60 degree Vee Twin 1.6cc

[edholly]

Had a hankering to build a twin and after recently building an ML Midge thought it would be a good basis to go down this line of thought.

There is a horizontally opposed Midge and it is called the Dragonfly, but I could find little about it on the Web. So I thought why not build a Vee. Some sketches and calculations showed a 90 degree Vee would have serious pumping losses compared to a single in fact 50% loss for same capacity, but a 60 degree Vee only loses 25% efficiency.

Looked at a couple of other single cylinder engines and settled on the Midge as the underpinnings for stroke / bore etc.

I have carved the crankcase so far - invested about 6 hours with a pencil and paper and another 15 or so in making it. Made a couple of errors so it is going to be a little bit different to my original design, but it should still run the same. The main error was boring the 2nd cylinder hole in line with the first, instead of staggered the width of the conrod. Now this has a plus in that the engine will look neater and a bit lighter, but it has committed me to a fork and knife conrod. I am beefing up the crankpin from 3.2 to 4 mm, but I figure this may not be necessary as the load from double the capacity is spread over 2 pulses not one big hit.

Other specs are as per Midge 10mm bore, 10.16mm stroke, however it will have a ball bearing either end of a longer crankshaft, the crankpin will be longer to take both throws of the conrods, the engine will have a small bolt on plenum chamber above the Vee for a single venturi to feed into and the gases then spread internally to the port. You can see this division in the centre of the flat between the cylinders and the screw holes to bolt the chamber to.

It certainly should run I think, and I can't wait to hear the double pulse per revolution to see what it sounds like.

I will have to learn how to use CAD as I think this engine would make a good little project for those that want something a bit different.

I think I will call it the Butterfly, on another Forum a couple of other names have been suggested, but I think Butterfly sounds pretty good to go with Midge and Dragonfly.

Here's a couple of photos of work so far.

 

[kadora]

I will watch your progress closely.
Fingers crossed.

V twin admirer Kadora.

 

[barnesrickw]

What kind of crankshaft magic are you using? I've been trying to figure out a two cycle v-twin for a while. Looked into this

And this






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[dman]

the pumping loss is only 29.3% for a 90* twin but the phasing is all goofy, the truth though is the lower end pumping has more to do with start and idle than power, if you have a reed rather than timing everything and it can pass through the reed directly to the cylinder at the appropriate time then you will have ample pumping energy in the exhaust. also a 90* case doesn't mean you have to share a crank pin. you could build it as 90* but have 45* pulse seperation if you use seperate pins. anyway just throwing things out there.

at 60* you are only losing 13.4% and at 45* (like a harley davidson, if you wanted to dress it up like a real engine or something) you are down to 7.6%

 

[mu38&Bg#]

Common crankcase V-twin two strokes have been done before. A friend of mine had one made with Cox cylinders, though I don't think he ever ran it. The guy who built it said it ran fine.

 

[Niels Abildgaard]

Hello Edholly.
When You prove, as I am sure You will ,that power is as good as from two single cylinders next step is to make a 90 degree where balance is nearly perfect.
When You get there and if You want help to calculate counterweigth size ,send me a mail.
You are probably about to prove that two cylinder two stroke construction has been much to complicated for the last 100 years.Better late than never.

 

[johnny1320]

I don't believe that a common crankcase v2 can work, not without forced induction. I have done CAD models of this and found one cylinder doesn't charger properly. I know of one other person that tried it and he had nothing but problems. the only other way I could see it work is to off set the crank pins.

 

[dman]

i may just have to build a 90* one with tuned pipes and maybe reed valves to show it can work... i wouldn't think it's the best compromise but there is no reason that it won't work. it may have more misses on one cyl that the other and not be great for a production engine at large scale but i really believe the design is workable at least on a demonstration level, maybe even a racing level where certain things can be less refined. but 60deg is probably a better compromise.

 

[johnny1320]

You are more than welcome to prove me wrong but I think you will find that the crankcase scavaging Will be less than adequate.

 

[Niels Abildgaard]

You are more than welcome to prove me wrong but I think you will find that the crankcase scavaging Will be less than adequate.

What is less than adequate?

 

[johnny1320]

In a 90 deg engine you will be lucky if it starts and stays running, reed valve or not, I have made CAD drawing of this design because a friend of mine was building a 90deg v twin two stroke using Zenoah cylinders pistons and rods with a custom case, they could not get it to stay running. In the CAD drawing I found that one cylinder is rising while one is on the down stroke, this pretty much cancels out the crankcase vacuum and pressure, I didn't look at a 60deg design but I believe the closer the angle the better it will work and the farther apart the worse it gets. In the end my friend made one cylinder a dummy to get the look of a V twin, I would say an offset crank pin would work or some kind of forced induction.

 

[barnesrickw]

Based on this old drawing, I think your right. Closer is better.




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[dman]

agreed that closer angles have less risk of problems. the point of greatest volume under the pistons is when the crank pin is split between the two cylinders, so it's 45 degrees ahead of one cylinder and 45 degrees behind another. when you work it all out the pumping volume is the sine of an angle of half the included angle between the pistons. you can see that as you get to under 60 degrees the losses are negligible, and even at 90 degrees i don't see why you couldn't increase the timing angles of the ports to compensate.

the problems may start with phasing. it may be difficult to take advantage of the whole lower end volume with the pumping being 45degees out of phase from each piston. the other problems would be from interference of the transfer ports. gas has momentum and since one piston will uncover it's intake port before another it will get good velocity before the other one is open. when the second pistons ports are uncovered flow will persist in the first port and might prevent the second from getting started when engine speeds are low and the transfer ports aren't providing restriction. this would be a big problem especially if the engine manages to build compression in the crankcase before piston one starts to flow where the flow may temporarily exceed the rate the crankcase volume changes and actually scavenge exhaust into the transfer ports of the trailing cylinder... i can spend all day trying to visualize every part of the cycle but it's not something to work out today. you might need to add rotary valves in the transfer ports or crank cutouts they expose the ports to shorten the intake cycle of the leading cylinder and keep things flowing on the trailing cyinder late in the cycle. but those are problems for a different day...

when the engine is at speed though it believe the exhaust scavenging will create all the flow needed at the appropriate time in the cycle. things could be rough at low speeds but there is no reason it wouldn't run like hell at high speeds.

at 60degrees i think there will be a good chance of success and it looks good too!

 

[johnny1320]

agreed that closer angles have less risk of problems.

There is no risk of problems, there are problems! Good luck!:wall:

 

[johnny1320]

OK I just did a CAD of a 60deg Vtwin at those angles I think it may work. the pistons are almost in unison although the leading bank will draw the air fuel back out of the trailing cylinder, and the trailing will charge the leading cylinder.timming will be a big factor more than 60deg and it gets worse

 

[edholly]

Have just about got the crankshaft finished along with the crankcase. Next job is to make the conrods and see if I can get them to fit with the space available. Will be very tight as usual and might have to relieve the rear of the cylinder a bit which won't affect the seal.

But will come to a bit of a halt for a little while, my Brabham BT6 has been invited to the Historic Demonstrations at the Australian GP next weekend, so about to get it ready then off at sparrow's fart next Wed morning for Melbourne. We get about 15 minutes track time Thu, Fri, Sat, Sun - what they like as around 7/10ths. Geared to do 160mph will see 140+ if its dry - wet - well lets just say I have the utmost respect for the bravery of those 1960s drivers - my car is ex Hulme (Works Formula Junior) 63, Gardner 64, Levis 65/6 and Stone 67/8. I did see 155mph over at Pukekoe in NZ when I raced it over there in 2010 and it is a wonderful car - we have won a lot of races together since I bought it in 2006.

But can't wait to get back home and back on the lathe and mill - found in recent times my passion for these little engines is really strong - and satisfies that "want to make something" instinct I've had since a kid.

Apologies for going off topic ....

When I posted the note above - didn't realize this had gone to a second page - thanks heaps to you guys that have looked at phasing and gas flow - yes - the main reason for 60 degrees was to still get reasonable pumping effect that 90 degrees gets robbed of. I am thinking of making the lead cylinder's transfer port hole slightly smaller than the trailing one, to try to lesson the lead cylinder's advantage - crude yes - scientific no - but its easy to open the hole later.

 

[edholly]

OK back from the GP - On the track everyday but Sat was the best day - 20+ minutes and finally learnt the track enough to have a decent run, weren't many cars left in front at the chequered flag but it was only a demo. The car would have felt at home running in the AGP's 1964, 1965 and 1966 and NZGP's 65,66,67 and 68.

Couldn't wait to get back to tackle the fork and blade conrod - which is now done. I think the rest of the bits except plenum chamber venturi, will be as made for the Midge - which I had 6 flights of in the Cardinal this morning.

One more thing to answer down the track - will I be able to assemble with pistons on the rods - or will I have to assemble piston onto rod after putting rods on the crankpin - time will tell.

 

[kadora]

Hello
Maybe you will be interested in this pumping effect of my half machined
four stroke V tween.
Kadora

[ame]http://youtu.be/2m15RXbRM5Q[/ame]

 

[gbritnell]

Why worry about the pumping/scavenging effects of a V-twin 2 cycle engine with a common crankcase. There were many twin and triple cylinder 2 cycle motorcycle engines built that ran great, just put a crankcase wall between the two cylinders. Yes you couldn't use a forked type rod arrangement but you could still make a V-twin if that is your goal.
gbritnell

 

[gbritnell]

Kadora,
It's true that you will get crankcase pumping effects from your engine it's just that they won't have the proper timing and efficiency to allow the engine to run well. 2 cycle engines while seeming quite simple are in fact engineered for high performance.
gbritnell