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Valve Angles, Cylinder Heads and their Designers

One hopes that WB’s obsession with valve and cylinder head design is not subject to cure. His articles on the subject over the years have been much enjoyed and his most recent contribution has served as inspiration for some research and reflection on my part.

Readers might like to know that the unusual cylinder head shown in the 1921 patent illustration was employed almost without change on the ‘Stromboli’ engine designed by Ettore Lanzoretti Spina for ELS Engines Ltd, a small engineering consultancy with offices at 29 Spring Gardens, Whitehall. One of the directors of the ELS Engines Ltd was Hedley Thomson, who was also the General Manager of Peter Hooker Ltd, an old, high quality engineering works located in Walthamstow.

Hedley Thomson had come from Arrol-Johnston of Glasgow, as had Frank Halford and John Brodie, who, along with Dr Eric Moult, Rod Banks, and WC Devereux, formed part of the galaxy of engineering talent resident at Hooker’s in the early 1920s. All later became prominent in the British aero-engine and motor industry. During the 1914-18 war, Hooker’s built Gnome rotary engines, and in the postwar period — until the bankruptcy of the firm in 1927 — they acted as something of an extension of the Royal Aeronautical Establishment, Farnborough. Because the RAE was prohibited from building engines of its own design, work was farmed out to private companies like Hooker’s.

Not only was the ‘Stomboli’ actually built in Peter Hooker’s shops, much of the development, and all of the testing, was done at the Walthamstow works. The ‘Stromboli’ was a giant of an engine, a tall in-line six-cylinder petrol (a diesel version was planned) engine of 170 litres capacity. It was designed as a powerplant for airships, a follow-on to the Beardmore Tornado used in R101, but it never left the test bench, nor did it ever produce its expected output of 1500 bhp. At first, this deficiency was attributed to weaknesses in the design of the engine’s crankshaft and crankcase. After a few minutes on the test bed, running at only 60% power, most of the bearings were left, in the words or DR Pye of the Air Ministry, “hanging like elegant bracelets on the shaft.”

The bottom end problems were eventually cured by a major redesign by RAE engineers. In fact, the RAE went further and designed a completely new and elegant lightweight crankcase based on Warren truss principles. There were also problems with the top end, but these were never remedied before work ceased altogether following the loss of the airship R101. Each cylinder (300mm x 400mm) had a head much like the one pictured in the illustration ( Motor Sport November, 1990, p 1222). There were eight 66mm valves (72mm were intended for production versions), four inlets and four exhausts, disposed in two groups peripherally around the head with their stems converging toward the centre at an angle of 30° (later reduced to 28°). Each group of valves was operated by pushrod and rocker arm bearing on a sliding sleeve. One of the few differences between the patent drawing and the actual engine was the shape of the rocker arm which in the ‘Stromboli’ was given a long swan neck. In fact, the valve gear was a main source of trouble; the rocker arms twisted under power, and the resulting force was transmitted unevenly to the valves, causing frequent breakage. The volumetric efficiency of the design was quite good, but the shape of the combustion chamber left a lot to be desired. Even on the engine’s normal 20% benzol (74 octane fuel), pre-detonation took place often enough so that testing had to be done in the end using a brew made up of 50% benzol.

As WB pointed out, the second name on the patent used in design of the cylinder head of the ELS engine was KJ Thomson, who, with his brother Hedley, became involved with JG Parry Thomas, remembered now chiefly for his land speed record attempt and racing exploits of the 1920s. However, Parry Thomas had been Chief Engineer at Leyland’s during the Great War. There he designed an highly innovative, double-row X-8 OHC aeroengine of 16.67-litres. The engine was designed to meet the same requirement for which AJ Rowledge of Napier’s designed the W-12 Lion. The Leyland engine was in many ways an ambitious and advanced design, and it went one better than the Napier having four banks of cylinders with a shaft and bevel gear-driven overhead camshaft for each cylinder bank. Four valves per cylinder were employed, disposed at a very modern, narrow angle of 30°, the same as WO Bentley would later choose when he built his 3-litre car. The engine was also unusually advanced in another respect having massively oversquare cylinder dimensions (6″ x 4.5″). Consequently, it possessed a quite remarkable piston area/ frontal area ratio. In almost every area of engine design, Parry Thomas’ X-8 demonstrated individuality and complexity. A two-throw, three-bearing water-cooled crankshaft and air-cooled pistons were among the distinctive features given the Leyland X-8.

Work began at Leyland in May 1917, but progress was slow. By April 1918, however, development work on a single-cylinder test engine was advanced enough for Parry Thomas to anticipate that his X-8 would produce at least 420 bhp. Peak horsepower on the test cylinder measured 53 bhp at 3500 rpm, and at normal ratings, and a developed eight-cylinder engine was expected to produce 450 bhp at take-off and 300 bhp at 10,000 ft. When a complete X-8 engine was at last assembled towards the end of 1918, it was commendably light, weighing only 549 lbs (about 660 lbs with radiator, etc), one pound under the design weight, but this was about its only achievement. On the test bed, weaknesses of the design soon became apparent. Valves broke frequently, a fault that was only remedied by changing the camshaft profile and by introducing a modified head with smaller and lighter inlet valves of 1.75″ (the same size as the exhausts) replacing the original 2.25″ ones. Pistons, rings, and gudgeon pins also broke repeatedly during test runs. By early 1919, the engine was still in trouble and had not yet been run at full power. When at last an attempt was made in April 1919 to run the X-8 full throttle, it ended in disaster. The white metal main bearings were run, the master rod broke, and four-cylinder liners were smashed as the engine virtually destroyed itself. By this time, especially now that the war was over, the Ministry had had enough and saw little need to continue support for the project. In any case, Rowledge already had the Napier Lion giving a reliable 450 bhp.

Though the X-8 engine was cancelled, this wartime design left something of a peacetime legacy. At the 1920 Olympia Show, Leyland displayed a luxury motorcar powered by a straight-eight 7.3-litre engine designed by Parry Thomas. This engine was also an OHC design and used the same and very unusual multi-layer leaf springs found on the X-8 aero-engine to close the valves (only two per cylinder in the automobile engine). That Parry Thomas decided to use this type of valve gear is not too surprising since this was one of the few features of the X-8 which had shown itself under test conditions to work satisfactorily.

The design and development histories of the ELS ‘Stromboli’ and Leyland X-8 engines tell us something about the engineering knowledge and skill of their respective designers. For example, the valve mechanism of Lanzerotti’s design shows ingenuity of a sort; there is an apparent economy in the way he managed to open 48 valves with only one camshaft and 12 pushrods. Such economies result in lightness, a primary objective of any aero-engine designer and the ‘Stromboli was light for its size, weighing in at an estimated 4000 lbs in its final version, with aluminium heads and replacement crankcase. But apart from the valve mechanism, the rest of his design had little to recommend it. The shape of the combustion chamber was horrible and shows little understanding of thermodynamics or the combustion process. Even with four spark plugs per cylinder and relatively high octane fuel, combustion in the ‘Stromboli’ cylinders was — to put it mildly — erratic.

The bottom end of the original (Lanzerotti) design was even more of a nightmare. Here, lightness again was the overriding objective.

Lanzerotti went so far as to cast holes in the main webs! Like many Italian engines of the period, much effort had been expended in making it appear attractive from the outside. Less thought, however, had been given to the strength of internal machinery and stressing seems to have been a skill unknown or unpractised by the designer. All that can be said in favour of the ‘Stromboli’ is that it was an engine that lived up to its name; when Rod Banks was running tests on the single-cylinder ‘Stomboli’, the head fractured and was blown off like the plug of a volcano (after which it was named). On another test, this time of the six-cylinder engine, Banks watched the flywheel describe an ellipse before twisting the crankshaft into something resembling a pretzel.

The Leyland X-8 aero-engine of Parry Thomas displayed even greater signs of ingenuity; it positively bridled with innovations and showed scant regard for convention. Clearly, here was a designer with a lively mind, eager to innovate, but also one lacking sufficient discipline and concern for tried and true engineering principles. For example, as the Ministry’s resident engineer noted, there are easier and simpler ways to keep main bearings cool than by employing a water-cooled crankshaft. Several of the engine innovations were considered unnecessarily complex and unlikely to yield additional power. Moreover, some aspects of the design had not been examined thoroughly in advance. The question of valve size illustrates this criticism. When, as the result of experiment, it was found that smaller valves and higher gas velocity produced better results than the oversized valves of the original design, Parry Thomas seemed surprised that gas velocity might be critical, as the Ministry’s engineer observed.

If all this discussion of aero-engines and their designers seems somewhat remote from the world of automobiles, one must remember that in the period up to the Second World War a very considerable overlap existed between these two worlds of piston engine design. Parry Thomas designed engines for both aircraft and automobiles. Roy Fedden of Bristol’s and Rowledge of Napier’s did the same, and, of course, WO Bentley and Sir Henry Royce demonstrated their engineering skills in both arenas. By examining and comparing aero-engine designs, one can see in better perspective the talents and accomplishments of Britain’s motor engineers of this period.

The aero-engines produced by both Bentley and Royce, for example, were outstanding. Bentley’s BR1 and BR2 designs for the Admiralty were undoubtedly the best rotary engines of the war, and Royce produced three highly successful wartime designs. He had the Eagle up and running (and running reliably) in less than seven months! In contrast, the Leyland X-8 of Parry Thomas was still unable to complete a test run at full power after two and a half years of development. Few cases better illustrate a classic dictum of motor engineering that development is design done slowly and expensively. Looked at another way, a comparison of the development histories of the Leyland X-8 and the Rolls-Royce Eagle gives telling proof of the thoroughness with which Royce studied a design problem in the preliminary stage and his exceptional attention to detail throughout the design process. In contrast to Parry Thomas, Royce eschewed innovation, preferring instead to achieve his targets, as Donald Bastow observed, by taking “limited steps from a known main basis of proved reliability.” Much the same could be said of the engineering philosophy of Bentley. With this in mind, it comes as little surprise to learn that in the postwar period several of the very best automobiles in terms of performance and reliability came from the design offices that had produced the most successful wartime aero-engines: Rolls-Royce, Bentley, and Hispano-Suiza. — DDH