Modem Carburettor Development
POPULAR INSTRUMENTS NOW AVAILABLE FOR THE SPORTS CAR OW NER.
II’ is certainly a far cry from the wick and surface carburettors of the early days of motoring to the almost foolproof instruments on the market to-day, but the problems are the same in general as they have always been. The main requirement of all carburettors is that they should, supply to the engine, under all conditions of speed and load, a mixture of air and finely divided fuel in constant proportions, except for slight modification to this proportion under certain special conditions.
The difficulty of solving the problem hasincreased as more has been found out about carburation, for every step is accompanied by parallel steps in engine design, by which increased revs, and acceleration have been obtained. A further minor problem has been introduced by the fact that modern petrol, blended to give power without detonation and also to be sold at a remarkably low price, is not so easily vapourised, as the very volatile spirit obtainable many years ago.
This means that arrangements must be made for preheating the mixture, and also to provide a rich mixture for starting without swamping the cylinder with liquid fuel, to the detriment of the lubricant on the walls.
The requirements of a good instrument are more easily outlined than are the methods of fulfilling them, and the fact that widely differing methods and, devices are used to get the same result does not necessarily mean that one method is better than others.
Carburation is still a problem, like so many in motoring, which has to be solved by a series of compromises, and the ingenuity and resource of designers has resulted in, the efficient and reliable instruments which are in use today.
All carburettors use a jet through which the fuel passes, it being drawn through it by the depression in the choke tube in which it is situated, due to the air passing through this tube on its way to the engine.
The simple carburettor woidd consist of such a jet and venturi, coupled with a device such as a float chamber which maintains a constant level of petroj in the jet.
p Unfortunately, this will not supply a constant proportion of fuel and air owing to the fact that the increase of air Speed and consequently the depression in the choke, does not have a proportionate effect on the fuel in the jet. This is due to the different viscosities of the two fluids, and the fact that one is a liquid and one a gas, and is governed by different laws. . This results in, far too much increase in the supply of fuel with increase in speed glvmg an over rich mixture at high speeds and, too weak at low. Therefore, it is
obvious that the first requirement of the carburettor is some method of compensating automatically for this error, either by introducing extra air at high speed or extra fuel at low speeds or some combination of these methods. This can be done by compensating jets, by variable jets with taper needle control, by valves operated by suction, or again by combinations of these.
The fuel must also be finely divided so that it will be completely evaporated on entering the cylinder, and the control must be such as to give maximum power at full throttle without having a rich and. consequently wasteful setting at ordinary cruising speeds.
On all car engines, and especially those of sports and racing cars, instant response to sudden throttle openings is essential.
When running light, the pressure in, the induction system is very low and the whole of the fuel is evaporated. When the throltle is opened suddenly it will use to nearly atmospheric pressure, and the fuel will no longer be evaporated fully unless heated considerably. This will reduce the volumetric efficiency, and. therefore the remedy is to supply momentarily an over rich-mixture, of which the proportion of fuel deposited owing to the increased pressure, will not unduly weaken the mixture. Conditions will then settle down to the normal full power state when a layer of deposited fuel covers the walls of the induction system. This temporary extra dose of fuel can either be supplied by storing a quantity of fuel when running idle in a small well round the jet to be delivered on opening
the throttle, or by means of a pimp operated by the throttle. It is important to avoid having to maintain an over-rich mixture continuously to give acceleration, as was done in early racing carburettors, as this is wasteful, and the modern instrument is extremely economical in, spite of its excellent performance.
No carburation problem can be considered without reference to the distribution system, and the performance of the best carburettor can easily be maimed by a faulty design of induction pipe. Special inductions system for increasing the perfornaance of some models are now marketed in, conjunction with the carburettor sets, that supplied by V. W. Derrin.gton for the Wolseley Hornet being an exmple.
This instrument differs from the more usual fixed, choke type in that the choke area is varied automatically by a suction operated disc controlling the choke piston. The depression immediately behind the choke space is communicated to the upper side of the suction disc by an opening in the lower part of the choke piston. The lower side of the suction disc is at atmospheric pressure. The piston which rises and falls according to the depression in the choke also operates a tapered needle in the main jet, the shape of which therefore controls the amount of fuel passing for any given depression. Both these features are thus entirely automatic and operate independently of the throttle position, but in relation to the demand of the engine. This instrument is used on many racing and sports cars of today and has well proved the efficiency of its somewhat unusual design.
The model A Stromberg, which is here illustrated., is especially suitable for small high speed engines, while the down, draft which this firm was one of the first to develop has been of great service in getting the utmost performance out of many sports engines.
The chief features of this model are a positive acting, throttle operated, pump, giving an extra supply of fnel for a definite period to aid, acceleration. As this only comes into action on accelerating, it allows a normally economical setting to be used throughout the range.
Incorporated, with the choking device for starting from cold is an automatic relief valve preventing excessive strangling with its d,eleterious effect on the cylinder walls.
The name of Zenith is well known to all motorists, and their instruments have always been noted for sound design and performance. (Continued overleaf)