Correspondence

From Dr Fred Starr

Dear Editor

I enjoyed reading the comments by George Latham on Ed Marshall’s first paper on sleeve valve engine development, and Ed’s rejoinder. Ed and I have had a number of useful chats on the merits of sleeve and poppet valves. Ed’s paper on sleeve valve was excellent and we all look forward to Part II.

In some eyes I am seen as an apostle of the poppet valve, through the presentations I have made on this subject and the papers in the journal. This is far from true. My interest in this subject stems from the realization that the poppet valve was the first engineering component which had to withstand stress at high temperature. And only now, in the light of modern metallurgical theories, can we explain why some materials worked and others did not.

In theory, as a metallurgist, I could have done a similar job in discussing the work that was done on sleeve valve alloys. However, although the basic principles of tribology are well understood, they are not much use in predicting how materials will behave, especially where the wear environment is ‘difficult’, as in the case of the sleeve valve. A paper explaining the technical background behind the development of sleeve valve alloys and coatings is probably thirty years in the future. Personally, having been involved in a few industrial problems involving wear, in which we were always at the ‘let’s try this’ stage, I think tribology should be written with a ‘y’ rather than an ‘I’.

Turning now to Mr Latham’s points and what Ed Marshall had to say, in my view, despite claimed superiority of the sleeve valve over poppet designs, there was not much to choose between them in terms of output-per-litre. The 3500 hp 36.7 litre Sabre engine sounds incredible, but this power was obtained using water-methanol injection, and the Merlin could reach similar specific powers using this technique. The elimination of the poppet valve gear on the sleeve would lower the engine height, but there might be an increase in engine width.

The biggest advantage of the sleeve valve was that by eliminating the risk of a poppet valve failure and the consequent need for valve removal, it allowed an engine designer to conceive engines with a very large number of cylinders and high power outputs. Maintenance needs would then be kept at a reasonable level. The same considerations were at work with the American radial engines, which used only two poppet valves per cylinder, whereas in Britain we used four in an effort to get the highest possible power.

Against this is the cost of manufacture. The sleeve valve engine required three high-tolerance cylinders to be machined and ground. The sleeves had to be made of a complex austenitic steel with a high proof stress. As this material had to have good wear properties, it had to be nitrided, requiring controlled heating in a nitrogen furnace. This compares with a poppet valve engine needing only the internal surface of the piston liner to be bored true, and then a trivial bit of machining on the cylinder head to take the valves. I do feel that a ‘special relationship’ had formed between Ricardo and the Air Ministry which kept up the R&D on sleeve valves when the need for it was no longer so vital.

Turning now to a couple of other points, there is still some confusion over superchargers and turbocharger terminology. In the USA, by the late 1930s engines used in commercial transports were ‘ground boosted’, that is the supercharger was driven via a gear off the engine and gave a lot of power at sea level, this being needed to reduce take-off distance. In practice, the pilot and flight engineer had to juggle the throttle during take off to keep the boost pressure within limits, since propeller rpm and engine speed rose as the aircraft got faster. In Britain, because of military considerations, gear-driven supercharging was used to give high power at altitude. This meant that if the throttle was fully open at sea level, engines would be ‘overboosted’ and liable to blow up. Hence barometric control was used to prevent misuse of the throttle at near-ground level, a feature absent on American fighters. Some German aircraft drove the supercharger off a variable-speed hydraulic drive; this giving optimum performance at all altitudes.

Turbocharging, where the exhaust gases from the engine drove a turbine which drove the supercharger compressor, was pioneered in America. The materials for the turbines were progressively improved from 1923 onwards. Turbocharging was useful for transport aircraft, as it improved overall engine efficiency. But it also proved invaluable for long-distance fighter and bomber aircraft and allowed really high altitude flying. But the ductwork, to carry the air from the compressor to the engine, and the exhaust gases from the engine to the turbocharger turbine, made for a very bulky installation. It was only feasible in fighters like the P47 Thunderbolt and the P38 Lightning.

Finally it was only ‘aftercoolers’, not intercoolers, which were generally used in WWII fighters, the intention being to cool the air-fuel mixture after it had left the outlet of the supercharger or turbocharger compressor. This reduced the risk of preignition of the charge. The main aim of an intercooler is to reduce the inlet temperature to the second compressor of a two-stage supercharger system. Such systems were needed when a high pressure rise was required from the supercharger. The intercooler reduced compression work, but an aftercooler would always be needed. I think only Rolls-Royce claimed to use intercooling but the unit used was quite crude, apparently.

From Mr E. L. Marshall

Dear Editor

I very much appreciate Dr Fred Starr’s comments on my paper and the subsequent correspondence. I would respond as follows:

My initial paper covered Ricardo’s sleeve valve work up to the end of the research phase and the start of the development of full-scale engines. The main point of this paper was to demonstrate the way in which Ricardo, almost alone, championed the sleeve valve as the way to overcome the combustion problems of the day. I am currently working on a second paper to take the story forward into his work assisting with the development of full scale sleeve valve engines and consequently, in the interests of brevity, I will leave my response to Fred’s comments regarding the Napier Sabre, manufacturing and maintenance costs, and other full-scale engine matters to be addressed in my second paper. I would however, like to comment on the subject of pressure charging.

In the 1920s pressure charging was in its infancy and the inter-cooling referred to in my paper concerns the terminology of the day, meaning cooling of the charge between exit from the compressor and entry to the engine cylinder, as in today’s turbocharged and inter-cooled automotive diesel engines. At that time the sophistication of multi-stage compressors had not yet reached the piston aero engine so Fred’s point about inter- or after-cooling had not arisen. During the 1920s the RAE at Farnborough carried out comprehensive evaluations of various types of compressors, both fixed displacement engine driven, and exhaust driven turbines and Ricardo was guided by their work.

Prior to the conference of March 1925, which I refer to in the paper, the Air Ministry asked a number of authorities to submit a note on the subject of supercharging with a view to:

(a)Maintaining ground level power at altitude.

(b)Obtaining a uniform increase in power output throughout the whole range of altitude.

Most authorities, with the exception of Ricardo, responded positively. Ricardo, for the benefit of discussion, decided to play ‘Devil’s Advocate’ and in a document, too lengthy to quote here in full, he pleaded the case against supercharging adding that in fact, he was quite open to conviction.¹

He started by covering the general view that increasing charge density increases the pressure and temperature of the combustion process, thereby increasing the intensity of heat flow in the valve area and hence the tendency for detonation to occur. This necessitates a reduction in compression ratio which in turn reduces thermal efficiency. If inter-cooling is employed to lower cylinder intake temperatures the inter-cooler invariably increases head resistance (drag). He went on to discuss the pros and cons of three alternative methods of increasing the weight of air taken into the cylinder per minute, namely, increasing the size of the cylinder, increasing the speed of rotation or reducing the temperature of the air by using a fuel with a high latent heat of evaporation. For reasons of increased weight, increased drag or complicated operation he dismissed them all. He concluded by pointing out that he had not dealt with the case of a sleeve valve engine employing stratified supercharging (as I described on pp. 80–82 of 83.1 of the journal), partly because he was clearly prejudiced in favour of this method and partly because it was at an earlier stage of development but, he went on to point out that:

(a)It did not lower efficiency, it increased it.

(b)It provided an altitude/power characteristic within the range possible with a propeller of fixed pitch.

(c)By lowering temperature it reduced heat flow.

(d)No air cooler was needed and a very much smaller supercharger was required.

(e)Pipe work was reduced to a minimum and neither the carburettors nor the induction system were under pressure.

(f)It provided automatic altitude compensation.

He also pointed out that, at that time, the world altitude record was held by a French engine running on a specially blended fuel, which actually out-performed a supercharged version of the same engine.