Fuel Air Mixture Ratio for Glow Engines...

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.

wrote in message news: snipped-for-privacy@4ax.com...

Dunno.

Pix, no details:

Discussion with links: From that page, a commercial product: 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:

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

snipped-for-privacy@nowhere.nil wrote:

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

This ratio sounds more like what I'd expect for methanol. An earlier posted stated 15:1, which as I recollect is approximately the correct ratio for gasoline, not alcohol.

In any case, the right answer to more power is a bigger and/or better engine......

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.

Right. Gasoline engines will fire on mixtures of 8:1 to 18:1, and the stoichiometric ratio, which is the point at which all oxygen and all fuel is burned, with none of either left over, is 15:1. Best power comes at about 12:1 because the stoichiometric is an ideal and not really achievable unless the engine runs REALLY slow. Alcohol/nitro engines are going to have different numbers as Brian Hamton pointed out, probably a lot richer, judging by the horrible fuel mileages they get.

An old saying among car buffs: "The only real substitute for cubic inches is more cubic inches."

Dan

The "chemically correct" mix for Methanol is 15.5% - that is, the weight of the Methanol would be 15.5% of the weight of the air in ideal combustion. For Gasoline it's 6.5%, for Nitromethane 58.9%.

In terms of "heating value", a pound of Nitromethane produces 5,000 BTUs, a pound of Methanol 9,600 and a pound of Gasoline 20,600. Clearly Gasoline is superior in terms of efficiency. But we can burn a much "richer" mixture of the others, so ...

Per pound of AIR in the combustion process (ideally) Gasoline provides 1,340 BTUs, Methanol 1,480 and Nitromethane 2,940.

Pass the Nitro! :)

Dennis ___________________________________________________________

[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