Off Boost (Turbo)

My turbos on my Volvo D4-300's don't build up much if any boost until the engines build revs. Is there any danger in running the engines for extended periods at a speed in which the turbos don't spool up? Such as 1,800 rpm.

When I say "danger" I'm talking about the turbos getting buildup or something like that.

Thanks,
Mike

The short answer is no.

Usually the question you pose is something like: Is it ok to run my engine at low rpms for extended periods. The answer to that question is yes as long as the engine is up to operating temperatures.

For that engine I would say anything above 1,500 rpm is ok, less for lower speed engines like the Cummins, Perkins or Lehmans.

David

I prefer to cruise my boat just under where the turbo kicks in.

1. turbo's last longer
2. Engine burns less fuel
3. Its much quieter

I do like to run up to nearly full throttle for 30-45 sec every trip. Not sure if diesels build up carbon like gas engines but its just a habit I picked up.

No problem running at low load other than the engine outliving your grand kids.

I asked for that same of my Cummins common rail and Cummins replied I can run very well even from + 850rpm even 24/7. You also have a common rail, check that the machine runs the right temperatures for cooling and oil, so I think it's OK.


My answers:
Hi,

I have a Cummins 5.9 qsb 380hp marine engine 2009 and it reaches max rmp 3065 which is perfect my Nordic Tug 37.

I have read a lot of conflicting opinions on the engine to run at low rmp a long time, because the machine may damage the carbon and etc. Some say it's ok to run this type engine at low rmp if the coolant remains in the correct slot.

The time i run my engine is most often 850-1300 rpm since the boat is most economical in this rmp area and temperature is ok. Cummins runs fine and does not smoke any, exhaust pipe mouth environment does not show any black carbon.

What is the manufacturer's view of low load low rmp almost always, whether it is ok or damaging the engine?

It would be great to have an expert answer, all the web instead of rumors.


And Cummins say to me:
This is fine for our engines. It is not suggested to Idle (650-750rpm) for long periods. Generally speaking, you can idle for about 20 minutes or so at this range and be okay. If you plan to idle longer than 20 minutes or so it is suggested that you ramp up your RPM to about 800-1000. Working the engine under a light load/rpm is fine. This is not uncommon for some our engines (like Generators which normally work at 1800 RPM or less) and will not cause any undue harm.
Thank you for contacting Cummins.
Katelyn
Customer Care Representative


In fact I trust what Cummins says about other wise men ...
NBs

I run my 450C mechanical Cummins a LOT at low load. 600-650rpm for hours on end fishing, 950 for hull speed cruising. Two trips from NC to the Keys and back at 950 when fuel was expensive. But a good bit of 1800-2000rpm up on plane. At about 3600hrs and 18yrs old. No issues.

A high output diesel running at low load looks a whole lot like a similar size non-turbo engine rated at much less. The turbo just coasts along doing very little in any sense until you add power.

Still a good idea to power up once in a while to burn the carbon off. On mine, I get some smoke when I go up to 1800 after fishing, but it cooks off in a few minutes. No need to go full power to do this, 1800 on mine is about 35% of rated power. If I go up to 2100, no more smoke so it got clean at 1800.

I am a little queasy running mine at 950 or 600 for as long as I do. I do not think that is best for the engine. But that is what I want it to do, I'll take my lumps. But so far, no lumps.

Ski in NC said:

I run my 450C mechanical Cummins a LOT at low load. 600-650rpm for hours on end fishing, 950 for hull speed cruising. Two trips from NC to the Keys and back at 950 when fuel was expensive. But a good bit of 1800-2000rpm up on plane. At about 3600hrs and 18yrs old. No issues.

A high output diesel running at low load looks a whole lot like a similar size non-turbo engine rated at much less. The turbo just coasts along doing very little in any sense until you add power.

Still a good idea to power up once in a while to burn the carbon off. On mine, I get some smoke when I go up to 1800 after fishing, but it cooks off in a few minutes. No need to go full power to do this, 1800 on mine is about 35% of rated power. If I go up to 2100, no more smoke so it got clean at 1800.

I am a little queasy running mine at 950 or 600 for as long as I do. I do not think that is best for the engine. But that is what I want it to do, I'll take my lumps. But so far, no lumps.

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Hi

Could you think of installing a troling valve for your gear, could you run a slightly larger rmp and boat speed for a suitable fishing (same 600-950rmp)

Nbs

Boost is controlled by limiters called wastegates on nearly all diesels made during the past many decades. Without the wastegate the engine may well blow up say many as fuel delivery is advanced. Some larger engines have external wastegates but most have internal. At lower fuel settings the waste gate allows bypass of exhaust gas thus little if any turbo boost.

On my engines the waste gate starts diverting at vey low RPMs, say around 20% power draw.

Waste gates are not that common. My engine is without. And waste gates only open when charge air pressure gets up to the WG setpoint which is max boost. Many WG diesels the WG does not open anywhere along the prop load curve.

NBS- Considered the trolling valve, but since my engine seems to put up with my abuse, I'll avoid trading that for putting the gear at risk. I can put cylinder kits in my engine in a weekend for less than rebuilding that gear!!

Ski in NC said:

Waste gates are not that common.

Click to expand...

Humm, the lists for marine diesels with waste gates is pretty long according to my memory. So to double check amongst others I note D Marchand started a long thread on boatdiesel September 3, 2015. A very long list of WG engines was developed. Many smart contributors on thread 56966

I was under the impression trolling valves varied the clutch pressure to change the shaft speed & were supposed to be used at idle only. At least all of the ones I've seen operated that way.

sunchaser said:

Humm, the lists for marine diesels with waste gates is pretty long according to my memory. So to double check amongst others I note D Marchand started a long thread on boatdiesel September 3, 2015. A very long list of WG engines was developed. Many smart contributors on thread 56966

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I have been aboard maybe 50 to 60 turbocharged diesel boats and never saw one with a waste gate.

I went back to the thread I started over on boatdiesel that sunchaser referenced. Here is a partial list of marine engines with waste gates provided by Tony Athens:

Yanmar 4LHA 230-240, Yanmar 6LP 315 hp, Cummins 6CTA 480CE, Cummins QSB 5.9 425Hp & above, QSB 6.7 above 425HP, Cummins QSC 8.3 all, QSL9 -- all

I am surprised at sunchaser's statement that his wastegate opens at 20% load. That is unusual. Wastegates are usually closed at the peak torque rpms and only begin to open near rated rpm at wot.

But I am not sure why the presence of a wastegate is relevant to the OP's question, but in the interest of accuracy, here is the list.

David

jleonard said:

I have been aboard maybe 50 to 60 turbocharged diesel boats and never saw one with a waste gate.

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Check out the boatdiesel thread I mentioned in post # 10.

Most wastegates on lower HP engines are internal and not viewable until turbo opened up. Similar to your exhaust valves, no seeum until something removed. If interested, the internet is loaded with wastegate technology "how they work and why needed" articles.

For those with flying experience the Continental and Lycoming engines use oil pressure to control the wastegate. Most marine and auto wastegates are spring loaded for bypassing exhaust when turbo pressure builds up.

Lots of Volvos have WG also.

All the marine WG's have some features that are visible on turbo exterior.

Point of the WG on a marine engine is to use a smaller turbo so you get boost at a lower rpm. Makes planing a heavy boat easier. But then up near full power the little turbo would either make too much boost or choke exhaust flow. So WG dumps some exhaust around turbine.

All the marine units I have seen open simply on compressor pressure. Above X psi, starts to open. Aero units likely have a more complicated control algorithm due to altitude changes. There you probably want lower boost limit at low altitude to protect engine from detonation and overload, but at higher altitude take advantage of as much boost as you can get.

I have dyno'd marine engines with wastegates. Both the WG did not open until we went over rated rpm at full power. At rated power and rpm, it was right at the edge of opening. Give lever a little push and it would open and you could see boost drop. But valve was still on the seat, lightly.

I would like to see variable geometry turbines on marine engines. Got one in my car and the control over boost is really neat.

Unfortunately as an industry we can't seem to keep sea water out of our turbos, so VGT would not live long. That's probably why we don't see them.

djmarchand said:

I am surprised at sunchaser's statement that his wastegate opens at 20% load. That is unusual. David

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I am just parroting Cat/Perkins. They told me my mechanical turbo design was intended to have the turbo wastegate begin to open (whistle it does) at lower fuel and exhaust flows to deal with EPA and CARB diesel soot design limits. The wastegate is not on-off but sequential

Fact or fiction, doesn't make any sense to me either. But emissions chasing has spawned some weird things.

My little D2-75 has a wastegate. Part of my engine checks is to verify the rod moves after a long shutdown period. Recommended by the manual.

MichaelB1969 said:

My turbos on my Volvo D4-300's don't build up much if any boost until the engines build revs. Is there any danger in running the engines for extended periods at a speed in which the turbos don't spool up? Such as 1,800 rpm.

When I say "danger" I'm talking about the turbos getting buildup or something like that.

Thanks,
Mike

Click to expand...

I don’t see much point in discussing this without some good numbers.
Turbo boost, speed and EGT at 100 rpm intervals across the operating range would be a good discussion opener.
Also not sure what you mean by “spool up”, turbos spin whenever the motor is operating, faster or slower relative to exhaust gas volume, Excluding waste gate control.
Turbos are engineered to operate most effectively in a specific range, if it does not work on your rig, it’s back to the drawing board to re engineer propeller size and possibly gear ratio to “fit”the engines parameters to your particular vessels carachteristics.

"When I say "danger" I'm talking about the turbos getting buildup or something like that."

It has been my understanding that some making boost keeps the turbo cleaner.

In other words working a bit , rather than just being an exhaust restriction .