Re: GAS-L: Wasted throttle power

[Andrew Heggie <andrew.heggie@dtn.ntl.com>]

On Tue, 20 Feb 2001 18:01:42 EST, Tom Reed wrote:

>Dear All:
>
>Having taught thermodynamics and studied the Carnot and Otto cycles, I was 
>impressed by the LINEAR increase in efficiency with compression ratio in the 
>range CR  6-12.  (Above 12 frictional forces reduce the advantages.)

My (limited) understanding is that the maximum otto cycle conversion
of heat to motion is given by Eff=1-CR^(1-k) where CR is the
compression ratio and k is the ratio of specific heats at constant
pressure and constant volume for the working gas.

With air and k at 1.4 this gives:
cr:1	Conversion
2	0.242141717
3	0.355605985
4	0.425650823
5	0.474694439
6	0.511640658
7	0.54084345
8	0.564724718
9	0.584756353
9.35	0.591045157
10	0.601892829
11	0.616784624
12	0.629892828
13	0.641554891
14	0.652024405
15	0.661496241
16	0.670123022
17	0.678026275
18	0.685304163
19	0.692036987
20	0.698291183
21	0.704122267
22	0.709577053
23	0.71469534
24	0.719511214
25	0.724054068
26	0.728349405
27	0.732419479
28	0.736283813

This is theoretical and non-linear (just cut and paste it into Excel
and produce a chart to see).

On plotting figures for a typical carburetted engine in typical
available ranges:
cr:1	Conversion at full throttle, brake thermal efficiency
6	0.25
7	0.28
8	0.3
9	0.32
10	0.33

To all intents and purposes it *is* on an approximate straight line
part of the curve. It does also fit the theoretical curve and achieves
about .54 of the theoretical maximum at full power. Unfortunately it
only gives one part of the operating characteristics and I have no
other data available.

Whilst keeping abreast of developments in the modern car world we
should bear in mind that a lot of the performance is due to fuel
injection and closed loop control which will not easily be achieved
with a simple gas mixer.

>
>Therefore, I am puzzled by the SAAB variable compression ratio approach, 
>since backing down CR would reduce cycle efficiency.   Unfortunately, real 
>life is always a little more complex than any given solution, so we keep 
>getting in deeper.

I think Kevin adequately covered this point, the Saab design allows
the use of a high cr at part load without it becoming too high a cr at
full load.
AJH