Fuel Econ and Hiway Mode

[Guenther, Max [ASCO/PA]]

What I meant is this,
	The effective compression ratio changes with throttle position.  The
closer to atmospheric pressure in a manifold(in a naturally aspirated
engine) the higher the effective compression ration.  The more stuff you
have in the cylinder the higher the pressure in the cylinder, regardless of
whether or not it is compressible.  At idle or part throttle your 11.5 to 1
454 is obviously developing less cylinder pressure than at a point when the
throttle is wide open(at the moment before ignition).  Yes the actual
compression ratio is the same but when you compress a gas which is at less
than atmospheric pressure the end pressure will obviously be less than if
you compressed it the same amount but had started at atmospheric pressure.
This causes a severe loss of efficiency(generally, increased compression
ratio increases efficiency (freshman thermo)).  Its why an engine is more
efficient at WOT.  It may not make a huge difference but to say that there
is none, I would have to disagree.  (but I have been wrong before;-)

I believe this is why a small engine running at max output is much more
efficient than a large one running at half even with more reciprocating mass
etc

Max


No flames please;-)
  



> >I don't think he was implying that inert gasses aren't compressible,
> >just that...more gas volume (but not more petrol) + same cylinder volume
> =
> >higher compression=better efficiency.   (of course, that's assuming all 
> >other
> >things being equal...which they never are!)
> 
> Are you thinking that the EGR is in addition to the regular charge via 
> pressure or something?  EGR replaces an equal volume of charge.  I don't
> see 
> any change in pressure.  I stand by my very-small-rocks theory.
> 
> Jeremy
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