Author Topic: Building a twin cylinder inline i.c. engine  (Read 15004 times)

Offline Brian Rupnow

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Re: Building a twin cylinder inline i.c. engine
« Reply #255 on: March 27, 2024, 06:12:21 PM »
Right now, I'm kind of lost in the forest. The fuel/carburation side of this engine seems to be okay. I have played with the ignition timing a bit and got back to where the engine is firing some of the time while being rotated. The easiest thing to try is a different fuel, so I will try some automotive gasoline in the tank to see if that makes any difference.  I could take out one piston and con rod and try to get it running as a single cylinder engine. That requires a considerable amount of work and I'm not sure I want to go there yet.

Offline Brian Rupnow

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Re: Building a twin cylinder inline i.c. engine
« Reply #256 on: March 28, 2024, 05:54:59 PM »
A big Thank You going out to Nelson from Kansas. Nelson has a tabletop cnc machine and used it to make four cams for me which DO have the flank radius machined into them. I will drill and tap the set screw holes and harden the cams myself. For all the people out there who said that a tabletop cnc machine is not capable of machining metal, these certainly look good to me. If I don't use them on this current engine, they will be used in a future engine.---Brian
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Offline Jasonb

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Re: Building a twin cylinder inline i.c. engine
« Reply #257 on: March 28, 2024, 06:15:19 PM »
What happened to the ones you already made with rounded flanks that you decided to swap out for the flat flank ones?

Can't you just put those back in

Offline Brian Rupnow

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Re: Building a twin cylinder inline i.c. engine
« Reply #258 on: March 28, 2024, 08:01:27 PM »
Jason---the ones I made are setting in a jar and will be used in a future engine.---Brian

Offline Mike R

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Re: Building a twin cylinder inline i.c. engine
« Reply #259 on: March 28, 2024, 08:59:45 PM »
Laurentic--Yes the compression ratio was changed. Was it changed enough to keep the engine from running? I doubt it very much. I didn't do an actual calculation on it.---Brian



Brian,


What was the thickness of the spacer under the cylinders? To me it looks like its about 1/8" (3.175mm)?  That is going to have a significant impact on the compression ratio of the engine. 


The following are assumptions as we don't have all the details, but I think it gives a good idea as to what may still be wrong with the internal combustion process (induction, compression, ignition, expansion and exhaust). 
Assuming that you designed it initially with the a 6:1 compression ratio (as a copy of Malcom Strides Bobcat), using your modified 25.4mm x 22mm bore and stroke we can reverse engineer a few things:
total displacement = 11.15cc with compression ratio 6:1 results in a design intent of 1.86cc combustion chamber volume


adding the volume the spacer adds (assuming 3.175mm) = 1.61cc  plus the original chamber volume of 1.86cc = 3.47cc combustion chamber volume as built.
working that out 11.15 to 3.47 is roughly a 3.2:1 compression ratio - likely too low IMHO.


Brian, you have been asked a few times about how the engine behaves with a bump test - its not a pointless exercise - it will help validate if there is compression, and how much is there.  We don't need a psi # to know if there is no compression, just spin the engine by hand - is there resistance to compression once per rev and if so does it feel good (snap over TDC) or is it "soft"? Take a video as you do it, and share,  if you really want some help from those on this forum. 




Suggestions for anyone else looking for help on why their engine will not run:


1.  provide all possible info:
   Is it built to an existing design or is it a new or modified design?
      If modified - what did you change?  details matter when things aren't working.
                    In this case Brian has changed a fair amount I understand:
   Cam tower height,
   valve length
   valve spring
   Cylinders spaced up (i.e. reduced compression ratio)
   induction (twin carb)
   bore and stroke,
        o-ring piston ring
   crankcase size,
   etc. , etc. to the point that I think it is effectively a new design (along with all the possible errors that come with a new design).


2.  Can you provide a video?  If so it should show if possible:
   a.  slowly turning the engine over - is there compression resistance as its turned over? For multi cylinder is there resistance as each cylinder comes to its compression stroke?  Does the engine "snap" over TDC, or is it "soft" when turning over.
   b.  If spark ignition, does it spark when turned over (assuming coil ignition or CDI, not magneto) and spark approximately when it should (just before TDC on compression stroke)?
   c.  slowly turning the engine over, show direction of rotation, and if possible the valve actuation (for each cylinder if multi cylinder)




Regards,


Mike


Offline Laurentic

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Re: Building a twin cylinder inline i.c. engine
« Reply #260 on: March 28, 2024, 10:35:02 PM »
Mike R - you make a number of really valid points, and I had forgotten the shim under the cylinder (and the fact I had queried it) and not picked it up how big it is in the photo.  Wow, that is a big shim!

Chris

Offline sid pileski

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Re: Building a twin cylinder inline i.c. engine
« Reply #261 on: Today at 12:44:09 AM »
I thought the shim was in the previous failed engine, if I recall correctly???

Sid

 

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