spiralcopter
Airman
- 16
- Dec 21, 2021
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Its a long time since I read up on Ducatis. When I first started riding bikes Hailwood won on one at the Isle of Man but that was a 864cc V twin. Then what I read was the rev limit was set by piston speed, the desmo valve system used less energy and allowed better valve timing, the valves could open and close more sharply. The modern Moto GP bikes are 1000cc V4s. What is high revving depends on cylinders and displacement, Mike Hailwoods 250cc six cylinder revved to just short of 20,000RPM.It occurs to me that at extremely high RPMs, sleeve valves would have the advantage that they do not suffer from valve float, as the valves are positively driven rather than spring-loaded. Ducati motorcycle engines can be revved to over 10,000 RPM in part because they have positively driven desmodromic valves.
On the other hand, I have it on the authority of someone who has worked on sleeve valve aero engines that the valve timing geartrain will essentially explode if you try and run them too fast. So clearly there are limits.
Was this actually a relevant advantage? Did anyone ever get a sleeve valve to rev high enough that the lack of valve float mattered? It occurs to me that the Sabre, with its displacement divided into over double the usual number of cylinders, should have been able to support high RPMs if only they could keep the valve timing geartrain (and everything else) from having problems.
One must bear in mind that the sleeves are quite heavy. In the aircraft engines I have studied their weight varies from 10 to 15 lb. That is a lot of reciprocating mass whose inertial forces must be borne by the drive train.It occurs to me that at extremely high RPMs, sleeve valves would have the advantage that they do not suffer from valve float, as the valves are positively driven rather than spring-loaded. Ducati motorcycle engines can be revved to over 10,000 RPM in part because they have positively driven desmodromic valves.
On the other hand, I have it on the authority of someone who has worked on sleeve valve aero engines that the valve timing geartrain will essentially explode if you try and run them too fast. So clearly there are limits.
Was this actually a relevant advantage? Did anyone ever get a sleeve valve to rev high enough that the lack of valve float mattered? It occurs to me that the Sabre, with its displacement divided into over double the usual number of cylinders, should have been able to support high RPMs if only they could keep the valve timing geartrain (and everything else) from having problems.
I think there was another issue with Bristol and Napier, they couldnt mass produce the sleeves to the tolerances and quality required.Any mechanical device run outside the design envelope is operating in the safety margin.
My favourite definition of an engineer is: 'Any idiot can design a bridge that won't fall down, but it takes an engineer to design a bridge that will only just not fall down."
If the sleeve valve engines are "unreliable" when operating over their design speed, it is not the fault of the engine.
My theory is that they would be more reliable if the correct oil with the correct additives is used.
The notorious Centaurus
I´m aware that it´s common practise, at least in the USA, to substitute a Wright Cyclone R-3350 for the original Bristol Centaurus in Sea Fury rebuilds. This is said to be due to the unreliability of the BC and, though realizing that it´s a complex design, I´ve always wondered what causes the...www.key.aero
I think there was another issue with Bristol and Napier, they couldnt mass produce the sleeves to the tolerances and quality required.
I'm curious as to what that accident was.To state they couldn't mass produce them isn't a fair reflection of the whole story.
Bristol has no issues with tolerance until they had to move to mass up production. The issue took 4 years to solve (by accident as it turned out).
After late 1939, it was other issues that held them up. By 1941, production was in full swing.
Napier struggled until Banks forced Bristol to give them a Taurus sleeve and the technology to make them - late 1942/early 1943.
Within a few years of the end of the war, the civilian Hercules was being used successfully and was still being sold until 1968. They only stopped when RR took over and refused to make any more.
Calum, then please explain why the post-war Centaurus had substatially higher power on 100/130 than the poppet-valve R-3350? I love your book but on this issue I side with Harry Ricardo.
Harry Ricardos interest in Sleeves was from an era before poppet valves had reached the high degree of metallurgical refinement
using a completely different engine to say sleeves are better whilst saying "don't compare it with THAT" one.One should not compare a turbocompounded variant to a non-compound one. The Centaurus 661 maximum is 2700 hp at 2800 rpm on 100/130 while the R-3350 as fitted in the AD-4 is 2700 hp at 2900 rpm on 115/145. To me the same power from a slight smaller size on fuel with 15 points lower PN is a massive advantage.
And Ricardo advocates sleeve-valves in his book written in the 1950s, after those metallurgical advantages.
That sleeve-valves are not used today means nothing. After all, how many decades it took for direct injection to be universally adopted in cars? VHS won over Beta even though the latter was much superior.
What did they have to do to get that power?Calum, then please explain why the post-war Centaurus had substatially higher power on 100/130 than the poppet-valve R-3350? I love your book but on this issue I side with Harry Ricardo.