Motorcycle V8

[DannytheRadomski]

I didn't know who to share this with, so I chose you guys. Look at this engine configuration: http://thekneeslider.com/ducati-elenore-v8-update/

[tromoly]

That is really interesting, the pistons kind of do a "push-me-pull-you" thing on the crankshaft. I wonder what effect that has on main bearing life.

[Z]

Much as I like interesting new engine ideas, I wouldn't be putting any money on this one. To repeat, I hope the builder has fun doing something different, but I see no significant advantages to this design, over the many other ways of doing multi-cylinder engines.

The two inner pistons on each bank need two piston pins at right-angles to each other. They therefore should also have four thrust faces to carry the side loads. These pistons are shown as conventional "slipper" pistons (ie. with only two thrust faces), but that may just be a deficiency in the CAD model.

Whenever one of the two inner pistons is on its power stroke, and assuming the other is not (else why have it, other than for smoother power delivery?), then that side's main conrod will be under a large sideways bending load. This will also put a large sideways thrust load on the bigend of that conrod, and likewise at its smallend/piston.

Each of the four "rockers" has a central pivot fixed to crankcase which acts somewhat like a crankshaft main bearing (similar loads). This pivot bearing is subject to oscillating (ie. reversing) motion. This can be a good thing if the oscillating motions are very small and thus low speed (like on the smallend/piston-pins). But the geometry suggests the motions are quite large, giving rubbing velocities similar to crank mains, so the "smallend" advantage is lost. The reversing motion suggests that conventional hydrodynamic bearings would not work too well, so needle rollers probably required (this is not necessarily a problem, just noted here).

As mentioned by one of the posters (Tom?) on the linked forum, the inner and outer pistons have oppositely phased "secondary" harmonics. This gives the engine better balance than a conventional "flat-plane" V8 (which has bad balance!), but the valve timing and general tuning would have to be different for inner and outer pistons. IMO (ie. debatable) the greater "dwell" at TDC of the outer pistons is BAD for thermal efficiency.

In summary, and not considering overall "package shape", I reckon a radial is a much better solution. It has the same very simple single-throw crank, fewer other parts giving less scope for frictional losses, equally good dynamic balance, but much better structural balance between the many gas pressure and inertial forces. Note that for even power pulses a four-stroke radial should have an odd number of cylinders, so say R7 or R9 in this case. A two-stroke radial (many have been built!) can have any number of cylinders, such as an R8 here.

Z

[tromoly]

Just noticed that the article was posted in 2010, with not much out there after that. Still an interesting concept, wish there was more out there on the end result.