Industrial engine ignition cut

[itsme]

Hi there,

I have built a tachometer for a Honda GX160 industrial engine (like a lawnmower engine) using an Atmel microcontroller and hall effect sensor. This engine has got a mechanical governor, but I want to get rid of that (even though it works well and is perfectly reliable) and replace it with an electronic rev limiter that is operated by the tachometer.

On the coil of these engines, there is a small connection. If this connection is connected to the engine casing, it will cut the ignition. I want to work an electronic switch into my design that will ground this connection whenever the speed goes over a certain limit - this is how the speed limiting will work.

What I need some help with, is what sort of electronic components I should be using to achieve this. I'm not entirely sure what the connection on the coil is actually connected to, but I assume it is probably the primary coil??? I need something that can use the 5V signal from the microcontroller to open or close this circuit. I was thinking maybe something like a MOSFET would work, but does anyone know what kind I would need? Unfortunately, I'm not quite sure what voltage and current it would have to handle, so maybe someone knows something about that too (I think a slight overkill may be in order here)?

In case this seems like an odd question (when I've just made a tachometer), I've just taught myself how to program these microcontrollers in the last month, so most of this electronics stuff is still quite new to me.

I hope it all makes sense!

Regards
Warren

[NC Cams]

Until/unless you know how much current and/or voltage and polarity of same that needs to be controlled from the shut down pin, it is pretty hard to recommend something.

In looking thru my MOSFET catalog, I can't seem to find one that has specs that will work with a "I'm not entirely sure what the connection on the coil is actually connected to..." circuit.

However, an IRLZ44 is logic level (will run at 5 gate volts - we've run them at 10 but NO HIGHER) and will sink a bunch of current when gate voltage is applied and is pretty cheap.

HINT: be careful with how much hysterisis you put into the circuit as hard cutting the engine can lead to rough treatment and muffler explosions, especially if the engine is carbureted...

8-)

[itsme]

Hi,

Thanks for the reply.

I couldn't find anything on a "I'm not entirely sure what the connection on the coil is actually connected to..." circuit either - that's why I asked here (although that may not have been clear in the original post).

I have tried connecting a voltmeter across this 'mystery' pin and the crankcase when the engine was running, but it read 0V. I tried connecting an ameter up between the 'mystery' pin and the crankcase as well, and as expected, the engine cut, but it didn't seem to show that any current was flowing (unless it was less than 10mA - due to the scale of the ameter). I was using an old, cheap multimeter for this, as I wasn't sure what to expect, but it still works just fine.

Are there any other suggestions on how I could test this pin, besides what I have already done?

I think my next step will probably be a slightly crude one, where I might just buy some MOSFETs (like the IRLZ44) and then play around with them until something happens (either positive or negative).

Regards
Warren

[Al_The_Man]

Most small engines use a magneto ignition when the magnet passes the coil the points open, the shut off usually shorts the points out, so all you have to do is shunt the primary coil, a hexfet like IRF630 I would think would be ample.
Although I think my approach would be to replace the govenor with a simple servo system , as in cruise control.
Al.

[lwill]

The connection on the coil is usually on the primary that shorts out the points or their equivilent to ground. No opening and closing of points, on spark. Since the primary is a obviously a coil, all pricausions of using a coil with electronics apply, it can generate reverse EMF (spices kicking back) in the thousands of volts and can easily damage most components (let the smoke out). If it were only a kill circuit, the simple solution would be to use a low voltage relay controled by the electronics thus isolating the two. You may want to search the WEB for automotive ignition circuits and see how they control the coil. Try searching for CDI (Capacitive Discharge Ignition) circuits. There use to be a lot of home brew designes around before the days of fuel injection or MSD boxes. Here is an example http://www.sportdevices.com/ignition/ignition.htm

Good luck, I have blown out several lest lights by accidentaly probing the simmilar signal on cars because of the huge spikes.

[lwill]

Buy the way, rev limiting as you are suggesting is good for over reving, but under normal runing conditions, cutting the spark will slow the engine down, but the engine will still be developing vacume and sucking in fuel. When the spark is returned the engine will breifly run over rich. This may not be a big deal on the scale you are dealing with, but can cause problems on a larger scale. On big engines running NOS, one of the things you never want to do is hit the rev limiter with the nitros engaged, the engine will load up with fuel and NO2, and when the spark returns, BANG! you got a bomb. This is why NOS has a bad rap when not run right.
Sorry, I got off topic.
Also consistantly running rich will wash the lubicant off the cylender walls shortening the life of the rings and mix fuel with the oil ruining the oil and treaten the bearings.
This is why engines control the spead with a throttle that controls the fuel/air mixture not the ignition.

[Zippi]

Just a thought...

If you're already reading RPM's with the Atmel, why not take a Radio Controlled Servo and control the carb directly, set speed with a pot tied to the Atmel? That'd be a more efficient governor, I think...

Those RC Servos just need a pulse to control the throw, there're articles out there on the specifics, I vaguely remember 1.0 mS to 2.0 mS being the pulse range. I've run one off a PIC some time ago. They're pretty easy to work with, and you can run it with 5 volts. I will say though, it may not be the most reliable solution, depending on what you're ultimately using the motor for. You could build in some mechanical limits with the servo, such as a way to throttle it down if the controller loses power.

You could also use an electromagnet w/ a spring to pull it back (solenoid), drive that with a mosfet and PWM signal from the Atmel. Tie it to the governor. If power is lost, the engine will stall, as the carb throttle will return to shut-off position. This is probably the best way to do this. If an RC servo fails, it can swing high, and stay there.

Just a few thoughts...

[NC Cams]

If you are dealing with inductive loads that come directly from the ignition coil, the IRLZ44 is not the best bet. If the mystery pin is merely driving gate or a sinking a collector voltage/current, it would probably be OK.

If you are linking to an ignition coil or magneto directly, you'd be better off working with a high avalanche rated FET ala the IRF741. IR lists this a being suitable for direcly driving a high energy ignition coil in applications note AN-969.

BTW, IWILL' s comments elaborate into WHY I suggested to carefully taylor the rev cut hysterisis characteristics, especially if you're dealing with a carbureted engine. If it is EFI, simply cut the fuel off completly and ignore the spark.....

[vulcom1]

There are switchs for gas model aircraft engines operated by the receiver that would probably work. It connects the coil to ground the same as a regular kill switch but receiver operated for emergency. You may think about this along with if you are limiting revs that it could be set up to fire at odd cycles where you would have a miss but not completely kill the engine. If you then back off the throttle below the rev limit it would then run fine. It comes to mind from my old days in Armour when my ferret was mechanically limited with flyweights but I could also see it being done electronically. Not sure how you would do it but maybe somebody else has some idea's.
John