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Ideal 40 hp/3000 rpm Direct drive inverted Two-stroke single cylinder engine?

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Niels

Well-Known Member
Joined
Aug 15, 2019
Messages
436
On another thread it is discussed if a half VW is the ideal 40 hp direct drive fourstroke,and I think it is.
Very close to at any rate and unbeatable in price
A single cylinder two stroke that makes 40hp/3000 rpm will be just as unpleasant torque variation wise and lower mass,so let us dream .
Yamaha,Evinrude,Suzuki and Mercury has stopped making big two stroke outboard with fuel injection for various reasons.
We can find realistic numbers for achieveable piston velocities, mean effective pressure etc.
Most modern and biggest Yamaha I know of has cylinders of 93*82mm (557ccm) and makes 50 propshaft hp at 5000 motor rpm
This is Mean effectivePressure of ca 8.1 bar.
This is very high and possible because the exhaust from three cylinders in a bank interact.
Engine can run at piston speed of 15m per sec if wanted.
Combined with 3000 rpm it means maximum stroke can be 150mm.
The bore must then be at least 100 for clearance between cylinder and conrod.
This is 1.2 liter that combined with 50 rps and 8.1 bar gives 65 horsepower.
We ask for 40 and a single cylinder MEP of 8.1 is unrealistic unless we accept a tuned exhaust pipe or turbo.
Lets try 7bar.
This needs 855ccm and can be made by a 90 mm times 135mm cylinder.
One exhaust valve of 45mm diameter,two of 32mm or four of 23mm will pass exhaust from 40 hp.
 
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