If you ever did the "spinning wheel" ride at an amusement park, the one where you are on the inner circumference of the wheel and the wheel becomes perpendicular to the ground:
At the top, for a short moment, you experience close to 0 G (and close to 2 G at the bottom). So if you had the cylinders of the engine arranged in a circle, with the circle spinning at the right speed, and ignition happening at the top of the spin, you could have the instant of ignition happen at close to zero G.