Why are high RPMs bad for diesels?

[reed1v]

Any engine, if run at max rpms will start to scorch inside the cylinders due to excess heat. Also lubrication will start to burn off and the metal components(pistons, etc) will start to deform. Great way to ruin an engine. Try it sometime. Run your car in low gear at high speeds and see what happens. True for both diesel and gas engines.

[01kiwijohn]

If the prop is pitched correctly should be no problem to run at governed rpm. Pumping lots of air through it and not labouring is what they love.

[Scaramanga F25]

I often remind myself that boat motors are always pushing a boat uphill as opposed to other uses. They never coast.

[Pat Rains]

Originally Posted by Kenomac

Turbo diesels like to be run at higher rpm. If run continuously at low rpm, be prepared to replace the turbo which will eventually plug up, overheat and be ruined.

On long passages (offshore TransPac or TransAt) where you must run a diesel engine at its recommended lower RPMs for consecutive days in order to retain maximum fuel range, it's wise to occasionally (like maybe once every 2 - 3 hours) rev it up to its higher RPMs for short periods (maybe 10 - 15 minutes) in order to blow any accumulated carbon off the turbo blades. Some engine mfgers say to do this turbo-cleaning high rev until you see nice clean exhaust, no black in it. ... This is a real pain in the butsky when you're bashing into head seas, so we often run down swell before starting a turbo-cleaning high rev, then U-turn back up to slow bash mode.

Charter boats with diesels are an interesting situation. Although charterers may be less likely to undertake long passages, just short quick hops; they're probably not very concerned with engine-life maintenance.

[a64pilot]

RPM alone wont build much heat, reason is there isn't much fuel being consumed, therefore not much heat. Governor will only add as much fuel as necessary to reach governed RPM, more of course and RPM goes up.
Now you want to get her hot? Load her up to where she starts to blow smoke, then hold her there, the smoke of course is partially burned fuel, and that puts combustion temps and pressure about as high as you can get them.
A little shot of propane or Nitrous will clear the smoke right up though.


Here is a common rail Duramax Diesel pickup truck as I said common rails aint your Grandfathers Diesels

https://www.youtube.com/watch?v=gfXwBm9L25U

[HighTemp]

I have heard from proponents of both schools of thought. One saying no more than 2500 rpm, and the other swearing that running close to WOT is the way to do it.
Questions:
Don't most modern marine diesel engines come with governors or throttle stops installed for the very purpose of not allowing one to over rev the engine?
and
Doesn't the load on the engine, as a result of prop size and pitch, hull design, etc..somewhat define the maximum RPM that the power plant can turn? I know that on my car, we limit the revs to 8200 RPM in 1st through 5th, but due to aerodynamic drag, it can't even get to redline in 6th.

I am certainly no marine diesel mechanic, but in my way of thinking, to improve reliability and extend engine life, it is more about running your engine in the most efficient portion of the power band, given your boats setup, than assigning some "one size fits all" ( I constantly hear 2500 RPM max ) do not exceed RPM number.

[a64pilot]

What kind of car do you have with an 8200 redline?
But yes, Diesels even old ones are governed just like your car is, hold it to the floor, the rev limiter will catch it, but you don't drive around town at 8000 RPM do you? Of course not, and I wouldn't run my boat motor at WOT long either.
Airplanes you do, but that is a different story for many reasons, for one thing they accept relatively short engine lives.

[JPCorrigan]

Great discussion from some extremely well educated people. Thank you all, I learned a great deal. I wish it would continue with synthetic oils in a Diesel.

[Dockhead]

Originally Posted by HighTemp

I have heard from proponents of both schools of thought. One saying no more than 2500 rpm, and the other swearing that running close to WOT is the way to do it.
Questions:
Don't most modern marine diesel engines come with governors or throttle stops installed for the very purpose of not allowing one to over rev the engine?
and
Doesn't the load on the engine, as a result of prop size and pitch, hull design, etc..somewhat define the maximum RPM that the power plant can turn? I know that on my car, we limit the revs to 8200 RPM in 1st through 5th, but due to aerodynamic drag, it can't even get to redline in 6th.

I am certainly no marine diesel mechanic, but in my way of thinking, to improve reliability and extend engine life, it is more about running your engine in the most efficient portion of the power band, given your boats setup, than assigning some " size fits all" ( I constantly hear 2500 ROPM max ) do not exceed RPM number.

Over-revving the engine -- that is, running it outside its permitted rev range is one thing. It's another question whether it's good or bad for it to run it within its permitted rev range, but for long periods of time. These are different questions.

You may not be a diesel mechanic, but I absolutely agree with your statement that "running your engine in the most efficient portion of the power band." Actually, I think you can tell whether the engine is happy by its SOUND. Mechanical harmony has a distinctive sound. My Yanmar is redlined at 3900, but clearly likes running at 1800 -- 2300 better than above that. I'm sure you can run it at 3000 all day if you want, but I also sure that the engine won't last as long.

Some people say that diesels like to run at their torque peaks. That would be about 2800 for my engine, but turbo motors will have more torque, pushed further up the rev range, so maybe this concept doesn't apply to them.

[HighTemp]

Originally Posted by a64pilot

What kind of car do you have with an 8200 redline?
But yes, Diesels even old ones are governed just like your car is, hold it to the floor, the rev limiter will catch it, but you don't drive around town at 8000 RPM do you? Of course not, and I wouldn't run my boat motor at WOT long either.
Airplanes you do, but that is a different story for many reasons, for one thing they accept relatively short engine lives.

I have a Nissan GT-R that, at one point, I apparently thought wasn't fast enough, so I spent entirely too much money making it go faster, and I still sit in traffic on the freeway...... And no, I don't run around at 8000 RPM very often.

Are the governors on marine diesels specifically set to limit the rev to say 80% of the theoretical maximum? I am trying to equate it to my car in that the engine is built to turn as high as 8600, but we electronically limit fuel and ignition timing to 8200 for "safety" purposes.

[SV DestinyAscen]

I don't think it is an rpm issue.

Engines, whether gasoline or diesel, have optimal running conditions. Most engine wear occurs during cold start up, or sudden shutdown (especially a problem for turbochargers, large mass engines with a lot of thermal mass).

Most charters don't care about boat, so they may not give the engine time to warm up, or idle before shutoff.

[Dockhead]

Originally Posted by a64pilot

What kind of car do you have with an 8200 redline?
But yes, Diesels even old ones are governed just like your car is, hold it to the floor, the rev limiter will catch it, but you don't drive around town at 8000 RPM do you? Of course not, and I wouldn't run my boat motor at WOT long either.
Airplanes you do, but that is a different story for many reasons, for one thing they accept relatively short engine lives.

But a governor on a diesel doesn't work the same way as on a petrol/gas engine.

A spark-ignited engine has power modulated by a throttle plate which restricts intake tract, creating a partial vacuum in the intake manifold and reducing the mass of mixture in the cylinders. This is one reason why they are less efficient than diesels -- pumping losses -- the engine expends energy pumping the mixture past the throttle plate. So the gas pedal or throttle lever is attached to this throttle plate. A governor, if the engine has one (my old Porsche 911S had a centrifugal distributor rotor which would disconnect the power to the spark plugs over 7500 RPM) is a separate mechanism, from the throttle.

A diesel doesn't have any throttle plate. The intake tract is always open and the mass of air flowing through the engine is always constant for a given RPM. The gas pedal or throttle lever is attached to the governor. The governor controls the fuel rack -- it pulls the rack in or out to inject more or less fuel to maintain the engine speed at the level ordered by the gas pedal position.

So on a diesel the governor is not a safety device; it's a primary part of the fuel system. On a diesel the gas pedal or throttle lever controls just the RPM of the engine; the power output is varied by the governor depending on the load, by injecting more or less fuel.

[HighTemp]

Originally Posted by Dockhead

But a governor on a diesel doesn't work the same way as on a petrol/gas engine.

A spark-ignited engine has power modulated by a throttle plate which restricts intake tract, creating a partial vacuum in the intake manifold and reducing the mass of mixture in the cylinders. This is one reason why they are less efficient than diesels -- pumping losses -- the engine expends energy pumping the mixture past the throttle plate. So the gas pedal or throttle lever is attached to this throttle plate. A governor, if the engine has one (my old Porsche 911S had a centrifugal distributor rotor which would disconnect the power to the spark plugs over 7500 RPM) is a separate mechanism, from the throttle.

A diesel doesn't have any throttle plate. The intake tract is always open and the mass of air flowing through the engine is always constant for a given RPM. The gas pedal or throttle lever is attached to the governor. The governor controls the fuel rack -- it pulls the rack in or out to inject more or less fuel to maintain the engine speed at the level ordered by the gas pedal position.

So on a diesel the governor is not a safety device; it's a primary part of the fuel system. On a diesel the gas pedal or throttle lever controls just the RPM of the engine; the power output is varied by the governor depending on the load, by injecting more or less fuel.

Thank you for that explanation. Makes sense.

[uncle stinkybob]

Originally Posted by a64pilot

OK, where to start?
First RPM of any reciprocating engine isn't the issue, it is piston speed, meaning long stroke engines must be run at lower RPM in order to keep piston speed within reasonable limits, this is why performance motorcycle motors can be run at very high RPM all day long with no harm, short strokes.
The RPM limit of a Diesel has nothing at all to do with structural limits, it has to do with fuel delivery, but sometimes a designer will accept the fuel delivery RPM limit and spec a long stroke that closely matches the fuel limit to enhance Torque.
A spark ignition engine for example can continuously spray fuel onto the backside of the intake valve, then when the valve opens, the engine will inhale the fuel, but combustion does not begin until the spark plug fires. This is one very good reason a spark ignition engine can run at such low fuel pressure as in 30 PSI or so, it has all day to deliver the fuel.
I Diesel engine delivers fuel directly into the combustion chamber, whether it has a pre-chamber or not, and fuel burn begins the instant fuel becomes available, so you have to begin an injection pulse at exactly the right moment and get the fuel in there fast, as the piston isn't waiting on you. This interval gets shorter and shorter as RPM increases, pretty quickly even with insanely high fuel pressures, there is just not enough time to get the fuel in there, that is what limits Diesel RPM.
A common rail Diesel will run fuel pressures above 30,000 PSI, yes that is not a mis-type, 30,000 PSI and they are THE hot rod Diesels, I have a 6.6 Diesel truck I have had for years, a common rail and let me tell you compared to our Grandfathers Diesels, it is a completely different animal.
On it's Tachometer, it has two distinct redlines, it goes hashed at 3,500 RPM, that is the fuel delivery limit, but during heavy downhill braking, its acceptable to allow the engine to turn up to 5,000 RPM, now we are getting close to the mechanical limit.

In short usually an engine design will allow limited high power which equals high RPM cause HP = RPM x Torque if you need it, My Yanmar 4-JHE for example allows operation at 3600 RPM for one hour, 44 HP, but continuous only 3500 or 40 HP, usually limiting factor may be heat.

Then there was a study done previous to WWII by the Brits that showed engine wear increased non linearly, meaning if you say double RPM you more than triple wear, presumably it due to the higher kinetic forces of a recip engine, remember that piston has to stop and reverse direction at every stroke, that stopping and starting has tremendous energy that has to be absorbed by the bearings, plus of course higher RPM means everything is happening more often in a given time frame, so of course higher wear.

I was looking to see if someone got it right. It's all about piston speed.

[Cheechako]

I don't know that running a diesel at it's higher rpm rating is exceedingly bad for it. Diesels are governed and constrained by the prop pitch, so I don't think charterers are able run them "too fast".

However, it's obvious that some engine manufacturers believe higher rpms = shorter life. Many commercial diesels are rated at different rpm for different use. A big one might be rated at 3000 rpm for recreational intermittent use. but the same engine might be rated at 2400 rpm for continuous use.