[rebel-builders] Rebel - engine choices - fuel economy --- was 0-2
Posted: Sun Feb 19, 2012 5:02 pm
To me the volumetric efficiency is the ratio of the actual volume of air/fuel mixture ingested compared to the theoretical maximum of fuel air mixture. When you say you get 110% volumetric efficiency how do you figure that you get 10% more than the theoretical maximum? I can recall when I did my Masters degree in Engineering another student asking the Prof (a PhD who literally 'wrote the book' we were using) if supercharging wouldn't give you more than 100%. HIs response was you have to look at the conditions, that is you now have a much higher manifold pressure so the theoretical maximum amount of air that can be stuffed into the piston is much higher. Any restriction in the inlet is going to cause flow losses and you will end up with less than 100% volumetric efficiency unless you stop the piston at BDC and wait until it is completely full and the flow has stopped (no flow losses then). And this assumes that there are no residual gasses from the last cycle (100% scavenging effic
iency - also very diffi
c
ult to obtain).
Having said that, there is no doubt that in 99% of the cases a four valve head is going to flow better than a 2 valve head.
Dave Jackson, MSc PEng> Date: Sat, 15 Nov 2008 20:26:30 -0500> From: klehman@albedo.net> To: rebel-builders@dcsol.com> Subject: Re: [rebel-builders] Rebel - engine choices - fuel economy --- was 0-2> > Hi Paul> > This stuff gets interesting. The 4 valve soob can get almost 110% > volumetric efficiency at 4400 rpm as well. That compares to around 75% > for a stock Lycoming. So that also magnifies the higher compression > efficiency. At 21"MAP I'm not really taking much advantage of that in > cruise but I'd guess my SFC at full power is dramatically lower than the > Lyc at full power and full rich. And of course with liquid cooling, the > soob (and maybe the rotax?) enriches very little at full power compared > to a Lyc.> Ken> > C&P Kucera wrote:> > I agree, fuel injection helps spread the fuel around so that it burns more > > efficiently.> > One other thing is having higher compression ratio, this has a big effect on > > increasing piston engine efficiency, auto engines, Rotax, diesels
are > > designed to han
d
le higher cr, Lyco,Conti, etc are not designed for it, > > pressures and temperatures climb with higher cr, more energy is extracted > > from same amount of fuel as the cr goes up.> > > > Paul> > > > 453R> > > > > > -----------------------------------------------------------------> List archives located at: https://mail.dcsol.com/login> username "rebel" password "builder"> Unsubscribe: rebel-builders-unsubscribe@dcsol.com> List administrator: mike.davis@dcsol.com> -----------------------------------------------------------------> > >
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iency - also very diffi
c
ult to obtain).
Having said that, there is no doubt that in 99% of the cases a four valve head is going to flow better than a 2 valve head.
Dave Jackson, MSc PEng> Date: Sat, 15 Nov 2008 20:26:30 -0500> From: klehman@albedo.net> To: rebel-builders@dcsol.com> Subject: Re: [rebel-builders] Rebel - engine choices - fuel economy --- was 0-2> > Hi Paul> > This stuff gets interesting. The 4 valve soob can get almost 110% > volumetric efficiency at 4400 rpm as well. That compares to around 75% > for a stock Lycoming. So that also magnifies the higher compression > efficiency. At 21"MAP I'm not really taking much advantage of that in > cruise but I'd guess my SFC at full power is dramatically lower than the > Lyc at full power and full rich. And of course with liquid cooling, the > soob (and maybe the rotax?) enriches very little at full power compared > to a Lyc.> Ken> > C&P Kucera wrote:> > I agree, fuel injection helps spread the fuel around so that it burns more > > efficiently.> > One other thing is having higher compression ratio, this has a big effect on > > increasing piston engine efficiency, auto engines, Rotax, diesels
are > > designed to han
d
le higher cr, Lyco,Conti, etc are not designed for it, > > pressures and temperatures climb with higher cr, more energy is extracted > > from same amount of fuel as the cr goes up.> > > > Paul> > > > 453R> > > > > > -----------------------------------------------------------------> List archives located at: https://mail.dcsol.com/login> username "rebel" password "builder"> Unsubscribe: rebel-builders-unsubscribe@dcsol.com> List administrator: mike.davis@dcsol.com> -----------------------------------------------------------------> > >
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