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'[EE]: Update 2 - Spark ignition for model engines'
2002\05\20@062042 by Alan Gorham

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Hello again

I have slight progress to report!

After my query on Friday about spark gap I was ready to do some test running at the weekend.

I set the gap initally to 0.015" or 0.381mm. Peter Peres recommended a gap between 0.3 and 0.6 mm and Bill Colville told me that he used between 0.010" and 0.015". I compromised!

With this setting I was getting a fat, healthy spark, so I was hopeful of seeing some action.

I ran the engine on glow again to warm it up and to broadly check that the mixture was set.
Then the spark gear was refitted and I began flicking. The engine was firing every flick but would not continue.
Also, about every fifth flick it seemed to backfire and blow air out of the carburrettor.

Now bear in mind that I had the ignition timing at 5 degrees after TDC; I thought that I might as well try retarding it a bit
more to try and stop the backfire. I went to 10 degrees after TDC and found that the engine would not fire every flick.
Moving to 15 degrees after TDC the engine didn't fire at all.

This is where I left it. I think that I could try leaning the mixture a little and having another attempt at 5 degrees after TDC.
I'm running on straight methanol/castor fuel at this stage FYI.
Anyone any ideas on what could be better?

I feel that I'm pretty close to success here and that just a small tweak is needed!


PS: Roman, as you can see I did get the sparking going. As yet there's no carbon build up. I just had to set the gap correctly and went to an extra cell in the ignition battery!

Thanks

Alan
Embedded Systems Engineer
Microtima Ltd
Ouseburn Mews
3-7 Stepney Bank
Newcastle upon Tyne
NE1 2PW

Tel: 0191 2304411
Fax: 0191 2304422

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2002\05\20@063714 by Alan B. Pearce

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>Now bear in mind that I had the ignition timing at 5 degrees
>after TDC; I thought that I might as well try retarding it a
>bit more to try and stop the backfire. I went to 10 degrees
>after TDC and found that the engine would not fire every flick.
>Moving to 15 degrees after TDC the engine didn't fire at all.

Based on this I think I would have tried moving the firing to closer to TDC,
say a couple of degrees after TDC.

To actually get to a point of running reliably it may need to rapidly change
to several degrees before TDC to get sufficient burn time for the charge to
evacuate and get the next charge into the cylinder. On the basis that it
would fire at 5 degrees after TDC, but not run, this may be your real
problem.

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2002\05\20@084646 by Alan Gorham

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Hi


>Based on this I think I would have tried moving the firing to closer to
TDC,
>say a couple of degrees after TDC.


I would have tried this, but I wanted to check if it was sensible first.
How does this explain the 'backfiring' behaviour I observed at the retarded
ignition point
and how will advancing the ignition slightly cure this?


>To actually get to a point of running reliably it may need to rapidly
change
>to several degrees before TDC to get sufficient burn time for the charge to
>evacuate and get the next charge into the cylinder. On the basis that it
>would fire at 5 degrees after TDC, but not run, this may be your real
>problem.

I've been thinking about vintage spark ignition engines which are
essentially fixed speed motors.
The ignition was simply set retarded for starting and then once running,
advanced until maximum power achieved.

If this is true, then assuming I set the throttle on my slightly more modern
R/C engine to a fixed setting, I should be able to start the engine at a
fixed ignition setting?
Or are you implying that more modern porting design means that the ignition
point needs to be twiddled between the point of flicking the prop and the
engine starting - if so this sounds tricky!

Thanks

Alan

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2002\05\20@090620 by Alan B. Pearce

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>If this is true, then assuming I set the throttle on
>my slightly more modern R/C engine to a fixed setting,
>I should be able to start the engine at a fixed ignition setting?

Umm, yes to a point. I suspect that you may be able to find a setting that
will start and keep running, but do remember that your motor is rather more
than "slightly" more modern than a model T engine. A model T would do in
revs/min what yours would do in revs/sec - well maybe not quite that ratio,
but not that far off it.

>Or are you implying that more modern porting design
>means that the ignition point needs to be twiddled
>between the point of flicking the prop and the
>engine starting - if so this sounds tricky!

Well I was thinking in terms of electronic adjustment of the advance being
done automatically as the engine started. :)

Bear in mind that the fuel burn time in the cylinder may be a significantly
higher percentage of the rotational angle than the fixed manual adjustment
on the model T engine mentioned above. This may mean that you need a more
pro-active means of adjusting the advance as the motor starts than a manual
adjustment.

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2002\05\20@093541 by Alan Gorham

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>Umm, yes to a point. I suspect that you may be able to find a setting that
>will start and keep running, but do remember that your motor is rather more
>than "slightly" more modern than a model T engine.


OooK!

When I said 'vintage spark ignition engine' I meant a model engine
from the 1950's or 60's. Engines of this type had their ignition timing
adjusted by hand
and I have been working on the principle of being able to start the engine
at a fixed igniton and throttle setting.

When (if!) this works I want to move on to the automatic advance and retard
so that I can use the throttle.

>Well I was thinking in terms of electronic adjustment of the advance being
>done automatically as the engine started. :)

I half guessed that you were hinting at electronic means of twiddling the
ignition to get the engine running, but this is
very definitely getting ahed of where I want to be.

Thanks

Alan

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2002\05\20@104445 by Alan B. Pearce

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>When I said 'vintage spark ignition engine' I meant
>a model engine from the 1950's or 60's. Engines of
>this type had their ignition timing adjusted by hand
>and I have been working on the principle of being able
>to start the engine at a fixed igniton and throttle setting.

I wonder what weight of flywheel they had. I don't think there is very much
flywheel on the modern glowplug engine. I suspect that they may use the
inertia of the flywheel to keep the mass moving until the advance is
adjusted.

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2002\05\20@110325 by Michael Rigby-Jones

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> -----Original Message-----
> From: Alan Gorham [SMTP:@spam@alanKILLspamspamMICROTIMA.CO.UK]
> Sent: Monday, May 20, 2002 1:16 PM
> To:   KILLspamPICLISTKILLspamspamMITVMA.MIT.EDU
> Subject:      Re: [EE]: Update 2 - Spark ignition for model engines
>
> Hi
>
>
> >Based on this I think I would have tried moving the firing to closer to
> TDC,
> >say a couple of degrees after TDC.
>
>
> I would have tried this, but I wanted to check if it was sensible first.
> How does this explain the 'backfiring' behaviour I observed at the
> retarded
> ignition point
> and how will advancing the ignition slightly cure this?
>
I suspect you were slowly filling the exhaust with fuel during your attempts
to start it.  With very retarded ignition, the mixture is still burning when
it exits the exhaust port, which ignites the fuel stored within.  The result
is a backfire, quite an impressive one on a full size engine :o)

Regards

Mike

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2002\05\20@112123 by Michael Rigby-Jones

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{Quote hidden}

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2002\05\20@112842 by Alan Gorham

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>> How does this explain the 'backfiring' behaviour I observed at the
>> retarded
>> ignition point
>> and how will advancing the ignition slightly cure this?
>>
>I suspect you were slowly filling the exhaust with fuel during your
attempts
>to start it.  With very retarded ignition, the mixture is still burning
when
>it exits the exhaust port, which ignites the fuel stored within.  The
result
>is a backfire, quite an impressive one on a full size engine :o)


Well, I doubt it!    ;-)
I've been running the engine with an open exhaust port so there has been no
place for
a liquid fuel charge to collect.
I mentioned earlier that at the retarded ignition point the engine was
firing every flick of
the prop, so it must have been partially burning the fuel in the combustion
chamber?

Thanks

Alan

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2002\05\20@123903 by Eoin Ross

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My take on it is that there is still combustion occurring on the intake stroke
The incoming fuel ignites while the intake port is open (and the exhaust is closed)
- therefore the gas exits via the intake.

Taking the timing towards TDC will give a greater burn-time and avoid the scenario hopefully.

Eoin

{Quote hidden}

Well, I doubt it!    ;-)
I've been running the engine with an open exhaust port so there has been no
place for a liquid fuel charge to collect.
I mentioned earlier that at the retarded ignition point the engine was firing every flick of the prop, so it must have been partially burning the fuel in the combustion chamber?
Thanks
Alan

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2002\05\20@145920 by Peter L. Peres

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I think that you should thin the fuel (add methanol) and keep the spark
point at 5 degrees after TDC for starting.

As you have found out backblows are nasty. I don't know what you mean by
'blowing air out of the carb'. If you have a proper fuel system and the
throttle is open that should be a short, solid column of fire. It is
called a carb fire. I was not joking whan I suggested that you fuel the
engine with a pipette while trying this out, and do not let it run until
you are sure it won't bang on you.

You need to find out why the timing is unstable. Revert to glow, and fit a
neon bulb to the HV coil instead of the spark. Use a bright one. I found
some bright green fluorescent ones. They have phosphor on the glass and
something that makes uv inside. Shine bright green when lit.  Make a mark
on the crankshaft and on the body at TDC, start the engine, illuminate the
mark with the neon bulb, and see if it moves around when you use the
throttle. If it does you need to fix it until it stays put (the bulb
should light exactly when the mark is 5 degrees beyond the mark on the
casing if you set 5 degrees after TDC, and it should stay there no matter
what the RPM is if you have no timing advance correction). On a normal
engine you would do this with a strobe.

Imho do not fit the spark plug again until you get this sorted out
permanently. You are playing with fire and you were very lucky.

Peter

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2002\05\20@150146 by Peter L. Peres

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On Mon, 20 May 2002, Alan B. Pearce wrote:

>>When I said 'vintage spark ignition engine' I meant
>>a model engine from the 1950's or 60's. Engines of
>>this type had their ignition timing adjusted by hand
>>and I have been working on the principle of being able
>>to start the engine at a fixed igniton and throttle setting.
>
>I wonder what weight of flywheel they had. I don't think there is very much
>flywheel on the modern glowplug engine. I suspect that they may use the
>inertia of the flywheel to keep the mass moving until the advance is
>adjusted.

The flywheel was the wooden propeller blades. Nowadays it is smaller
(nylon etc).

Peter

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2002\05\20@150149 by Peter L. Peres

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On Mon, 20 May 2002, Alan Gorham wrote:

>If this is true, then assuming I set the throttle on my slightly more modern
>R/C engine to a fixed setting, I should be able to start the engine at a
>fixed ignition setting?

No, it has to be retarded at start or you'll have a carb fire.

>Or are you implying that more modern porting design means that the ignition
>point needs to be twiddled between the point of flicking the prop and the
>engine starting - if so this sounds tricky!

You have some time to un-retard the timing, it is not so critical, just as
long as you run the engine rich and do not allow it to pick up rpm
(throttle) while in this mode. Running a few tens of seconds with retarded
ignition was used deliberately to heat up engines (fragile exhaust
manifolds ?) quicker or so I read.

On a valveless (ported) 2-stroke if the spark is retarded very much you
can have the charge ports open while there is flame in the cylinder. This
will cause a cranckase combustion followed by a hiccup (the next
compression cycle will compress only exhaust gas and will not detonate).
It will also overstress all the gaskets in the cranckase and may blow out
any reed valves the engine may be using there. Alcohool burns slowly, it
may be this what you are seeing. Excess air will help it to burn faster,
maybe change the starting mixture (lean it).

Also typically glowplugs need no choke and sparked engines need a choke or
priming. The reason is the same as above.

Peter

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2002\05\20@155158 by Dennis Hoskins

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You guys have raised my curiosity level.  If variable ignition timing is so important on ported model engines,  why do slightly larger (read weedeater) engines with fixed timing not have the same problems.  Could it be the flywheel or maybe the recoil starter?  I just know that they start , idle and run well well at high speed, all on fixed timing setting.

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2002\05\20@164720 by Peter L. Peres

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On Mon, 20 May 2002, Dennis Hoskins wrote:

>You guys have raised my curiosity level.  If variable ignition timing is
>so important on ported model engines, why do slightly larger (read
>weedeater) engines with fixed timing not have the same problems.  Could
>it be the flywheel or maybe the recoil starter?  I just know that they
>start , idle and run well well at high speed, all on fixed timing
>setting.

The timing setting is not as fixed as you think it is. They have magneto
ignition and that has a certain amount of built-in advance. The recoil
starter is crucial for proper start of a non-retarded engine. The firing
point is probably at TDC when started and moves back a little due to the
magneto at high speed (so giving advance).

Also 'idle' and 'high speed' are pretty close together for lawnmovers,
leafblowers etc. They do not care about efficiency and they don't expect
to be safe (as in no engine failures in flight) and durable.

Peter

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2002\05\20@185130 by Michael Rigby-Jones

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> -----Original Message-----
> From: Alan Gorham [SMTP:KILLspamalanspamBeGonespamMICROTIMA.CO.UK]
> Sent: Monday, May 20, 2002 9:55 AM
> To:   EraseMEPICLISTspamEraseMEMITVMA.MIT.EDU
> Subject:      [EE]: Update 2 - Spark ignition for model engines
>
> I ran the engine on glow again to warm it up and to broadly check that the
> mixture was set.
> Then the spark gear was refitted and I began flicking. The engine was
> firing every flick but would not continue.
> Also, about every fifth flick it seemed to backfire and blow air out of
> the carburrettor.
>
> Now bear in mind that I had the ignition timing at 5 degrees after TDC; I
> thought that I might as well try retarding it a bit
> more to try and stop the backfire. I went to 10 degrees after TDC and
> found that the engine would not fire every flick.
> Moving to 15 degrees after TDC the engine didn't fire at all.
>
It sounds like the engine is very retarded.  You wouldn't normaly have a
static advance set anywhere *after* TDC.  Usualy a static advance would be
set something like 5-10 degrees BTDC.

Regards

Mike

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2002\05\21@041204 by Alan B. Pearce

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>I just know that they start , idle and run well well
>at high speed, all on fixed timing setting

Do these not use a magneto? I suspect the pole pieces are shaped to give
some automatic advance with speed change. See some discussion on this very
early in this thread, before it reached "update 2".

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2002\05\21@044848 by Michael Rigby-Jones

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> -----Original Message-----
> From: Alan Gorham [SMTP:.....alanspam_OUTspamMICROTIMA.CO.UK]
> Sent: Monday, May 20, 2002 1:16 PM
> To:   TakeThisOuTPICLIST.....spamTakeThisOuTMITVMA.MIT.EDU
> Subject:      Re: [EE]: Update 2 - Spark ignition for model engines
>
> Hi
>
>
> >Based on this I think I would have tried moving the firing to closer to
> TDC,
> >say a couple of degrees after TDC.
>
>
> I would have tried this, but I wanted to check if it was sensible first.
> How does this explain the 'backfiring' behaviour I observed at the
> retarded
> ignition point
> and how will advancing the ignition slightly cure this?
>
I suspect you were slowly filling the exhaust with fuel during your attempts
to start it.  With very retarded ignition, the mixture is still burning when
it exits the exhaust port, which ignites the fuel stored within.  The result
is a backfire, quite an impressive one on a full size engine :o)

Regards

Mike

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2002\05\21@084647 by Alan Gorham

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HI to everyone who has contributed to this.

There seems to be some differences of opinion as to my next step here.

Eion, Alan and Michael are advocating moving my sensor to give a firing point closer to but still after TDC.

Peter is advocating keeping the firing point at 5 degrees after TDC and maybe thinning the fuel.

Let me clarify exactly what I have been trying to do and see if we can get a consensus:

I want to end up with a unit that retro-fits to a model glowplug engine for spark ignition. I have seen commercial units that do this and they seem to be fit and forget. Look at: http://www.nelsonhobby.com/ignition/samples/samples.htm for an example that uses a single Hall sensor.
I will need a unit that does automatic advance and retard eventually and I plan on using the trusty PIC for this. However, I want to prove my ignition system to myself first.

I have been working on the assumption that a fixed speed engine where the ignition is retarded for starting and the advanced for running would be the way to prove the ignition system. This assumption stems from the fact that this is the way model spark ignition engines started out.

The engine I am using is a 1960's Merco (English) 0.29 cu. in. two stroke. This is an R/C engine (i.e. with throttle) but I have fixed the throttle at idle speed for testing. I think this may be a problem area - do you think I should actually try to start with the throttle open and the ignition retarded?

I had intended trying to start with the throttle open at first, but old habits die hard and I always start my glow engines at idle. Presumably a retarded ignition point for full throttle is not necessarily retarded the same amount at idle speed?

Peter - the above might explain why I did not see your 'solid column of flame' when I saw a backfire? I'll be trying the pipette method in future, but tell me - this is only to prevent a big backfire, right? I won't be totally sure that the engine will continue to run at an ignition setting that does not cause a bang?

Anyway, thanks for all the input, people!

Alan

Embedded Systems Engineer
Microtima Ltd
Ouseburn Mews
3-7 Stepney Bank
Newcastle upon Tyne
NE1 2PW

Tel: 0191 2304411
Fax: 0191 2304422

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2002\05\21@091617 by Morgan Olsson

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Hej Alan Gorham. Tack för ditt meddelande 14:41 2002-05-21 enligt nedan:
>Look at: http://www.nelsonhobby.com/ignition/samples/samples.htm
In the text:

" The retaining ring is rotated in position such that the spark plug fires when the piston is at top dead center (maximum upwards travel) position. "

/Morgan

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2002\05\21@093145 by michael brown

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Are you using gasoline mixed with oil? (I'm assuming that you are, as the
engine won't last long without it)  I noticed that the web site you
referenced says "the Super Tiger _90 and larger_ glow engines can be
operated with gasoline and still use their standard carburetor".  Since they
qualify their answer, I'm wondering if engine size has any significance.

You said, in an earlier post, that you were *not* using a muffler.  If so,
then how are you pressure feeding the fuel to the carburetor?  Glow fuel
seems to be slightly more viscous than gasoline/oil mixture, perhaps the
engine is worn to the point that piston blow-by is causing the back-firing.
Do you have access to an electric starter?  These do wonders for starting
engines that don't want to run.  I have a .40 model that can't be started
any other way (cheap, ringless engine that has had many gallons of fuel run
through it).

The electric starter spins the engine much faster than you could possibly
turn it by hand, plus it overcomes kick-back.  Not to mention the safety
issue.  This may help you to establish your base timing as you can tell when
it starts firing correctly by ear.  You would also be able to seat the
needle valve initially and then slowly open it.  Since the gasoline is
supposed to be used more efficiently, then setting the needle to where it
ran good on glow fuel would be too rich of a setting for a gasoline mixture.

Michael Brown
Instant Net Solutions
http://www.KillerPCs.net

"In the land of the blind, he who has one eye is king"

{Original Message removed}

2002\05\21@094432 by Bill Colville

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> The engine I am using is a 1960's Merco (English) 0.29 cu. in.
two stroke. This is an R/C engine (i.e. with throttle) but I have fixed
the throttle at idle speed for testing. I think this may be a problem
area - do you think I should actually try to start with the throttle
open and the ignition retarded?

Alan,
My experience is with model engines of the 1930's and 40's
running on a mixture of 3 parts white gasoline and 1 part SAE 70
weight motor oil. Some people added about 5% ether or other
souping-up agent but I never found it necessary. Of course the
porting on those older engines was more conservative than the
current engines. I don't know how your Merco might compare,
although being of 1960's vintage, I suspect it is still rather
conservative.

Anyway, the starting procedure then was to set the ignition
timing to TDC, open up the needle valve a turn or so to richen
up the mixture, then either choke it once or twice with the venturi
closed off or squirt a small prime into the exhaust port. Usually one
or two flips of the prop would have it running. After starting, advance
the timing, there was usually about 30 degrees of advance available,
then close the needle valve to lean the mixture out for maximum
RPM.

These older engines did not have throttles, and I am not familiar
with the Merco throttle. If it restricts the opening in the venturi at
the idle setting, I would suggest starting with it in whichever
position allows the most air through.

There was a discussion of fixed timing previously. Some of the
racing engines of that period, a Hornet 60 comes to mind, had the
timing fixed at maximum advance, and was almost impossible to
start by hand flipping the propeller. It started very easily if an
electric starter was available.

Sorry for the long post. I hope it helps.

Bill

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2002\05\21@101148 by Alan Gorham

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In the text:

" The retaining ring is rotated in position such that the spark plug fires
when the piston is at top dead center (maximum upwards travel) position. "

/Morgan


And also in the text!:

"Starting of the ProSpark equipped engine is easier because the engine is
started with the piston at top dead center in the retarded spark position.
There are no thrown propellers or kick backs."

Alan

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2002\05\21@101611 by Alan Gorham

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>Of course the porting on those older engines was more conservative than the
>current engines. I don't know how your Merco might compare, although being
of 1960's vintage, I suspect it is still rather
>conservative.

Dead right!



>Anyway, the starting procedure then was to set the ignition
>timing to TDC, open up the needle valve a turn or so to richen
>up the mixture, then either choke it once or twice with the venturi
>closed off or squirt a small prime into the exhaust port. Usually one
>or two flips of the prop would have it running. After starting, advance
>the timing, there was usually about 30 degrees of advance available,
>then close the needle valve to lean the mixture out for maximum
>RPM.

Interesting to note that you were able to hand start, this is what I am
aiming at.
Also interesting to hear that you started at TDC and had such a large amount
of advance!

>These older engines did not have throttles, and I am not familiar
>with the Merco throttle. If it restricts the opening in the venturi at
>the idle setting, I would suggest starting with it in whichever
>position allows the most air through.

I'm going to try starting at the full open throttle position. However, I'd
prefer to be able to start at idle, but maybe I need the automatic advance
and retard unit to do this.

>There was a discussion of fixed timing previously. Some of the
>racing engines of that period, a Hornet 60 comes to mind, had the
>timing fixed at maximum advance, and was almost impossible to
>start by hand flipping the propeller. It started very easily if an
>electric starter was available.

That's what I've heard (and what I'd expect!).

>Sorry for the long post. I hope it helps.

It did - confirmed a few thoughts about the way ignition engines work(ed)!

Thanks.

Alan

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2002\05\21@102852 by Alan Gorham

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>Are you using gasoline mixed with oil? (I'm assuming that you are, as the
>engine won't last long without it)  I noticed that the web site you
>referenced says "the Super Tiger _90 and larger_ glow engines can be
>operated with gasoline and still use their standard carburetor".  Since
they
>qualify their answer, I'm wondering if engine size has any significance.


I'm still using methanol based fuel at the moment. I fully intend to move to
petrol when everything is working.
I think that the reference in the website has more to do with carburettor
jet size and the fineness of the thread on the needle valve.
Large Super Tigre engines have a very fine thread on the needle which allows
the mixture to be set very lean for petrol running.


>You said, in an earlier post, that you were *not* using a muffler.  If so,
>then how are you pressure feeding the fuel to the carburetor?  Glow fuel
>seems to be slightly more viscous than gasoline/oil mixture, perhaps the
>engine is worn to the point that piston blow-by is causing the back-firing.
>Do you have access to an electric starter?  These do wonders for starting
>engines that don't want to run.  I have a .40 model that can't be started
>any other way (cheap, ringless engine that has had many gallons of fuel run
>through it).


I have *never* found pressure feed necessary. The very fact that the
carburettor acts as venturi tends to do it for me.
Remember, I'm only interested in bench running at this stage, not 3-D
aerobatics!
The engine in question is old and ringless but starts first or second flick
by hand when it is run as a glowmotor.
Remember also that I am still using methanol based fuel for spark ignition
at this stage.

>The electric starter spins the engine much faster than you could possibly
>turn it by hand, plus it overcomes kick-back.  Not to mention the safety
>issue.  This may help you to establish your base timing as you can tell
when
>it starts firing correctly by ear.  You would also be able to seat the
>needle valve initially and then slowly open it.  Since the gasoline is
>supposed to be used more efficiently, then setting the needle to where it
>ran good on glow fuel would be too rich of a setting for a gasoline
mixture.
>


The website that I referenced mentions easy hand starting. Now, I *know*
that they use clever electronics to retard the ignition point for starting
and low throttle operation, but it has been my contention all along that if
I pick the right ignition point I can hand start my engine and have it keep
running at this setting. This totally ignores any fiddling with advance to
speed up the engine.
I don't need that yet.
I may use an electric starter ( the shame!) if it proves ultimately
necessary, but I hope it won't :-)

Thanks

Alan

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2002\05\21@103614 by michael brown

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Morgan notes:
>> In the text:
>>
>> " The retaining ring is rotated in position such that the spark plug
fires
>> when the piston is at top dead center (maximum upwards travel) position.
"

Then Alan points out:
> And also in the text!:
>
> "Starting of the ProSpark equipped engine is easier because the engine is
> started with the piston at top dead center in the retarded spark position.

Sounds like a typical datasheet to me.  ;-)

> There are no thrown propellers or kick backs."

Aw, where's the fun in that?  ;-)

michael

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2002\05\21@152937 by Peter L. Peres

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On Tue, 21 May 2002, Alan Gorham wrote:

>I had intended trying to start with the throttle open at first, but old
>habits die hard and I always start my glow engines at idle. Presumably a
>retarded ignition point for full throttle is not necessarily retarded the
>same amount at idle speed?

The retarding is connected with fuel richness (which sets flame speed
among other things). Lean fuel goes with less advance (faster burn)  wrt
the spark at the same speed and conditions than rich fuel. The purpose of
active timing control is to counteract this effect. Thus in theory you
could use very rich fuel and fixed advance to produce the detonation at or
after TDC required for starting. But don't bet on this.

>Peter - the above might explain why I did not see your 'solid column of
>flame' when I saw a backfire? I'll be trying the pipette method in
>future, but tell me - this is only to prevent a big backfire, right? I
>won't be totally sure that the engine will continue to run at an ignition
>setting that does not cause a bang?

Backfires are different from carb fires. At a backfire the flame goes
through the exhaust. At a carb/manifold fire the intake path is open to
the flame. The intake path and manifold is normally full of perfectly
mixed fuel and delicate equipment (like the carb), and it is not designed
to take direct flame temperatures, or the force of the explosion. Flying
air filters and even carbs or aluminium intake manifolds stripped off
engine blocks are not unheard of in this context.

A 2-stroke will not emit any column of fire through the carb, no matter
what happens to the ignition, because the cylinder charge porting and the
inlet porting from the intake manifold to the crankcase are never open at
the same time, exception being taken for reed valved engines where the
imperfectly closing (or blown out!) reeds can admit the flame back into
the intake manifold. It can only happen on a four stroke, where this path
can be open (inlet valve open = cylinder connected to inlet manifold and
carb). Anyway treat the air intake and exhaust with respect while testing
the ignition.

You seem to have a unstable point detection and running the engine timed
slightly after TDC (3 degrees or so) will avoid any accidents until you
clarify the instability. This reduces the accuracy requirements to 'only'
1%. This is why I 'recommend' it. Keep in mind that your shaft angle
sensor needs to be repeatably accurate to the nearest degree at least for
stable ignition characteristics. This is better than 0.3% required
accuracy fyi. This was discussed before on this list. A dodgy jury rigged
detector plagued by shaft play and missing speed correction will not cut
it here. The timing point detector is a precision device and must be
implemented as such.

An engine timed for exact TDC spark needs to be started vigorously because
if it is slow during TDC, the explosion will produce little torque (crank
wrist pin is inline with cylinder pin at TDC) and the force may score the
pin or bend the linkage. This especially since you need to change the
compression (lower it) to change from alcohool to gas probably. If the
timing is slightly beyond TDC you can just 'flick it over' slowly and it
will start immediately. This is the way small stationary engines and demo
engines are started (by pushing the flywheel over by hand). Also old old
aircraft apparently.

As procedure I'd suggest that you close the needle valve completely and
use a graded pipette to prime the engine through the carb before each
start. Put a metered quantity in, then ignition off, turn the engine over
about twice to work the fuel in, ignition on, flick to start. Repeat,
while changing the amount of priming fuel. Drill this procedure until you
do it automatically. Turning the prop while the ignition is forgotten on
is not good (ouch). The ignition MUST be designed such that it will not
spark either when turned on, or off, no matter how quickly and how
'imperfectly' (see threads about PICs not resetting when voltage does not
drop completely etc). Use a high quality switch.

Once you have the priming figured out, repeat as above but start opening
the needle valve a little every time. After a while the engine should
start and run. The settings will be different for a warm engine. Be
prepared to close the throttle as needed immediately after start, and to
adjust the timing towards more advance as Bill C. said.

The throttle (air obstruction plate) should be open a third or more during
start, but be prepared to close it after start to the idle position.

Peter

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2002\05\21@154035 by miked

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This might be useful.
http://www.modelflight.com/ignition.html

> > The engine I am using is a 1960's Merco (English) 0.29 cu. in.
> two stroke. This is an R/C engine (i.e. with throttle) but I have fixed
> the throttle at idle speed for testing. I think this may be a problem area
> - do you think I should actually try to start with the throttle open and
> the ignition retarded?


Michiana R/C Choppers
http://www.mrcc.info
@spam@mikedspam_OUTspam.....mrcc.info

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2002\05\21@184344 by Michael Rigby-Jones

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Oop's sorry for the multiple posts, something screwy going on I think (list
server is uspoosed to filter them out isn't it?)

Cheers

Mike

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2002\05\22@005004 by Michael Rigby-Jones

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{Quote hidden}

That almost seems to be saying that TDC *is* the retarded position.
Although I've never had experience with spark igntion model engines, I have
rebuilt many motorcycle and car engines, and invariably if my first guess at
the igntion timing was too far retarded the engine would fire but not run,
and major backfires would occaisionaly result.

Out of interest have you reduced the compression ratio of the engine?  Glow
engines run very high compression because alcohol can resist detonation far
better than petrol.

Regards

Mike

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2002\05\22@105954 by Alan Gorham

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Hi

>That almost seems to be saying that TDC *is* the retarded position.
>Although I've never had experience with spark igntion model engines, I have
>rebuilt many motorcycle and car engines, and invariably if my first guess
at
>the igntion timing was too far retarded the engine would fire but not run,
>and major backfires would occaisionaly result.

The behaviour you describe sounds most likely what I am seeing.


>Out of interest have you reduced the compression ratio of the engine?  Glow
>engines run very high compression because alcohol can resist detonation far
>better than petrol.

I'm still running on methanol at the moment. However, I have uncovered an
article in an old R/C magazine that described a homemade electronic ignition
conversion. The author described how he reduced the compression ratio as you
said, until he had a running spark igntion setup and then took it back to
it's original value.

The puzzling thing is that commercial spark conversion kits *do not* mention
anything about changing the compression ratio.


Thanks

Alan

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2002\05\22@232054 by Michael Johnston

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Alan
I think to decrease compression you need to shim the cylnder head. I
received a cooper shim with my rossi 45 and the instructions said to use
this if you plan on using a tuned pipe exhaust. What size is this engine you
are converting?  Mike Johnston
{Original Message removed}

2002\05\24@165634 by Gareth Bennett

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When using methanol  based fuel, the ignition timing usually requires MORE
advance than a conventional petrol powered engine, you could try starting at
10 Deg Before TDC and see how you go. Methanol is a slower burning fuel than
petrol. And needs more advance than usual to get good performance (And with
the anti knock properties, can stand more advance) But this has quite a lot
to do with compression ratio, etc etc.
{Original Message removed}

2002\05\25@202046 by Brandon Fosdick

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Alan Gorham wrote:
> Well, I doubt it!    ;-)
> I've been running the engine with an open exhaust port so there has been no
> place for
> a liquid fuel charge to collect.

What do you mean by open exhaust? Did you take the muffler off? In my experience
model airplane engines don't run very well without the muffler. I had one plane
where the muffler fell off in flight and the engine stopped very quickly. I've
also found that they're very hard to start without the muffler too. Although I
did have a Saito 4-stroke (forget what size) that would start ok but wouldn't
run higher than idle. It was loud too.

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2002\05\25@202206 by Brandon Fosdick

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"Peter L. Peres" wrote:
> Also typically glowplugs need no choke and sparked engines need a choke or
> priming. The reason is the same as above.

I've never seen a glowplug engine that didn't need to be primed, especially when
started cold.

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2002\05\26@143036 by Michael Johnston

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I agree with that mike johnston
----- Original Message -----
From: "Brandon Fosdick" <bfozEraseMEspam@spam@TERRANDEV.COM>
To: <RemoveMEPICLISTspamspamBeGoneMITVMA.MIT.EDU>
Sent: Saturday, May 25, 2002 7:21 PM
Subject: Re: [EE]: Update 2 - Spark ignition for model engines


> "Peter L. Peres" wrote:
> > Also typically glowplugs need no choke and sparked engines need a choke
or
> > priming. The reason is the same as above.
>
> I've never seen a glowplug engine that didn't need to be primed,
especially when
> started cold.
>
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2002\05\26@143651 by Michael Johnston

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Brandon
On the contrary Model Airplane engines didn't have mufflers before the late
70's. What most of them had was a baffle tied to the throttle arm. When the
engine was at idle the baffle was accross the exhaust port of the engine
with just enough give to let the exhaust escape. The purpose of the baffle
was to give a little back pressure and to the engine warm which is what
muffler's on the engines do now days. Mike Johnston
{Original Message removed}

2002\05\27@061749 by Alan Gorham

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Hi

>What do you mean by open exhaust? Did you take the muffler off? In my
experience
>model airplane engines don't run very well without the muffler. I had one
plane
>where the muffler fell off in flight and the engine stopped very quickly.

This is probably due to the change in mixture setting caused by the change
in back pressure when the muffler drops off.
Engines *do* run perfectly well without a muffler, but I would'nt recommend
it for the obvious reason.


>I've also found that they're very hard to start without the muffler too.

Again, that is only true if you try to start at the mixture settings you use
when the muffler is fitted.

It's pretty academic anyway, because I can't think of any practical
situation where anyone would want to use a 2 stroke without a muffler. I
only did it because I'm using an old and useless engine that does not have a
muffler for my tinkering :-)

Alan

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2002\05\27@133125 by Peter L. Peres

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On Sat, 25 May 2002, Brandon Fosdick wrote:

>"Peter L. Peres" wrote:
>> Also typically glowplugs need no choke and sparked engines need a choke or
>> priming. The reason is the same as above.
>
>I've never seen a glowplug engine that didn't need to be primed, especially when
>started cold.

If you have a fuel pump on board and the corresponding linkage between
throttle and needle valve (obligatory in this setup) all you need to do is
flick the throttle up for a couple of seconds to get priming (unlike a
fuel injection that takes care of this for you).

If you electric-start a glowplug you don't need priming, just a little
more cranking than usual.

Peter

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2002\05\27@143648 by Peter L. Peres

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On Mon, 27 May 2002, Alan Gorham wrote:

>It's pretty academic anyway, because I can't think of any practical
>situation where anyone would want to use a 2 stroke without a muffler. I
>only did it because I'm using an old and useless engine that does not have a
>muffler for my tinkering :-)

You mean, like a cheap, powerful, loud Black Widow ? ;-)

(Where you can tell if the mixture is right by looking into the the
exhaust ports with engine *running*, to see its bright orange reflection
on the piston top a BDC - and can't hear anything at all afterwards)

Peter

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