Avoid “dieseling” in hydraulic cylinders.

24/03/2020 11:01:55 AM

When air is mixed with the hydraulic oil in your cylinders it can create the conditions for an extreme phenomenon known as “hydraulic dieseling”. This happens when bubbles of air mixed with oil undergo extreme pressurisation, causing them to explode in much the same way that diesel fuel ignites within an engine cylinder. Does that sound bad to you? It is! Tiny bubbles of exploding gas will quickly burn cylinder seals and lead to hardening, cracking and cylinder seal failure. In this article, we’ll explore why this happens and how to avoid hydraulic dieseling.

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Hydraulic dieseling is caused by air in hydraulic fluid:


The conditions for dieseling arise when air is allowed into the hydraulic system. There are many ways that this could occur. Apart from obvious leaks, hydraulic oil contains dissolved air naturally. This is not usually a problem, however under certain circumstances, dissolved air can be pulled out of oil to form air bubbles (a phenomenon known as gaseous cavitation), which sets up the conditions for dieseling to occur. One way or the other, air is responsible.

  1. Air introduced during filling or flushing.
    Dieseling can result from not purging the cylinder’s chambers of air during commissioning. Filling or flushing are both opportunities for air to enter the system and steps must be taken to minimise this likelihood.

  2. Air bubbles caused by negative pressure.
    If the system is not sufficiently well designed, air can be drawn past the cylinder rod seals under negative pressure as the cylinder retracts. Negative pressure also causes the solubility of oil to drop which allows dissolved air to escape and form air pockets.

  3. Loose hoses or connectors.
    All points of connection are prime locations for leaks to occur.

  4. Clogged filters and strainers
    Obstructions in the hydraulic system such as those caused by clogged filters can cause extreme pressure drops that causes the solubility of the oil to be reduced, which allows the formation of dissolved air to form into air bubbles.

  5. Faulty float valves
    Float valves control the descent of a hydraulic cylinder under load. If the float valve is faulty, it can create negative pressure within the cylinder which allows air to be drawn under the cylinders rod seals.

  6. Pump inlet problems
    Leaks in suction lines or low reservoir oil level will allow free air to enter into the inlet of the pump. The free air will become entrained air as it exits the pump and the oil is compressed.

  7. Worn hydraulic pump components.
    Worn pump shaft seals and other components increase the likelihood of air being drawn into the system.


Avoid dieseling in hydraulic cylinders


When bubbles of air and oil are allowed to form in hydraulic fluid, it sets up the condition for an explosive event that is the same - in principle - to the combustion process that powers diesel engines. This phenomenon quickly burns hydraulic seals and leads to hydraulic cylinder failure. So now that you know how air bubbles can form in hydraulic oil, setting up the conditions for dieseling, let’s consider some steps you can take to minimise air entering the system.


  1. Pre-fill and bleed your hydraulic system prior to start-up. Fill the cylinder with clean hydraulic fluid through the service ports before connecting the hoses.
  2. Take care to purge all air from cylinders by cycling the cylinder through the length of its stroke several times without load.
  3. Check the operation and adjustment of valves at prescribed intervals.
  4. Check and maintain all working parts of the cylinder through a prescribed schedule of preventative maintenance.
  5. Ensure the cylinder is designed to meet the needs of the application in order to ensure optimal performance and reduce negative pressure events.


Following correct commissioning and maintenance procedures at all times will help minimise the presence of air in your hydraulic cylinder and reduce the likelihood of hydraulic dieseling. This is one more reason to conduct a thorough inspection and repair procedure whenever you encounter a leaky cylinder seal. Because replacing a burnt seal without addressing the underlying dieseling will usually necessitate another repair in short order. For more information, please contact our repair centres nationwide.


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