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PostPosted: August 30, 2016, 11:03 am 
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I think every 2-stroke person has mused about this, it looks so simple. But the crank case pressures in a high rev'ing 2-stroke are a lot higher than your think. It might work for some low rev diesel engine, but nobody has ever made it work for a performance vehicle. It would be the perfect solution, 4-stroke bottom end, positive oiling, the perfect solution.

Do the maths and give it a try.

Graham[/quote]

Hmmm.
Why would a regular 2-stroke need super high crankcase pressure?
Unblown 4-strokes manage to charge the combustion chamber quite well on just atmospheric pressure ~15psi
Add a blower and you can easily triple that. Seems like that would be enough.

Maybe I should build one. I wonder what would be a good donor engine.


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PostPosted: August 30, 2016, 12:24 pm 
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A 4-stroke as the assistance of a vacuum as the piston falls and the inlet valve is open to draw the fresh charge into the cylinder. A 2-stroke has no such vacuum as it has no valves and a gaping hole is the side of the cylinder, the exhaust port.


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PostPosted: August 30, 2016, 12:31 pm 
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Roots-type charger, forcing air directly into the cylinder at BDC (not pressurizing the crankcase) and real exhaust valves(several of them). Add a fuel injector and a spark plug in there. Easy peasy, if we are talking about building an engine from scratch and not trying to modify an existing 2 or 4 stroke engine. Maybe start with a wet sleeve 4-stroke engine that could be modified a bit more easily. Convert the intake valve to a 2nd exhaust valve and grind a new cam, add some intake ports o-ring sealed to the OD of the wet sleeve. Hmmmmmm........

You might just end up with the same power and weight as a supercharged 4 stroke......

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PostPosted: August 30, 2016, 1:46 pm 
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FastG wrote:
A 4-stroke as the assistance of a vacuum as the piston falls and the inlet valve is open to draw the fresh charge into the cylinder. A 2-stroke has no such vacuum as it has no valves and a gaping hole is the side of the cylinder, the exhaust port.


Yes, I'm aware of that. I'm just saying that if 15 psi vacuum is enough to charge a 4-stroke cylinder, then 15 psi of pressure should be enough to charge a 2-stroke.


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PostPosted: August 30, 2016, 4:12 pm 
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In reality, there isn't even 15 psi difference between the cylinder being filled and atmospheric pressure pushing the air in with a naturally aspirated 4 stroke. I wonder how hard it would be to convert a 4 stroke car engine to a 2 stroke by driving the camshaft 1:1 to the crank. It would need a positive displacement supercharger like the old Detroit diesels had, but you could also turbo it like they did. Would probably need a custom camshaft too. A 2 stroke doesn't have to use ports in the cylinder wall and a pressurized crankcase, that's just one of the simplest ways to build one. And the old Detroit's could be made to rev. Here's a video of a 12V71 going to 5800 rpm. It sure sounds good.

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PostPosted: September 1, 2016, 3:29 pm 
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Richard, take a look at this book. It's a standard reference I think on 2 stroke engine design.

http://dragonfly75.com/motorbike/2StrokeDesign.pdf

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