The piezo stack is linked to the control valve of the fuel injector via a fuel-filled hydraulic coupler. The upper piston of
the coupler is a larger diameter than the lower piston. This difference in diameter causes an increase in the
linear movement of the lower piston (more travel).
Note: If the hydraulic coupler is not full of fuel, the lower piston will not move and fuel will not be injected into the
combustion chamber. The hydraulic coupler is supplied with fuel by the low pressure fuel pump when the key is
turned on and from return fuel when the engine is running.