Bump valve uniflow engine

[MJM460]

Thanks Andrew.  I hope the discussion has answered your original question and helped you understand how the various dimensions work together to make a working engine.

Those off cuts look like they were made for the part.  A great start to your engine.

MJM460

[astroud]

Progress has been made and I am nearly ready for the first run.
The crankshaft is 3/8"  diameter in anticipation of having to overcome quite high compression ratios.
The conrod is 3 piece Loctite'd together, the centre section being a 6mm tube from a radio aerial. The ball in the inlet valve has a biro spring to keep it against the seat as this is a horizontal runner. The piston has no rings, just a couple of shallow oil grooves.
I decided there is no exact science for the timing, the single exhaust port is 1/4" dia and starts to open when the piston is half down its stroke and the piston bumper starts to open the ball valve at approximately 1/8" BTDC.

[PaulR]

What a little beast, I guess it's going to be pretty loud...

[Sanjay F]

Looking forward to seeing this one running 

[astroud]

After a bit of fettling she is a runner ! I needed to increase the valve opening from 1/16" to  1/8" and increase the inlet pressure to about 50 psi. After building engines to run on the lowest pressure it was quite a gulp moment to wind the pressure up and I had supply pipes blowing off their barbs until I wired them on. Midway through the runs the engine would suddenly start to race away making me step back and the first time it happened I feared for the integrity of the engine.
It must hit a resonance spot, a rapid pressure reduction causes it to eventually slow. Unless I give it a good flick to start it will just oscillate backwards and forwards.
<a href="https://www.youtube.com/watch?v=wxMrnPKdZP0" target="_blank">http://www.youtube.com/watch?v=wxMrnPKdZP0</a>

[Sanjay F]

Nice! Very purposeful sound 

[PaulR]

Great stuff Andrew, really nice. These uniflow engines do make some interesting random sounds that's for sure but generally it reminds me of a taxiing WWII aircraft.

50 psi is pretty scary though!

[MJM460]

Hi Andy, good to see it running successfully.  Congratulations on a runner.

It might be worth a little experimenting to see if you can modify something simple to get it running at lower pressure.  If you have a look at Paul’s Bump valve engine thread you will see he had his running at much lower pressure, so the required pressure is more a matter of design details than a limit of the bump valve principle.

We talked earlier about the effect of compression ratio, and I suspect the main difference between your engine and Paul’s is the compression ratio, though I don’t know enough of the critical dimensions of either design to do a more precise analysis. 

Is it possible that your compression ratio is relatively high?  Could you reduce the compression ratio by playing with your conrod length?  Or introducing more clearance volume by modifying the head design?

It migh also be worth playing with the exhaust opening.  Starting to open at half the stroke seems a little early to me, but it may be worth increasing the exhaust by adding a second port, perhaps a little nearer the bottom dead centre, in order to exhaust more gas so less work is required to recompress on the return stroke.

Alternatively, on to the next project!

A runner is always a good result.  Further experimenting only if it is your interest.

MJM460

[astroud]

Hi MJM
Thanks for your thoughts. I originally thought I would build the engine and then experiment with the settings as you suggest principally because it was my first uniflow and I doubted I could get it to run. Now it does I am having second thoughts about changing anything.

More playing has shown it likes 50 psi to start but can then be turned down to about 30 psi and it will run nicely there. If I could get a fast enough flick speed on starting it would probably start with a lower pressure, if not high enough it just bounces backwards and forwards which is quite entertaining.

I had thought about a variable exhaust port position but could not come up with any easy design. The compression ratio is high, the calculated clearance at tdc is 0.080". I could shave off from the head locating spigot which is about 0.125" deep  but no so easy to put it back if needed....I want to swap out the 1/4" ball bearing valve for a smaller one but having trouble finding one on the workshop.

I will do some more simple playing, then go onwards and upwards to something else.

Andrew

[MJM460]

Hi Andrew, I can understand moving on.

If you ever had the inclination to experiment after a rest on this project, you could experiment with increasing the clearance volume by making a spacer to install between the head and cylinder.  Remember the extra volume only has to be volume, it does not have to be the same diameter as the bore.  You would need to extend the valve pin to suit.

Rather than simply giving it with your fingers, you could try starting it with a leather lace, the same technique as used for diesel engines in a model boat.  (You will need to clamp the base down )

What is the next project?

MJM460

[Jasonb]

String wrapped around a flywheel will also get it going as will a rubber faced drum in a cordless drill held against a flywheel rim

A good way to allow for some experimentation is to use a separate wrist/gudgeon pin holder then you can just use shims to alter the position of the piston and also add larger shims to the crown to alter compression ratio.

[astroud]

Hi Andrew, I can understand moving on.

If you ever had the inclination to experiment after a rest on this project, you could experiment with increasing the clearance volume by making a spacer to install between the head and cylinder.  Remember the extra volume only has to be volume, it does not have to be the same diameter as the bore.  You would need to extend the valve pin to suit.

Rather than simply giving it with your fingers, you could try starting it with a leather lace, the same technique as used for diesel engines in a model boat.  (You will need to clamp the base down )

What is the next project?

Not sure about the next project, I have a half finished IC engine which I should get back to, I baulked at making the cams and valves as being beyond my skills but should have a go. Otherwise buoyed by getting the uniflow to run a multi cylinder uniflow with possibly valves according to Chuck Fellows' thoughts and designs which were published here several years ago.

Andrew

[astroud]

String wrapped around a flywheel will also get it going as will a rubber faced drum in a cordless drill held against a flywheel rim

A good way to allow for some experimentation is to use a separate wrist/gudgeon pin holder then you can just use shims to alter the position of the piston and also add larger shims to the crown to alter compression ratio.

Thanks Jason, something to remember for the future. My conrod is a 6mm tube with little and big ends stuck on so I can unstick it, make a new shorter central section and stick it back together again.

Andrew

[astroud]

After playing with the engine I noticed if I unscrew the air valve the engine would slow down so I decided to build another air valve with a variable position valve meaning I would be able to move the ball closer or further away from the piston activating prong. I achieved about 0.15 movement which I had seen from my testing was enough.
The photo below shows the design, basically a moveable spool with the ball and spring within the housing. I found out after testing the air pressure within the spool can push the cap on the opposite end to the ball seat off its seating resulting in a loud clacking sound when running. The only way I see to fix this is either solder or Loctite or a much stronger return spring but it is not a problem if I keep the pressure below 40 psi so I will leave as is for the moment.
I have some smaller diameter ball bearings coming to see what impact they might have.

If you can watch the video to the end it does its party trick and starts oscillating backwards and forwards.

Andrew

<a href="https://www.youtube.com/watch?v=Tlg3QlicZzg" target="_blank">http://www.youtube.com/watch?v=Tlg3QlicZzg</a>