60 degree Vee Twin 1.6cc

[edholly]

Have just run a back to back test on the Owen Mate original and the Owen Mate Vee Twin or as I referred to it here the V-HOT.

Using an 11 x 4 Turnigy wood prop the Owen Mate gave 6500 rpm and the Vee Twin gave 7350 rpm.

After running the Vee at the Cowra Oily Hand weekend have wanted to do a comparison, same fuel, same atmospherics etc ... and I must admit that I am surprised that the Vee Twin is actually quite a bit more power than the original single.

The puzzling thing is, moving the No.2 cylinder compression lever makes no appreciable difference to the power/revs - until it gets to an overcompressed point, in other words No.2 appears to contribute little from the "explosion" within the cylinder - but more than compensates by supercharging the No1 cylinder.

After the Cowra weekend's running I regained a lot of respect for this engine and although No.2 doesn't appear tp contribute much it must be doing so as the cylinder muff does get hot and burnt mixture does emit from the exhaust the same as No.1

So - conclusion now changed to - yes it is a success, and it's nowhere near as complex to make as the inline twin - so would welcome anyone else that wanted to make it. Now maybe if a CAD drawing was done it would be very helpful ...

Here's the latest running - a 5 second video

And again - thanks to all of you who encouraged me to build this one and gave advice along the way.

Oh, there a video of the engine running at Cowra ... taken by Peter Vanderwaterbeem me with my best hat on too ... thanks Peter !
https://www.facebook.com/video.php?v=847470441953266&set=vb.500961986604115&type=2&theater

Ed

[ame]https://www.youtube.com/watch?v=FOeYqfAFxTM&feature=youtu.be[/ame]

 

[Niels Abildgaard]

Congratulations.
All of us get older with time and few get wiser and have the class to admit it.
The power gain relative to original is the rpm relation cubed ie 40%.
Not bad for a prototype.
you have mentioned engine weigth/mass somewhere I cannot find and the Vtwin is comfotably good also.
If I was You better balance were my next goal.Look at a Ducatti V2 crank.
It needs more counterweigth than feasible on an overhung crank.Either split bottom conrods or a pressed up crank.
If You give us bore and stroke,piston and conrod mass,conrod length and crankpin dimension I will sketch one.

 

[GailInNM]

Ed,
Interesting numbers and conclusions. I appreciate you posting them.
The engine looks and runs great.
Thm:Thm:
Gail in NM

 

[edholly]

Neils, Gail, you have opened my eyes somewhat to the true power of this 4cc Vee Twin.

If you go back to my first post in this thread, I estimated that the efficiency of a 60 degree Vee Twin would be 75% that of a single.

Now the Owen Mate is 2cc, the Vee Twin Mate is double at 4cc.

Neils says, (which I wasn't aware of) the power is the cube of the revs, 7350 cubed is 44.4% more than 6500 cubed. So if the Owen Mate makes 100% then the Vee Twin makes 144.4% not double which would be 200% of course.

Now 144 out of 200 is 72% - not far off my estimate of 75% -SO - however the Vee Twin does it, by extra pumping into No.1 or actually making power out of No.2 it does show it's efficiency is within the realms of the theoretical maximum of 75%.

Mmm, I look at it now sitting here on the shelf beside me with quite a bit of respect now ...

cheers

Ed

 

[dman]

Some of what is said above is correct to the best of the ability to test but isn't the whole story. For one the calculation the delta in rpm cubed shows the change in power output is technically correct. This should be about the change in power exerted to the propeller. However this only proves the engine makes 44% more power at at different point in rpm which may be after torque rapidly falls. It only shows that the change in power is at minimum 44%, it may be much more on a dyno where full torque can be measured at identical rpms. It also only proves the torque is at minimum 27-28% more. Which makes isn't any more relevant than the power comparison because likewise the rpms are different, but shows a performance increase much less than your the 75% guess..

As far as the math saying the pumping is 75% well this is flawed. As is the 50% number for 90degrees. These numbers are what you get when you look at volumes of one cylinder at tdc and bdc. However these points are not the points where the crankcase has maximum or minimum volume. In fact a 60degree twin crankcase pumps 86.6% the volume of the ideal value. At 45 degrees we are up over 90%. A 90degree engine can pump 70.7%. Now what I ha e to wonder is how much pressure remains in the cylinder during the transfer phase (remember exhaust is open at this point so it won't be much) and can you use the high velocity in the transfer ports at high rpm to give a venturi effect to extract additional air from the carb and not restrict the cylinder to the crankcase air? Or use the momentum of the transfer ports and/or exhaust scavenging to ocerscavenge the crankcase and pull in more air from the carb via a reed valve?

Obviously there are many design variables and reaching full potential from an odly phased common case two stroke will be tricky... I do enjoy this thread. And love to see that thing running. Maybe the rpm tests can be repeated with a bigger or smaller prop, perhaps a different load will show a larger delta without special fabrication. Or a dyno can be mocked up with a fish scale on an arm fixed to a variable brake like an electric motor shunted through a large potentiometer or a current regulator, or a centrifugal water pump with a valve on the inlet. Really any number of things can be used as a dyno. You could mount the engine so that it swivels on the axis of the output shaft and support it with a fish scale. This way any number of things can be used as a brake including a flywheel or propeller. This could be easier than a fully inertia dyno because torque would be measured not calculated so fast data acquisition on the rpms wouldn't be needed to measure changes in rpm over time. A setup where the engine torques its own case and a non adjustable brake might not be great for reading power unless you can read rpm and torque simultaneously and integrated in real time. If you're not an electronics wiz it can be done mechanically or you can just compare peak torque

 

[Niels Abildgaard]

A V2 90 degree common crank two stroke will be smoother than silk and a lot of trouble in full size and model engines come from imperfect balance.
It has been stated that power cannot be equal from the two cylinders,but has not yet been tried with tuned exhausts.
Power /mass was very good for the 60 degree so it is not a dead end.Yet at least.
To get best balance,pistons need be light and equal mass.
If the following cylinder is made bigger in bore,work and maximum downforce can be equal and as maximum firing pressure is lower so piston masses can be equal.
Cooling fin area needed is more or less the same and seen from the outside it will be nearly impossible to se the bigger bore.
Also as maximun cylinder pressure is less,wall thicknes can be less as well.

 

[Brian Rupnow]

Ed--Congratulations on "Going boldly where no man has gone before"---You do nice work. And I must say, I'm very pleased to see that I'm not the only guy who designs something, screws it up while machining it, then changes the drawing to reflect what I built!!!---Brian

 

[edholly]

Neils, would be good for sure to compare a 90 to a 60 Vee Twin. I have other projects (an old Lotus Elite and giving the Brabham BT6 a birthday) lined up for next year or more so won't be able to build too much over this time frame, although I snuck in a few hours this morn on my .5cc engine ... find I enjoy this more now than car work!

Brian, you make me laugh with your caption ... thanks for the compliment ! Must say the 60 degree thing was a bit of a passion and the end result was well worth the effort. It wasn't that much harder to build than a single but much more rewarding, especially the double Owen Mate one, as it truely is a useable engine.

cheers ... Ed

 

[dman]

It was discussed earlier about the pumping imbalance because the leading cylinder is already flowing before the trailing cylinder ports are open and that at this point the piston velocity is already near its highest. And that this was maybe offset with a small port on the leading cylinder. But what if the perimeter of the crank was used as a rotary valve that controls flow to the transfer ports and the leading cylinder transfer port was interrupted as the trailing cylinder reached the proper angle?

I think this could work on a 45 or 60 degree twin. A 90degree twin might need something additional for a smooth running engine because the angles are pretty far off. The engine may balance well but it could have other issues with dead cylinders under certain conditions.

 

[Niels Abildgaard]

Aircraft engines ,be they small or big are not really judged on power per cubic but on power/mass and fuel consumption.
Tests on a 175 cubic ccm cylinder showed best fuel consumption at a mixture of roughly half exhaust and half fresh charge.
The bigger following cylinder idea of mine deserves a test but I am getting old.
The problem is the crankshaft.You can buy small two stroke engines from China for peanuts and thus get ignition and cylinders and pistons but a suitable two rod crankshaft taking more than put,putting is a problem.