Another version of how not to build a model engine

[sorveltaja]

Nick, the vapour carburetor is most certainly next thing to test, if the current carb doesn't give good results.

I soldered the parts together, and noticed that the carb throat was filled with solder also.

Took it apart, drilled the holes open, and removed rest of the solder. After cleaning the parts, plan B was to try super glue. It seems to work better in this case .

[sorveltaja]

I had to add an extension piece for carb, so that it's possible to put in place and remove it without removing breaker/cam combo:


Now the carb throat has 0.7mm(~0.027") diameter, and the engine fires with that, but doesn't run.

Next thing is to drill the throat to 0.8mm(~0.031"). Cleaning the carb after drilling might be more difficult, without having to disassembe it, as it's little more complex, than the previous ones.

[sbwhart]

Thats developing into a great little engine:- 

Stew

[sorveltaja]

Thanks for kind comments .

Here is a cross section sketch of that carburetor:


Basically the principle is same, that has been used at least on small commercial glow engine's carbs. That concept makes it possible to make a carb, that's needle doesn't intrude across the throat.

[Bernd]

S,

I don't see any "venturi", just a straight hole to the engine in your drawing. I recall that the glow plug engines all had some kind of venturi on the carburator before the fuel got mixed in with the air. 

Bernd

[NickG]

S,

THat extension to the inlet manifold might affect things. You have now brought another variable into the system and you don't know the effects.

Good work though, hope you find a solution.

We want to see a video too!!

nick

[sorveltaja]

Bernd, you're right, there isn't venturi. I have wondered, how does one affect the engine's behaviour . Many of the old engines have just straight tube as venturi.

Here is an example of an engine, that is roughly about the same size as the current project, although it's two-stroke: http://www.modelenginenews.org/schroeder09/index.html

I'm not sure, if 2 and 4 stroker carbs are comparable, but on that Schroeder's 09, the carb throat is quite big (4.37mm(0.172")).

I'm bit confused about those .

Nick, the extension piece doesn't seem to affect, as the engine behaves like it did before it.

Anyway, I'm also looking forward to shoot a video of it running. If lucky, at this week.

 

[John Hill]

At the risk of showing myself to be a complete Wally I will venture my thoughts on the venturi...

A venturi causes a change in pressure in the airflow and I assume a carb with a venture will more readily atomise or evan vaporise the fuel.  So I assume that where a venturi is not used the fuel must be more volatile.

[Bernd]

Ok, I looked at the Schroeder 09. I can see that it dosen't have a vebturi.

Well it was guess as to what might be wrong. Looks like you don't need one to run the engine.

Keep up the good work. I'm sure you'll get it straightened out eventually.

Bernd

[bogstandard]

You will find that you will get very little fuel suction as you have it.

The way the one you showed works is the spraybar acts as a backwards venturi. Instead of having the curved in walls of a standard venturi, speeding up the air flow and thus the drop in pressure, the airflow around the spraybar speeds up the airflow and it is that which causes the pressure drop, and so the fuel is sucked into the inlet. It is not as efficient as a venturi, but it is good enough for these small engines.

C-o-C explains the principle.


Bogs

[Darren]

I think the problem was too much fuel and not enough control ?

[AdeV]

A venturi causes a drop in air pressure (swapping pressure for velocity). This serves two purposes: It draws fuel into the airstream by the difference in atmospheric pressure; and, as a happy coincidence, the faster-moving air helps to atomise the fuel.

I think that the "venturi" effect in the Schroeder engine is created by the tapered cut (creates a large surface area at the end of the tube), the air would be accelerated as soon as it's in the main body of the tube. The effect will be quite small & as a result inefficient, but I suppose on an engine that size, you can trade efficiency for simplicity.


With regards to this engine, I suspect if you cut a slight taper in both sides of the main carb body and open out the fuel/air mix passage into the engine, as per the diagram below, it'll work better. Go easy on the size increases & test often, that way you shouldn't overshoot the sweet spot.



There's a pretty good explanation here: http://www.explainthatstuff.com/how-carburetors-work.html

This page may also help you out a bit: http://www.hooked-on-rc-airplanes.com/nitro-engine-tuning-tips.html

I see Bogs has come up with a better explanation of the Schroeder carb than the one I'd stumbled around at.

[bogstandard]

You have to be very careful with angles in a venturi, if they are the wrong combination, you can actually get the air stalling and causing all sorts of problems, with dead air in places you don't want

I have just made a venturi for my spraymist system using a couple of basic angles that I know work. That is 22 degrees for going in and 6 degrees coming out, both inclusive, but I did away with the 6 degree one because of the way I will be using it. That should provide a steady pressure drop and hence suction once the engine is turning over.


Bogs

[AdeV]

You have to be very careful with angles in a venturi, if they are the wrong combination, you can actually get the air stalling and causing all sorts of problems, with dead air in places you don't want

I should point out that IANAFD (I Am Not A Fluid Dynamicist), so yes - if the OP can find the correct angles for a basic carb, that would be better. Or use the spraybar, which is probably easier to machine anyway.

[bogstandard]

I was picking up on what you had said Ade, not criticising, but trying to be of assistance

I have stated before, I am no super expert on it, but I do tend to remember bits of two years of aerodynamics that I was force fed many many years ago.

As you suggested, in this instance, a spraybar would be the easiest way to go.


Bogs

[AdeV]

I was picking up on what you had said Ade, not criticising, but trying to be of assistance

Don't worry, I took it to be an assist rather than a crit. And, after I read your post, it occurred to me that every model carb I've ever seen has quite a steep taper on the inlet, not a nice shallow one like wot I drew. IstillNAFD, though

[sorveltaja]

I must admit, that I enjoy reading the discussion about that carb thing .

There sure isn't plain right or wrong aspects, but rather options, of how things can be achieved.

As much as I'd like to test variations of tapered venturi's, lack of required tooling keeps me on that basic straight tube -venturi schema.

In the mean time, testing goes slowly, but evenly. With 0.8mm carb throat, the engine fires, and almost runs.

In between, I tested also with the previous carb(1.9mm throat), and it has turned out to be a kind of reference. With that, the engine runs, making it possible to check once in a while, that valves are seating ok, spark timing is ok, etc.

[sorveltaja]

After some more testing with 1.9mm and 0.8mm carbs, the engine wants to run only at high speed with both.

Part of the problem might well be the spring loaded intake valve, having tension, that allows only limited rpm range:


It's kind of difficult to adjust its spring tension .

I think I'll start a new camshaft. With it, the intake timing should be more constant, than that spring loaded, regardless of rpm .

[NickG]

That is a good point S. But many slow running engines have an atmospheric intake valve. The spring must be really very weak in such a system so that the suction of the piston will open the valve. From what you are saying it's too strong and the engine can only open it at high rpm?

Nick

[sorveltaja]

Nick, it's something I wonder also. When turning the engine slowly by hand, the inlet valve makes snorty sound, indicating that it lets the air in, and also, that the spring tension isn't too strong.

I may be wrong though. But for example, on hit'n'miss engines, I assume, the spring tension isn't as critical, because there is a governor, that effectively prevents the engine from running too fast. 

Hmm... should I mod the current engine to hit'n'miss. Tempting option, but we'll see.

[sorveltaja]

Before making whole new camshaft, I'll try to revive the old one:            Rest of the over-filed inlet cam machined off, and new cam blank to be                                                                                                       silver soldered:
       

Before..                                                          ...and after cleaning. Cam is now ready for forming:

[NickG]

S,

this might work better by as you say relating the timing directly to engine speed - as long as you get the timing right. Great stuff, fingers crossed it works. 

Nick

[sorveltaja]

Forming the inlet cam entirely by filing isn't too attracting option, at least for me, so i made a quick 'n' dirty jig, so most of the excess material can be machined off in lathe:


Grub screws keep the camshaft in place, allowing the shaft to be rotated between machining:


This simplified jig doesn't necessarily need the tailstock support, as only light cuts are taken.

There are no dials to measure the angles, instead I'm going to perform the machining process by following the basic instinct hunch.

If it fails, with that jig, it's easier to make new cams anyway .

[John Hill]

If you think there is a problem with the inlet valve maybe it is bouncing instead of closing cleanly?  If it is bouncing I think you will see a small cloud of fuel being blown out of the carb?

I greatly admire your progress on this engine!

[sorveltaja]

John, can't say for sure, would bouncing be the problem, as I haven't noticed fuel blown out of the carb, except on cases, if there is impurities in the fuel, that gets to inlet valve, causing it to fail.

Usually compressed air is enough to clean the valves.

Before next running tests, I'm going to make a fuel filter.