Looking for DME BIN files

[johnb]

If you're talking about my test, I just measured the voltage at the battery with one channel, and the current on the coil wire (ground side of the coil->DME) with a current clamp probe, with the engine running. The software knows about the conversion of current->voltage in the probe, so it was able to display the actual current in amps.

[Tom]

If you're talking about my test, I just measured the voltage at the battery with one channel, and the current on the coil wire (ground side of the coil->DME) with a current clamp probe, with the engine running. The software knows about the conversion of current->voltage in the probe, so it was able to display the actual current in amps.

Not necessarily your set up, I was just mulling how to bench test the charge time. But testing on the car is probably much easier, though I was thinking you both controlled the voltage and I can't think how to do that on a running car. Presumably it will be close to 13.8 (maybe a wee bit higher), but I was thinking a regulated power supply would be ideal just to see how long it takes to charge, but my power supply is too wimpy to supply 9 amps.

[Rogue_Ant]

...I was thinking a regulated power supply would be ideal just to see how long it takes to charge, but my power supply is too wimpy to supply 9 amps.

Put a decent amount of capacitance in parallel with the power-supply.

Coil_energy_Capacitance.PNG (15.83 KiB) Viewed 472 times

Something like an automotive audio capacitor, 1 Farad would make the voltage pretty solid for a single charge cycle:

https://www.amazon.com/RECOIL-Storage-R ... B08HQWXZY7

[Tom]

...I was thinking a regulated power supply would be ideal just to see how long it takes to charge, but my power supply is too wimpy to supply 9 amps.

Put a decent amount of capacitance in parallel with the power-supply.

Coil_energy_Capacitance.PNG


Something like an automotive audio capacitor, 1 Farad would make the voltage pretty solid for a single charge cycle:

https://www.amazon.com/RECOIL-Storage-R ... B08HQWXZY7

You got me poking around amazon and to my surprise they have a 10amp supply for 5 cents less than the capacitor...https://www.amazon.com/Adjustments-Adju ... r=1-5&th=1

[Rogue_Ant]

But another thing that made me think something had to be wrong was the dwell times. If it takes 6.5ms to fully charge the coil, why doesn't the dwell map use that time when it's available? If I read the dwell map correctly, the time drops below that 6.5ms once you get above 500rpm or so, even though there's plenty of available time up to 4000rpm and beyond (for typical running voltages).

Likely the DME doesn't have the thermal budget for continuously charging the stock coil to 9 amps.
Integrating the loss of the driver, I come up with nearly 90% increase in loss (heat) charging to 9 amps (6.5ms) compared to the stock 4.6ms (7.3 amps).
The DME already gets warm, doubling the loss would probably get pretty hot.

[Tom]

Please check my math, but here is my first pass at showing the dwell times in milliseconds (stock chip/map).

dwell in ms.jpg (251.3 KiB) Viewed 437 times

[johnb]

Please check my math, but here is my first pass at showing the dwell times in milliseconds (stock chip/map).


dwell in ms.jpg

That looks like a perfect match for the one that was posted on RL way back, that we talked about here a few weeks ago: viewtopic.php?t=3574&start=150#p49458 (except that one is rounded)

[johnb]

Here's what I don't get though. Why are the dwell times so long for low rpm? Worst case, at ~14v it takes no more than 6.5ms to fully charge. But the dwell map calls for more than 10 times that at the lowest rpm. What does the extra dwell time achieve?

Is there something happening at low rpm besides the voltage drop that makes it charge more slowly?

[Rogue_Ant]

Here's what I don't get though. Why are the dwell times so long for low rpm? Worst case, at ~14v it takes no more than 6.5ms to fully charge. But the dwell map calls for more than 10 times that at the lowest rpm. What does the extra dwell time achieve?

Is there something happening at low rpm besides the voltage drop that makes it charge more slowly?

A couple of considerations: Firstly, during cranking the fuel-air mixture is generally not well mixed, and can be difficult to ignite. To get the best chance, we need to ensure the spark has the maximum energy possible.

It isn't that it needs that much time to charge. At 6.0 volts, the coils is basically as high of current as it can get (a little less than 6 amps) after 40ms, the longer time doesn't improve spark energy.

Background info:
The DME updates its rpm measurement every T1 interrupt (you calculated 11.5ms earlier). It is a count of how many teeth have passed since the last interrupt. At 40rpm, there is only 1 tooth counted for the 11.5ms duration. A small increase in actual crank speed will dramatically increase the DME's measured rpm. A count of 2 teeth is 80rpm; 3 teeth is 120rpm, etc.


Ok, now when first cranking the engine, the starter motor is accelerating the crankshaft. The DME might first calculate 40rpm, but due to the extreme quantization, that measurement can quickly change from 40rpm to something much higher. By having a long dwell time, the DME is ensuring that even if the rpms dramatically change, the coil will have had plenty of time to charge.

Additional factor, is that if you look at the crankshaft speed during cranking, it is not consistent. It slows when a cylinder is in the compression stroke, and accelerates after that cylinder hits TDC (this should be true even without spark or fuel).

This is guarding against the accelerations that happen during starting / low-rpm, to ensure the coil has plenty of time to build energy. We can see this as the DME has uses generally the same number of ('half')teeth required for the lower rpms:

DME_Dwell_raw.PNG (17.56 KiB) Viewed 419 times

To charge and fire the coil confidently, we want multiple teeth to have passed. The table's minimum of 10 is equal to 5 teeth, which at 40rpm is approximately 57ms.

Finally, the 'on-time' of the coil vs 'off-time' at low rpms is pretty small. So, even though it is on for a long time, the off time is extremely long, giving a lot of time for the driver & coil to cool off. And cranking operation is not considered as a continuous operating mode, rather a transient.

[johnb]

Here's what I don't get though. Why are the dwell times so long for low rpm? Worst case, at ~14v it takes no more than 6.5ms to fully charge. But the dwell map calls for more than 10 times that at the lowest rpm. What does the extra dwell time achieve?

Is there something happening at low rpm besides the voltage drop that makes it charge more slowly?

A couple of considerations: Firstly, during cranking the fuel-air mixture is generally not well mixed, and can be difficult to ignite. To bet the best chance, we need to ensure the spark has the maximum energy possible.

It isn't that it needs that much time to charge. At 6.0 volts, the coils is basically as high of current as it can get (a little less than 6 amps) after 40ms, the longer time doesn't improve spark energy.

Background info:
The DME updates its rpm measurement every T1 interrupt (you calculated 11.5ms earlier). It is a count of how many teeth have passed since the last interrupt. At 40rpm, there is only 1 tooth counted for the 11.5ms duration. A small increase in actual crank speed will dramatically increase the DME's measured rpm. A count of 2 teeth is 80rpm; 3 teeth is 120rpm, etc.


Ok, now when first cranking the engine, the starter motor is accelerating the crankshaft. The DME might first calculate 40rpm, but due to the extreme quantization, that measurement can quickly change from 40rpm to something much higher. By having a long dwell time, the DME is ensuring that even if the rpms dramatically change, the coil will have had plenty of time to charge.

Additional factor, is that if you look at the crankshaft speed during cranking, it is not consistent. It slows when a cylinder is in the compression stroke, and accelerates after that cylinder hits TDC (this should be true even without spark or fuel).

This is guarding against the accelerations that happen during starting / low-rpm, to ensure the coil has plenty of time to build energy. We can see this as the DME has uses generally the same number of ('half')teeth required for the lower rpms:

DME_Dwell_raw.PNG

To charge and fire the coil confidently, we want multiple teeth to have passed. The table's minimum of 10 is equal to 5 teeth, which at 40rpm is approximately 57ms.

Finally, the 'on-time' of the coil vs 'off-time' at low rpms is pretty small. So, even though it is on for a long time, the overall duty-cycle is not high giving a lot of time for the driver & coil to cool off. And cranking operation is not considered as a continuous operating mode, rather a transient.

Thanks, that explains it...I realize now that in converting the dwell map values to time we're making the assumption of constant crank speed, which might be reasonable in some conditions but not at 40rpm!