Wittaker "Firefly" 0.46 Glow Engine

[Jasonb]

The build series for this engine started in Model Engineer magazine a few issues back but there were few photos to go with the text. As it was meant to be a beginners engine I decided to knock one up and take a series of photos that could be posted on the ME website for anyone thinking of building the engine.


The full set of construction photos and my brief descriptions can be found here for those interested, if not  here is the video and a couple of stills.


<a href="https://www.youtube.com/watch?v=AvbnjyrHq8M" target="_blank">http://www.youtube.com/watch?v=AvbnjyrHq8M</a>









J

EDIT should have said the drawings can also be downloaded from the following, there is also a fun ?D rendering that can be manipulated.

http://homepage.ntlworld.com/glenn.royds/RCM&E/

[chuck foster]

every nice engine jason 

chuck

[doubletop]

Jason

I was aware you were making it but hadn't appreciated that you were getting on so well. A beautiful job, but I don't think you'll get that workmate airborne

Pete

[steamer]

Nice build thread Jason!

Dave

[NickG]

Lovely work as always Jason, it looks great and runs brilliantly.

I've got a couple of questions, not directly related to this but to glow plug engines in general you may be able to help with - I'll post here if that's ok, incase it enlightens anybody else at the same time!

I'm currently in the process of designing a very simple engine, along the principals of one of Jan Ridders' engines however to uncomplicate the ignition system I was thinking of using a glow plug. From the way in which they work I understand that the compression ration needs to be higher than would normally be the case, I was thinking around 10:1 - do you know whether this would be ok?

I've heard glow fuel is basically methanol and it can have varying degrees of nitromethane added but doesn't need any for low performance? What do you run yours on?

Finally, do you think Jan's vapour type carb would work with such an engine or wouldn't the methanol give off enough vapour to work in that way? I guess you'd have timing issues if you tried to use petrol with a glow plug?

Thanks,

Nick

[Jasonb]

I've not really done much with glow engines but certainly with the 2 strokes the compression ratio needs to be higher, not sure if its the same if you use the glow plug on a 4-stroke.

There is also 15-20% oil in the fuel, I was running it on some 20year old 5% nitro which I suspect was a bit flat.

I've not used a vapour carb/tank so can't be sure but it should work as the fuel is quite volatile but that would be for just methanol and some other means of oiling as the oil may not vaporise in teh tank.

J

PS Had a tacho on it last weekend and got about 7,200rpm with the dud fuel.

[NickG]

Thanks Jason, might give it a go then but leave provision incase it needs to be converted to spark ignition. I may need to up the CR as I had planned for 10:1, only a matter of a smaller recess in the head though. Forgot about needing oil in the fuel, that must be an issue with the vapour tank.

Wow, 7200 rpm = impressive! Is that a butterfly on the carb you are pulling with the wire? or a sort of choke?

Nick

[Jasonb]

Its a cylinder or "barrel" with a cross drilled hole, as it is rotated more of the hole opens up and allows more air through. You can see it on this exploaded view.

http://homepage.ntlworld.com/glenn.royds/RCM&E/RC2095-1.pdf

[Admiral_dk]

A Small Glow engine normally has a compression ratio in the range of 7.5:1 to 8:1 - go higher and be prepared to get into trouble with detonation unless you really know how to make a perfect squish band and the necessary calculations for the exact fuel you'll be using 

A really nice build Jason 

[NickG]

Ah right, I see now. Thanks Jason, guess this carb would work as a side draught too, or maybe could put such a carb straight on the top of the cyl head on my design?

That's interesting Admiral, I thought it would need to be higher than a conventional  petrol engine, more like a compression ignition for glow plug, thanks for the info. So How does one know when it's going to fire? i.e what can adjust the ignition timing?

Cheers,

Nick

[Jasonb]

The carb will work any way you want it as it is designed for an aerobatic aircraft that will get spun all over the place

[Admiral_dk]

The timing takes care of it self, so to speak - when you throttle back the plug runs cooler and therefor fires the combustion later > reducing the timing advance and when you give it the beans the plug runs hotter and it advances the timing.

If you really need to adjust it, you change the heatrange of the plug or the amount of Nitro in the mix.

[NickG]

The carb will work any way you want it as it is designed for an aerobatic aircraft that will get spun all over the place

  

Thanks again Admiral, that makes sense. Guess the CR is quite critical though as it's a combination of heat from compression and the glow plug that sets it off, rather than a juicy spark. So whereas in a petrol engine with 95 octane you could get away with maybe anything from 5 or 6 to 10 or 11 :1, as you can set the ignition timing independently. In a glow plug it needs to be low enough so as not to cause detonation but high enough to make sure it still fires before TDC. I will see what needs to be done to aim for 7.5:1, if there's not enough meat in the head I guess I could put a slight dish in the piston crown.

Sorry for going a bit off topic Jason.

[Admiral_dk]

You could go for 8:1 and add a thicker gasket if you're got too much compression.

[NickG]

good idea, thanks!