Overall Compression Ratios of the Big Pistons
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Overall Compression Ratios of the Big Pistons
The big piston engines such as the Merlin, Griffon, Napier Sabre, R-3350 & R-2800 are quoted as having compression ratios between 6:1 and 7:1. Clearly you wouldn't need fuel with the lead content that 115/145 avgas has if this were the whole story. The answer of course, is that the engines were either supercharged or turbocharged. What I haven't seen quoted is the effective compression ratio when the boost is taken into consideration as well. Have you any figures?
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It's not the mechanical compression ratio that matters but the actual max pressure in the combustion chamber just before ignition.
Some years ago, an American motor engineer designed a camshaft which allowed the intake valve to close later than normal. By doing this he effectively lowered the peak pressure in the combustion chamber; this then allowed him to increase mechanical compression ratio to about 14:1 without pre-ignition using 'standard' unleaded fuel of about 86 octane thus making the engine more efficient.
Typically a 7 litre dragster engine which runs a high boost Roots type mechanical supercharger and burns a nitro methane/methanol mix of fuel produces about 3,000hp and has a compression ratio of 5 or 6 to 1.
Some years ago, an American motor engineer designed a camshaft which allowed the intake valve to close later than normal. By doing this he effectively lowered the peak pressure in the combustion chamber; this then allowed him to increase mechanical compression ratio to about 14:1 without pre-ignition using 'standard' unleaded fuel of about 86 octane thus making the engine more efficient.
Typically a 7 litre dragster engine which runs a high boost Roots type mechanical supercharger and burns a nitro methane/methanol mix of fuel produces about 3,000hp and has a compression ratio of 5 or 6 to 1.
Last edited by chevvron; 29th Jul 2017 at 13:08.
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Thank you for the replies. Cyclic 35's link to a converter gave me what I was looking for.
As an example, a RR Griffon 57 at 6:1 compression ratio with 25psi boost at sea level has an effective compression ratio of 16.2:1.
Now I can see why all that tetraethyl lead was required!
As an example, a RR Griffon 57 at 6:1 compression ratio with 25psi boost at sea level has an effective compression ratio of 16.2:1.
Now I can see why all that tetraethyl lead was required!
Small correction on the Top Fuel Dragsters...that's more like 8000hp.
About a 1000hp per cylinder.
And it needs the equivalent of a plasma welding arc to ignite the fuel at those compression levels.
Hence the massive dual ignition systems.
About a 1000hp per cylinder.
And it needs the equivalent of a plasma welding arc to ignite the fuel at those compression levels.
Hence the massive dual ignition systems.
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Thank you for the replies. Cyclic 35's link to a converter gave me what I was looking for.
As an example, a RR Griffon 57 at 6:1 compression ratio with 25psi boost at sea level has an effective compression ratio of 16.2:1.
Now I can see why all that tetraethyl lead was required!
As an example, a RR Griffon 57 at 6:1 compression ratio with 25psi boost at sea level has an effective compression ratio of 16.2:1.
Now I can see why all that tetraethyl lead was required!
