Remaned has identified the following technical points for the Ford Focus
The amount injected is proportional to the injection time (pulse) and rail pressure. The flow graphs in relation to pulse time and rail pressure are called the injector characteristics.
Common Rail system injectors are very high precision parts. They are capable of injecting amounts varying from 0.5 to 100 mg/cp under pressures from 150 to 1800 bar. The precision needed requires very small manufacturing tolerances (μm) on jet diameters and on the operating play between the different moving parts. However, machining variations, load losses, any contact between moving parts and magnetic forces cause variances between one injector and another. Because of this, flow variations can reach 5 mg/cp. In other words, that means that the same pulse applied to two injectors could produce a difference of 5 mg/cp.
It is impossible to control an engine effectively with such a variance between injectors. So it is necessary to apply a correction so that the required amount of diesel is injected whatever may be the initial characteristics of the injector. For this to happen, it is necessary to know the injector characteristic and correct the pulse applied to the injector as a function of the difference between this characteristic and that used in the DCU.
The characteristic recorded in the DCU is called the target. It is the mean characteristic of flows measured from a representative batch of injectors. This target is used to convert the flow demand Q in pulse time T. However this pulse cannot be applied directly to the injector since the characteristics of this latter are different from the target. It is therefore necessary to correct the pulse time T using the particular injector characteristic This characteristic is determined by measuring the flows for different pressure values. The C2I is a modelling procedure for these
The injectors are tested at the end of the manufacturing line. The measurements are processed to work out correction coefficients. These coefficients are written on the injector in two forms, as follows: in a data matrix code and in alphanumeric characters.
To correct the flow differences between injectors and the target, each injector is marked with the flow difference between the actual pulse time and the target pulse time as a function of flow, for a given
• The corresponding corrections be then determined for pilot and principal flows from the graph, for four pressures (200, 800, 1200 and 1600 bar). That produces eight coefficients.
• From this we get a (Pressure, Flow and Correction) card that gives us the correction to apply to the target pulse to obtain the required flow.