The kinetic energy of the flywheel keeps things moving when burning gases aren't moving the pistons. The evidence of this is demonstrated on the two flavors of the motorcycle I own. One version (BMW R1200GS Adventure) has a heavier flywheel than the other (non-Adventure). I've ridden both. With the light flywheel it's easier to kill it pulling from a dead stop because there isn't as much mass to overcome the lack of subtlety in my clutch hand. On the Adventure version that I own, sloppy clutch work that often comes with trying to get a 600 lb. "dirt bike" out of a mud hole is ignored by the more massive flywheel. Even when the cylinders are barely able to fire because I'm one sniff of clutch away from killing the engine, the bigger flywheel will keep things turning for a wee bit longer before I kill it and fall over in the mud.
As for the answers parallel to mine that say it's the mass of the other pistons, the mass of a piston is trivial in comparison to that of the flywheel. Even lawn mowers mentioned in one example have flywheels (at least the ones I owned always did). The mass of a tiny lawn mower piston is going to have a hard time against the relatively large mass of a lawn mower blade with no flywheel.
To speak to your two points: 1. My motorcycle's pistons lie on their sides, gravity has no practical effect.
2. Power stroke movement is caused by the expansion of burning gases, period. All other factors, such as flywheel inertia, contribute relatively minimally.
As for the answers parallel to mine that say it's the mass of the other pistons, the mass of a piston is trivial in comparison to that of the flywheel. Even lawn mowers mentioned in one example have flywheels (at least the ones I owned always did). The mass of a tiny lawn mower piston is going to have a hard time against the relatively large mass of a lawn mower blade with no flywheel.
To speak to your two points: 1. My motorcycle's pistons lie on their sides, gravity has no practical effect.
2. Power stroke movement is caused by the expansion of burning gases, period. All other factors, such as flywheel inertia, contribute relatively minimally.