Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: mobydick94927 on September 24, 2014, 04:50:44 PM
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V-12 Supercharger Boost Problems
Some of the aero racing engine guys claim that the Merlin engine could withstand enormous supercharger boost levels, while the Allison was more prone to high boost damage.
Apparently, this is due to the way that the crankshaft end of the cylinder liner was secured and positioned. The Allison used a large, threaded ring torqued to 2200 ft-lbs. The RR Merlin somehow, mysteriously used an O-ring, strangely enough.
Reportedly, the Merlin's O-ring arrangement allowed the cylinder liner to expand a little under extreme conditions, without warping. In the rigidly-fixed Allison, the cylinder liner would distort and eat the piston under extremely high boost.
This prompts a swarm of questions:
Most importantly, does anyone have more information on this issue, especially how RR came to the O-ring solution in the first place?
Can anyone eleborate on how the Merlin O-ring was employed?
How did the DB 60x secure its liners - and did it have similar boost-related problems? Do racers use the DB 60x engine?
Liner distortion seems to imply that maximizing the Allison's power through supercharger development would be limited by this Achilles heel. Supercharger development accounted for most of the Merlin's 2000 hp achievement, while the larger Allison maxed out around 1550 hp.
My source for this information is an article in Air & Space Magazine, "Masters of the V-12":
http://www.airspacemag.com/flight-today/masters-of-the-v-12-3039083/
The only additional bit of information I have on this is that the Merlin cylinder liners had to be re-torqued every 25 hours, while the Allison's were more set-and-forget. So there was a price to the increased performance.
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First post ?
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One notable point between the Allison and the Merlin was the Allison was developed with a strong influence of cost to make and speed of production where the Merlin had more emphasis put on performance and less on cost and speed of production. This of course is a broad overview statement.
If no answer here try;
http://www.enginehistory.org/ (http://www.enginehistory.org/)
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The Merlin came from a long heritage of Rolls Royce racing engines.
(http://upload.wikimedia.org/wikipedia/commons/thumb/2/24/RollsRoyceR%28ScienceMuseum%29.JPG/300px-RollsRoyceR%28ScienceMuseum%29.JPG)
This is the Rolls Royce R engine, the granddaddy of the Merlin and Griffon. In 1931 this engine produced almost 2,800 hp and propelled the Supermarine S6.B to more than 400 mph at sea level. 8 years before the war.
(http://upload.wikimedia.org/wikipedia/commons/e/ec/Supermarine_S.6B_ExCC.jpg)
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Amazing.
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Which is why there are stories of pilots forgetting about putting the WEP on and running it for like 30mins or more without or only minor any issues.
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Which is why there are stories of pilots forgetting about putting the WEP on and running it for like 30mins or more without or only minor any issues.
The injection tank could well be empty before thirty minutes on some planes.
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The injection tank could well be empty before thirty minutes on some planes.
I think he was referring to engines that didnt use additives for WEP,the merlin was simply over-boosted and the only way the ground crew knew it was used was by the broken wire in the throttle!
:salute
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I think he was referring to engines that didnt use additives for WEP,the merlin was simply over-boosted and the only way the ground crew knew it was used was by the broken wire in the throttle!
:salute
Yes, you are right. I had noted on "some planes" but should have put "some planes" first in the statement to add clarity. My bad.
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As I continue to research this issue, one thing I have learned is that one has to be careful about WHICH Merlin engine is under consideration. Rolls-Royce unfortunately applied the Merlin nameplate to at least 2 and possibly 3 different engine lineages during the WW2 era.
It turns out that there is nothing linear and direct about Merlin engine history. It is all very twisted, convoluted and haphazard. This may be due to the fact that old Mr Royce himself died in 1933 and the company was thrown into some disarray. Merlin engine development fell into some unfortunate hands, it appears.
Back to the O-ring: I am suspecting that the O-ring came about with the Kestrel engine, which had wet cylinder liners. Does anyone know of this?
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The injection tank could well be empty before thirty minutes on some planes.
I've never heard of a Merlin engine wartime application with ADI. ADI is a poor-man's intercooler and the two-stage Merlins have intercoolers. I know of modified Merlins in Reno racers where they've replaced the intercooler with ADI (among a whole lot of other mods). Some of them run at close to 4,000 hp.
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Back to the O-ring: I am suspecting that the O-ring came about with the Kestrel engine, which had wet cylinder liners. Does anyone know of this?
Did you try the engine history site I posted earlier? They would be more likely to have your answer.
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I have checked out enginehistory.com and other sites - and none of them answers my questions directly. My original question centered around the use of O-rings in the Merlin engine(s).
After a lot of senseless work, I have developed a theory. Mind you, this is only my own guess - there is no guarantee that it is correct. But it makes a rough sort of sense to me, poor fool.
First, I need to refine the basic issue: it appears that the Merlin engines could withstand the extreme temperatures and pressures of incredibly high supercharger boost, while the Allisons could not. IF - this is true, then why should it be so?
What I have divined - after a long and winding road - is that the Merlin's advantage came about as the happy accident of an unfortunate decision. It was not due to any genius or foresight on the part of Rolls-Royce management or engineering. It was simply dumb luck.
To put this in perspective, we might consider the common ancestor of the the RR-Merlin and the Allison V-1710 to to be the Curtiss D-12. This engine was developed in the 1920's and powered the winners of the 1923 and 1925 Schneider Trophy races.
When Allison developed their V-12 engine around 1931, they used the Curtiss D-12 as "inspiration " for their design. The Allison appeared to be basically a Curtiss D-12, scaled up, with a supercharger added. Like the Curtiss, it had Dry Cylinder Liners.
The immediate ancestor of the RR Merlin engine was the RR Kestrel engine. Rolls Royce also used the Curtiss D-12 engine as "inspiration" for their Kestrel design. They made 3 critical differences from the D-12 design, however. One of these was the use of Wet Cylinder Liners, instead of the Curtiss's original Dry Cyliner Liners.
Herein lies the difference: Dry Cylinder Liners are reinforced and supported by the cylinder block metal - Wet Cylinder Liners are not. Therefore, in general, Wet Liners must be made stronger and more robust than Dry Liners.
The Dry Liners of the Allison engine could therefore be made of *relatively* slight and willowy metal shells. The Wet Liners of the RR Kestrel engine were forced to be more massive.
Understand that in the 1920's and 1930's that nobody, absolutely nobody, could anticipate the extreme demands of supercharger boost that were available in the 1940's. It was simply terra incognita.
Therefore, just by the accident of using thicker, stronger Wet Liners, the Rolls-Royce engines had more resilient cylinder liners that were able to stand up to the rigors of super-duper supercharging.
The later generation of Merlin engines, beginning with Merlin Mark 61, used dry liners. I suspect that RR just carried over the design of the earlier Merlin Mark II's wet cylinder liners. The use of O-rings may have had little if anything to do with the robustness of the Merlin engines. It was just the strength and robustness of the cylinder liners.
Some people have commented that the development of the Merlin engine(s) was a triumph of development over design. That is to say that Rolls-Royce management had their heads up their butts. My theory supports this case. If it were not for the "happy" concatenation of more-or-less random events and mistakes, the Merlin engine might never have been fully developed, the Battle of Britain may have been lost, and European history may have been very, very different.
For the want of a nail, a horseshoe was lost. For the want of a horseshoe, a horse was lost. For the want of a horse, a knight was lost. For the want of a knight, a battle was lost. For the want of a battle, a kingdom was lost. Eh?
The Postscript is that the performance of the Allison engine might have been vastly improved by the use of more robust cylinder liners. Alas, by the time this issue became pertinent, the piston engine was on its way out in favor of the jet.
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Dumb luck? More like poor speculation of people many years after the fact.
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A lot of factors go into design decisions. I have been in on many such meetings. I promise you the design decision on wet or dry cylinders in both companies was well thought out.
The Allison people would have known that wet cylinders cool better but as we know though the manufacturing process at Allison was geared toward production speed and cost. That is to say, how fast can we make these engines and how much can we make on each engine.
RR-Merlin decision at that time may have been based on in-house experience with wet cylinder block design, available manufacturing machines, engine weight specifications and so on.
Both companies would have had an overall design philosophy and design decisions would be guided by that. You can see that today in modern car manufactures.
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As I continue to research this issue, one thing I have learned is that one has to be careful about WHICH Merlin engine is under consideration. Rolls-Royce unfortunately applied the Merlin nameplate to at least 2 and possibly 3 different engine lineages during the WW2 era.
What would be these engine lineages?
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If you want information on high horsepower Allison engines, ask the unlimited hydroplane guys.
This one beat the turbines and I feel a better set up engine could still do that.
(http://www.thunderboats.org/history/images/history0479a.jpg)
Here's a video of it in action.
https://www.youtube.com/watch?v=4FMP9crlA7w
A friend builds merlins for tractor pulling but his engines are way beyond even the engines you see at reno.
If you're familiar with a merlin, then the picture below might not look very familiar but it's been taken further than anybody else has taken one and makes huge hp at far less boost than the merlin guys at reno.
(http://i5.photobucket.com/albums/y171/morerevsm3/Picture254.jpg)
And here's his cam drive setup........again....way beyond what anybody else has done whether Mr. Nixon or Mr. Barrow.
(http://i5.photobucket.com/albums/y171/morerevsm3/miscellaneous/Picture011_zps1559778d.jpg)
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As far as I can tell, the 3 lineages of the Merlin engine were:
Merlin Mark I - an all-new design featuring the Ramp Head concept. It was unsuccessful.
Merlin Mark II - an old design, the Kestrel, scaled up from 22 to 27 liters displacement.
This was the Battle of Britain design.
Merlin Mark LXI (61) - an all-new design, neither Ramp Head nor Kestrel. Introduced in England in 1942 and in the U.S. (Packard) in 1941. This became the Mustang engine, as well as late-model Spits, etc.
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Understand that in the 1920's and 1930's that nobody, absolutely nobody, could anticipate the extreme demands of supercharger boost that were available in the 1940's. It was simply terra incognita.
What boost level do you think the Rolls-Royce R engine used in the Supermarine S6.B was at in 1931 in order to produce almost three times the power of the Merlin II in a Spitfire Mk I in 1940?
I've also never read that the Merlin II was simply a scaled up Kestrel. Nor that the Merlin 60 series was a completely new design rather than a change to the supercharger built into the engine. You'll need to support those claims.
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The first American version of the Merlin was the Packard Merlin 28 (Mark XX (first production Merlin XX, 4 July 1940)), designated the V-1650-1. The first Packard Merlin ran in Aug 1941. The first Merlin 60 engine ran in March 1941.
The V-1650-1 was had a single stage, 2 speed supercharger.
Karnak, the heads of the Merlin II were scaled up from the Kestrel. Only 172 Merlin Is were made.
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One of my main sources was enginehistory . org , in the article:
The Rolls-Royce Merlin Aero Engine
by J. SJ. Wells
Regarding the change from Merlin Mk I to Mk II, Quote:
"At Rolls-Royce, instead of taking off their jackets and sorting the Merlin problems out, they chose to simply dust off the old Kestrel, enlarge the capacity to 27 litres and put it back into production using the spurious title, Merlin II."
This article gives an interesting history of the Merlin(s) beginning in 1932, including a discussion of the early, unsuccessful Ramp Head (Mark I) engine, of which only about 175 were produced.
Regarding the tremendous power of the "R" racing engines:
The "R" engines were enormous at 37 liters displacement - around 600 cubic inches larger than the Merlin (which itself was about 300 cubic inches larger than the Kestrel). The R engines were flat-out racing engines - they only had to last for one race, thus they could be tweaked beyond all reason. The "R" engines were run on custom-made blends of fuel while the Merlin was limited to whatever was available in quantity in wartime. According to one source, the "R" engine could have 18 psi boost. The "R" engines were essentially "hand-made" and 19 were produced.
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Yes, the Rolls Royce R engine led indirectly to the Griffon (Spit14); also a 37-litre engine.
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Regarding the tremendous power of the "R" racing engines:
The "R" engines were enormous at 37 liters displacement - around 600 cubic inches larger than the Merlin (which itself was about 300 cubic inches larger than the Kestrel). The R engines were flat-out racing engines - they only had to last for one race, thus they could be tweaked beyond all reason. The "R" engines were run on custom-made blends of fuel while the Merlin was limited to whatever was available in quantity in wartime. According to one source, the "R" engine could have 18 psi boost. The "R" engines were essentially "hand-made" and 19 were produced.
Absolutely, but that they had been built (and I assume other engines from other companies/nations in the same vein) indicates that the need for ever more power was foreseeable and the boost pressures needed were understood.
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The insider details of Merlin supercharger history are fortunately preserved in a filmclip available on youtube:
https://www.youtube.com/watch?v=by4lH2whhjk
(Sir) Stanley Hooker was hired by Rolls Royce with no particular assignment in mind.
He eventually took over management of the Merlin supercharger program.
Check it out.