Glazed Bores

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Andrew_Fanner

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Don't understand this myself but apparently old big diesels working lazily on the river can suffer. I am told that there are oil additives that can help.

1. What are they/who makes them/what cost
2. Are they in fact just snake oil

Thanks for any advice.
 
Answers: 1. None that I know of. 2. Probably.

Read latest YM for what I hope is a clarification, and as near as I can manage to understand the true situation.

Probably the worst advice I have ever read on a cure for bore glazing is to tip an abrasive, Vim has been mentioned but I have seen others, into the air cleaner. If ever there was a better short term 'cure' that caused massive long term damage, this is it.
 
As you suggested, it's a problem seen in under loaded diesels which can in extreme cases cause other problems. It is however not really as prevalent as most would make out. Unless you have other symptoms like oil burning or overheating the best advice is "if it ain't broke don't mend it" Additives are not really a cure anyway.
 
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Oh I thought were talking about the residents of The Lounge

[/ QUOTE ] You should know. Of your 50 most recent posts there are more in the Lounge than on any other forum.

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In order for the piston rings in an internal combustion engine to seal effectively against the cylider liner and thus give good compression a film of luboil is needed between the liner and ring. An engine in good condition will have crosswise 'hatching' abraded into its surface when it is honed and itis these scartches that form the oil bearing surface.
Glazing is the deposit of varnish in these scratches by burning the lub oil at low temperature when running at low loads ,as well as polishing of the surface. This varnish fills in the honing scratches until there is a smooth surface which will no longer support an oil film and therefore there is a loss of compression and in many cases carry over of luboil into the combustion chamber causing smoking and difficult starting. The only effective cure is to rehone the cylinders. I know of no additive (which is not a contituent of good quality oils) which will make any difference. There is an argument for using lesser quality oils and changing often, as some of the components which make up the varnish come form the oil additives, but I have always subscribed to the idea that you should follow the manufacturer's recommendations for oil specs.
 
Miller Oils do a "Glaze Buster Oil" which I confess to having used some years ago on a Thornycroft 90(BMC 1500 Diesel). But I don't think it made any difference - the compression was low on two cylinders and I eventually replaced the engine
 
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Glazing is the deposit of varnish in these scratches by burning the lub oil at low temperature when running at low loads ,as well as polishing of the surface.

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I was told that glazing and polishing are entirely different problems with very different causes.

As I understand it glazing is caused by excessively HIGH (not low) temperatures. The excessive temperature is the result of an excessively lean mixture at low power outputs, caused by the way that most of the engines we use achieve low power outputs by limiting the fuel injected but without proportionately reducing (throttling) the air intaken.

Why they don't have a more sophisticated power output control system is a mystery to me. The other mystery is the way that many craft (e.g work and harbour-masters' launches, water taxis, etc.) have their engines running at tickover, or low output, most of the time without apparent ill-effect.
 
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Glazing is the deposit of varnish in these scratches by burning the lub oil at low temperature when running at low loads ,as well as polishing of the surface.

I was told that glazing and polishing are entirely different problems with very different causes.

As I understand it glazing is caused by excessively HIGH (not low) temperatures. The excessive temperature is the result of an excessively lean mixture at low power outputs, caused by the way that most of the engines we use achieve low power outputs by limiting the fuel injected but without proportionately reducing (throttling) the air intaken.

Why they don't have a more sophisticated power output control system is a mystery to me. The other mystery is the way that many craft (e.g work and harbour-masters' launches, water taxis, etc.) have their engines running at tickover, or low output, most of the time without apparent ill-effect.

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You were told correctly - glazing is the deposit of varnish on the surface of the liner, whereas polishing is a result of the wear between the piston ring and the liner surface. However, they tend to work in tandem- the more the liner glazes, the worse the lub oil retention on its surface and therefore the worse the lubrication between the liner and piston ring leading to polishing of the surfaces. As to glazing being caused by high temperatures at low power this is not correct. Diesel engine exhaust temperatures increase with increasing power output. On some large 2 stroke medium speed engines, exhaust temperatures at low power are barely sufficient to burn the lub oil at all. Even on four strokes tickover and low power exhaust temperatures will typically not exceed 150 - 200 deg C. Think you are confusing this with a spark ignition petrol engine.
 
my recollection of a mag article some years ago referred to the 'cup of coffee' that destroyed your engine, and referred to the practise of letting the engine warm up in neutral, prior to departure. The article stated that marine diesel engines had a constant air aperture, and that engine speed/power was controlled by the injection of diesel. The article then compared the marine diesel engine to a bunsen burner which of course burns hottest when there is more air allowed into the gas mix, ie weak fuel/strong air mix ( same as diesel engine tickover) when the air is restricted the burner is cooler /forums/images/graemlins/cool.gif /forums/images/graemlins/grin.gif same situation as when you open the diesel engine throttle, mix becomes diesel strong therfore flame is cooler. /forums/images/graemlins/cool.gif
was the article wrong - or did I misunderstand /forums/images/graemlins/tongue.gif
 
I'm looking at the readouts from our two main engines as I type. Engines are both on low power as we are standing by the rig at the moment. Exhaust temperature average is about 300 deg C within a 25 degree range on both engines. Full load exhaust temperatures are normally around 400 - 430 degrees C. At MCR exhaust temps can reach just under 500 deg C. Exhaust temperature is related to how much fuel you are burning. Take the analogy of a blow torch - you adjust the heat by opening or closing the valve and controlling the amount of fuel. The air aperture remains the same size - just as in a diesel. To adjust the heat of a bunsen burner you adjust the size of the orifice thus admitting more air and therefore more EFFICIENT combustion which is why the falme changes from yellow to blue.
 
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