Diesel Engine Design Life

[vyv_cox]

I have some doubts about the above. For simplicity let's confine ourselves to crankshaft bearings as jdc has done. The analysis ignores the concept of a threshold engine speed below which there may be negligible wear ..analogous to the situation with fatigue damage. Such a threshold might be 1000-1500 rpm in a typical small marine diesel. I agree that higher rpm risks increased wear rate but my guess is that that increase is pretty small up to 2500 rpm and only becomes serious near the maximum design rpm. Anyway let's hear from the "Italian Tune Up" advocates.
My own VP 2002 dates back to 1987 and I have a Kubota tractor of 1983 vintage ..don't know their engine hours.

I agree that the advice is contrary to my understanding. Having been involved in engines, and particularly bearing life, for most of my working career, I have never heard that bearings have a life measured in revolutions. By far the biggest factor in the life of every type of engine, from small petrol to large gas turbine, is the number of starts they have undergone. Wear and stress levels are greatly increased during the start process. For industrial gas turbines charts are published as a guide to lifetime according to starts, but this is very probably not done for small diesels, although it is for big ones.

 

[vyv_cox]

>Don't worry, that assertion by KE has been repeated many times and rejected by the majority of forumites on every occasion.

I see you still haven't called Volvo to check what I said, nor has anyone else who has posted here, probably because you will all be proved wrong and have to admit it. I don't doubt that well looked after engines can last longer if of well looked after but if Volvo say the design life of modern engines is 8,000 hours I believe them, why should they lie.

Design life is just a number, not necessarily based on anything. Most refinery and offshore equipment in my experience has a design life of 25 years. I know many machines that have doubled this and I know one steam pump in a USA refinery that has been running reliably for over 100 years. All of this equipment runs constantly, doing 8000 hours per year.

 

[reeac]

I agree that the advice is contrary to my understanding. Having been involved in engines, and particularly bearing life, for most of my working career, I have never heard that bearings have a life measured in revolutions. By far the biggest factor in the life of every type of engine, from small petrol to large gas turbine, is the number of starts they have undergone. Wear and stress levels are greatly increased during the start process. For industrial gas turbines charts are published as a guide to lifetime according to starts, but this is very probably not done for small diesels, although it is for big ones.

Regarding number of starts, I read somewhere that car engine designers had to modify bearing materials to counter premature wear caused by the stop/start innovation.

 

[vyv_cox]

Regarding number of starts, I read somewhere that car engine designers had to modify bearing materials to counter premature wear caused by the stop/start innovation.

Could well be the case. Diesel engines have almost universally used copper-lead bearings with a couple of different overlays. I guess that lead-indium overlays would give slightly more resistance to dry starting than lead-tin, although I am not sure of that. Current petrol engines all use some variation of aluminium alloys, mostly with tin but others are known. Hardened shafts are needed but this is mostly to cope with very low viscosity lubricants.

 

[fisherman]

One point: I believe my hour meter runs as a clock regardless of engine speed. Many of my hours will be at low revs while hauling gear.

 

[KINGFISHER 9]

Regarding number of starts, I read somewhere that car engine designers had to modify bearing materials to counter premature wear caused by the stop/start innovation.

One of my cars is a 3 litre diesel Jaguar ... luckily it has a switch for the rather annoying stop-start gizmo, and the engine will also stay running when you stop with the aircon on.

 

[blxm]

Just had a spare few mins and read through all these posts. All interesting stuff. As a non expert I am taking away......
Design life of an engine is an approximation of a likely usable life given an undefined expected usage schedule. No real use to me in the real world

Actual life of an engine. Massively dependant on usage. Average small yacht engines have a very different usage pattern from commercial engines and so comparison is almost irellevent.

Quality of Oil in my 2008 yanmar is irrelevant as long as it exceeds the minimum, would be difficult with oils readily available. given the tiny amount of oil used may as well change each season reguardless of the fact that it may not actually need it.

 

[MM5AHO]

We have a forklift at work with >30,000 hours - original engine. Naturally aspirated diesel with hydraulic drive. It has had a new head (cracked old one at about 19000 hrs), but all else original. Its still going. Look after a diesel and it will last a long time.
I have a 1965 Lister generator 19kVA, still going well, not burning oil, unknown hours.

Update: 35000 and still going.

 

[Jegs]

I believe that Cummins "life" some engines for a certain amount of fuel consumed, might be more accurate than hours?

ATB,

John G

ps Where's LateStarter when we need him?

 

[DownWest]

Might this also have something to do with piston speeds? We bought a house in '79 which shared a Lister Start-O-Matic with the neighbour. The set was already near 30yrs old and I gave it new rings and a valve grind. Worked fine until the Grid arrived. 1500rpm for around 6hrs a day. Bit more on wash days ) Very heavy low stressed engine.

 

[jdc]

I agree that the advice is contrary to my understanding. Having been involved in engines, and particularly bearing life, for most of my working career, I have never heard that bearings have a life measured in revolutions...

Well, the point of my trivial and much over symplified analysis was to point out ony one (of many) mechanisms whereby 'life' as measured in hours alone is potentially misleading. But nonetheless, despite my bowing to Vyv's superior practical experience, bearing life is specified in revolutions.

SKF say for instance

Bearing life and load ratings
The individual life of a rolling bearing is expressed as the number of revolutions or the number of operating hours at a given speed that the bearing is capable of enduring before the first sign of metal fatigue (spalling) occurs on a raceway of the inner or outer ring or a rolling element.

And the lifetime formula within ISO 281 gives the life in millions of revolutions.

 

[rotrax]

Harley Davidson made the Servi-Car-a three wheel trike made from the front end of one of their side valve '45's, subsequently the later Shovel Head model.

The San Francisco Police Dept were having serious problems with low milage severe cylinder wear and outrageous oil consumption.

After a couple of years the factory sent a man to take a look.

He followed a Servi-Car rider around the streets of SF while the parking meters were being emptied of coins. Thats what they were used for-they had a secure compartment for placing the bags of coins in, which were also very heavy.

The rider left the machine idleing at full retard-manual control on the '45's-and went down the line of meters taking the coins out and bagging them.

The machines were low milage, but high hours and at full retard were getting very hot.

When the Police Dept were instructed to stop the practice of leaving the Servi-Cars idleing the problem went away.

jdc's information re bearings is new to me also, introduced no doubt as part of the ISO quality standard.

In 1969 I dropped an exhaust valve into the combustion chamber of my pre war 350cc Triumph single cylinder race bike. During the rebuild I honed out the hardened steel con rod liner for the big end, hand stoned the raised bit in the middle of the crankpin using a lathe, flat stone and parrafin and rebuilt the crowded roller big end with 1 thou O/S 1/4x1/4 rollers.

That big end is still in use today and has been raced successfully every season since 1969.

I put the extended life down to two things-using the correct gear ratio to avoid over reving and high quality pre war parts.

The crankpin and conrod liner were made by R&M from top quality steel and were hardened correctly.

I always used a good quality 20/50 oil-what I could pinch from work-and I had modified the oil pump by fitting a Triumph Twin one and put in a stronger relief valve spring.

Very empirical I know, but when I last stripped the engine it had about 2 thou play in the rod/crankpin area measured dry.

Squirting a bit of oil down the hollow timing side mainshaft made that clearance dissapear instantly.

As a representative motorcycle of its era I never used a rev counter or measured the hours, but I can say without fear of contradiction it was the best big end I ever used!

 

[vyv_cox]

Well, the point of my trivial and much over symplified analysis was to point out ony one (of many) mechanisms whereby 'life' as measured in hours alone is potentially misleading. But nonetheless, despite my bowing to Vyv's superior practical experience, bearing life is specified in revolutions.

SKF say for instance

Bearing life and load ratings
The individual life of a rolling bearing is expressed as the number of revolutions or the number of operating hours at a given speed that the bearing is capable of enduring before the first sign of metal fatigue (spalling) occurs on a raceway of the inner or outer ring or a rolling element.

And the lifetime formula within ISO 281 gives the life in millions of revolutions.

That statement refers to rolling element bearings, which are a totally different case from the plain bearings used in diesel engines.

 

[Chris_Robb]

Interesting discussion, but life span on a yacht engine is largely down to how you use it, how you maintain it, and if you leave it for months with saltwater in the heat exchange side.

Having followed a number of threads on different aspects of maintenance, I decided this year to remove the salt water side - heat exchanger, Oil cooler (turbo engine) and exhaust bend, following pictures of destroyed engine following an oil cooler failure. The heat exchanger had been off in 2011.

Just as well - the engineer in Turkey sends pictures at every stage of dismantling using Whatsapp, generally with a thumbs up sign of going OK. With the Oil cooler - all looked fine even when the tube stack was stripped from the ali housing. They he cleaned it - covered in oil - water is in the tubes - and it came up the most beautiful pink. The next photo after that was the stack housing - crumbling to pieces - and a thumbs down. Replacement sourced from his stock of broken engines - €400 instead of £1500 - all stripped and quality assured.

Next with the thumbs down was the exhaust bend - cracked and caked and ready to fail - a new Inox one made locally for half the cost of the volvo job

I dont think that would have survived another season and if it had failed that would have been another stat - 3,000 to 4,000 hours (or 26 years) for a Volvo 2003T.

As it is, I would imagine with much of the rest of the ancillaries already replaced, I hope that it will have along and happy life.

I am putting in a change over switch this year to take fresh water from the tank so when I leave the boat for a month - I will wash through. (ANY reasons not to????)

Using Whats App was a wonderful way to keep on top of a job in Marmaris Turkey - we ended up replacing engine mounts (very bad) and exhaust rubber pipe in a very bad state (spare carried) and all the rubber diesel hoses on the engine. I was able to answer the engineer back in minutes to authorise and discuss - by using whatsapp to phone - and to feel sure that the job was being done well.

 

[jdc]

That statement refers to rolling element bearings, which are a totally different case from the plain bearings used in diesel engines.

Very good point, I was indeed using info on roller bearings. But are you sure that the wear of plain bearings is NOT a funtion of number of rotations? Reductio ad absurdum, are you saying that 1 hour at 3,000 rpm at maximum load produces tye same wear as 1 hour at 1000 rpm with no load? Surely their wear must be somewhat higher at high engine speed and associated power than it is at low speed?

 

[NormanB]

I wonder how many small boat diesel engines have such a long life that they die of old age and wear out? ( The engine 'life' question).
In the small boat category I suspect most suffer catastrophic failure related to salt water getting into the engine - at a time when they are merely 'middle age'.

 

[pandroid]

The handbook that came with my HR says: "pleasure craft diesels do not wear out, they just rust away"

 

[Daydream believer]

Very good point, I was indeed using info on roller bearings. But are you sure that the wear of plain bearings is NOT a funtion of number of rotations? Reductio ad absurdum, are you saying that 1 hour at 3,000 rpm at maximum load produces tye same wear as 1 hour at 1000 rpm with no load? Surely their wear must be somewhat higher at high engine speed and associated power than it is at low speed?

Just following this thread & wondered if there are 2 different things here
Roller bearings ( quote above)- as in bearings with rollers instead of balls
Rolling element ( V Cox)- as in wheels where a side load always applied as opposed to say a shaft where the load is more central
Is there a difference or is " rolling element" the same as " roller bearing"
And does not the 2 different uses ( centrifugal alignment & wheel load) make a difference to the life of the bearing?
Or shall I just keep quiet & carry on reading ??????

 

[NormanB]

Rolling element is a generic term that applies to both rollers and balls. It is not a design specific term, like location, thrust et al.

 

[Heckler]

Just following this thread & wondered if there are 2 different things here
Roller bearings ( quote above)- as in bearings with rollers instead of balls
Rolling element ( V Cox)- as in wheels where a side load always applied as opposed to say a shaft where the load is more central
Is there a difference or is " rolling element" the same as " roller bearing"
And does not the 2 different uses ( centrifugal alignment & wheel load) make a difference to the life of the bearing?
Or shall I just keep quiet & carry on reading ??????

Diesel and petrol engines use plain shell bearings, not ball or rollers except in some very old ones such as the Saab discussed here. A diesel egine is inherently long lasting because it is built more robustly and also because it delivers its power lower diwn the rev range. Roughly speaking it delivers its power at half the revs of a petrol engine and so in a given time frame it only does half the "distance" When I overhauled serious long hour/ mileage diesels I would see wear on the side of the big end shells which correlated with the firing pressure side. Usually the tin/lead flashing would have disappeared, almost as if a high spot had worn off. The worn bearings would look worn but on measuring them, the flashing wear would hardly register. Changing the oil and filters regularly extends the life. Spalling etc which happens to ball and rollers doesnt happen to plain shell bearings. Also dont forget we have moved in to shafts running directly on high engineered aluminium alloys, again they last fir ever as long as the oil etc is changed. The overhead cam in my MD22 is in perfect condition after 18 years and many thousands of hours.
Stu