CH25S MDI tach?

[darkminion_17]

[Gompers]

I was under the impression that wasted spark refers to the spark plug firing on the exhaust stroke as well as the compression stroke.

The two coils fire at different times, (90 degrees apart from one another) but they both fire every time the magnet goes by, so once every 360 degrees of flywheel rotation. The cylinder only has a compression stroke once every 720 degrees of flywheel rotation (4 stroke engine). So one of those fires is "wasted".

On a non-wasted spark engine, like a K series, the engine only fires once every 720 degrees of rotation, since the cam travels at half the speed of the crank, and ignition is timed off the cam, not off the flywheel.

I believe (though I could be wrong...haven't ever really messed with one) that the SAM on a CH22/25 engine just advances the timing based on engine RPM, but doesn't actually work like a fully electronic ignition would in a non-wasted spark engine, only firing the cylinder once every 720 degrees.

It's also why when you do static timing on a KT series, which has two "ignition" lobes on the cam, you should set it, rotate the engine 360 degrees and check it/set it again. It can be off quite a bit depending on the condition of the camshaft lobes. The KT manual says to set it between the two.

So, on a 4 cycle wasted spark engine, any spark plug firing will give you a 360 degree rotation of the engine. On a non-wasted spark engine, it would give you 720 degrees of rotation. That's why you would theoretically need to divide by 2 to get the engine RPM.

[cooperino]

Gompers,
You might very well be 100% correct. So I looked for some explanation. Here is what I found

The wasted spark system is an ignition system used in some four-stroke cycle internal combustion engines. In a wasted spark system, the spark plugs fire in pairs, with one plug in a cylinder on its compression stroke and the other plug in a cylinder on its exhaust stroke. The extra spark during the exhaust stroke has no effect and is thus "wasted". This design halves the number of components necessary in a typical ignition system, while the extra spark, against much reduced dielectric resistance, barely impacts the lifespan of modern ignition components. In a typical engine, it requires only about 2–3 kV to fire the cylinder on its exhaust stroke. The remaining coil energy is available to fire the spark plug in the cylinder on its compression stroke (typically about 8 to 12 kV).


So to me it looks like they call a wasted spark system one that fires spark plugs simultaneously or in pairs as they put it. The spark in this case is not the same to both cylinders. There is a difference in the amount of energy of spark from one cylinder to the other.. "something I didnt realize till reading this"

The other thing in question that I cant seem to find an answer to is in a SAM system. Does something in that system make each cylinder only fire on compression? I know its advances but it seems like there is more to it than that.

I like the conversation. I like learning how things like this work. I think SAMMAC probably has a good idea of how SAM "spark advance modules " work.

[RAC65]

So the SAM system fires each time the flywheel comes around. With no cam position sensor it has to since it has no way of knowing which stroke it's on. The original ch25s SAM system uses 2 wire coils and apparently also uses the signal from just 1 coil for a useable tach signal on the kill wire coming from the SAM box.
The retrofit MDI system while still using 2 coils and firing every revolution has 1 wire coil packs with just the kill wire. There is also no "black box" as on the SAM system.
Changing out to the MDI also required changing the flywheel. Still just 1 firing magnet unit bolted to the flywheel but its in a different place relative to the crank key slot.

[RAC65]

BTW.... I still need a way to separate a tach signal from the kill wires and boost the signal to run the tach. :-)

[Gompers]

Originally Posted by cooperino

Gompers,
You might very well be 100% correct. So I looked for some explanation. Here is what I found

The wasted spark system is an ignition system used in some four-stroke cycle internal combustion engines. In a wasted spark system, the spark plugs fire in pairs, with one plug in a cylinder on its compression stroke and the other plug in a cylinder on its exhaust stroke. The extra spark during the exhaust stroke has no effect and is thus "wasted". This design halves the number of components necessary in a typical ignition system, while the extra spark, against much reduced dielectric resistance, barely impacts the lifespan of modern ignition components. In a typical engine, it requires only about 2–3 kV to fire the cylinder on its exhaust stroke. The remaining coil energy is available to fire the spark plug in the cylinder on its compression stroke (typically about 8 to 12 kV).


So to me it looks like they call a wasted spark system one that fires spark plugs simultaneously or in pairs as they put it. The spark in this case is not the same to both cylinders. There is a difference in the amount of energy of spark from one cylinder to the other.. "something I didnt realize till reading this"

The other thing in question that I cant seem to find an answer to is in a SAM system. Does something in that system make each cylinder only fire on compression? I know its advances but it seems like there is more to it than that.

I like the conversation. I like learning how things like this work. I think SAMMAC probably has a good idea of how SAM "spark advance modules " work.

Wasted spark was used back in the day to simplify the ignition system and is used today to reduce emissions. The plugs would only fire in pairs if they they have cylinders with strokes that are 360 degrees out from one another. Otherwise, you don't have a reduction in ignition parts. This is how M18 and KT17s work. One magneto (for a M) and one coil (for a KT) with two leads that fire at once, since the cylinders are 180 degrees opposed.

Also, pretty much all single cylinder magneto ignition small engines are wasted spark since the magneto fires every time the flywheel magnet comes around.

From wikipedia: "Most single cylinder [four-stroke] engines use the wasted spark system in order to capitalise on the simplicity and reliability of the flywheel magneto. These engines need a flywheel to run smoothly, and the heavy current-generating magnets help provide the momentum while delivering a zero-maintenance drive to the ignition system. Bolted to the end of the crankshaft, this flywheel rotates twice for each compression stroke."

Obviously there's no second plug to fire in this case.

This won't work on V twins with shared crank journals since the firing interval is not equal. So one piston is 90 degrees away from TDC when the other one is at TDC, regardless of which one is on compression and which is on exhaust. Each cylinder has it's own ignition system and it fires on every revolution, hence "wasted spark". It's basically two individual ignition systems each running like a single cylinder engine would.

As RAC says above, there's no cam position sensor to know which cylinder is on which stroke so it fires every time. I suspect this was done to keep things simple(r).

[cooperino]

Originally Posted by Gompers

Wasted spark was used back in the day to simplify the ignition system and is used today to reduce emissions. The plugs would only fire in pairs if they they have cylinders with strokes that are 360 degrees out from one another. Otherwise, you don't have a reduction in ignition parts. This is how M18 and KT17s work. One magneto (for a M) and one coil (for a KT) with two leads that fire at once, since the cylinders are 180 degrees opposed.

Also, pretty much all single cylinder magneto ignition small engines are wasted spark since the magneto fires every time the flywheel magnet comes around.

From wikipedia: "Most single cylinder [four-stroke] engines use the wasted spark system in order to capitalise on the simplicity and reliability of the flywheel magneto. These engines need a flywheel to run smoothly, and the heavy current-generating magnets help provide the momentum while delivering a zero-maintenance drive to the ignition system. Bolted to the end of the crankshaft, this flywheel rotates twice for each compression stroke."

Obviously there's no second plug to fire in this case.

This won't work on V twins with shared crank journals since the firing interval is not equal. So one piston is 90 degrees away from TDC when the other one is at TDC, regardless of which one is on compression and which is on exhaust. Each cylinder has it's own ignition system and it fires on every revolution, hence "wasted spark". It's basically two individual ignition systems each running like a single cylinder engine would.

As RAC says above, there's no cam position sensor to know which cylinder is on which stroke so it fires every time. I suspect this was done to keep things simple(r).

Very good read and well written explanation.. thank you!

[Gompers]

Originally Posted by cooperino

Very good read and well written explanation.. thank you!

Anytime! I'm far from an expert on these things, but love to learn from others and share what I've learned. The KT timing thing was especially interesting to me first time I timed one, I couldn't figure out why it wouldn't stay put. Turned out the engine I was working on had one lobe worn worse than the other for whatever reason, and it was a couple degrees different between the two.

Now we just need to get OP his tach working. Inductive pickup works well for hooking up an OScope, but probably doesn't deliver enough of a pulse to drive the tach.

I can't think of any super simple things off the top of my head. Some sort of inductive pickup with a simple pullup circuit to trigger a voltage spike might do it. Probably simpler just to replace the tach, but I know that's not what you're looking for.

[RAC65]

Yeah, tried the inductive pick-up, no dice. Had a MSD GMR pick-up that I think does pull a signal from the cut-out wire but the output signal from the GMR unit isn't high enough voltage to drive the tach. The tach will run from a dummy ignition using a early 4 pin GM HEI ignition and coil with a Chrysler pick-up. but the coil has to be in the circuit to run the tach, the signal from the 4 pin HEI on its own won't run this tach.
Never tried it, but what about pulling signal from stator wire going to rectifier/regulator?

[cooperino]

I think the CD system is what he is trying to achieve. I would be looking into inductive CD tachometers