Faulty Induction Pipes.
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By L. Mantel!.
[An article entitled" Valve Timing and Petrol Consumption," by the present author, which we published in our issue for the 30th of April, evoked, to oar knowledge, so much interest b..ith amongst manufacturers and amongst the rapidly-increasing body of owners who considerably understand the mechanism of their vehicles and desire to increase that understanding, that we hove been prompted to invite Mr. _Monica, who writes so attractively of petrol-engine problems, to investigate for our readers the subject which is entitled above. Alterations to induction passage cross-sections are beyond the capacity of the average owner, but a knowledge of possible short-comings in that direction is certain to be useful to him. Manufacturers and designers will welcome the present article for obvious reasons, we feel sure.—Et).]
In a recent issue of THE COMMERCIAL MOTOR the Editor was good enough to give publication to some of the author's views concerning the relation of valve timing to petrol economy.
As the question of consumption in its strict relationship to power cannot fail to be of interest, perhaps the consideration of a few other factors in economic design may not be unrenninerative.
There is little doubt that one of the errors which is most frequently met with in the design of older types of engines and unfortunately in more than one modern production, is an mdifferentIy4lesigned induction system.
The faults which one may find here are, broadly speaking, two in number, viz : inconsistent area and incorrect cross section.
Another Reason for High Consumption.
The author has already endeavoured to show in what manner careless or unscientific cam cutting is productive of fuel waste' owing to the induction pulsations which result therefrom, and which, for reasons already dealt with, cause deposition of liquid fuel on the interior surface of the induction tract. While correction of the cams will enormously improve the economy of the engine in the majority of instances, there are certain eases in which no amount of cam or carburetter alterations seem to have quite the desired effect. Assuming ignition to be in order and internal frictional losses reduced to a reasonable minimum the trouble can frequently be traced to faulty induction design.
Errors in this department no doubt arise largely from a misconception as to the nature of the substance passing up the induction pipe, which is almost universally termed " gas."
The Mixture Not a Gas.
It is well to understand in the first place that this mixture is by no means "gas.'' It would in fact he scarcely correct even to describe it as vapour, for there is reason to believe that although vapour enters to an extent into its constitution, the moving mass is in reality principally composed of spray, more or less finely divided and held in suspension by the velocity or turbulence of the accompanying column of air in which it is borne.
Could it be truly described as gas or-vapour, then induction and inlet-earn design would be almost nil as economic factors, and the fuel consumption problem which has so long been a thorn in the side of manufacturers would no longer be the bogey which many find it.
What Happens in the Column of Mixture.
Perhaps the best nay to appreciate the type of problem with which one has here to contend is to imagine a section of the induction gases gra
phically set out. Hare it will be seen that in a cylindrical induction pipe the highestvelocity, the " thinnest " mixture, and the nearest approach to a true vapour are to be found in the centre at normal gas speeds. Then, as one considers the imaginary annuli of mixture spreading outwards from the central point, the induction rate progressively decreases, the B16 mixture becomes heavier and more " wet," until at the actual internal walls of the pipe, velocity has fallen oft to comparatively nil, and tile petrol exists virtually as a layer of liquid.
In a sectional front elevation, as it were of the induction column, with the above conditions diagrammatically represented by a curve, this would roughly take a parabolic form at average speeds. As tire induction velocity increases, however, the curve is gradually flattened in the centre, until at very high speeds this portion extends almost across the section and then suddenly falls off as it approaches the walls, or, expressed in other words, the constant-velocity area increases with the induction rate, the atomization and degree of " dryness " following suit in a direct ratio.
The Reason for Adequate Induction Area.
The argument to be drawn here is obvious, a Lulling off in velocity is disadvantageous to economy owing to local deposition of spray, and two fairly conclusive corollaries naturally follow from this.
firstly, that the induction system throughout should be of suitable diameter to give, on one hand the necessary homogeneity of mixture, without, on the other hand being sufficiently constricted to cause wire drawing, that is, to impose upon the engine too great a mechanical effort in effecting the inspiration of its firing charge. Secondly, that the induction system should be of constant area throughout.
Now in these two respects it is fairly safe to asseri; that the majority of engines leave a little to be desired.
Respecting the actual mean area itself, this depends to a large extent upon circumstances. Having in view the broad fact. that homogeneity of mixture and prevention of deposition are the conditions to bc aimed at, the induction value, or current pulsations as effected by valve timing must be considered in combination with gas velocity. Obviously therefore in an engine baying good valve timing a much larger induction area can be given than is the case with one in which induction pulsations are present to an undue extent.
An Engine's Suction Power Decreases as it Wears.
It is, however, better to err in having the tract a little too small than by giving it too great an area.
for, it is well that, whatever may be the induction value of the engine when new, assuming the valves to be well timed, it invariably depreciates after a few months wear, owing to loss of spring ternper, tappet clearance, and other causes which do not generally come within the ken of the average owner of commercial vehicles.
As these factors develop, the induction area, which may have been admirably suited to an originally welltuned valve system, becomes proportionally too large as imperfections set in.
As a Rule Induction Areas Might be Decreased.
There is little doubt that the great majority of vehicles at present on the road would derive a certain amount of economic advantage were the induction system decreased in area throughout.
The average 3-tonner on the market at present, having an engine that develops from 36 L.p. to 40 h.p. at speeds of 1100 to 1400 r.p.m. has an induetion sys
tem of in. to 2 in. diameter or even more. This, for commercial purposes is, in the author's opinion, unnecessarily large.
It, is true that a very open system with constant area, big valves, etc., will frequently improve the top of the power curve in a high-speed engine, but generally at the expense of the middle and lower parts, and almost invariably to the detriment., in a greater or lesser degree, of flexibility at low speeds and fuel economy. And although this open system has its advantages in a certain small proportion of the commetcial-motor output which is used for fireengine purposes and similar designs, where high power over long periods is necessary, the author submits that in the large majority of cases it is quite uncalled for. Flexibility, economy and good pulling at medium speeds are much more desirable features at which to aim.
A Compromise as to the Right Crcss Section.
The reasons for this effect of area upon efficiency will, of course, be tolerably obvious to the majority of engineers. In the whole scheme of induction design with the present system of carburation, it is a question, on the one hand, of getting the necessary amount of gas with the least possible effort on the part of the engine, and, on the other hand, of keeping the imperfectly-vaporized spray in suspension during its passage through the induction system.
The former is palpably a question of freedom—or in other words a large-diameter tract, while the latter is, on the contrary, a case of velocity as produced by a suitably-constructed passage.
It will be seen therefore that—to use ti homely phrase—we are " Between the devil and the deep sea." A compromise is necessary according to the nature of the work the engine is required to do. Academically speaking a variable induction tract is the obvious solution, but mechanically this is, of course, impossible owing to the complications involved in carrying out such a scheme.
Fantastically-shaped Induction Pipes.
Now we conic to the question cf coast ailey of area. It was at one time quite usual to see induction pipcs of the most fantastic shapes, varying from those whidi were provided with a large bulbous enla ifee !tent in the centre of the Y or T-shaped design to others in which the pipe gradually spread out its upper cools to cover perhaps a double port ; and many were the fanciful theories broached in support of the advantmges to he obtained from these little embellishments.
The Fetish of the Bulbous Pipe Junction.
One of the most common—and in the light of modern advancement, most amusing ideas sx as that in some mysterious way the provision of a spherical enlargement at the central joint aided the vaporization end equalized the distribution.
Now although we are far from being entirely
tait with the vagaries of an induction current, the majority of designers know perfectly the fallacy of introducing at any point of the induction pipe a sudden enlargement in area, for the net result of such an experiment shows that all it apparently does is to cause a local reduction in the velocity with the inevitable result—local deposition of petrol—and in this connection it is only fair to add that the majority of four-cylinder induction pipes are of quite reasonable shape, most of them taking the form of a simple T or Y.
Special Designs for Special Purposes.
Much ingenuity has at various times been expended. on induction pipes of special shape to promote constant current direction, and thus a tendency to stir
charge the cylinders. Some of these designs—noticeably the figure-eight types —give a most marked improvement in volumetric efficiency at very high engine speed, and are undoubtedly par excellence for racing purposes, but their benefits only become evident at extreme gas velocities, and it is questionable if they produce any improvement whatever at low and medium gas rates.
It is not here therefore, that one can find food for argument. But, what happens inside the port `‘ Many designers apparently forget that even though this area is within the cylinder casting, hidden from view, and surrounded by a water jacket it is virtually still the induction pipe.
The Induction Pipes in the Cylinder Casting.
The impression which is apparently abroad is that once the gas has been "nursed" up to the valve port the heat of the surroundings does away with the necessity for bestowing any further attention upon maintenance of its velocity. Hence, the space left between the port and the valve is frequently a straggling and indeterminate area of considerable capacity, and generally dimensioned so as to accommodate itself to the other departments in the casting rather than with any idea of treating it as an induction pipe.
When the Mixture Gets Inside.
it is quite likely that in the early days of .680 spirit the heat of the cylinder would have been ample to prevent deposition even in the presence of the whirls, eddies and dead areas resulting from an irregular ante, chamber of no particular shape, and having a mean diameter much greater than that of the external pipe. But it is very sure that the spray from modern .760 and .780 petrol cannot be treated in this manner. Firstly because it is too short. a time in this area to get enough heat by convection or surface contact, and secondly, because, even if it were thoroughly warmed up, only certain of the lighter parts of the fuel would become vaporized ; the rest would simply deposit or pass into the cylinder in such a " wet" state as to bo useless for combustion.
The Only Way is to Heat the Mixture.
When one ha.s to deal with an engine designed like this, there is usually only one cure, that of strongly heating the induction gases by applying extra hot air at the main carburetter intake. Theoretically this tends to reduce power owing to rarefaction of the charge due to heat expansion, and such is, in fact, the case with a normal engine, but under conditions like the above the improvement effected by preventing deposition and stratification more than balances any loss from rarefaction.
It is also necessary to increase by a fair amount the choke area of the carburetter so as to effect the double purpose of providing a higher gas velocity at the defective part and compensate in a measure for the rarefaction by giving the engine a larger volume.
A Symptom which is Readily Diagnosed.
The most prevalent symptom in an engine designed in this way is a pronoueed inclination to fire in the carburetter both when cold and—in bad cases—even after the engine has got thoroughly heated up. This is probably the direct result of stratification or " wet " and " lumpy " mixture as produced both by the conditions dealt with above and by bad valve timing.
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That it proceeds from slow combustion, or a c()/1 (Wiwi equivalent thereto, is, of course, logically obvious, for actual ignition in the carburetter is duc to the incoming charge, during the induction stroke, being fired, as it enters the combustion head, by burning remnants of the last explosion. • The fact or there being any flame whatever left in the cylinder after the conclusion of the exhaust stroke is, of course, proof positive that the development of the explosion must have been abnormally irregular. It
might be perhaps well to add a word of warning here as to the advisability of checking the ignition in cases of this sort.
An explosion lag can readily be produced either by mechanical faults in the magneto or by incorrect timing, and after checking the latter it is a good plan to disconnect the H.T. cable from its plug terminal and arrange a in. spark gap to earth. If now, on racing up the engine, the spark becomes weaker or ceases altogether, magneto shortcomings are immediately ascertained and will generally be found in the setting of the break or in the strength of the contact-breaker spring.
These, however, being in order and a suitable degree of advance provided, induction design or bad valve timing can often be cited as the aggressor. In the foregoing it is assumed throughout that a carburetter of recognized merits is fitted—correctly tuned to the engine and in good order generally, for it is needless to observe that the symptoms can be almost exactly reproduced by incorrect adjustment or defective carburetter design.
It is Not Always the Fault of the Magneto or the Carburetter.
Granted therefore that these points are in order, the intelligent examination of a confirmed incurable" on the lines suggested in the present article may do much towards elucidating troubles which have baffled the owner or repairers, owing to their field of investi_ gation having been limited by the assumption that the magneto or carburetter must necessarily provide the cause.