Diesel engine bore glazing

[sgr143]

It's generally said that running the engine for long periods not under load gives rise to the bores getting glazed. Now I'm quite happy to accept that as a matter of experience, but I am still a bit puzzled as to just why this happens. (I'm an academic metallurgist, so I worry about this kind of thing for a living...).

No load means somewhat lower gas pressures in the cylinders, but I can't see what difference that makes. I can't see that it would make any difference to the pressure the rings exert on the bores. The engine would run cooler for a while, but it shouldn't take that long, loaded or otherwise, to get up to the temperature set by the coolant thermostat; after which, no difference?

So I'm a bit stuck for an explanation. Any ideas?

Steve

 

[paul-essex]

I think hours is the only reason for glazing most engines in most uses in life run light loaded

 

[nigelmercier]

I thought the problem was that the bores became worn for a smaller distance than the maximum travel.

 

[oldharry]

Lots of odd ideas about bore glazing, but basically it is caused by running an engine cool, as at fast tickover allowing oil and combustion byproducts to condense on the cylinder wall forming a golden coloured hard lacquer glaze. This is super smooth and prevents proper lubrication of the rings so preventing them from sealing properly. Running an engine off load means the pistons, rings and bores come up to working temperature much more slowly so condensation occurs for much longer. Many diesels will never achieve full working temperature if left ticking over or just running an alternator as the cooling systems are designed to cool on load and can over cool the engine on light loads.

When the engine is built the bores are deliberately left rough with peaks and valleys in the bore walls. Running in consists of wearing'plateaus' on the ridges, allowing the valleys to retain oil to lubricate the rings and help create a seal. Glazing deposits fill in the valleys preventing oil being held in the grooves. Long term wear has the same result and reboring recuts the grooves. Modern engines often have the plateaus machined in to the bore, allowing the engine to develop full power from new, and resulting in longer ring life, as they have not had to cut the plateaus. Much more on the Cox engineering website.

 

[PaulRainbow]

Mr Cox explains it well :

https://coxengineering.sharepoint.com/Pages/Boreglazing.aspx

 

[Topcat47]

That's a pretty good explanation of the mechanism. Honing the bores can remove the glaze but of course the best answer is to run the engine flat out under load from time to time. This burns off the glaze but has to be done before it really builds up so periodic "Italian Tune Ups" are the best way to prevent it.

 

[sgr143]

I looked over Vyv Cox's website BTW : very informative; and the materials bit, which is my patch of knowledge, is sound (so many people get fatigue utterly wrong :Vyv doesn't). But it didn't quite answer my question, which is really about what the composition of the glaze is, and what reactions form it; and just why this happens more under lightly loaded running (though agreed that lower temperature is likely a large part of that).

I'll go and ask around: Must be someone who knows around my university. Or someone who knows who knows...

Steve

 

[Freebee]

I once worked with some aussi.s who claimed the way to cure bore glazing was to add vim to the intake of the running engine!!!@

 

[sgr143]

I once worked with some aussi.s who claimed the way to cure bore glazing was to add vim to the intake of the running engine!!!@

Yup, that might work. But considering the trouble normally taken to stop abrasives from getting into engines, possibly a cure that's worse than the problem. A bit like curing herpes by cutting... no, I'll stop there.

 

[RichardS]

I thought the problem was that the bores became worn for a smaller distance than the maximum travel.

That sounds a bit like an engine with rubber conrods. I've got a 4 cylinder engine in the garage with 2 conrods and 4 pistons which is a similar concept I suppose, as those 2 central pistons travel a much smaller distance than the maximum travel! :ambivalence:

Richard

 

[Colvic Watson]

I once worked with some aussi.s who claimed the way to cure bore glazing was to add vim to the intake of the running engine!!!@

More likely to just cut vertical grooves and exacerbate the problem.

 

[CAPTAIN FANTASTIC]

Lubrication is the key process that causes glazing. Low engines revs causes poor lubrication due to low oil pressure (depending on the engine design) which in turn wears down components. Glazing is mainly due to oil compounds being impregnated on the surface on the metal/bores. Older engines suffer significantly more from glazing mainly because the oil used was/is "old fashion" mineral oils, which is poor lubricant in comparison to "modern" synthetic oils of superior lubricating qualities and stable, able to lubricate at low rpm, and at high temperatures. My 45 years old diesel engine runs on fully synthetic oil to ensure efficient lubrication under wide temperature range and I am sure that the cylinders are likely to be glazed from work and various lubricants over the years.

 

[nigelmercier]

I once worked with some aussi.s who claimed the way to cure bore glazing was to add vim to the intake of the running engine!!!@

With model plane engines it was quite common to pour in metal polish.

 

[vyv_cox]

I tried to make the point on the website that low load running in itself does not cause glazing. Discussing this with Skipper Stu highlighted that industrial oil field engines run for months on tickover, waiting for their moment of glory when they are required to deliver full power. These engines do not suffer bore glazing.

I worked a great deal with reciprocating compressors, which regularly suffer from liquids in the bores when warm gas comes into contact with cool bores. I believe this is the mechanism of glazing in engines, and found several references that agree. The worst way to run an engine for battery charging is to start it up from cold, not in gear, so that combustion products have a long period when condensation on the bores is maximised.

Perhaps the main reason that glazing is less of a problem today than it was is a combination of better machining when the engine is built and reductions in fuel sulphur content. Combustion products are the weak organic acids such as sulphonic, carbonic, nitronic, etc. Elimination of the sulphonic is a big bonus.

 

[RichardS]

With model plane engines it was quite common to pour in metal polish.

I would have thought that would cause bore polishing .... which sounds like one has just replaced one problem (glazing) with another (polishing).

Richard

 

[rustybarge]

Cylinders are honed on the cylinder surface for one reason only: to retain oil to lubricate the piston rings. A slow idling engine produces a lot a soot because a diesel is a high compression heat engine that does not burn heavy diesel oil cleanly at low revs.

That soot and carbon fill up the tiny grooves in the honed surface leading to zero lubrication, ring wear and pressure blow by.

In the good old days you had to run in a new diesel at full load / Max continuous revs to bed in the bores.

 

[reeac]

Stand by for lots of protests re. using synthetic oil on an old engine! Actually, I was told by a former Mobil man that synthetic oils have intrinsically poorer lubricity than mineral oils so they have to add compounds to remedy this.

 

[vyv_cox]

Stand by for lots of protests re. using synthetic oil on an old engine! Actually, I was told by a former Mobil man that synthetic oils have intrinsically poorer lubricity than mineral oils so they have to add compounds to remedy this.

Not sure that is correct. I have a lot of experience of synthetic industrial lubricants as opposed to engine ones. These have no additives at all, just the straight product. We would use synthetics for problem machines where mineral ones had problems. Best one was a big compressor whose bearings ran very hot. Oil was changed and the temperature went down. When the machine was finally opened and checked it was found that the maker had misread radius for diameter and the clearance was half what it should be.

 

[RichardS]

Not sure that is correct. I have a lot of experience of synthetic industrial lubricants as opposed to engine ones. These have no additives at all, just the straight product. We would use synthetics for problem machines where mineral ones had problems. Best one was a big compressor whose bearings ran very hot. Oil was changed and the temperature went down. When the machine was finally opened and checked it was found that the maker had misread radius for diameter and the clearance was half what it should be.

Absolutely correct.

When I started using Mobil 1 synthetic in the mid-70s (there was no other!) the advice from Mobil was to run-in new engines with mineral oil until the first oil change as, in those days, manufacturing tolerances were not so good (no CNC milling) and a new engine run on synthetic would never run-in properly because of insufficent friction.

That advice alone convinced me to use sythetic oil from that day forward, even though I was paying the equivalent of £20 a litre in those days.

Richard

 

[Heckler]

Lubrication is the key process that causes glazing. Low engines revs causes poor lubrication due to low oil pressure (depending on the engine design) which in turn wears down components. Glazing is mainly due to oil compounds being impregnated on the surface on the metal/bores. Older engines suffer significantly more from glazing mainly because the oil used was/is "old fashion" mineral oils, which is poor lubricant in comparison to "modern" synthetic oils of superior lubricating qualities and stable, able to lubricate at low rpm, and at high temperatures. My 45 years old diesel engine runs on fully synthetic oil to ensure efficient lubrication under wide temperature range and I am sure that the cylinders are likely to be glazed from work and various lubricants over the years.

I think that your thoughts on low revs and low oil pressure cause glazing are wide of the mark.
Stu