In this entry, I continue the theme regarding ISTA/Expert mode.
Next engine test (after Rough run) in the list: Combustion time.
Combustion time characterizes parameters of the creating of the spark. If the spark is too “short” or too “weak”, or there is no spark at all – problems with the burning of the fuel are guaranteed. On another side, you have to remember: correct spark parameters do not guarantee the ignition of the fuel (even more – a good burning of it). The fuel will not ignite if the proportion of it and air will not be correct.
In addition, these DI engines have their own specifics – the fuel mixture in the combustion chamber is not Homogeneous (congeneric) as for the “old” engines. Even as if correct (Stoichiometric – fuel mixture with Lambda 1.00) readings of the Lambda probe do not guarantee that the fuel mixture is “correct” directly near the spark plug – in the place where it is ignited. If everything is fine with the spark, other potential causes (for example, injectors) of bad fuel burning can be investigated.
How does DME measure Combustion time?
DME measures time from the moment when the management transistor of the ignition coil is closed  to the moment when the voltage on the active output of the coil drops till the relative 0: . For initial information see here.
In the image: signal on the primary winding of the ignition coil. (4) – Combustion time. Time-lapse between  and .
What should be Combustion time?
Combustion should be in the range of 1 .. 3.5 ms. These are the numbers from the explanatory material of ISTA. And that is all that BMW says about it. And yes, a vague test follows that the Combustion time also depends on the air gap of the spark plug, RPM, etc. What can I add?
Some Combustion times for quick control. With new/good spark plugs:
a. in idle, the Combustion time should be 2.1 .. 2.3 ms;
b. 1000 ..1500 RPM; with a very low required torque (around 20 .. 40 Nm) Combustion time should be 2.2 .. 2.5 ms;
c. in Overrun mode, Combustion time is max: around 2.5 .. 2.8 ms;
d. in the mode of large throttle and 1000 .. 2000 RPM Combustion time is minimal: around 1.0 ms.
Basic relationship for the length of the Combustion time:
a. the larger the air gap of the spark plug, the shorter the Combustion time;
b. the larger the required torque (and more fuel + air in the cylinder), the shorter the Combustion time;
c. the lower the compression of the cylinder and larger losses, the longer the Combustion time;
d. if the spark in the combustion chamber is not forming at all, Combustion time is concise (close to 0 ms);
e. if the spark forms correctly, but the fuel does not ignite for some reason (misfire), Combustion time is long – close to the max value.
Combustion time (scale 4), idle:
Low torque and Overrun:
The red curve – accelerator position. As we see, when the torque increases, Combustion time shrinks.
Large throttle and Overrun:
Pressing the accelerator pedal for 60% (red curve), Combustion time drops to minimums: around 1 ms.
Note: B58 continues to activate the ignition coils in the Overrun mode (in this mode, as we understand, the fuel is not injected in cylinders, so – the spark is formed for “clean air”).
Legend of the image:
Why the Combustion time “behaves” exactly like this? I will try to explain max simple and compact:
a. 2 .. 3 ms before the moment of the necessary sparkle, DME opens the Ignition transistor; to the primary winding of the ignition coil, the onboard voltage is supplied, and the energy starts to build in the ignition coil;
b. after these 2 .. 3 ms, the voltage in the Ip coils primary winding has reached 6 .. 9 A (amperes); the total accumulated energy W = 1/2 * Ip * Up * ton, where Up = onboard value (14.0 V); ton = energy accumulation time.
c. Ignition transistor is closed; the ignition coils start to give the accumulated energy to the secondary winding; at this moment, the spark starts to form;
d. if the coil transformation coefficient is (typical value, excluding N43/N53, for which it is 2x higher) 1:50, in the secondary winding the current of (6 .. 9)/50 = 120 .. 180 mA is flowing;
e. if the “least resistance” path are the electrodes of the spark plug (so it should be), the spark is created between the electrodes of the spark plug – in the combustion chamber;
f. at the moment, when the spark is created, the voltage (kV or thousands of Volts) between electrodes (very simplified) Ue = Lg * P, where Lg = size of the air gap/mm; P – pressure in the burning environment. So – the larger the pressure, the larger the voltage between the electrodes of the spark plug;
g. power of the spark: E = I * Ue, where I – previously (in point d) calculated current of the secondary winding;
h. total energy, which will be accumulated in the ignition coil (mentioned in point b), now is used to create the spark: W = E * t, where t = time, in which the spar “burns”.
Maybe at the first moment, it seems complicated, but rephrasing in the “human language”: initially, the electrical energy is accumulated, then – released. The spark can be “powerful”, but then it will “glow” for a shorter time, it can be “weaker”, then it will be longer. The reason is simple – the amount of energy, which was accumulated, is limited. We can release this energy quickly, with a “big bang” or be done slowly and “quietly”.
The second aspect, the “equivalent” air gap between the spark plug electrodes increases proportionally of the pressure. So, if at room pressure the air gap of the spark plug is 0.6 mm and 600 V is necessary to maintain the spark, in conditions of 10 bar pressure, 10 times larger pressure or 6000 V will be necessary! And here we return to the previous paragraph – the larger the “boom”, the shorter the time when all energy is consumed. If there is a lot of fuel in the cylinder when it ignites, the pressure in the combustion chamber increases swiftly and significantly – even till 50+ bar. The voltage, which is necessary for the spark to continue to “live”, also increases. The higher the voltage, the larger the “power” of the spark, but the previously accumulated energy is used more swiftly. We can see it also in the graphs – when pressing the accelerator pedal; the Combustion time reduces from 2.5 to 1.0 ms. Instead, in the Overrun mode, the pressure in the burning chamber remains low (because there is no fuel to burn), and the spark is “less powerful”, but it burns max long: even till 2.5 .. 2.8 ms.
Not so complicated, isn’t it?
Instead, if the spark in the burning chamber is not created at all (for example, misfire happens because the electrodes of the spark plug are wet from the fuel):
a. voltage in the secondary winding of the spark plug is very high (even 20 .. 30.000 V) because the fuel, which made the electrodes wet, serves as the insulator and “increases” the air gap significantly;
b. via the “easiest way” (which is not the electrodes of the spark plug this time, but typically – insulator of the ignition coil, or the insulator of the spark plug itself) the energy, accumulated in the coil, flows to the Ground, creating the arc discharge in the “wrong place”: outside the combustion chamber;
c. as the voltage mentioned in point a is high (much higher than in the case of a regular spark), the energy accumulated by the ignition coil is used very fast. The “wrong” spark is very bright/”powerful”, but it “glows” for a concise moment. This short Combustion time is noticed by the DME, and the misfire is registered.
a. a quick test – I compare the Combustion timing with the samples;
b. increased Combustion timing: either reduced air gap of the spark plug or reduced compression/pressure in the cylinder;
c. reduced Combustion time: increased air gap in the spark plug (the spark plug is worn out) or increased pressure in the combustion chamber (detonation, residues);
d. concise Combustion time (close to 0 ms): “normal” spark is not created at all – misfire guaranteed;
e. very long Combustion time: the spark is created, but the fuel does not ignite – misfire.
Note: we have to remember the specifics of this DI engine. In a range of very low required torque, and if the spark plugs are worn out, the spark in the combustion chamber is created (and Combustion time is correct), but the fuel ignites poorly, irregularly. As there is not much fuel in the cylinder, the pressure increase is also small at the moment of the fuel ignition. Accordingly – in this mode, the Combustion time does not change significantly depending on the fuel is ignited or not. It has to be kept in mind – in this mode, even a bad fuel ignition could be possible, even not taking into account as if successful spark. And also, the Combustion data will be comparingly correct, not taking into account the shivering of the engine, which can be felt very good. Read more here.