I continued to work on the crankshaft. I have already established the bore to be .75″ and now I want to confirm the stroke. A stroke of .75″ is a good place to start. I develop a model representing the volume inside the cylinder with the piston at the top of the stroke and at the bottom. The valves are at 45 degrees with respect to the top of the piston so the roof of the combustion chamber is a cone. The flat spot at the top is where the spark plug will be mounted.
The numbers circled in red in the photos below are the volumes of the two models.
Dividing them gives the compression ratio. .39 cubic inches : .06 cubic inches or a compression ratio of 6.5:1. This is a good compression ratio for a model engine, I personally don’t want to go any higher. I think that model engines with a higher compression ratio don’t start as well, don’t idle as well and have a larger tendency to blow out head gaskets.
I create a simple model of a piston and connecting rod, knowing that these are just to give me something to work out the crankshaft dimensions and clearances. I will revisit both later.
Above is a cut away looking at where the piston ends up at the bottom of the stroke and to see if the cylinder sleeve hits the crankcase.
Here the piston is at the top of its stroke.
I started out with a simple internal space inside the crankcase, but the pictures below show that I have interference and need to open up the space inside the crankcase.
Above the connecting rod hits the top of the crankcase space.
Above the connecting rod cap screw again hits the side of the crankcase.
Here I open up the internal crankcase space and shorten the cylinder sleeve. When I finalize the piston and connecting rod later, I will need to revisit these interference areas.
This is my solution for the front ball bearing holder for the crankshaft is shown above. The green part marked by the arrow will house the bearing and then screw into the front of the crankcase. I have made sure the bearing holder and its retaining screws clear the gears. I should mention that my plan is to split the crankcase at the crankshaft.
The above photo is a cutaway showing how the front crankshaft bearing holder is mounted to the front of the crankcase.
The rear crankshaft bearing holder is much simpler as it does not interfere with anything like the timing gears in the front.
To be continued….