Algodoo

this is an air friction engine and density engine in one
left collision = airfriction off
upper collision = make heavy
right collision = much airfriction
lower collision = make light

diff to an air engine:
faster
no motor hinge needed

diff to a density engine:
more power at low rpm
has speed limit on the thing its driving

new thing:
the faster it goes (m/s), the faster the rpm

EDIT:
srry, i cant help the img is screwed up
but i can replace it by a picture