I noticed the lack of 3 cylinders representing, what made me think "where did the two cylinder go?"
There were quite a few in the early days. I don't know if it was just power marketing that wiped em out or if there is something inherently wrong with them (too pulsey?) for a car. Might be a good "range extender" though... (though I'm expecting to see some floating piston/linear alternator thingies to start appearing there)
Flat twin engine - Wikipedia, the free encyclopedia