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Old Dec 26, 2007, 07:02 PM
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UK
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Question
Running a glowplug engine on petrol/oil/solvent mix?

Any experience on this subject much appreciated!!

I'm looking at initially mixing approx 20% oil + 80% petrol, and maybe some kind of solvent (white spirit etc. to aid mixing - prob. less than 5%)

The flashpoint of petrol is -40C, auto-ignition is about 250C (high to prevent knock)
The flashpoint of methanol (usual model plane fuel I believe?) is 11C, auto-ignition at 455C.
It seems plausible that the engine could theoretically run on either. Compression is very low, so knock is not an issue. Ignition looks likely for petrol fuel.

I will have to run engine much leaner with petrol, i assume? Will this cause problems with engine temperature/explosion risk? With same fuel flow as model plane fuel, i'd be producing about 2x more power (based on specific energy content of fuel).
I don't know how lean i can tune typical cheap .75 glowplug test engine, or could I just increase the lubrication oil content of the fuel? I was thinking of using a fairly non-combustable synthetic. i guess extra oil will just come out of exhaust, but too much will cause higher compression, loss of swept volume, and probably in severe excess, a bent connecting rod.
In ideal world, petrol needs 14.7:1 mixture for complete combustion, ~12.5:1 for max power.
With methanol, engine needs 5:1-6:1 mixture, but much higher fuel flow to produce same power output.

Can anyone confirm if this approach is likely to work, or should I be putting on kevlar helmet / riot gear so that I don't get crankcase or piston in the brain?....

Thanks in advance for any knowledge.
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Old Dec 26, 2007, 09:11 PM
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Adelaide, South Australia
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It's possible that a glow engine would run on straight petrol/oil mix but you'd need power applied to the plug continuously because there'd be no catalytic reaction on the element to keep it hot. Also there'd be no way to time the ignition point accurately because, with the continuously glowing plug, any contact with petrol would begin ignition. With methanol, the ignition depends on several things including the temperature of the coil, surface area of the coil, cavity size and compression pressure. All these things have to be within certain limits before the methanol can ignite.

While petrol gives close to the same power as methanol (roughly double the heat energy but close to half the mass needed to mix with the same mass of air) this causes a problem with cooling. A lot of a 2 stroke's cooling comes from the internal evaporation of the fuel inside the crankcase. With petrol you only have half the mass but to make matters worse petrol has a very low latent heat of evaporation compared to methanol so you end up with about 15% the cooling affect compared to methanol. A large heat sink head like used on helicopters or even cars might help there.

Lubrication could be a problem in that with only needing half the mass of fuel then this of course means only half as much oil is passing through each cycle. If an engine absolutely needed a minimum of 15% oil with methanol then it could need up to 30% oil with petrol. Petrol however is slightly lubricating in itself so the actual amount would be slightly less than the 30%. Oil content has no real affect on compression because the volume passing through on each cycle is incredibly small, on the order of 1/2000th the volume of the combustion chamber.

Overall I think the biggest problem is the ignition timing because it's uncontrollable with a glow plug. It'll act like a glowing piece of carbon in a car engine giving preignition leading to detonation. Experimenting with lowered compression may help reduce this but that'll also severely reduce the engine's efficiency.

As an after thought, if you had a means of controlling the power sent to the glowplug you could vary it's temperature which might give a somewhat controllable ignition point.
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Old Dec 27, 2007, 10:33 AM
that tree ate my plane
rotccapt's Avatar
oklahoma city oklahoma
Joined Apr 2004
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what if you were to add a small amount of methonal to the mix. i have been told that you can add up to 50% metonal to an ignition engine and it will work and also the same with glow. with the methonal you wiould still get the catalistic properties for the glow plug. just a thought
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Old Dec 27, 2007, 05:45 PM
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UK
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Thanks for the replies.
I completely overlooked ignition timing. I could build a hall effect sensor triggered capacitive discharge ignition, and maybe hack together a spark plug that fits the glow plug thread. It's certainly easier than trying to empirically drop the compression ratio by lopping bits of the head or piston to see what happens, and hoping the engine does not get destroyed by the experimentation. I don't have tooling to achieve this.
The suggestion about controlling plug temperature with constant current is also interesting. It might need closed loop feedback system with temp sensor, and something like PWM controller, to maintain a constant temperature sufficiently accurately though?
It might be better to go the other way, and dieselise, by jacking up the compression ratio?
It all seems a lot of work... and the plane is already dangerous enough, being a very heavy home built riveted aluminum/softwood frame prototype with MDF skin.
I think I'll go with the commercial methanol/synthetic mix for test flying, and come back to looking at more easily available high volume fuels as a project later.
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Old Dec 27, 2007, 06:28 PM
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Wayne V's Avatar
London, Ontario, Canada
Joined Jan 2006
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You might get some use out of this thread
http://www.rcuniverse.com/forum/m_6165393/tm.htm

This one is more for converted engines but some good info on mixing gas and 10/10 airplane glow fuel
http://www.rcuniverse.com/forum/m_1267448/tm.htm
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Old Dec 27, 2007, 09:18 PM
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twchdyn's Avatar
rhyl
Joined Jul 2005
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look into control line and free flight motors. lots of spark ignition motors, not necessarily running on gasoline/petrol.

Agreed with the ignition timing problem. with glow as you probably know. different plug lengths can be used to good effect to increase compression and advance timing. also differet heat plugs to advance/retard timing.

there are some comercially available kits to convert glow motors to spark ignition. could be as simple as modifying the head to accept a spark plug and using an electronic ignition system. but at such a small size motor the extra weight of ignition and battery and the reduced power output make it (in my own fickle opinion) not much use.

But on the other hand id like to see it suceed and please prove me wrong. dont be put off. now if you got a spark ignition dry sump 4 stroke running reliably, that would be impressive!!!
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Old Dec 30, 2007, 06:12 PM
smothered in tradegy
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why
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Old Dec 30, 2007, 09:54 PM
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Beaumont tx
Joined Oct 2004
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There used to be conversion sets (points, condensor, coil, etc) to convert glow to gas. Have no idea whether they're still in use, but they worked OK for the larger engines. Never saw a .40 size work, but .90 Super Tigres did fine.

howell
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Old Dec 30, 2007, 10:26 PM
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London, Ontario, Canada
Joined Jan 2006
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So if i can run a 2 stroke gas engine on 1/3 glow-2/3 gas with a glow plug i'm pretty sure you could do it on a glow to gas setup, i might have to try it. Straight gas is another story though.
http://www.rcgroups.com/forums/showthread.php?t=793079
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