Author Topic: New Vertical Twin Design  (Read 2042 times)

Offline Roger B

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New Vertical Twin Design
« on: January 25, 2018, 12:00:06 PM »
I am designing a new version of my vertical four stroke engine and would like to bounce a few ideas around.

<a href="https://www.youtube.com/watch?v=9FOV1FHP8ww" target="_blank">http://www.youtube.com/watch?v=9FOV1FHP8ww</a>

The original is 16mm bore and 15.5mm stroke. In the same space I can increase this to 20mm bore 19mm stroke and so just about double the capacity. I will also make it as a twin which increases the overall length of the crankcase by about 50%.

If I make a twin I can choose 180° or 360° firing and get two different sounds and two different out of balances. This choice also affects the choice of ignition system.

I can use conventional points and coil with a distributor running at camshaft speed. In the 180° case the cam lobes and distributor connections must be at 90°.

I can use one of the CDI type systems on the market driven at crankshaft or camshaft speed with various numbers of wasted sparks. Any recommendations for systems? I used an RCexel unit on the original. Are sparks at any of the unwanted dead centres a problem? At the BDC at the start of the compression stroke there is combustible mixture in the cylinder. One of the systems designed for V twins would allow me to choose 180° or 360° (I would just have to make a second crankshaft and camshaft).

The original used a brass cylinder liner for various reasons. For this version I could use brass or cast iron. Any thoughts? Brass will not corrode so much in contact with the cooling water and has a similar coefficient of expansion to the aluminium cylinder.

I am still playing around with the cylinder head layout to get suitable water passages in as well as with the crankshaft so it will go in the keats for easy machining. The current state of the design is attached below.
Best regards

Roger

Offline AlexS

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Re: New Vertical Twin Design
« Reply #1 on: January 26, 2018, 07:09:23 PM »
Nice start of design your new twin. What about a 360 degree firing? So the pistons moves together. In comparison with a 180 degrees you have more force vibration. But you can reduce that with some first or even second order balancing shafts? Like a one cylinder. It would be more complicated to build, but with two first order balancing shafts the only vibration are in the y-axis?

Offline Art K

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Re: New Vertical Twin Design
« Reply #2 on: January 26, 2018, 07:33:30 PM »
Roger,
It's good to see the start of a new engine. I liked the PDF of the basic outline of the engine. Just out of curiosity what happens if you have the crank throws 90 degrees apart rather than 0 or 180? I think I saw that on a race prepped Norton 850cc engine. Does that even out the balance a bit? so it vibrates less.
Art
"The beautiful thing about learning is that no one can take it away from you" B.B. King

Offline Roger B

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Re: New Vertical Twin Design
« Reply #3 on: January 26, 2018, 08:07:45 PM »
Thank you both  :ThumbsUp: As ever my answer is I'm not sure  :headscratch:  ::) With this size engine the out of balance forces are not going to be that big. My original 25cc horizontal engine tends to vibrate all over the place but the mk2 with simple balance weights is quite stable.
Best regards

Roger

Offline kuhncw

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Re: New Vertical Twin Design
« Reply #4 on: January 27, 2018, 08:11:19 PM »
Hi Roger,

I like your design.  It is similar to the Bob Shores Silver Bullet.  As you probably know, the Silver Bullet fires 360 degrees apart using waste fire ignition. 

I built a Silver Bullet, but changed the crank and cam to fire 180 degrees apart, as that was the sound I wanted.  At the moment, I'm waste firing it with a two lead coil, a Hall sensor, and two magnets at 90 degrees driven by the camshaft.  As you mentioned, one cylinder always fires into an intake stroke, but that causes no problems and the engine runs well.  With the pistons moving out of phase with each other, there is little need for a crankcase breather.  If the pistons were traveling together, I'd need a breather.

I'd prefer cast iron cylinder liners, or maybe steel, over the brass.  You might consider a larger diameter flywheel if you like a lower rpm idle. 

Unless you plan on sustained running at mid rpm or more, I'm not convinced cooling channels in the head are worth it unless you can manage more than a couple drilled passages.  My Bullet has a dry head and will run idle with a few excursions to higher rpm for a good 10 minutes or so with no problems.

Roger, I doubt I've told you anything you did not already know, but I wanted to give you my thoughts.  Good luck and I'll be following your build.

Chuck

Offline Admiral_dk

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Re: New Vertical Twin Design
« Reply #5 on: January 27, 2018, 08:43:09 PM »
A lot of modern motorcycles, both street and racing, has 45-90 degree crank offset on twins - the reason is to avoid the "dead spots" you get with a 180 or 360 degree crank ... and an almost even firing order like a 360 degree engine.

This kind of crank + camshaft, will be an extra challenge on our kind of engines - and probably not likely to reward the extra effort ....

Offline Roger B

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Re: New Vertical Twin Design
« Reply #6 on: January 31, 2018, 08:03:21 AM »
Thank you Chuck. I thought that waste fire would be ok on the inlet stroke, confirmation is always good  :ThumbsUp: I am still not certain between brass or CI for the liners both have benefits and disadvantages  :headscratch:  I intend to run this engine under load as I have with the previous two so cooling will be essential. I am intending to fit a water pump as thermosyphon cooling seems insufficient with these small pipes and passages. The vertical engine boils very rapidly when put under load with the generator.

Thank you Per, as you say anything other than 180 or 360° is probably only extra complication with this sort of design. It might be of interest if Strictly Business decides to make a twin  ;)
Best regards

Roger

 

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