Today I thought I would give a little intro to piston ring issues and particularly sticking and wearing causing blow-by. This is made based on a series of questions on the topic I received which can be read as standalone answers but I have tried to combine into a single article on the topic. Many of you will already be familiar with blow-by and the general layout of a combustion engine and particularly a piston ring and piston assembly, but just briefly to start with, a simple intro for those who need a refresher in the explainer box below.
Engine blow-by is characterised by the escape of high-pressure gases from the engine’s cylinders into the crankcase. This can cause damage to the engine and its components, and can lead to costly repairs. One of the primary causes of engine blow-by is worn, sticking or damaged rings. Rings are small, circular components that are located in the engine’s cylinders and are used to seal the cylinder and create a controlled combustion chamber. When the rings are worn, stuck or damaged, they can no longer create a proper seal, which can allow high-pressure gases to escape into the crankcase.
Quick recap on what is a piston ring and how they cause blow-by.
Now you have a a refresher on the basics, if you have ever fitted a piston or piston ring you will know it is an exceptionally tight fit and you would be forgiven for believing there was no room for movement between the two and indeed that there was no gap between these and the cylinder bore.
“Piston rings actually move closer and further away from the piston during the combustion process as part of the design “
Adam – Founder of Learn Oil Analysis
At the microscopic level there is a lot going on though. Firstly the final sealing is actually performed by the lubricant rather than the rings. That makes sense as without a final layer of lubricant you would have metal to metal contact.
What you may not know is the piston rings actually move during the combustion process perpendicular (at right angles) to the piston movement. This is because the top rings often have tapering to allow combustion gases behind and to press the rings further outwards during combustion to reduce gas escape. Once the pressure drops they retract inward to allow smooth passage of the piston back up. On the bottom side the rings slightly twist the angle of contact (similar to how you change your trowel for plastering or knife to ice a cake). This twisting allows a fine blade like effect to scrape oil down on the downward stroke but a more flatter surface on the upward stroke to prevent drawing oil back up. A variation on these are often called scraper rings for this purpose.
Hopefully you now get the idea that the rings being able to move smoothly and create an effective seal is important for the engine to work. Failure to do so means there is poor sealing between the upper and lower cylinder.
So what can go wrong?
Piston rings main two issues are sticking and wearing. Sticking causes them not to move freely as they should to switch between their different positions of the stroke. Wear equally causes them to not function as well too as a damaged ring cannot seal as well as it should.
What causes sticking rings?
Generally this is oxidation of the lubricant. As an oil ages oxygen molecules and free radicals cause the relatively short chain oil molecules to link together until they form thick, sticky gummy deposits often termed varnish. These coat components and prevent things moving properly. This is not a problem exclusive to engines and I have whole articles written on the subject of varnish. These in an engine are linked to low base number, high acid number and oxidation.
What causes wearing rings?
It is worth remembering the that Silicon and aluminum in oil analysis can indicate dirt ingress into the engine. This is important because dirt is one of the most common causes of ring wear, which is a type of wear that occurs on the rings that seal the gap between the piston and cylinder wall in an internal combustion engine. Ring wear can have several negative effects on the engine, including increased oil consumption, reduced power output, and increased emissions. Other causes of ring wear include improper break-in procedures, improper lubrication, and running the engine at excessive speeds or under heavy loads for extended periods of time.
Hence hopefully you get the idea, that it is important to regularly check the oil and address any issues with dirt ingress or ring wear as soon as possible to prevent further damage to the engine.
What happens when rings stop working properly and how can oil analysis detect it?
Rings play a crucial role in the functioning of an engine. They need to move freely in order to seal the combustion chamber and prevent the escape of gases. If the rings become stuck, it can cause a number of problems. For example, oil may flow in one wrong direction while gases go in another wrong direction, leading to inefficient combustion and potentially damaging the engine. If oil goes the wrong direction up to the piston crown it burns. Burnt oil can produce ash, which can lead to coking (a build-up of carbon) in the engine. Additionally, the escape of combustion gases can cause a range of issues, including reduced power and efficiency.
The escape of combustion gases can have a number of negative impacts on an engine all detected with oil analysis.
One of the main issues of combustion gases going into the crank case is that fuel follows. Fuel lowers viscosity, leading to increased wear on the engine. Additionally, soot produced during combustion can agglomerate and create abrasives, which can cause further wear and increase the viscosity of the oil. These wear particles, viscosity changes, soot and wear can oil be detected in an oil sample.
Solid carbon particles, such as soot, can increase the viscosity of a fluid due to their thixotropic nature. Thixotropy refers to the ability of a substance to change viscosity in response to external stimuli, such as shear stress or temperature. Some materials, including soot suspensions, can become more fluid when subjected to shear stress (such as when they are stirred or agitated), but will revert to a more viscous state when the shear stress is removed. This can cause the viscosity of the fluid to increase over time as the soot particles settle and the fluid becomes less agitated.
Quick question from Tim in Canada:Why does soot increase the viscosity of an oil.?
In addition to their thixotropic nature, the small size and high surface area of soot particles can also contribute to their ability to increase the viscosity of a fluid. The small size of the particles allows them to easily suspend in the fluid, while their high surface area allows them to interact with and potentially adhere to one another, further increasing the viscosity defined as resistance to flow. In some circumstances an oil can become solid with very high soot called gelling under certain conditions.
Another problem of combustion gases passing to the crank case is combustion gases can also include water and nitrogen oxides (NOx), which can lead to the formation of nitric acid. This acid can be corrosive and can cause damage to the engine and other parts of the vehicle. It is important to ensure that combustion gases are properly sealed within the engine combustion chamber in order to prevent these negative impacts and ensure the engine operates efficiently and effectively. Although the gases are not detected directly in the oil, the acids formed are with high acid and low base numbers of an oil.
So how do I confirm there is an issue with sticking or worn rings after it is suggested by an oil analysis?
One way to confirm the presence of worn rings is through oil analysis. If the oil analysis data suggests the presence of worn rings, there are several tests that a mechanic can perform to confirm this diagnosis. These tests may include:
Compression test
A compression test measures the pressure inside the engine’s cylinders and can help to identify any problems with the rings. To perform this test, the mechanic will remove the spark plugs and use a compression gauge to measure the pressure in each cylinder. If the pressure is low in one or more cylinders, it could indicate a problem with the rings.
Leak-down test
A leak-down test measures the amount of air that escapes from the engine’s cylinders when the engine is not running. To perform this test, the mechanic will remove the spark plugs and inject pressurised air into the cylinders. If a significant amount of air escapes, it could indicate a problem with the rings.
Oil consumption test
An oil consumption test measures the amount of oil that is being consumed by the engine over a period of time. If the engine is consuming more oil than usual, it could be a sign of worn rings or other problems. This is quite common test performed and we even have a field of fluid added on our sample registration process that customers can fill in at each sample so we can help customers monitor issues.
Summary
In conclusion, engine blow-by can be caused by a variety of factors, but worn or stuck rings are a common cause. By performing tests such as oil analysis, a compression test, leak-down test, and oil consumption test, a mechanic can confirm the presence of worn rings and take the necessary steps to repair or replace them before the problem becomes more serious.
Regular oil analysis is the best single action you can take to head problems off early and reduce the risk of a catastrophic failure. If you would like to chat about your piston rings, pistons, bearings, liners or any other part of your engine or indeed any of your machinery please reach out and click the contact us button on the bottom right of the screen.