Model Forum / Radio Controlled / Air Models / June 2004

    The chemically correct ratio is somewhere close to 15-1 A/F. Useable
range varies from slobbering rich at 8-1 to "hot and dry" at around 20-1.
Best power is achieved at around 12.5-1. All these numbers can vary based on
loading, air temperature, RPM, humidity, and the position of the moon on any
given Sunday.
    The main problem with a supercharger is the tendency to increase the
effective compression ratio of the engine which must be compensated for by
shiming the head to avoid advancing the timing too much. So it is entirely
possible to design an engine that runs like a banshee at full throttle but
is a bear to start and won't idle well.
    Give it a try, nothing ventured nothing gained. But I think you will
find that it would be much simplier to just fit a larger engine.

Dunno.

Pix, no details:
http://www.rb-rc.com/rc_supercharger/rc_supercharger.htm

Discussion with links:
http://www.rcuniverse.com/forum/really%25%25%25_superchargers/m_1790945/tm.htm

From that page, a commercial product:
http://www.rbinnovations.com/
http://www.rbinnovations.com/newsite/main/customer%20rides.htm

OH, I've got an OS 1.20 supercharged (factory built).  Bought by a
friend of mine.  We haven't been very successful with it.  It's got a
pretty bad reputation.  A factory sponsored pilot won an FAI
pattern world title with it in the 1990s, but the engine never won a
big following.  Most guys stuck with the YS engines, which have
a method of roughly doubling the air pressure provided on the
combustion stroke.

I think the OS uses a "roots blower," whatever the heck that is.

Here's a long discussion from some cars guys:
http://www.rcgroups.com/forums/showthread.php?t=97159&page=1&pp=15

Why not fool around with what you want to do and let us know
how it works out?  :o)

                    Marty

For methanol the ratio for maximum power is anywhere between 4.5 and
6.5:1 but when you start adding nitro it gets a bit trickier. Nitro has
a starting point of 2.5:1 but can be run at 0.5:1 or even richer and
still provide max power. This broad range of acceptable ratios for nitro
is one reason it makes an engine easier to tune. You can be ham-fisted
and get away with it :)

For 2x boost your supercharger would have to displace twice as much air
as the engine size (roughly speaking) so for a .60 size engine it'd have
to pump 1.20 cub inches/rev. This would need a quite powerful electric
motor because it has to overcome the friction of the supercharger as
well as pump and compress the air. A centrifugal impellor is impractical
because of the small size needed. Even a slot car motor that spins over
100,000 revs wouldn't be fast enough.

Brian Hampton
Adelaide, South Oz

O.S. had on a 1.20 4-cycle, as I recall...

If your talking about 2-stroke engines, the design of model engines has the
exhaust ports open past the closing of the inlet so that supercharging is
not possible.  There are 2-strokes of different designs which can be
supercharged but are not practical in the sizes which we use in models.

For 4-strokes, supercharging is possible.  OS had one using a roots type
blower driven from the crank but I do not know their boost.  And there is
one using 2 strokes of crank pressure per power stroke to gain about a 25%
boost. Grand Prix engines of the 50's and 60's used 2 stages of roots
blowers and gained pressure boost of about 4:1.  These were driven off the
crank and are not really practical for an electric motor to drive as they
have to have speed sync'd to crank.  Centrifugal blowers for a model engine
would have to run about 100,000 and would require too much power for most
model size motors.  And the inertia loads on the gears would limit their use
driven from the crank.

For   ANY engine, supercharged or not, the max power fuel/air ratio is about
10% to 18% richer than chemically correct depending upon the fuel being
used.

Ray S.

[For those keeping score at home, sorry for the repeat of this
message--after I posted it, I realized how easily I could return
to the old thread--Agent keeps a copy of all the posts I've
made in the past.]

From Clarence Lee's column, RCM, January 2004, p. 42:

"The Walbro carburetors are intended for gasoline operation
and gasoline burns at a lean air/fuel ratio of 14.7:1 and rich
at about 12.5:1.  Methanol, on the other hand, burns at a
lean air/fuel ratio of 6.45:1 and rich at about 4:1.  As such,
a Walbro carburetor intended for gasoline usage will not
pass sufficient fuel if used with methanol."

I know that the question asked was only about methanol,
but it seems best to give the quotation as it stands in
RCM rather than paraphrasing it.

                Marty