What causes engine blow-by?
Blow-by in an internal combustion engine is where the expanding gases caused by the heat of combustion are allowed to escape past the piston into the crankcase. Escaping gases cause a loss of power and efficiency (since any gases that escape are not pushing on the piston to drive the crankshaft).Blow-by is supposed to be prevented by the piston rings. These are metal rings that fit into grooves around the head of the piston. They are almost circular, and are compressed down as the piston is inserted into the cylinder, so that they push outwards against the cylinder wall, sealing the piston into the cylinder. Multiple rings are used to keep a good seal, since a single ring is not a perfect seal (having a small gap where the two ends of the ring meet).Blow-by can be caused by:Stuck piston rings. Over time, carbon and other gunk can build up around the rings, sticking them into their grooves so that they don’t expand to seal against the cylinder wall. The main way to prevent this is regular oil changes.Worn/scored cylinder walls. Dirt and other detritus in the cylinder can be carried along with the piston, scoring the wall and creating space where gas can escape past the rings.Worn rings. The rings are supposed to be sized such that, once fitted into the cylinder, there is only a very small gap between the two ends. If the ring is worn, or the wrong size has been fitted, the gap may be larger than it should be, allowing more gas to escape.Broken rings. The rings are essentially compressed springs, and they can fracture. If this happens, they will no longer push against the cylinder walls, and therefore they will not seal properly.Incorrectly-fitted rings. The rings have a gap at one point around the edge. The gap is very small (see above), and they are supposed to be fitted so that those gaps do not line up (so that escaping gases would have to travel a long, tortuous route around the circumference of the piston to follow the route through the gaps). If they are incorrectly fitted so that the gaps line up, this would make it easier for the gases to leak out.