Why the Piezo injectors were introduced in the internal combustion engines? Because of their prompt reaction! Exactly the prompt reaction allows the petrol engine to work in Stratified charge and BMW Direct Injection diesel engines (making the main injections already at the burning phase of the diesel fuel).
Unfortunately, as it often happens in life, everything has its price. But, before we move to the week spots of the Piezo injectors, very shortly – about the reaction problems of the “old” injectors. Why the “old” injectors couldn’t get “fast”?
There are two reasons.
1. Mechanical inertia. The nozzle of the regular injectors is moved by the electromagnet. Each electromagnet has to have a core of magnetic material – it also will be moved. Unfortunately, this core is relatively heavy. It takes time to move this core.
2. The winding, which is used in the electromagnet, has an inductance – it is an “integral part” of each winding. Unfortunately, the inductance limits the speed of the increase of the current (dI), when supplying the voltage (U). Very simple: we connect the injector to the onboard voltage of the car (12V), but at the first moment nothing happens – after some time when the current at the connection will rise and its power is enough to move the core of the electromagnet. Yes this problem can be solved by supplying an increased voltage at the first moment (actually it happens: to the injectors of 5V at the first moment the 12V voltage is supplied), but also there are limitations – even the voltage of 100 .. 200V will not solve the impact of the first problem.
And now – let’s talk about the modern Piezo injectors. They don’t have any of the two previously mentioned problems (a heavy electromagnet and inductance problem). Everything look’s very nice and beautiful? Unfortunately…
The Piezo element (an element, which changes the size, if the voltage is supplied) has pronounced properties of the capacitor. The capacitor, as know, accumulate the energy when its supplied to it, and able to give away this energy, when we try to “take away” it. It as if wouldn’t be anything special, but – the giving and “taking” away of the energy also takes time! Yes, while trying the avoid the “brakes” in the performance of the injectors, we got one. Yes, it’s different, yes, it’s less, but still – it exists!
This time “the brake” manifests itself differently – when swiftly supplying the voltage (U) to the injector or “dropping” it, the voltage in the connection (dI/dt) at the first moment is very high – there is a short-circuit in the chain. The high current (several tenths of ampers – they are provided by the management transistors of DME) limits the speed of increase/drop of U on the pins of the injector. Significantly higher current is not a solution because it would increase the heating of the DME, also the short-term overload of the injector itself would be huge.
Here, how the signal of the Piezo looks on the clips of the injectors (idle, N43/N53 series engine, 150 .. 200 Bar Rail pressure) – far from a perfect “on/off” rectangle.
The injector opening, indicated by INPA/ISTA is around 200us, but, as we see – at least a half of this time the management signal of the injector rises and drops, trying to reach the required value (0 or 130V)! Even, if we assume, that the injector itself has no mechanical inertia at all, there are problems with its management! Also, a real management signal at the moment of rising/dropping is not with perfectly straight lines!
What happens at the marked breaking points?
There could be, for example, a change of the parameters of the output of the management transistor (so-called Miller capacity changes), or changes in parameters of the injector itself, for example – moving of the nozzle.
It is clear, that in static conditions (for example, if the 130V are supplied to the injector) everything is fine, but – how the injector will behave in conditions of such “inclined” management signal?
Unfortunately, it is not possible to precisely predict the behavior of the injector in such conditions! Exactly, for this reason, the scattering of the injector offset corridor of the delay encoding (data, which are indicated on the injector by the manufacturer) differs for +/-42% or – two times! The flowrate data (the efficiency of the injector in static conditions), theoretically could differ for only +/-17%, in reality – even less. So – the injectors are very accurate for “slow” modes, but inaccurate for the “quick” modes. One more nuance, which has to be taken in the account – in the test stands of the manufacturer, very expensive and accurate management hubs, the car solutions are much cheaper and inaccurate! Yes, to solve this problem, the MSD management units use one management transistor for the whole bank (2 or 3 cylinders), but – still, the solution is not so accurate as in the lab of the manufacturer.
And finally – the offset/delay parameter of the injector at the manufacturer’s lab is measured at an exact temperature, with an accurate Rail pressure. Will the injector behave identically in different conditions? Of course, no – nobody can give a guaranty for it!
If we look menu ../F5/Shift+F6/F5, you can see, that in idle a very wide corridor for the injectors is allowed – their opening time could differ for even 4 (!!!) times!
Here, the week spot of the Piezo injectors – they are very inaccurate by short openings. Exactly, for this reason, a modern DME’s are so complicated, they create the adaptation maps of the injectors in such a complicated way and use only complicated solutions in measuring the injectors! Short openings – idle and a range of very low load – these are the modes when the efficiency of the injectors can be felt the most!
Is not that the manufactured don’t know about these problems. In addition to complicated algorithms of creating the adaptations, also other solutions are used: for N54, N55, and N63 series engines the Rail pressure in idle (for N54 also in the run) is reduced to 50 Bar. 4 times lower Rail pressure – 4 times longer injection time, many times smaller problem of very short openings. Unfortunately, for N43/N55 series engines, this solution is not possible because the Stratified charge requires a principle of very short injection packs.
Exactly due to the problem, mentioned before, for N43/N53 the Stratified charge is so critically necessary. Exactly the Stratified charge is necessary to measure the parameters of each injector by very short openings, including leaking (spilling) and the quality of the atomization of the fuel injected. If the engine doesn’t use Stratified charge, the mechanical efficiency of cylinders in idle and in conditions of partial load can differ (and usually also are different) thoroughly – there will be vibrations, increased CO/HC in the exhaust gases, misfires. Yes, there weren’t such problems with the old injectors – the scattering of their parameters was incomparably smaller. Unfortunately, nothing is perfect. Also, piezo injectors are not.