glow or spark ingition, difference?

[Debian]

Could someone please explain the differences between theese two ignition systems?
I thought there was only diesel engine (without spark plug) and 4 or 2 stroke engine with spark plug.
Never heard about "glow" type, maybe just I can't translate the word "glow" itself!
thank You

Paolo

 

[Twinsquirrel]

Hi Debian,

A glow engine is kind of half way between a Diesel (compression ignition) and a spark ignition engine, it uses a coiled wire in the glow plug to which retains the heat from the previous combustion cycle to ignite the compressed fuel/air mix on the next cycle. The fuel used is different too, normally a mix of Methanol and a lubricating oil, sometimes a little nitro-methane for an extra kick (it also lowers combustion temps). The plug is usually heated electrically during start up.

A glow plug isn't needed in a Diesel engine as the compression ratios are much higher and energy used to compress the fuel/air mix is enough to cause combustion. In a Diesel car glow plugs are used at start up to assist the fuel to vapourise aiding combustion.

Hope that helps/is correct

David

 

[Bogstandard]

Paolo,

Unlike Davids explanation, which is correct, I will confuse you a little more by going a bit deeper.

Maybe the easiest translation would be, unlike a spark plug that fires across a gap with a spark to ignite the fuel, a glow plug is a similar design but has a small coil of wire in the tip instead of a gap as a spark plug has. When connected to a battery (for engine starting) the coil 'GLOWS' red hot and so the fuel starts to burn and causes the engine to fire. After a short warm up period by the engine, the coil will continue to glow without the battery being connected, by way of the combustion in the cylinder.
This glow is there permanently then while the engine is running, and only usually stops glowing when either the engine runs out of fuel, or the throttle is closed down so combustion cannot take place.

Here is a technical description

http://en.wikipedia.org/wiki/Glow_engine

Certain types of 2 and 4 stroke petrol engines can be run using a glow plug instead of a spark plug, as long as the fuel is the correct one for ignition.

People who design and make IC engines might use glow plugs at an early stage of development, to prove the engine will run without going to the trouble of fitting a proper spark plug ignition system.

As far as I know, you cannot use glow plugs on something like a hit or miss engine, because the intermittent firing will not allow the plug to heat up enough to fire by itself, I suppose if you left the battery connected, then it would run.

I hope that this has helped to explain it to you, and not confused you even more.


John

 

[Debian]

David and John thank's You both for the clear and satisfactory explanation! I was not able to find such a good description surfing on google.
Ignition in i.c. model engine is something yet weird to me, I can't easily find interesting documentation, and maybe I should buy the Bob Shoore book's. Right now only a cloud of strange names running across my head (point, buzz, coil, rare earth magnet, battery, spark..........) but I really can't combine them!
Thaks again

paolo

 

[DICKEYBIRD]

Just to add a little fuel to the fire, (sorry) model engine glow plug coils are mostly platinum which catalytically reacts with the methanol in the fuel. This is a major contributing factor in sustaining the heat required for combustion. AFAIK they won't run without it.

Additionally, under perfect conditions, I've personally hand started a .40 cu. in. glow engine without a battery connected.

 

[T70MkIII]

I'm sure I've seen a glow plug ignited Challenger V8. How would you get the mixture fire at the right time for a 4 stroke? Surely it would ignite as soon as (and while) the inlet valve is open?

Thanks.

Regards
Richard

 

[wareagle]

I'm sure I've seen a glow plug ignited Challenger V8. How would you get the mixture fire at the right time for a 4 stroke? Surely it would ignite as soon as (and while) the inlet valve is open?

Not nessecarily. With the proper fuel, it is totally doable with glow ignition. There is the component of compression. When the fuel/air mixture is compressed, it lowers the flash point of the mixture to below the temperature of the glow plug. Once the temperature reaches the ignition temperature the fuel/air mixture then begins its burn.

Gasoline nor diesel would not be the fuel used in a glow ignition engine. Gasoline's flash point would pretty much start its burn while the intake valve is open as you have said. Diesel takes a lot of heat (usually from high compression) to ignite, so it isn't a viable fuel for this type of engine.

A mix of methanol and oil, typically with a low percentage of nitro is what is used in a glow ignition engine. Methanol is a cool (in relative terms) burning fuel, and the nitro methane is a hotter burning fuel, and also produces its own oxygen during combustion. The oil is for lubrication of the engine. In an engine with a wet or dry sump oil system, the oil could be left from the fuel, though a tiny bit would help lubricate the valve train.

Hope this helps!

 

[T70MkIII]

Thanks very much, wareagle - that makes sense now, and makes the idea of doing a 1/4 scale V8 with glow plugs seem all that more attractive to me.

 

[wareagle]

A methanol based fuel is also nice from a safety point of view as compared to gasoline. Lower vapor pressure, less volitile, doesn't burn quite as redily, and burns a lot less explosively. Makes it safer to store in places you would never store gasoline. Methanol can also help with cooling and engine - Knew a guy that ran a sprint car on methanol and would actually have frost on the manifold before the engine got warmed up.

You do need to be somewhat careful about compression ratios. if you get to high a compression ratio with a glow engine you will get detonation or pre-ignition and it will generate a lot more heat and also be rather hard on the mechanical components.

Ahhh, another sprinter!! Those things are lots of fun! And yes, it is not unusual to see ice on the injectors!

Regarding the characteristics of the various fuels... All very true. One drawback with methanol is if it catches fire, the flames can invisible. Makes for bad situation, so be very careful when handling it!! Keep a fire extinguisher available when handling it.

On the compression and pre-ignition/detonation issues, that can happen with any fuel if the compression ratio is too high. And it can definately hammer an engine apart!

Thanks very much, wareagle - that makes sense now, and makes the idea of doing a 1/4 scale V8 with glow plugs seem all that more attractive to me.

kf2qd brings up a good pointon the compression ratios. I suggest doing some research before embarking on the journey. You may have to make adjustments to timing, cam geometry and bore/stroke to get the engine to live with glow ignition. I have never tried a glow ignition on a four stroker, so I am kind of playing a guessing game here. Just food for thought.

 

[DICKEYBIRD]

I have never tried a glow ignition on a four stroker, so I am kind of playing a guessing game here. Just food for thought.

There's a plethora of very successful 4-stroke model aircraft engines around to gather data from. Saito, OS, Enya, Thunder Tiger and Magnum to name a few. The most common displacements range between .30 and 1.50 cu. in for single cyl. engines.

The metallurgy, bore/stroke ratio, compression, valve size, cam timing and glow plug heat ranges have all been optimized for easy starting, excellent power/weight ratio, almost perfect throttling and long life. A feller could visit a local r/c field and ask around for a crashed one to pick up cheap and dissect to gather data. One could emulate any of the production engines' specs and incorporate them into your own multi cyl. engine and almost guarantee success. An r/c field is also a great place to get the word out and pick up a little cash for doing simple machining jobs.

 

[T70MkIII]

I appreciate the advice, guys, and am researching away (your replies definitely count in that regard).