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Date: Sat, 08 May 2004 17:11:29 GMT
From: -Bob- <uctraingNOSPAMMEnospamanet.com>
Subject: Re: Compression ratio w/turbo blowing ?


On Sat, 08 May 2004 17:27:13 +0100, Johannes H Andersen <johsnospamfitter_nos_pam.com> wrote: > > >-Bob- wrote: >> >> On Sat, 8 May 2004 17:21:29 +0200, "MH" <nomailnospamere.no> wrote: >> >> >The compression ratio does not depend on the boost pressure, it is the ratio >> >of the smallest and largest volume in one cylinder (piston at TDC and at >> >BDC). >> >see http://en.wikipedia.org/wiki/Compression_ratio >> > >> >Max boost pressure in a FPT 900 is 0,7 bar (over atmospheric pressure = 1 >> >bar), at that pressure the waste gate will (or should) open. >> >> But wouldn't the "effective" compression ratio be higher since the >> charge is denser than a normal charge ? Or does the turbo simply make >> the pump run faster ? > >But such an "effective" compression ratio wouldn't be constant, since >the cylinders don't fill so readily at higher rpm. But the geometric >compression ratio is lowered to compensate for the higher degree of >filling of the cylinders due to the turbo pressure, effectively >increasing the volume of the engine compared to an NA engine. (Positive >displacement engine). Hence the compressed space must also be larger. So a turbo motor runs faster because the effective _volume_ of the engine is increased not because the effective flow is increased ? (noting that as the motor runs faster, flow will go up too). In other words, cramming a denser charge in a cyl that would normally only hold an atmospheric charge effectively makes the motor run as if it was larger ? Does anyone (engineer) attempt to measure this as an equivalency ? What I mean is... if you have 14lbs of boost, the charge will be "X" times more dense, can we predict that the motor will generate "Xn" times more power (ignoring the obvious layman's 'standard' that we usually see 100hp/liter in most production turbo's).

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