Blow-by gas is a mixture of air, fuel, combustion gases, and vaporized oil that leaks past the piston rings during the combustion process in an internal combustion engine. This is typically a small amount and is accounted for in the engine design through the use of a positive crankcase ventilation (PCV) system that recirculates the blow-by gases back into the intake manifold to be burned off during combustion.
However, when an engine ages or wears out, the gaps between the cylinder walls and the piston rings can become larger. This can lead to an increased amount of blow-by gases. Here's how it can contribute to oil consumption:
- When more blow-by gases leak past the piston rings, more oil vapor is included in that mixture. This vapor can condense and form droplets in the PCV system, which are then carried back into the combustion chamber to be burned off, leading to increased oil consumption.
- Excessive blow-by can also cause more oil to be splashed onto the cylinder walls. If too much oil ends up on the cylinder walls, the piston rings may not be able to scrape it all back down into the oil pan. The excess oil on the cylinder walls can then get burned in the combustion process.
- Further, if the increased blow-by gases overwhelm the capacity of the PCV system, it can cause pressure to build up in the crankcase. This high pressure can force oil through seals and gaskets, leading to oil leaks. This is another way that blow-by can contribute to oil consumption.
So, while a certain amount of blow-by is normal and expected in an operating engine, excessive blow-by can lead to increased oil consumption due to the reasons above.