Dark Hydraulic Oil? It could be due to Micro-Dieseling



Isn’t 1093 degree Celsius (2,000 degrees F) hot?

At this temperature, aluminum, copper, gold and iron would have melted. Stainless steel and carbon steel would glow red.  Your Thanksgiving turkey would turn into a charred mess in a second. What is so significant about 2,000 degrees? Did you know many hydraulic systems can create temperatures in this range?

Diesel Engine Working & Cavitation:

Most people are familiar with diesel engines and how they work, which includes a rapid compression of air.  When you inject a fuel source, it creates an explosion.  Did you know that micro-dieseling is similar?  A front-end loader that runs a diesel engine is a macro level (or big picture) of what it does on the micro (or smaller) level in a hydraulic pump. As oil enters the suction side of the pump it is subjected to the lowest pressure in the system.  Dissolved air is released from the oil.  As the oil travels through the pump, the pressure increases rapidly which causes rapid air bubble compression.  This process of rapid air compression in the presence of a fuel source basically describes how a diesel engine operates.  This phenomenon without the thermal event is also called cavitation.  You may not hear micro-dieseling occur unlike most severe cases of cavitation.  It may be subtle, but if your hydraulic oil is darkening during use, there is a thermal event occurring and there is a good chance it is micro-dieseling.


Cavitation is the process of formation of bubbles in a liquid, typically by the movement of a propeller through it.  If the gas bubbles pass through a high pressure zone, they will violently collapse. This alone can cause serious reliability issues with the machine component in terms of vibration, noise, surface damage and potentially failure. 

What is micro dieseling?

Micro-dieseling in simple terms is a pressure-induced thermal degradation.

How Micro-dieseling Occur?

An air bubble transiting from a low-pressure area to a high-pressure area as it passes through the pump.  The rapid compression of these air bubbles around a fuel source, hydraulic oil, generates an enormous amount of heat and causes thermal breakdown of the oil surrounding the air bubble. Micro-dieseling is caused when an air bubble in the oil is rapidly and adiabatically compressed causing extreme local temperature increases.  An air bubble will move from a low or negative pressure area to a high-pressure zone and through adiabatic compression get heated to very high temperatures.



  • These temperatures are high enough to carbonize oil at the bubble interface, resulting in carbon byproducts (sludge and varnish) as well as increased oil degradation (oxidation).
  • Oil thermal breakdown.
  • Pressure spikes.
  • The cavitational erosion of the hydraulic pump and other components.
  • Thermal event, ignition that burns a part of the oil turns oil into dark color.

Method of prevention:

It is possible to stop the root cause of the problem – the bubbles. If you can control the bubble population, you can control micro-dieseling.

Checkpoints to determine the cause of air build-up:

  • Pressure drop through an orifice,
  • Pressure drop through pipes and hoses,
  • Turbulence from valves opening and closing,
  • Shock waves due to sudden closing of valves and cessation of pump operation,
  • Pressure drop due to the sudden opening of a valve,
  • External force on a piston rod,
  • Suction resistance,
  • Plunging of fluid at the return to the tank,
  • Inadequate net positive suction head available (NPSHA) relative to the net positive suction head required (NPSHR) in centrifugal pumps,
  • Suction-side recirculation to sub-best efficiency point (BEP) operation of centrifugal pumps,
  • Nearly dry operation of a pump due to insufficient fluid volume.

Control measures:

  • Leaks to be investigated and sealed,
  • Aeration and sudden change in pressure to be avoided,
  • In a tank top filter assembly, the stand-pipe or diffuser should be long enough to ensure oil is returned below the reservoir oil level, even at the lowest level during operation,
  • Check for improper pump location.

Keep this in mind.  If your oil darkens, there is a thermal event occurring in your system.  It could be a result of micro-dieseling.

Easy Onboard Test Method:

Compare Fresh hydraulic oils with a representative oil sample drawn from the system. The Visual appearance will tell you what we mean.

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