Question

Nearly all conventional piston engines have flywheels on them to smooth out engine vibrations caused by the thrust of individual piston firings. Why does the flywheel have this effect?

Short answer

Flywheel has very large moment of inertia which smoothens the vibrations.

Step-by-step solution

Concept of law of conservation of angular momentum.

The law of conservation of angular momentum states that there will be no change in the angular momentum unless an external torque acts on the object. Angular momentum of a system is conserved as long as there is no external torque acting on the system.

Reason for flywheel smoothing the vibrations.

Sudden piston torques have negligible effect on the flywheel due to its very large moment of inertia. There is significant torque on combination of every piston which causes vibrations and these flywheels smoothens the vibrations.