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Old 05-07-2012, 12:48 PM   #1
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Detroit two stroke

There has been some comment on the relative inefficiency of the two stroke Detroit when compared to a 4 stroke diesel engine. I'm wondering if there may be some confusion based on the difference between a two stroke gas engine and a four stroke gas engine. Modern electronically controlled four stroke/ two stroke gas or diesel engines are more efficient than there mechanically controlled ancestors. However are the older mechanically controlled 4 strokes cleaner and more fuel efficient than the two stroke Detroit? If so why and how do you know this. When I look at the difference what I see is an engine firing twice as often burning the same fuel controlled by mechanical injection into the combustion chamber same as the 4 stroke diesel. The difference being that the intake charge is through ports in the cylinder liner at the bottom of the piston stroke rather than valves in the head. A roots blower pressurizes the air in a plenum that charges the cylinder through these ports. This allows the 4 valves in the head to be exhaust valves. The expanding hot gases from combustion escape through these valves aided by the fresh charge from the pressurized combustion air entering through the intake ports. These engines fire on every revolution thus twice as often as a four stroke. MTU continues to manufacture these engines for military and industrial use . Over 3,500,000 of these engines were built between 1940 and now with an estimated 300,000 still at work. So my question is this, are they really less efficient? We know they are very durable. Are we comparing the efficient modern electronically controlled diesel with old technology? Is the Lehman , Volvo , Perkins, more efficient than the Detroit?
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Old 05-07-2012, 02:37 PM   #2
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Scary,
Do'nt mean to sound like a smart ass but to me it's a no brainer.

2 strokes over scavenge.

That is they exhaust more than their exhaust and tend to leave less air in the chamber when the compression stroke begins. This leaves less air in chamber to compress so less pressure is present at TDC. Also this extra air cools the cylinder causing thermal loss and less efficiency.

If the blower pumps more air into the chamber than it can use it's energy lost. And if the blower pumps less than is required or needed less pressure will result.

Everything is not always right in a 2 stroke. There is always some hot oxygen starved air remaining in the cylinder when the compression starts. And the blower is required to pump a bit of excess air out into the exhaust ...... a waste of energy.

Lats but by no means least is that the plower takes gobs of power to run.

The above applies only to DD type diesels w poppet exhaust valves.

This may not all be totally correct but you prolly get the idea.
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Old 05-08-2012, 05:29 PM   #3
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How much efficiency loss are we talking about? 10%? 20%. The reason I ask is that I'm under the impression that the venerable DD is one of if not just about the most reliable engine out there (in trawler HP ratings). SO......say you burn 300 gals of fuel/yr. The DD would burn 330-360 or roughly $2-300 more per year given a more frequent oil change schedule. And the engines are usually exponentially cheaper to buy if repowering than a new whiz-bang EPA fair haired pet of the month.
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Old 05-08-2012, 07:01 PM   #4
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twisted71,
That's what I hear/read. Sound insulation could solve most of the noise problem I think and the fuel burn is'nt much more but having not lived w a DD I'm not sure about the oil leakage. If I had a bigger trawler and needed to repower I'd give the Detroit's serious consideration.
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Old 05-08-2012, 07:23 PM   #5
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When we took Perry's (Otisguy) Hatteras 48 LRC from Panama to Guatemala last January. 800 miles offshore the 4-53's (na) burned just under 4 gallons/hour and used a gallon of oil each. 1800 rpm averaged about 8 knots. Noisy in the engine room but the Hatteras quiets them down to a purrrr. Love those engines and love that boat.
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Old 05-08-2012, 08:39 PM   #6
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I have a similar 48lrc

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When we took Perry's (Otisguy) Hatteras 48 LRC from Panama to Guatemala last January. 800 miles offshore the 4-53's (na) burned just under 4 gallons/hour and used a gallon of oil each. 1800 rpm averaged about 8 knots. Noisy in the engine room but the Hatteras quiets them down to a purrrr. Love those engines and love that boat.
I have similar fuel burn, covered 666 knmiles open ocean and burned 290 gallons of fuel and 2 quarts of oil. Airseps or catch cans solve most of the oil leaks. I believe one of the other members has a rare 48LRC with Lehmans. It would interesting to compare his fuel burn at 8knts. I ran a little slower at 1600 - 1700 as we had fairly high winds in the mid 30's and as much as 45knts off the Mendocino coast. Following seas may have given me a boost. With 1450 gallons of fuel I've got close to 3000 kmiles of range at 8knt. Slow to 6knt and it adds at least another 1000.
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Old 05-08-2012, 09:06 PM   #7
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Quote:
Originally Posted by Anode View Post
When we took Perry's (Otisguy) Hatteras 48 LRC from Panama to Guatemala last January. 800 miles offshore the 4-53's (na) burned just under 4 gallons/hour and used a gallon of oil each. 1800 rpm averaged about 8 knots. Noisy in the engine room but the Hatteras quiets them down to a purrrr. Love those engines and love that boat.
That just gave me a fat wood!
I've been sceptical of the p.o. statement that the boat gets 1mile/gallon. Gensets running and all. Maybe It will get a little better. I wish I had some good audio of them running. Sounds awesome, engines running and all. Until I ran out diesel on the port main. These are the easiest diesel engines to get running again i've ever worked on. I can't wait to get rolling.
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Old 05-08-2012, 10:56 PM   #8
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Scary

The genset business and diesel irrigation pumps were dominated by DDs until the early 70s. As the cost of fuel went up the 4 strokes started paying off quickly in fuel savings. For genset use, a 4 stroke from the 80s was not nearly as efficient as those that came along a decade later as rudimentary computer controls and JWAC improvements were realized. Although years ago, I seem to remember the DDs were about 25% more in fuel burn as gensets vs say a Cat 343. We calculated that if GM gave us the engines they were not cost effective vs Cats

It is astounding what the big diesels can do today with onboard precise fuel mapping. In my last big job we purchased many large haul trucks from Cat with 4000 HP engines that could maintain full power at 15,000 feet in South American mines. MTU and Cummins were a year or two behind.
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Old 05-08-2012, 11:19 PM   #9
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Doesn't this all depend on how one measures "efficiency"? My former Seagull five (or was it six?) horsepower, two-stroke engine (paired to my twenty-foot pocket cutter) seemed efficient, as far as weight and size goes.

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Old 05-09-2012, 01:27 AM   #10
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Tom,
I do'nt think the fuel burn of a gen can be realistically compared to a boat engine.......unless the gen is running at 50 to 75% constant load. Some gens run at 5% load most of the time. A seemingly sharp fellow on Boat Diesel said a DD should be about the same fuel burn as an old Lehman. Compare w a turbo Cummins is way different.
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Old 05-09-2012, 06:43 AM   #11
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The DD 2 strokes will produce about 16Hp from each gallon of fuel,
IF they are run in the proper range , usually at 60%+ of rated power.

The same engine will have ratings at various RPM 1200 prime generator , 1500 Euro 50cps or US 60CPS at 1800 for a demand gen set.

All will get similar fuel burn with proper loading (over 60%) at the selected RPM.

AS boat engines IF the selection was proper by the NA and not screwed up by the addvert guys , the efficiency is just fine.

Most more "efficient " old 4 stroke engines (18hp/gal/hr) have the same problem ,although the loading has to be higher at the selected RPM, usually closer to 80%.

Underload a 4 stroke and they get as fuelish as as other engine , producing 12 or less HP per gallon an hour.

"2 strokes over scavenge.

That is they exhaust more than their exhaust and tend to leave less air in the chamber when the compression stroke begins. This leaves less air in chamber to compress so less pressure is present at TDC. Also this extra air cools the cylinder causing thermal loss and less efficiency."

Not really , when the exhaust valves close there is usually a posative (tho minor ) pressure in the cylinder , so the engine is not considered supercharged, with perhaps 1 lb of boost.

The amount of scavenging air is higher than in most 4 strokes , but hardle enough to overcool the cylinder . The DD engineers had since 1938 to fine tune the cam timing. The exhaust valves open (as they must) before the scavenging blower ports are uncovered , but un like a chain saw the exhaust valves are closed before the scavenge ports are covered , causing the boost.

The only folks I know of that did this with out valves was PUCH motorcycles , that used a "split single" design that uncovered the exhaust first , but was able to close it before the intake was closed.

A modern large industrial diesel can create 22 to 24HP from a gal of diesel, BUT the electric injectors and brain box are a constant risk to an electrical glitch , or lightning side strike.

New multi-fuel LARGE!!! ship engines are over 60% efficient , and very complex , even the 2 strokes.

None of their tech is useful down in the 3-5 gph engines we need.

The best hope for "efficiency" is probably VW , BMW or Mercedes diesels , marinized .
But the high initial and later parts cost doesn't make sense in 200 -400 hours a year boat use. Especially where weight is not a huge problem.

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Old 05-09-2012, 07:24 AM   #12
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The best hope for "efficiency" is probably VW , BMW or Mercedes diesels , marinized .
A year or so back, I delivered a sailboat from Hollywood, Fl to Barnegat, NJ. Conditions were such we motored or motor-sailed most of the way. I developed a love hate relationship with this little Mercedes. When you're not buying the fuel, all you want them to do is get you there. It did with a little coaxing.
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Old 05-09-2012, 11:22 AM   #13
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The Eagle has a 1977, DD 671, 165 hp, natural green leaker. As near as I can tell we get 1.5 to 2 miles per gallon at hull speed, 1500 rpm, 7 to 10 knots. I been told our 671 is one of the cleanest, very small/few leaks and smoothest running DD 671. The larges oil leak is actually from the Allison transmission real seal, not the 671. The DD 671 few/small leaks are taken care of by plastic containers secured to the engine, and/or absorption pad under the engine. The only reason I bought a single engine boat is because the Eagle had a DD 671 engine, which is one of the most reliable, proofing engines in the world.

Besides, if there is additional fuel cost, itís a very small % of the over all cost of owning a boat for most pleasure boats.
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Old 05-09-2012, 12:02 PM   #14
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Eric

Gensets are the great equalizers in fuel burn discussions. For the past 30 or so years published genset data closely matches field reality. That is why the DDs disappeared.

For boats, you'll find on boatdiesel those who have swapped DDs for modern engines routinely report a 20 to 30 % fuel improvement at the same speed in the same vessel. In my mining experience there is no debate, much worse (please don't ask me to verify reports and findings from 30 years ago, long since buried) for the DDs over hundreds of large engines operating at 70 - 100% load. For large yachts with say 16V92s, which direction do you think buyers and owners are running, yup away.

Why did the Cat 3208 become so instantly popular 30 years ago in over the road use, it replaced 6-71s. Even without taking into account far worse emissions from a DD, the big users voted with their checkbooks many decades ago - 4 stroke all the way.

DDs are great Forum talking points if you have them in a vessel which sits at the dock like Phil and FF. If you travel many thousands of miles per year in your boat they are not so great. At the end of the day though, it really doen't matter, these discussions are not for business, they are for fun.
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Old 05-09-2012, 12:04 PM   #15
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Test performance of 48lrc detroit 453n

This data was produced by Motor Boat and sailing in 1976:
knots 4 nautical mpg 4
5kn- 3.5 knmpg
6kn- 3 knmpg
7kn-1.75 knmpg
8kn-1.33 knmpg
9kn-1 knmpg
10kn-.75 knmpg
This boat was stabilized like mine and my boat seems to perform about the same. My boat weighed 66,000lb at haul out. If you look at the brochures for Nordhavn and Kady Krogan their big advertized ranges are all at speed at or below 6 knts. They also tested this boat running on a single 453 with the other engine in neutral prop free wheeling. The engine running at 1800 rpm gave the same speed as both running a 1600 rpm giving a effective 44% increase in range.
When I bought this boat I considered re-powering with a couple of JD's, the 60k plus to do it buy's a lifetime of cruising fuel or maybe a trip twice around the world. The ease and low expense of working on these old Detroit's, parts availability in third world counties like Mexico or some logging camp in Canada persuaded me to keep them in the boat. So I spent the money updating electronics and waste treatment. When I brought the boat down from Washington to Oregon I found the combination of waves and 1/2 knt current had me running a 1200 rpm to time my approach to the Columbia River Bar at dawn. I was still maintaining 6.33 knts doing better than 3 kmpg. I'm sure when I return to the frozen north the waves and current will work against me probably knocking down to 2knmg, still not bad.
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Old 05-09-2012, 12:17 PM   #16
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Cat 3208

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Eric

Gensets are the great equalizers in fuel burn discussions. For the past 30 or so years published genset data closely matches field reality. That is why the DDs disappeared.

For boats, you'll find on boatdiesel those who have swapped DDs for modern engines routinely report a 20 to 30 % fuel improvement at the same speed in the same vessel. In my mining experience there is no debate, much worse (please don't ask me to verify reports and findings from 30 years ago, long since buried) for the DDs over hundreds of large engines operating at 70 - 100% load. For large yachts with say 16V92s, which direction do you think buyers and owners are running, yup away.

Why did the Cat 3208 become so instantly popular 30 years ago in over the road use, it replaced 6-71s. Even without taking into account far worse emissions from a DD, the big users voted with their checkbooks many decades ago - 4 stroke all the way.

DDs are great Forum talking points if you have them in a vessel which sits at the dock like Phil and FF. If you travel many thousands of miles per year in your boat they are not so great. At the end of the day though, it really doen't matter, these discussions are not for business, they are for fun.
The 3208 was built as a light duty truck engine for Ford light duty trucks. Much like the 5.9 Cummins that started as a Ag pump engine and got morphed into light duty marine applications and Dodge pickup engines. They are not sleeved engines and are not considered to be heavy duty commercial engines.
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Old 05-09-2012, 12:32 PM   #17
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The 3208 was built as a light duty truck engine for Ford light duty trucks. Much like the 5.9 Cummins that started as a Ag pump engine and got morphed into light duty marine applications and Dodge pickup engines. They are not sleeved engines and are not considered to be heavy duty commercial engines.
A snip of 3208 history...

Since information from Caterpillar is always held very tight within the family it was very hard to get an accurate history of how the relationship all came about, so if I’m a bit off in my timeline, forgive me. I assure you, it won’t affect the content of this article.
The bottom line is, Ford had a truck line with mammoth 477 and 534 cid gasoline engines and realized it needed a diesel for its fleet vehicles if they wanted to stay in the game. Somewhere in the late 1960s or early ’70s Caterpillar seemed to have an interest in getting into the medium duty fleet truck engine business, or Ford coerced them into it – opinions vary as to which is the most likely scenario. The point is, Ford had the trucks and Cat had the engines.
From 1968-1974 The Ford 6000 series trucks used the Caterpillar 1140 150 hp and 1145 175 hp diesel engines. In that same time, the 7000 and 8000 series had the Cat 1150 200 hp diesel. These early renditions were considered inexpensive “throw away” engines. There were very few oversize or undersize parts available, at least from the OEM. Obviously those were different times and certainly part of a much more vibrant economy, however the entrepreneurial spirit of engine remanufacturers and the automotive aftermarket never accepted the “disposable engine” mentality and so the story continued.

It's a little heavy for pickup trucks...especially back in the 70's...

People can talk all they want about "industrial" and "real marine engines"...well the first time I see a trawler driver treat their 3208 like a garbage truck or bus driver ( zero to full throttle every few feet to a city block for 8 or more hours a day...well then ya got me!
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Old 05-09-2012, 12:41 PM   #18
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Right you are Scary, but the (theoretically) better operating cost per mile is why the 3208s were born, not to mention their lighter weight which then can be translated into hauling freight vs DD iron. Business decisions enter into making the change discussion, although not on this Forum nor should they. Not asked, but why did the Eurropeans never strongly follow the DD route? You've got it, their much higher fuel costs. Nor Cat who has had to sell into the foreign markets for something approaching 80 years.

I clearly see why you like your 53s, no reason not to. Hatt made the right decision at the time.
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Old 05-09-2012, 08:01 PM   #19
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I'm not knocking the 3208 or 5.9 or the 444 Navistar

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Right you are Scary, but the (theoretically) better operating cost per mile is why the 3208s were born, not to mention their lighter weight which then can be translated into hauling freight vs DD iron. Business decisions enter into making the change discussion, although not on this Forum nor should they. Not asked, but why did the Eurropeans never strongly follow the DD route? You've got it, their much higher fuel costs. Nor Cat who has had to sell into the foreign markets for something approaching 80 years.

I clearly see why you like your 53s, no reason not to. Hatt made the right decision at the time.
All of these engines have proven their merit, my real point is that at the time of the roll out and early production of the DD 2 strokes they were close to the same efficiency as the 4 strokes of that period. Most all engines including the 4 cycle DD's produced later have cleaner emissions and better fuel efficiency, but so do all gas engines. Better metals and machine tolerances plus market demand and regulation have forced manufacturers to step up. Remember what happened when Chrysler rolled out the 5 year warranty in 1965, all of a sudden those Fords and Chevy's that were lucky to run 100,000 miles stopped burning oil and started lasting several hundred thousand. Things change as market demands. I really wanted to start a conversation and give the credit to the DD 53,71,92 series engines they deserve. Are they equal to any of the newer engines as to economy or emissions, no. They are however incredibly durable, that's one thing you can't take away from them.
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Old 05-09-2012, 10:05 PM   #20
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Or their smoothness.
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