ESO 3. Two and four stroke engine

Animation and how a two stroke engine works [1] and [2].  Watch this video [4]
How four stroke engine works [3].

[1] http://www.animatedengines.com/twostroke.html
[2] http://library.thinkquest.org/C006011/english/sites/2_taktmotor.php3
[3] Four stroke engine video: https://www.youtube.com/watch?v=_y6PS-2j2Ug
[4] How stuff works https://www.youtube.com/watch?v=LuCUmQ9FxMU&playnext=1