The fuel is air compressed to 4, 350 pounds per square inch, or 300 times the pressure of the air you breathe.

FORBES: Driving on Air

Instead of sparking the ignition when the piston is close to top dead center (when the air is compressed), that spark occurs as the piston is pulling away, when the air pressure is at its most turbulent, creating much faster ignition and more power.

FORBES: How To Get 50 MPG Out Of Your Gasoline Engine