What is Micro Dieseling of Lubricating Oils?

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Microdieseling is pressure-induced thermal degradation of oil. An air bubble moves from a low or negative pressure zone to an area of high pressure. The air undergoes adiabatic compression and heats up to 1000 degrees Celsius. These extreme temperatures can carbonize oil at the bubble interface resulting in sludge, soot, tar & varnish and increased oil oxidation.

For micro-dieseling to occur we require two key ingredients: entrained air and a rapid increase in pressure. Let’s look at how each manifests in a hydraulic system.


These are the four types of air-in oil contamination

  • Dissolved Air – invisible air, just like the carbon dioxide trapped in a bottle of coke before opening
  • Entrained air – unstable microscopic air bubbles
  • Trapped pockets or air free from dead zones, high areas
  • Foam – akin to the head on a beer or foam in a glass of coke

For micro-dieseling to occur, we need air bubbles to enter to pump. Entrained air will do this naturally, but hydraulic systems are designed to operate with a regular amount of entrained air. Issues tend to arise when an abnormally high amount of entrained air enters the system by:

  • Pump operation is almost dry due to insufficient fluid volume
  • Insufficient residence time

Alternatively we may convert dissolved air into entrained air by reducing the suction side pressure. This reduces gas solubility of the oil.

  • An orifice can experience pressure drop
  • Pressure drops through pipes and via hoses
  • Turbulence caused by valve opening and closing
  • Shock waves caused by sudden valve closings and cessation pump operation
  • Pressure drop caused by sudden valve opening
  • Piston rods are subject to external force
  • Suction resistance

Rapid pressure increase

Rapid increases in pressure are generally brought about by the activity of pumps. In hydraulic systems this means that the location for micro dieseling is similar to cavitation. 

What follows is a compression of the air bubble, which contains hydrocarbons at the oil interface. You will already be familiar with how a diesel engine works. During compression, the mixture of the sprayed gasoline and the rising temperature of the compressed air results in ignition. Your oil will turn black because the burnt fuel is not fully converted to water or carbon dioxide, forming soot and tars. This is normal for diesel engines. Engine oils contain detergents and dispersants to handle high levels of soot without needing to be changed.

When the same phenomenon occurs in hydraulic oils, it leads to:

  • Oil darkening (soot loading)
  • Wear (soot is abrasive)
  • Valve sticking (due to build-up of tars and varnish)
  • Increased bulk oil temperature (high temperature air bubbles equalise with oil system)
  • Accelerated oxidation
  • Decreased oil life