RPM for a healthy diesel engine

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cmedsailor

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Engine manufactures state a "cruising" rpm that is the optimum to use, in terms of performance, consumption and life of the engine. A lot of (useful) discussions exist in this forum for the relation between rpm and fuel consumption. This is a different question now:
If cruising rpm is say 75-80% of manimum rpm (I believe we all agree on this) what are the long term consequences of running usually an engine at lower (say 60-65%) than cruising rpm? (NOT in terms of consumption).
And since I have heart people saying "it's good to run a diesel engine hard" same question if running at higher (say 90%) rpm than cruising. (again ignore consumption please).
Thanks
 
Prolonged use at low power tends to lead to glazed bores and worn piston rings. This is made worse by the use of synthetic or semi-synthetic oils in normally aspirated engines. Marine diesels tend to run much slower and cooler that auto diesels anyway, even when turbo-charged which exacerbates matters.

At "crusing speeds" diesels are running at their optimum, getting the right amount of fuel for the air charge to burn efficiently and the engine to maintain a good operating temperature which means that the lubricating oil is doing the best job possible.

Petrol engines tend to run hotter than diesels and they run at higher RPM for a given power delivery so the effects are less noticeable.
 
I suspect the answer is "two tenths of sweet sod all" to use an inelegant expression. Provided the engine is run hard enough to get the oil up to operating temperature, then you can be sure that it is designed to operate at varying speeds. Indeed, one way of damaging an engine is to run it at constant speed for a prolonged period - dont ask me how I know.

Bore glazing occurred when the oil type oil "cooked" to form almost a varnish and this filled the spiral hone markes in the cylinder walls. These hone marks are there to carry the oil which the piston rings slide on and which makes the cylinder seal. We have one or two oil company experts on the forums who will be able to pronounce on the subject, but what I was told by Castrol technical was that glazing as such is now very uncommon - what you are more likely to get these days with synthetic oils is polishing of the bores.

So provided you dont a/ leave the engine to idle for prolonged periods b/ leave to run at constant throttle for prolonged periods c/ run it long enough and hard enough to warm it up properly, I wouldnt worry. Engines are consumeables anyway
 
I'm sceptical about this "diesel engines need periodic thrashing" argument. It goes against my instinct for mechanical sympathy which says that an engine would last longest if run at a constant speed, preferably continuously, for its entire life. That is certainly the case with old fashioned heavy diesels used as stationary generators before the days of universal grid power. Some ran for 50 years virtually non-stop.

May be I am wrong, but I'd like to hear the reults of real measured experiments running diesels to the end of their lives under a range of conditions.
 
I think it depends on how the load is matched to the engine's torque/power curve as well, which will be a function of prop and seastate etc.
There have been a few threads on here about overheating, so I guess the cooling system and raw water flow vs rpm can be important too!
I would have thought that diesel engines were fairly immune to the fatigue effects of running at constant RPM, as many of them are used for generators. Personally, I tend to set the engine where the noise and vibration aren't too bad, but both of my boats had fairly noisy racing boat type installations. And I prefer to spend a day in Yarmouth or Cherbg rather than motor all the way across the channel. 12 hours of constant noise is bad for me, sod the engine!
 
The Yanmar workshop manual suggests that their engines be run flat out for 5 minutes after two hours of running at low speeds. I suspect that their 'low speeds' refers more to battery charging than to motoring at less than about 75% of maximum revs.

My normal cruising speed is 2000 to 2200 reves, with a maximum of 3600. Periodically, although not every two hours, I run flat out for a couple of minutes or so.
 
Looking at a torque curve there is a range where it has its highest value. For example on my Volvo MD2030 this is 62NM at 2000rpm, rising to 63NM at 2400rpm and droping to 61NM at 2800rpm. In any speed below or above this range the increase/decrease is much higher. What does this tells us? That optimum is around 2400rpm? (70% of maximum 3400-3500).
 
Without wishing to insult other posters that I recognise are trying to give their best opinions, a lot of rubbish is talked generally about this subject and there is a great deal of "folklore".
There are several pertinent points to consider.

The first is common to any engine, petrol, diesel or indeed any other fuel and that is temperature and time at temperature. When stationary, as many marine diesels are for long periods there is condensation. This condensation is at it's worst when the engine has been running in a damp atmosphere and cools down but it happens all the time. This causes the lubricating oil to become emulsified with water and this water can cause corrosion. The cure is to get the engine good and hot to drive this off as steam. RPM does not matter greatly but the engine needs to be made to work to reach temperature and it needs to be hot for at least an hour or two.

The second is our old friend carbon build up on pistons, combustion chambers and valve stems. This is clearly at it's worst when the engine is run slowly for short periods when the engine never reaches full operating temperature and optimum combustion efficiency. The cure here is to avoid slow running especially for short periods.

The third is indeed bore glazing where, if the engine is only ever run slowly at low temperatures there can be excessive polishing of bores and gumming up of rings.
These conditions are often seen to have the same cause but in fact with modern oils the latter is not so much of a problem if the oil is changed regularly and never allowed to build up sludge due to emulsification (see first para)

What this all means in practice is that running at peak efficiency means adopting the attitude that about 80% throttle is best. At 60% it's probably still OK but less than this, especially for short periods, will cause problems in the long term. More than this probably won't cause harm in moderate bursts but is not going to improve matters and will as you indicate be fuel inefficient. If you do have glazing, cokeing up, or other problems however, a "good hammering" at full revs for an hour or so can often get it up to high temperature, bust the glaze, get rid of moisture and free gummed rings.

Hope this puts things in perspective....
 
Nice points BoatMike.

And to put things even more into perspective, some types of diesel engine are used in cars, in vans, as boat engines, and also as generators. Add to that that they'll have different 'ratings' (max revs) for 'commercial' and 'leisure' use, and that those same engines may be branded as Volvo, Perkins, Sabre, you name it . . .

Change the oil regularly; if it's out of use for any period, take steps to minimise corrosion. Don't worry about the revs; in boats that's a secondary consideration for diesel engine life.
 
[ QUOTE ]
If you do have glazing, cokeing up, or other problems however, a "good hammering" at full revs for an hour or so can often get it up to high temperature, bust the glaze, get rid of moisture and free gummed rings.

Hope this puts things in perspective....

[/ QUOTE ]

I think this is what happened with my engine. I was testing my propeller "capabilities" and maximum rpm I could get out of the engine. During the first trial I managed to get up to 3200 rpm. In the second trial I got 3400 rpm (with difficulty) and smoke coming out of the exaust. In the last trial I got very very easily the maximum 3500 rpm claimed by the manufacturer and absolutely no smoke or "oh my God, it'll explore" noises from the engine!!!
Boatmike helpful as always. Thanks.
 
(just for info) : Most injection pumps have a rating for max revs stamped onto the ID plate.

Raw water cooled engines usually run a lot cooler than heat-exchanger ones for good reason - to reduce salt deposits in cooling tracts. Older Perkins etc. had a manual valve fitted instead of thermostat to alter the water flow to optimise temp.

Talking to mechanics out here who actually repair items instead of just exchanging - they laugh at the work engine hard theory. They tell me that its good when engine is varied rpm when run, but also that when engine is running sweetly and minimal vibration - it's OK to run for long periods like that.

Poster above mentions stationary engines at fixed revs going for years. Ships generators do that day after day after day for year on year .... they don't even have throttles as most people think of - they have constant speed governors. They also run on lower quality diesel than we use !
 
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