Back to the 2 pairs of pistons. Pair one starts at 1 and pair two at 3. Applause if you do that :) I like!
Then there are 4 piston movements. We begin with the piston extended. Chamber is empty. 1. pull the piston back to suck in oil and oxygen. 2. move forward to compress the stuff you pulled in (close the ewall). 3. light the gas and move the piston back. 4. open the exit ewall and push the piston forward to kick the burned fuel out. Begin at 1 again.
There is stuff missing: First there are 2 pairs of pistons. The pistons in a pair are doing the same movement. the other 2 are doing the same, but half a cycle behind the other pair.