Redline on electronic diesels

Just wondered if electronic diesels(in this case QSB Cummins) have a redline that is electronically limited. IOW, if the boat was underpropped would it spin to it's physical limit or would there be some computer/electronic limit keeping it from reaching its physical limit. My buddy is wondering if he is underpropped. He currently makes 2850RPM on a 2800RPM enigne. He seems to think he could be underpropped and thinks the computer is keeping it from revving any higher.

He may be underpropped.
I'd expect all diesels have a rev limiter, either mechanical linkage or electronic.
Does it have the same maximum rpm in neutral?

If the engine has a C Cruse package fitted Idle can be adjusted through the C Cruse but not the RPM limit .The RPM limit can be adjusted through the ECU.

I can't speak for Cummins but all cat electronic diesel engines have a overspeed safety as well as a high idle limit that's factory programmed. If I were to connect to an engine and make any changes to safeties and some other setting's I would have to get factory passwords and it also logs my com adapter sn#. So if there are any issues with the changes made they can track it back to the person who made them.

As to your buddies engine. He would need to see if the Cummins shows % of load to be sure.

Mechanically governed engines have a slop of about 200-300 rpm above rated that the engine will rev at wot in neutral. This is the nature of mechanical governors. It takes that much over rpm for the governor to pull back the fuel flow to limit rpm, less under load probably.

Electronically governed engines are a lot tighter, maybe as much as 50 rpm. As a result you need to evaluate propping by looking at percent load at wot, not just rpm.

Your friend's engine might be propped right, but look a the load that the engine pulls at 2,850 rpm. Should be 90-95% load at wot.

David

Old school diesels are governed. Modern diesels have overspeed safeties. Everything in life can fail. I have seen more than one mechanic crawl out of the engine room naked after smothering a runaway diesel.

He is propped perfectly. You want to be 50-100 rpm over at WOT to ensure you are not overloading the engines

The QSB is limited to about 70 or so rpm above rated rpm. So a 2800 rated engine will only go to like 2870 no load. Starting about 2850 (??) the fuel rated declines down to near nothing at 2870. Much tighter governor control than the old mechanical governors, which Dave was correct, is about 200-300rpm of "droop". Electronic maybe 70-80rpm of droop.

To tell if it is seriously under propped, look on display for "% load" at full power. If at 100%, governor has not backed off fuel. If at 90%, gov has backed off. If at 80% he could definitely add more wheel.

Thanks guys. Not everyone understood the question. So the electronic diesel definitely has an electronic limit...good to know. That is exactly what I was asking.

"He is propped perfectly. You want to be 50-100 rpm over at WOT to ensure you are not overloading the engines"

If the boat planes like a sport fish , or tows water skiers , this is true.

Id the boat is a displacement cruiser , it is true the engines will not be overloaded at cruise , but they may not be efficient, burning more fuel and running higher RPM .

FF said:

"He is propped perfectly. You want to be 50-100 rpm over at WOT to ensure you are not overloading the engines"

If the boat planes like a sport fish , or tows water skiers , this is true.

Id the boat is a displacement cruiser , it is true the engines will not be overloaded at cruise , but they may not be efficient, burning more fuel and running higher RPM .

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He is a planing boat. And the person that you quoted misunderstood the question. His answer is a good answer for a MECHANICAL engine. I was asking if the redlines on electronic diesels are electronically limited thusly hiding an underpropped condition. And the answer to that question seems to be YES.

Whether electronic or mechanical, all diesels are "governed" in the same manner. Put throttle to full in neutral and you will find out what the "no load" or "high idle" rpm is. Both systems do the same thing, but the electronic has much more precise control. Just different ways to do the same thing.

So the QSB 380 high idle is probably 3080 for a 3000rpm engine, the mechanical 6BTA 370 rated at 3000, high idle is probably 3200 or so.

Many operators are still thinking gas engines which have no equivalent governor. Some do have rev limiters that cut the ignition, but that is not really the same thing. With a gasser, it is not normal (nor healthy!!) to shove the trottle to full in neutral. But with a diesel it is a standard check of the governor settings. We do it routinely with diesels.

In fact, one main reason for the governor on a diesel is that rpm is simply not stable without a governor. Set the rack somewhere fixed and engine will either rev to the stars or slow down and stall. Inherently unstable unless something (governor) is actively controlling fuel rate. A gasser with throttle is the opposite, inherently stable for a given throttle position.

Thanks Ski. I did mention the revving in neutral test and he kind of cringed. The problem is it is a Meridian. And Meridian, like many mass producers of boats, does their own gauge package....and the load percentage is not included. He is working on getting a better gauge set up.

What parameters does the "computer" measure to come up with a load percentage?

On a QSB, the two inputs to the governor are throttle position and rpm. So zero throttle is 600 and 100% throttle is 3080. If you set it at 50% throttle it will go to some intermediate rpm (straight line proportion has it at 1840, but it need not be along that line). So without any load it will go to that rpm and gov will control fuel rate to hold that rpm steady. No load probably about 10% load as some is needed just to spin the engine and churn the juices. If you could gradually add load, rpms will stay about constant, fuel rate will increase until load hits 100%, then any more load past that will bog the engine and rpm will drop.

In old skool diesel lingo, it is called a "variable speed gov" or generator governor. All it does is try to hold the rpm selected regardless of load.

So the governor inputs are throttle position, rpm and some sensor readings. Governor output is injector opening times, timing and rail pressure. There is an algorithm in the computer that converts the various sensor readings (rpm, rail pressure) combined with inj open times and calculates an expected burn rate and %load. It does not measure those things directly, but looks up on a chart from dyno data and displays that.

Highway vehicles use a different type governor (road load governor) that mimicks the behavior of a gasser. 8.2 Detroit is an example of a road load gov on a boat, and most folks don't like it.

Thanks Ski. So is the power lever on an electronic diesel just a rheostat(fly by wire) or is it mechanically linked to the governor? Sorry for all of the questions. It has taken me a long time to understand how a mechanical governor works and now they went and changed the rules on me!!!

Yea, I think some sort of rheostat aka variable resistor. I think there are two in each so it throws an error if the two don't agree, for safety. No mechanical connection from throttle to governor, all electrical/electronic.

Ski in NC said:

Highway vehicles use a different type governor (road load governor) that mimicks the behavior of a gasser. 8.2 Detroit is an example of a road load gov on a boat, and most folks don't like it.

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Interesting that the 8.2 Detroit has a road load type governor. I helped a friend move his boat with an 8.2 500 miles over a week and didn't feel anything different. But we were only using a fraction of its hp.

I suspect a marine type rpm governor would be much more noticeable in a road vehicle and probably unusable.

David

The only way you see an 8.2 act different than a var spd gov is when you try to rev it in neutral. If you raise rpm to say 1000, then it will shoot up to like 2000. It will not just sit there at 1500 like a normal marine engine.

If you drive the boat normally (start at idle, put in gear, add power) then it does nothing weird.

Baker said:

Thanks Ski. So is the power lever on an electronic diesel just a rheostat(fly by wire) or is it mechanically linked to the governor? Sorry for all of the questions. It has taken me a long time to understand how a mechanical governor works and now they went and changed the rules on me!!!

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It's going to vary from one mfg to another. But for us(cat) we use one of three different throttle inputs depending on application. Pwm is most common but some ecm's can be controlled via 0-5vdc or 4-20ma. In 0-5v mode you could use a trim pot to control speed by using the ecm's 5vdc reference. I know I have worked with a couple mtu's and they used 0-5vdc for speed control also.

Also most mechanical injection pumps have a flyweight setup to govern high idle. But this will vary from one engine to another. The most of smaller cat engine's are setup that way. When you get into large bore stuff like the 3500 series they rely on the governor to limit engine speed. The engine only has a fuel rack that's connected to the injectors.

fyi, off topic just a little. But, modern gas cars have rpm limiters, and they can limit at much less than redline, if out of gear. Just to name one case. Its all up to what the programmer puts in.

Ski in NC said:

So the QSB 380 high idle is probably 3080 for a 3000rpm engine, the mechanical 6BTA 370 rated at 3000, high idle is probably 3200 or so.

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On my original Cummins QSB 5.9L 380hp it only reached 2925rpm on sea trial. Ideally, it should have reached 3000-3050 meaning I am very slightly under propped. Not a problem considering how I run the boat. If I had a planing boat, I'd be happy with the 3050 rpm that the OPs engine hit at WOT during the sea trial.

dhays said:

On my original Cummins QSB 5.9L 380hp it only reached 2925rpm on sea trial. Ideally, it should have reached 3000-3050 meaning I am very slightly under propped. Not a problem considering how I run the boat. If I had a planing boat, I'd be happy with the 3050 rpm that the OPs engine hit at WOT during the sea trial.

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I think you mean you are slightly OVER propped? Since the engine doesn't reach full expected RPM?

twistedtree said:

I think you mean you are slightly OVER propped? Since the engine doesn't reach full expected RPM?

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I am pretty sure that is what he meant!

twistedtree said:

I think you mean you are slightly OVER propped? Since the engine doesn't reach full expected RPM?

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Baker said:

I am pretty sure that is what he meant!

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Yup, you are both correct. Over propped. Senior moment...

The boat builder would match the engine transmission and prop so why mess with it?
If you want to try and make improvements, have the prop tuned to a higher standard.
If you start change the prop with the hopes the a governor will protect your engine and drive line from damage is just a tad bit foolish IMO.
The alternative is to contact the builder and ask what prop specs he would recommend to accomplish your goals.

My AT does not have a ''load meter". It does have a RPM readout.

OldDan1943 said:

The boat builder would match the engine transmission and prop so why mess with it?
If you want to try and make improvements, have the prop tuned to a higher standard.
If you start change the prop with the hopes the a governor will protect your engine and drive line from damage is just a tad bit foolish IMO.
The alternative is to contact the builder and ask what prop specs he would recommend to accomplish your goals.

My AT does not have a ''load meter". It does have a RPM readout.

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Not sure you understood the question. And the builder USUALLY props the boat to spec on an empty dry boat. Boats gain weight over time. Anyway, the purpose of this post is if there is any performance left on the table. He may even gain efficiency by adding pitch if he can. When I purchased my boat it was underpropped. I added some pitch and gained a knot or two and am right on the money with the prop. It ain't a science my friend....as much as you(or the manufacturer) might think it is. And nobody is trying to do anything with the hopes the governor is going to catch it.

Baker said:

Not sure you understood the question. And the builder USUALLY props the boat to spec on an empty dry boat. Boats gain weight over time. Anyway, the purpose of this post is if there is any performance left on the table. He may even gain efficiency by adding pitch if he can. When I purchased my boat it was underpropped. I added some pitch and gained a knot or two and am right on the money with the prop. It ain't a science my friend....as much as you(or the manufacturer) might think it is. And nobody is trying to do anything with the hopes the governor is going to catch it.

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Ah ha, now I do understand the question, I think.
Now, 'we" have to determine how much weight we added to the boat.