Perhaps You Didn't Know . .
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TECHNICAL or semi -technical matter can frequently be stodgy and hence not generally acceptable. Dealt with conversationally, it beconies much more palatable and it is hoped that you may find the answers given by Mr. Jones of some value, either as knowledge to be stored or for immediate application.
'Member of the party (depicted above) : "Some years ago, Mr. Jones, it used to be quite common to see air funnels on carburetters. The mouth of the funnel would face forward in order to catch the air. it was, of course, arranged outside the bonnet. Would such a device give a degree of supercharge, and, if so, to what extent would power be increased?"
Not So Simple.
Mr. Jones: "Speaking generally, the idea is valueless from the point of view of producing a supercharger effect. At a speed of, say, 50 m.p.h., one might expect another horse-power, but certainly not more, and, in any case, the matter was not so simple as you suggest. In order to balance the increased pressure in the intake, it was necessary to install a system of piping to boost up the ' atmosphere' in the float chamber and petrol tank as well." "That reminds me of a point that has often puzzled me. I understand that when vehicles are to be operated at high altitudes, an adjustment has to be made to the carburetter to compensate for the lower atmospheric pressure, but that when they are supercharged no such adjustment is necessary. I should have thought it would have been all the more necessary, because by multiplying the pressure, you surely also multiply the error."
Mr. Jones: "I appreciate your difficulty, but in point of fact the error is not multiplied. To begin with, the unsuperchargerl engine requires a small jet at high altitudes, because the air is less dense and, therefore, will not contain enough oxygen to enable the original quantity of petrol, which is practically incompressible, to be burned.
"By means of a supercharger you add, say, one atmosphere to the pressure and, accordingly, instead, for example, of a 2-per-cent. excess of fuel, you get only a 1-per-cent. excess on the total. Alternatively, you can think of the supercharger as supplying gas to the engine up to a predetermined maximum quantity which is always available, whilst the amount required is regulated by the throttle."
M.P. : "Assuming that a vehicle has been running on a high-grade spirit, what adjustments, if any, would become necessary when changing to a lower grade?"
Mr. Jones: "As low-grade fuels do not normally require so much air, the obvious course would be to enrichen the mixture. It might be necessary to alter the ignition timing. Some distributors have an octane selector to provide for this."
Do Carburetter Jets Wear?
M.P. : "I have often wondered whether the continuous flow of petrol through a jet tends to increase the size of the jet aperture. If this be so, what is the probable mileage at which the effect of the increased size of jet would begin to manifest itself? The same remarks would possibly apply also to injector nozzles on oil engines."
Mr. Jones: "Of course, wear does take place, and after, say, 200,000 miles, which is not unreasonable for a vehicle which has been in continuous service for a number of years, the jet would definitely pass more fuel. In view of the rate at which wear takes place, it would be difficult to say at what mileage the increased bore would affect consumption.
"In the case of a high-velocitydischarge jet, such as an injector nozzle, the more serious aspect is wear caused by the valve. This is definite and appreciable. Such a jet may be reconditioned about three times before it is finally discarded."
M.P. : "Taking two identical vehicles equipped. with engines having similar induction systems and carburetter settings, and running under identical conditions, what is the most probable cause of one engine consuming fuel at a higher rate than the other? "
Mr. Jones: "Provided that running records—a common source of error— have been carefully kept, the most probable causes are driving methods and the general freeness of the chassis as a whole. Many drivers adopt the habit of coasting downhill, which certainly makes a difference to a petrol engine, but not to an oil engine, as the governor will cut the fuel off fully if the engine be revving fast with the accelerator pedal released.
"The man who 'drives on his brakes' will show a poor return as compared with the man who takes his foot off the accelerator early and uses his brakes with less vigour and frequency. Much fuel can be wasted bv indifferent methods of starting up. f have seen engines started on full choke and left to run for minutes on end with the engine ' hunting ' through an excess of petrol."
Cylinder-bore Wear.
M.P. : "Such a practice would surely lead to excessive cylinder-bore wear? "
Mr. Jones: "It is a well-established fact that continual starting and stopping are responsible for rapid wear of the bores and this is, of course, accentuated under cold weather conditions when the choke is being used constantly. The neat fuel simply washes the oil away, and until it is replaced abrasion will go on.
"The latest theory of excessive cylinder-bore wear is corrosion—a problem which the oil companies have set themselves to solve. It is contended that, when the cylinder walls are cool, acid vapour produced by combustion condenses upon them and eats into the metal. Such corrosion is, of course, accelerated when the film of oil has been washed off the iron surface."
M.P. : "Talking of cylinder-bore wear, what would be the most likely explanation of excessive petrol consumption following a rebore? Should not the increase in power make for reduced consumption under similar load and operating conditions ?"
Mr. Jones. : "When you say 'exces sive,' do you mean a slight increase. as compared with results obtained prior to the rebore, or something abnormal?"
M.P. : " Prior to the rebore we were getting 12 m.p.g., but now the figure is about 10-10i m.p.g."
Mr. Jones: "It must be remembered that we are dealing with a larger engine, the increase in capacity depending, of course, upon the amount of metal removed. I should expect a slight increase on this account, but not 20 per cent. Moreover, the added power should partly counteract the increased consumption.
" I have heard of cases where a rebore has resulted in a marked improvement all round. Much depends upon the condition of the unit prior to the rebore. Rebored engines usually run a bit stiff at first, and it may take several thousands of miles before the engine has properly settled down. It is clearly unfair to make direct comparisons." M.P.: "What is meant by a ' flat spot,' Mr. Jones? What effect does it have on running and how can it be cured? "
Mr. Jones : " A 'flat spot' is the term given to a period of hesitancy in pick-up as the accelerator is depressed. It may be only slight, or it may persist over an appreciable movement of the accelerator pedal. It is caused either by a choked jet or by a bad combination of jets. In the latter case, fuel starvation occurs when passing from one jet to another.
" In a carburetter which formerly was in order, the instrument should be thoroughly cleaned, particular attention being paid to the pilot and compensating jets. Readjustment of the slow-running jet will often cure a flat spot. Normally, however, a carburetter, once properly tuned for a particular engine, should give a consistent no-trouble performance.
"Well, gentlemen, I am afraid can't stop longer to-night, by.: I hope to see you again in the near future."
M.P. : "Thanks, Mr. Jones. It's been very interesting. We shall look forward to seeing you again."