RCU Forums - Glow plug

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Does the glow plug has any other function than heating the fuel to start the engine?
Someone said to me that it does on the engine operation, without the glow driver connected.

Thanks,

Nilo

Thata a good questions !

YES THE ENG CAN START WITH JUST TURNING OVER THE ENG AT A HIGH RATE OF SPEED,THE CHEM ON THE END OF THE PLUG AND THE CHEM IN THE FDUEL IS WHAT MAKES THE ENG FIRE UP, IT HAS HAPPEN AND WILL HAPPEN BE CAREFUL IT CAN HAPPEN

Yes. It is what keeps the engine running. Once the engine starts, the compression keeps it glowing.

The platinum filament in the glow plug acts as a catalyst to the fuel and it burns on contact. However, the reaction needs to have heat to react properly, hence having to make it glow with a battery.

Once the engine is running, the heat of the combustion provides the hot environment for the catalytic action to burn the fuel.

-David C.

Yes David,

I guess you got the point here.
If the material which the Glow active part is made from( tungsten?) has some kind of "heat momentum", that when the engine fires keeps itself hot, then in the next turn it will help the explosion and so forth, even without the drive connected. Would it be what you're saying?

Nilo

How about adding another glow plug question? I told my LHS I needed a spare glow plug for my EVO 46 trainer engine. They picked out the right? one, and I bought it. It's a lot longer than the plug that came with the engine. Is this right? Good? Bad? Thank you sirs. - Joe

If the Glow plug is long enough, the cilynder could hit it and cause a serious damage.

Nilo

Cylinder probably won't hit it seeing as it doesn't move ;)
However the piston might :D

As long as the engine turns easily with the plug in, you should be good to go.

To the original question: It is not HEAT that triggers the explosion, as David pointed out, it is a chemical reaction between the filament in the plug (Which is made of Platinum) and the alcohol in the fuel.

Heat only HELPS the reaction happen. So technically, an engine CAN start without a glowdriver (and I have heard of cases where this has happened).

Quote:

ORIGINAL: nilo

Does the glow plug has any other function than heating the fuel to start the engine?
Someone said to me that it does on the engine operation, without the glow driver connected.

Thanks,

Nilo

I think there have been a number of mis-leading answers to this question....

Firstly, the filament in-and of itself does not contribute chemically, nor catalytically to the combustion. It's only function is to produce a heat source. The length of the filament (because it is a coil) is surprising, and has a substantial surface area which is good for heat transfer.

Fuel combustion is a funny thing. Regular "gas" (that you put in your car) is a prime example. The actual fuel is non-flammable. It is the fuel vapour that burns.... and it only burns in a discrete boundary of oxygen/vapour concentrations at a given pressure... Too much vapour, and there is no combustion. Too much oxygen, and there is no combustion. Too little pressure, and there is no combustion. Too much ... etc. All combustion takes place at a temperature point that is specific to the pressure and mix of the fuel.

To proove this, just look at a candle.... Candles are quite safe, until they are lit. Candles are not considered to be explosive, although, in the right environment they can explode.

Your engine works as follows:
fuel/air is mixed in just the right proportions inside the carbeurattor. The fuel is atomised (sprayed very finely into the air) so that the fuel has a large surface area. The larger the surface area, the quicker the fuel vaporizes. The fuel/air mixture gets fed into the cylinder. The glow-plug is glowing inside the cylinder. It does not cause any combustion yet becuase there is not enough pressure. The Up-stroke of the piston compresses the fuel. Because your engine is well tuned, the fuel/air is fully compressed fully before it combusts. The fuel-air that is near the glow element combusts first, because it is the most heated. The heat, combined with the pressure brings the remaining un-combusted fuel above the flash-point temperature, and the explosion that started near the glow plug progresses through all the fuel/air mix.

Basically, combustion only happens in your engine when the fuel/air mixture is just right, and the mixture is significanly pressurised, and a heat source is applied. At that point, the mixture combusts. Combustion is an exothermic reaction (causing heat), and this heat is partially used to heat the glow filament. Enough heat is retained in the filament that on the next compression cycle of the engine there is enough heat left to initiate the next combustion.

To start the engine, you need the heat in the filamnt, otherwise the fuel/air mix will not reach the flash-point.

It is theoretically possible to tune your engine so that the fuel will combust without a heated glow-plug, but I don't believe that a normal engine will be able to obtain enough compression. If you were to use a different fuel though, you may get different results.

If there is enough residual heat in the engine, and glow plug then the fuel may combust without actually charging the glow plug. i.e. If your engine has been running, and you stop it, then it may be possible to re-start it without actually re-appling the current to the glow plug.

Because of all these things, your most efficient engine operation will be affected by air temperature, humidity, pressure, and fuel content. This is why it is important to re-tune your hish-end engine operation every time you go to the field.

So, The glow plug is just a filament that stores heat! It is specailly designed so that it does not cause ignition too early in the engine cycle (or too late). Changing the length, size, and composition of the filament will all affect it's operation. A longer glow-plug (with a longer filament - more surface area) may advance the combustion. possibly causing back-fires (until you re-tune the engine to operate under the new conditions).

So, there is an amateur intro to combustion. (mostly learned from automobile maintenance).

gus

In the time it took me to write my response, MinnFlyer responds with a contradictory response .... so ... I had better apply my homework to glow engines .....

For what it's worth. Platinum is like gold .... very inert. I would be VERY SURPRISED if platinum reacted CHEMICALLY to anything in our fuel. Otherwise it would not be used as jewelry, etc....

gus

gus... we can't read so much text !

There is a reaction between platinum and methanol that is significant in a glow engine. It 'aint a deisel!

google the following: "platinum methanol reaction"

Here is a science experiment for budding chemists: http://chemlearn.chem.indiana.edu/demos/OscMeth.htm

There IS a catalytic reaction involving the Platinum .... but, Platinum is not the catalyst, the mathanol is!

go figure.

http://www.wrcs.org.au/articles/10.htm

http://www.fly-imaa.org/imaa/hfartic.../v11-1-44.html

http://www.holdfastmac.com.au/howglowplug.html

Learn something new every day.

gus

Platinum is the catalyst and acts as a substitue for a spark, flame, or other energy source, to initiate combustion.

Normally, combustion requires an energy source to break the H2 (that's subscript 2) bonds so that
the hydrogen atoms are free to combine with oxygen.

Platinum does the same thing - i.e. it break up hydrogen molecules into a pair of hydrogen atoms.
In the presence of oxygen, combustion follows.

I'm no platinum expert, I just read this:
http://www.chem.umn.edu/services/lec..._platinum.html

Minni flyer and Jim Trainer are correct, there is a chemical reaction; and, it can and has taken place without external heat (glow plug battery connected) although it is a rare occurance in actual practice.

I'm really surprised about the glow plug engineering and the theory of chemical reaction did sound weird on the beginning.
If I do remember the old study days, this catalyst reaction would withdraw material from the glow plug on every reaction?
If so, the platinum material in the glow plug would fade away requiring its replacement or usually its life time is so long that this concern is irrelevant?

Nilo

Nice description Gus, but there is a catalytic reaction between the methanol and the platinum. It's very similar to a catalytic heater you can get for safe combustion in heater in, for example, a boat .

-David C.

Nilo,

If you are really intrested in this subject look over in the engine forum I remember reading some really good explanitions regarding this subject. many people tend to think the glow plug works exactly like a spark plug but that isn't the case a spark plug needs an external power source to provide the juice for a spark where as because of the chemical reaction a glow plug is self sustaining.
Glow plugs can a do wear out. I'm not sure of the exact cause just that sometime they don't glow.:D

If I remember my chemistry correctly, a catalyst is a substance which lowers the energy of activation necessary to start a chemical reaction, without itself being involved in the reaction. So whatever the catalyst metal in the glowplug coil, it shouldn't be used up. That's not to say that the coil doesn't degrade due to heat, mechanical shock, carbon deposition, etc.

Jim

It never ceases to amaze me how complicated our little hobby actually is! :D

Aqueous gasolone is non-flamable? You drop a match in a 5-gallon gasoline can and tell me it's not flammable... :D

Gasoline will only burn as a VAPOUR, with the right amount of air in the mix...

It is really quite fasinating...

Here are some references:
http://www.google.com/url?sa=U&start...ver.htm&e=7800
http://www.ccohs.ca/oshanswers/chemi...able/flam.html
http://www.brownoil.com/msdsgasoline3.htm
http://www.mapl.com/msds/msds/127mar019.html#5
This is a really good one... How to make a bomb...
http://www.ohscanada.com/virtual-iss...le/acprev.html

What it comes down to, is that Gasoline ONLY BURNS (under normal "room" temperatures/pressures) when it is in a vapour form, and it is in a concentration of between about 1.4% and 7.5% AND an ignition source is present. Any other concentration, and combustion simply does not happen.

So, If there is not enough air inside your gas tank, I will happily drop a match in!

gus

This is true, I have seen an experiment where an electric arc was generated below the surface of a tank of gasoline (don't try this at home kids!) resulting in only bubbles and smoke. Only when that arc was brought above the surface did it ignite.

BTW Gus, don't feel bad, I only learned about the platinum reaction thing about 6 mos ago (and it was right here on RCU!)

I agree with the catylitic explination, and no the platnum coil isn't "used up" in the reaction at all.

Plugs fail over time for other reasons such as:
- build up of carbon or burnt oil residue on the coil. Carbon being a result of the burning fuel and oil.
- build up of silicon deposits on the coil. Some fuel has anti-foaming additives that will eventually leave deposits on the coil. I've actually seen plugs that glow quite brightly when hooked to a battery, but as soon as the glow ignitor is removed, the engine dies. The cause was a fine layer of stuff on the coil that prevented enough methonol from getting to the platnum.
- melting. If you run the engine too hot or lean, it's not uncommon for the combustion temps to get too hot for the coil.
- other metal particles. Sometimes you can actually see little "beads" on the coil where metal has melted on. The metal might come from anywhere, but it's most common in new engines being broken in.
- dirt or other particles can sometimes get in the engine and literally just break up the coil.
- too much compression can sometimes slowly suck the coil right out of the plug, sometimes it's not slow at all.

On the other hand, if you are careful with your engine, you can have plugs last for years. I've had single plugs last for multiple seasons until I get stupid with the needle valve and kill it with an overly lean run or something.