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A portion of the air-fuel mixture is drawn downward by the piston and seeps past the piston rings into the crankcase. This leaked gas, known as blow-by, is inevitable. Explore more engine visuals.
Alexey Dudoladov/the Agency Collection/Getty ImagesUnless you're an automotive enthusiast, the term 'positive crankcase ventilation' might sound intimidating and complex. However, it's simpler than it appears. By the end of our explanation, you'll find it straightforward. To achieve this, we'll provide a brief overview of how internal combustion engines, commonly found in cars, operate. Ready? Let's dive in!
Internal combustion engines are designed with a series of hollow cylinders, each housing a piston that moves up and down. An air-fuel mixture is delivered through the intake manifold to each cylinder's intake valve, where a spark plug ignites the mixture in the combustion chamber. The resulting explosion forces the piston downward, rotating the crankshaft. This rotation not only returns the piston to its original position but also drives the car's transmission, propelling the vehicle. Simultaneously, the piston expels the leftover gases through the exhaust valve.
However, some of the air-fuel mixture is drawn downward by the piston and escapes past the piston rings into the crankcase, the protective housing for the crankshaft. This escaping gas, called blow-by, is unavoidable and problematic because the unburned fuel can contaminate the system. Before the 1960s, blow-by gases were vented directly into the atmosphere. The introduction of positive crankcase ventilation (PCV) in the early 1960s marked the beginning of automotive emission control systems.
Positive crankcase ventilation works by redirecting blow-by gases through a PCV valve to the intake manifold, where they are reintroduced into the cylinders for combustion. However, these gases, which are mostly air, can dilute the fuel-air mixture, making it too lean for efficient combustion. Therefore, recycling is optimal during low-speed driving or idling. At idle, the intake manifold's lower pressure draws the gases through the PCV valve. At higher speeds, the manifold's pressure increases, reducing gas flow. If the manifold pressure exceeds the crankcase pressure, the PCV valve closes to prevent backflow, ensuring gases don't re-enter the crankcase.
PCV System Oil and Air Separator
The crankcase serves as an oil reservoir, typically located beneath the crankshaft. While the crankshaft and oil are meant to remain separate, oil vapors can mix with blow-by gases. Recirculating these vapors into the cylinders can make the fuel-air mixture overly combustible, akin to reducing gasoline's octane rating. This can slightly hinder performance in some engines and cause backfiring in older models. Additionally, oil vapors can form a film in the air intake, gradually obstructing airflow. While not critical for non-performance vehicles, some prefer using an oil and air separator to prevent these issues.
An oil and air separator removes oil from blow-by gases before they re-enter the intake manifold, redirecting the oil to the crankcase or a catch can. Not all vehicles come equipped with separators, but they can be added as aftermarket components or even homemade. Common designs include mesh filters that trap oil droplets while allowing air to pass, tube-based systems that separate oil via gravity, and centrifuges that force oil to the sides for collection. These systems help maintain engine efficiency and prevent oil buildup in the intake system.
