Electronic Carburetor/Roto 85 FS Install

[TonyHallo]

Built the electronic control for the ZAMA carb used on the Sthil M-Tronic chainsaw a few months ago. It uses a Seeeduino XAIO board and programed in Arduino. There is a GitHub site describing the project and quite a few discussions on other airplane sites. The original project involved using the solenoid out of ZAMA and making a housing to control the fuel flow on glow engines converted to gas. I plan to use the ZAMA as others have.

The photo above is the original design where it used an Adafruit BMP280 sensor to measure the barometric pressure and temperature, this was connected via I2C to the controller. With the I2C communication, if the sensor failed for any reason, the controller would fail in an undetermined mode. Sometimes it would quit, sometimes it would continue at the same rate as prior but could be controlled.
Replaced the BMP280 with 2 discrete analog sensors and added code when the sensor values were outside normal range, the sensor value falls back to a set number. This was tested by disconnecting each of four wires going to the controller.
Now it is time to mount this to the Roto 85 FS that powered my Champ until it met it's demise. This engine was purchased around late 2018. During the built the carburetor flange was broken on the intake manifold. Little info was available at that time and I purchased the engine from an importer on Ebay. Made up a piece of aluminum that is screwed and glued on with JB weld. The repair worked well over the 5 year period that I flew the plane. The second engine I purchased a few years ago has a much more robust intake manifold.
The hole in the intake is 1/2" in diameter, the original Walbro WT962 has a 14mm bore, and the new Zama needs a 20 MM bore to accommodate the black plastic piece. The plastic piece sticks out about 1/2" from the flange on the carb, see second photo. I plan the make a isolator plate out of 3/8" G10. The plastic piece is there for the idle circuit.

 

[pawnshopmike]

You fancy.

 

[acerc]

I have no words for of the things you guys are doing other than, WOW, Amazing.

 

[dhal22]

I have no words for of the things you guys are doing other than, WOW, Amazing.

I got no words as I have no idea what’s going on……..

 

[TonyHallo]

Have the carb mounted. Used the original heat insulator and made a new one 3/8" thick out of G10 material.

Since the solenoid pulse width is set by the throttle I want accurate and repeatable throttle opening. The throttle servo is controlled via a exponential curve as normally done with our engines while the solenoid uses a separate curve to control the fuel flow. The fuel curve looks more like a butterfly valve Cv curve. I expect the final curve will look similar to the curve below.

The servo is mounted very close to the using a 1 to 1 linkage.

Need to make a few gaskets and the engine should be ready for a test run and tune.

 

[Snoopy1]

Like what you are doing very interesting. Do you need to run a regulated fuel pressure to the carburetor.

 

[dhal22]

I want a Roto 85 inline if it will fit in my next project…..

 

[TonyHallo]

Like what you are doing very interesting. Do you need to run a regulated fuel pressure to the carburetor.

No regulator, the carb is just like a Walbro. Basically the solenoid replaces the needle valves. There is a pump and diaphragm on the carb.

 

[Snoopy1]

Ok so if I understand this setup. No H and L needles, the servo operates the throttle plate. On the other end of the throttle plate shaft is a rotary sensor feed which feeds back to an electronic device this device sends a signal to a solenoid of some sort which injects fuel into the Venture relative to throttle plate position. ?

So the graph show above the X axis is throttle plate angle and the Y axis is fuel flow.

 

[TonyHallo]

Not quite, the controller reads the pulse width from the throttle stick, separate channel from the throttle plate. The pulse width is converted into the solenoid pulse and corrected for ambient temperature and pressure. So in my case I have the throttle plate on an exponential curve and the throttle channel for the fuel control uses the curve above. This curve will be tuned to the engine at various points, I can use up to a 17 point curve in OpenTx. For tuning I'm starting with the curve above and have the the S1 slider set to add or subtract 15 % from the base. I plan to tune the engine at each click on the throttle ratchet. Each click is approximately 10% move from -100% to 100%. I have the radio set to read the position of the throttle stick and need to add a function to read the fuel channel.
If this works as expected then I do plan to look at adding a potentiometer in the future but that will take a little more effort and code as the fuel map will need added to the controller. Right now the fuel map is the curve in the radio and very easy to change. Using the pot will require a controller flash to change the curve.
I plan to use the slow function on throttle opening, when the engine was in the Champ the throttle was set to 1 second slow on opening so that is what will use here as well. I do not plan to add the slow function to the fuel control channel so this will act as an accelerator pump function.