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Old 12-07-2020, 07:15 PM   #1
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Diesel Two Stroke

I have been doing a bit of reading on Detroit Diesel and the 2 stroke design. They are very interesting. I found this YouTube video very informative on they way they work. Because I had such a hard time finding a good explanation I thought I would share should some one be looking in the future.
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Old 12-07-2020, 07:48 PM   #2
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Wow!, having taken apart numerous lawn mower two strokes, outboard motors and chain saw engines I had a rudimentary knowledge of two strokes. Being a gear head I also have a pretty good understanding of a four stroke.

This guy really clearly explained the workings of the Screaming Green Leakers, the Detroit two stroke.

pete
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Old 12-07-2020, 07:54 PM   #3
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I have 2 Detroits in my boat and I love them. Not sure why but I love the sound of those old girls. Also not the best in terms of fuel efficiency but hard to beat in terms of simplicity and reliability.
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Old 12-07-2020, 08:07 PM   #4
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Great engines. IMO the most reliable diesels of their class ever created by mankind. Much better than 4-strokes in that regards. No injection pump. Unit injectors which are easy to replace. Bypass oil filters. "Built in" fuel polishing system due to its huge volume of cooling diesel pumped each hour.

My Jimmies were from WW2. Very little drip - nothing like the stories you hear about. Rather poor fuel efficiency but if you only cruise 100 hours per year then that's no big deal.

Really, really, really loud. I would wear foam ear plugs underneath my ear muffs when I went into the engine room.
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Old 12-07-2020, 08:08 PM   #5
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Noisy two-strokes and Volvos are deal killers for some of us.
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Old 12-07-2020, 08:14 PM   #6
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For being designed in the 1930's, these engines have stood the test of time. Two-cycle Detroits are still in wide service throughout the world. The commonality of parts in a given engine series (53, 71, 92 etc) has never been duplicated.
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Old 12-07-2020, 08:36 PM   #7
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For being designed in the 1930's, these engines have stood the test of time. Two-cycle Detroits are still in wide service throughout the world. The commonality of parts in a given engine series (53, 71, 92 etc) has never been duplicated.
Doesn't matter to me as a pleasure boat owner not interested in everything aboard being drowned out by exhaust noise. I say this based on my own experiences. YMMV.
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Old 12-07-2020, 11:30 PM   #8
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Doesn't matter to me as a pleasure boat owner not interested in everything aboard being drowned out by exhaust noise. I say this based on my own experiences. YMMV.
Sometimes that’s a good thing.
I say this based on my own experiences.

Twin 8V-92TA’s in the house!
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Old 12-08-2020, 08:25 AM   #9
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" Rather poor fuel efficiency but if you only cruise 100 hours per year then that's no big deal."

The DD fuel use is usually on par with other installed engines. One hassle is they need to be loaded at about 60% to obtain 16HP/Gallon of fuel use.

This is done by specifying the right number of cylinders and injector size for the intended use. That is why there are so many cylinder sizes and choices.

It is true on a test bed the 4 strokes can claim better fuel burn BUT they too must be matched to the HP /RPM desired .

Either way a huge engine at tiny HP is not efficient.

Sadly engine size is often claimed "bigger is better" .
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Old 12-08-2020, 08:44 AM   #10
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The DD fuel use is usually on par with other installed engines. One hassle is they need to be loaded at about 60% to obtain 16HP/Gallon of fuel use.

This is done by specifying the right number of cylinders and injector size for the intended use. That is why there are so many cylinder sizes and choices.

It is true on a test bed the 4 strokes can claim better fuel burn BUT they too must be matched to the HP /RPM desired .

Either way a huge engine at tiny HP is not efficient.

Sadly engine size is often claimed "bigger is better" .

That's true that engines tend to be less efficient at light load. Gas engines are the most extreme example of this. N/A diesels suffer less, turbo diesels suffer even less typically. Detroits (especially non-turbo) lose more efficiency at light load than most modern diesels. And even at peak efficiency, they're less efficient. 16 HP / gal isn't good by modern standards. Most modern turbodiesels are in the 18 - 20 HP / gal range at peak efficiency and have a wider range of good efficiency.

As an example, based on the Cummins datasheets, a 380hp QSB 6.7 putting out 205hp is making just shy of 19.4 HP / gal. At WOT, it's about 18.3 HP/gal. For a Yanmar 8LV 370hp, the efficiency according to the datasheet is shockingly good. At 200hp, it's burning 9.9 gal/hr, which comes to 20.2 HP / gal. At WOT, it's about 19.5 HP / gal.
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Old 12-08-2020, 08:52 AM   #11
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Long before emissions issues killed the DD two strokes they were being abandoned by large fleet high hour industrial users. This demise was due to fuel usage and weight making them uneconomical.

The weight factor became a concern for long haul truckers and dirt movers who made profits by net tons hauled. In high hour generator use they were being replaced 40 years or more ago to lower costs per KWH.

Obviously if one has them in a low hour usage old boat they are perfectly acceptable and often sought after by aficionados. The cult factor is huge, just like the 57 Chevy. Perfectly acceptable for a reason to love them.

I've a relative with 16V92s in a boat. The increased fuel costs are not an issue in comparison to other vessel costs. His experience and shared by others means the DD 2 strokes will be around for awhile yet.
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Old 12-08-2020, 11:17 AM   #12
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Quote:
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Sometimes that’s a good thing.
I say this based on my own experiences.

Twin 8V-92TA’s in the house!
LOL. I'll raise you four 12V92TAs through the Panama Canal (New Jersey to San Diego) for the third fastest piloted transit in the canal's history.
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Old 12-08-2020, 12:57 PM   #13
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FF wrote;
“Either way a huge engine at tiny HP is not efficient.

Sadly engine size is often claimed "bigger is better" .”

From what I read only a small portion of the energy in fuel goes to do actual work. The majority is heat loss through numerous heat sinks like exhaust, cooling water, out from cylinder walls and combustion chamber surfaces.
And the bigger an engine we employ to get the job done the bigger the hest loss. The job is a fixed amount of energy needed and used but with bigger and bigger engines more and more heat is being lost. Lost to heat sea water and the atmosphere. But a little engine running hard looses the least amount of heat. And of course our engines are “heat engines”.
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Old 12-08-2020, 01:06 PM   #14
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I certainly did not buy my boat for its fuel efficiency. Note avatar.
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Old 12-08-2020, 01:19 PM   #15
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rslifkin wrote;
“As an example, based on the Cummins datasheets, a 380hp QSB 6.7 putting out 205hp is making just shy of 19.4 HP / gal. At WOT, it's about 18.3 HP/gal. For a Yanmar 8LV 370hp, the efficiency according to the datasheet is shockingly good. At 200hp, it's burning 9.9 gal/hr, which comes to 20.2 HP / gal. At WOT, it's about 19.5 HP / gal.”

Yes we’ve all heard the essence of that.
But almost nobody on this forum would run their engine that hard.

50 to 60% load is too high for most here judging from the conversation we read.

Re the DD it should be compared at typical loads and I’m not saying the DD will be far better or worse but it probably will be different and that is where they should be compared. But I fear at a 40% load the DD may even be worse re fuel burn.
But significantly the DD won’t burn many parts or maintenance otherwise. Could swing the favorable rating to the DD ..... ?

We have a tendency to think of new and old as more or less black and white. As an example many will use synthetic lube oil proclaiming it’s better. It is. But to what degree may be a very small degree indeed.

And people like to be seen as modern and progressive as users of things the represent the brand new stuff the only the best informed have. Whether or not it’s better or not can be a very different matter.
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Old 12-08-2020, 01:35 PM   #16
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LOL. I'll raise you four 12V92TAs through the Panama Canal (New Jersey to San Diego) for the third fastest piloted transit in the canal's history.
You base your condemnation of the DD's noise based on that ???
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Old 12-08-2020, 01:51 PM   #17
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The big brother to the Jimmy (EMD) is still in production, including EPA Tier IV variants with SCR Systems, Electronic Engine Controls (still using unit-pump injection), Separate loop after-cooling and closed crankcase ventilation.

https://www.progressrail.com/en/Segm...y_Engines.html

Unfortunately as with Jimmy's they are retiring faster than we can sell new ones, but at least we can sell something today
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Old 12-08-2020, 02:18 PM   #18
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Interesting mr. pacopico.
Thanks
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Old 12-08-2020, 02:29 PM   #19
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You base your condemnation of the DD's noise based on that ???
That you be silly of me, wouldn't it?

No I base my comparison of much lower power that that. And all you need do is heard one of those recreational level DDs light off in a marina to know how objectionable they are. Then there is the wait while that noisy thing goes away to wherever the owner is bound. Can't happen fast enough for all in the marina with four cycle anythings.
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Old 12-08-2020, 02:40 PM   #20
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I bought all my private boats because they had Detroit 2 cycles. Usually IL671s. I like the reliability. It doesn't matter how little fuel your engine needs if you're stuck with a dead engine.

Most people that don't like Detroits don't understand them and probably let the usual marina mechanic work on them.
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