Way back in the 1940's there was a gentleman named Russell Bourke who had an idea.
He tinkered around and engineered a mechanical wonder in his home workshop in 1954.
It is difficult to describe, but I will give it a shot and then point you to some links that has videos and a working diagram that will give you a more detailed idea of his vision.
His engine is a pancake design that has two cylinders . Theoretically, you could marry as many together as you wanted for any type of horse power or torque situation you could reasonably dream up.
The opposing pistons are mated to a single connecting rod with a unique triple slipper bearing. The compression ratio could be varied from 8:1 to 20 to 1! It was tested with several different types of fuel and could run on unleaded gas, diesel, Coleman lantern fuel, natural gas etc. .
The original engine was spec'd at 30 cubic inches and tested at 35 HP @ 5,000 RPM. The emissions emitted were Carbon Dioxide, and water vapor.It could be carburated or fuel injected, run a capacitor discharge ignition or use compression to fire the fuel for diesel.
He built a four cylinder version later that was rated at 400 Cubic inches and was static estimated to put out 200 HP and 500 FT LBS. of torque at only 2,000 RPM!
They were also fabulously fuel efficient, running air to fuel ratios of 30:1 and even 50 : 1 with an exhaust temperature of only 190-240 degrees!
Enough out of me, go check out the website thoroughly, it is flat out fascinating.
Here is a working diagram so you can wrap your head around the design ,and here is the home page, be sure and check out the videos.
I would love to see one of these stuffed under the hood of a car.