A piston ring collapses radially when a positive load differential oriented radially inwards is present between the outer and inner diameters of the ring. The load distribution and orientation depends both on the radial pressure of the ring and its distribution and on the pressure from combustion that builds up in front of and behind the ring.
The alternating ring contact in the piston groove is controlled firstly by the inertia and frictional forces and secondly by the gas forces acting over and under the piston ring. A negative pressure differential (pressure over and under the ring) can cause the rings to lift from the bottom groove side on the expansion stroke [21, 38], which will lead to increased blowby.

Remedial actions:

• Avoid radial collapse by choosing another running face geometry and/or another radial pressure distribution
• Retard pressure build-up under the top ring e.g. by using a ring in the 2^nd groove that seals less effectively
• Increase the volume under the ring
• Piston groove inclination
• Decrease the inertia forces
• Adjust the axial clearance