while looking at that came across this, which seems like the way to do it. steam for an added efficiency stroke: https://en.wikipedia.org/wiki/Six-stroke_engin

while looking at that came across this, which seems like the way to do it. steam for an added efficiency stroke: https://en.wikipedia.org/wiki/Six-stroke_engine#M4+2 https://en.wikipedia.org/wiki/Six-stroke_engine#M4+2