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

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

Hi Willy, sorry to hear about your fall.  We don’t seem to bounce like when we were younger.

I hope you are back on your feet soon.

Best wishes

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

Hi Charles, And not a very efficient compressor at that.

Still due credit to you for picking up the comment in Benson and Rayman’s book.  I should have refreshed my memory of the book earlier.

I did however consider including of how the pressure would contribute to to the valve opening, but decided it was a matter of degree, and my post was already too long without going into the detail of valve area exposed to the cylinder pressure and spring loading.  But thanks for introducing it.

MJM460