It is hard to count the decades since Siemens and Bosch developed DME for the petrol engines. The primary things should be in order! Would it be logical? Yes, it would! Well, let’s see how is Bosch, with the latest MEVD 17.2.9 management installed on the B58 series engine, doing.
Let’s see the basic things of the ignition.
TIS says the multi-ignition is used.
In ISTA, even more exact information – multi-ignition is used in all engine performance modes till 2000 RPM.
And here – the actual oscillogram of the ignition:
As we see, events happen after each 180ms (5.5 times during a second) – so: one on each combustion cycle.
And more – exactly in the moment of discharge:
Such a “picture” I saw of a cold, partially warmed, and warmed-up engine – both in idle and increased RPM.
Where is the multi-ignition here?
Yes, this would be a question to Bosch here – where is multi-ignition? The answer – there is no multi-ignition! Yes, the DME of the previous generation had. But… Oh, they were Siemens DMEs!
What happened to the multi-ignition? No error messages in the DME. In addition, there should not be any connection with the error messages – whatever, why switch off this feature?
Why switch off?
I have the answer. The answer – what went wrong this time.
Pay attention to the marked:
This is the moment when the DME Ignition switch has just closed, and the voltage in the circuit rises rapidly. In the marked point, the voltage has reached around 420V and suddenly, with a breach, drops swiftly.
A short digression is in place here. What kind of Ignition switches are used by MEVD 17 DME? EcoSpark(R) 2 (2nd generation) switches, for example, FGD3040 or NDG8201. Let’s see the Datasheet of these switches:
Here, the max allowed voltage: is 400V. Above this voltage, the built-in Overvoltage protection works. What happens at the moment when this protection works? Transistors are repeatedly opened. The energy accumulated in the injector is dissipated to the transistor at the moment of switching. This power is released as heat.
Why is such excess happening? There are several reasons:
a. as we see, the voltage in the primary circuit of the ignition coil grows with the speed of around 150V/us. The critical limit (400V) is reached by around 3us. Instead, to create an arc in the spark plug, a longer time is necessary (at least 5 .. 10us). During these first us, the voltage grows uncontrollably;
b. in addition to the “perfect” ignition coil, there is the so-called “unrelated” inductance, which (would) create a voltage surge even if the spark would ignite immediately. The reason is that very rapidly, a huge amount of energy is released;
c. an additional voltage spike is created by the wires from DME to the ignition coils. Each wire is (although small) inductance; it should not be forgotten!
One more interesting nuance, see the marked:
Shutdown (current fall) time for these switches can differ for even 7(!) times! Accordingly – depending on the batch/specimen (not even talking temperature), the switch can shut down when it desires. Accordingly – in one case, the problem described in this entry will be larger, in another case – smaller.
I noticed this problem already for the DME of MSD80 generation, description here.
What is the most essential such repeated (emergency) opening of the switches:
a. reduces useful energy – part of the energy is lost;
b. heating of these switches increases significantly – the lost energy is released in them;
c.. the creation of the spark can be unstable – as we see, the voltage in the circuit drops, and – self-explanatory – an arc is disrupted at that moment.
In addition to all these problems – uncontrolled and increased voltage prematurely damages the ignition coils!
To solve all these problems (to my MSD80), I installed snubbers, as shown in the link above. After installing the snubbers:
a. the heating of the ignition management switches was reduced significantly (I evaluated it by using a thermal camera, not “by senses”);
b. lifespan of the ignition coils increased significantly. If before installing the snubbers, I replaced the ignition coils “in a circle”, then after their installation – so rare that I even forget where and when I bought any of them.
All symptoms indicate that:
a. Bosch hasn’t made any conclusions; the Overvoltage problems are in “full throttle”;
b. ignition switches have been too hot (due to repeated opening), and due to this, the multi-ignition was not an option;
c. unstable idle and misfires during it, without any illusions – consequences of the previously mentioned problem.
For the test, I prepared snubber from 01uF and 33R. After connecting:
Here, the maximum voltage has dropped from 420V to 260V; no “shivering” of the voltage was observed.
In this oscillogram, you can see how stable and even the arc is! As from the school book!
For comparison – how the spark is created without snubber:
A complete nightmare happens here! It looks like the transistor repeatedly opens, even several times! The voltage shivers – the arc is not stable, for sure!
Here is one more graph (on the image, right side):
In this image, we can see that by high temperatures, these switches start to experience troubles (inductance of the B58 ignition coil is 1.0 .. 1.5 mH) already by the current of 15A. This (connected with the heating problem of the switches) looks like one more reason to switch off the multi-ignition.
Compared to the ignition coils of the previous generation, the inductance of the primary winding is reduced from 2.5 .. 3.0mH to 1.0 .. 1.5mH, at the same time INCREASING the charge time of the coil (from 2.2ms to 2.5ms), which means – the current, which flows via switches, wires, connections, has increased significantly (to 15 .. 18A)!
Finally, there could be one more reason to switch off the multi-ignition – the connection of the ignition coils of this generation is not able to withstand such a load. The contacts of the connector are so (in size) tiny that the common sense says – regular 15 .. 18A current impulses via them is a severe overload.
Because of these ignition problems, it is not a surprise that some of the users of these engines have observed that the smoothness of the idle gets better if the air gap of the spark plug is reduced from the default 0.75 mm to 0.50 mm. By reducing the air gap, the Overvoltage problem is reduced, and less energy is necessary to maintain the arc. Yes, of course, this is a fight with the consequences, not the cause.
Let’s see the technical parameters of the ignition coil:
Let’s calculate the primary (maximal) voltage: 29000/80=363V. So, the maximum allowed voltage in the primary circuit is 363V! Instead, by 420V (the actual voltage) of primary voltage, the 33.600V voltage in the secondary circuit is created, which is 16% (or 4600V) more than the max allowed.
In the Datasheet, exactly 29000V is indicated, making it clear that even the limit of 30000V is not acceptable. But simultaneously – serially, constantly (in each cycle, not occasionally/in emergency cases), the limit, defined by the manufacturer itself, is overreached! I think that each technically thinking person has doubts regarding the longevity of the ignition coils in such work conditions.
A cherry on the cake is the description of the manufacturer itself regarding the Ignition system. If two paragraphs of text “regarding nothing” can be forgiven, then the graphic is a masterpiece:
If I curve still is approximately true, then the U curve (green color) is complete nonsense. Even assuming that the developers of the descriptions have forgotten to mention the use of a 1:10 probe, the situation does not get better. BMW – what is this? Who drew this, who confirmed this, and who hasn’t fixed this for years? It is interesting how many ignition systems have been “fixed” thanks to this masterpiece.
If BMW AG can’t find it at all (and are not able to make any measurements by themselves), I offer this image, found on the Internet (it is not perfect, but at least it is correct).
For the engine of my car, an increased unevenness of the cylinders No.2 and No.6 in dile was observed. Taking into account the firing order (1/5/3/6/2/4), it is clear that the solving of the problem should be started with cylinder No.6.
Why exactly the cylinder No.6 could be the most problematic for these engines?
In the marked area – additional sound insulation above HPFP. Yes, the pump works very loudly, and not without the reason this insulation is introduced. But, by submitting this insulation, the air exchange in the area of the ignition coil of cylinder No.6 is completely paralyzed. The ignition coil works in an abnormal temperature mode. Its connection wires were petrified! More about overheating read here.
Here are at least four (!) reasons why the multi-ignition is switched off and why these engines have and will have problems with the ignition.