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  1. #1

    Effects of Running Diesel at Low RPM

    With all the talk of running your boat at hull speed to save gas I thought I would post a question to see what specific engine problems one potentially faces by not running at ‘cruising rpm’ for extended periods. I have a 43 DC with Cummins VT 903s (turbos).
    Last edited by sgharford; 05-29-2008 at 12:53 PM.

  2. #2

    Re: Effects of Running Diesel at Low RPM

    It's temperature related. If the engine cooling system will allow the engine to reach proper operating temp at low RPM, then it's not an issue. Car engines can do this from idle on up; boat engines' cooling systems - at least older ones - are usually unable to achieve proper operating temp at low RPM (and often can't hold it down to proper operating temp high RPM either!)

    Consequently, lots of folks run the engines up to fairly high RPM periodically to "blow" everything out. Other folks question whether that really does anything useful but it often looks impressive and the smoke cloud may kill a lot of mosquitos. That's not a bad thing!

  3. Re: Effects of Running Diesel at Low RPM

    See Sam's for prior posts on this subject.

    I just tested my own 1972 8V71TI's at 1500 RPM and found turbo exhaust temps about 475 to almost 500 degrees...perhaps a bit cool for extended period operation, but not bad. Mike had previously suggested 525 degrees or higher might be acceptable...

    My engines are slightly overpropped and turn up about 2150 rather than 2300 at WOT. When I speed up to 1800 or 1900 RPM I get no smoke after perhaps 5 to 10 hours of 1500 RPM operation, slight smoke at around 15 to 20 hours operation at 1500 RPM which clears quickly...maybe 30 seconds maximum...I use 170 degree thermostats.

    Four cycle turbos, like Cummins, may have somewhat different operating characteristics and requirements.
    Rob Brueckner
    former 1972 48ft YF, 'Lazy Days'
    Boating isn't a matter of life and death: it's more important than that.

  4. #4

    Re: Effects of Running Diesel at Low RPM

    I tried a few searches related to this but could not find anything in depth. Seen various information pertaining to this in other post but nothing solely revolving around the issue. I always thought the biggest deal with not running hard was clogging the exhaust elbow (this came from my sailboat days after I installed an 18 HP Yanmar in my 30’ sailboat. Think someone else was calling these elbows "surge tubes" the other day).

    What happens to a turbo if exhaust gas not hot enough? Is it just lose of efficiency and/or do you hurt them mechanically as well?

  5. #5

    Re: Effects of Running Diesel at Low RPM

    Quote Originally Posted by MikeP View Post
    It's temperature related. If the engine cooling system will allow the engine to reach proper operating temp at low RPM, then it's not an issue. Car engines can do this from idle on up; boat engines' cooling systems - at least older ones - are usually unable to achieve proper operating temp at low RPM (and often can't hold it down to proper operating temp high RPM either!)

    Consequently, lots of folks run the engines up to fairly high RPM periodically to "blow" everything out. Other folks question whether that really does anything useful but it often looks impressive and the smoke cloud may kill a lot of mosquitos. That's not a bad thing!

    Jim Stitz the long time J & T service manager said the same thing. Engine runs at design temp you are good to go. Too cool and raw fuel leaves the exhaust after not being combusted in the chamber.

    Older Burgers would swap injectors to make the long run to Puerto Rico or Mexico. remove the 105mm and use 90mm for long range. Change the jumper lines.
    "A man must know his limitations" Clint Eastwood

  6. #6

    Re: Effects of Running Diesel at Low RPM

    It is my understanding that low load operation, at least in DDs, does not generate sufficient combustion chamber pressure to force the rings into the cylinder walls with sufficient force. This causes glazing of the cylinder walls and increased blow-by of combustion gasses past the rings into the crankcase. Eventually blow-by deposits form behind the rings and they become stuck. At some point the a ring breaks and wrecks the cylinder.

    If the engine is not brought to normal operating temperature (oil temperature) for a sufficiently long period there is not enough heat to evaporate moisture in the oil that comes from the blow-by gasses. The carbon in the fuel burns to form CO2 (and CO if combustion is incomplete). The hydrogen burns to form water. CO2 and water forms carbonic acid. The sulfur in the fuel forms (after a couple of chemical reactions) sulphuric acid.

    Increased blow-by of the nasties above from low load operation adds significantly to the normal rate of oil contamination. The additive package in the oil will give out much sooner and allow the acids to start working on the rod, main and cam bearings.

    At least on a normally aspirated engine, it is not the RPM that matters, but the load. If you want to cruise for long periods at low RPM, prop up so that the engine is normally loaded and you will avoid all of this trouble. Of course you have to be sure your gear can take the loads imposed at the lower speed (oil pump capacity and hydraulic pressure on the discs) and that the shaft can take the increased torque. Most of the gears and shafts in our boats probably can.

  7. #7

    Re: Effects of Running Diesel at Low RPM

    The water jacket temperature can and should be at operating range at low RPM but your still doing long term harm because the combustion temp is to low. There is no direct link to jacket temp and combustion temp you could be 180 at idle no load with no combustion temp at all. Or you could be 150 at 2300 with high combustion temps. On DDs around 400 is the absolute minimum 475 to 700 is perfect. On 4 stroke engines the ranges are much higher.

    Brian

  8. #8

    Re: Effects of Running Diesel at Low RPM

    Detroit Diesel specifically recommends running 8V71TIs at least 900 RPM wherever possible to limit carbon formation in the rings, but of course, it's not always possible.

    We put 6,000 miles on our 8V71TIs last year and a lot of it had to be at lower speeds, like 900 to 1,200 RPMs. Based on this forum, I ran them up on plane at about 1950 RPMs at least once per day for at least 45 minutes to get them to operating temps. Of course, there were lots of days when I couldn't, like in the Trent Severn canal, but I did heat them up when I could. It's fun too!

    We plan to cruise the Tennessee river system this Fall and then go to Florida in January to April. I'll probably go at hull speed a lot to save fuel, so I'll do the same to heat them up daily.

    Doug
    Last edited by Nonchalant1; 05-29-2008 at 09:45 PM.

  9. #9

    Re: Effects of Running Diesel at Low RPM

    Do I understand this correctly - the reason for running under “load” is important as it corresponds to combustion chamber temperature? Since diesels use a constant volume of air in compression and only vary the amount of fuel sprayed in, I assume the additional fuel needed to run at the same RPM under load is what is causing the increased combustion temperatures. Additionally, more fuel leads to bigger bang which is higher pressure (higher pressure also corresponds to higher temp in fixed volume – whole working principal behind diesels) which leads to more fuel burned completely in combustion, pushing the rings out, and clearing out deposits.

    So in order to really pull this off correctly one would need to mount temperature sensors somewhere in the exhaust line, correct? Would you place temp sensors before the turbos, which would seem to make sense to get most accurate reading.

  10. #10

    Re: Effects of Running Diesel at Low RPM

    Pyrometers are mounted in the exhaust manifold before the turbos. They can give you some great info on your engine. On a vee, you would have one on each bank.
    Sky Cheney
    1985 53EDMY, Hull #CN759, "Rebecca"
    ELYC on White Lake--Montague, MI

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