Skip Firing with MS3X

[vishnu]

Hi,

I'm using megasquirt to control an engine which requires to be injected and fired only once every 15 cycles. So, at present I'm taking the CAM signal to a custom made circuit built by my colleagues, which will give out a cam pulse every 15 cycles and the first 14 will be turned off.

In this case, I believe that the RPM will not be synced because the tach pulse would be sent on the 15th cycle and when the megasquirt looks for the CAM signal on the 16th cycle, it would not be present.

Am i thinking right in assuming that the trigger pulse for the injector sent by the CAM sensor in the 15th cycle would inject the fuel in the next cycle (that is 16th) ?


P.S.: Since I was not able to figure out a way to tweak the code for skip firing, we have ended up building up an electronic circuit for the same.

Thanks.

[billr]

I gotta ask... what kind of beast is this??? Give us more details of what you are trying to do. "Hit-and-miss" speed/power regulation has been pretty much obsolete for the last 100 years! I'm thinking you will need to fool the crank input, not just the cam. Actually, I'm thinking an "auxiliary" crank wheel, driven by a belt or gearing at 1/15th crank speed would be the easiest way.

[vishnu]

Hi,

I'm presently running a research engine which has optical piston access. Hence, it is required that the engine should be run on skip firing mode, otherwise the quartz piston would get heated up easily.

Oh, i see. I was just wondering if there was any other soft method to fool the crank input. That's why posted the question here, thinking anyone of you would have tried earlier. Anyways, thank you very much

[billr]

Well, maybe there is some other way. Maybe use something like a 6-tooth crank wheel and set TS for 90 teeth for the crank, then use your divider to fake-out the cam signal? Still, it seems like a little bit of gearing at the crank wheel would be simpler; especially if this is a research engine and the extra space for a jack-shaft off of the crank is irrelevant.

[elaw]

It may not be absolutely perfect, but why not try the rev limiter?

Set it so it cuts spark & fuel at whatever RPM the engine reaches after 1 firing, then set the hysteresis so the fuel + spark remain cut until the engine does another 15 revolutions (during which time I'd expect the RPM to drop a lot).

[vishnu]

Well, maybe there is some other way. Maybe use something like a 6-tooth crank wheel and set TS for 90 teeth for the crank, then use your divider to fake-out the cam signal? Still, it seems like a little bit of gearing at the crank wheel would be simpler; especially if this is a research engine and the extra space for a jack-shaft off of the crank is irrelevant.

Thanks for the suggestion!

[vishnu]

It may not be absolutely perfect, but why not try the rev limiter?

Set it so it cuts spark & fuel at whatever RPM the engine reaches after 1 firing, then set the hysteresis so the fuel + spark remain cut until the engine does another 15 revolutions (during which time I'd expect the RPM to drop a lot).

Hi Eric,

I could prevent the RPM from dropping down because we have a dyno which will keep it steady in the same RPM. I thought the Rev limiter would cut only the spark but not the fuel.

Glad to know that it cuts the fuel too until 'n' number of revolutions. Will try it out.
Thank you

[vishnu]

Eric,

Is there a way to set the spark and fuel cut for exactly 15 revolutions in the Tuner Studio? I'm able to set only RPM for hysteresis, which is a very rough approximation and am guessing it wouldn't work well everytime for the same 15 revolutions.

Any suggestions?

[jsmcortina]

Nobody has ever asked this before and it is most unusual request, so no, there's no easy way to do this.

James

[kaeman]

probably easiest way would be run a toothed wheel with missing tooth ie. 36-1 or 24-1, and set it up with a 15 to 1 reduction, then just tell the unit its a 1 cylinder with 36-1 or 24-1 tooth wheel on the cam, not the crank.

[elaw]

Or if you're more electronically inclined, built a count-to-16 circuit that hooks into the injector driver and coil driver signals, and inhibits them for all except one count.

[jamies]

what about if you run alcohol / methanol as a fuel as it wont get all that hot?

[vishnu]

I'm using Tuner Studio version 2.6.14 and I couldn't see the "Cut x sparks out of Y events" option under the REV Limiter tab, as it was in previous versions of TS.

Kindly suggest

[dontz125]

The option to 'cut x sparks from y events' is a feature of the firmware actually running the MS3; TS is the tuning software, but it can only change what the developers allow.

[billr]

Besides, do you really want to just "cut spark", leaving fuel still going in? I don't think there is anything simpler than dividing-down the injector pulses, as "elaw" suggested; you wouldn't even have to bother with the spark, it will do nothing with no fuel during the 14 "neutered" cycles.

[vishnu]

Besides, do you really want to just "cut spark", leaving fuel still going in? I don't think there is anything simpler than dividing-down the injector pulses, as "elaw" suggested; you wouldn't even have to bother with the spark, it will do nothing with no fuel during the 14 "neutered" cycles.

Thank you! Yeah, I understand. But was just curious to find out why that "cut x sparks out of y events" option is no more seen in the tunerstudio version 2.6.14

[jsmcortina]

But was just curious to find out why that "cut x sparks out of y events" option is no more seen in the tunerstudio version 2.6.14

The TunerStudio version is irrelevant. The firmware version drives the settings.

James