Flame eater questions

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Stilldrillin:
Some crisp pics here, of an original engine..... http://www.ebay.co.uk/itm/VACUUM-ENGINE-SOLAR-3-FLAME-EATER-/270825644643?pt=UK_Trains_Railway_Models&hash=item3f0e776263

David D

NickG:
Hi Andrew,

Just seen this, I've built three of these, all runners (last one on Monday night!) but they can be very tricky to get to run. First thing to check is the friction ..take the cam off, leave the valve open and see if it spins over nice and freely. I would say it should do 15 to 20 revolutions with a firm flick over of the flywheel. The next thing is, you are quite right, that spring does look a bit strong to me.

The cam should shut the valve fully shut about 45 degrees before bottom dead centre. To clarify, the piston is on it's outward stroke and the valve shuts before it reaches it's extremity, roughly 45 degrees but it shouldn't be too critical, both cam operated engines I made I set by eye and they ran 1st time. The overlap of the valve with the port shouldn't be too much ... about 1mm or so will do but noticed yours has a turned up end on the valve so take this into account with the overlap. Although I can see why you've done it, this feature is not the best idea as it will not aid sealing. If it's thin enough however, it should still work. What gauge shim is it made from?

When the pressure increases inside the cylinder to above atmospheric, the valve (again if thin enough) will push off its face so it's sort of automatic opening. As long as the cam is right, the spring will open it. It must be fully open again by top dead centre to start the cycle again.

The other thing to ensure is that you have a big flame covering the whole port if you can. This isn't critical on some engines but that is quite a big port you have there.

Hope this helps.There must be many of these engines under the bench destined never to run though as it's easy to get frustrated with them!

geoff_p:
Gents, I've just joined Modders and have found your topic, with Nick's insights. I am at the 'frustrating' stage with my first Flame-eater, trying to get it to go.

My valve is graphite/carbon - an electric-motor brush - moved up-and-down on the cylinder head, and 'farts' quite nicely on compression. For the moment the valve spring is an elastic band (mounted away from the flame, I hasten to add) so I can easily vary its tension. The valve is operated through a sort of bell-crank from a cam.

The port is about 3/4 of the piston-diameter by perhaps 3/8 wide, and the head has been thinned-and-tapered in the port area.

But the cam is giving me heartache! OK, Nick, you suggest closing about 45 degree before BDC, but when should it open?

I have written some CNC-code so I can make different cams fairly easily, and so far have tried 90 degree dwell, 120, and 180 dwell all with quite snappy opening and closing actions. However, none of them work!

Any advice will be hugely appreciated,

Geoff
Thailand

NickG:
Hi Geoff,

Is your flame eater to your own design? The port sounds a bit large to me if I'm honest, would it be simple to make it smaller? There are so many parameters that affect when the valve should open! Myself and a member of our club were discussing it a few weeks back at an exhibition. We were surprised to find that mine opened well before TDC on poppin - at least 45 degrees.

However, when the engine is actually running, because of the valve design on poppin, you can get a kind of latching effect, where the 'vacuum' or sub-atmospheric pressure in the cylinder will keep the thin valve against the port face until the pressures equalise and it is ready to open.

If your cam is such that the valve is forcibly opened at a set point, if you open it too late, the pressure in the cylinder will start to rise above atmospheric and you will be trying to compress the gas, sapping power. It's a trade off with the how long the power stroke is, I would try around 45 deg before top dead centre as a starting point.

Another guy on here called madjackghengis did quite a few successful flame eaters and a lot of experimentation. We worked out that one of the good design points of 'poppin' was the whole valve mechanism ... low friction pivots, low spring tension required and a very thin valve that lent itself to sealing well. Also, the cam forcibly shut the valve at a pre-determined point, however, it didn't forcibly open it again, that was essentially left to its own devices providing the spring pressure wasn't too great. This meant it wasn't that critical and relatively easy to get a running engine ... both of my poppin engines ran with the first flick of the flywheel which was a revelation considering the Jan Ridders one I made!

The cam dwell is 110 degrees on poppin but as I said it's very insensitive to cam timing ... I set both by eye and both ran that well I didn't bother messing around with it!

Hope this is of some use and you can get it running ... they are fantastic to watch, don't give up, I nearly did on my first one but glad I persevered. Incidentally, that still doesn't run too well, takes a long time to warm up and is very tempremental. I am planning to one day make a graphite piston and valve for it.

Best regards,

Nick

geoff_p:
Many thanks Nick, for that detailed reply.

Yes, it's my own 'design' (though that is a rather posh term for it) and is made from my own castings. Originally it was going to be a hit-n-miss petrol engine but I have to save-up for some gears, so I thought perhaps I can use the parts for something-else in the meantime.

Since most flame-lickers I've seen have cooling-fins I thought the water-hopper cooling should be good.

The main bearings are ball-races and the Big-end runs on graphite/carbon - two motor-brushes held together and bored out. The crankshaft is cast aluminium, and the graphite & aluminium combination seems to offer very little friction - it spins very easily.

The 25 dia x 30 stroke cylinder and piston are both aluminium and match very well: with a hint of oil they have only a little blow-by.

The valve is another carbon-brush, 17 x 11mm, for a port which is 13 x 6, and is held against the port-face by spring-tension (emulated here by an elastic-band until I can find my box of springs!) and that same tension acts downwards on the lever-arm to keep the cam-follower in contact with the cam.

This morning I made another cam, very similar to Nick's and calculated to be 90 degrees opening-dwell but although the engine pops-and-farts with it, I didn't get a run. I'll try again later, varying the timing.
Geoff

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