to mix with what air exists therein. In my opinion, which is based on extensive experiment, this system is far from conducive to the best results, because the air in the cylinders of the engine not only varies greatly in quantity— it may be under compression—but it may be to a greater or less degree already carburetted by petrol vapour, and if neat acetylene in an indefinite quantity is allowed to enter a cylinder which already contains almost sufficiently carburetted air, the risk of obtaining a mixture too rich to give sufficiently powerful effects is great. A system which ensures much more certain results is one in which the acetylene is diluted to a certain extent with air before it enters the engine cylinders. This system has several advantages. In the first place it makes use of an explosive mixture, and therefore does not require the special mixing arrangements inside the cylinders which had been found necessary when using neat acetylene in order to ensure the thorough mixing of the gas with the cylinder contents, and in the second place, the use of acetylene being optional, the mixture, if it should be found to be too rich, can be further diluted. In order to get the mixture to the right cylinder, that is, the one whose piston is on the firing stroke, the simplest plan is to have a pump for each cylinder, and as the pumps are small the whole may be combined in a unit and made to form quite a small and neat fitting, and with this pump low pressure acetylene can be used and the amount of mixture be accurately measured and delivered in definite quantities. Obviously the advantages of such a device lie in its extreme simplicity and in the fact that it can be fitted to any car without vital alteration to the framework or engine. The amount of acetylene gas used is extremely small, as may be gathered from the fact that the acetylene supply opening, to the pump, is very slightly larger than the opening in a large size burner, so that the amount of gas used would be rather less than that used in two head lights kept burning during the period of operating the pump—a matter of seconds of course. Dissolved acetylene provides acetylene lighting in its most convenient form, and the same supply can be made to answer the double purpose—and simultaneously—of starting and lighting. It is a poor case in which the necessity of, decrying one's opponents arises, and it would be futile to attempt criticism against the effectiveness of the electric and compressed air starters in view of recent R.A.C. trials, but when the fact is considered that, with modern carburetters and ignition systems, one pull over of the starting handle is in many cases sufficient, the question arises whether the complication and weight involved in these systems is not in the nature of using a steam hammer to drive a tin-tack. It is a good claim for the compressed air system that it gives tyre inflation, but the detachable wheel has reduced roadside requirements in this direction with a consequent reduction in percentage of its advantages. The electric system combines electric lighting and the acetylene system acetylene lighting. In connection with the electric system there is a point I should like to raise, and that is how long the accumulators stand the heavy overload. In a chart published by one of the American papers it was shown that something like half a horse-power was required to turn an engine of medium size at a moderate starting speed. Now, 0.5 h.p. = 373 watts, and for a 24 volt battery to give an output of 373 watts would necessitate a discharge at the rate of 15.5 ampères, a serious demand from a battery the normal rate of discharge from which is two to three ampères. The remaining starting system on the list, the mechanical, is at the present moment most prominent in the form of a spring starter. Given an engine in perfect condition and ready to start on the before-mentioned first pull over, this system should be effective and reliable, but it can claim no pretension to facilitate starting on those occasions, on which the acetylene starter excels, when, owing to some slight starting fault, difficulty is experienced in getting the engine to pick up on its own carburetter. Finally, when it is remembered that most engines are already, under certain conditions, self-starting, does not the obvious solution of the problem seem to lie in the direction of bringing those conditions about in the most simple and direct manner? | ||