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Old 05-14-2018, 10:16 AM   #41
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Hard to find the consumption map for a particular engine though. In your example, from 1000 - 2000 rpm at ordinarily achievable prop loads, the consumption looks pretty flat.

I also though max torque was always achieved at 100% load. That is unlikely at 1400 rpm on a small marine diesel, unless you have a variable pitch prop. Looking at the fuel consumption map, it doesn't appear that a variable pitch prop would offer much savings in fuel.
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Old 05-14-2018, 12:33 PM   #42
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FF,
Thanks for finally giving us some more info on the fuel map.

What engines that are common to fairly common in trawlers have an acessable fuel map? Or/and how does one find it?

There’s Steve DeAntonio and Tony Athens and probably a half a dozen other “experts” that have comunicated far and wide that have ventured an opinion on the load issue. Then there’s engineers. What you read or hear from manufacturers mostly comes from engineers. That’s why I prefer the opinions from engineers. And of course a sanitation engineer dosn’t measure up for our purposes.

Then there’s common sense. Would you buy a car that had never exceeded 30mph? There’s thrmostats that artificially raise the coolant temp to much much higher that it otherwise be. One should think about why they put them in engines. Ever seen an engine since WWI that lacked the thermostat? Running engines at optimum temps has long been a must do thing.
And there has been much written about engine oil and how hot it should be w the engine working. And of course there are different oils.

Much to consider for a pleasureboat skipper. When I was a young man very seriously sludged up engines were common. My father intrusted me w the job of “fixing” his 6cyl 1960 Chevrolet in about 1964. I ran the car w mostly kerosene and a quart or two of 50w oil. Didn’t save it. Many people switched to the then new detergent oils but very often considerable oil leaking and/or blowby and smoking would result. Theromstats, synthetic oil, detergents, viscosity improvers, ring design and other things have been implimented that help reduce the problems associated w sludging. But sludging is still an issue. Some makes and year models of newer cars are chronically burdened w sludging.

So to dismiss this issue on old diesel trawlers seems a knee jerk reaction.
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Old 05-14-2018, 02:34 PM   #43
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Quote:
Originally Posted by bayview View Post
Mark
" Max. torque is achieved at 1400 RPM, a 25% load."

How did you determine the torque at less than the normally presented max load curve?
From the engine performance chart JD once had at its website.
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Old 05-14-2018, 03:23 PM   #44
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Mark,
How do you (or do you) know/think the 1400rpm torque is at what your boat sees while underway or what the engine produces at WOT at 1400rpm on the dyno?
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Old 05-14-2018, 07:04 PM   #45
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Eric: based on JD chart. I don't have boat-specific info other than fuel consumption, RPM, and speed. Also, my understanding is that max torque is achieved at less than WOT.
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Old 05-14-2018, 07:32 PM   #46
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I can't claim a perfect understanding, but here's what I think happens:

The engine torque curve goes up and right and flattens. The prop absorption curve goes right and bends up. On a diesel there is no "throttle", rather there is a lever that sets the rpm governor set point. In turn, the rpm governor controls the fuel rack, and will attempt to add or pull fueling so that the governor commanded rpm is met. On a more modern common rail diesel this is all done electronically rather than mechanically, but works the same otherwise.

When you set 1400 rpm on the "throttle" - really the rpm governor - if there is enough torque in the engine to reach that speed it will add fuel until it does. If there is not enough torque, it will not reach that rpm and will be effectively overfueled, smoking. If there is more than enough torque, it will reach that rpm and the fuel rack will be pulled back by the governor, maintaining the rpm at a lower torque than maximum available.

A properly sized and pitched prop can only absorb the maximum torque the engine can produce at one point: maximum rated rpm, where the two curves meet. Anything less than that there is excess torque available, the rack will be pulled back, and the engine will be operating at less than full torque load. This difference is shown as the vertical space between the engine and prop torque curves supplied by the manufacturer.

So while maximum torque can be achieved by the diesel at less than WOT (in your case at 1400 rpm), it will not be operating that way on a boat. On a tractor let's say, you could set the "throttle" for 1400 rpm, then load the engine. As you increase the load, fuel will be added by the governor to maintain speed. When the load reaches the maximum torque the engine can produce, the governor and fuel rack can do no more, and the engine will slow down and begin to labor and smoke.

Common rail engines can recognize this condition and protect themselves from overfueling, but a mechanically governed engine isn't that smart.
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