Marine Diesels vs Truck Diesels - SailNet Community
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post #1 of 26 Old 02-25-2011 Thread Starter
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Marine Diesels vs Truck Diesels

Why is there so much talk of how sensative marine diesel engines are to idle and running rpm (best to run at high rpm) when diesel truck and equipment engines do not seem to have this problem? There are many semi rigs that sit idling there engines to keep the sleeper warm in the winter or idle for hours in traffic jams and these engines get many hours on them before overhaul.

Seems marine diesel are too sensative. Whats the story?
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post #2 of 26 Old 02-25-2011
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I've often wondered the same thing. Diesels are generally overbuilt and very rugged engines.

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post #3 of 26 Old 02-25-2011
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I think a lot has to do with how trucks are used vs boats. Trucks may idle a long time, but when driven they will be up and down through the rev range constantly. In a boat most often the engine runs at a constant rpm, sometimes for hours. I'm not an expert, but that is a big difference.

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post #4 of 26 Old 02-25-2011
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It appears there are different schools of thought on idling a diesel or any engine. The issues would be in-efficient cylinder/bore oiling and loads. The forces on the crank are smoother as speed increases, and oiling is more reliable.

Car and Boat diesels are generally the same class:
High-speed (approximately 1,000 rpm and greater) engines are used to power trucks (lorries), buses, tractors, cars, yachts, compressors, pumps and small electrical generators. As of 2008, most high-speed engines have direct injection. Many modern engines, particularly in on-highway applications, have common rail direct injection, which is cleaner burning.

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post #5 of 26 Old 02-25-2011
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Many truck and marine engines are the same basic engine. Having owned and operated about a hundred truck and equipment diesel engines as well as several marine engines I have never subscribed to the common belief/advice that marine engines will be harmed unless they are run at 80+-% of full RPM. It is very important to have the correct size and pitch prop which allows you to reach the full operation RPM when motoring in gear. An oversized or pitched prop is just like trying to climb a hill in a truck in too low a gear. It builds excessive heat both in the cooling system and in the exhaust and is extremely hard on the engine. As far as I'm concerned you shouldn't run a marine engine MORE than 80% for extended periods, but I fail to see how any damage can be done by running at a lower speed as long as the engine develops enough heat to run at normal operating temperature. When breaking in a new diesel I have found that running it hard will lessen the chance of the rings not seating but after that I tend to baby them by running them around 60-70% and I've yet to have any of the supposed problems caused by this (fouled injectors, carbon build up). In my opinion more damage is caused by short run times where the engine never gets up to operating temp or running the wrong prop than anything else (except perhaps lack of normal maintenance). Long idle times are a waste of fuel, and with seawater cooling the engine it won't be at normal operating temp so you won't be getting complete combustion of the fuel which CAN foul injectors, etc.

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post #6 of 26 Old 02-25-2011
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This is from "Gas Versus Diesel" by David Pascoe. "In pleasure craft use, diesels not only don't run continuously, but they are often rarely run. And in this case, it is the disuse that leads to their early demise. The reason for this is due to corrosion. An engine that is not running, especially for extended period of time like weeks, yet alone months, develops internal corrosion in all parts of the systems so that wear is greatly accelerated. An engine that is running all the time precludes most of this corrosion from occurring. Diesel engines in pleasure craft almost never wear out; they break down due to corrosion damage and other maintenance deficiencies."

It sounds right. Hope it helps.

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post #7 of 26 Old 02-25-2011
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For what it's worth, the manual for my Yanmar 2gm20 (see attached) advises one to "race" the engine every couple of hours when idling for extended periods.
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post #8 of 26 Old 02-26-2011
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Being truck driver and former owner of a few highway semi's I should say that idling of modern truck engine is a bad thing. Engines don't take it well and manuals say to bring RPM's up from 600 to about 1200 if engine is left idling overnight. Most trucks you see idling actually running at higher RPM than idle.

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post #9 of 26 Old 02-26-2011
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I think the real issue isn't idling the engine, but that has to be balanced off by running the engine hard enough to blow out the carbon deposits occasionally. If that isn't done, then idling is going to be a bad thing.


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post #10 of 26 Old 02-26-2011
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It's also a heat issue, a diesel needs to be run fairly hard to get it up to normal operating temperature for complete combustion, idling a diesel, particulary straight after an initial start to charge batteries does not allow this. Further, some makes of engine are more susceptible to cylinder glazing under these conditions due to poor oil control on the cylinder walls by the oil control rings of the piston. This is where the thoughts of running hard at at least 80% is required to achieve cylinder pressures that allow the piston and oil control rings to do their job properly whereas in actual fact it's probably just a way of overcoming bad design, manufacturing and materials.
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