A minor matter

This heading does not refer to the still-popular Morris Minor, nor will it interest those vintage-car folk who might think it is about the Calthotpe Minor. It has been sparked off by an epoch-making discourse which appeared last year in Autocar, in which Edward Eves told how the young Louis Renault and his three-year-old Renault Fils et Cie pioneered the side-valve engine and several other innovations as well. Eves showed that such an engine was used for the 1902 / 03 Model-K Renault and a minor mystery arose only because it was not clear whether such an engine had featured in the Renault light-car that won for Marcel Renault the important Paris-Vienna race of 1902 against opposition from far larger cars, or whether that racing voiturette had retained automatic or atmospherically-opened inlet valves. The matter was, in my view, very convincingly cleared-up by Kent Karslake in our issue of last February, so this outpouring is not about the Paris-Vienna Renault but about the valve arrangements on veteran cars in general. In passing, though, I would say that while this minor mystery was in the public eye, Motor Sport asked Renault’s PRO for a photograph of the engine of the P-V racing-car in their Paris Museum. The reply was that the only one available was of the complete car. Apparently no-one was prepared to take a camera and obtain a picture for us. However, no sleep was lost, as the engine of the car in the Museum is rumoured to be non-original, anyway. . . .

The first four-stroke petrol engines had to have inlet and exhaust valves and it was logical to put the latter beside the cylinder, as a side valve, as it had to be mechanically lifted from its seat on the exhaust stroke, and operate it from a cam in the engine’s base-chamber, but to use an inlet valve in the head, sucked open by the descending piston and closed against the pressure of a light valve spring. This was beautifully simple, especially with a single-cylinder engine. The disadvantage of letting nature open the inlet valve was that it did not respond until the pistol had descended some considerable distance downwards, on its inlet-stroke, because before this there was insufficient “vacuum” to overcome the action of the valve spring. Nor could the inlet valve be kept open after b.d.c., for as the piston rose on its compression-stroke there was no suction to pull the valve down. For this reason, and to get as large an inlet valve area as possible to combat its timing deficiencies, automatic inlets were located in the head, above the piston. Had they been placed at the side of the cylinder, they would have had to be sucked open against not only valve-spring pressure, but their own weight, and I doubt if it would have worked, but if anyone knows of such an engine I stand corrected. . . .

When greater efficiency was called for and valve timing and overlap began to be understood, inlet valves were also mechanically operated. and it seemed logical to put it or them at the opposite side of the cylinder(s) to the exhaust valves — the T-head arrangement — with a separate cam or camshaft to prod them, Mercedes probably being the first to do this, in 1901. At the expense of a wide cylinder block this enabled cross-flow of induction and exhaust to be achieved, and it enabled large valves to be used. The T-head engine survived up to WWI, a notable example being the sporting Alfonso Hispano Suiza.

When Mr. Eves, in the aforesaid article, wrote that “Louis Renault had the distinction of putting into production the World’s first side-valve engine”, what he really meant was the first side-by-side valve, L-head, engine. It was logical to line up all the valves, inlet and exhaust, of a four-cylinder engine along one side of the crankcase and actuate them from a common camshaft, an idea, Eves says, brought to Renault Fils et Cie by M. Viet, De Dion Bouton’s designer, whose ingenious layout apparently found no favour with his then-masters. Overhead valves, whether opened by push-rods and rockers on an overhead camshaft, or camshafts, were in the future. So, for the moment at all events, I can spare you such minor matters as the different reasons why later engines sometimes had four overhead valves per cylinder, which should be larger, the inlet or the exhaust valve (remembering that, for instance, Mercedes used one inlet and two exhaust valves per cylinder for their 37/90 hp. engine, whereas Ettore Bugatti opted for two inlets and a single exhaust valve per cylinder on most of his cars), or which production car had four side-valves per “pot”. — W.B.