F=ma applied to slider crank piston motion

[Kalus]

I'm struggling to apply F=ma to the motion of a slider crank, more specifically the piston. I want to find out how much force is nessesary to keep the piston in motion. Essentially, in the mechanism there is acceleration and deceleration, does that mean only at some points in the cycle there is force needed to move the piston?

Using the equations derived for the motion of a piston:

http://upload.wikimedia.org/wikipedia/en/math/1/6/8/1686ee2f8b1d67ce1eb1aa4fb4b0daac.png

Gives you this:

 

[Kalus]

Sorry, I should add why I'm asking this. I have the following diagram:

Where f_g is the combustion force, f_j is the inertial force and f is the resultant force. I'm trying to work out what's providing the force to overcome the acceleration around 650 to 90 degrees.

 

[256bits]

shaft flywheel