THE MOTOR

Evening Post, Volume CV, Issue 124, 26 May 1923, Page 23

THE MOTOR

ECONOMICAL RUNNING

(By 'Autos.")

It',is, remarkable how very few people, appreciate the great importance of good, slow running from the point of view of economy, writes L. Mantell in " The Motor " ■ (London). The general impresEion appears to be that the length of the petrol bill roust of necessity be affected only by the " power" or ordinary adjustment of their carburettor. It will generally be argued that as the slow-running department only functions when the engine is idling, this part may be dismissed as a negligible factor in the economical scheme. It never occurs to them that each time the throttle is closed not only is the slow-running jet brought into operation, but generally in the least advantageous manner, in that the engine is not, a? a rule, idling at all, but is,' on the contrary, being pushed round by the transmission at speeds far in excess of the ordinary idling r.p.m., and thus the 'slow-running delivery orifice, being genela-ly unprovided \vitb any corrective device, is made to supply " through the nose," as it were, during that time. - ' Motorists also seem delightfully oblivious to the fact that in town driving the throttle is frequently more often in the closed than in the open position, and that even on an average country run there are very few routes where it is not necessary to take the foot off the accelerator scores of times each journey, and a very large percentage of the gross distance is done at about one-quarter throttle. _ All this should be patent to anyone who cares to observe intelligently the movements of his or her right foot over a given journey, but analytical observation of this kind is not a strong point •with the average private owner, and so the outwardly innocent slow-running' device carries on its" iniquities all unsuspected, while its big brother in the choke tube has unjust contumely heaped upon it when the m.p.g. is not all it ought to be. Come down to hard facts, however, and see exactly what bearing the slowrunning arrangements hive upon the consumption and to what extent they are adversely affected by maladjustment or. certain defective engine conditions. HOW AIB LEAKS AFFECT CON- ; sumption. The first and most frequent cause of chronic petroj waste is air leakage. It does not matter whether this is due to engine causes or throttle-spindle wear, if there is an appreciable amount of air entering the induction system by illegitimate channels of whatever kind, the trouble commences, and this is the way in which it operates:— Consider-the area of throttle opening jiecessary to make an engine idle pro perly. It is very small indeed, a mere line, in fact, and this little gap passes ft minute volume of air carburetted with a correctly combustible -atio of petrol. Now, remembering i ; at this area of air entrance is of necessity very small, it should be obvious that a comparatively small leakage may easily represent an increase of 50 j>er cent, or 100 per cent. in the gross amount of air passing into the combustion head, therefore—and this is the vital point—the slow-running petrol supply must in such a case, and at that position of the thorttle, be increased by a similar proportion in order to maintain the combustibility of the .mixture. ' . j- HI.V TO DETECT LEAKS .. / That, however, does not matter very greatly in itself, for we are still dealing iVdth very small volumes, .but immediately the throttle is opened a, little the •Original area is enormously increased, £nd whereas the proportion of the leakage area to the legitimate throttle area ?nay originally have been, say, 50 per cent., it. now btcomes, by comparison, quite a negligible increase. Not so the jet, however, for remember that it had to be enriched by exactly the same proportion as the leakage bore to the orig-. inal volume of air, and as that increase remains a constant quantity one is saddled with a permanent over-dose, which sets in immediately the throttle is opened beyond the idling position, and continues throughout the range of operation of the slow-running jet; this varies in different makes of carburettors, but is usually extended to about a quarter or a third throttle. If the carburettor fitted is one of those in which the auxiliary output is in the nature of a fixed jet of definite size it is easy to detect 'from the size necessary in relation to the cubic capacity of the engine whether a leakage is present or not, for, although engines of different characteristics will require vastly different main settings, the auxiliary or slow-running supply follows very closely the cylinder\capacity, regardless of other details of design which come into, play later in the cprve. Motors of 1100 c.c. to 1160 c.c. will, if the induction tract bo light, take an auxiliary setting which consumes! about 10 c.c. or 12 c.c. at an ordinary idling spesd of about 250 r.p.m. An average 15 h.p. car, having an. engine somewhere between two and three 1 litres capacity, should consume, when idling, from 20 c.o. to 24 c.c. per ininute. The limits are tolerably elastio here, for, if the engine and carburettor are free from leakage, the amount used in: this manner is comparatively negligible, but it should increase in exact proportion with the speed of idling when the throttle is gradually opened over the full range of the slow running. If induction leakages as described are in evidence the relative rate of the auxiliary output v/ill increase quite disproportionately with the speed when the gradual opening commences, and this is the direct cause of the waste. It is well-known that to make an old engine "tick over" is generally found to be an expensive matter, and the reason is that there is almost invariably a fair amount of inlet valve stem clearance via which air leaks into the induction system.

The result is that an inordinately rich auxiliary setting is required before the engine will throttle down. Probably the driver does not know that it is rich, for the reason that the majority of carburettors- provide a slow-running supply either via needle-throttled orifices or by means of a fised-jet orifice, with an adjustable suction thereon. In neither of these cases is it possible for an amateur to estimate the actual petrol flow, but where a calibrated jet is employed, and engines which should run on a definite size refuse to idle until this has been considerably increased, leakage may be diagnosed with certainty. TRACING THE TROUBLE. It is well in such cases to be sure that the carburation and not the ignition is at fault, and it is very easy to decidi> upon this by the simple experiment of Blowing down the engine. as much as pqs sible without stopping it and flooding slightly the carburettor. If the engine immediately picks up, the requirement of a larger jet than normal is indicated, and, therefore, leakage proved, but.if the flooding has "no effect one jnay usually suspect the ignition 1. While correction of the leakage and readjustment of. .the., carburettor is of j

course, the soundest way to tackle this trouble when its presence has been established, its effects can be, to a great extent, mitigated in certain carburetters by resetting. . In order to explain exactly how this is done on* must endeavour to picture the general nature of a throttle mixture curve. Most of the cars at present on the road are fitted with open carburet tors having either two jets or what amounts in effect to two jets. One of thase is mounted in a choke tube or in a restricted area of some kind, and tho other is mounted, in such a way that cither a high velocity or high suction is applied to it at low throttle positions, so that when the velocity in the choke tube falls too low to draw upon the main supply the smaller jet comes into aotion and takes care "of the slow running. Now, in the conditions described, the waste occurs, principally because the ranges of action of these two jets overlap each other.. Normally, the auxiliary mixture is supposed graduto weaken off as the main gathers strength upon opening the throttle progressively, and in a correctly adjusted carburetter these respective actions should exactly balance each other, so as \p maintain a perfect mixture throughout the movement. In the case in point, however, our trouble is a local rich area due to a surfeit from 1 the auxiliary department when it should be weakening ofi and • thus overlapping the main supply. As we cannot avoid the rich section on the auxiliary jet-curve, unless by stoppong the.leakage, the other alternative is to retard the entrance of the main output, and this can be effected either by using a larger choke and tuning a jet to it, or, if the carburetter has a compensating jet, a reduction of this in addition to a bigger choke setting, will, if correctly done, go a certain distance towards effecting a cura^ The action of either of these readjustments is to weaken tlie main mixture where the auxiliary supply is overlapping it with a strong one, and thus tends to restore the balance throughout the curve. In the case of a constant-vacuum carburetter the 1 trouble can also be tackled to an extent, not by raising the needle or metering the pin as a whole— which many people would, no doubt, attempt—but by very carefully making a small flat at the neck of the pin just where it coincides with the jet orifice when at the idling position. This re- i quires very careful manipulation, and it is likely that in the hands of an amateut more than one pin will be spoiled before the required effects are obtained, but, with caje, it can be done. THE ONLY SUEE REMEDY. All this, however, is purely in th# nature of a compromise, the only sure way. is to cure the trouble at its source, and m ffiiat connection one is inclined to wonder how long it will be before makers recognise the necessity for pro> viding some'kind of adjustable inlet stem sealer on their engines. To the best of the writer's knowledge there is no such fitment on any standard car. The owner, never realising the real root of his trouble, suspects his carburetter in nine cases out of ten, increases the output of the slow-running arrangements to get easier starting and good idling. Down goes the mileage per gallon, and the poor- carburetter gets all | the blame.

One of the most interesting entries for the forthcoming .French Grand Prix, the premier motor-car road race contest of Europe, is that of a 12-cylinder "Delage." As the event is limited to cars not exceeding a cubic capacity of 2000 C.C. (2 litres), the designer of this car has been restricted to a very small bore in his .12 cylinder "Delage," the bore being only 42 millimetres with a stroke of 90 millimetres. Fancy a bore of slightly over If inches! What a difference to the sin and 6in bores 'used in racing cars some 12 years back. . The adoption of a twelve-cylinder engine fpr racing purposes is really a confirmation of the present practice towards small bore high-speed engines. For ordinary use, under the present status of automobile engineering, M. Delage considers that six cylinders are sufficient; but for racing purposes there is an advantage in going beyond this figure, for the weight of reciprocating parts is reduced, the danger of breakage is lessened, and it becomes possible to run at a rniich higher number of revolutions. It must not be concluded that'because Delage will race with a. two-litre "twelve" this type will be offered to the public in the near future; it is the type most suited, in the opinion of the Delage engineers to this particular set of rules; and, while it undoubtedly will have'an influence on the normal models, that influence is not likely to make itself manifest to the public for a considerable time. r „. • The twelve-cylinder Delage will be of the V type, with the two blocks offset so asto allow the, connecting rods to be mounted side by side. ■ There will be two valves in :the head,; with an overhead camshaft The entries for this classic event total 118 cars, comprising three Fiats, three j Sunbeams, three Bplland-Pilains, four Voisins, four Bugattis, and one Delage. With the exception of the'Delage these cars will all have either 6 or 8-cylinder engines. The event will be decided on 2nd July. A Fiat carried off last year's race whilst an American "Dussenberg" was successful in 1921. •.

There seems, still some divergence of opinion amongst designers as.to the'arrangement of wind-screens. The American screens are generally inclined. In England and on the Continent, they are generally vertical. The glare of shop windows when driving in town is probably better guarded against by the rearwardly inclined screen, but for all ordinary work the vertical -screen seems to fill the bill best. But there is much room for improvement in screens. One very important matter is the use of unsplinterable glass. It seems almost ridiculous that any firm should, in these days, put on a screen of ordinary rolled plate, when glass may be obtained which minimises the risk of serious injury by being practically unbreakable and unsplinterable. It is true that the cost is higher, but we cannot say that it is so much higher as to make it dearer in the long run to fit it than to use the, plain variety. There are risks other than the merely personal one connected with the use of ordinary glass for screens. A passenger may be injured, in a sudden stop, through a crash with the screen, and the owner would probably be responsible. These third party risks are generally heavy ones, and should be avoided. Safety screens proved the most direct method of avoiding this particular kind, and they protect the owner as well. Screens are generally too narrow. They could, with advantage, be made at least four inches wider than is general in touring cars. Heavy frames are to be avoided. .. They look unsightly, they obscure the driver's,view, and they are causes of screens-cracking. Light metal frames with spring metal packing for the glass, give the best results, and are least subject to fracture. The American screens generally are good, but the finish is not a patch on those fitted to the leading European makes.,