Diesel effect in the hydraulic cylinder

The diesel effect is a cause of internal wear in hydraulic cylinders. Preventing it is part of the design of hydraulic systems. This can be achieved through design measures. However, it is just as important to calculate the flows in the system precisely in advance and to operate it in normal operation only within its load limits.

Oil + pressure = explosion

From a chemical-physical perspective, the diesel effect in a hydraulic cylinder is in fact almost the same as what is induced in diesel engines: strong compression causes an oil-gas mixture to explode. In the hydraulic system, these explosions generate high, destructive forces and create unwanted particles that cause further damage in the system.

Diesel effect in hydraulic cylinders: air is required

To cause oil to burn explosively through compression, oxygen is required. In a closed hydraulic system, oxygen is present only above the fill level of the hydraulic tank. This area normally contains ambient air with an oxygen content of 21%. This oxygen is unproblematic as long as it is not mixed into the hydraulic oil through poor design. If, for example, inlets are installed too high and the hydraulic oil runs into the reservoir like a waterfall, this can lead to this undesired enrichment of the oil with air.

However, the usual way air enters the hydraulic system is through leaks. These occur primarily at mechanically stressed seals, such as the sealing lips of hydraulic cylinders. Especially here, cavitation—as the second main cause of internal damage—can cause damage to the seals. The diesel effect in the hydraulic cylinder is therefore not directly related to cavitation as long as the oil is degassed. Indirectly, however, it can certainly be a cause of the diesel effect.

Even seals that appear intact can allow quantities of air into the hydraulic oil due to the high suction pressures that occur when the hydraulic cylinder extends. A visual inspection is therefore often not sufficient to find and remedy these leaks.

Consequences of the diesel effect in hydraulic cylinders

The explosions caused by the diesel effect are microscopically small. However, they are sufficient to generate shock waves that can erode material from seals, inner walls, or hydraulic motors (radial or linear). This damage often goes unnoticed for a long time until it becomes apparent through more serious damage. In addition, the eroded material as well as the residues from the micro-explosion heavily contaminate the hydraulic oil. Rapid blackening of the hydraulic oil, for example, is an indication that a diesel effect is occurring somewhere in the system.

Countermeasures against the diesel effect in the hydraulic cylinder

The most important measure to prevent the diesel effect in the hydraulic cylinder is the precise calculation and design of all flows and components. Only when the pressures and back pressures are known and the components are designed with generous safety margins can gas ingress largely be avoided. This also includes regular maintenance with the preventive replacement of critical components, such as seals.

In addition, hydraulic engineering today offers powerful components that can prevent air from being entrained into the hydraulic oil. These include, for example, bleed valves for hydraulic cylinders or degassing modules for tanks and lines. At the latest once the diesel effect has been identified in the hydraulic system, these modules should be installed.