Interesting idea - - - - wonder what they would be for longevity.
Hi Joe,
A few comments on Atkinson's cycle engines.
His first - and most curious arrangement - has one firing stroke every turn of the wheel. But is has 2 pistons in the cylinder, and the "intake stroke is 1/4 ~ 1/3rd of the cylinder's displacement. The power stroke is twice as long - hence the "higher" efficiency.
So I say this has "1.3" - or 0.33 of 1-stroke per revolution where the piston go back and fro = 2 strokes. I.E. it can "suck" a charge = 1/6th (0.167 = 17%) of the total RMS displacement....
Now thinking of a "modern conventional" 4 stroke engine, it sucks charge for 1 stroke of the 4 strokes making a full firing cycle. Therefore I rate this as 1/4 = 25% of the RMS displacement of the engine...
I studied the cycle (for fun) a year or so back: see below adobe file.
The 25% beat the 16% when engineers of the time looked at "Power" from engines rather than "efficiency" = because a smaller (cheaper" engine could be bought for the same "power" requirement. The "cheaper" sold products, rather than "engineering efficiency".
Reliability and durability come from a combination of materials, design, maintenance and workload. Your car engine is relatively cheap, yet lasts for 150,000 miles before you replace (scrap) the car (typically), yet the engine isn't worn-out. That is longlevity in another view!
But these engines are lovely to watch as running models.
https://birkpetersens.blogspot.com/2013/01/atkinson-cycle-engine.html
K2