The Why of Carburetter Adjustments,
If you've noticed an error in this article please click here to report it so we can fix it.
A Number of Practical Hints and Explanations with Regard to such Operations as "Joggling the Float," "Giving Her More Air," "Enriching the Mixture," etc.
By an Engineer-Correspondent.
The writer, during the past few weeks, has had occasion to question a large number of driers on technical points relating to the functions of the various component parts of a commercial-motor vehicle and of the methods of adjustment of such parts as require that operation from time to time.
In nearly every case drivers gave quite lucid answers as to how they would proceed to adjust the brakes, or clutch, and some of them possessed a very fair knowledge of the methods of adjustment of magnetos. and ignition gear, but when tackled on the question of carburetter adjustment, very many of the drivers, whilst perfectly clear concerning the methods of adjustment of the particular types of carburetters with which their engines were equipped, were absolutely ignorant of the basic principles of carburetter adjustment. The following notes upon the first principles of carburetter adjustment, will, it is therefore presumed, be appreciated. They will be continued, according to current notes.
The Carburetter is a Clever Compromise.
There can be no doubt that. the petrol engine has been very greatly improved in general construction since Uottlieb Daimler introduced his first four-cycle model to the European market., but, it is very doubtful whether the carburetter has been improved at the same rate as have all the other parts which go to make up a complete engine. There have been many changes in the designs of carburetters, and there will be many more before finality is reached ; alterations and " improvements " are frequently announced by makers, and there could be no more interesting study for a young engineer with a taste for experimental research than a careful consideration of the carburetter requirements of engine builders. Perhaps it would not, be incorrect to state that at the present time, there is no such thing as a perfect carburetter ; true, many of them are better than others, and some are so exceptionally good as almost to satisfy all the condit:ons of practical application to motor vehicles.
The Practical Requirements which a Carburetter must Fulfil,
The four principal requiiernents of a carburetter are :—(a) the supply, at all speeds of the engine, of a sufficient quantity of suitable gas to enable the engine to give out its utmost power ; (b) that it shall be economical in use ; (c) that it shall be simple in construction and consist of few parts ; and (d) that the working of its parts shall be as silent as possible.
Many engineers have attempted to meet the first and second of these requirements by making the carburetter " automatic " in its action, but in their endeavours to do so, they have so complicated their designs by the addition of numerous air valves, springs, and other delicate parts, that the noise arising from the working of these parts has precluded the attainment of the fourth requirement.
Relative Rates of Flow of Petrol and Air.
In order to determine the proper act:on of a carburetter one should first learn how petrol and air tlow under the influence of varying degrees of suction. According to authoritative books on the subject the flow of a liquid through a tube is proportional to the difference of pressure that exists at the two ends of the tube, but the flow of air follows a very different law, and this difference is due to the fact that when flowing through an orifice leading from one chamber at a given pressure, to another chamber at a lower pressure, the air expands as it enters the latter chamber, and a portion of the energy which produced the flow is used up during the period of expansion. In other words, the expansion of the air as it leaves the tube from the high-pressure chamber tends to check the flow.
The mathematician would explain the relative rates of flow of petrol and air by stating that the flow of petrol through a tube is proportional to the difference between the absolute pressures at the two ends of the tube, whereas the flow of air is proportional to the square root of the difference between the squares of the absolute pressures which exist at the two ends of the tube.
A carburetter should be so designed that two curves which are drawn to represent the rates of the flow of air and petrol should, under all conditions of running, bear the same relation to each other, in order that the petrol spray may be taken up by, and mixed with, the air so as to form a gas of constant. explosive value. This is not an easy problem and the -larger the engine the more difficult it becomes. The functions of a carburetter are to convert the petrol into gas and to mix it thoroughly with the air so as to deliver to the cylMders a perfectly evenly mixed vapour.
There are Three Broad Types.
The three principal types of carburetters which are in use at the present time are :--(a) the mechanical or hand-regulated type : Os) carburetters that are more or less automatic in their action, and are provided with auxiliary air valves ; and (e) those
...t which the amount of fuel delivered by the jet is controlled either mechanically or by the suction of tne engine, so that the strength of the mixture remains the same for all suction speed. Included in the last-named class are special devices, such as the White and Poppe, the Polyrhoe, the Zenith, the Claudel-Hobson, and other well-known types. The mechanical type, is that in which the air and petrol passages are opened and closed by hand, at the will of the driver ; in this class of carburetter the air and petrol passages remain of the same dimensions until altered by the driver, and the correct mixture, therefore, cannot be assured for more than one piston speed with each setting ; this is a serious limitation.
Automatic Carburetters Preferred as a Rule.
Carburetters, which really belong to the first of the three classes named, do not often appeal to the average owner, although skilled drivers prefer to have the condition of the mixture under their own control. The principles laid down later apply to all types of carburetters ; each maker issues special inetruei ions relating to the adjustment of ivs own particular device, and a driver should make a point of securing a copy of such instructions, for, as has already been stated, many drivers, although experienced in the adjustment of one particular make, are quite at sea when they handle, for the first Lime, a carburetter which differs in details of construction to any that they have seen before ; it is desirable, therefore, to have a fair knowledge of the principles which govern all adjustments.
When an engine refuses to start, or will not accelerate, or will not run at a low speed, it may be taken for granted that the mixture is either too weak or too rich, but in order to determine which of these causes is the correct one, the driver must be guided by other and more subtle symptoms. Of the more pronounced indications of poor mixture, that of over-heating, if not caused by a stoppage in the cooling-water pipes, the slipping of the fan belt, or the failure of the circulating pump, is generally caused by the presence of too much petrol in the mixture. Black smoke emitted from the silencer also indicates an excess of petrol, whilst popping in the carburetter is a sure indication that the mixture is not rich enough, and that in causequence it burns very slowly inside the cylinder, and
the combustion of the charge is not complete before the inlet valve opens for the next admission stroke. Popping was not known in the days when engines were fitted with atmospheric inlet valves.
Starting an Engine.
With any spray carburetter the mixture for starting the engine must be obtained by priming the jet or 'joggling the float," as it is more commonly called ; by doing this either too much, or too little, petrol may be admitted, and the mixture that is tornied during the first few revolutions of the crankshaft may probably be so bad that the engine will not
start. In order to ascertain the real cause of the engine's refusal to start—assuming, of course, that the responsibility really does rest with the carburetter — one should observe whether any petrol drips from the float chamber, after allowing sufficient time for the float chamber to fill after turning on the petrol ; at the first sign of dripping an attempt should be made to start the engine. If it still refuses to start, the carburetter should be "joggled" again and the attempt to start repeated. Should the result still be unsatisfactory, the petrol must be shut off at the tank and some of the petrol drained off, so that when the next attempt to start is made the mixture will be weaker. One or two experiments of this kind will soon demonstrate how much priming is required for any particular make of carburetter. The size of the main air passage has a lot to do with the amount of joggling" that is required in order to start from cold ; when the engine is warm the required amount of " joggling" is very small.
Raising or Lowering the Jet.
If the engine starts, but almost immediately slows down, holding down the float should be tried ; if the engine responds to this action by immediately accelerating, a weak mixture evidently was the cause of the trouble, and the driver should at once adjust
the float so as to raise the petrol level. As a rule, the level should be not more than 1-16th in. below the tip of the jet. If, instead of accelerating when the float is held down, the engine should stop, then undoubtedly the cause of ii s stopping is due to too much petrol passing the jet, arid correction could also be made for this by adjusting the li-ight of the float so as to lower the level of the petrol in the jet.