When Good Oils Go Bad Part 5
By Steven Lumley, technical manager, WearCheck
This edition of WearCheck’s “When Good Oils Go Bad” series gives insight into microdieseling.
There’s a creature in the ocean that kills its prey without ever touching it.
Say hello to my little friend – Synalpheus fritzmuelleri, better known as the pistol shrimp, and arguably the deadliest gunslinger in the sea.
At just 3-5cm in length, it hardly looks like a formidable predator. But this tiny shrimp has a rather dramatic way of proving that appearances can be deceiving.
With a snap of its oversized claw, it fires a cavitation bubble through the water at speeds of around 100 km/h. When that bubble collapses, it produces a shockwave reaching sound levels of up to 218 dB, louder than a jet engine at close range, and briefly generates temperatures of approximately 4720°C.
A tiny, collapsing bubble generating temperatures hotter than molten steel – its prey doesn’t stand a chance.
So, what does this little vignette have to do with microdieseling? Quite a lot, actually.
Because this isn’t just a marine biology curiosity – it’s a perfect demonstration of the destructive power of collapsing bubbles. The same fundamental phenomenon occurs in lubricated systems every day.
Not just in the ocean – inside your oil, thousands of times per second. Welcome to the explosive world of microdieseling.
When Air Isn’t Just Air
Air is one of the most overlooked contaminants in lubrication systems. It’s invisible, it’s everywhere and – under normal conditions – it doesn’t seem particularly harmful.
But I’m about to burst your bubble.
When air becomes entrained in oil (existing as tiny, unstable bubbles), it creates the perfect conditions for one of the most destructive degradation mechanisms in lubrication. Because under pressure, air doesn’t behave like a harmless contaminant – it behaves like fuel.
What Is Microdieseling?
Microdieseling is a form of pressure-induced thermal degradation, also known as compressive heating. It occurs when entrained air bubbles move from a low-pressure zone to a high-pressure zone within a system.
As this happens, the bubbles are rapidly compressed. Because this compression occurs faster than heat can dissipate (a process known as adiabatic compression), the temperature inside the bubble rises dramatically. In many cases, these temperatures exceed 1000°C.
At this point, something remarkable, and destructive, happens – the oil surrounding the bubble doesn’t just degrade, it burns.
Diesel Combustion on a Microscopic Scale
The term microdieseling is no exaggeration – it is called “dieseling”, because it mimics the combustion process of a diesel engine on a microscopic scale. Just like in a diesel engine, compressed air leads to ignition, but in this case, the “fuel” is the oil itself.
As the bubble collapses, the surrounding oil is exposed to extreme localised temperatures and the bubble interface becomes carbonised, resulting in partial combustion. The result is the formation of carbon (soot), tars, sludge and varnish-type, resinous deposits. And just like in a diesel engine, this combustion is incomplete, leaving behind solid carbon by-products that contaminate the oil and deposit on surfaces.
Why the Damage Varies
Not all microdieseling events are created equal.
The intensity of the bubble collapse, specifically the implosion pressure, determines the type of degradation products formed. High-intensity collapse tends to produce combustion-related by-products such as soot, tars and sludge. Lower intensity events are more likely to produce varnish, resins and coke-like deposits.
This explains why some systems experience heavy soot loading and rapid oil darkening, while others suffer from varnish, sticky deposits and valve stiction. Different conditions, same root cause.
More Than Just Oil Degradation
Microdieseling doesn’t just damage the oil – it affects the entire system. Each collapsing bubble generates extreme local temperatures, shockwaves and micro-jets of oil. Over time, these repeated micro-events lead to surface-pitting and erosion-increased wear, valve-sticking due to varnish deposits, and reduced efficiency with sluggish system response.
In hydraulic systems, the presence of air further reduces the oil’s ability to transmit pressure, resulting in that familiar “spongy” behaviour. So, while the explosions may be microscopic, the consequences are anything but.
What Causes Microdieseling?
For microdieseling to occur, two key ingredients are required- air in the system and a rapid change in pressure.
Air in the system
Air can enter or form in oil through:
- Suction-side leaks
- Low oil levels
- Turbulence and agitation
- Contamination or poor handling practices
It can also form when dissolved air comes out of solution due to pressure drops.
Rapid pressure changes
These are commonly created by:
- Flow restrictions and orifices
- Cavitation conditions
- Valve operation (opening and closing)
- Pumps (especially across suction and discharge zones)
In fact, microdieseling often occurs in the same regions where cavitation is present, but with an important difference: cavitation collapses bubbles – microdieseling ignites them.
Detecting Microdieseling Through Oil Analysis
Microdieseling is often a hidden problem, but it leaves behind a distinct signature. Unlike other degradation mechanisms, it combines elements of thermal stress, oxidation and combustion, resulting in a unique mix of indicators. Rapid oil-darkening due to soot-formation, increased insoluble content, elevated oxidation levels and high varnish potential are all common signs. The challenge, however, is that many of these indicators appear later in the degradation process, making early detection difficult without targeted analysis and a clear understanding of the underlying mechanism.
The key tests and indicators used to assess microdieseling and its associated degradation effects are summarised below.
The Real Problem
It is tempting to treat microdieseling as an oil failure, but it isn’t. Microdieseling is fundamentally an air-management problem. As long as air is present in the system and subjected to rapid pressure changes, the conditions for these microscopic combustion events will exist. Control the air and you control the problem.
Final Thought
While great in champagne, tiny air bubbles can have very different consequences in lubricating oils and the systems they protect. Those tiny bubbles drifting through your oil may look harmless. But, under the right conditions, they become something far more destructive – collapsing, igniting and releasing bursts of energy powerful enough to degrade the oil, damage components and ultimately shorten equipment life.
In lubrication, it is often the smallest things that cause the biggest problems, and when it comes to microdieseling, those tiny bubbles are anything but harmless.
Look out for the next instalment of this series, where we explore Electrostatic Spark Discharge – when static strikes back.
Please visit www.wearcheck.co.za or contact WearCheck on marketing@wearcheck.co.za or +27 (31) 700-5460.