The Greatest Engines of All Time
by LJK Setright
One of the highlights of ETi's Unusual Engines series, for the late LJK Setright, there was no better motor than the Napier Sabre.
Its history was brief, its performance was always questioned by heavy-handed censorship, its makers were treated badly by government and industry alike, and a vicious rumor-mongering operation sought to play down its achievements. It only featured in two types of operational aircraft, both of which left something to be desired; but with it, those two applications achieved things that no other aircraft could - such things as might only be achieved by a combination of overwhelming power with exemplary reliability. World War II might have taken very different turns without it but despite this, Rolls-Royce, having failed to match it, even attempted to have its production stopped.
||The Sabre represents the apotheosis of the engine designers art in obtaining the greatest possible power from the least possible bulk and with minimal frontal area. It had as many cylinders as could comfortably be fitted into any design, proportioned to exploit the basic advantage of presenting a vast total piston area, combined with an unusually high rate of crankshaft rotation. And then there were the sleeve valves...
Power had long been Napier's stock-in-trade. The company's design and development team was tiny but rich in talent. Other riches had escaped them, and their hopes of getting something done about it were not much better, for the aero-engine business at the time was viciously competitive and the Air Ministry persistently refused to give them any encouragement. Rolls-Royce, having since the previous decade considered Napier a very real threat, employed its powerful lobbying facilities to ensure that its competitor was kept at bay.
Unable to make headway with the production of the Sabre, Napier concentrated on its technical development. Employing a few little twin-cylinder research engines and some very complex and precise instrumentation of their own manufacture, Napier's engineers worked away perfecting the basic design, their ideas running further and further beyond the original concept of 2,000bhp - a remarkable feat when at the time R-R Merlin was just breaking into four figures.
|And what a refined design it was! The 24 cylinders were arranged in two tiers, each being tantamount to a flat-12 engine with its own six-throw crankshaft. These two crankshafts were coupled together by gearing at the nose, but by indirect means so that the dangers of transient overloads were eliminated: floating gears balanced the loading imparted to the drivetrain from each of the crankshafts, the float being axial and spring-inhibited in an elegant system that certain other firms should have copied but, lacking the wit to appreciate it, did not.
The cylinders were cast in light-alloy blocks studded and bolted to the flanks of the crankcase. In the bores were the valve sleeves, driven from worm gearing cut into the two half-speed shafts that ran, on right and on left, the full length of the engine. Other niceties were sometimes of established Napier style, notably the piston crowns with their generously radiused shoulders and considerable clearance from the bore progressively tapering to a narrower clearance just above a low-set wedge-section compression ring. This was never explained, and again nobody copied it, but I suspect that there was something very docile in the quenching process that went on in that tapering clearance. The engine as a whole looked, to people accustomed to popular notions of engine architecture, quite unfathomable - just a boxy lump of metal with four rows of spark plugs going in and two rows of exhaust stubs coming out. Sleeve-valve engines were mysterious, but in their time they did marvels: they were most marvellous during the late 1930s and the ensuing war years, when fuels were particularly critical.
Single-sleeve valves enabled a higher compression ratio to be used, or a higher supercharge pressure, and would also tolerate a higher admixture of lead in the fuel. With no exhaust poppets to grow incandescent, and with freedom to form a beautiful combustion chamber shape, detonation could be postponed even further. Coefficients of flow through the ports were greater than past poppet valves, more back pressure could be tolerated, and the sleeve actually reduced bore wear because it was always moving relative to the piston - even at top and bottom dead centers - and so the lubricating oil film was always energized. The shallow cylinder heads allowed the engine to be compacted within less frontal area, and the motions of the sleeves and their driving mechanism were altogether less vicious and limiting than the jerking and hammering of poppet valves
Add the particular virtues of the small-cylindered Napier layout, contributing high crankshaft rates and vast breathing capacity in relation to the physical size, and with all bearings (thinwall shells, to be copied by Vandervell), and shafts fully supported in a naturally rigid structure - the result could hardly fail to be a prodigy. So when, after nearly five patient years, Napier was grudgingly allowed to type test the Sabre at 2,O5Obhp and 3,70Orpm, it was already capable of very much more. Throughout its life it was dogged by this discrepancy between the truth and what was allowed to be published. The Air Ministry made sure that Rolls-Royce was kept fully informed of the project, enabling it to wage a campaign of suppression that varied from slander to sabotage.
The R-R people could be dangerously nasty when embarrassed. Some consolation came after Lord Beaverbrook was made Minister for Aircraft Production in 1940: he saw the promise of the Sabre and commanded facilities for its manufacture. It might never be built in huge numbers, might forever be handicapped by such things as the sequestration of all the best and most suitable airscrews for use only on R-R engines, might forever suffer from the lies spread about it; but it would be built, and flown, and strike fear into the enemy as no other engine ever did.
To this day it is fashionable to doubt the supposed power of the Sabre. My information came from the wartime R&D department at Napier, and refutes the popular opinion. In its first production form, for the Hawker Typhoon, the Sabre was modestly rated for combat at 2,615bhp at 3,850rpm and about 151b/in2 boost. Later it was given water/methanol injection, raising that to 3,055bhp at 171b/in2 - still a shadow of its real ability, for it was known to be capable of a sustained 3,750bhp. That was actually the most it was ever set to deliver in service, though there had been experimental engines built (featuring much higher boost from a three-stage supercharger - the production Sabre was quite modestly boosted, never being envisaged as a high-altitude engine) that it could deliver 5,5OObhp at 4,20Orpm, running at a boost of about 451b/in2 and a b.m.e.p. of 4671b/in2.
Nor was that 3,750bhp service a typical combat rating that might be maintained for only 15 minutes. It was a figure that could be sustained hour after hour, day after day: Napier tested the Sabre at that rating for 175 hours non-stop. The company's usual endurance test was more varied: 10 hours at cruise rating, three hours at climb rating, one hour at take-off power, and one hour at combat maximum, with the whole 15-hour cycle being repeated non-stop over and over again.
What this perhaps proved incidentally was the greater suitability of the sleeve valve for a liquid-cooled engine than for the air-cooled type, for Bristol was never able to display such results.
As it happens, the two firms were mutually supportive and the centrifugally cast austenitic steel sleeves for the Sabre were actually made by Bristol, although with different surface finishes from the bigger ones in Bristol engines.
In the last year of the war the Sabre displayed amazing reliability. When new it had its troubles, as a matter of course, as surely did every engine; once in service, given decent maintenance (which was not always possible) it gave few problems. One was persistent: it was liable to catch fire as it started. Starting was not by electric motor but by Coffman cartridge while the pilot worked the Kigass priming pump; part of the standard starting drill was to have a man standing by with a fire extinguisher aimed at the air intake until all was running smoothly.
However much the Sabre frightened the enemy, this quirk of behavior frightened some trainee pilots quite seriously. Had Napier been free to develop the direct fuel injection system it made before the war, the fire problem would have disappeared.
Ground-attack duties provided another yardstick with which to measure performance. The net weight of the Sabre was only 1,071kg; the fully fuelled Typhoon weighed 5,130kg - but it carried, as no lesser fighter could, two 454kg bombs under its wings as though they were not there. This included a fabulous climb from sea level to about 2,400m for a lo-hi-lo dive-bombing mission, or fully acrobatic jinks during a low-level attack. This aircraft, and its later and aerodynamically superior derivative the Tempest, also served well as nocturnal aircraft, whether for interception duties or for ground attack and interdiction, and pilots particularly welcomed the absence of glare from the exhausts.
The last word on the Tempest is that at low altitudes it was the fastest aircraft of the war. Even some Americans have conceded this. It was the only operational fighter that could overhaul a Vl in straight and level flight. Two Tempests were even seen to chase and catch (and destroy) an Me262 jet. There seems to have been nothing in the air (apart from the rockets some of them fired) that could accelerate so fast.
WHY I LOVE....
I am charmed by the original compressed-air rotary engine of Hargrave, as by the dainty Gnome and other rotaries that took the air up to 1918. I am captivated by the 1966 five-cylinder 125cc racing Honda, safe to 24,000rpm and knock-proof on 85 RON petrol. I feel profound respect for the utterly simple solid-fuel rocket engine, blasting into space with a rapidly lightening load of solid rubber and aluminum dust. Yet nothing that I know can unseat the overwhelming fascination inspired by the prodigious Napier Sabre, surely the most brilliant, most maligned, most iconoclastic, and most ferociously competent piston engine of all. Use the latest technology to make an engine like that now, and what marvels might ensue!
Engine Technology International March 2006
Reproduced with kind permission of UKIP Media & Events Ltd and with help from Steve Darnell
The Hawker Tempest Page Collection