Because some of you have much more experience with cars than I do... but I thought about something....
The only thing I calculated about this build is the displacement, the rest is just supposing that symmetry will do its job... I don’t have the knowledge (or time since this isn’t a real proyect) to carry out all the necesary calculations so that the engine actually runs at this time. So again, take it easy... its more of a thought experiment than a final design.
Why not have a 3 cylinder engine with a flat plane crank?
the advantage is that you remove the rocking motion as the firing order would be 1-3, 2; the center piston is made so that its displacement is exactly the same as the two smaller ones combined. I kept the stroke the same on both cylinders, I only reduced the bore of 1 and 3 to 70mm (the pictures don’t show it correctly)... in the end it should displace about a litre, with a stroke of 64mm.
In terms of rotating mas, in order to balance the crank, you do need to add more weight, in order to balance the cylinders you also need to have the mass of all the components of cylinder 2 (connecting rod, pin, piston rings... etc) be equal to the mass of all the components of the first and last cylinder. two cylinders will be almost square, meaning that it will have a turbulent power curve. my thoughts are that it would work like a 2 cyl engine, and at that stage it would make sense to reduce complexity and just make it a two cylinder motor.
its a very wierd build, and it is at a basic stage right now, just a passing thought I had today.... I also want to design an engine with pistons alike the Honda NR.