At the first moment, you can think – what interesting can there be?
But there is!
As already mentioned here, in Homogeneous mode, idle, MSD80 actually performs not one, but two injections! “Distance” between them is 20 ms, which means: around 40 .. 45 % of full (one) stroke. Obviously, BMW engineers have detected, that fuel, injected in two portions, creates better atomization (is better mixed with air).
Additionally, INPA/ISTA indicate the time of each injection (each/one from two identical).
Even more interesting is an obstacle, that directly after deleting old adaptations (during re-adapting of the engine) in idle MSD performs only one (!) injection cycle!
When the following conditions are fulfilled:
- flywheel adapted;
- MSD performs in full functionality mode (all probes are heated);
- initial creation of offset and multiplicative LTFT of banks has been performed,
MSD will turn on this mode of two injections in idle, Homogeneous mode.
Additionally, not only will turn on but – “with taste” (if we can speak out in such a way about a technical device).
Possibly, the question – why a such complicated thing is necessary – would be quite logical. Why not turn on this two injection mode already initially?
Two portion mode is more demanding to opening/closing parameters of injectors and their conformity to data, stored in MSD memory (coding data). If the fuel is injected in two portions, the opening time of each injector is reduced 2 times, the injector “spends” much longer time when opening and closing. Even its management signal (not taking into account inertia of injector itself) more and more moves away from ideal:
Here, how the injector management signal for the opening of around 0.2 ms (idle, two portion injection mode) looks. Far from “correct” rectangle!
Actually – the injector spends more time while opening and closing as in stable open position!
So – after deleting the adaptations, DME performs one longer injection session, because then the injector is stable open for a longer time, parameters of injector’s delay time (which are marked encoding with 1XX or 2XX code) have less impact. The significance of coding (parameters of injectors) will decrease not for 2, but 3 .. 4, or even 5 times! Even, if the injectors are not encoded or are encoded incorrectly, or they true parameters don’t match coding data, directly after deleting old adaptations the engine will work max stable.
Note: for long injector opening times flow-rate coding is intended (codes: 5XX; 6XX; 7XX), but flow-rate scattering is significantly (even 5 .. 10 times) narrower than a scattering of delay parameters – this parameter doesn’t gives fundamental problems while creating adaptations.
Coming back to the theme – how MSD switches from one injection portion to two!
If initially adaptation of LTFT (for banks) was successful, in idle MSD in sequence (starting with a 1-st cylinder, in firing order) switches each cylinder to two injections mode and immediately adapts the opening time of these cycles, evening the mechanical efficiency of the cylinder. This test (in case of correctly encoded and correctly performing injectors) is operative: around 10 seconds long.
When all cylinders are sequentially tested and adapted, after few seconds they are switched to two injection cycle mode, all in one time.
Here, in the image: 4-th (in firing order) cylinder test. As we see, injection times of all other cylinders are 2 times longer and “inappropriate” for the warm engine, idle. So, cylinder No. 1, 2 and 3 work in the mode of one cycle, cylinder No.4, which is switched to 2 cycle mode, is being tested.
When MSD switches back to one cycle injection? As usual, information from the manufacturer is 0. I can mention only my observations. Two times longer injection times as “regular ones” I’ve seen, if some of the cylinders have fundamental problems (fuel adjustment has finished with the fiasco, or Rough run data indicate completely incorrect values).
If you happen to see as if inadequate injection times – don’t be surprised, possibly, that MSD has switched from 2 or 3 cycle injection (because of some problem) has switched to the more simple, but more stable solution – fuel injection in one portion.