I'd almost swear I heard they strengthened the engine-block and boosted the compression-ratio.
How would you boost compression ratio?
Aboukir’s High-Altitude Spitfire (Original Modification)
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How would you boost compression ratio?
why would you?
That's not a field mod.
A masterpiece of understatement gumbyk.
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?
wuzak
Captain
different pistons, shave the surface of the head and/or block. From the description you provide it looks like they shaved the block (and cylinder liners) to raise compression.
So they basically reduced the volume inside the cylinders so the pressure would go up more?
No they didn't reduce the volume inside the cylinders, they reduced the volume in the combustion chambers.
The combustion chambers is on the top of the cylinders.
So that the fuel/air mixture in the cylinders is compressed into a smaller space when the pistons get to the top of their stroke.
The combustion chambers is on the top of the cylinders.
So that the fuel/air mixture in the cylinders is compressed into a smaller space when the pistons get to the top of their stroke.
Higher compression has nothing to do with producing more RPM.
It means you produce more HP per power stroke of each piston.
Zipper you're asking questions that indicate you have little knowledge of the fundamentals of the internal combustion engine.
You need to address this shortfall yourself rather than rely on this forums members to constantly educate you a tidbit at a time.
It means you produce more HP per power stroke of each piston.
Zipper you're asking questions that indicate you have little knowledge of the fundamentals of the internal combustion engine.
You need to address this shortfall yourself rather than rely on this forums members to constantly educate you a tidbit at a time.
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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.
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?
No idea what you are asking, what does "expressed" mean? What is the increase in power?
wuzak
Captain
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?
wuzak
Captain
Yes.
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?
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.
The Aboukir Spitfire used a single stage supercharger with two speeds.
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.
