Ancient History

[Darrell Norquay]

At 08:06 AM 9/6/96 -0500, Tom Cloud wrote:

>I haven't dug up my stuff about VE and CFM and displacement, but seems to
>me that an xxx CID engine is a pump that always pumps xxx CI of air every
>two revolutions (assuming a 4-stroke).  All that changes is the density
>(i.e. throttle closed = lower density A/F charge). 

This would only be true if the engine is a perfect air pump.  Due to a few
major factors and a zillion minor ones, the engine's VE changes quite
radically with RPM, and is in almost all cases <100%.  This means that if
the VE is 80% at a particular RPM, a cylinder is only packing 80% of it's
true volume of air/fuel mixture into it at that speed.  VE is related to,
but not to be confused with overall energy conversion efficiency, which is
much lower.

You can tune the intake runners and exhaust system to give 100% or even
greater than 100% VE's, but only over a very narrow RPM range.  If you tuned
the intake and exhaust paths to give optimum VE (which generally coincides
with the torque peak) at, say, 3000 RPM, and chose an overall gear ratio
that put 70 MPH right at 3000 RPM, in combination with some ignition advance
and fuel leanout, you could get phenomenal (comparatively) cruising fuel
mileage with any engine.  It'd be no fun at all to drive, though, unless it
had a 27 speed transmission... 

OEM's tune the system for a broad, flat VE (torque) curve, to give the
engine driveability, economy, and durability.  Aftermarket parts makers (and
DIY'ers) attempt to increase the peak output at the expense of driveability,
economy, and durability.  It should also be possible to increase the economy
at the expense of power output and driveability.  The area under the VE
curve, though, would stay pretty much the same for a given size NA engine,
no matter which approach you take.     TANSTAAFL.

>And I thought that running R-E-A-L lean meant burning things up (I KNOW it
>does!).  So, it takes a certain amount of fuel to just coast, much less
make >any power.

This is only really true at WOT.  Running lean, while trying to make maximum
power from the engine is a bad idea.  Different story at light loads or
cruise. I have seen industrial methane powered stationary engines running at
AFR's of 28:1, and they last a longggg time.  Once you get the engine
running, you can lean out the mixture until the engine is just making enough
power to balance the power required by the load, (overcoming drag, friction,
etc).  Ask any pilot, he will tell you that as a plane reaches cruise
altitude and speed, you lean it out until the engine is just making enough
power to maintain cruise conditions.  This gives maximum fuel economy.

>So, how does one take a 400+ CID engine and get that kind of mileage
>unless he's running a 1.7:1 rear end (i.e. L-O-W r's) -- even if he had
>100% VE ?

Fuel consumption is pretty much directly proportional to RPM (as is HP) for
a given displacement.  Fuel consumption is also tempered by the shape of the
VE curve.  Maximum efficiency occurs at the VE peak, which coincides with
minimum energy required to produce a given amount of power.  So, you're
right, a low numerical rear end ratio or overdrive does increase mileage,
but lowering RPM would also decrease HP and VE, and thus overall engine
output. If you set it all up jesssstt raaht, so that VE, torque, and HP all
peak at a low enough RPM, which also coincides with your normal cruising
speed, voila`, mega-MPG.  


regards
dn
dnorquay at awinc.com