Do fuel injection systems cavitate?
One of the themes which comes up regularly with fuel injection systems is the subject of cavitation. Many years ago I gave a talk on fuel injection systems and the question arose as to whether fuel injection systems cavitate. My answer then as now is that every fuel injection system cavitates. In fact cavitation is inherent in all hydraulic systems. However, not all cavitation leads to damage.
Cavitation is not restricted to fuel injection systems, but today as diesel fuel injection pressures have increased to levels in excess of 2000 bar, cavitation damage can be seen fairly regularly. Cavitation damage may not cause problems as the position in the system where the bubbles collapse is the important factor and if they collapse against a wall which is not a sealing surface or structurally important, no system damage is likely to occur. However, if the bubbles collapse against a sealing surface (valve seat, injector nozzle seat, etc.) then the system performance can deteriorate.
What is cavitation?
Cavitation is basically the formation of vapour bubbles in a hydraulic systems. Cavitation bubbles form where the local pressure falls below the vapour pressure of the liquid. When the local pressure increases again and exceeds the vapour pressure, the bubbles collapse. The majority of the bubbles collapse in the fluid without any obvious effect.
Cavitation damage occurs where the bubbles collapse against a surface and the extremely high local pressure generated by the collapse of the bubble can cause removal of the surface material. Over a long period of time the cavitation can cause malfunction of the system.
What causes cavitation?
Cavitation occurs in different parts of hydraulic systems due to local flow conditions. The first time I came across cavitation was in fuel injection systems with a fuel injection pump connected to a spring loaded injection nozzle via a long injection pipe. These systems used to suffer from cavitation in the fuel pipes due to the rapid end of injection or pressure drop. I will discuss this in a later episode.
Low pressure systems can suffer from cavitation, especially when the suction side of a pump is restricted and the pump tries to suck more fluid than can be supplied. In these cases the major symptoms are a lack of power and instability in flow which can lead to governing difficulties. However, in these pages I want to look at cavitation in high pressure systems where the pressures are large enough to cause damage to components.
During the suction stroke of a cam driven system, the pumping plunger pulls a depression in the fuel between the closing of the delivery port and the opening of the filling port in the barrel. Vapour bubbles form in the fuel for a brief period of time and the bubbles collapse as soon as fuel enters the pump chamber. Due to the low pressures involved (between 3 & 5 bar) the energy in the collapse of the bubbles is low and as the surfaces of the plunger and barrel are hardened no damage is normally seen.
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