What happens to the engine?
     

The effect of the remnant film is particularly noticeable at the piston rings (see illustration).
The face of the segment (2) is under load for 3/4 of the cycle and is in contact with face (1) for 1/4 of the cycle, during ingress of gas. The load during explosion is distributed on surface "s".
 
 

The more the engine is worn, the more the surface area "s" is reduced : the wear tends to push the sleeve back to position B (ovalising effect).
To ensure tightness, "seating" of the piston ring becomes more & more important. To avoid this classic problem, it is imperative that the piston ring maintains all its mobility. As surface "s" tends to diminish, the load per cm2 increases considerably & can cause the oil film to break & to escape.
 
In this case, one is left without a lubrication layer, with one "hard surface" (steel) interacting with a "soft surface" (piston made of light alloy).
This produces a veritable breaking effect on the piston ring movement, resulting in premature wear.
 
In these conditions, the piston ring looses its mobility, as well as the required tightness. It provokes :
 
• loss of compression
• the loss of oil into the cylinder, causing production of Co gases
• an increase in oil consumption
• calamine deposits in space (E), which impede the mobility of the piston ring & prevent it from doing its job.