Flame licker won't run, seems like nothing helps

[Ridddervold]

Hi you all, this is my first post on this forum so please be kind!

I have designed and manufactured a flame licker engine at the university workshop for a couple of months now, and the time has come when I should get it to run.. but I can't! I don't even get an honest attempt from it to start.

I have tried following:

• Sanding cylinder and piston with 1200 grit paper for hours until 100% smooth. There is a good seal.
• Making sure the valve and cylinder hole is 100% flat. There is a good seal here too.
• Erasing friction to a minimum.
• Three different stroke lengths and some valve timings.
• Placing the wick at different positions (the flame is good, clean and strong, I'm burning 90-95% alcohol)
• Three different flywheels; one large but light aluminum, one large and heavier aluminum and one small but quite heavy iron. I have never made any calculations of the size and weight though.
• Heating and cooling the cylinder before trying to start it

I have designed the engine myself. (I have attached the CAD-model.) The materials aren't quite right in the CAD-model, everything is made out of aluminium except the valve (brass) and the piston (teflon). I've only oiled the crank shaft slightly with very light oil, the cylinder and valve is all dry (possibly a bit of graphite). There is a spring retracting the valve, it is as soft possible.
When trying the different adjustments I've noticed that the engine from time to time makes popping noices when turning, both with and without the flame. The sound is stronger with the flame though. A problem (I think?) is that the valve seems too be blowing out air on the stroke when it's supposed to be sealing because of sub-atmospheric pressure.

I hope I can get some help from you guys since it seems like many of you have the proper knowledge and experience. When I get the engine running I get my engineering degree! 
Please excuse my spelling, I'm Swedish!

[mklotz]

Use the heaviest flywheel.

Really preheat the cylinder.  Despite the fact that it acts as a heat sink to quench the gas, it needs to be fairly hot.  At least that's my experience.  Preheat the valve as well.

Try some light oil in the cylinder.  Graphite is good at reducing friction but not so good at sealing.

Don't worry about your spelling.  It's better than 95% of the native English speakers on this forum.

[b.lindsey]

Erik,
First of all welcome to the forum. If possible we would like for you to post an introduction in the intro section about yourself. Things like what type of engineering you are studying, other experiences with model engines, what the school shop is like and so on.

As to the problem at hand, It might also help to post a few pictures of your engine for those that don't have the ability to open Solidworks files. A short video of you turning the engine over showing the crank position relative to the valve could also be of help in diagnosing the problem.  You seem to have address the usual things that prevent running though flame lickers are notoriously hard to get to run at times until everything is just so. Friction is of course the enemy in these engines. I have also found that the piston/cylinder seal can be too good at times. There seems to be a very fine line between sealing and adding friction, though a sloppy fit isn't good either.

From what you say I would lean towards a timing problem but that is more an opinion than a diagnosis at this point.  I am sure other members will offer ideas as well. Best wishes in getting it to run and finishing up your degree!!

Bill

[Arbalest]

Heating the cylinder, switching to 98% alcohol and using graphite in the bore all helped to get mine running.

[Ridddervold]

Thank you for your replies! I will take some pictures of it tomorrow and try to get a good video of it so you can see the valve timing and so. How heavy a flywheel am I looking for in this type of engine? I can make some calculations considering moment of inertia, crankpin length, assumed negative air pressure and bore but I think it is better to partake of your experiences.

I will get on with posting some information as soon as I get the time!

[Alan Haisley]

It would perhaps be useful to see a movie showing the flame being drawn in. Maybe it's just not drawing enough in or not at the right time. Good luck.

Alan

[PStechPaul]

I had no idea what a "flame licker" engine was, but I found a video that explains it somewhat. The hot gas is pulled into the cylinder through a valve which then closes, and as the gas cools it creates a vacuum (or pressure differential to atmospheric) which draws the piston back for another "gulp" of hot air...
 
<a href="https://www.youtube.com/watch?v=vkaz9rDJWQ4" target="_blank">http://www.youtube.com/watch?v=vkaz9rDJWQ4</a>

[Ridddervold]

I'll post some pics and a video in a couple of hours. A thought just crossed my mind though; can it be that the wall on the left side of the valve hole prevents gas from getting sucked in properly?

[Ian S C]

     I  Don't like the idea of using aluminium for the cylinder, it has high friction, and with the heat required to run one of these motors, maybe it is distorting,  also the piston is going to get very hot.   It would be better if the cylinder was at least lined with steel, or better still cast iron,  and the piston made of cast iron.  It would also be possible to make a good (maybe better),piston from graphite.
      My first flame licker took a year to get going, I made my second one 4 or 5 years ago, it still won't go.   Ian S C

[b.lindsey]

Ian, I agree with you as to the graphite piston. I am not sure I have heard of Teflon being used before. Even though it is more thermally stable than many/most plastics, I wonder about how the top of the piston would react being in closer contact to the direct heat of the flame. The Philip Duclos flame eater as shown in the video Paul posted does have an aluminum cylinder though and a steel piston as designed, even though many builders have changed that to graphite (as I should do with mine). Even so it is a tried and true runner, so I am not sure that aluminum per se is the culprit.

Bill

[fumopuc]

Hi Ridddervold, the combination of aluminum and teflon sounds a bit strange for me too. I do not have any experience with this. My one has a brass cylinder and a graphite piston. A graphite piston should run only and ever dry. No oil or other grease.
I made the mistake that my first piston has had to much clearance. After making a bigger one with nearly zero clearance it was a runner. http://www.modelenginemaker.com/index.php/topic,2836.0.html

[NickG]

Agree with all that has been said here - would be good to see some pics or convert the CAD models into a more readable format. I've made 3 flame lickers so far, the first one I had a lot of trouble with, more due to it's unorthodox valve design. That had a cast iron cylinder and piston. The next ones I made were to the poppin design but had graphite pistons and cast iron cylinders - both of these ran more or less with the first flick of the flywheel.

I wouldn't bother with calculations but the heaviest flywheel won't necessarily be the best, it may cause too much friction in the bearings which could be reduced by a lighter but larger diameter flywheel while not compromising the moment of inertia too much.

I think cast iron is the ideal cylinder material, I want to make a graphite piston for my first engine as I think that will make .

I am interested to see the way the valve operates - the cam should force the valve shut at around 45 degrees before bottom dead centre, then depending on the stiffness of the spring and the valve configuration, you'll get a 'latching' effect and the valve will open when it's good and ready in spite of what the cam is doing. i.e. when the pressure inside the cylinder becomes greater than atmospheric again the valve will be pushed off it's seat if allowed to do so. This is the best method, rather than forcibly opening the valve with a cam at a predetermined time.

We need to see schematics of the engine to speculate any more on this though.

Nick

[fcheslop]

Sorry Im not able to view youre drawings.
As Bill Iv had no problems using alloy for the cylinders as long as they are lubricated using graphite.
I have made two of my own design and have found that 110 to 120 degrees of valve duration works well and that the valve just covers the port by about 1mm max and that the valve is able to lift to allow for exhaust the spring pressure just needs to be enough to hold the valve onto the cylinder and no more
best wishes
frazer

[Ridddervold]

Thanks for all your answers! Here are some images of the engine. I've attached a brass plate between the cylinder and valve, sealed with high temperature silicone which works fine, no signs of melting or anything despite the heat of the flame.

Pictures are also showing the crank shaft, graphite in the cylinder (coming from the teflon that actually has a small percentage of it) and variable stroke lengths. I will post a video of it turning soon, the silicone was still sticky though, so I didn't mount the valve. The wobbling of the flywheel doesn't seem to have any impact, it spins for about 100 seconds on a firm push.

Edit: Managed to get a video uploaded to youtube:
<a href="https://www.youtube.com/watch?v=NJnKuO_r3a8" target="_blank">http://www.youtube.com/watch?v=NJnKuO_r3a8</a>

[Alan Haisley]

That brass plate looks pretty rough. It and the valve plate (not shown?) need to be near perfect flats to each other for the valve to seal.

Alan