Piston stroke and its implications
[CECIL CLUTTON, WHO IS FAST BECOMING A MOTORING WRITER OF NOTE, HERE DISCOURSES ON LONG-STROKE AND SHORT-STROKE ENGINES. ALTHOUGH A VINTAGE CULTIST, HE SAYS MUCH IN FAVOUR OF MODERATE PISTON STROKE, AND GOES ON TO OUTLINE A MOST ATTRACTIVE AND SENSATIONAL CAR OF 1⅔-LITRES CAPACITY.—ED.]
THE choice of bore and stroke for any size of engine has become so hazed by the gloomy shadow of our Treasury rating that the true ethics of the matter have been lost sight of on many occasions. It is nevertheless true that the optimum stroke lies within fairly close limits.
It is popularly supposed that if two engines have the same bore but different strokes, the longer-stroke engine is necessarily the more powerful, because it has the greater capacity. In the days when valve design and bottom end lubrication limited crankshaft revolutions to a maximum of some 3,500 per minute this was certainly true, because if both engines had the same mean effective pressure and the same maximum r.p.m., the total B.H.P. would clearly be directly proportional to the respective strokes.
Nowadays, however, it seems that there is no practical limit to the amount of revs. that can be dealt with from the aspect of bearing lubrication, so that the designer is only limited by the number of times per minute that he can persuade the valves to bob up and down and yet refrain from melting—and also by the maximum safe piston speed. For normal touring cars the valve question seems to become difficult between 5,000 and 5,500 r.p.m., while the pistons are seldom asked to surpass a speed of 3,500 feet per minute.
There is, however, some objection to reaching as high a piston speed as this, since it is generally agreed that the cruising revs. of any engine coincide with a piston speed of 2,500 f.p.m. Therefore, if peak revs. represent a piston speed of 3,500 f.p.m. it is clear that cruising revs. will be only five-sevenths of the peak, so that the sustained power output will not be appreciably more than 70 per cent. of the maximum. On the other hand, if the stroke is so short that the maximum piston speed does not exceed some 3,200 f.p.m., the sustained revolutions and power output can be as much as 80 per cent. of the maximum; and since this is a far more important characteristic than a very large maximum output, such an engine has a great advantage.
To take an example, the 3-litre Bentley (80 x148.6) has a peak of 3,500 r.p.m., but can only cruise at 2,500 r.p.m. The V12 Lagonda (75 x84.5), on the other hand, has a maximum of 5,300 r.p.m. and can cruise as near this maximum as 4,500 r.p.m. with complete mechanical safety. It must, however, be conceded that few, if any, other designers have succeeded in making a production engine safe at such speeds, but a cruising speed of 4,000 r.p.m. does not seem to call for particularly clever design. Such r.p.m., combined with a piston speed of about 2,500 f.p.m., implies a stroke of roughly 94mm., and assuming that such an engine could run up to a peak of 5,200 r.p.m., the piston speed even then would not reach 3,300 f.p.m.
It therefore appears that the optimum length of stroke lies somewhere between 85 and 95 mm.
With cruising revs. of 4,000 per minute, as just envisaged, and even such a low m.e.p. as 95 (which implies a very smooth and flexible engine), the sustained b.h.p. per litre will be as high as 29, although at 5,200 r.p.m. it will not be as much as 38.
An undoubted attraction of the long-stroke engine, which probably accounts for its introduction in the 1912-14 G.P. machines, even before the days of Treasury rating taxation, lay in the high ratio between the swept volume and the capacity of the cylinder head, which facilitated high compression ratios without recourse to funny shaped pistons. So effective were these early G.P. cars in this respect that even on the petrol then available, m.e.p.s of as much as 110 were not unknown. As against this, it is difficult to obtain a high compression ratio in a short-stroke engine, which must normally rely on sheer crankshaft speed to churn out the power; the VI2 Lagonda, for example, despite its peak of roughly 40 b.h.p. per litre, appears to operate on an m.e.p. of only about 90, and this undoubtedly accounts in large measure for its phenomenal smoothness at all engine speeds, despite a flat cylinder head.
To combine the additional advantage of high volumetric efficiency with a short-stroke engine, therefore, it is evidently necessary to use a supercharger, and with the aid of this device it is quite feasible to obtain an m.e.p. of as much as 130 in a touring engine; nor need this lead to harshness or inflexibility if the cylinder head is reasonably designed, and the cooling layout properly arranged (as witness the Type 57C Bugatti). Under such circumstances each litre of our 94 mm. bore engine will be capable of knocking back no less than fifty-two steeds on occasion, or forty as a regular thing.
The advantage of all this, of course, is that an exceedingly favourable weight to b.h.p. ratio can be obtained with quite a small car. To take an example: The types 320 and 327 B.M.W. make it clear that a comfortable and durable four-seater saloon can be brought inside a weight of one ton. To obtain reasonable acceleration on such a vehicle (say, 13 seconds from 0-60) a b.h.p. of about 80 is necessary. Again, to drag it in moderately streamlined form through the atmosphere at a sustained 80 m.p.h. (no unreasonable cruising speed these times) some 60 h.p. should suffice.
If we incorporate our 94 mm. stroke in a four-cylinder lightly supercharged engine of 75 mm. bore, giving a capacity of 1⅔-litres, working at an m.e.p. of 120, this is exactly what we shall get.
Such an engine would be sufficiently compact and light to be compatible with an overall weight of 20 cwt., whereas, in the old days, to obtain equivalent acceleration, cruising speed, reliability and comfort, nothing less than 5 or 6 litres would suffice. Large cars of this kind are, indeed, exceedingly attractive, but they are unhandy on country lanes or in thick traffic, and few these days have facilities for storing such monsters, nor for paying the absurd tax demanded from them. This ability to get high performance with a reasonably sized vehicle is, indeed, the only justification for high volumetric efficiency, and it is only within the last year or two that design has advanced to the point that high revs and volumetric efficiency have become available compatible with reliability and durability. Only recently, too, has suspension been improved to the point where first-rate road-holding can be combined with comfort in any but the largest of motor cars.
It is this advance that has outmoded the long-stroke engine—even from the viewpoint of tax-dodging expediency but few designers, except W. O. Bentley with the twelve-cylinder Lagonda, have yet fully exploited the fact.
Going a step further, there is no reason why our 1⅔-litre engine should not be practically as flexible as the V I 2 Lagonda, but equally as with the Lagonda, free use must be made of the gears if the most is to be made of the power available. To facilitate this without tiring the driver, a finger-tip controlled gearbox, of the Cotal variety, is absolutely essential, and a return to much closer ratios than are now usual must be insisted upon. On our specimen car, for example, with 28″ wheels, nothing wider than 4, 5.2, 7 and 11 to 1 would suffice to make possible the best use of the engine, giving maxima of 40, 63 and 84 m.p.h. on the gears, a cruising speed of 80 m.p.h. at 4,000 r.p.m in top, and a flat out maximum of up to 104 m.p.h., depending on the extent to which streamlining was employed. Incidentally, such an engine would be sufficiently compact to make it possible to take advantage of the joys of front wheel drive without unduly lengthening the wheelbase. Front wheel drive has other advantages beyond those of fast cornering and stability on wet roads, for it also makes it possible to reduce the overall height, thus both decreasing frontal area and lowering the centre of gravity, whereby soft suspension and stability may more easily be combined.
It is therefore interesting to consider how important a factor the choice of stroke has become in modern design, directly or indirectly affecting the layout and proportion of the entire vehicle.