Go Back   The 75 and ZT Owners Club Forums > The 75 and ZT Owners Club Forums > Technical Help Forum
Register FAQ Image Gallery Members List Calendar Mark Forums Read
Notices

Reply
 
Thread Tools Display Modes
Old 27th November 2020, 20:57   #11
COLVERT
This is my second home
 
R75 Saloon.

Join Date: Feb 2009
Location: France/or Devon.
Posts: 14,003
Thanks: 3,851
Thanked 2,167 Times in 1,816 Posts
Default

Quote:
Originally Posted by bl52krz View Post
When you consider how hot it gets when the engine is up to temperature, starting with the glow plugs in operation, is less of a problem . I have never had to use my glow plugs to start mine. Turn the key, and it turns over when it is down to -4/5 and it starts no problem.
Just curious but how do you know the glow plugs haven't got hot ???

The second you turn the key the plugs are powered up.

On some cars, maybe ours are different, even though the indicator light goes out the plugs continue to work until the engine reaches a slightly higher temperature.

On the general run of diesels and not ours with the high pressure rail the longer operation of the glow plugs tends to act like a choke.
COLVERT is offline   Reply With Quote
Old 27th November 2020, 21:02   #12
COLVERT
This is my second home
 
R75 Saloon.

Join Date: Feb 2009
Location: France/or Devon.
Posts: 14,003
Thanks: 3,851
Thanked 2,167 Times in 1,816 Posts
Default

Quote:
Originally Posted by clf View Post
I didnt think any diesels did this, as I thought they all ignited from.heat generated.by compression. Would be Interesting to.know what engines did this. I would.have thought this would have caused premature failure on a glow plug as well due to hot spots.

Sent from my SM-A600FN using Tapatalk
Actually at the start and the glow plugs are working the spray of diesel on to them keeps them cooler. Heat is what tends to do in glow plugs from, maybe, over time, the combustion chamber temperature at high revs.
COLVERT is offline   Reply With Quote
Old 27th November 2020, 21:04   #13
clf
This is my second home
 
clf's Avatar
 
MG ZT CDTi

Join Date: Jan 2014
Location: carrick
Posts: 7,859
Thanks: 3,494
Thanked 2,657 Times in 1,973 Posts
Default

Quote:
Originally Posted by COLVERT View Post
Just curious but how do you know the glow plugs haven't got hot ???

The second you turn the key the plugs are powered up.

On some cars, maybe ours are different, even though the indicator light goes out the plugs continue to work until the engine reaches a slightly higher temperature.

On the general run of diesels and not ours with the high pressure rail the longer operation of the glow plugs tends to act like a choke.
glow plug relay on most of our cars have almost certainly failed, from most of what I have read on them. Looking at the condition of my relay, I am sure it is dead too lol.
__________________


It is not gloss primer .............. it is duct tape silver!
clf is offline   Reply With Quote
Old 27th November 2020, 21:08   #14
clf
This is my second home
 
clf's Avatar
 
MG ZT CDTi

Join Date: Jan 2014
Location: carrick
Posts: 7,859
Thanks: 3,494
Thanked 2,657 Times in 1,973 Posts
Default

Quote:
Originally Posted by COLVERT View Post
Actually at the start and the glow plugs are working the spray of diesel on to them keeps them cooler. Heat is what tends to do in glow plugs from, maybe, over time, the combustion chamber temperature at high revs.
I can envisage that, however, if there is wet spots from diesel, via the spray, I would have thought that this would have ignited first or have been hotter due to a larger (only tiny amounts of course) concentration of fuel. Or even some form of build up of residue to cause an issue on the outer surface of the glow plug.

I dont know how it works at all, but it is just how I 'see' it. To me it seems a flawed system in terms of longevity.
__________________


It is not gloss primer .............. it is duct tape silver!
clf is offline   Reply With Quote
Old 28th November 2020, 22:47   #15
COLVERT
This is my second home
 
R75 Saloon.

Join Date: Feb 2009
Location: France/or Devon.
Posts: 14,003
Thanks: 3,851
Thanked 2,167 Times in 1,816 Posts
Default

Interesting thread this.------I'd like to add a bit more about the ignition of petrol diesel in car engines and in the process probably bore the pants off any readers.----

To get maximum power out of the various fuels the correct air to fuel ratio is very important. For the sake of argument let's say around 12/1 air to fuel.

First Mr clf in the above post. You say that fuel hitting a working glow plug will catch alight. Almost certainly so but the mixture would so rich it would produce hardly any power and would also be a slow burn. ( rich mixtures burning, produce the least heat. )


Now back to the enrichment of cold fuels on start up and how and why this is necessary.

The older petrol engines had a manual choke. This was used to enrich the mixture.---When cold petrol was drawn into the engine it did not vapourise well. The petrol molecules were very large and well spaced out. In fact there was so much room between them that the 25,000 volt spark from the plug could actually miss hitting them.------No hit, no flame.

If you go from this micro level back up to the macro level the analogy you can see simply looking up to the sky above you at night. Looking at the milky way there are billions of stars and you might say they must touching each other. However, in fact, there can be tens of thousand of light years between.

( The speed of light being around 186,000 miles per second. )--

The analogy here between stars and fuel is, on the micro level, similar vast spaces exist between petrol molecules.

( the reason why those high voltage sparks miss their mark. ) To overcome this problem many, many more molecules of fuel are added by increasing the amount of fuel going into the engine. It reaches a point that it's impossible for the spark to miss the vastly increased targets.--Result===the engine fires up.------( More in the following post. Lol. )

Last edited by COLVERT; 28th November 2020 at 22:51..
COLVERT is offline   Reply With Quote
Old 28th November 2020, 23:06   #16
COLVERT
This is my second home
 
R75 Saloon.

Join Date: Feb 2009
Location: France/or Devon.
Posts: 14,003
Thanks: 3,851
Thanked 2,167 Times in 1,816 Posts
Default

The reason I'm doing a second post--( I know, I know, I do go on a bit. Lol. ) is that I have ,from time to time, lost some very large posts that it's taken me some time to compose so please bear with me . )---

Further to petrol engines.
In a minute or two from start up the engine is beginning to produce enough heat to cause the petrol molecules to break down into millions of smaller molecules. In this form the spark no longer has any trouble igniting them. The choke is no longer necessary and is switched off. This is done manually or slowly by a heat sensor in the water jacket.





PS. More to follow about diesels tomorrow. ( It's now midnight over here and I'm about change back into a pumpkin. Lol )----That bit is for the pantomime followers as we are getting closer to Xmas.



Night all.
COLVERT is offline   Reply With Quote
Old 28th November 2020, 23:29   #17
Mike Noc
This is my second home
 
Mike Noc's Avatar
 
Rover 75 CDT Manual Connoisseur SE, Rover 75 CDT Automatic Connoisseur SE & a Freelander Td4.

Join Date: Jul 2009
Location: Hampshire
Posts: 11,531
Thanks: 3,470
Thanked 3,119 Times in 2,247 Posts
Default

Quote:
Originally Posted by COLVERT View Post
To get maximum power out of the various fuels the correct air to fuel ratio is very important. For the sake of argument let's say around 12/1 air to fuel.

First Mr clf in the above post. You say that fuel hitting a working glow plug will catch alight. Almost certainly so but the mixture would so rich it would produce hardly any power and would also be a slow burn. ( rich mixtures burning, produce the least heat.)

Mixtures are very important in petrol engines but irrelevant in diesel engines. They aren't air throttled so take in the maximum amount of air on each induction stroke. The mass of air will vary depending on heat, density and how hard the turbo is working, but it will always be the total of whatever is available, so at idle there is loads of excess air in the combustion chambers.

There is no mixture like you get with petrol, the fuel burns on a flame front as it is injected.
Mike Noc is offline   Reply With Quote
Old 29th November 2020, 10:02   #18
COLVERT
This is my second home
 
R75 Saloon.

Join Date: Feb 2009
Location: France/or Devon.
Posts: 14,003
Thanks: 3,851
Thanked 2,167 Times in 1,816 Posts
Default

Quote:
Originally Posted by Mike Noc View Post
Mixtures are very important in petrol engines but irrelevant in diesel engines. They aren't air throttled so take in the maximum amount of air on each induction stroke. The mass of air will vary depending on heat, density and how hard the turbo is working, but it will always be the total of whatever is available, so at idle there is loads of excess air in the combustion chambers.

There is no mixture like you get with petrol, the fuel burns on a flame front as it is injected.
OH, Mike I am truly sorry about disagreeing with you about diesel mixtures but I have to. I hope I don't offend you in any way and you treat these posts as an argument to obtain factual information and not a put down from one person to another.

The mixture ( the ratio of air to fuel.) Constantly varies in a diesel engine due to the governance of the ECU.

The input of air, as you say, remains maximum to the requirements of the engine. In order to produce power according to the throttle pedal position the ECU will vary the amount of diesel into the mix. The richer the mix the greater amount of power produced by it.

It's because of this function by the ECU that so much more power and thus MPG can be produced by the engine for a specific quantity of fuel. ( ie one gallon for instance. )-- Ignition timing of course also plays an important part in this better economy too. ( There are a number of small additional things that help too. )



Wheeew. Here endeth todays sermon.---( Well it is Sunday after all. Lol. )



Whooops. I forgot to say------the total of air used will be the amount being processed at any given time and not the total of air available which can be far greater. ( dependent on the velocity of air in the input duct at any given time. )

Last edited by COLVERT; 29th November 2020 at 10:09..
COLVERT is offline   Reply With Quote
Old 29th November 2020, 11:11   #19
Mike Noc
This is my second home
 
Mike Noc's Avatar
 
Rover 75 CDT Manual Connoisseur SE, Rover 75 CDT Automatic Connoisseur SE & a Freelander Td4.

Join Date: Jul 2009
Location: Hampshire
Posts: 11,531
Thanks: 3,470
Thanked 3,119 Times in 2,247 Posts
Default

Always a friendly discussion with you Jon,

Yes diesel fuel does mix with air to burn, but not as in a petrol engine where the air and fuel is precisely measured, mixed and then ignited.

With a diesel the injected fuel mixes and ignites with air as it is injected - there is loads of excess air left if the engine is idling when very little fuel is injected and the same amount of air as would be there if you were injecting the maximum amount of fuel.

For this reason you can't run a diesel engine too lean - at idle they can happily run with an air fuel ratio of 120 to 1. Try doing that with a petrol engine.

That's also why you can run with the MAF disconnected too - on a petrol engine you need a precise measurement of the mass of air for the mixture, but on a diesel you just need to ensure that you have enough air available as you can never have too much.
Mike Noc is offline   Reply With Quote
Old 29th November 2020, 12:59   #20
clf
This is my second home
 
clf's Avatar
 
MG ZT CDTi

Join Date: Jan 2014
Location: carrick
Posts: 7,859
Thanks: 3,494
Thanked 2,657 Times in 1,973 Posts
Default

Quote:
Originally Posted by COLVERT View Post
In a minute or two from start up the engine is beginning to produce enough heat to cause the petrol molecules to break down into millions of smaller molecules. In this form the spark no longer has any trouble igniting them. The choke is no longer necessary and is switched off. This is done manually or slowly by a heat sensor in the water jacket.
Or a manual choke (that us poor folk had to use), it was a revelation though, with the bimetallic strip on the side of the carb (none of these fancy water jacket sensors - although it had one, but all it did was to cause illumination of a dash light, if the coolant overheated ). Manual choke taught me a little about the 'over-enrichment' of fuel delivery, which I understood to be, that there was too much petrol (it is important at this stage to call the fuel being described) being introduced for the amount of air, so therefore could not be atomised enough to explode, micro, macro and bluddy great droplets (relatively speaking of course). Effectively drowning the explosive force of combustion, and making the exhaust gas smell of petrol.

I know enough about the combustion process that I want to know about, especially nowadays with ecus controlling the mixture. My initial question was about the glow plug. The descriptions above from both you and Mike, (which I do appreciate, please do not get me wrong), would be better served for describing the benefits of bypassing the EGR?

I had never heard of, and did not think it happened, that diesel was sprayed directly onto a glow plug. As a practice I thought it would be both inefficient and destructive to the glow plug. I always thought a glow plug tip would be just in the ignition chamber, being hot, doing its thing, so when compression happened in cold conditions, the extra heat provided gave a more effective (read complete) burn, and therefore, a larger expansion of exhaust gases, ie more power.

With a working injector, it should be relatively fine. With a clogged injector, tiny droplets (not the micro/macro mist) of diesel would be sprayed/fall onto the glow plug tip. This would burn, and ignite any diesel vapour, creating combustion, effectively causing pre ignition (and therefore less power). BUT this is where I wondering. The fuel droplets from a clogged or inefficient injector, would land on and burn at the same point millions of times, as the glow plug doesnt move. This would surely leave a nanoscopic () residue. Over time, this residue, would also burn off millions of times. At that physical point, where the residue burns, would be hotter than the surrounding area of the glow plug tip. This would be a weak point, and would cause a failure of the glow plug tip casing, due to the tiny differences in heat expansion of the tip, leading to a failure of the glow plug.
__________________


It is not gloss primer .............. it is duct tape silver!
clf is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT. The time now is 23:41.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Copyright © 2006-2023, The Rover 75 & MG ZT Owners Club Ltd