Valve Timing and Petrol Consumption.
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By Mantel!.
[Editorial Note.—Whilst this journal primarily caters fa; the users of commercial motors of all kinds, the relative interests of manufacturers and repairers are by no means disregarded. The present article is particularly important for the manufacturer of petrol vehicles. II should also prove of interest to that growing body of users which is capable of taking a highly intelligent view of the technical problems involved in the plant they employ. It is our opinion that so much misapprehension exists as to the causes of occasional bad consumption and as to the true meaning of correct valve setting, that we are glad to ,g,e4.publicity to the following exposition of much
thought on this subject, which will be read and, we trust, found interesting, by a large body of our readers.]
The relation which the operation known as valve timing bears to the development of power in an internahcoinbustion engine is a subject which has from time to time been ably and thoroughly discussed in the technical Press by the various leviathans of the racing world ; but its significance as an economic factor scarcely seems to have received the recognition from designers which its importance certainly merits_ It is a subject of exceptional importance to commercial-vehicle owners and builders.
Valve-letting Differences Have Little Effect on
Power.
This oversight possibly arises from the fact that careful investigations have shown that, within certain limits, there is very little difference in resultant power between comparatively divergent valve settings, and the impression has conceivably been gathered from this that its relation to economy is likewise a negligible quantity. It is the intention, therefore, of the writer of the present article to show the manner in which consumption is at the mercy of valve timing and to detail as comprehensively as possible the physical and chemical laws bearing upon the subject.
Let us consider, first of all, the general scheme of timing in an ordinary four-stroke motor. The inlet valve opens at a point shortly after the commencement of the induction stroke, and closes at an apparently indeterminate distance past the bottom ; while the exhaust valve opens at a point eufficiently in advance of the termination of the firing stroke to ensure against cushioning on the succeeding exhaust cycle, and closes at, approximately, top centre.
The Widely-different Practice of Various Makers.
It is fairly evident from the nature of the valve timing on many commercial vehicles at present on the market that designers are under the impression that there is more ambiguity attached to the question than is actually the case, for they differ from each other by an incredible amount in some cases. For the bad economical results produced by certain flagrant departures the carburetter is usually blamed. The abortive attempts to remedy matters by means of carburation modifications appear on the surface, therefore, to be an argument to the discredit of carburetter designers, whereas, in actuality, the boot is frequently on the other foot.
In order to appreciate the position from all its standpoints, it might be well to deal in detail with the chemical and physical happenings which accompany the movements of the valves, and, for convenience, the exhaust might, perhaps, be taken first.
The primary functions of this valve are, of course. to clear the cylinder of exhaust gases during the cycle succeeding the firing stroke' but the exercise of a certain amount of finesse in the timing makes a considerable difference in the extent to which this is carried out.
The better to understand the nature of this refinement and its raison d'etre let us consider the chemi• cal laws hearing upon it.
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Carbon Dioxide Produced Retards Flame Propagation.
The chief products of the combustion of hydro. carbons are water vapour, nitrogen and carbon dioxide. The two first mentioned we can dismiss as of comparatively little importance, but not so the last one. It may not generally be known that this gas is probably the most powerful anti-combustion agent known to modern science ; the presence, there, fore, of quite small quantities in the firing charge will slow the flame rate to a considerable extent, hence the reason for getting rid of it as thoroughly as possible. During the exhaust stroke the ascending piston expels as much of -this burnt charge as is within its sphere of action, but when it comes to rest at the top of the stroke there are still the contents of the combustion head to be ejected. To close tha valve here, therefore, means the imprisonment of a considerable proportion of burnt gas, which contaminates the incoming charge to the detriment of the explosive efficiency.
The Suction Effect of the Exhaust.
In order to get rid, therefore, of the remnants of this carbon dioxide—or as much of it as is possible— advantage is taken of the exhaust inertia. Exactly to comprehend the manner in which this operates, it is necessary to picture the column of exhaust gases moving with explosive velocity down the exhaust pipe, impelled by the residual pressure in the cylinder and by the action of the ascending piston.
When the initial influences which started the movement of this column of gas have ceased, viz., at the top centre, the impetus or momentum of the moving mass of gas immediately reacts upon the combustion head like an extractor pump, and, in a suitably-designed system, will actually produce a vacuum of several pounds.
Setting for Exhaust-valve Lag is Necessary. It will be seen from this that a certain degree of valve lag is absolutely necessary, and it is a noticeable fact that engines which do not have it always require a richer mixture to compensate for the residual carbon dioxide. It must be here impressed that the presence of adequate exhaust springs is assumed, for, i obviously, n an undersprung valve the natural la,s consequent upon valve inertia may quite easily represent a considerable number of degrees of the cralA circle, and, although it may give fairly good results at a certain speed where the inertia lag synchronizes with the correct degree of mechanical retardation. it will rapidly impose an efficiency limit as the revolutions mount up beyond the critical point. Thus are repairers, and even manufacturers, frequently confused by symptoms which are apparently contradictory to the above principles of auto-extraction, and they will often be heard to express the opinion that the top centre is the best point to close the exhaust valve.
What actually happens is, of course, obvious ; they employ a large valve with a light spring, and possibly design the cam with abrupt faces. The result is that the valve automatically provides the lag which should be present by design and setting—an exceedingly doubtful principle, for the reason that the closing point in such a case is very variable, and may be anywhere between top centre and 30 degrees late, twein ding to engine speed.
A Strong Exhaust Spring and Correct Setting Necessary.
Such an engine is bound to be erratic in respect of economy, and will generally be found best at a certain definite speed. A cam, therefore, designed to close the valve from 10 degrees to le degrees late. according to conditions, with a spring sufficiently strong to ensure that the tappet will follow the cam absolutely at all speeds, is unquestionably the better method, and will provide a more regular consumption curve.
It 'night, perhaps, be well to consider at this point the conditions governing the opening of the inlet valve, for the question of overlap with the exhaust seems to be an exceedingly vexed one,designers being prone to formulate theories based upon trial and error investigations on one type of engine.
Open the Inlet Before the Exhaust Closes.
At first glance it might appear that opening the inlet before the exhaust closes must, be incorrect, but in view of the minus pressures which momentarily obLIM for a few degrees beyond the top centre, it is obvious that reversal of direction of the gas—which surface reasoning naturally anticipates --cannot occur, and the action of relieving the inertia in the induction pipe is, on mature :consideration, a perfectly sound principle, a.nd one which is certainly conducive to good scavenging if carried to the correct degree. It becomes, therefore, a question of how much overlap is advisable.
The Consideration of Combustion-head Scavenging.
here, now, we get up against a variety of conflicting factors ; first of all the design of the combustion head must be considered. If it be of such a nature that the inlet gases can sweep the cylinder before reaching the exhaust. valve, a• larger overlap can be given than is the case with certain types of L-headed engines, in which the gas merely conies out of one valve and goes down the other. This, of course, is simply waste without scavenging. Again, engines which have a very clear and straight exhaust way, with long exhaust pipe and free silencer situated at its farther end, will give a higher degree of negative pressure than a carelessly-dimensioned and designed system, and in such an engine a. smaller overlap is evidently required. Thirdly, we have to consider the height of the exhaust cam, the weight of the valve, the strength of the spring, and the speed of the engine, in order to arrive at an approximate estimation of the degree of automatic closing lag, if, indeed, it be present at all, for, palpably, this will affect the overlap.
The Need for Exhaust Lag and Inlet Overlap.
To sum up, therefore, a T-headed engine, with tangential exhaust cams of moderate lift and strong springs, presents the typical conditions suitable for a few degrees of overlap. Conversely, an L-headed engine, having its exhaust cams, valves, and springs of such a design as is likely to produce bounce or lag is probably better without overlap. It must not be thought that this is a mere example of academic hairsplitting : the exhaust lag and inlet overlap constitute in reality the most potent factors in economy, and a variation of very few degrees can make an astonishing difference in the strength of mixture. necessary for running the engine satisfactorily. Before leaving this question of automatic scavenging, it may be well to consider the action thereon of the exhaust-opening point, This is a matter upon which it is difficult to give any very accurate figures, for it is one of those factors, the effect of which must be estimated by deduction in the absence of any reliable mechanical or chemical means of indication.
The opening of this valve is usually calculated with a view to exhaust clearance solely, and its effect as a scavenging agent upon other cylinders seldom appears to be taken seriously into consideration.
The Effect of Exhaust Suction in Other Cylinders.
As above stated, it is difficult to say exactly what effect this has upon scavenging other cylinders than its own, but there is reason to believe that a lateopening point—provided it does not cushion the piston during the succeeding cycle—is conducive to good scavenging, for the reason that, when the piston of the exhausting cylinder in question is at the end of its firing stroke, there is anothee. piston which has just completed its exhaust stroke and is at top centre. Tr the pipes, therefore, lead from each port into a common pipe by gradual curves, it would seem reasonable that the momentum of the gases in the latter must have an inter-extractor action upon the piston at the top of the stroke to a degree which varies with the speed of the gases, or, in other words, with the lateness of the opening point of the previous cylinder in order of firing.
Apart entirely from the speculative aspect of the question, however, there is little doubt that, in the majority of engines, the later one can open the exhaust without causing actual back pressure, the better is the economic result, and it is difficult to assign this benefit to any -cause other than the above, for the effect upon driving torque after 45 degrees before bottom is negligible.
The Problem of Inlet Closing.
The only remaining point now for consideration is the closing of the inlet. Here we have an entirely different set of conditions to contend with. The problems that, up to the present, have been discussed are, primarily, of achemical nature ; but the remaining one is purely physical. Consider, now, the progress of the induction stroke. Theoretically, the cylinder has received its quantum of gas at the bottom of the stroke, which would thus appear to be the psychological moment to close the valve.
In actuality, however, it is by no means full ; in fact, it might surprise many people to know that in a very high-speed engine with comparatively small valves it is not much more than half full at this point when turning at a high rate of revolution.
In order, therefore, to permit of a more complete inspiration, it becomes necessary to retard the valve an appreciable distance past the bottom centre ; but how much to retardit is a matter requiring great judgment, for, if closed too late, the ascending piston will cause a set-back in the induction current, and if closed too soon another species of pulsation is set up, due to the rebound of the gas. In the former -case considerable waste is occasioned by ejection of the spray, while in the latter, although the loss is much less pronounced, there is a noticeable falling-off in economy, due to the disturbance produced in the air and petrol columns by pulsations.
The Relative Inertia of Petrol and Air.
It has been customary to estimate the relationship of air and petrol on a basis of velocity solely ; but the extremely contradictory results obtained thereby have convinced investigators that othec factors enter into the case, and these are now believed to be induction pulsations, as produced by either valve timing or design of the exhaust system. To comprehend the nature and effect of these pulsations, it is only necessary to picture the general arrangement of 'the carburatinF device and induction system. Here we have two distinct substances of different natures and weights, a column of air in a pipe, and a column of petrol in a jet or jets. The former, being light and, therefore, having little inertia, will respond readily to pulsations ; but the Cl petrol, on the other hand, being a body of infinitely greater weight and, therefore, comparatively great inertia, will not respond so readily. It is, accordingly, easy to see, in view of this difference, that, while violent fluctuations have very little effect upon the amount of air which enters the cylinders, the efflux of petrol, on the other hand, is influenced in inverse proportion to the degree in which current vibration is present. It might, possibly, appear from this that an increase in the jet orifice is all that is needed to compensate for the restriction in output due to the above condition ; but, in actuality, this is not so, for practical investigation shows that, as the rhythm becomes more pronounced the column impulses, gradually lose their synchronism—that of the petrol lagging behind that of the air. As this progresses it obviously presents increasingly unfavourable conditions for spraying. and whereas, with well-timed valves, the impulses are fairly synchronic and the petrol well atomized, the reverse tends to produce a very " wet " mixture which readily deposits, is carried into the cylinders in a liquid state, and is eventually ejected unburnt at the exhaust.
There. is a popular idea that the nature of the spray itself has a very considerable bearing upon -atomization ; but the author has experimented at some lent,..th with different spraying devices and finds that, at high speeds, there is practically no difference between a simple jet and the most elaborate spraying arrangement. At low speeds, where the velocity past the jet is small, there is certainly a slight difference, but it is quite insignificant compared with that produced by the variations in the closing time of the inlet.
And now as to the actual figure of closing lag, this, like all the other points, depends upon the usual conditions, size of the valve, height of the lift, etc. ; in short, upon all those factors which govern the rate of induction. In an engine having large valves, high lift and low induction vacuum, it might come as early as 15 degrees for ordinary commercial speeds, while for an engine the reverse of above, having a low inductive capacity, it might possibly ascend to 35 degrees with economic advantage.
In conclusion, the author would like to impress that, in offering these views, the subject is merely touched upon in a very general manner, and he would point oat the impossibility of dealing in limited space with the various side issues and special conditions which must necessarily present themselves in a discussion of this sort. Also, it is well to recollect that, when the carburetter has been adjusted to suit a certain induction valve, any appreciable variation of this by retiming must be followed by a suitable alteration of the mixture to meet the new requirements, otherwise the immediate effect of correcting the opening and closing points of the valves may be increased fuel consumption, and, in the author's opinion, the tardiness of many manufacturers in recognizing the significance of valve tinting as a factor in economy is mainly due to their failure to appreciate the importance of accurately balancing the one with the other after every experimental alteration.