2.1 The most common fuel injection pumps (monobloc
or "jerk" pumps) comprise a plunger moving up and down in a barrel
which contains ports for fuel to enter and leave. The pump is designed
to provide the variable fuel flow required for the engine to operate
under fluctuating load or rpm, by adjustment of the plunger delivery
stroke. At a point determined by the engine's fuel requirement, the
plunger will uncover the ports and the internal pressures of between
800 bar and 1500 bar will be spilled back into the fuel supply and
spill piping.
2.2 Each injection pump action generates high
magnitude spill pressures followed by periods of reduced pressure.
As a result, maximum pressure differences exist between successive
injection pumps in the engine firing order. The pressure differences
accelerate columns of fuel within the piping system and when combined
with the action of the circulating pump relief valve, can cause cavitation
and reflected pressure waves. Cavitation implosions occur quickly,
and can induce very short duration pressure pulses in excess of 100
bar.
2.3 Tests have determined that the magnitude of
pressure pulses in the fuel system of a typical medium speed engine
installation is greatest at 40% to 60% engine load, and will reach
60 to 80 bar. The pulses are approximately 8 times the nominal pressure
of the system. High-speed engines such as those installed on high-speed
craft generate higher injection pressures and it is likely that the
fuel system will experience correspondingly higher pressure pulses.
2.4 High pressure pulses lead to vibration and
fatigue and are responsible for many failures of equipment such as
thermostats, pressostats and mechanical dampers. The failure of fuel
lines and their components will invariably involve fatigue and the
initiation of fractures due to tensile stress.