Debating Dimensions

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Looking at the specification of the new Volkswagen-backed Seat Marbella, I was reminded of a classic discussion which took place before the First World War between the great L H Pomeroy Snr, designer of such famous motor cars as the “Prince Henry” and 30/98 Vauxhalls, and the race-loving Louis Coatalen, brilliant Chief Engineer of the Sunbeam Motor Car Co of Wolverhampton. Why?

Well, I noticed that the Marbella 850 has the “over-square” engine dimensions (65mm x 63.5mm) expected of a small engine — and that ancient crossing-of-swords (or perhaps should I say con-rods) was about short-stroke versus long-stroke engines.

In very elementary terms, there are two means of increasing the power-output of an internal-combustion engine. Either you increase its capacity or you increase its rotational speed. These two distinctions were more evident in 1911, when the aforementioned argument occupied the learned members of the Institution of Automotive Engineers, than they are today. In those times unsophisticated valve-gear precluded high engine-speeds and, if these were achieved, there was a limit to piston speed.

Returning to the Seat Marbella, a longer stroke would obviously be feasible, without running into the realms of excessive piston speed, as the engine peaks at 5450 rpm. But I am glad the designer did not resort to this because, as it is, the smaller of the two Marbella engines has a capacity of 843cc, and the larger Marbella 900GL (with the stroke increased by 4.1/2mm and peak speed down by 50 rpm) is of 903cc.

The size of the 850L reminds me of those little economy cars of the vintage years which made a gallant attempt to unseat (no pun intended!) the success of the Austin Seven of some 100cc less — the Singer Junior, Triumph Super 7, Morris Minor, and other 850cc offerings, and after the war the Mini Minor — while the cylinder-capacity of the Marbella 900GL is within 30cc of that of the immortal Ford 8. And in spite of pundits who tell us that engines of more like one-litre, pulling higher gear-ratios, will give better economy allied to brighter performance, I have always found that the smaller the engine, the smaller the petrol bills. . .

That hotly-debated issue of 1912 was over which was preferable, a short-stroke or a long-stroke power-unit. Pomeroy opted for the former, Coatalen doggedly for the latter.

That this was an acrimonous debate was demonstrated when the academic Dr F W Lanchester FRS, replying to a paper read by Pomeroy before the IAE in 1911, retorted: “It seems strange to have to tell an engineer in the twentieth century that a large engine, other things being equal, cannot safely be run at so high an rpm as a small one, and considering how thoroughly it is established that piston-speed is the criterion rather than revolution-speed, it would seem almost an impertinence to offer to the Institution a demonstration of the fallacy of any such doctrine as that which has the author’s support.”

Pomeroy came back with the loaded comment: “I am sorry Mr Lanchester is such a busy man, for I presume part of my paper missed him. I will just read it again, because I knew Mr Lanchester would come along with exactly the words he has done . . .” The arguments were resumed in the motoring press in 1912, this time betvveen Pomeroy and Coatalen.

Whereas Pomeroy relied on theoretical proof (and was right, if possibly before his time), Coatalen put up racing successes as evidence of his long-stroke beliefs. He was able to quote the 1-2-3 grand-slam of his 80mm x 149mm 3-litre Sunbeams in the 1912 Coupe de l’Auto (which was, incidentally, the first British victory in an important contest since Edge and the Napier had won the 1902 Gordon Bennett Trophy, whereas it is often left to Segrave’s Sunbeam success in the 1923 French GP to claim that fame).

In response, Pomeroy could quote the fine performance at Brooklands of his 89.7mm x 118mm 3-litre Vauxhalls, one of which was the first car of 22hp RAC rating to officially exceed 100 mph. Even he was not averse to introducing the exception that proves the rule, however, for he put a notably long-stroke 80mm x 200mm engine into one of his Vauxhalls in the hope of being the first to do 100mph in the 16hp class. This car was timed at 97.67mph over the Brooklands half-mile in 1911 before a con-rod broke and a piston was ejected onto the concrete with some force . . .

There is little point in labouring the arguments of the famous protaganists in detail, but Pomeroy contended that the short-stroke engine vvas lighter and less likely to overheat, and that more power and efficiency could be attained by adopting a maximum piston area, while Coatalen believed that unless an engine was so large that no chassis would accommodate it, the stresses feared by his rival would never be reached.

These days the short-stroke theory prevails and we are accustomed even to “square” and “over-square” cylinder dimensions.

In racing prior to WW1, it was often capacity limitations which drove designers who believed in large piston-areas to adopt comparatively short piston-strokes, having “used up” the available capacity in the bore of their pistons. Then, when limited-bore regulations were introduced, for voiturette races particularly, the reverse applied; a long stroke was necessary if anything like a decent-sized engine was to be produced, leading to Peugeot’s 1910 two-cylinder racing engine with a 280rnm stroke, a power-unit so lofty that the driver’s forward vision was impeded! Even this did not represent the peak — a Corré La Licorne had a De Dion engine with a 300mm stroke!

If we look at the runners in that dramatic 1914 French GP at Lyon, we see that the victorious Mercedes, built to the race-stipulation of 4.1/2 litres, were of 93mm x 145mm, their piston-speed at a peak 2800 rpm being 3070 ft/min. That was about the norm for racing engines at the time, although after the war rising crankshaft speeds raised piston pace to 4000 ft/min in the case of the wonderful 1.1/2-litre straight-eight Delage, and 4300 for an ERA engine. This caused no problems, even for that abnormally long-stroke 1910 Peugeot.

The short-stroke engine became commonplace for production cars, even though Harry Ricardo said there is little to choose between a long or short stroke where thermal efficiency is concerned, and something to be said for the former in respect of mechanical efficiency (power will be developed at a lower speed, therefore with lower mechanical losses). In America in 1933 P M Heldt made the interesting point that if a designer needed to put a multi-cylinder in a chassis previously using a power-unit with fewer cylinders, he would be forced to reduce cylinder-bore to make the new engine fit under the existing bonnet, and to maintain the desired swept-volume would then have to use a longer stroke than was originally intended.

However, the long-stroke engine is associated more with vintage cars, of which the 80mm x 149mm 3-litre Bentley, 98mm x 150mm E-type 30/98 and several 1.1/2-litre sportscars with a 100mm or even 120mm stroke are examples. The low-revving, long-stroke engine was thought better at slogging up hills; the late Kent Karslake used to say that although his 1912 80mm x 180mm Alfonso Hispano Suiza would be outpaced by a Brescia Bugatti on the level, it could close the gap up a long gradient (conversely, however, FT Jane quoted his short-stroke 145mm x 120mm racing Benz as an admirable hillclimber due to its short stroke).

By the end of the vintage years, the longest piston-stroke found in a production car seems to have been 150mm, in the bigger blown six-cylinder Mercedes-Benz engines.

Five years later, the longest-stroke engine on the market was that of the 32/34hp six-cylinder Minerva, and that was by then a pretty antiquated motor-car. I doubt if there is anything approaching this among 1988 cars, so, after all these years, the theories of L H Pomeroy, the celebrated Vauxhall and Daimler designer, are apparently vindicated. WB

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