Diesels at idle; risky?

[diver dave]

Odd title, right?
Anyway, I got to thinking about the way diesels are governed and how that might be an issue at idle, IF there is a driveline (or grounding) problem.

I have heard stories, that I cannot authenticate, of trawler diesels spinning off their mounts due to a line fouling the shaft.

So, perhaps my question is this. How much fuel flow is fed when a diesel is jammed up at idle (not allowed to reach idle rpm)? And, is this different with electronic vs mechanical injection?

One of the benefits of diesels are the wide range of fuel to air mixes that still allow combustion, and the constant rpm nature of the governers when in seas, towing, etc.

 

[High Wire]

My friends lobster boat sucked in a pool cover and a plastic trash can that stopped it immediately from 2000 rpm cruise. No damage to mounts, transmission, or struts either time.
300 hp Cat 3116 with mechanical FI.
At idle in gear it would just stop.

 

[twistedtree]

If the mounts break, it’s because the mounts are broken.

 

[psneeld]

I have choked off a dozen or more engines for a variety of reasons and never broke or damaged a motor mount.

I will say high speed vessels where the props can be pulled back far enough can damage mounts.

 

[Island Cessna]

For most of our Diesel engines the torque capability peaks in the mid rpm range, and drops off at both high and low rpm. If the mounts can withstand full throttle torque they can resist idle speed torque too.

 

[diver dave]

OK, I see the urban legend of the spinning engine due to a loose dock line can be debunked.

One time I came across a D6 in the woods. I managed to start it and ran it around for awhile but couldn't for the life of me figure out how to stop it. Tried to stall it into a large tree, no dice. Long story short, I found the fuel cut-off.

 

[Larry M]

We wrapped a line around the shaft on our sailboat that killed the engine. Shaft, mounts, strut, transmison we’re all good. The cutless bearing was toast though. It broke the nitral loose from the navel brass. A new cutless bearing and we were good to go. We probably motored for 20 hours with no issues other than low rpms we had a little vibration till we could get to a yard to haul out.

 

[markpierce]

I have choked off a dozen or more engines for a variety of reasons and never broke or damaged a motor mount.

I will say high speed vessels where the props can be pulled back far enough can damage mounts.

Can imagine a quick/instant change from forward to reverse at higher-than-idle engine speed will be a problem.

 

[psneeld]

Can imagine a quick/instant change from forward to reverse at higher-than-idle engine speed will be a problem.

Actually I was referring to running aground at 25+ nmph.

 

[markpierce]

Actually I was referring to running aground at 25+ nmph.

Can't imagine a toy-but-true trawler traveling more than a small fraction of that speed.

 

[cool beans]

uhhhh....this is cartoon physics?

Thinking about it, what mechanics in an internal combustion engine, after having stopped rotation of the crank externally, would then cause the block to start rotating around the crank shaft?

Unless I'm missing something...or excess beer

 

[psneeld]

my comment about running aground has nothing to do with tbe engine rotating more....

just yanking back on everything.....

 

[diver dave]

uhhhh....this is cartoon physics?

Thinking about it, what mechanics in an internal combustion engine, after having stopped rotation of the crank externally, would then cause the block to start rotating around the crank shaft?

Unless I'm missing something...or excess beer

funny you mention that. Just 2 days I saw an IC engine designed to rotate the engine around the crank. It was a early aircraft engine. Two different radial designs did that. The crank was fastened to the frame, the prop attached to the crankcase.

 

[Sabre602]

uhhhh....this is cartoon physics?

Thinking about it, what mechanics in an internal combustion engine, after having stopped rotation of the crank externally, would then cause the block to start rotating around the crank shaft?

Unless I'm missing something...or excess beer

Yup, the cartoon of the airplane spinning when the prop is stopped! With this logic, a powerful enough diesel would capsize the boat if the prop got wrapped....

 

[Lepke]

Commercial boats rarely have rubber motor mounts like yachts. Bigger steel fishing boats have the engines directly bolted to the engine beds. If something were to quickly stop a prop, because of the reduction gear, a coupling, the shaft or prop blades would fail. The clutch should release before an engine would break its mounts. Maybe before shaft or prop damage.

 

[FF]

"Just 2 days I saw an IC engine designed to rotate the engine around the crank."

Those early engines had great cylinder cooling!

 

[Surface Interval]

Bronze propellers will absorb a lot of shock from a propeller strike. Stainless propeller would be a different story but rarely see that on recreational boats.

 

[jleonard]

I have a trawler friend who backed over a line that had one end cleated.
He pulled the engine off it's mounts.
38 Marine trader with single Lehman 120.
It can happen.

 

[diver dave]

I have a trawler friend who backed over a line that had one end cleated.

He pulled the engine off it's mounts.

38 Marine trader with single Lehman 120.

It can happen.

Says a lot about the cleat[emoji848]

 

[FF]

"Says a lot about the cleat"

He pulled the engine off it's mounts.
38 Marine trader with single Lehman 120.

It sez even more about the boat build.

 

[psneeld]

Passed along to me by someone in the field...

" How much fuel flow is fed when a diesel is jammed up at idle (not allowed to reach idle rpm)?"

Enough to try to maintain the idle rpm setting. If the drive line is "jammed up" and not able to turn the engine will not turn fast enough to burn any additional fuel or produce enough power to accept the load. The short version is the engine will either lug along smoking like crazy at less than idle rpm and power or it will stall because it is overloaded.

And, is this different with electronic vs mechanical injection?"

Not sure but I think the electronic governor will prevent idle fuel to exceed a programmed amount and the engine would be stopped much sooner with no dramatic display of black smoke.

 

[MYTraveler]

uhhhh....this is cartoon physics?

Thinking about it, what mechanics in an internal combustion engine, after having stopped rotation of the crank externally, would then cause the block to start rotating around the crank shaft?

Unless I'm missing something...or excess beer

Newton's Third Law of Motion covers this -- for every action there is an equal and opposite reaction. Whatever torque an engine is putting into the shaft, there is corresponding torque on the engine mounts. If the shaft were stopped, the engine's torque would increase, up to its max torque at its current rpm, and that increase would be felt by the motor mounts. The thing is, the mounts must be designed to comfortably handle the max torque, so absent bad mounts, the engine shouldn't rip off the mounts.

 

[Ski in NC]

Yep, if you snub out an engine and it breaks mounts loose, mounts were not spec'd or designed right.

 

[Fletcher500]

Sir Newton to the rescue. I was thinking about this as it relates to the Gyro that will be installed in our boat that is under construction. It is literally trying to rip a hole out off the hull due to the generated torque. The area supporting it and attachment hardware is therefore bult up to handle the expected load per Seakeeper specs. Similar principle.

 

[psneeld]

also passed along....

"If the shaft were stopped, the engine's torque would increase ..."

......snip.....it's a diesel, not an electric motor or reciprocating steam engine ... if the shaft were stopped it would produce zero torque.

Going back to the OP's question, if the engine was idling an reduction in idle rpm will reduce the little torque that is available. It will very rapidly reduce the torque and stall the engine.

......snip.....The shafts were pulled out because something pulled on the shaft while the boat was moving ahead, the engine mounts broke because of any number of reasons, not one of them having anything to do with the rpm of an idling engine.

Since several posters mentioned issues at higher than idle rpm, a glance at your engine's propeller curve provides the answer. If the engine is operating at or near max power it has almost certainly passed the peak torque rpm, the engine would not see any more torque than normal max torque less than max power operation. That engine operating at max power would increase torque as the "jammed up" shaft rpm dropped to the max torque level but it would probably pass through max torque to zero torque before much of anything would happen.

The torque shown in the power and prop curves is derived by "jamming up" the output in a test cell, the engine doesn't know if it is twisting against a test cell brake or a stray mooring line. If the engine could produce damaging torque at idle it would be displayed on the torque curve as a vertical line at zero rpm, it wouldn't be a curve.

As I wrote before, the torque available at idle is so little that the only sign of a problem would be sudden silence.

 

[diver dave]

Hard to find torque curves below 1000 rpm for tractor motors. I've been looking. That maybe is due to the loading condition required at the test rig would not result in a running engine.

But, the line caught on the shaft would also be subject to a more interesting loading effect. That of stopping the inertia of all the moving mass. Shaft, tranny, prop, flywheel, crank, etc all in motion, at some point without the benefit of actual combustion. All on the way down to zero rpm.

That fact remains though of the torque formulas. The 3 or 4 foot wide spacing on the motor mounts will see far less force than the dock line wrapped on a 2" dia shaft, or even around the 3" diameter prop hub.

 

[alormaria]

I wrapped a poly tarp around my prop at about 7 kts. It stopped the engine.

The next year I had the engine pulled for other work and the yard said some of my engine mounts were broken so they replaced them all. They also found the shaft (brass) was "corkscrewed". Seemingly straight but twisted around on it's axis. Replaced it with an "aquamet" stainless shaft.

I watch where I'm going now much more carefully in the spring and after rain.

 

[ABfish]

A dump truck is not a boat, but.....I've seen bad drivers stall engines thousands of times with no immediate adverse affect. I've also seen broken universal joints from revving the engine and trying to pull out of a hole.

This does raise an interesting point, though; on heavy trucks, I've always thought of the u-joints as the "fuse" in the driveline. What's the weak point in marine drive train?

 

[psneeld]

Nothing really....thats why they sell things called drivesavers.

 

[Ski in NC]

A dump truck is not a boat, but.....I've seen bad drivers stall engines thousands of times with no immediate adverse affect. I've also seen broken universal joints from revving the engine and trying to pull out of a hole.

This does raise an interesting point, though; on heavy trucks, I've always thought of the u-joints as the "fuse" in the driveline. What's the weak point in marine drive train?

Fortunately for us on boats, what protects the driveline from shocks is the
WATER!! Hard to break the things like on truck drivelines when your wheels are in the water. Can be done, but you really have to rodeo the controls.