Ability to reverse the firing order on demand?

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benckj
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Re: Ability to reverse the firing order on demand?

Post by benckj »

I’m having a chuckle as well because I recall starting old tractors by setting crank position before firing a charge. If you got it wrong it ran in reverse which was great fun driving. Being a diesel or actually kerosene there was no ignition timing to worry about.

Another funny story from a work colleague described how he lost power on a tractor while climbing a hill. When he pushed in clutch he rolled backwards rather quickly and then popped the clutch only to start engine backwards which reall made him move in wrong direction. The seat has a permanent pucker mark.

Sorry for stories but can’t help with problem. Enjoying the read though.
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billr
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Re: Ability to reverse the firing order on demand?

Post by billr »

I've let myself get sucked into this deeper than intended, but need some clarifications:

This is an inline-6, correct?

Do you have any pictures of that type crank? The only one I've found so far won't correlate with the firing order posted (either fwd or rev). What I am seeing would fire either 1-6-3-2-5-4 or the reverse.
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Re: Ability to reverse the firing order on demand?

Post by stobl »

Is an online 6, yes.

I'm searching for an actual photo of the be crank, but here's the microfiche of it:

https://www.marineengine.com/parts/merc ... d-assembly

In this, page 2 second column is for the engine:
https://www.google.com/url?sa=t&source= ... onPbwgrdXj

And here's a view of the race version of the motor mark 75h. Identical except for a few performance tweaks.(no starter, exhaust, etc)

https://images4.sw-cdn.net/product/pict ... 814536.jpg
billr
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Re: Ability to reverse the firing order on demand?

Post by billr »

Thanks, that info is plenty adequate. I may not be able to help you much, but it will amuse me to study what all is required to reverse that beast.

I noted that the firing order changed at one point, accounted for by dual markings on the dizzy cap, and the twin coils in the photo.

So far, your best help here (from somebody else!) is to use table switching and not have to use two CKP sensors. I'm chagrined that I didn't think of that.

Jean B, I'm not saying you are wrong about the 120-240 problem, but I don't quite understand it yet and want to study more before agreeing or challenging. At any rate, I wanted to get us past any discussions of V6s, as I was pretty sure the old Mercs were alone in being inline.

PS: I don't think it matters, but is #1 the top cylinder or the bottom? Also, that racing engine shows the pull rope cranking CW looking down on the engine, is that the forward direction (1-6-4-2-5-3 for you)?
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Re: Ability to reverse the firing order on demand?

Post by racingmini_mtl »

billr wrote:Jean B, I'm not saying you are wrong about the 120-240 problem, but I don't quite understand it yet and want to study more before agreeing or challenging. At any rate, I wanted to get us past any discussions of V6s, as I was pretty sure the old Mercs were alone in being inline.
Bill my example was actually more valid for a 3-cylinder (or a 6 cylinder with a 3-plane crank) as I thought this engine only had 3 ignition coils firing every 120 degrees but if you change 120-240 for 60-300 that's still valid.

The easy way of looking at it should be to think of a distributor. If you start from cylinder 1 going one way you have one firing sequence and if you reverse the direction, you have a different sequence. This is the equivalent of the different angles I mentioned.

With a firing order of 1-6-4-2-5-3 reversing becomes 1-3-5-2-4-6 so with sparks A-B-C-D-E-F, you would need to change spark B from cylinder 6 to cylinder 3, C from 4 to 5, E from 5 to 4, F from 3 to 6 and sparks A and D stay with cylinders 1 and 2. Of course, that assumes you have a correct reference to cylinder 1 when going in the reverse direction which is why you would need a second CKP sensor that gives the correct tooth 1 angle in the reverse direction without changing the parameters (which is not something you can do without modifying the code).

Jean
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billr
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Re: Ability to reverse the firing order on demand?

Post by billr »

What about the suggestion to use table-switching, with really goofy advance values in the "reverse" table to compensate for the #1 spark angle being in a new position? Wouldn't that eliminate the need for a second CKP sensor? I realize that may put the missing-tooth gap in a non-ideal location relative to compression loads on the crank, but it would only be for occasional use (reverse) and I think a 6-cylinder 2-stroke has pretty tame cranking anyway.
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Re: Ability to reverse the firing order on demand?

Post by racingmini_mtl »

Using table switching might be possible but I think you're very likely to end up with a needed timing value outside the allowed range.
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stobl
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Re: Ability to reverse the firing order on demand?

Post by stobl »

Jean,

Lets say the sensor is 40 degrees prior to #1 TDC. IIRC I think the max this engine does is about 30* advance. That would place it 20 degrees after #3 TDC (going in reverse, it would be 20 degrees prior to #3 TDC). With table switching, couldn't you still use the single sensor, but change the firing order to #3 being the first to fire after the missing tooth? Although as I write this out, along with the reverse firing order table change, you would need to have a second trigger offset that would switch with the firing order table change.

So for forward: [40* trigger angle] 1-6-4-2-5-3
Reverse: [20* trigger angle] 3-5-2-4-6-1

Or is there something I'm missing? I don't see why there would be a need for a second trigger if you could switch the tables along with a separate trigger offset angle. Sorry, I didn't clearly state that in the beginning, but that was my thought process with this. Or even- split the difference- 30 degree trigger offset between the cylinders #3 and #1 so it's identical for both directions? Being as reverse direction is only used for a few seconds within a day worth of use, and then only used at slightly above idle speeds, a max of 20 degree advance wouldn't be horrible. Then all you need is to establish that once the missing tooth is seen by the sensor, start with #3. My thought is that it would be "simpler" to give the board the alternate firing order upon receiving the "ignition table switch" input signal.

sort of,

if reverse signal = true
trigger advance = 20
firing table: A=3 B=5, etc....

As the ECU can't tell which direction the engine is spinning, all it knows it that after the missing tooth, 40* or so comes TDC for the first cylinder in the firing order. If you can change that first cylinder from #1 to #3, i don't see why it wouldn't work? (along with trigger offset change)


Blake
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Re: Ability to reverse the firing order on demand?

Post by LAV1000 »

I am more into hardware, and would be switching the 6 logical outputs and the second crank sensor.
I would experiment whit these ones:
https://nl.mouser.com/ProductDetail/Tos ... pYGjM9M%3d
billr
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Re: Ability to reverse the firing order on demand?

Post by billr »

Table switching can take care of the sensor offset angle, so you can use a single-sensor with a "bias" added to all cells in the second table. However, the coils are hard-wired and have to be switched via logic as suggested; or use a dizzy that automatically switches the single-coil output. (Yes, Jean, I understand it now).

I realize we are talking in circles here, but keep in mind that you can not do the below, spark outputs always go A-B-C-...

"If you can change that first cylinder from #1 to #3, ..."
stobl
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Re: Ability to reverse the firing order on demand?

Post by stobl »

Darn. I just reread the manual section on firing order too. For some reason I thought the firing order could be changed/ outputs rearranged. Looks like it's back to a distributor retrofit and msd box then :-(. Thanks everyone for the discussion.
billr
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Re: Ability to reverse the firing order on demand?

Post by billr »

No need to give up yet. Several of us feel this can be easily handled with just a few simple and inexpensive logic chips. The biggest hassle will be cost and space for six COP/CNP units.

However, how is "water-resistance" handled with the stock system; special plug wires/boots dizzy and such? The basic MS units aren't at all water-proof, and the well-protected MS3Pro is kind of pricey. There are certainly less expensive ways to put a basic MS and the special logic you need in a sealed enclosure, but your maybe-wet environment needs to be considered as we discuss your proposed install.
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Re: Ability to reverse the firing order on demand?

Post by stobl »

I'm not completely turned off yet, I'm just not too experience with the chips. I'll have to do a little more research.

As for water incursion- the shroud on the engine for the most part keeps water out with the air inlet on the rear, however it does get a bit wet due to there being no airbox or filters on the carbs. Worst case, I can throw the megasquirt in a small pelican case and use some sealed bulkhead connectors. Or, it doesn't even necessarily have to be on/in the engine. The ecu can be on the boat itself with the wires passed through to the engine. If I go that way (Most likely) there would be a max of 30" or so from the engine to the rear storage bay on the boat where the fuel tank and lifejackets/etc. are. That is fairly sealed off with the cover on it.

I'm not too worried about space for the coils though. Essentially the profile of the engine (Looking down) is a straight line, with the starter and distributor turning it into a "T". Once the distributor is out of the way, it opens up quite a bit of space on that side of the motor.
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Re: Ability to reverse the firing order on demand?

Post by LAV1000 »

stobl wrote:I'm not completely turned off yet, I'm just not too experience with the chips. I'll have to do a little more research.
Those chips are just analog device's packed together in a convenient package.
If you put the ms unit inside the boot use separated looms for your coils and sensors.
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Re: Ability to reverse the firing order on demand?

Post by stobl »

Since I'm seeing this picture a bit more clearly now- in regards to the trigger offset angle, would table switching be difficult to implement for that? I'd still like to keep a single sensor, but "switch" the coil paths depending on direction.

So if I'm thinking this through correctly- Outputs from MS would split and run through two ICs- one will connect to the forward rotation firing pattern, and the other will connect to the reverse direction firing pattern? With a manual switch determining which gates are open? So two sets of trigger wires would feed to each coil?

If a second crank sensor is used and switched the same way, wouldn't they have to have identical offsets? (as to not have to adjust the settings?)


Thanks again for all the help,
Blake
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Re: Ability to reverse the firing order on demand?

Post by racingmini_mtl »

stobl wrote:If a second crank sensor is used and switched the same way, wouldn't they have to have identical offsets? (as to not have to adjust the settings?)
Yes but running in the reverse direction. So if the missing tooth is located at a certain angle before TDC, the second sensor would need to be located such that the missing tooth is located at the same angle but after TDC (which will become before TDC in the reverse direction).

Hopefully that's not too confusing.

Jean
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Re: Ability to reverse the firing order on demand?

Post by billr »

Yes, two sets of trigger wires to each coil driver, only one set at a time enabled by the switching mechanism. That could be either simple mechanical switches or electronic components. Note that only 4 coils would have to be switched, #1 and #2 don't change when rotation is reversed.

I see Jean just addressed the "second sensor" issue, but let me add that a single sensor would only require adding a constant value to all cells in the forward table for reverse. That will give very odd values in the second table, but is actually very easy to make. You need either table-switching or two (switched) sensors, but not both.
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Re: Ability to reverse the firing order on demand?

Post by billr »

Well, I think the OP has already suggested this, but I'm just catching on...

If a single CKP sensor is located such that the center of the wheel missing-tooth gap passes the sensor at TDC or BDC, then the #1 angle will be the same in either direction; no need for either table-switching or a second sensor. This is getting easier and easier!
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Re: Ability to reverse the firing order on demand?

Post by LAV1000 »

billr wrote:Well, I think the OP has already suggested this, but I'm just catching on...

If a single CKP sensor is located such that the center of the wheel missing-tooth gap passes the sensor at TDC or BDC, then the #1 angle will be the same in either direction; no need for either table-switching or a second sensor. This is getting easier and easier!
I don't know if this works ?
Say you put the sensor at TDC and you need ignition at 25 BTDC does it work whit -25 or is gonna be 335 and almost 1 revolution later ?
Sensor at BDC, also take a needed 25 BTDC can you put in 155 degrees after BDC ?

Then there is the point that it is not adviced to locate the sensor at the TDC or BDC.
Because at those points the crank almost stops and there is no accurate speed measurement of the crank.

Could it work whit a 12-1-12-1 setup, symmetrical missing teeth ?
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Re: Ability to reverse the firing order on demand?

Post by elaw »

LAV1000 wrote:
billr wrote:I don't know if this works ?
Say you put the sensor at TDC and you need ignition at 25 BTDC does it work whit -25 or is gonna be 335 and almost 1 revolution later ?
I think with a basic "missing tooth wheel" setup, the Megasquirt is smart enough to make it work. It knows the gap is at TDC, and therefore knows that to fire a spark some amount BTDC, it needs to wait until some number of teeth have passed since the gap, then some number of microseconds, then fire the spark. I'm oversimplifying a bit but you get the idea.

The important point being that as another poster indicated, if the center of the gap is exactly at TDC, the timing calculation will be the same regardless of which way the engine is turning.
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