Author Topic: Glow Plug Ignition Timing  (Read 1818 times)

Offline RayW

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Glow Plug Ignition Timing
« on: February 22, 2017, 08:40:22 AM »
Can anybody please explain to me how ignition timing works on a hit and miss glow plug engine?
With a normal spark ignition four stroke, fuel is drawn in on the induction, or suction stroke, compressed on the compression stroke, and ignited by the spark occurring at the end of the compression stroke.

With an engine such as my Otto, which uses propane and has glow plug ignition, surely the gas/air mixture will ignite on the induction stroke, as soon as it comes into contact with the hot glow plug, or will it not ignite until compressed on the compression stroke?

Also, the intake of gas to the inlet chamber beneath the inlet valve, is controlled by a hit and miss pecker operated valve in the carburettor, whose operation is dependent on engine speed. As the engine slows down, the pecker opens the valve to admit another shot of gas to the inlet chamber. This means that the gas valve will not necessarily operate as the same time that the inlet valve is opening as there is no synchronisation between the two. Presumably, gas is simply held in the inlet chamber until the inlet valve next opens?

Sorry if these seem like daft questions, but I really need to get this clear if I am ever going to get the Otto to run. As I have reported in my Otto thread, I can get some firing when turning the flywheel with an electric drill, but not sign of anything when turning it more slowly by hand.

Ray

Offline Jasonb

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Re: Glow Plug Ignition Timing
« Reply #1 on: February 22, 2017, 09:12:20 AM »
I've not run one of these engines with the permanent supply to a glow plug but imagine it works like a hot tube which is always hot.

J

Offline RayW

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Re: Glow Plug Ignition Timing
« Reply #2 on: February 22, 2017, 12:17:30 PM »
Yes Jason, I understand that. I used to operate a 12 1/2 horsepower hot bulb Hornsby engine at a local windmill, but that had an injector which injected fuel into the cylinder on the compression stroke so that ignition took place somewhere around TDC.
The point I am making about the Otto is that if my assumption is correct, and the fuel ignites as it is drawn into the cylinder, then the engine must be firing on the induction stroke, rather than on the compression stroke. How early in the induction stroke that occurs will depend on how quickly after TDC the inlet valve opens and admits the gas/air mix. As the valve is atmospheric you have no control over that and it will depend to some extent on how much suction there is, and the strength of the inlet valve spring.
I would be interested to hear from anyone who is already running an Otto on gas with a glow plug as I know that there are some out there who have done so successfully, so obviously it does work! It's just that this 70 year old is struggling to get his head round it!
Ray

Offline Rustkolector

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Re: Glow Plug Ignition Timing
« Reply #3 on: February 22, 2017, 07:09:03 PM »
I have a Mietz & Weiss hot bulb 2 stroke oil engine that injects fuel just after closing the exhaust port on the compression stroke. The fuel does not ignite until it is compressed into the hot bulb. Precise timing of the fuel ignition is based on how hot the bulb is. It will be hotter under load than at light load. Early M&W engines used a water drip into the intake air stream to control pre-ignition. This was manually adjusted by the operator. Later M&W engines dripped water into the intake automatically through a condenser that would drip progressively more water the harder the engine worked and less at low load. The system worked in that pre-ignition was effectively prevented. Unfortunately the water had few if any lubricating properties.

Further, on two cylinder model spark plug engines using a 180 degree crankshaft (John Deere type 2 cylinder) using a waste fire ignition system, one cylinder is always fired at the bottom of its fuel intake stroke. It does not ignite that fuel charge. The air fuel mix ignites only when it is put under compression.

Jeff

Offline Dave Otto

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Re: Glow Plug Ignition Timing
« Reply #4 on: February 22, 2017, 07:40:47 PM »
Can anybody please explain to me how ignition timing works on a hit and miss glow plug engine?
With a normal spark ignition four stroke, fuel is drawn in on the induction, or suction stroke, compressed on the compression stroke, and ignited by the spark occurring at the end of the compression stroke.

With an engine such as my Otto, which uses propane and has glow plug ignition, surely the gas/air mixture will ignite on the induction stroke, as soon as it comes into contact with the hot glow plug, or will it not ignite until compressed on the compression stroke?

Also, the intake of gas to the inlet chamber beneath the inlet valve, is controlled by a hit and miss pecker operated valve in the carburettor, whose operation is dependent on engine speed. As the engine slows down, the pecker opens the valve to admit another shot of gas to the inlet chamber. This means that the gas valve will not necessarily operate as the same time that the inlet valve is opening as there is no synchronisation between the two. Presumably, gas is simply held in the inlet chamber until the inlet valve next opens?

Sorry if these seem like daft questions, but I really need to get this clear if I am ever going to get the Otto to run. As I have reported in my Otto thread, I can get some firing when turning the flywheel with an electric drill, but not sign of anything when turning it more slowly by hand.

Hi Ray

It seems to me that because the fuel valve pecker is is tied to an eccentric connected to the cam shaft the time that it actuates when called on by the governor will always happen at the same time in the cycle of the engine. If you turn the engine over manually and slow enough that the governor isn't active the pecker will open the valve at the same place every other revolution. Therefor I would think that this event needs to be timed with the intake stroke of the engine.

This is the only way you are going to get a consistent fuel mixture and with propane that mixture is more critical than with petrol. On a for cycle engine with a hot tube (not hot bulb) the fuel mixture is forced by compression up in to the tube until it reaches the point in the tube that it ignites. I'm not exactly sure how the glow plug is going to react; but my guess is with a proper mixture that it will only go off  under compression. I have seen one of these engines run in person on a glow plug and it ran quite nicely.

Also, I'm not sure if you are using a demand regulator and if not this will also contribute to fuel mixtures issues. The engine needs to pull the fuel that it needs by vacuum and not be force fed.

Sorry if I'm telling you something that you already know.
Dave

Offline RayW

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Re: Glow Plug Ignition Timing
« Reply #5 on: February 22, 2017, 09:36:52 PM »
Thank you Jeff and Dave for your thoughts. At the moment Dave, I am just using a continuous very low pressure gas supply controlled by an adjustable regulator on the gas bottle, not a demand valve,
 as this has worked successfully on both my Wyvern and on my full size half horsepower Stuart. I had already considered that might be an issue and downloaded plans for a demand valve, so looks as if that might be the next project.
The only guidance that Heinz Kornmuller gives regarding valve timing is that the exhaust valve should be open at TDC. Presumably, it should actually just about be closing at TDC, so that there is minimal or no overlap with the inlet valve opening to allow maximum suction on the induction stroke.




Ray

Offline Dave Otto

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Re: Glow Plug Ignition Timing
« Reply #6 on: February 22, 2017, 10:37:50 PM »
HI Ray

Not saying that what you are currently doing is wrong or won't work; but a proper demand valve would probably make setting the mixture easier and more consistent.
Where is your fuel valve opening in relation to where the piston is?

Dave

Offline racinjason

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Re: Glow Plug Ignition Timing
« Reply #7 on: February 22, 2017, 10:44:29 PM »
A glow plug even with power applied still requires the increase in cylinder temperature caused by the compression of the fuel load to ignite the fuel load, that's why it doesn't ignite the fuel in the crankcase of a 2 stroke when the transfer ports open.
 Cheers Jason

Offline RayW

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Re: Glow Plug Ignition Timing
« Reply #8 on: February 23, 2017, 11:28:02 AM »
Thanks to all for the suggestions and advice. As you will see from my Otto thread today, we now have a running engine!
Ray

Offline NickG

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Re: Glow Plug Ignition Timing
« Reply #9 on: February 28, 2017, 08:29:18 PM »
Well done Ray, yes as the others said the fuel / air mix isn't volatile enough to ignite by the glow plug at ambient temp and pressure.


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