Blow by in a gas engine is the uncontrolled passage of combustion gases past the piston rings into the crankcase. This phenomenon is a direct consequence of the internal pressures generated during the combustion cycle and the necessary gaps required for piston ring operation. Understanding the specific pathways and root causes is essential for diagnosing reduced performance, increased oil consumption, and potential long-term engine damage.
How Combustion Pressure Escapes
The primary mechanism for blow by is the pressure differential created when the cylinder fires. During the power stroke, the explosion forces gases against the piston crown. The piston rings, specifically the compression rings, are designed to seal this gap. However, the rings must maintain a slight gap to allow for thermal expansion and oil film lubrication. High cylinder pressures, often found in turbocharged or high-compression engines, can force gases past these gaps. Additionally, ring end gaps aligned improperly or worn cylinder walls provide direct channels for these gases to bypass the sealing action.
The Role of Piston Ring Dynamics
The effectiveness of piston rings is not static; it relies on consistent and correct seating against the ring lands and the cylinder wall. Blow by often originates when these rings fail to maintain this seal. Ring flutter, where the ring vibrates against the cylinder wall at high RPMs, can prevent proper sealing. Furthermore, incorrect ring gaps or using low-quality rings that do not maintain tension contribute significantly to the issue. The rings are designed to ride on a thin layer of oil, and if this oil control is lost, pressure can build up behind the ring and force it against the bore.
Contributing Mechanical Factors
While ring and cylinder condition are primary, other engine components play a critical role in managing blow by. The Positive Crankcase Ventilation (PCV) system is designed to manage the gases that do escape. If the PCV valve is clogged or the system is blocked, pressure builds in the crankcase, overwhelming the oil separator and forcing gases back past the rings. Similarly, excessive wear in the cylinder bore, such as scoring or tapering, reduces the surface area available for the rings to seal, creating gaps that grow worse as the engine ages.
Identifying the Symptoms
Drivers often notice the effects of blow by before they understand the cause. A common indicator is blue smoke from the exhaust, which signifies oil being burned in the combustion chamber as a result of blow by contamination. A sudden drop in oil level without visible external leaks is another red flag. Moreover, a misfire or rough idle can occur if the pressure leak disrupts the optimal combustion pressure required to drive the piston efficiently. Recognizing these signs early can prevent more severe internal damage.
Preventative Measures and Solutions
Addressing blow by requires a dual approach: managing the pressure and maintaining the sealing components. Regular oil changes with high-quality detergent oils help keep combustion byproducts in suspension and maintain ring flexibility. Monitoring and cleaning the PCV valve and associated hoses ensure the crankcase remains at proper pressure. For more severe cases, such as significant cylinder bore wear or persistent ring failure, a rebuild with new pistons and rings is necessary to restore the pressure seal and eliminate the blow by path.
