Diesel Engine Design Life

Trundlebug said:
You appear to be missing one important point: the usage pattern and environment of marine engines.
One of the main reasons why engine manufacturers recommend an annual oil change is because over the winter the engine sits unused for up to 6 months in a damp, salty and cold environment.
As the oil gets contaminated during diesel engine use by acidic and abrasive particles, the additives break down as you correctly state. Engine manufacturers therefore recommend an annual oil change, which should be done (although I'm not sure they state this) at the end of the season so it sits with fresh oil over the winter and gets maximum protection. It isn't "backside covering" there is genuine reasoning behind it.

Other engine types don't have this usage pattern, they're used far more regularly so the number of usage hours between oil intervals becomes far more relevant and more important, hence the use of semi and fully synthetic oils.

Switching to semi or fully synthetic oil in a marine engine doesn't give you any benefits in this respect (extended service intervals): they recommend annual oil changes exactly because of the above reason, so you don't get contaminated oil sitting next to the bearing surfaces for several months in the cold and damp.
You simply don't get any benefit from using oil that can (in other applications) last for longer (i.e. more running hours) between changes - running hours aren't the determining factor in a marine engine.
Click to expand...

But if any of that were the case why does the manufacturer of the petrol or diesel engine for your car/motorcycle (which might even be the same manufacturer as your boat engine) also stipulate annual oil changes even for very low usage engines?

Of course, it might be that car and motorcycle engine manufacturers are covering their backsides / optimising their revenue stream (delete as applicable depending how cynical you are!) whilst marine engine manufacturers are totally above that sort of shenanigans. You can decide for yourself whether that sounds likely! :rolleyes:

Richard
 
Some users seem to think synthetic lubricant is close to holy water in its properties. It isn't. Synthetic lubricant in motor oils is just a highly refined form of the same old hydrocarbons that all the others use. Because it is synthesised from gas its molecular length is far more regulated than distilled oils, which may have molecular lengths anything between 8 and 30 carbon atoms. In high temperature engines run for extended periods use of conventional oils causes deposition of gums and sludge, which is much reduced in synthetics.

A test of the life of oils at high temperature is the Rotating Bomb Oxidation Test (RBOT) in which a round flask containing oil is rotated while a temperature of about 120 degrees (from memory) is maintained. Once the oil fails it gels up and becomes almost solid. Distilled oils have an RBOT of 100 - 200 or so hours, whereas I recall some synthetics whose RBOT was 5000 hours and still going. There is clearly little need for this in cold-running yacht auxiliaries. At low running temperatures their TBN will deteriorate just as quickly as that of distilled oils, as the acids produced during combustion are not reacted by additives designed to work at temperatures considerably higher.

An additional property of the even size of the molecules in synthetics is that they flow better, so that oil pressure is maintained in bearings with very close clearances for considerably longer. When colleagues were heavily involved in specifying lubricants for Ferrari F1 cars I was amazed at the exceedingly small bearing clearances they were running. This is also valuable in cam and tappet lubrication, where lift rates are enormous in F1 and other high performance cars. It also contributes to improved fuel consumption in high performance cars.

All this is a million miles away from yacht engines.
 
vyv_cox said:
Some users seem to think synthetic lubricant is close to holy water in its properties. It isn't. Synthetic lubricant in motor oils is just a highly refined form of the same old hydrocarbons that all the others use. ......
Click to expand...
That's not true at all, with the possible exception of mickey mouse brands which are desparate to slap a synthetic label on the cheapest possible product.
http://www.mne.psu.edu/chang/me462/syn_vs_min.pdf
 
lw395 said:
There is one good reason to change your oil at least annually.
If you ever want to sell your boat, a lot of people are going to ask about such things.
If you narrow the choice of buyers to those that share your exact opinions about extended oil changes, it may be on your hands for a long time.
Click to expand...

That's a good point about the perception of others.

I'll be honest and say that I've only ever had one boat (well, one boat with an engine or two) so if and when it comes to selling it I will have to cross that bridge.

However, I have a lot of experience with car and bikes and have sold many which have no service history apart from the details which write in the service booklet. When I advertise the car or bike I always include a full description of how the engine might well have gone up 3 - 5 years with no oil change and/or changes might have been done at 20k miles or whatever. I also include photos of the engine the last time I worked on it and offer to put the car up on ramps or whip off the cam cover .... basically whatever the buyer wants to see, he gets to see.

Typically I would say my cars sell well above book value and usually 50% - 100% above ostensibly similar vehicles being advertised at the same time. Of course, they are usually bought by like-minded home-mechanic enthusiasts so I'll have to hope that there are some similarly-minded boat enthusiasts around if the time ever comes. :)

Richard
 
lw395 said:
That's not true at all, with the possible exception of mickey mouse brands which are desparate to slap a synthetic label on the cheapest possible product.
http://www.mne.psu.edu/chang/me462/syn_vs_min.pdf
Click to expand...

It is totally true if you only include the polyalphaolefins paragraph, which is the one applicable to motor oils. The paper discusses a huge range of synthetic lubricants other than motor oils. Shell's synthetic motor oils are made in the PAO plant at Stanlow. Well, they were, don't know since they sold it but I assume there is little change.
 
Tranona said:
Doesn't ruin engines, just not necessary for the type of engines we use in small yachts.
Click to expand...

Iunderstood that modern synthetic oils can cause bore polishing in engines not specifically designed for it. My md 2020 is based on a perkins block. This is old technology but very reliable.
I also understood that once ised for a while engine oil developes an acid. One should not leave the oil standing in the engine because this acid is detrimental. I change my engine oil at the end of every season.along with other things like filters. I see no point in taking a chance on a boat. Breaking down at sea is not like conking out on the M25 ( although less expensive)
 
vyv_cox said:
Some users seem to think synthetic lubricant is close to holy water in its properties.

A test of the life of oils at high temperature is the Rotating Bomb Oxidation Test (RBOT) in which a round flask containing oil is rotated while a temperature of about 120 degrees (from memory) is maintained. Once the oil fails it gels up and becomes almost solid. Distilled oils have an RBOT of 100 - 200 or so hours, whereas I recall some synthetics whose RBOT was 5000 hours and still going.

An additional property of the even size of the molecules in synthetics is that they flow better, so that oil pressure is maintained in bearings with very close clearances for considerably longer. When colleagues were heavily involved in specifying lubricants for Ferrari F1 cars I was amazed at the exceedingly small bearing clearances they were running. This is also valuable in cam and tappet lubrication, where lift rates are enormous in F1 and other high performance cars. It also contributes to improved fuel consumption in high performance cars.
Click to expand...

In my limited experience all that is true and it's sufficient to convince me that what is best for my V8 which legally cannot exceed 2.5k revs in top gear is also great for a marine engine which also rarely exceeds 2.5k revs. I doubt whether the difference in engine running temperatures of a V8 at virtual idle or an in-line 3 at close to max revs are much different.

However, as in all things, you pays your money and you takes you choice. :)

Richard
 
vyv_cox said:
It is totally true if you only include the polyalphaolefins paragraph, which is the one applicable to motor oils. The paper discusses a huge range of synthetic lubricants other than motor oils. Shell's synthetic motor oils are made in the PAO plant at Stanlow. Well, they were, don't know since they sold it but I assume there is little change.
Click to expand...

A lot of people don't rate Shell oil for this reason.
The better brands use a lot of esters AFAIK.
 
Good way of answering this is look around on youtube at all the ancient diesel engines running. They seem to last forever with proper servicing. I don't think they ever die, they just mature.
 
As to engine life. I had a chat with the Perkins distributor about this and was told it is common for commercial boats to go to 40,000 hours with one rebuild. This is for the 6ltr 6 cylinder Perkins. More life is expected if not heavily loaded than if heavily loaded. Sort of obvious I suppose. High reving types may well be worse than this.

I also know commercial vehicle diesels very often exceed 1,000,000 miles with a rebuild, so maybe 25,000 hours.
 
lw395 said:
I was always told..... all sorts of things that are half true in certain cases.
Click to expand...

+1

A graph of engine load against carbon build-up would start with low load / high carbon build up at the origin and climb to high load / low carbon build up at high load.

A similar graph of engine load against metal-to-metal wear would start with low load / low metal wear and go in the opposite direction.

A third graph would show fuel consumption against load and so on.

From the graphs there will be a "sweet spot" which might be the revs that a fixed-load generator would be set to run at. Diffferent applications will have a different "sweet spot".

As you say, there's usually an element of truth in the "old sayings" but it's never the whole story.

Richard
 
Daydream believer said:
That is odd because I was always told diesel engines performed better if under load rather than running light
Click to expand...

There is a need for more clarity between terms such as "Running Light" and "idling". In the case of idling, there is insufficient oil pressure and poor oil distribution and therefore higher rate of wear, also the engine does not reach the optimum temperature resulting in additional wear. "Running Light" is good, any engine under load will wear out faster.
 
CAPTAIN FANTASTIC said:
There is a need for more clarity between terms such as "Running Light" and "idling". In the case of idling, there is insufficient oil pressure and poor oil distribution and therefore higher rate of wear, also the engine does not reach the optimum temperature resulting in additional wear. "Running Light" is good, any engine under load will wear out faster.
Click to expand...

All of the last few posts might be true in some circumstances but not others depending upon the design parameters of the engine. It's really a "how long is a piece of string" type of argument. For example, one of my engines is designed to develop full oil pressure at idle. When the engine is revved a bypass valve diverts the "excess" pressure so the gauge reads like it's fixed whether the engine is doing 750 rpm or 5000 rpm. Bearing wise, it could probably run forever at idle .... it would probably die from coking-up first. :(

Richard
 
RichardS said:
+1

A graph of engine load against carbon build-up would start with low load / high carbon build up at the origin and climb to high load / low carbon build up at high load.

A similar graph of engine load against metal-to-metal wear would start with low load / low metal wear and go in the opposite direction.

A third graph would show fuel consumption against load and so on.

From the graphs there will be a "sweet spot" which might be the revs that a fixed-load generator would be set to run at. Diffferent applications will have a different "sweet spot".

As you say, there's usually an element of truth in the "old sayings" but it's never the whole story.

Richard
Click to expand...

Hi Richard, this quote is more useful than many others so far.Explains a lot, guess we should get good at posting graphics?
 
RichardS said:
When the engine is revved a bypass valve diverts the "excess" pressure so the gauge reads like it's fixed whether the engine is doing 750 rpm or 5000 rpm.

Richard
Click to expand...

As in a Rootes 1750cc petrol engine
Damned valve was a short length of capped tube with a spring for tension. A bit of dirt would get in it & it would stick open & the engine would loose oil pressure
I do not suppose many can remember the Humber Sceptres & Hunters etc
 
vyv_cox said:
Some years ago a TV motoring show, possibly Top Gear in a previous version, tried to find the Mercedes car with the highest mileage. A taxi in Egypt was found with more than 1,000,000 miles on the clock, equivalent to 25,000 hours at 40 mph. Several others were found with mileages approaching that, mostly on the original engine.
Click to expand...

 
Last edited:
Daydream believer said:
As in a Rootes 1750cc petrol engine
Damned valve was a short length of capped tube with a spring for tension. A bit of dirt would get in it & it would stick open & the engine would loose oil pressure
I do not suppose many can remember the Humber Sceptres & Hunters etc
Click to expand...

I think you mean 1725cc?, yes remember them well!
After I re-engined ny first car, with a 1725 from a scrappy, there was no oil pressure because the pressure reg valve stuck open.
AFAIK, every engine with an oil pump has a presuure regulator valve but this is the only one I've had stick completely open.
To be fair, the engine had stood around a while and went on for years afterwards.
 
You must log in or register to reply here.

Share this page

Top