Here's a brain teaser for you: These Yanmar 2 cylinders are uneven-firing. They fire 180 degrees apart then nothing for another 540 degrees of crankshaft rotation. Seems like that would have some impact on making that 'rocking couple' uneven, and even harder to design for in a broad-rpm engine.
I'll bet somebody in Yanmar gave up doing calculations at some point and said 'ok enough theory just make the lower end massive'.
California, that's funny! Thanks for the chuckle. I confess I've not looked into the firing vs crank inertia on my Yanmar. Congratulations to you for doing so. But I kinda doubt if they just "gave up" doing calculations!
Those are pretty easy to do and have been a basic & well understood part of engine design for a long time.
As my friend Charlie says, "Simple dimple just like beer & apple pie!" *
*(BTW, Charlie is a kindergarden teacher, author, & beer brewer who is also a nuclear engineer - when he says something is "simple" my mental alarm bells go into immediate frantic hyperdrive .... )
The geometry of the crank and cam lobes can be changed easy enough on paper and during development to put the firing in any imaginable relationship to the rotating masses. After that, it's just a matter of the designer running the numbers, picking one set of workable answers, and then getting prepared for endless design meetings if what emerges is an unusual or oddball solution......
But that's how progress is made.
I agree with what you say. It does seem making the lower end more massive would always be a valid solution. That's a good brain-teasing thought experiment because if you make the lower end rotating parts massive enough, then you should be able to cause both the reciprocating weight and the firing impulses to become relatively insignificant in comparison to the rotating mass. Changing RPM becomes a problem.....But now that I think on it, lots of great big early engines were built in exactly that manner. It wouldn't surprise me if big ocean-going ship engines still were. Doing that does require an equally massive engine block of course. That may not be problem in a cargo ship where you are looking for ballast anyway.
But if you decide to go with that solution on a lightweight modern tractor, better invite the bean counters to the design meetings.
If the financial & legal guys are going to go ballistic you sure wouldn't want anyone to miss the fun...
Yes, Wankels are cool.
Enjoy!
rScotty
- who is prepared to swear on a stack of Marvel Comics that I certainly never, ever presented a goofball design just to watch the fireworks.
