Glow plugs - how do they work?: Motoring Motoring Discussion forum

Motoring Discussion > Glow plugs - how do they work?	Miscellaneous
Thread Author: Mr Moo	Replies: 45

New Sat 31 Dec 11 07:35

Glow plugs - how do they work? - Mr Moo

I know the basic function of a glow plug is to raise the temperature inside the cylinder, so that sufficient heat is generated to ignite the diesel fuel on starting However, is the main purpose to actually heat the air inside the cylinder, so that it is hot enough to initiate combustion on the compression cycle of the engine turning over, or is it more designed to heat the metal lump of the cylinder head and block, so that when the engine turns over and generates heat through the compression process, the heat is not lost / dissipated to the cold metal straight away?

On previous diesles, if I cycled the glow plugs a couple of times in extremely cold weather, before actually cranking the engine, I got a 'cleaner' start, with less lumpy running for the first few seconds, which is presumably because I'm ensuring that the metal of the cylinder head is being warmed up a little through each cycle of the glowplugs.

Interestingly, on the new Golf 2.0 TDi (which has a DPF), it won't physically let you crank the engine over until the glow plug light goes off. Even at ambient temperatues of 8 - 10 degrees, where I'm certain it would start easily with no glowplugs, it wants to allow the glowplugs a couple of seconds before it will crank. Presumably this is in order to create the cleanest cold start it can, thus reducing the soot and emissions that a less good cold start would create and minimising the cack that clogs the DPF.

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New Sat 31 Dec 11 09:52

Glow plugs - how do they work? - Manatee

I admit I don't know, but the question caught my interest. This is a waffly article -

en.wikipedia.org/wiki/Glowplug

which suggests the idea is to heat the "core of the engine block", whatever that is, to a "specified temperature". Doesn't sound convincing to me, at least for the pre-start phase, given that the light goes out in a couple of seconds. Maybe it just creates a nucleus of heat to help the fuel ignite initially.

The article does make the point that the glow plugs don't these days necessarily switch off when the light goes out, which confirms my understanding from other sources.

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New Sat 31 Dec 11 10:28

Glow plugs - how do they work? - Zero

A glow plug cant heat a block, or head. Its a great chunk of iron and would need a nuclear generator pumping out 5 million gazilliawatts to heat up the block and head in 5 seconds.

The glow plug provides a localised hot spot where the flame front can start under compression.

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New Sat 31 Dec 11 11:04

Glow plugs - how do they work? - sherlock47

I am at one with Z on this, however it does raise the question as to why multiple glow plug cycles does result in smoother running at start up in cold weather?

PS talking old stle XUD diesels - with, as far as I know, no post start timer.

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New Sat 31 Dec 11 11:07

Glow plugs - how do they work? - Old Navy

>> I am at one with Z on this, however it does raise the question as
>> to why multiple glow plug cycles does result in smoother running at start up in
>> cold weather?
>>
>> PS talking old stle XUD diesels - with, as far as I know, no post
>> start timer.
>>

Because the XUD engines are indirect injection diesels which do need glow plug heat to start, and the glow plugs needed longer to heat up than the latest ones.

Last edited by: Old Navy on Sat 31 Dec 11 at 11:09

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New Sat 31 Dec 11 11:15

Glow plugs - how do they work? - Old Navy

EDIT:-

The lack of post start heating may be down to the lack of emission regulations at that time.

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New Sat 31 Dec 11 10:46

Glow plugs - how do they work? - Old Navy

I don't know either.

But. :-)

I have owned diesel cars since 1989, so have gone through the transition from indirect injection through direct injection and on to the second(?) generation common rail system. As I am a mechanical anorak I read any articles I come across. My amateur take (and experience) on it is that direct injection diesels do not need glow plugs to start at average UK temperatures. Glow plugs are not big enough to heat the metal parts of the engine. The do provide a "hot spot" to aid combustion and do remain on for a period after engine start to aid emission control. The usual cause of smokey starts is dirty injectors, more likely with the latest systems with micron sized injector holes. this can easily be cured with a good injector cleaning additive.

Feel free to put me right if this is rubbish.

Last edited by: Old Navy on Sat 31 Dec 11 at 10:47

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New Sat 31 Dec 11 11:14

Glow plugs - how do they work? - Number_Cruncher

As the piston nears the top of the stroke in a diesel engine, there is a lot of air movement - either in the swirl chamber of an indirect injection engine, or air squishing into the bowl in the piston top.

The glow plug tip protrudes into this flow, and keeps the air hot enough to support combustion.

The glow plug tip is designed to ensure there is a thermal resistance between the hot glow plug tip and the seal between the glow plug body and the cylinder head. i.e., the glow plug is designed NOT to heat up the metal of the cylinder head.

If glow plugs are over tightened on assembly (perhaps via the use of an incorrect thread lubricant) this thermal barrier can be crushed and short circuited. There will be no warning of this - as the glow plug will run cooler, it will last longer, but, the glow plug will not be working effectively.

The tiny droplets of fuel in the spray and the air local to that droplet need to be hot enough for that droplet to begin to burn. Even in a hot engine, fuel which ends up in contact with the cylinder wall and/or piston top is effectively quenched and lost to the combustion process, and hence responsible for HC and particulate emissions.

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1

New Sat 31 Dec 11 11:24

Glow plugs - how do they work? - Old Navy

Cheers N_C, another layer for my anorak. :-)

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New Sat 31 Dec 11 11:36

Glow plugs - how do they work? - Number_Cruncher

Much more here:-

www.beru.com/download/produkte/TI04_en.pdf

The drawing on page 6 shows the thermal resistance I was mentioning - it is labelled as the "annular gap".

As you can see, the annular gap means the condction path between the element tip and the cylinder head is long. If the glow plug is over tightened - either too much torque, or incorrect lubrication on installation, the taper seat can close up the annular gap, thus short circuiting the thermal resistance - giving a short heat path to the cylinder head.

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New Sat 31 Dec 11 12:09

Glow plugs - how do they work? - Old Navy

>> Much more here:-
>>
>> www.beru.com/download/produkte/TI04_en.pdf
>>

That counts as a whole new anorak, thanks!

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New Sat 31 Dec 11 12:47

Glow plugs - how do they work? - Cliff Pope

So basically they work a bit like temporary spark plugs, only are on continuously rather than a series of sparks?

Anything hot will do. We had an old marine diesel that had a wick in the air intake. One soaked it in fuel, then an electric heater got it so hot it was smoking and about to ignite, and after about 10 seconds turned the starter. The flame was sucked into the cylinders and the engine started with a bang. After that it ran by itself, generating its own heat.

If the electric heater failed, a flaming rag on a stick would work. Or perhaps Quickstart and a cigarette lighter.

Last edited by: Cliff Pope on Sat 31 Dec 11 at 12:49

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New Sat 31 Dec 11 13:22

Glow plugs - how do they work? - Robin O'Reliant

So how hot does the tip of the glowplug actually get?

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New Sat 31 Dec 11 13:26

Glow plugs - how do they work? - Zero

this hot

www.youtube.com/watch?v=LB3sVD_wD7Y

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New Sat 31 Dec 11 14:04

Glow plugs - how do they work? - henry k

>> this hot
>>
>> www.youtube.com/watch?v=LB3sVD_wD7Y
>>
Why pick me out for a warning !!!

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New Sat 31 Dec 11 13:47

Glow plugs - how do they work? - Old Navy

It says 1,000°C in N_Cs link.

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New Sat 31 Dec 11 19:31

Glow plugs - how do they work? - Dave_

>> So how hot does the tip of the glowplug actually get?

I did once actually try testing a suspected dud glowplug by powering it when removed from the engine whilst holding the business end. I discovered that they heat up very, very, very quickly.

Last edited by: Dave_TDCi on Sat 31 Dec 11 at 19:32

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New Sat 31 Dec 11 19:43

Glow plugs - how do they work? - Number_Cruncher

Turning Dave_TDCi's point around a bit, it's also really surprising how quickly a wire which is shorted out across a car battery heats up. The importance of including correct fusing and good practice in wire routing when making modifications or repairs cannot really be over-emphasised.

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New Sat 31 Dec 11 19:51

Glow plugs - how do they work? - Dave_

>> The importance of including correct fusing and good practice in wire routing when making modifications or
>> repairs cannot really be over-emphasised.

I learned a lot about auto electrics in the early 1990s from practical experience. My friend fitted his MkI Astra with a pair of spotlights (whatever happened to those!?) and promptly melted half the wiring behind his dashboard. That was fun to rewire so it would work again.

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New Sat 31 Dec 11 21:44

Glow plugs - how do they work? - -

Whilst we're on glow plugs, here's how i used to test them in situ if there were any suspected of failing.

Take ammeter and wire to each glow plug individually with clip on wire straight to live for supply, obviously after removing the heavy supply wire that powers them all.
IIRC a healthy old fashioned glow plug would take around 20 to 25 amps of current whilst heating up.
I suppose if you had a big enough ammeter you could simply check the current draw at the battery but you wouldn't be able to tell if one was weakening.

I've heard that this method isn't any use as a duff plug could still drain the same current whilst not giving a suitable glow, but all i can say is this method has worked reliably for me for many years....and would the sod who borrowed my old Lucas dashboard ammeter which was worth its weight in gold for this type of thing please return it..;)

Any comments on this method which saved removing glow plugs unecessarily.

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New Sat 31 Dec 11 22:06

Glow plugs - how do they work? - Number_Cruncher

I think it's difficult to think of a better test which is still practical.

I imagine ninety odd percent of glow plugs are condemned by low current resistance checks using a DMM.

In my sloppy way, I have tested glow plugs by wiring them in series with a apre sealed beam headlap - if the lamp lights....

Of only historic interest now are the glow plugs wired in series, where the open circuit failure of one plug would stop ALL the plugs from working.

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New Sat 31 Dec 11 22:14

Glow plugs - how do they work? - -

>> Of only historic interest now are the glow plugs wired in series, where the open
>> circuit failure of one plug would stop ALL the plugs from working.

Please tell me i've read that wrong N-C, what was wrong with the old way, eventually you'd get one or two failing but the resulting lumpy misfire accompanied by white smoke till the non working cylinders fired up fully soon warned you of possible failures.

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New Sat 31 Dec 11 22:27

Glow plugs - how do they work? - Shiny

Most cars under 10 years old and some older ones up to around 14 years old such as VAG/BMW have self-regulating plugs and they keep the glow plugs on even after the telltale light has extinguished, and for up to 3 minutes after starting, so these days, glow plug cycling has no effect.

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New Sat 31 Dec 11 22:43

Glow plugs - how do they work? - Number_Cruncher

Other way round GB! - the older systems were series connected, and worked by strict union rules - one out : all out!!

W123, and Land Rovers spring to mind as obvious examples. Most cars from the early 80's onwards have been connected in parallel, and work as you've described.

As for afterglow, that's now quite long in the tooth. My 1990 Nova 1.5 TD would drop the glow voltage to 7V after the engine started, and my 1995 W124 has afterglow for a few minutes - you can hear the combustion sound change as the plugs switch off - more so in cold weather, and more so when the engine has poorer compressions.

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New Sun 1 Jan 12 09:40

Glow plugs - how do they work? - Hard Cheese

The 2nd sentence in Z's first post above sums it up succinctly ...

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New Sun 1 Jan 12 09:46

Glow plugs - how do they work? - Cliff Pope

>> Other way round GB! - the older systems were series connected, and worked by strict
>> union rules - one out : all out!!
>>
>>

Like some kinds of christmas lights!

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New Sun 1 Jan 12 15:19

Glow plugs - how do they work? - Bigtee

Not all diesels have plugs in the head if at all some have air warmers in the inlet manifold it's like a bulb element it heats the cold air sucked into the engine and with great compression the engine starts.

Our Cummins use no device just compression.

Never tested my Vectra yet unless it's broken i don't play the last system i did check gave 5 secs at start up and when started shut off for 5 secs and came back on for 5 secs repeated 3x times, the longer interval in winter ran with the coolant temp sensor so if cold was longer when hot hardly anything for after glow.

Best way of testing a glow plug is to remove it and see if it glows, it can be done with a multi meter in the head but you can't see the glow pattern.

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New Sun 1 Jan 12 15:32

Glow plugs - how do they work? - bathtub tom

My old Maestro made it evidently clear when the plugs switched off - the headlamps suddenly went much brighter.

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New Sun 1 Jan 12 19:59

Glow plugs - how do they work? - Harleyman

it's also really surprising how quickly a wire which
>> is shorted out across a car battery heats up.
>>
>>
>>

One reason why, as an apprentice mechanic in the Army, I was taught never to wear a wristwatch with a metal strap, or any other jewellery, when working on vehicles.

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New Tue 3 Jan 12 04:38

Glow plugs - how do they work? - Pat

I've dithered about replying to this thread (I'm not always taken seriously here!), but on balance I think I should thank Number_Cruncher.

My first encounter with anything mechanical was at the age of 7or 8 when I was having a holiday on my Uncle's farm way out in the Fen at Turves.
He had a POW called Hans, who I adored, and Hans liked me too.
One day that I'll never forget was when Hans said I could help him service the generator that produced electricity for the farm and house.
I got gloriously covered in oil and grease, and I was taught how to use a spanner to undo previously loosened nuts! I passed thinghim hime and learned the names. I laid parts out in rows incorrectttect order to hand back to him.
It was absolutely the best day in my life and I loved every minute of it.
At the end of it the copper had been used to heat some water, instead of boiling potatoes for the pigs, and I was dumped in a tin bathreturnedreurned to my rightful colour!

This experience gave me a love of engines, transmissions and anything mechanical. I always wanted to understand that fuel went in here and power came out there, but how did that work?

I'm waffling here but in all the years that have followed I never understood just how glow plugs worked until now and I found it fascinating reading.

What I still can't understand is why I still have that thirst for knowledge about things that will never be of any use to me now, at my time of life?

Threads like this are the real reason I stuck around on here in the early days, and from all of us who read and learn, but never comment, I'd like to say a big thank you:)

Pat

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1

New Tue 3 Jan 12 07:50

Glow plugs - how do they work? - Old Navy

>> What I still can't understand is why I still have that thirst for knowledge about
>> things that will never be of any use to me now, at my time of
>> life?
>>
>> Pat
>>

We are from a similar mould, with the obvious gender modifications. :-)

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New Tue 3 Jan 12 08:28

Glow plugs - how do they work? - Pat

Yes, well......:)

Pat

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New Tue 3 Jan 12 08:30

Glow plugs - how do they work? - Pat

Edit to add that the typo's are not typo's but I assume IE9 not being compatible with the software.

Cursor sticks and won't move.

Pat

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New Tue 3 Jan 12 08:34

Glow plugs - how do they work? - Old Navy

>> Edit to add that the typo's are not typo's but I assume IE9 not being
>> compatible with the software.
>>
>> Cursor sticks and won't move.
>>
>> Pat
>>

I use IE9 and sometimes have that problem on this site. I find that it is the cursor marker that is in the wrong place, but it types where it has been put even if the cursor has not moved. If that makes sense.

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New Tue 3 Jan 12 08:52

Glow plugs - how do they work? - Zero

On this site on IE9 the cursor just disappears completely sometimes and you have no idea where the typing point is till you see the letters appearing.

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New Tue 3 Jan 12 10:55

Glow plugs - how do they work? - Pat

That's our excuse and we're sticking to it:)

Pat

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New Thu 5 Jan 12 11:55

Glow plugs - how do they work? - Kithmo

>>
>> The article does make the point that the glow plugs don't these days necessarily switch
>> off when the light goes out, which confirms my understanding from other sources.
>>
I was always under the impression that the light went out to tell you when you could start turning the engine over, not to tell you when the glow plugs went off.

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New Thu 5 Jan 12 12:27

Glow plugs - how do they work? - CGNorwich

As an aside I used to make model aircraft using glow plug engines My understanding of these engines was that the plug continued to glow when the powers source was disconnected and the engine was running. Model engine fuel contains methanol and a catalytic reaction between the platinum in the plug and the methanol cause the plug to glow continuously when the engine is running

This is obviously different to how a diesel engine works . Diesel engines are used in models but are heavier than glow plug engines.

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New Thu 5 Jan 12 12:34

Glow plugs - how do they work? - Cliff Pope

I think a hot spot alone will work, without any catalytic action.
I have always understood that old fashioned semi-diesels were really only scaled up glow-motors. One heated up the bulb first with a blowlamp, and then once the engine was running it maintained the necessary heat itself.

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New Thu 5 Jan 12 12:36

Glow plugs - how do they work? - DP

>> As an aside I used to make model aircraft using glow plug engines My
>> understanding of these engines was that the plug continued to glow when the powers source
>> was disconnected and the engine was running. Model engine fuel contains methanol and a catalytic
>> reaction between the platinum in the plug and the methanol cause the plug to glow
>> continuously when the engine is running

The heat from compression is also an important factor in keeping the plug glowing.

These are incredible little engines. I have an R/C car with a little .15 glow engine. It actually makes 1.2 hp at 29,000 RPM! :-)

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New Thu 5 Jan 12 15:22

Glow plugs - how do they work? - Dave_

>> an R/C car with a little .15 glow engine. It actually makes 1.2 hp at 29,000 RPM! :-)

Is that nearly a kilowatt from 15cc? I wish bigger engines had a similar bhp/litre figure :(

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New Thu 5 Jan 12 19:53

Glow plugs - how do they work? - MJM

I think from my aeromodelling days that the engine capacity is in cubic inches.

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New Thu 5 Jan 12 20:24

Glow plugs - how do they work? - bathtub tom

I had a Merlin diesel back in the '60s.

0.75cc IIRC.

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New Thu 5 Jan 12 20:47

Glow plugs - how do they work? - CGNorwich

Got me browsing - I'm liking the look of the radial engine! (2nd from last).

alshobbies.com/shop/cat.php?id=616

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New Thu 5 Jan 12 21:09

Glow plugs - how do they work? - Zero

Good lord, My sons last two cars didnt cost that much together.

Think that makes 8 cylinders and 4 litres. (if you were to smash them together)

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New Fri 6 Jan 12 08:07

Glow plugs - how do they work? - Dave

I have a little Yanmar tractor, probably 30 years old, that has something called Thermo Start. I think it was also fitted to some Massey Fergusons.

There is a little plastic pot that is filled with diesel, and a pipe from this to the inlet manifold. In the manifold is a small glowplug. When the ignition key is turned anti clockwise it opens a valve from the pot and energizes the glow plug. Diesel then drips onto the glowplug and starts to smoulder/burn. There's also a decompression lever. So it get's 10 seconds with the thermo start, then crank the engine with no compression, and when up to a good speed and the oil ight out, flip the compression lever over and it starts.

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