Do high revs shorten engine life?

[silver-fox]

My question to the engineers out there is....

"If I habitually run at full throttle will I shorten my engine life?

Here is some background information.

My 55hp Thornycroft engine is a marinised 4 cylinder Ford diesel engine. The manual says it can be used at a constant max of 3000rpm going up to 4000rpm for 1 hour in 10.

As a cautious owner, who also wants to conserve fuel, I cruise at between 1800-2000 revs giving a speed of around 6 knots. With the throttle fully open (and a clean hulll and prop) the engine will reach 2700 revs and 8 knots. Needless to say, the engine is significantly noisier once it gets above 2200 revs.

I remember reading somewhere that most engine wear takes place in the first minute or so of use as the engine gets up to operating temperature, if that is true then subsequent operating revs within manufacturers guideline would seem to have little effect on life.

 

[Tranona]

Most marine engines die of neglect and misuse rather than use. Highly unlikely that you will wear out the mechanical through use. What they don't like is the common yachty use of start, run for 10 minutes, not reaching operating temperature, shut down then same at the end of the passage. Continuous running within operating range is fine and over 10000 hours is not unusual. If you can only get 2700 at WOT suggests your prop is a bit too "big". You should be able to reach maximum continuous rated revs. As you can see from the other engine thread, running continuously at low revs is not best - you need to run it at higher revs from time to time - so your "cautious" approach is not necessarily advantageous for engine life.

 

[macd]

You're confusing two different forms of wear: one due to excessive friction (notably, as you mention, on start-up when cold) and one due to loadings. High revs in a warm engine fall into the latter category: loadings on big and small ends and main bearings, in particular, which rise as the square of engine speed. That said, most diesels are happy to be run pretty close to flat-out for hours on end. And, as Tranona wrote, most marine donks rot to death.

 

[Fire99]

I'll echo the Macd and Tranona. Chances of engine failure due to running 'high' revs = Very Slim. Chances of engine failure due to spending more hours than not sitting around doing nothing or very short runs where engine never gets up to full operating temperature = Significantly higher.

 

[pvb]

I always run my engine (Volvo 2003T) for at least 20-30 minutes so that it gets up to temperature, and I always make sure I have a regular blast at full throttle. It does no harm at all, but is I believe beneficial. At over 20 years old, the turbo and exhaust elbow are still in fine condition.

 

[ianj99]

I remember reading somewhere that most engine wear takes place in the first minute or so of use as the engine gets up to operating temperature, if that is true then subsequent operating revs within manufacturers guideline would seem to have little effect on life.

This is quite true.
Poor maintenance is also a big factor in engine longevity.
Overloading any engine will increase wear so provided you back off the throttle until the max rpm drops slightly, you won't be doing this. Its those who slam the throttle against its stop with black smoke pouring from the exhaust who suffer the consequences.

My 33yr old 4108 is operated in a similar manner to your Ford and runs as smooth and quietly as a 4108 can, has 60psi oil pressure, good starting etc. It will only do 2400rpm flat out at the hull speed of about 7kts but cruises economically at 2000rpm and 6kts. (thanks to extra sound proofing, Supermount engine mounts and a Centa Flex M coupling its pleasingly quiet at all speeds including at max rpm (in gear)

I recommend the use of ZX1 oil additive / friction reducer. This is claimed to provide big benefits during those first few minutes of running of a cold engine and there is no mistaking the friction reducing effect. MY old 4108 cranks over so fast you'd think it had no compression!
http://team-zx1.com

Ian
PS lets not have a debate on additives, its personal choice and ZX1 has worked in all the vehicles I've used it in, does not contain ptfe, graphite or molyslip. Watch their Youtube videos.

 

[lw395]

One thing to note is that some marine engines don't seem to have much reserve in the cooling department, so running them continuously hard can be bad.
There is no airflow over the sump to cool the oil unlike a taxi or lorry.
If the cooling system is designed around not thrashing it much, best not to thrash it much...
Cars and stuff other than generators are not intended for developing full power for hours on end.

I'm in a minority of one on here in thinking synthetic oils can be good, because their residual film gives much better lubrication at start up.
There are two sides to the debate, same as with additives.

 

[pvb]

I'm in a minority of one on here in thinking synthetic oils can be good, because their residual film gives much better lubrication at start up.
There are two sides to the debate, same as with additives.

You're in a minority of two - I've always used synthetic oil in my Volvo 2003T.

 

[ianj99]

You're in a minority of two - I've always used synthetic oil in my Volvo 2003T.

I used to use synthetic in everything but now use Millers semisynthetic and ZX1. You get the cold start protection & friction reduction of the ZX1 and the cost effectiveness of the cheaper oil which I change often. The overall cost is probably about the same so its personal choice.

The soot contamination does nothing for any oil's lubricating quality and I am totally opposed to the use of so called long life oils and extended service intervals now common place on road vehicles. Its far better to use a cheaper oil and change it twice as often.

Ian

 

[Lakesailor]

Every design of engine, and indeed every single engine, is slightly different. If you have a mechanical ear you can detect the "sweet spot" where the engine will run with least vibration. Some motors run smoothly near max revs others at a lower speed. It's mainly to do with harmonics and out-of-balance vibration. Some motors have balance shafts, some use massive flywheels and crank pulley dampers to smooth them out. But an engine will have a rev range at which it's smoothest. Find that speed and it will run for ever.

 

[GrahamM376]

Cars and stuff other than generators are not intended for developing full power for hours on end.

I'm in a minority of one on here in thinking synthetic oils can be good, because their residual film gives much better lubrication at start up.

Many modern "marine" engines are marinised plant engines, designed to run at high constant revs all day.

Use of synthetic oils is bad for many engines, always use the spec given by the engine manufacturer.

 

[Marsupial]

One thing to note is that some marine engines don't seem to have much reserve in the cooling department, so running them continuously hard can be bad.
There is no airflow over the sump to cool the oil unlike a taxi or lorry.
If the cooling system is designed around not thrashing it much, best not to thrash it much...
Cars and stuff other than generators are not intended for developing full power for hours on end.

I'm in a minority of one on here in thinking synthetic oils can be good, because their residual film gives much better lubrication at start up.
There are two sides to the debate, same as with additives.

Properly installed with bilge blowers, injected exhausts and heat exchangers of the correct size marine engines engines should run at their continuous rating without overheating. All the car installations I was involved in required the engine to be capable of running flat out for a tank full of fuel (300 miles ish) in 40 deg C ambient.

The oil temperature is an issue with marinised engines and "modern" high speed varieties should have a larger sump capacity (than the road version) and an oil cooler to cope. Sea water cooled versions often lack the oil cooler but do have a large sump capacity. But this means it takes longer for the engine to reach operating temp so we are back to low temperatures and premature wear.

Use the correct oil for the engine

 

[Norman_E]

My thoughts: Marine diesels rarely seem to fail though main and big end bearing wear, but many suffer from glazed bores. Most diesel engines are very strongly engineered, and despite the higher compression pressures than petrol engines they are relatively unstressed because maximum revs are low, typically below 4,000. Stresses due to high speed appear to increase as the square of the speed, but running a diesel engine close to its maximum continuous rev limit does no harm provided it has been allowed to warm up slowly, whereas running it for hours on end at half revs can lead to glazed bores. On my somewhat over engined boat the best cruising speed is achieved at about 2,500 revs whereas maximum continuous rating is 3,400. My solution is to five it a 5 or ten minute burst of full revs every so often and watch the boat try to climb up its bow wave. At 5,500 hours I reckon a Yanmar 4JH series is about run in.

EDIT: For start up lubrication Gardner had the best solution. They fitted a hand pump to circulate the oil and raise the oil pressure before operating the starter!

 

[Keith-i]

All this cold start business and not letting the engine warm up always seems spurious to me. If you do 50 cold starts a year, whether you let the engine reach operating temp or not is irrelevant. It is still 50 cold starts. The difference of course is whether you try and get it to warm up quickly, or leave it idling for extended periods. Gunning it straight after start-up clearly isn't wise, but I think leaving it on tick over with no load is just as bad. Start up and drive off gently.

 

[reeac]

Except that if the engine gets up to full temp. then you'll get rid of any condensed combustion products that have got entrained in the oil. Generally, it seems to me, boats don't need re-engining for 25-30 years or so so the treatment that they receive can't be too bad for them.

 

[srah1953]

There seems wide consensus that a well treated marine diesel will, essentially, last for ever. One poster said that he/she always runs the engine for 20-30 mins, but the problem for me, and I suggest a lot of other people, is that for most sailing which will constitute day sails, except for the annual cruise, you only need to run the engine long enough to get out of the harbour and up with the sails. It doesn't seem logical (and psychologically difficult to do) to keep the engine running when there is no further need for it, and, therefore, the engine cannot, perforce, be well treated!

 

[PeterMcK]

For what it's worth, as an engineer I've ignored virtually all the engine preservation advice you find on these threads, other than to give the engine reasonable doses of maintenance.

When I raced the boat, I didn't carry enough diesel to go through the warm-up before use capers, or heat fully before closing down. When I cruised the boat, I had it over propped, never needing anything like full engine rpm to achieve hull speed; and when I needed to charge the batteries whilst anchored or sailing, I let the engine idle, generally for an hour or two per day.

At over 30 years old and 12,000 hours, the engine was still giving full power, starting easily and using almost no oil. I removed it to give it new engine mounts (one being inaccessible was worrying me), a badly needed repaint, and I thought while I'm at it, a top overhaul, new big ends, mains and timing chain, new seals and a look at the bores.

On inspection, it was obvious I could have run it indefinitely, almost certainly for another ten years or more, without major attention.

The engine was a BMC/Thornycroft T90, standard fitment in my Moody 33 Mk2. However, by coincidence, the very week I opened up the engine, I was offered a BMC/Thornycroft T108, the bored out and improved version of the T90, almost a straight swap, with only 500 hours on it and at a price I couldn't refuse, stored just across the road from the boat.

I've overpropped the new setup even more to take advantage of the 50% extra torque the T108 produces, reaching hull speed at only 1800 rpm, well short of the engine's 3000 rpm maximum continuous rating. I continue to charge batteries idling when it suits me. I expect to get the same deal from it as I did from its predecessor (if I live that long!).

I've owned the boat for 20 of its 34 years and this is how I've always run it. If that did the old engine any harm, it wasn't very obvious.

I often wonder how many of the purveyors of commandments and scare stories can say the same; how much is based on experience, how much is recycled alarmist nonsense, the grain of truth becoming the message.

But to get back to the OP's question. In short, yes, it is indisputable from an engineering perspective that high revs must shorten engine life because higher piston velocities come with higher friction and wear (IIRC wear is proportional to the cube of mean piston speed), and inertia loadings on all the reciprocating components, a major factor in wear of bearings, valves, camshafts, chains, etc., increase in proportion to the square of the rpm. A big engine running slowly will always outlive a small one running flat out.

And as an earlier poster alluded, you won't damage an engine by judicious over propping provided you don't run it constantly up to its fully loaded torque (which in fact is almost certainly its 1 hour rating); back it off 50 or 100 rpm or, better, set the governed speed to suit the setup.

 

[JackFrobisher]

... set the governed speed to suit the setup.

What does that mean, please? I have a lot of experience with petrol engines but not so much with diesels ...

 

[ianj99]

For what it's worth, as an engineer I've ignored virtually all the engine preservation advice you find on these threads, other than to give the engine reasonable doses of maintenance.

When I raced the boat, I didn't carry enough diesel to go through the warm-up before use capers, or heat fully before closing down. When I cruised the boat, I had it over propped, never needing anything like full engine rpm to achieve hull speed; and when I needed to charge the batteries whilst anchored or sailing, I let the engine idle, generally for an hour or two per day.

At over 30 years old and 12,000 hours, the engine was still giving full power, starting easily and using almost no oil. I removed it to give it new engine mounts (one being inaccessible was worrying me), a badly needed repaint, and I thought while I'm at it, a top overhaul, new big ends, mains and timing chain, new seals and a look at the bores.

On inspection, it was obvious I could have run it indefinitely, almost certainly for another ten years or more, without major attention.

The engine was a BMC/Thornycroft T90, standard fitment in my Moody 33 Mk2. However, by coincidence, the very week I opened up the engine, I was offered a BMC/Thornycroft T108, the bored out and improved version of the T90, almost a straight swap, with only 500 hours on it and at a price I couldn't refuse, stored just across the road from the boat.

I've overpropped the new setup even more to take advantage of the 50% extra torque the T108 produces, reaching hull speed at only 1800 rpm, well short of the engine's 3000 rpm maximum continuous rating. I continue to charge batteries idling when it suits me. I expect to get the same deal from it as I did from its predecessor (if I live that long!).

I've owned the boat for 20 of its 34 years and this is how I've always run it. If that did the old engine any harm, it wasn't very obvious.

And as an earlier poster alluded, you won't damage an engine by judicious over propping provided you don't run it constantly up to its fully loaded torque (which in fact is almost certainly its 1 hour rating); back it off 50 or 100 rpm or, better, set the governed speed to suit the setup.

How refreshing to hear from someone speaking from experience as I have done, instead from one of the many armchair experts who are strong on theory and short on first hand experience.

Ian

 

[ianj99]

What does that mean, please? I have a lot of experience with petrol engines but not so much with diesels ...

It means adjust the throttle stop screw on the injection pump so you cannot use more throttle than is good for the engine.

Or as was said earlier, just open the throttle until the revs stop increasing and then back it off by 50-100rpm and use this as the max power setting. You can use those extra few rpm for emergency / short term use (20mins).

Ian