As the name suggests, the concept of diesel effect refers to the combustion process in a diesel engine.
Research by cylinder manufacturers in conjunction with leading hydraulic component manufacturers has made it possible to explain this phenomenon. Oil in a hydraulic system often contains small amounts of air, which in some cases can form “bubbles” that also contain oil vapour. The diesel effect occurs as a result of cavitation, a phenomenon that occurs during fluid flow.
The formation of air bubbles in hydraulic fluid is caused by the phenomenon of cavitations that occur during the flow of the fluid. The cavitating phenomenon actually consists of the vaporisation of the working fluid and subsequent condensation of the vapour bubbles. This condensation of vapour bubbles occurs under high pressure conditions. The presence of these entrained gases reduces the effective mass coefficient of the liquid, causing instability in the operation of the actuators.
The phenomenon of cavitations consists, in fact, in the vaporisation of the working liquid and, subsequently, the condensation of the vapour bubbles. This condensation of vapour bubbles occurs under high pressure conditions.
Prezența gazelor antrenate reduce coeficientul de masă efectiv al lichidului, cauzând instabilitatea în funcționare a elementelor de acționare. Dacă bulele de aer generate de cavitație, care conțin și particule de ulei, sunt supuse unei presiuni ridicate. Temperatura din interiorul bulelor crește dramatic.
This sudden rise in temperature generates the diesel effect, i.e. combustion in the hydraulic system. The combustion process takes place in milliseconds. If the air-oil mixture reaches a ratio of 7.5 to 1 by weight and a temperature between 200-250°C, it can ignite spontaneously, as in the case of diesel cycle engines. Favourable conditions are easier to achieve the higher the working pressure, e.g. over 100 bar.
Typically, in a hydraulic cylinder, burn due to the DIESEL effect initially occurs in an area of about 15-20% of the total circumference of the rail, i.e. in the area opposite the zone of maximum friction, where the radial load of the piston is located, where cavitation can occur.
The higher the quality of the pressure seal inside the cylinder, the greater the possibility of micro-explosion of the fluid.
Consequences of cavitations and the Diesel effect
Cavitation can have a variety of negative effects, including material damage to pump housings, relief valves, suction valves and therefore guide and sealing elements. Negative effects of cavitations include changes in flow characteristics, reduced pump and gear efficiency, pressure spikes exceeding system pressure.
Efectul Diesel provoacă creșteri localizate ale temperaturii și se manifestă prin înnegrirea uleiului. Crearea de reziduuri de ardere și distrugerea inelelor de ghidare și a garniturilor. Consecințele cavitației și ale efectului diesel nu sunt întotdeauna vizibile imediat. Acestea sunt adesea observate atunci când este deja prea târziu și este necesară o reparație a cilindrilor hidraulici.
Given the serious consequences of cavitation and the diesel effect, appropriate measures must be taken to prevent these phenomena from occurring.
Since it is impossible to produce sealing systems capable of withstanding fluid explosion, the best protection is to remove air with the help of the valve.
An alternative that does not eliminate the phenomenon, but reduces its consequences, is to protect the gasket by fitting a square section phosphor bronze ring. This does not prevent the Diesel effect from occurring, but it reduces the action of the flame on the seal and prolongs the life of the system.