I'd almost swear I heard they strengthened the engine-block and boosted the compression-ratio.
How would you boost compression ratio?
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I'd almost swear I heard they strengthened the engine-block and boosted the compression-ratio.I don't think they did anything to the engine.
why would you?I'd almost swear I heard they strengthened the engine-block and boosted the compression-ratio.
How would you boost compression ratio?
I'd almost swear I heard they strengthened the engine-block and boosted the compression-ratio.
How would you boost compression ratio?
A masterpiece of understatement gumbyk.That's not a field mod.
Well, it was technically a maintenance-depot mod: I remember reading about it, of all places, Wikipedia. Normally, this isn't a suitable source, but the source was "Spitfire: The History" by Eric B. Morgan and Edward Shacklady, which I own. The source listed pages 149, 155-156.That's not a field mod.
So, that said -- how do you drive up a piston-engine's compression-ratio? Increase the RPM?.......Another successful Middle East modification, again by No 103 MU, was the high altitude VC developed to meet the high flying Ju86P twin engined, photo-reconnaissance aircraft. Allied naval movements from Egyptian bases, so vital to the defense of the Middle East, were being monitored by a team of four Ju86s and there was little the RAF could do when the German aircraft flew over at heights of 37,000 ft plus. Normal operation ceiling of the VC was approximately 36,000 ft, but at that height it had to be an exceptional airplane and was, normally, an unstable gun platform. The staff of No 103 MU, practically built their own high altitude fighter by taking a new VC (BP985) that had arrived at the depot on 4 May 1942 and stripping it of all unnecessary weight.
........It was not feasible to increase the supercharger gear ratio so it was decided to raise the compression ratio of the Merlin 46 by as much as possible. The cylinder block was modified by hand as were the liners. A four blade de Havilland 45/1 Hydromatic propeller was installed, plus an Aboukir filter which incorporated the 91/2 gallon oil tank. All armoured plate was removed together with the Hispano cannon installation leaving just the two inner .303 Browning machine guns. Finally, extended wing tips of local manufacture were added.
So, that said -- how do you drive up a piston-engine's compression-ratio? Increase the RPM?
So they basically reduced the volume inside the cylinders so the pressure would go up more?different pistons, shave the surface of the head and/or block.
The compression ratio, is the ratio of the volume of the cylinder at BTC compared to when it is at TDC. Basically how much of the swept volume is compressed into the "combustion chamber" which is what is left when the piston is at the top of its stroke. With a supercharger or turbo charger the compression ratio of the engine is a bit academic because the gas is already compressed when the piston is at bottom dead centre. This is what all the discussions of pressure and fuel grades is all about. A Merlin or any other supercharged engine has a very low compression ratio compared to any normally aspirated high performance engine.Well, it was technically a maintenance-depot mod: I remember reading about it, of all places, Wikipedia. Normally, this isn't a suitable source, but the source was "Spitfire: The History" by Eric B. Morgan and Edward Shacklady, which I own. The source listed pages 149, 155-156.
While page 149 didn't seem to have anything of use, page 155-156 did: Under "MIDDLE EAST MODS", there's the following entry
So, that said -- how do you drive up a piston-engine's compression-ratio? Increase the RPM?
I understand that. What I'm curious about is how the increase in power would be expressed.The compression ratio, is the ratio of the volume of the cylinder at BTC compared to when it is at TDC. Basically how much of the swept volume is compressed into the "combustion chamber" which is what is left when the piston is at the top of its stroke.
No idea what you are asking, what does "expressed" mean? What is the increase in power?I understand that. What I'm curious about is how the increase in power would be expressed.
Spin the shaft faster?
Absorb more power by adjusting the propeller for the given engine/propeller RPM?
I understand that. What I'm curious about is how the increase in power would be expressed.
Spin the shaft faster?
Absorb more power by adjusting the propeller for the given engine/propeller RPM?
Is it possible that they increased the compression ratio to get a slight improvement at very high altitude trading lower performance at lower altitude because using maximum boost would cause detonation?The extra power would allow the propeller to be run at a coarser pitch.
The engine speed would not necessarily change, and certainly wouldn't for a WW2 aero engine.
Usually for a higher compression ratio you can't run as much boost. For this particular application that wasn't as important, since the performance gain they were seeking was well above the engine's critical altitude.
Is it possible that they increased the compression ratio to get a slight improvement at very high altitude trading lower performance at lower altitude because using maximum boost would cause detonation?
It isn't my "major" but as I understand it the higher you go the less the twin supercharger works. Eventually you reach a stage where the total effect of the twin supercharger and intercooler delivers a charge that is at the level (or close to) sea level atmospheric pressure. Then you use the "tricks" that an atmospheric engine uses, like increasing the compression ratio. As I see it the engine was optimised for one use and would be of little use anywhere but extreme altitude.Okay, so the modifications would serve to drive up the normal pressure within the cylinders, which would increase horsepower for a given throttle setting, and allow the propeller to be driven at a higher pitch setting for the same RPM. With the pressure within the cylinders being higher, the risk of detonation would be higher and, the engine would need a bit of throttling at lower altitudes, in order to prevent this, and a greater throttling loss would occur in this area?
The Aboukir Spitfire used a single stage supercharger with two speeds.It isn't my "major" but as I understand it the higher you go the less the twin supercharger works.
Its the same principle. The density of the exploding charge is what governs the power output. To achieve that density the supercharger becomes less effective the higher you go. The Merlin and Allison engines are not highly tuned for atmospheric engines the tuning is in the supercharger or turbo set up. A compression ratio of 6-1 is "cooking" in a normally aspirated engine.The Aboukir Spitfire used a single stage supercharger with two speeds.