Why not?: Flywheel as crank position sensor

[NewbVetteGuy]

As I was reading the install instructions for my crank position sensor / trigger wheel, I had to wonder why flywheels aren't ever used as crank position sensors?

They're HUGE and have tons of teeth, which should make them freakishly accurate...

I understand you just can't file off a tooth to mark TDC, but a strong earth magnet put close to a tooth and a combination of a hall effect sensor and a magnetic sensor seems like it would work... (or Hall Effect + Optical Sensor and a shiny metal chrome-plated coated tooth, even...)


Is it just that at high RPM with a huge number of teeth, it's hard to differentiate one tooth from another or is it just that installing a crank trigger on the front of the engine is easier? (Popping off a dust cover and installing a sensor or two doesn't seem THAT hard...)


I'm also curious why for sequential port injection or with COP ignition EFI systems still require a cam sync sensor for engines that don't have variable valve tech. If you accurately know where the crank is and the crank is connected to the cam via a chain, or gears, then you also know where the cam is -it doesn't seem necessary to actually measure where the cam is in this scenario as if you know where the crank is, and the two are connected by a chain / gear, then for every degree you know where the cam is if you degreed it when you installed it, right?... (Is it just that the EFI system designers don't want to have to rely upon users to have accurately degree in the cam?)


Just overly curious again...

Adam

[chevyfreak]

Only one i know off is hyster fortens forklift. They use a speed sensor on the ring gear to tell system manager if engine is running. Not as a trigger but it does measure rpm of engine.

Chevyfreak.

[arlancam509]

the cam position signal is required to tell the ECU whether the piston at top dead center is pooooshing out exhaust or compressing the fresh mixture (when going with sequential injection). if you have a 4 cylinder engine, you don't need the cam position signal, but you sure do with an eight.

[Kevin Johnson]

Shape of the teeth.

[BenE64]

They can build up metal filings from the starter motor engagement which messes the signal up. Most diesel engines i work on now have a speed sensor on the fly wheel, but reference timing from another position sensor.

[strokersix]

Flywheel electric power generation is in the near future, at least for agricultural machinery which I am a little bit familiar with. Picking up a timing signal should be much easier I would think. I'm no sparky just a mechanical guy so I'm sure it's more complicated.

[mk e]

The flywheel is used all the time for crank position senor....the ring gear on the flywheel is not though because you need a missing or added tooth/teeth and a gear just won't work as a gear like that. Older setups used the ring gear for rpm readings, but I've not seen that since the 80s.

[NewbVetteGuy]

All the replies made sense, except for the shape of the teeth.

I believe it. I just don't understand it.
Can anyone elaborate on that one?


Adam

[Kevin Johnson]

All the replies made sense, except for the shape of the teeth.

I believe it. I just don't understand it.
Can anyone elaborate on that one?


Adam

I was asked several times to make trigger wheels and so looked at the designs for the teeth. I suppose you could make anything work to some degree.

https://www.google.com/search?q=gear+to ... e&ie=UTF-8

https://www.google.com/search?rlz=1C1CH ... HH30iaxaM:

[mk e]

All the replies made sense, except for the shape of the teeth.

I believe it. I just don't understand it.
Can anyone elaborate on that one?


Adam

I was asked several times to make trigger wheels and so looked at the designs for the teeth. I suppose you could make anything work to some degree.

https://www.google.com/search?q=gear+to ... e&ie=UTF-8

https://www.google.com/search?rlz=1C1CH ... HH30iaxaM:

The 80s ferraris had a sensor on the flywheel teeth. It was a Bosch VR type. I guess the important thing is that the core be about the width on the flat of the tooth to give you a nice sharp fast + to - drop....you get a flat in the voltage curve when they mismatch and ECUs hate that, yes I learned that the hard way. I think the teeth would be too narrow for any hall type senor I've ever seen...but for sure there is a Bosch vr available that reads ring gear teeth really well.

[Orr89rocz]

Yeah the logic in most computers reads the passing of the edge of the tooth as a digital rising or falling signal. The tooth being square would be the type needed for the hall effect type triggers

Msd flying magnet type might work ok for a signal. Not sure if you put in 4 magnets or much more. Not familiar with that type.

[Kevin Johnson]

Maybe, just maybe, Ferrari has different expectations as to how John Q. Public will treat their product.

https://www.youtube.com/watch?v=5b1aRop-UbU

https://www.google.com/search?rlz=1C1CH ... 94&bih=542

[Newold1]

For very accurate spark and injector timing on a 90 V-8 engine for an example you need a sensor timing of 60x or 24x for example. These timing reluctors allow breaking down the timing into 90 TDC events on each cylinder into 8 equal degree amounts with 2 missing teeth to provide the location of TDC usually 15 or 12.5 degrees before TDC for initial timing advance.

You could have large resolution on a chevy flywheel teeth count like 168 or 152 but when you delete the required missing teeth for TDC orientation the starter motor would loose contact on starting and tear up some serious teeth.

This is why it is much easier and doable to mount a proper reluctor somewhere on the crankshaft, crankshaft snout or even on a keyed damper. The shape and wear of flywheel starter teeth as already mentioned in this post also don't work well as reluctor teeth shape.

[mk e]

You'd be surprised....

I can't remember what it was hooked to exactly....I think it provided and rpm signal to the magnetic marelli ignition units ( the v8 had a pair of 4cyl units). The actual trigger signal came from a pair of pins on the back of the flywheel, then each box had it's own pickup mounted 90 degrees apart. Then a pair of distributors to send it to the right cylinder. It worked.

I did a few efi conversations by adding more pins on the back to make a 36-1 pattern and just using 1 of the 3 oem pickups. I added a cam position to the last couple, I haven't done one in probably 10 years now, 308/328 are considered collectable rather than junk these days so now they stay stock

[stealth]

OEM on Audi engine in my TT