Understanding Blowby in an Engine

The Impact of Blowby in Engine Operation: A Comprehensive Guide

Blowby is a phenomenon that occurs in internal combustion engines where combustion gases escape past the piston rings and enter the crankcase. While often overlooked, blowby can significantly impact engine performance and longevity. This article delves into what blowby is, how it forms, and its effects on an engine, aiming to provide a comprehensive understanding of this crucial aspect of engine operation.

What is a piston ring gap?

All piston rings are formed in a circle. But to install the ring on the piston, it must have an opening. This opening is referred to as the ring gap”. During installation the installer expands the ring with a ring expander tool and inserts it onto the piston. When first installed, the gap is quite large as shown in the image #1 below. However, once the piston rings are compressed and installed in the cylinder head, the ring gap closes as shown in image #2.

Piston rings installed on piston but not staggered. This image just shows how large the piston ring gap is before compression.

At the factory, the assemblers install the rings so the gaps are staggered around the piston. The staggered ring gaps reduce blowby in an engine

To prevent excessive blowby, the installer staggers the location of the ring gaps so the blowby gas encounters a maze-like path out of the cylinder. See image #3

Once the piston is installed in the cylinder, the gap closes, but it’s never fully closed

However, no matter how small the gap or how well the gaps are staggered, blowby manages to escape past the piston ring.

What’s in Blowby?

Ideally, the piston rings should create a near-perfect seal between the combustion chamber and the crankcase. However, in practice, some amount of gas inevitably escapes past the rings. These gases primarily consist of unburned fuel, air, and combustion by-products like carbon dioxide, carbon monoxide, soot, and various hydrocarbons.

Factors that determine how much blowby an engine has

• Piston Ring Tension and Wear—Despite engineering precision, piston rings can never create a perfect seal. Some carmakers install low-tension piston rings to reduce friction and improve gas mileage. Some technicians believe that those low-tension piston rings create more blowby. However, the engineers disagree on this. But there’s no denying that over time, all piston rings wear, increasing the gap between the piston rings and the cylinder, allowing more gases to escape.
• Ring Flutter and Blowby—  At high engine speeds, piston rings can experience a phenomenon known as “ring flutter,” where the rings momentarily lose contact with the cylinder wall, providing a pathway for gases to leak into the crankcase.
• Cylinder Wall Condition— The condition of the cylinder walls also plays a significant role. Any scoring, pitting, or wear can enhance the likelihood of blowby.
• Engine Load and Temperature— Higher engine loads and temperatures can increase the pressure differential across the piston rings, exacerbating blowby.

The Effects of Blowby on Engine Performance

Blowby can have several detrimental effects on an engine’s performance and longevity:

• Blowby Contaminates Engine Oil— The combustion gases that constitute blowby carry with them unburned fuel and combustion by-products like water and soot. When these gases enter the crankcase, they can contaminate the engine oil, reducing its lubricating properties and accelerating wear on engine components. The amount of unburned fuel and water is much higher during cold start-ups due to the much higher air/fuel mixture needed to start a cold engine. In addition, the cold cylinder head tends to quench the combustion early, causing more unburned fuel, soot, and water to enter the crankcase. If the vehicle isn’t driven long enough to heat the oil to the point where it can evaporate the fuel and water and long enough to filter out the soot, those contaminates will stay in the oil, causing sludge formation and oil additive depletion.
• Increased Crankcase Pressure— The presence of blowby gases in the crankcase increases the internal pressure. If this pressure is not properly vented or routed to the intake manifold to be burned, it can lead to oil leaks and potentially blow seals and gaskets.
• Oil Consumption— Blowby can lead to increased oil consumption. Contaminated oil can lead to the formation of sludge, which can block oil passages and lead to inadequate lubrication.
• Reduced Engine Efficiency— Blowby reduces the overall efficiency of an engine. The loss of combustion pressure means less force is exerted on the piston, resulting in reduced power output.
• Emissions— Blowby gases, when vented back into the intake system, can increase the level of pollutants in the exhaust, affecting the vehicle’s emissions performance and potentially leading to failed emissions tests.

How to mitigate the effects of blowby

While blowby cannot be entirely eliminated, several strategies can be employed to minimize its effects:

• PCV System— Positive Crankcase Ventilation (PCV) is the single most important feature in reducing the harmful effects of blowby on crankcase oil. The PCV system uses manifold vacuum to suck blowby gases from the crankcase back into the intake manifold, where they are burned again. Keeping the PCV system in good working order is crucial for managing blowby.

• Avoiding short trips, especially in cold weather— Short trips of less than 5 miles in cold weather and less than 10 miles in warm weather prevent the oil from reaching its full operating temperature of around 200°F, which is high enough to evaporate the fuel and water from the oil. If you start a cold engine and drive short trips, the contaminants in the blowby stay in the oil, causing acid formation, sludge, and anti-corrosion additive depletion.

• Regular Maintenance— Ensuring regular maintenance, including oil changes and using high-quality oil, can help in managing blowby. Fresh oil maintains its lubricating properties better and can help in sealing the piston rings more effectively.

©, 2019 Rick Muscoplat