Ignition Problem

[David CambridgeParticipant @davidcambridge45658]

For my first machining project I’m building the hit and miss engine designed by David Kerzel. I’m at the stage that I have every major component made (at least twice!) and now I’m into problem solving.

I think there are probably a few things wrong, and a few parts to make again, but in the first instance I’m having problems with ignition. I’m using a model plane electronic ignition and Hall probe affair to provide the spark to a Rimfire spark plug. However, the spark looks quite week and in fact I can’t get it to light my gas oven hob, or even a cloud of butane (all due safety precautions taken!). Does this seem right, or do you think I’m missing something ?

David

Edited By David Cambridge on 26/04/2015 13:52:26

[David CambridgeParticipant @davidcambridge45658]

Ignition Problem

[MichaelRParticipant @michaelr]

David,

You say you are using a model aircraft ignition system, I'm not a expert but these are designed for high speed engines and have built in advance and retard which could be a problem on a slow running Hit and Miss engine.

Maybe someone will come forward who knows a bit about these systems and using them on a slow running engine.

I use a Minimag ignition module with a hall sensor on my Hit and Miss engine.

Mike.

[JasonBModerator @jasonb]

Yes as Stick says if you are using one of the Rcexl ignitions then they tend to have built in auto advance which is more suited to the higher reving RC plane engines and will throw your s off by about 30 degrees

These ignitions are also designed to have their hall sensor on the crankshaft, if you have fitted the sensor to the timing gear then that will also be giving it false rpm readings.

As Bogs says valve and piston seal are the usual causes of problems. Do you get a good bounce on the compression when flicking the flywheel around, should be something like the start of this video.

As for the valves you want a really weak spring on the inlet, as you turn the engine over by hand you should just be able to see it opening but more likely hear it.

Virtually all the hit & miss engiens I have made have had very sensative fuel needles even after making them with a shallower taper, you really only want to open the needle a crack. Also check your fuel tank level, the fuel can drain back into the tank between hits if it is too low or flood the carb if too high.

How are you turning the engine over to start it? if using a cordless drill then make sure its not too fast an making the governor weights throw out and holding the exhaust valve open.

J

[Neil WyattModerator @neilwyatt]

Gas is much harder to light than petrol.

Neil

[David CambridgeParticipant @davidcambridge45658]

Thanks all, and as usual excellent advice from everybody.

This is my first engine and my first model engineering project – and as a consequence I’m now faced with the problem that I’ve built something that looks notionally correct but doesn’t work and I’ve no experience on what could be wrong. I’ve loads of variables and only when they are all correct will it work! So where to start ?

My compression is certainly not brilliant and one of this week’s jobs is to remake the piston rings, and then bed things in again using external mechanical power. Having said that, if I swap out the cylinder head for a blanking plate there is more than enough vacuum to pull the crank shaft round from 180 back to 360 after hand turning to the 180 point. With the blanking pate in place, compression bounce would perhaps be overstating things but there is enough compression to stop rotation. With the real cylinder head in place I am still getting vacuum (enough to suck fuel up) and I’m still getting some compression but it’s also obvious that my valves are leaking to some extent. Of course, I’ve no idea what level leaks are OK – are we talking ultra-high vacuum precision, or a little bit leaky is no big deal?

Jason’s videos are interesting, and certainly when I’ve had the engine spinning under external mechanical power the valves seem to be working. The exhaust spring came courtesy of a pen, and the intake spring was wound from music wire. They seem to operate exactly like the valves Jason shows so I think that bit is OK. As of yet I’ve not built the governor mechanism.

Neil mentions that petrol will be easier to light in comparison to gas, but that’s a double edged sword and is why I’m using gas. I figure that getting everything covered in petrol isn’t a clever thing to be doing, whereas gas will blow away.

Anyway, and on balance I’m being steered by comments above that indicate that the RCXEL isn’t necessarily the best choice of kit for my application, and after a very helpful email from Julian at minimag and the benefit of shared experience above I’ll try the minimag kit. I’m thinking that as long as I have fuel, air and and a hot spark I should be able to get combustion even if there are other problems that stop the engine working – and I’ve proved to myself that at the moment I can’t. But then again – it's all guess work!

David

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