During the combustion event, the intent of the engine design is to contain the combustion gases in the combustion chamber to load the piston and drive the piston to turn the crankshaft. It is the intent of the power cylinder system to seal the gases and to control the oil film on the power cylinder parts. The gas that escapes around the rings and piston is referred to as blow-by gases or just blow-by. The power cylinder system and individual components are designed to seal the gases. The understanding associated with which features affect blow-by is important in the engine design and development process.

When diagnosing blow-by problems the engineer must be aware that other engine systems can affect blow-by. These systems are as follows: Power cylinder system (piston, rings, liner); Head gasket; Cylinder head system (valves, valve stem guides and seals, etc.); Turbocharger; Supercharger; Air compressors.

This document describes general methods for determining the causes of high blow-by so that it might be corrected.