Hall trigger ignition on hit'n'miss exhaust pushrod?

[rklopp]

I am building a Breisch/Peters 1/2-scale Olds Type R hit-and-miss. I want to use a Hall effect sensor as an ignition trigger, with triggering from a magnet on the exhaust valve pushrod, but I am concerned about double-triggering. The plans call for old-school Kettering points that trigger from a ramped cam on the pushrod. With points, the ignition will fire when the points open, but will not fire when the points re-close. They re-close 360 crank degrees from firing, meaning at or just before TDC at the start of the intake stroke. If I use a Hall sensor, the ignition will re-fire at this point. Do I risk lighting off the fresh charge just as it's coming into the cylinder, perhaps causing a backfire? Has anyone successfully run a hit-and-miss with a Hall sensor triggered from the pushrod?

I have a Breisch 1/3-scale Associated Hired Man with a Hall sensor triggered by a magnet on the cam gear, so there's not risk of double-firing. I would like to avoid putting a magnet on the cam gear on the Olds, if only for "authenticity." (Yeah, I know, Hall sensors are not exactly authentic, but neither are over-scale points.)

 

[danallen]

I am not sure how you would time the spark with such a set up. The exhaust push rod doesn't move from the beginning of the intake cycle until after the end of the power stroke.

 

[Jasonb]

Depends on the shape of the cam, plenty of engines running with ignitors that are tripped off the push rod.

Should not be a problem with the double spark as the fuel/air mixture is unlikely to ignite until it is compressed

 

[rklopp]

I am not sure how you would time the spark with such a set up. The exhaust push rod doesn't move from the beginning of the intake cycle until after the end of the power stroke.

The cam has an extra small ramp section to trigger the ignition. During this phase, there is a lot of intentional slack in the valve works, and the valve does not lift off its seat.

 

[rklopp]

Depends on the shape of the cam, plenty of engines running with ignitors that are tripped off the push rod.

Should not be a problem with the double spark as the fuel/air mixture is unlikely to ignite until it is compressed

I thought the compressed mixture was harder to light off. Or is it just harder (takes more voltage) to jump a spark across a gap within a compressed mixture?

 

[Jasonb]

When running hot tube ignition the tube is red hot all the time but engine will only fire when the piston is at or near the top of the compression stroke so therefore does not light as it is being drawn into the cylinder or compressed so should be much the same with a spark

 

[danallen]

The cam has an extra small ramp section to trigger the ignition. During this phase, there is a lot of intentional slack in the valve works, and the valve does not lift off its seat.

Ok. I understand that. How would you get double firing if the pushrod moves once per cam revolution?

 

[Jasonb]

Once as it pushes and then again as it moves back passing the hall sensor each way.

Pushrods for ignitors tend to have some form of one way "ratchet pawl" so only trip the ignitor one per revolution of the cam

 

[rklopp]

Once as it pushes and then again as it moves back passing the hall sensor each way.

Pushrods for ignitors tend to have some form of one way "ratchet pawl" so only trip the ignitor one per revolution of the cam

Exactly. The pushrod makes a round trip out and back passing the trigger point going both ways.

 

[idahoan]

On a hot tube engine you need the compression to push the mixture into the heated portion of the tube, I would think that if you had an ideal mixture coming in through the intake valve, the spark plug would not have any trouble lighting it off.
I considered this option on my Little Brother engines but decided to hide the hall sensor behind the cam gear.

Dave

 

[Nerd1000]

I thought the compressed mixture was harder to light off. Or is it just harder (takes more voltage) to jump a spark across a gap within a compressed mixture?

It's harder to make the spark, but compression heating should actually make the compressed mixture easier to ignite once the ignition source is provided.

A lot of older Harley Davidson motorcycles use a wasted spark ignition system, and thanks to their V-angle this means that one cylinder gets a spark during its intake stroke. This apparently does not light off the uncompressed mixture, so depending on your circumstances you might get away with it on your engine.

 

[Jasonb]

Certainly many model engines using wasted spark without a problem and as for the model aero "diesel" engines that have no ignition source but just compress the fuel/air mix they work Ok too.

Also if a single spark will ignite the mix what is the advantage of a buzz coil that gives us many sparks at or around the point we want the engine to fire in the hope that one of those sparks will work, I was under the impression they provided sparks over a period and at some time in that period the mix would be at the right compression to fire making the need to set the timing less exact

 

[rklopp]

There’s no risk of double-ignition on a model “Diesel” because they are two-stroke. They can only fire once per working cycle. My concern is firing twice in the four-stroke working cycle, with the extra firing occurring early in the intake stroke.

 

[Jasonb]

My point was that they fire when the air/fuel mixture is compressed which is when yours is most likely to fire.

If you build an engine where it is easy to advance and retard the ignition timing while it is running there will only be a certain band of adjustment before it simply won't fire. The band is usually fairly equal around TDC

 

[rklopp]

To answer my original post query, triggering on the pushrod works fine with no accidental firing during the intake stroke when the pushrod is on its return trip. I fired up the engine for the first time this evening using a temporary fuel tank and it fired right up. This made my day! I still have to get the mixture and governor tuned.

 

[awake]

Sounds like you've got it sorted ... but if it turns out, down the road, that you really do need to suppress that second spark, you would only need to add a bit more to the electronics of the electronic ignition - basically add a "flip-flop" to the circuit, though there is probably a bit more to it than that. (Isn't there always?)

 

[Vietti]

FWIW my last engine is running on a mag and I'm turning the mag at twice engine speed so there is a spark at the top and bottom of every stroke! Did it to increase mag output and make hand start feasible. Works with no ill effects though if running rich I sometimes get a strange sound from the exhaust as the spark plug ignites a residual fuel charge. Doesn't effect the engine or RPMs, I wouldn't try it on a high speed engine, could get ugly! I use waste spark on other engines with no ill effects.

 

[william_b_noble]

the way to solve your problem is to have two adjacent hall effect sensors. When A trips before B, spark. When B trips before A, no spark. very simple logic - spark when A and NOT B - one 74LS00* will do the trick.

* or a 7400 or a 5400 or any equivalent - you can also do that same logic with discrete parts if you want. Off the top of my head, a 2n2222 and a couple of resistors will do the trick