Variable Compression, Variable Displacement you decide

[jb24 at chrysler.com]

You gotta love a group that is always trying to understand how to make
more power.
Couple of points for this thread:

1.  VE is technically just based on volume throughput given a reference
density.  Supercharged (both turbo and crank-driven) engines typically
have higher flow losses and therefore lower VE through the cylinder
itself (at the above-atmospheric reference density).

2.  Thermodynamically, compressor efficiencies will reduce the overall
heat conversion efficiency.  Therefore more stages typically have
greater inefficiencies.

3.  Crank-driven superchargers do take more power to spin than turbos
because of losses in the drive-belt.  All else being equal, for a given
gross output turbos will have the higher net output (although only
fractions of a percent different).

4.  Taking a stock motor and putting a bar of boost into it won't
double your peak atmospheric-referenced VE.  Inlet and exhaust tuning
and associated valve timing can contribute 20% or more to the peak VE.
Race car engines and stock motorcycle engines have in the neighborhood
of 110% peak VE without supercharging.  Race-tuned two-strokes run
somewhere in the range of 125% VE, however they only manage to burn
about 70% of that at peak power (the rest goes out the exhaust), thus
only make somewhat more than 160% of the specific power of race
four-strokes.

5.  The best measure of engine output is Brake Mean Effective
Pressure.  This pressure is the average of the pressure on the
expansion stroke to get a given torque, and is calculated based on
dynamometer measurements (brakes).  Automotive applications are around
3-7 bar BMEP.  Race engines can get up around 11 bar.  Supercharged
engines at the peak of F1 turbos hit 22 bar with 5 bar of boost.  22
bar was about 1,000 hp / liter out of a 1.5 liter V6.  Horsepower is
BMEP times rpm and a correction factor.

John Bucknell is jb24 at chrysler.com
---------------------- Forwarded by John R Bucknell/JTE/Chrysler on
02/17/98 05:13 PM ---------------------------

        owner-diy_efi @ efi332.eng.ohio-state.edu
        02/17/98 04:24 PM
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Subject: Re: Variable Compression, Variable Displacement you decide

Roger Heflin wrote:

> torque corves, therefore they aren't equivalent.    Also the number of
> compression stages and the number of expansions stages has nothing
> to do with the efficiency.  Thermodynamics says more stages don't
> increase efficiency.

I'm going to disagree on this point (from a purely theoretical
standpoint). If you have a cycle on the T-S chart (temp and entropy, I
think) a multistage cycle can get you more efficiency than just one
large single stage.

>  The reason a supercharge is generally
> less efficient is that they compress more air than is needed, and
substantially
> increase the pressure in the intake more than is necessary.

I don't understand this point. I thought more air (higher boost) was
always better. You have more air, so you add more fuel to keep the right
AF ratio, and vroom! More power :) If you are talking about this from an
efficiency point of view, I understand then :)

Andris - mech E student struggling with Heat xfer at the moment