blow-by
In an internal combustion engine, any burnt or unburnt matter escaping from the cylinder, past the (worn) rings on the piston, and into the crankcase.
blow-by: combustion gases leaking past piston rings
Blow-by occurs when high-pressure gases from the combustion chamber escape downward past the piston rings into the crankcase during the power and compression strokes. This leakage is not a design feature but a consequence of wear. As piston rings age, their sealing surfaces round and gap, allowing gases to slip by. In a new engine, blow-by is minimal, typically 0.5 to 2 percent of the air intake volume at idle. By 100,000 miles, it can exceed 5 percent in a worn engine.
The gases that blow by are a mixture of unburnt fuel, combustion products (mainly carbon dioxide and water vapor), and soot. This mixture pressurizes the crankcase, creating back pressure that forces oil past seals and gaskets, causing leaks. The water vapor condenses on cold cylinder walls and mixes with oil, forming a milky sludge that degrades oil quality and clogs passages.
Detection and consequences
Mechanics detect excessive blow-by by measuring crankcase pressure or by observing oil mist exiting the positive crankcase ventilation (PCV) system. A simple roadside check involves placing a cloth over the dipstick tube while the engine idles; strong gas flow indicates heavy blow-by. High blow-by raises fuel consumption, lowers power output, and forces the PCV system to work harder to evacuate the excess pressure.
The term itself comes from the action: gases blow past the rings and by the piston into the crankcase. Blow-by is inevitable with piston rings, which are designed to maintain contact through springiness rather than hermetic sealing. Ring blow-by increases when engines run at light load, when cylinder temperatures drop and ring tension weakens, or when fuel dilutes the oil and reduces its viscosity on the ring surfaces.
Ring wear accelerates in engines that run short, cold trips without reaching full operating temperature, in vehicles with high-mileage intervals between oil changes, or in engines operating on heavily ethanol-blended fuels. Diesel engines, which operate at higher peak pressures, typically experience more blow-by than gasoline engines of equivalent age.