Why are high RPMs bad for diesels?

[thruska]

Originally Posted by ErikFinn

Finally someone speaks the matter of the truth to the point. The title of this thread couldn't be further from the truth and is pure BS.

sub removed, cheers.

In a nutshell, thats it.
You cannot lug a diesel indefintely and get away with it for very long.
I think the expression is , the piston want to change holes.

[sartorst]

When an engine is running with no load unassuming injectors are spraying properly warmed up etc the burn takes place at the top portion of the cylinder and all fuel is consumed as the load increases the burn continues further down the cylinder until a point where all off the fuel is not being burned before reaching the end of the cycle leaving raw fuel to wash oil from the cylinders = wear
If a mechanical engine failure occurs within governed rpm it was a weakness exposed it was going to fail sooner are later lets face it most of these engines are old technology
Proof of the theory of the unburnt fuel is that the carbureted engines were worn out by 100 k miles particularly if not tuned the same engine when fuel injected would go 200k miles then mechanical failures started showing up !
A diesel engine with improperly adjusted injectors or pump or improperly pitched will wear much faster
Note also that a higher weight oil will take up the spaces created by wear in an older engine

[foufou]

Why aren't there governors on charter boat diesels? Or any diesel engine?

[Dockhead]

Originally Posted by foufou

Why aren't there governors on charter boat diesels? Or any diesel engine?

All diesels have governors. They can't run with out them. Read the explanations further up in the thread.

[Dockhead]

Originally Posted by sartorst

When an engine is running with no load unassuming injectors are spraying properly warmed up etc the burn takes place at the top portion of the cylinder and all fuel is consumed as the load increases the burn continues further down the cylinder until a point where all off the fuel is not being burned before reaching the end of the cycle leaving raw fuel to wash oil from the cylinders = wear
If a mechanical engine failure occurs within governed rpm it was a weakness exposed it was going to fail sooner are later lets face it most of these engines are old technology
Proof of the theory of the unburnt fuel is that the carbureted engines were worn out by 100 k miles particularly if not tuned the same engine when fuel injected would go 200k miles then mechanical failures started showing up !
A diesel engine with improperly adjusted injectors or pump or improperly pitched will wear much faster
Note also that a higher weight oil will take up the spaces created by wear in an older engine

No question that running a diesel engine with too little load for the RPM is bad for it.

But from that does not follow logically that it's good for it to run it at maximum load and at redline all the time.

The engine may be designed to last a reasonable amount of time at max load and redline, or it may not -- for example, recreational engines are often only rated to use max RPM and load intermittently.

In any case, more stress and load, and higher piston speed, than a reasonable minimum need to keep the combustion chamber warm and rings seated, will of course shorten the life of a diesel engine just like any other mechanism.

A really heavy duty engine -- a Ford Lehman, say, or Perkins M92, might not mind too much running at max RPM. Redline is only 2400 and specific output is very low. The M92 is 4000cc for only 82 horsepower. The same engines will hate much more running at low load -- because the very large combustion chambers (the Perkins is 1000cc per cylinder) take a fair amount of load to start to warm them up.

But an engine like mine -- Yanmar 4JH3 HTE, 100 horsepower out of only 2000cc -- will be under a lot of stress when putting out 100 horsepower at 3900 RPM, with the turbo putting out 2 bars of boost. Such an engine will be much happier and last much longer putting out say 40 horsepower (the same horsepower/liter as the Perkins at full power), and 20 horsepower (and say 1800 RPM) is fine for these engines, much better for it than running at redline.

Nor will your car, whether it's diesel or gas, like to run at redline all day long. It might not fly apart, if it's well designed and built, but its life will be shortened. More RPM, more power output = more internal stress -- you can't get around this simple fact.

[Pete7]

Originally Posted by Dockhead

A really heavy duty engine -- a Ford Lehman, say, or Perkins M92, might not mind too much running at max RPM. Redline is only 2400 and specific output is very low. The M92 is 4000cc for only 82 horsepower. The same engines will hate much more running at low load -- because the very large combustion chambers (the Perkins is 1000cc per cylinder) take a fair amount of load to start to warm them up.

Indeed, my Ford Dover engine diesel rib had a 4.2L engine in it and that was 4 cylinders so over a litre per cylinder. That is a 30 year old block producing 200 geegees on the dyno. I took a great deal of effort to start and warm that engine up before opening up the throttle and letting the turbo do the work. Equally after a run gave the engine several minutes to allow the turbo and working parts to cool down and any hot spots to equal out. Conveniently the trip from the marina to the harbour entrance did this rather nicely.

Linda asked about charter boats, that sort of mechanical sympathy just doesn't happen on a hire boat. Arrive at full speed, tie up and engine off. Now what happens to the oil in the turbo bearing which isn't being pumped around anymore. Same thing happens on a motorway/freeway. After a long drive owners pull off park up and switch off allowing the engine to slowly cook its self.

[SV Bacchus]

Wow, this sorta went all over the place and the consensus I get is: there is no one right way. It appears all should read their manuals, do their research and do what is right for their power plant.

I have several comments or questions and rather than one long post I will follow this with several short ones. Folks can comment, laugh or ignore; whatever their pleasure!

[SV Bacchus]

First area I want to mention, as it impacts a few owners here: buying a used charter yacht and how it was used.

Until this thread I had never thought about, or even read, the high speed, get their quick mentality (which seems very plausible because most are not used to the speed of a sailboat) and then quick shutdown being detrimental.

That all seems to make sense and is now something to consider as we look at our next boat.

However, I always thought the big issue was excessive running, possibly above idle, under no load (on mooring or at anchor) to charge the batteries.

Isn't the no load scenario worse than the high speed quick shutdown? I offer that because most of those areas are close enough they are not at high speed for days on end.

[SV Bacchus]

Next is the excessive idling being [I]very bad[I]for a Diesel.

My dad is a retired railroader and worked evenings. Mom and I would sometimes meet him at 11PM so I could go for a quick ride on the engine. He worked in a switching yard.

I noticed they never shut the engine down and one day I asked him why. He said that after working all day the crew would have to sit around for 15 minutes while the engine idled to let the temperature even out in the engine, no hotspots. Then in the morning the early crew would have to start it up (which is where most wear occurs) and let it idle for 30 minutes to get up to operating temperature before starting work. He said heavy loads on a cold motor was bad, bad, bad.

Dad added that even though it was, l believe (excuse me, was a long time ago) 12 cylinders and about 1000 horsepower it would idle all night just sipping diesel. The manufacturer and the railroad had long ago determine it was the most economical way to treat the engines.

Make no mistake, these are very big cylinders and pistons so they were very low RPM engines. But they were never shut down if being used for the next shift, only if not being used for a few days or in the shop for routine maintenance.

When you pay millions for a locomotive it pays to do what the engine manufacturer says.

So the question here is: Can lower RPM Diesel engine tolerate idle better than higher RPM? Tolerate it even to the point it isn't damaging?

I ask because boats are a much different animal than a locomotive but a low RPM Diesel is still a Diesel.

[SV Bacchus]

Finally, the Indiana farm boy coming out in me.

I must be a gear head because I can tell by sound when an engine is at a good speed (for the most part). With farm equipment it seems implements controlled engine speed more than Diesel requirements. Some things, like a sickle bar would literally tear itself apart if operated well above where it should.

We are presently looking at a 97 model 45' Leopard that has upgraded engines by the PO but is still running stock props. He only went up about 10HP per side saying "it's good to have the extra power when you need it".

No engineering studies, nothing. He says just run it at the RPMs listed for the stock engines. Kinda makes sense, after all, what's 10HP right? Hmmmm, I'm not so sure. If it becomes ours I will do some experimenting and listen to my ears.

When people upgrade power plants, how to they determine proper props for the size/weight of the vessel coupled with the new HP rating? Or do they even bother and just go with their ears?

[lindabarzini]

Thanks for all the great responses. I was basically trying to understand why charter boats are considered to be worth less than private ownership.

As far as diesel engines go, it may not necessarily be bad to have a chartered boat, especially if it was professionally maintained. In my more limited experience, I have always found diesel engines work better the more they are run. They start right up and basically can run all day without complaining, especially with oil changes every 50 hours and a Racor fuel filter.

[Sailmonkey]

It's not just the engines on a charter boat. I for one will beat the snot out of the sails and rigging also on a charter boat.......faster is more fun (but too expensive for my private ownership).


Sent from my iPhone using Cruisers Sailing Forum

[Dockhead]

Originally Posted by Sailmonkey

It's not just the engines on a charter boat. I for one will beat the snot out of the sails and rigging also on a charter boat.......faster is more fun (but too expensive for my private ownership).


Sent from my iPhone using Cruisers Sailing Forum

Exactly.

And not just that they've been beaten up -- it's also that they have been used so much. Typical privately owned boat is used three or four weeks plus some weekends a year, by one owner who knows the boat and cares about it. Charter boat is used by an endless parade of people over the whole season, three or four or six months a year continuous, and many of the charterers are not boat owners at all and fairly clueless. Naturally they are beat up and worn out and in a completely different condition from a boat owned by a private person for his own use. I wouldn't buy an ex charter boat, myself.

[skipgundlach]

Nigel Calder, in a SSCA seminar, recommended you treat your sailboat diesel like a racer's at least once a month, for at least an hour, to clean out all the stuff mentioned by others.

One of my best and earliest cruising buddies had a Perkins 4-154 which always had black smoke out the back. I gave him that advice; he tried it and it went away, never to return.

We have the same engine; it's redlined at 3600, continuous duty at 3000; we normally put it at 2500 for the sweet spot mentioned earlier.

Now, if someone could only find a solution to the inevitable rear seal leak on these engines...

L8R

Skip

[Thorne]

My 2 cents.

I grew up on a farm. We daily ran diesel tractors that we had to fix if they broke down.

This is what we found.

It all matters.

From the type of oil you use, to the quality of diesel you use, to the cleanliness of the filters, to the change of the oil in the air breathers ( pretty dusty where we were). From the load you put on the motor, to the number of RPM's you run it at.

For example even if you run a lighter oil and you lug your motor down with a heavy load it will eventually gum up.

Over rev your motor even with a light oil and it will eventually scorch the cylinders.

Now on a tractor you can change the gear or pull the implement out of the ground just a bit, or put a smaller implement on to lighten the load on the motor but you NEVER lug the motor down.

The top end of the throttle on a large tractor is set for "Operating Speed", meaning that is the "Happy" speed for the engine and that is where you run the motor, All Day, Every Day. If you don't you are costing the business money. When you get off the tractor "turn it off, unless you just got thru running it at full load and speed, normally you idle down the rows back to the barn so it's cool enough"

Right size tractor, right size implement, right forward gear, and run at full throttle.

That having been said most tractors have a "Highway" pedal. That increases the top speed of the governor to actual safe engine speed and is only used for running down the highway occasionally.

How does this equate to a boat?

Right prop ( right load ), right oil ( manufactures recommended oil please ), full throttle ( set by the installer to be around 80% of engine max ), clean fuel and your motor will hum along for years like mine has and our tractors used to. Don't idle too much, but don't be afraid the engine will fail if you run it at full throttle. Other things might not like the stress ( think transmission ) but the motor should be fine.

So if your in doubt get the specs and check it out, don't take a chance.

Of course Turbos change everything.

Still check your specs.

Great thread by the way.