air fuel mix

[bouncebounce crunch]

Say a low end or air bleed is set at 60% air 40% fuel. open your throttle the mix is the same but more of it is sucked in.

Is this correct?

Edited By bouncebounce crunch on 05/04/2016 12:58:13

[Dave Hopkin]

That's my understanding of how a carburetor operates, the butterfly value regulates air flow, the air flow over the needle value determines how much fuel is drawn in - more air equals more fuel and visa versa - whether its a true linear change though - I have no idea!

[Don Fry]

Not linear, that is why injection systems are universal in cars nowadays, carbs can't be set up to pass emission regulations, poison the catalyst etc

[Peter Beeney]

bbc,

If I remember correctly, in very general terms the combustible air to fuel ratio is approximately 14 to 1, and I suspect that at the slow running point this will be about the same. However, when the throttle is opened the conditions change, and again in very general terms a richer mixture is required/supplied for a short period of time. There is an explanation for this, but it does get slightly involved, it probably depends on the reason for your query.

This would be relating to petrol fuelled engines, though, methanol guzzlers are just that, I believe, so maybe the slow running / throttle opening situation will be different. Although the same atmospheric conditions apply. I’m of the school that believes one of the reasons why our model engines are relatively so powerful is simply because of the amount of fuel they are able to burn. Compared to your car, it seems to me that as a performance comparison you might soon have to consider gallons to the mile…

PB

[bert baker]

Does it matter as long as it works.

What are you trying to do

[cymaz]

Never mind the %'s it's how reliably it's starts and runs, that's for 2 stroke glow. Setting up the needles properly is 99% of the game

Though I can see the point of your question.

 

Edited By cymaz on 05/04/2016 18:19:44

[Don Fry]

Bouncebouncecrunch, to answer the question, at full chat, yes, at tick over, yes, if both mixture controls are perfect. Not likely. At all other revs, no they will not be perfect. We hope, good enough to run.

A brand of moter, MDS, (aka, many dead sticks), no knowledge personally, but I bet, nothing wrong with the power plant, lousy carb, and lousy owners.

It is hard to make our moters run nicely. Look at the Laser postings, where Jon of Laser bears his soul about getting his 180 petrol towards production quality.

Want another route, but open another bag of worms, go leccy.

If this game was easy, we would not bother.

[Nigel R]

open your throttle the mix is the same but more of it is sucked in

 

basically, yes.

top end mix is set with the main needle, get the idle mix right with the idle needle (also affects throttling), get the idle speed right with the servo travel

our glow carbs are a very low tech route to setting fuel air mix, but we don't really care about efficiency or emissions, just that the motor runs reliably.

also, glow engines will run on a very wide range of air fuel mixtures, I think we get away with having very simple carbs. petrol is a bit more fussy

 

Edited By SuperNash on 06/04/2016 16:58:42

[Nigel R]

 

I’m of the school that believes one of the reasons why our model engines are relatively so powerful is simply because of the amount of fuel they are able to burn.

We burn over twice as much fuel in a glow.

More energy in petrol though.

Overall I agree. There's a good reason why (some) race cars use methanol.

 

Natural gas: 17.2
Gasoline: 14.7
Propane: 15.5
Ethanol: 9
Methanol: 6.4
Hydrogen: 34
Diesel: 14.6

 

Edited By SuperNash on 06/04/2016 16:58:56

Edited By SuperNash on 06/04/2016 16:59:24

[bouncebounce crunch]

Thank you all for your responses.

The percentages (fuel air mix) were hypothetical and not actual. What i was trying to make clearer is, Does an air fuel mix change from low to high throttle or just more volume of the exact same mix?

60-40 will be the same at idle as full throttle.

Cheers bbc.

 

Edited By bouncebounce crunch on 07/04/2016 07:43:27

[Cuban8]

Interesting subject, so I had a read through this **LINK**

Section on Bernoulli's Principle, Reynolds Numbers and viscosity stretches my 'O' level Physics but does seem to make sense (I think!)

[Peter Beeney]

bbc,

In relation to petrol engines with carburettors in motor vehicles definitely not, during the actual transition from the low to high throttle position the mixture is made very rich; there are reasons for this which we could discuss if you wish. I don’t know by how much though. However, modern engines with management systems that can take care of every conceivable running condition, all at the same time, and can then pass it on to an injector in a twinkle may be rather different.
In relation to our model engines it’s quite possible the same is also true to some extent; I’m afraid this is something that I’ve never really paid much attention to. Maybe something to tinker with on a rainy day, starting with the relationship between the needle valve and the throttle barrel opening.

In my experience I’ve generally found model engines to be fairly critical of carb settings, the ratio has to be about right for really good running, i. e. efficient. Admittedly the carbs are relatively pretty crude, although some of the expensive engines are better in this respect, and maybe a lot of the higher pricing is due to the more sophisticated carburettors. Also from a colleague that looked into this, burnt methanol has very little in the way of unwanted emissions; but I’m not that convinced about the oil content; although most of it appears to be thrown out as unburnt anyway, but it still maybe considered a pollutant. Overall though, fortunately not a massive issue in the general scheme of things…

My personal line of thinking about ‘60-40 will be the same at idle as full throttle’ would be that at idle the mixture would be rather richer than weaker; but there might not be a lot in it…

PB

[bouncebounce crunch]

Yes! back for more punishment.

So now we generally have established that an air fuel mix at low setting or idle is the same or very close to the same mixture at high throttle, just a wider throat to suck it all in to the burn chamber.

Am I near or off the mark?

bbc

[Bob Cotsford]

In theory, yes. In practice the idle mix may be a little richer to allow the engine to accelerate smoothly as you open the throttle.

The mix for methanol is nearer 6.5:1 air:methanol, but factor in oil content and you can be nearer 5:1 for air:glow fuel. Nitro content will also affect this ratio.

[J D 8]

Pretty much right, but too large a throat for the size of motor and there will not be enough "suck" to get the fuel in. That was the problem with the MDS engines mentioned by Donald above and we did fit carbs from other makes with good results for reliability at the cost of a loss of some top end performance.

[Nigel R]

Posted by Peter Beeney on 07/04/2016 09:42:11:

However, modern engines with management systems that can take care of every conceivable running condition, all at the same time, and can then pass it on to an injector in a twinkle may be rather different.

A few of my colleagues have worked on engine management systems, word is that acceleration enrichment is still done by these systems.

[bouncebounce crunch]

Thank you Gentlemen.

Bob and John, You will probably get blame, harassment and We will eat your biscuit type threats..

this exercise was a basic Carby set and leave scenario for fiddlers.

I welcome more input. RCM&E may find this a very interesting magazine article.

bbc

[Denis Watkins]

All the Air Bleed Carbs are set and leave, apart from one or two clicks on the main needle over winter and summer. There is an ingress of dust and dirt and pollen over the year so the needle will need a wipe ocassionally, but just count it out and put it back where it came from. Clean fuel and informed care will lead to a trouble free IC life