Willdo

I have gleaned quite a bit of information recently from the forum on CD ignition systems.
Also, I see that RCIGN and some others have successfully used glowplug ignition on "gas engines".

What I really want to understand is:-
At what timing BTDC does the charge decide to fire?
What makes it decide?
Is it governed by compression ratio?
Is it governed by temperature?
Do different grades of plug change the timing?
Does engine wear affect the timing?
Do glowplugs work best with Alcohol fuels?
Can they work with petrol (gasoline)?
Is nitro necessary?
Is starting easy?

Possibly many of these questions have been answered before, but these are the ones which interest me most and I thought that it would be good to have them all answered on one thread if possible.

Antique

My Feedback: (4)

It fires when conditions are right..
Temp, glow plug type, compression..
Some glow plug manufacturers make different grades, from hot to cold..
Engine wear, if the compression decreases, will affect the timing..
glow plugs work best ONLY with methanol..
Nitro is not necessary, depends on the performance you want..Our 289 cc twin race engine went 234 mph using FAI fuel...
Starting is easy, just bump backwards like a glow engine...

rmh

Ralph gave you the GOOD points of glow ignition
And if you understand the engines - this setup works very well
BUT- as you asked - ALL are affected.
The really negative point is that ignition occurrs when the pressure tempurature combo is "correct for combustion"
the position of the piston can be ANYWHERE.
The glow plug can even be dead.
"how so???"
alky and platinum react - alky makes the platinum glow and that creates enough additional temp for the fuel to ignite.
It is a cataylitic reaction.

Some engines will start with no initial warming of the plug - really-
IF you up the compression to gain more torque -you can easily induce a pre ignition.
also you can -as the thermal rise sets in, go straight to detonation .
So the guys who know glow-- add nitro, change compression /oil quantity/ pipe settings -on and on --to obtain proper "timing.
The best power setups we ever did , were on spark ignition, alky and nitro-
and tuned exhaust.
this setup had perfect table manners - instant starting- extremely smooth ,low idle - flawless transition and more top end than same engine /pipe on glow only would do.
Why?
the timing was controlled .
On a flat out full bore racing setup - you simply chuck everything in favor of best WOT performance
so once you balance compression (including exhaust setup) -- you are only left with possible runaway temp - which causes some real problems .
My newest ignition setups have a tattletale tach - so I can see how fast I am really turning in the air .
so far I have not used it for max power tuning -just been too durn busy.

thirty years ago --we used an audio tach- to read in air rpm - it works if you know how to use it.
basically it was an audio pitch calibrated to scale - - you listened and tried to shift the audio till it matched the sound of the engine in the air
under ideal conditions you could get very close readings
This is the same kind of setup -50 years ago we used to calibrate our clarinets- really--
it was called a stroboscope - and you matched each tone on the instrument with a calibrated signal and watched a spinning wheel lamp, appear to slow and become stationary - meaning you were synchronized.
Bottom line - yeh you can use glow on gas engines -but it is NOT a plug and play setup

Antique

My Feedback: (4)

Probably depends on the engine...I made a few G23 glow engines from old parts, the cases were broken off at the rear, so I just milled the back flush.....Put on a WT449 from a glow 23, and a glow plug adapter with a McCoy 9 plug in it, G38 muffler, and played...8000 rpm with an 18-6 Mejzlik..Started just like a small glow engine, good idle and transition....
Less than 2 lbs total weight...

Willdo

Looks like it's a good system for specialised competition machines where the experts are prepared to tinker to get the last dregs of power, but not for general flying.
I may still experiment a little though, ( just for fun you understand ).

Seems I am still tied to using alcohol fuel, which brings me to another couple of questions:-

1. Approximately what CR does alcohol require to operate efficiently?
It would be hard to raise the CR adequately on a chainsaw type motor without a seperate head to gain the maximum efficiency from alcohol, wouldn't it?
I could probably do it with some drastic measures though. ( I don't think that skimming the bottom of the cylinder method would be adequate).

2. When using alcohol fuel, - say on a chainsaw type motor.
If it uses twice the volume of fuel / oil mix it would when using petrol ( gas ) , would the oil content have to be cut down accordingly?
eg. would say 50:1 on petrol need to be cut to 100:1 for alcohol, to deliver the equivalent amount of oil? ( because of the higher volume of fuel going in).

I realize that these are not all ignition questions, (they would probably be relevant too if I was using CD ignition), but they all tie in together I think.
I have always wondered about these problems, so maybe some of you guys can help.

Thanks very much for the answers so far, - most helpful!

rmh

Good questions --
CR- can be pretty high----providing the temp doesn't shoot up .
numbers?
pretty confusing,as two strokers are not like four strokers, so actual compressed area is different - but 10-1 is fine - and if you use spark to control the actual ignition point - even higher - -on four cycle/sparkplug /alcohol stuff -the compression goes waaay higher .
basically you have to try it.
oil content - must go UP-
good oil content may be 10-1
on regular bushed glow stuff - 4-1 is not uncommon.
alky has no lube qualities - it dries.
my little 2300 ST engine -on spark and 5% nitro and 20% oil -remainder alky was a super setup -over 9000 on 18x8 APC props
real problem ?
It drank like crazy!
I finally threw in the towel on my alky stuff - hell -I couln't carry enough fuel to the field to fly more than a few times!
power comes from fuel burned -these engines ar e just air pumps which burn.
Some small gassers are pretty good subjects for conversion .
Others - not -
the old Tartan twins took a fair bit of port enlarging, etc., to get decent flow .
Converting glow engines to gasoline - can be just as confusing but if you like to experiment - do it - modelling was once apastime for the experimenters - now many are guys who prefer to order from the menu - assemble and go fly -

Willdo

It looks like I'd be tied to alky fuel. and if I did my sums correctly it would cost me at least four times the price of petrol (gas) fuel, (would use twice as much volume, at at least twice the $/gal). Not to mention the shorter flights, or heavier fuel load. So I'd better think carefully.

Interesting to see that Ralph successfully uses straight pipes, and Dick successfully uses a tuned pipe, with glow ignition. I was conditioned to believe that a two stroke couldn't really operate successfully without tuned pipes, spark ignition, and 30:1 and upwards fuel/oil ratio,(played around with motorcycles too long) now it looks like the aircraft boys have taken another path completely!
Good to see that there are other ways of doing things of course.

As Ralph knows, high concentrations of oil in the fuel bother me, but he doesn't seem to have any trouble, and he has mentioned 10:1, (I do understand that plain bearings may need a lot), but 4:1? - 4:1?? - doesn't bear thinking about!
Please tell me you're joking, - well aren't you?

It looks to me that nothing is terribly critical, and there are no hard and fast rules, so trial and error must be the way to go, which suits me! I don't like step by step manuals.

I'm one of the older school , I prefer the challenge and pleasure of experimenting instead of going out and buying something ready made, ( except CD ignitions of course).
However, whatever turns us on makes us happy.

Maybe I'd better learn to fly properly before I start all this - I can stagger into the air and get it down in one piece so far!

tkg

My Feedback: (1)

Petrol has some lube values, alky has none. Most engines with a full set of bearings can live at 5% or so of oil with alky. A bushed rod is a 10% at least.

Antique

My Feedback: (4)

We would like to use tuned pipes on our race planes but the rules don't allow for outside exhausts, and trying to put a tuned pipe inside the fuselage doesn't work very well, although it's been tried..We put a two into one tuned pipe (Aerrow 200 twin) inside the fuse of a Stiletto...It was insulated pretty well, but the aileron wire somehow got melted and killed the receiver..The resulting 200 mph crash left a looong gouge in the desert and took about 3 hours to find all the leftover pieces...The pipe looked like an accordian
We use 8 to 10 oz of oil/gallon of fuel in the engine partly to carry away some of the heat and just to be safe..It works VERY well, try it sometime....Excess oil does not degrade the perormance at all, it's just messy....4-1 is not a joke, my 445 picked up 200 rpm, from 6800 to 7000, on gasoline using that mix....The idea that less oil in the fuel makes better performance is an urban legend...Been there, the performance of our race engine speaks for itself..Two championships and the same glow plugs (McCoy #9) lasted racing seasons..They're still in the engine...

Willdo

Well it looks like I somehow managed to change my "ignition thread " into a "lube" thread but thats just as interesting anyway, and they're both tied in together.
Thanks TKG, Dick and Ralph, for your comments, any more additions will be very welcome.

BTW, Ralph, I'm building a throttle servo controlled advance/retard unit for your CD ignition, I was also going to try a CH coil, but I found 3 secondhand motorcycle ones which I'll try out first.
I'm only doing it as I find the time, so it'll be slow, I think the company prefers me to work on their stuff.