Timing issues

[Philip Lochner]

Dear Sirs
 
Background:
I would very much like to install an MS2 into my Jaguar XJS V12 (I've got the WOLF3D in there at the moment - doing ignition only). At 6500rpm, with a single coil and distributor this will require a spark every 1.538msec.  I am somewhat concerned that the MS2 is going to have ignition related issues at such high revs.  MegaSquirt-II scales down both the charge and discharge times proportionally.  The issue is this: Let's say I've got dwell on 4ms and spark duration on 0.4ms.  Then, with MS2, when dwell is down to 2ms, spark duration will be down to 0.2ms which means you could start charging the coil while its still busy discharging. If its OK to charge a coil while its still busy discharging (I truly don't know) then don't bother to read further but please just inform me accordingly.  If not, please hear me out.
 
It seems to me that a typical coil needs AT LEAST 0.2msec to discharge. On the Jag I have found a setting of 0.4msec to work well as this leaves about 1.1msec to recharge the coil which worked fine up to 6200rpm (self imposed rev limit).
 
The Wolf3D offers two changeable coil related fields (please note the words in capitals in relation to the words "charge" and "discharge"):
1) "MAXIMUM charge time" (set to 4ms in my case) and
2) "MINIMUM discharge time" (set to 0.4ms in my case)
 
The WOLF manufacturer confirmed that Minimum discharge time overrides MAXIMUM charge time, meaning that it will allow the coil to discharge for NOT LESS THAN the time set and whatever time is left to the next ignition event will be used for charging the coil up to a max of the "Max charge time".
 
Presentation:
On the MS2 things appear to be slightly different.
 
I have three concerns :
 
1) on the  " Ignition options  " screen  is a field called "MAXIMUM spark duration".  The word  " maximum "  alludes to the possibility that the figure entered   there is  a maximum figure which can reduce under certain circumstances.   Should this not be a MINIMUM  rather than a maximum? 
2) My second concern comes from  the words "tries to wait":
[*]Maximum Spark Duration (ms) (max_spk_dur) is the amount of time MegaSquirt-II tries to wait before starting another charging cycle.
What does "tries to wait" mean?  Why should it EVER have to TRY to wait?  It seems to me that it MUST wait at least this amount of time. 
3)  Under "Configuring Megasquirt2", all the way down to the bottom we read:  
Maximum Dwell Duration (ms) (max_coil_dur) is the longest period the coil is allowed to charge - too long and it can burn out your coil, too short and the spark may be weak. Normally this value is between 2.0 and 4.0 milliseconds. For the GM HEI 'in-cap' coil, a value of 3.5 milliseconds is typical.  
 
MegaSquirt-II attempts to set up a charge/ discharge cycle that charges the coil for the specified charge time, discharges it for at least the specified spark duration time, and then remains in the discharge state until it is time to start charging for the next cylinder. At high rpm, there may not be time to fit in the specified charge + discharge times. In this case, MegaSquirt-II scales down both the charge and discharge times proportionally so they just fit within the time between tach pulses.   
 
Proposal:
 In view of the above, would it be sensible to change the code so it will read  as follows? 
"MegaSquirt-II scales down both the charge and discharge times until is reaches the "Minimum  Spark Duration"  time  after which the discharge time will remain constant at not less than the set figure.  From then on, only the charge time will be further reduced (resulting in weaker sparks as the revs climb...) so that the sum of charge and discharge time will fit between the tach pulses.

 
Kind regards
Philip Lochner
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Posted by email.

[Bruce Bowling]

First off, the RPMs here are pushing the envelope with a V12 with a saturated ignition coil setup. Not to say it cannot be done, but a fast regenerating CDI unit may be a better option compared to very short discharge times on a conventional coil.

First, if is, from an electrical point, it is O.K. to re-energize an ignition coil during the flyback phase. Switch-mode power supplies do it all the time. All you are doing is re-introducing current within the primary. In fact there are some "plasma" ignitions that apply a fast square-wave pusletrain to a ignition coil in order to prolong the burn duration. Now, the coil may get real hot, since the overall duty is high - keep this in mind.

I know the goal would be to burn as long as possible. But the dwell and associated burn times are related. Basically, the flyback pulse in the primary circuit depends on the amount of core saturation, this is related to how long the current is applied to the coil.

One thing you need to also consider is that (I assume) this is a single coil with a distributor. The rotor within the distributor points to the spark plug to fire. This needs to be phased up, and there needs to be a time where the coil is not discharging (burn time) while the rotor is in between distributor cap posts. If the coil is discarging when the rotor is in between posts, the spark could jump from the previous post to the next post before it was intended, with the result of advanced spark.

The best thing you can do is to select a coil with a low primary resistance and inductance, this will allow the coil to charge up quicker during the short dwell time. There are MSD coils with low inductance that would be a good fit, I believe the racing coils are really low inductance.

To do this right, we will need the resistance and inductance of the primary circuit, the turns ratio of the coil, the secondary resistance and capacitance, and the voltage drop across the ignition driver.

- Bruce

[midnightblue]

Jaguar had recognised this issue on the HE version on the 5.3 V12. To try and help the coil charging they wired in a second coil with the primary in parallel to the main coil, and the HT ouput filled with epoxy.

I can only assume that this was designed as a "charge reservior" to aid the main coil, providing a high current source for high RPM operation.

I'm not sure if this would be enough to be helpful, particularly up at 6500 rpm, but it's an interesting concept.

[Philip Lochner]

Hi Bruce
 
I fitted an MSD 6A to the Jag, but only AFTER I got things working nicely with a single coil (0.8Ohm primary, dont have the other specs) and distributor with rotor phased at 22BTDC.  Perhaps the fact that I live at 5000feet asl has something to do with it.  I did not notice any improvement at high rpms but oddly things did improve a low RPM and idle - which I THINK has to do with higher voltages on the primary and also the multiple spark being generated per compression stroke.
 
While still running on the single coil  setup I found spark above 4500rpm to be very sensitive to the the discharge time setting (as this affected the charge time directly) on the ECU which is why I thought I should raise this topic.
 
Thanks for the reply as I'm confident that since I now have the MSD anyway, I should be able to make the MS2 work on the Jag.
 
PS: Will Ultra Megasquirt allow for two oxygen sensors each being used to autotune is own VE table?
 
Kind regards
Philip Lochner

-----Original Message-----
From: Bruce Bowling [mailto:bbowling@earthlink.net]
Sent: 21 March 2006 01:39 AM
To: megasquirt2@msefi.com
Subject: Timing issues



First off, the RPMs here are pushing the envelope with a V12 with a saturated ignition coil setup. Not to say it cannot be done, but a fast regenerating CDI unit may be a better option compared to very short discharge times on a conventional coil.

First, if is, from an electrical point, it is O.K. to re-energize an ignition coil during the flyback phase. Switch-mode power supplies do it all the time. All you are doing is re-introducing current within the primary. In fact there are some "plasma" ignitions that apply a fast square-wave pusletrain to a ignition coil in order to prolong the burn duration. Now, the coil may get real hot, since the overall duty is high - keep this in mind.

I know the goal would be to burn as long as possible. But the dwell and associated burn times are related. Basically, the flyback pulse in the primary circuit depends on the amount of core saturation, this is related to how long the current is applied to the coil.

One thing you need to also consider is that (I assume) this is a single coil with a distributor. The rotor within the distributor points to the spark plug to fire. This needs to be phased up, and there needs to be a time where the coil is not discharging (burn time) while the rotor is in between distributor cap posts. If the coil is discarging when the rotor is in between posts, the spark could jump from the previous post to the next post before it was intended, with the result of advanced spark.

The best thing you can do is to select a coil with a low primary resistance and inductance, this will allow the coil to charge up quicker during the short dwell time. There are MSD coils with low inductance that would be a good fit, I believe the racing coils are really low inductance.

To do this right, we will need the resistance and inductance of the primary circuit, the turns ratio of the coil, the secondary resistance and capacitance, and the voltage drop across the ignition driver.

- Bruce



This post is at: http://www.msefi.com/viewtopic.php?p=109014#109014

Posted by email.

[grippo]

PS: Will Ultra Megasquirt allow for two oxygen sensors each being used to autotune is own VE table?

Posted by email.[/quote]

MS II v2.6x does this right now - but I don't believe anyone has tested it.