The banner above is an advertisment - if it asks you to download software, please ignore.
Site News - 3/26 M Car Covers (by State of Nine) | 12/12 Make Amazon Pay Saabnet!
Date: Sat, 08 May 2004 18:40:32 +0100
From: Johannes H Andersen <johsnospamfitter_nos_pam.com>
Subject: Re: Compression ratio w/turbo blowing ?


-Bob- wrote: > > 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). It doesn't necessarily run faster, it produces more power at same rpm. Since the cylinder filling is less restrictive at low rpm, the turbo pressure moves the torque curve towards lower rpm. That is provided that there is enough gas flow to spin the turbo wheel. You typically see high performance NA engines have peak torque around 4000-5000 rpm, while turbo engines have peak torque at 2000-3000 rpm. The positive displacement effect works particularly well at low rpm, the engine feels larger than the bhp number suggests, even taking into account the peak power figure of the turbo machinery. > 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). Well I know some fluid dynamics, but I'm not an mechanical engineer. The relationship is complicated since it is not a direct relationship between boost and effective volume. This is because of the inevitable flow resistances through the many nooks and crannies before the gas mixture ends up in the cylinders. And these flow resistances grows non-linearly with flow speed.

Return to Main Index
StateOfNine.com
SaabClub.com
Jak Stoll Performance
M Car Covers
Ad Available

The content on this site may not be republished without permission. Copyright © 1988-2024 - The Saab Network - saabnet.com.
For usage guidelines, see the Mission & Privacy Notice.
[Contact | Site Map | Saabnet.com on Facebook | Saabnet.com on Twitter | Shop Amazon via TSN | Site Donations]