I was planning to have a mature design, proven by means of a running engine before posting anything about my V12... Anyway, the pictures are screen captures from my evolving SolidWorks model.
The engine is just over 11 inches long and is intended for a scale model airplane. Bore and stroke are 1 & 3/4 inch respectively to produce a displacement just shy of 120 CC.
The idea is to produce usable power at 6000 RPM, rather than the much higher speeds typical of single cylinder models with this bore and stroke. This reduces stresses and should produce a sound with the second harmonic of a Rolls Royce Merlin V12 at 3000 RPM – better than the whine of a typical model engine.
The two combustion chamber designs shown result in a compression ratio of around 10:1 . The intent is to use glowplug model fuel with spark ignition - glowplug model engines typically have compression ratios in the range of 7:1 to 9:1 .
Many of the design elements were deliberately simplified to facilitate milling on a benchtop size manual machine. A rotary 4rth axis will be used to produce the crankshaft and crankcase regardless of manual or CNC control. Some parts, such as the connecting rods and cylinder heads exist in variants optimized for manual or CNC machining.
The design is far from finished. The valve-train exists only in my head. I have not yet decided if the overhead cams will use bucket tappets or rockers to take the “wiping load” exerted by the cam lobes. The crankcase will also be lightened to achieve a more rounded external appearance.
I have a number of different connecting rod designs. COSMOSXpress stress analysis shows that the conrod's piston pin bearing is strong enough, but it is the weak point in the design. The factor of safety can be brought up by making the conrod wider at the small end bearing - a width that would match the bearing area of the rod and piston where they contact the piston pin. This would require a variant of the piston design.
The stem of the conrod is stronger than it needs to be and it can be hollowed out and lightened as shown in the picture without reducing the factor of safety below 5.