The Evolution of the High Efficiency Engine By Capt. W. G. Aston
To the student of automobilism there has been nothing more interesting during the past twenty years than to watch the march of progress as expressed by the increasing efficiency of engines. For many years experimenters did a vast amount of work without seeming, as the phrase goes, “to get much forrader.”
All of a sudden, or so it appeared, about 1911 or 1912, they struck the right path, and from that time forward, development has taken place at an amazing rate. We have, in fact, almost got to the point of being able to say that the dimensions of an engine have little or no influence upon the power it can develop, for this is rather a function of the weight of fuel that it can contrive usefully to burn in a given time.
A mental picture of what has been accomplished in respect of engine efficiency, which, of course, one must measure in terms of bore and stroke, may be had on comparing the sort of engines with which Mr. Edge raced against Nazzaro in 1908, when the latter broke the world’s record for speed, and the sort of engine which the Sunbeam drivers will sit behind in this year’s Grand Prix.
The one was an enormous four-cylinder of goodness knows how many litres capacity, the other is a super-charger six-cylinder of only two litres, and the probability is, that in point of brake horse power, the latter, tiny little plant though it be, gives the greater output. In hunting higher efficiency, several basic problems have to be solved. The obvious way to get increased Power in an internal combustion engine is to work the motor at higher effective revolutions.
Now as crank shaft speeds are put up, so there is a great increase in the power absorbed, by the fact that the reciprocating Parts have to change their direction of travel and come to a dead stop twice in every revolution. Therefore, the first thing the designer has to do is to reduce these in weight. Formerly, pistons and connecting rods were copiously drilled, but more scientific methods have now supervened, and they are made of aluminium alloys, whereby amazing lightness is secured, whilst their strength is not impaired.
Another advantage of aluminium for these parts is that it helps very considerably to conduct the heat away from the piston. Recently a new kind of magnesium alloy has been exploited for pistons, whereby they can be made lighter than ever before, and by this factor alone, it is quite feasible that the revolutions of many engines will be put up by as much as fifteen or twenty per cent.
Getting Productive “Revs.”
Now, it is no use making an engine turn at a higher speed unless it gives an adequate output of horse power at such speeds. The ordinary output curve of an explosion engine rises quickly, remains horizontal for a short distance and then descends again. This drop is due first of all to the power absorbed in overcoming the inertia of the reciprocating parts, but is principally attributable to the fact that the cylinder at very high speeds, fails to fill itself with a reasonable weight of gas.
A brief mental calculation will show that in an engine running even at the reasonable speed of 4,000 r.p.m., the inlet gas has very little time indeed to get into the cylinder, in addition to which it has to pass through twisty ports and through the narrow opening afforded by the lift of the valve. In order to overcome this objection, designers of racing cars were at one time unanimous in exploiting the multiple valve principle, which gave them proportionately bigger valve openings with a reduction in the weight of the valves themselves.
More recently the tendency has been to use a single pair of valves per cylinder, and to mount them symmetrically in a hemispherical combustion head, in which the sparking plug is situated in the centre of turbulence.
This is not because the principle of multiple valves is incorrect, for four valves so arranged would certainly be better than two, but with the progress that has been made in engine efficiency, the organisers of the big races have brought down their dimension limits to such an. extent that the cylinder of the modern racing car is really not big enough for the four-valve scheme.
The mere word “efficiency,” in conjunction with any mechanism, implies the elimination to the greatest possible measure of friction, and this is a direction in which the designer of the racing car has done much valuable work, more, in fact, than is often appreciated by those who concern themselves only with touring vehicles. First of all there was the question of lubrication to be solved.
Whereas the old engine relied upon a clumsy splash system, in which luck played a great part, the modern engine receives a supply of oil forced under pressure to every working part. Moreover, special steels for such components as the crank shaft have been devised, which are not only rigid enough to stand the huge stresses imposed upon them by the transmission of power, but are of a nature which will run comfortably in bearings under the highest loads.
Some Recent Advances.
In many racing engines plain bearings have largely disappeared in favour of roller bearings, the frictionless working of which has enabled the most violent accelerations to be secured. This is a point of importance, because in the modern racing car which has to participate in road events, quick ” getaway ” after a stop, or instant adaptability on the part of the engine to a change of gear is absolutely essential.
In the search after higher efficiency careful attention has had to be paid to cooling principles, for a higher output of power from given cylinder dimensions means that the mean effective pressure above the piston must be proportionately increased, and with this is implied an increase in temperature. In earlier days much trouble was experienced in engines designed for high output by reason of the fact that the expansion of such things as valve seats was uneven.
At other points, too, there would be an insufficient flow of water, so that the metal became incandescent, and gave rise to violent preignition. The modern engine designer has learnt a good deal from these lessons, and now knows how to design engines in which no such trouble is ever likely to occur.
The final step in the development of the high efficiency motor which may lead to far bigger things than are at present apparent, is the introduction of the supercharger principle. This is of enormous value, not only because it increases the effective weight of gas pumped into the cylinder at high speeds, thereby increasing the effective revolutions, but also because it greatly enhances the power at low speeds, and consequently promotes an unusual ability in acceleration.
The reason for this is because the large valves and big valve lifts that are requisite for high revolutions tend to destroy the beneficent influence of turbulence at the lower end of the scale. Whereas the 1923 engine without the supercharger gave an immense amount of power once it had achieved about 3,000 r.p.m., but little or no power below this figure, its 1924 counterpart, fitted with the super-charger mechanism, gives an excellent output of horse power, i.e., about 25 per cent, more at the least, all along the scale.
In concluding this brief review of an absorbing subject, it is not too much to say from the data which at present are available that the one litre capacity engine, capable of giving its 100 b.h.p. with an almost complete absence of vibration and fuss, will be within the bounds of practical possibility inside the next year or two.