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Old 13-06-2010, 14:57   #1
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A Paen to Two-Stroke Engines

I just love two-stroke engines. Why? Let me count the ways.
A two-stroke engine has twice the power strokes for the same displacement. And twice the power strokes for the same RPM. So it has double the density of power-producing motions.
This is good. It means that, in comparison, four-stroke engines are doing a lot of messing around, before they get around to having a power stroke. They are doing Ė exhaust, intake, compression Ė pow! Itís like Ė letís do this, letís do that, then something else, and then maybe Ė A Power Stroke! While two strokes are more like: Pow! Pow! Pow! Pow! Pow!
I love the 25hp Mariner two-stroke outboard on my dinghy. But much more than that I love the 600cc, 120hp (!) Rotax engine in my snowmobile. That is one unbelievable power plant. In my life with engines (which have included 2.2 liter Porsche 911S engines, 4.4 liter BMW V8ís, Ducati Desmodromic and Honda 750 motorcycle mills, and other interesting motors), I canít think of a more impressive one. It has a specific output similar to a Formula 1 motor, but it is insanely light Ė it weighs 27 kilos (!!!). 120 horsepower, and 27 kilos Ė itís no wonder they are used as aircraft engines. But what is most brilliant about it is the way it feels. It will go from idle to redline in about 3 nanoseconds Ė you have power at your very fingertips. It is power incarnate; itís like your brain is directly connected to thrust with no intervening mechanisms. Itís like a continuous explosion going on under your right thumb. And it does all of this with a relaxed redline of 8000RPM, much less than four-stroke motorcycle engines I have been acquainted with. I have a thousand-odd hours on this particular motor (less than on my boatís main engine) without the slightest problem. You pull the recoil starter cord out until you feel compression, yank it a little, and it starts. A complete pussy cat, until you touch the throttle.
What is really interesting is the combination of the two-stroke operating principle with compression ignition, that is, the diesel cycle. Why is it that no one has ever made two stroke diesels other than Detroit Diesel? The Detroit Diesel uses a Rootes-type supercharger to build up a supply of compressed air that blows out the combustion products through four (!) open exhaust valves when the piston moves down to the bottom of the stroke. The cylinder now filled with fresh air, the piston moves up, ready for the next spurt of fuel and the next power stroke. So again you get Ė twice the density of power strokes, so twice the useful work, out of an engine of the same size, compared to a four-stroke. So you get much more power (approximately twice) for a given displacement, size, weight, bulk, cost of engine. Plus you have an easier time of balancing it, since youíve got power on every revolution; you donít have any ďemptyĒ down strokes. The Rootes blower saves the engine the two empty strokes Ė intake, and exhaust, doubling the effectiveness of the work of the engineís core. It seems like pure genius to me, compared to a four-stroke diesel. Detroit Diesel had fantastic success with them; millions were made, and I believe are still being made. But why did other makers not adopt the principle? I wish I had an engine like that on my boat.
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Old 13-06-2010, 16:26   #2
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Was at Mystic Seaport last week and saw something that, judging by your enthusiasm, would have made you weak in the knees

They have just got running a Weikmann (spelling may be wrong) 2 cycle, 2 cylinder, reed valve, dry sump, seperate crankcases with a oil-scavenging gallery, bevel gear driven external waterpump, chain driven fuel pump, manual cylinder priming petcocks, head and block seperate one piece casting for each cylinder, dog-clutched to a reversing pitch prop.

Bore 12 5/8", stroke 12 5/8" Built 1913
Met 3 of the engineers, part of the team which put 3300 hours in the rebuild. Will try to post a picture in near future
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Old 13-06-2010, 16:32   #3
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... They have just got running a Weikmann Wichmann ...
This one?
Wichmann Engine Co. type 2 AB
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Old 13-06-2010, 16:50   #4
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I have never heard anyone wax so eloquently about 2-strokes.

I still have a soft spot for the old 6-71 Detroits as well. The sound of one brings back memories of Sea Scouts in my youth. The 2-71 gensets bring back the same memories. All the Sea Scout boats had those engines back then. They were always ex military boats and engines. They were also relatively Sea Scout proof.

I once brought one back to life after it had been submerged for a few days. I flushed out the vital fluids and it fired right back up like nothing had happened. Cant do that with a computer controlled engine.

6-71.....


2-71...minus the radiator of course.
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Old 13-06-2010, 17:12   #5
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Thats the baby, Gord

A Stradivarias to a gearhead like me
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Old 13-06-2010, 18:44   #6
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I prefer a lower RPM world with fewer unburned hydrocarbons. Dave
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Old 13-06-2010, 19:39   #7
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Dockhead, two stroke diesels were also built by the Rootes, later Chrysler, in UK during 60's & 70's - the Commer TS3, interesting opposed piston design with 6 pistons moving in 3 cylinders. Quite a few found their way into boats in NZ, all leaked lots of oil like any English engine and made alot of noise.
There are a few prototype 2 strokes around, but reason MTU (Detroit owners)dropped them for commercial use is simple - emissions. However they are still manufactured for military use.
I think you will find some of those multi storied diesels built for ships are two strokes.
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Old 14-06-2010, 01:45   #8
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That is a beauty! You guys are right -- weak in the knees.

83 horsepower @ 250 rpm; one foot (plus) by one foot bore and stroke. Weighs 5 metric tons. Wow!

Looks like it was in continuous service until 1981.
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Old 14-06-2010, 05:01   #9
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David Pascoe did an excellent comparison:
Marine Engines - Comparing Diesel Types: Two Cycle, Four Cycle
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Old 14-06-2010, 05:59   #10
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I once owned a Commer dump truck in Australia which was powered by a Rootes TS3 supercharged boxer 2 stroke diesel,it was only 105 hp as i remember but they were draft horses,they used to put them in power boats back in the day(1960 -70s).
Steve.
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Old 14-06-2010, 07:55   #11
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Wartsila Sulzer still make 2-Stroke engines, so do MAN B&W

Wärtsilä RT-flex96C and RTA96C low-speed engines

MAN Diesel & Turbo - TWO-STROKE
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