The crankcase ventilation system is a critical but often overlooked aspect of internal combustion engine design that manages harmful gases created during engine operation. This system captures blow-by gases (combustion gases that leak past piston rings), prevents pressure buildup in the crankcase, and reduces environmental pollution. Modern crankcase ventilation systems consist of several precisely engineered components working together to maintain engine health, performance, and longevity while meeting environmental regulations. These sophisticated systems have evolved from simple road draft tubes to complex integrated systems that effectively manage crankcase gases through specialized valves, separators, and pathways.
Types of Modern Ventilation Systems
Modern crankcase ventilation systems can be categorized as:
1. Natural ventilation - an older type dependent on back pressure formed when the automobile is running, allowing crankcase gases to be extracted through the air cleaner and throttling valve
2. Positive ventilation - the prevalent modern approach that uses engine intake vacuum to draw gases into the intake manifold at the appropriate time for re-combustion
These systems can be further classified as "ingestive" (where gases return to the combustion chamber) or "non-ingestive" (where gases are filtered but not returned to the combustion chamber).
Core Components of Crankcase Ventilation Systems
PCV Valve: The System Regulator
The Positive Crankcase Ventilation (PCV) valve is the central controlling component of the modern crankcase ventilation system. This seemingly simple device is a spring-loaded valve with a sliding, tapered pintle inside that regulates the flow of gases from the crankcase to the intake manifold.
The PCV valve performs several critical functions:
1. It adjusts airflow based on engine operating conditions, controlling how much crankcase gas enters the intake system
2. It closes when manifold vacuum is high (such as at idle) to restrict crankcase gas flow, preventing the engine from running too lean
3. It opens wider under load or at higher RPM when more blow-by gases are produced and when the intake system can accommodate them without compromising performance
4. It acts as a flame arrester, closing when positive pressure develops in the intake system (such as during backfire events or in turbocharged engines under boost) to protect the crankcase from damage
In forced induction applications (turbocharged or supercharged engines), the PCV valve takes on additional importance. It must isolate the crankcase from the intake manifold when the latter is under positive pressure to prevent that pressure from further increasing crankcase pressure.
Oil Separator/Trap: Preventing Contamination
The oil separator is a crucial component that prevents oil from entering the intake system while allowing gases to pass. Two main types exist:
1. Centrifugal separators - use centrifugal force to separate oil from gases
2. Labyrinth-type separators - use a series of baffles and passages that cause oil droplets to condense and return to the crankcase
Oil separators work on the principle that liquid fractions (oil and fuel) have higher density than gases. As the blow-by gases pass through the separator, the heavier oil particles are removed and returned to the crankcase, while only the gaseous components continue through the system.
Oil separators can be mounted in different locations depending on engine design. On older BMW engines (pre-N52), the oil separator is placed outside the engine, while on newer engines like the BMW N53, it's integrated into the top of the cylinder block opposite cylinder No.5.
Breather System: The Fresh Air Inlet
For the PCV system to function properly, it requires a source of fresh air. This is provided by the crankcase breather, which is typically ducted from the engine's air filter or intake manifold.
The breather components include:
· Breather caps or vents (often located on valve covers)
· Filters to prevent contaminants from entering the crankcase
· Baffles to prevent oil mist from escaping through the breather
A properly designed breather system not only provides fresh air but also helps create a "scavenging effect" - a slight suction within the crankcase that aids in removing blow-by gases. The breather is generally placed as far as possible from the crankcase air outlet (where the PCV valve is located) to ensure efficient air circulation through the entire crankcase.
Connecting Hardware: Hoses, Tubes, and Fittings
The system's components are connected by various hoses, tubes, and fittings that create the pathways for gas flow. These include:
· Crankcase vent-pipes that direct gases from the crankcase
· PCV hoses that connect the PCV valve to the intake manifold
· Breather hoses that bring fresh air into the system
· Clamps and connectors that secure the connections and prevent leaks
A complete ventilation kit typically includes multiple hoses, clamps, gaskets, and connectors specific to the engine design. For example, a crankcase ventilation kit might contain components like:
· Clamps (14-27 mm)
· Oil trap hose block exit
· Various hoses for different connections
· Manifold gaskets
· Sealing rings
System Operation and Integration
How the Components Work Together
The crankcase ventilation system operates as an integrated whole:
1. Blow-by gases from combustion leak past the piston rings into the crankcase
2. Fresh air enters through the breather, mixing with and diluting these gases
3. The mixture flows through the crankcase, picking up additional blow-by gases
4. The gases pass through the oil separator, which removes oil droplets
5. The PCV valve regulates flow based on engine operating conditions
6. The cleansed gases enter the intake manifold to be burned during combustion
This continuous flow helps remove harmful gases and moisture from the crankcase while maintaining appropriate pressure levels.
System Variations by Engine Type
The specific design and component integration vary by engine type:
· Naturally-aspirated engines typically connect the extraction tube to the intake manifold
· Turbocharged engines require special considerations due to the potential for positive pressure in the intake manifold
· Two-stroke engines with crankcase compression design don't need a crankcase ventilation system because their normal operation involves sending crankcase gases to the combustion chamber
Modern engines may incorporate the ventilation system directly into the engine design, with integrated passages and chambers rather than external hoses and components.
Maintenance and Failure Points
Common Failures
The most common failure points in crankcase ventilation systems include:
1. Ruptured rubber membranes in PCV valves or breather diaphragms
2. Clogged oil separators, particularly in labyrinth-type designs where oil slag and dirt can accumulate on the walls
3. Carbon buildup in intake systems due to failing PCV systems
4. Blockage of PCV valves and associated pipes, leading to issues like unstable idle speed
These failures can lead to increased crankcase pressure, oil leaks, sludge formation, and decreased engine performance.
Diagnostic Signs
Signs of a failing crankcase ventilation system include:
1. Oil leaks from seals and gaskets due to excessive crankcase pressure
2. Thick whitish paste (a mixture of oil and water) visible when removing the oil filler cap
3. Oil consumption and blue exhaust smoke
4. Unstable idle speed or rough running
5. Moisture and sludge buildup in the oil
Maintenance Considerations
Proper maintenance of the crankcase ventilation system is essential for engine longevity:
1. Regular inspection of PCV valves and breathers for clogs or deterioration
2. Cleaning or replacing oil separators according to manufacturer recommendations
3. Checking hoses for cracks, blockages, or disconnections
4. Ensuring proper operation of the complete system, especially after engine modifications
For direct injection engines, carbon buildup on intake valves can be a particular concern, as fuel system cleaners added to the tank won't reach these deposits. Professional cleaning methods may be required.
Conclusion
The crankcase ventilation system, though often overlooked, plays a vital role in engine health, performance, and environmental compliance. Its components—PCV valves, oil separators, breathers, and connecting hardware—work together to manage crankcase gases, prevent pressure buildup, and reduce emissions. Understanding these components and their functions is essential for proper engine maintenance and troubleshooting.
As emissions regulations continue to tighten worldwide and engine designs evolve, crankcase ventilation systems will likely become increasingly sophisticated, with greater integration into engine designs and improved filtering and separation technologies. These advancements will continue to enhance engine efficiency, reduce environmental impact, and extend engine life by better managing the inevitable blow-by gases produced during combustion.
