Limited Coil Tachometers

[SteveSr]

This is absolutely fascinating, THANK YOU! I have been trying to find out how the stihl tach can read the limited coils and my other one can’t!

I need to read it a few more times to understand everything though.

I am not sure that this is correct. Here is how I think that it is commonly done:

Existing tachs that don't work on limited coils - count spark pulses over a relatively long interval like a half second and do some math based on the interval. This method won't work with limited coils.

Tachs that work with limited coils - Measure the time between adjacent sparks and do some math. This requires a more precise timebase i.e. more costly. You now have to figure out which intervals are valid and separate out those that aren't. More statistics, math.

I might be missing something here but I don't see any advantage (or reason) to determine the RPM that a particular coil starts limiting. This is already published in the specs. It also doesn't help you tune the engine either because you are still hitting the limiter.

 

[Vintage Engine Repairs]

By definition these tachometers are just counting sparks. When the coil limits and starts cutting off sparks your reading goes out the window! The faster the refresh rate the less accurate the reading and I really doubt that it would read a limited coil once it started limiting.

I haven’t seen a tach yet that will work once the limited coil RPM is reached. They go haywire and read 5K to 18K. But all of them I’ve used so far work below the limited rpm spec.

View attachment FullSizeRender.MOV

 

[KostasVolos]

I am not sure that this is correct. Here is how I think that it is commonly done:

Existing tachs that don't work on limited coils - count spark pulses over a relatively long interval like a half second and do some math based on the interval. This method won't work with limited coils.

Tachs that work with limited coils - Measure the time between adjacent sparks and do some math. This requires a more precise timebase i.e. more costly. You now have to figure out which intervals are valid and separate out those that aren't. More statistics, math.

I might be missing something here but I don't see any advantage (or reason) to determine the RPM that a particular coil starts limiting. This is already published in the specs. It also doesn't help you tune the engine either because you are still hitting the limiter.

Actually is a bit more complicated than that .
The "counters" used are decimal,
meaning that for every ten fire sparks the counter IC sends a + 5 VDC signal to the MCU ,which also "keeps the time".

For example : 15,000 rpm = 250 spark pulses per second .
The MOSFET sensor for each pulse triggers the counter to "count"from 0 to 9 and reset ,while sending a short signal to the MCU .So in one second ,the MCU will receive 25 pulses ,from
the 4017 family counter IC.
One pulse ( which is 10 pulses of spark plug ) every 4 milliseconds .Each pulse is processed and monitored by the MCU.
So at 15,000 rpm ,the available
processing time is 4 milliseconds.
For higher rpm figures ,the available processing time decreases and of course there's a limit of rpms the tachometers can effectively measure,due to hardware restrictions .
But the screen refresh rate is something else .
These 8-bit MCUs are not "multi -tasking" .The can not measure pulses and calculate them into rpm and at the same time showing "live" the results on a LCD screen . They either have to do the one or do the other. So they stop (completely )the processing of incoming pulses for a short period of time and update the screen ,before go back to counting and calculating .That's why the usual "low refresh rates" of these tachs.For higher precision (Better averaging )and more stable read outs.
.

https://www.build-electronic-circuits.com/4000-series-integrated-circuits/ic-4017/

 

[Vintage Engine Repairs]

You mentioned refresh rates in an earlier post, why have stihl chosen a slow refresh rate at around 1 second for their tachometers, rather than, like many other brands that use 0.5 seconds?

Edit: Is there a practical or useful reason (for the hardware / software) as to why they have this processing time

 

[KostasVolos]

You mentioned refresh rates in an earlier post, why have stihl chosen a slow refresh rate at around 1 second for their tachometers, rather than, like many other brands that use 0.5 seconds?

Edit: Is there a practical or useful reason (for the hardware / software) as to why they have this processing time

@15,000 rpm ( = 250 sparks per sec at a single cylinder 2T engine )

Tachometer #1 /refresh rate 1 sec
- Counter IC "sums" spark pulses to decades sending one pulse every 4 msec to MCU .
One break of 4 miliseconds for
screen updating .
249 pulses ( 4 msec spaced apart ) and 1x 4 msec screen updating = 1 second .

Tachometer #2 /refresh rate 0.5 sec

124 x 4 msec pulse processing + 4 msec screen updating + 124x 4 msec pulse processing + 4 msec screen updating = 250 x 4 msec = 1 second .

At the first case the MCU had a specimen of 249 counts to follow and process before it updates the screen ,while on the second case the specimen is 124 counts .
The precision of the first tach is higher ,but it is not as responsive to the rpm fluctuations as the second tach ,which in turn has less real time precision but is much more responsive to rpm fluctuations .
At first case the tach needed 4 msec during one second for a single screen updating ,while the second tach "spend" 8 msec for updating twice the screen .

The first tach during 5 seconds of measuring will show 5 measurements from hundreds of pulses .
The second tach will show double measurements of the same amount of pulses .
The first will provide more accurate readings on a steady throttle situation ( idle or WOT ) ,while the second tach is able to monitor more acurrate the acceleration rate and/or deceleration rate of an engine .

 

[Vintage Engine Repairs]

Could the above be why the stihl tach is able to process the set readings of a limited coil so accurately?

 

[KostasVolos]

Could the above be why the stihl tach is able to process the set readings of a limited coil so accurately?

No ,It's an added code routine.
A large sample of pulses will of course provide enough data and time for the appropriately programmed MCU to "distinguish" a rev limit mechanism ,if the latter all it does is fire the spark plug not in every crankshaft revolution ,but on every other .
Which is exactly what the rev limiting coils do.

At the 15,000 rpm of the example ,the MCU has a "pool" of 249 pulses .If the MCU measures that it took 8 msec for a pulse to be sensored ,instead of 4 msec which was "expected " ,it will normally either try to average between the 15,000 rpm and 7,500 rpm or output the smallest or larger one ,or output the last measured.If it is programmed
to "recognize" a possible alternating pattern of "x and x/2 " measurements as "rev limiter " ,
they will output a steady measurement depending on their
exact program routine about how
to calculate rpms in such occasion .

 

[Vintage Engine Repairs]

Very interesting, thanks again! That’s exactly what I have found, this stihl tach takes a second or two to settle down but then it holds (in the example of the stihl fs55 I have) a constant and unchanging rpm of
10030rpm like it’s locked on to some sort of equation. This said, the limiter seems to hold a perfect set rpm too so it could just be reading that of course.

 

[KostasVolos]

Although myself find it kinda vane .
For example ,my MS180 2-mix .
It has a rev limiting coil with max rpm at 14,000 .

Any tach will measure correctly up to 13,999 rpm .

( digital brains are VERY precise !
When a rev limiter is said that will 'cut off' at 14,000 rpm ,that's what it will do .When it will sense that the flywheel turns at exactly 14,000 rpm ,it will engage the whole electronic rev limiting sub -circuitry. Not at 13,999 rpm ,neither at 14,001 rpm but exactly at 14,000 rpm . )

Anymore than that ,is of no use ,since the saw is not allowed by design to rev more than 14,000 rpm ,no matter how lean the mixture is set .

I've opened the muffler lately to almost 125% of EPS@CW ( exhaust port surface at cylinder wall !!! ) and replaced the fixed main jet at the carb ,with one two sizes ( + 0.02 mm ) larger .
Removed the vacuum from the "dirty" metering diaphragm side and adjusted the metering lever flush with carb body /gasket machined surface.

At WOT (out of cut) I get a steady 12,800 rpm from an ordinary $10 tach.
I'm good with it and there's no need for further messing.
It four-strokes out of the cut ,cleans inside.

 

[Vintage Engine Repairs]

I have found rev limited coils have a variance. It’s reflective in the manual where is says “rev limited to…. +/- 800 rpm.

Thats interesting you say your ms 180 is limited. The ms 180s out here don’t have a rev limited coil, it’s unlimited!

I find the most useful thing about a tach that reads the limited coils is I get accurate reading right up to the limiter and then it holds. Wjth my other tach, when it gets close to the recommended rpm and the rev limiter kicks in, it can be 500 rpm or so away from the limited rpm but it starts to go crazy.

Now with the stihl tach I set it to just at or slightly below the limiter by a couple hundred rpm’s with attachment fitted and then check under load, make any tweaks and go from there.

Some engines you can differentiate the limiter from 4 stroking, other times it can be so tricky. At least for me anyway!

Edit:

If you have a tach that can’t read limited coils, stihl recommends setting it to 500 rpm below the limiter then turning the H screw in 1/8 a turn more. I’m sure it’s perfectly fine, but when saws cost thousands, the tach gives me peace of mind on customers equipment

 

[SteveSr]

I have found rev limited coils have a variance. It’s reflective in the manual where is says “rev limited to…. +/- 800 rpm.

This is likely true as the limited coils likely rely on analog components (with their inherent variability) to set the RPM limit. It is not likely digital. However, if it was digital there would also be variability in the time-base (oscillator) that determines the limit.

I find the most useful thing about a tach that reads the limited coils is I get accurate reading right up to the limiter and then it holds. Wjth my other tach, when it gets close to the recommended rpm and the rev limiter kicks in, it can be 500 rpm or so away from the limited rpm but it starts to go crazy.

The tach won't lie when it starts limiting sparks. It will go crazy indicating t hat you are starting to hit the limiter but maybe only occasionally.

Edit:

If you have a tach that can’t read limited coils, stihl recommends setting it to 500 rpm below the limiter then turning the H screw in 1/8 a turn more. I’m sure it’s perfectly fine, but when saws cost thousands, the tach gives me peace of mind on customers equipment

So where have you found this Stihl info that explains how to tune a machine with a limited coil? PM if you have to.

 

[Vintage Engine Repairs]

This is likely true as the limited coils likely rely on analog components (with their inherent variability) to set the RPM limit. It is not likely digital. However, if it was digital there would also be variability in the time-base (oscillator) that determines the limit.


The tach won't lie when it starts limiting sparks. It will go crazy indicating t hat you are starting to hit the limiter but maybe only occasionally.


So where have you found this Stihl info that explains how to tune a machine with a limited coil? PM if you have to.

I added a video on the other thread about tachs, where the stihl edt 9 can read limited coils.

Stihl fs55 in the example. Limiter is circa 10k in the manual,
View attachment FullSizeRender.MOV

Here is one of the documentation of tuning a limited coil with the method I mentioned above. Each service manual for each machine is slightly different, some older manuals seem to say “set it to ….” Which is the max speed the limited coil allows, others say to set it below the limiter and lean the H a certain amount. Some say to set it to the limiter and turn the H in a … of a turn more. Others say set it to just 4 stroke in and out, (below the limiter)


Read point 4 & 5 - stihl ms 461