[Gasification] ideal wood gas engine

[Daniel Chisholm]

On Fri, Feb 25, 2011 at 20:45, Toby Seiler <seilertechco at yahoo.com> wrote:

> I'm under the impression that a small amount of positive pressure on the
> intake has a large effect in the engine power, in comparison to a vacuum
> intake.  How do you think a gas and air pressure of 2-5 pounds or so, would
> affect the compression ratio / efficiency if one is using low pressure in
> producer gas making?
>
> Perhaps someone has density information/chart and the way to relates to
> compression ratios.
>

Toby perhaps the easiest way to keep things straight is to think in terms of
absolute pressure.

A normally aspirated engine running at wide open throttle has a manifold
pressure of about 15 psi absolute (or if you want to quibble for more
accuracy you could use 14.5 psia: atmospheric pressure is 14.7 psia, and
there's a small amount of vacuum due to the flow restriction of the wide
open carburetor)

If you supply 1.5psi of boost (so you manifold pressure is now +1.5psi
higher than atmospheric), you have raised your absolute pressure by 10% -
from 15.0psia to 16.5psia.  To a pretty good first approximation, this 10%
increase in absolute pressure will give you a 10% increase in horsepower.

Similarly a 3.0psi boost will give you about a 20% increase in horsepower,
and a 4.5psi boost will give you about a 30% increase in horsepower.

Compression ratios are multiplicative.  So if you are providing 20% boost
(3.0psi) to an engine with an 11:1 compression ratio, the overall effective
compression ratio is 1.20 times 11, for 13.2 to 1 compression ratio.  FWIW
the "1.2" is from the 20% boost, the "11" is the piston's compression)

Figuring out the engine's efficiency change is not as straightforward.  On
the one hand, increasing the compression ratio increases the engine's
thermodynamic efficiency (because the mean pressure and mean temperature to
increase).  Working against this though is that the engine's *expansion*
ratio has not been changed (the motion of the piston is what extracts work
from the gas).

While turbocharging an engine has a great deal of sex appeal, it probably
makes a lot more sense to keep Doug's advice in mind.  While it's not
completely true that "there is no substitute for cubic inches", that does
make an awfully good "zeroth law of engine selection".

Adding 40% more cubic inches is in many cases a *much* more elegant solution
to a problem than adding 20% more intake pressure...


-- 
- Daniel
Fredericton, NB  Canada
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