Management of piezo injectors of N43/N53 series engines is interesting and complicated enough, to pay it a special attention.
Testing of injector signals (do they correspond to normal) have to be done every time, when there is suspicion, that incorrect performance of injector could be the consequence of damaged DME or bad connections.
For management of injectors, MSD80/81 uses both connnections of injectors. To understand is the injector open or not, both signals (connections) should be observed. Even more – these signals have to be seen at the same time. To observe the signals, it’s very conveniently to use a digital oscilloscope with 2 or more channels.
MSD80/81 manages injectors in the following way:
- each bank has one common high-voltage (to 150 V) pulse generator;
- each injector has a separate MOSFET switch, which other pin of injector connects with Ground or leaves disconnected if the injector doesn’t need to be opened.
Why such – as if complicated system?
The opening time of injector has to be provided with a high accuracy. If we want the precision of opening 0.1 % (acceptable value) by opening 0.2 ms (minimal opening time: idle/Stratified charge), the opening precision has to be ensured with a resolution of 0.2 us! Such high accuracy can be ensured with specialized PWM output of CPU – amount of such outputs is limited for even powerful CPUs.
System, implemented by MSD, allows to use only two (not 4 or 6) high-resolution PWM outputs, only two (not 4 or 6) manageable pulse generators, only 2 (not 4 or 6) ADC inputs to measure high-voltage pulse signals (their voltage). Additionally, technical tolerances of the high-voltage pulse generators (differences from ideal) are equal for the whole bank, without differences for each cylinder.
And now – from theory to practice!
In the image: schematics (fragment) of N53 series engine injectors connection:
As we see, connections of injectors have no common connection! Both Pins of each injector are used for management! Of course, the connection of injectors is displayed very simplified: actually line 15 corresponds to the high-voltage source (to 150 V).
Here, the signals of both Pins of one injector (N43 series engine):
Blue color: output of the high-voltage pulse generator (Pin 1 of the injector); connection common for all injectors of one bank;
Yellow color – management signal of one of the injectors (Pin 2 of the injector).
As we see, high-voltage pulse generator creates an impulse for both injectors of one bank, but each injector of the banks is chosen sequently.
Here, at this moment the injector is not opened – signal on both inputs is the same, MOSFET switch (signal: yellow color) is closed.
Here, at this moment, the injector is activated – MOSFET switch, which manages signal in yellow color, is open: high-voltage pulse (in blue color) is supplied to the injector.
In an example, mentioned before, we see, that the injector is supplied with 2 impulses with a pause of 20 ms between them. MSD80/81, depending on work mode, use 1 .. 3 impulses for each injection cycle.
A short departure.
In the image: MSD80/81 injection strategy. True thou, the graphics don’t foresee, that both in idle or in partial load mode the engine can perform both in Homogeneous or in Stratified charge (in the graphic only the Stratified charge scenario can be found).
Returning to management signal of injectors.
So look’s the high-voltage impulse: idle, Homogeneous mode (one of two pulses).
Why this management impulse is far from ideal (rectangle)?
Piezo injector (as load) is a capacitor. To quickly charge or discharge the capacitor, a high current is necessary. The high current creates high losses (which are released in heat) in active elements (transistor of high-voltage pulse generator and power source), and also high load to a piezo element of injector itself. Obviously, the manufacturer has chosen a compromise between max precious management of injectors and technological possibilities. True thou, such long rise/fall time means, that the injector has to be in “half-open” (“analogous”) position for a long time (as we see, at least 100 + 100 us). At this moment it becomes clear, why injectors need such wide (+/-50 %) adaptation corridors (in idle/Stratified charge).