2 cylinder lawn mower with OEM fuel injection

[Automate]

Looking to put a uaEFI on a 2 cylinder engine with OEM fuel injection running a garden tractor.

Requirements are quite different than typical automotive engine.

90 dev V twin OEM fuel injection
Magnet on flywheel and spark coil for each cylinder
OEM ECU gets engine speed from spark coil
OEM ECU only has three sensors:
MAP
Air temperature
Water temperature

Runs at set speeds, so throttle response not important
No OEM throttle position sensor
3550 max RPMs, so no spark advance
Has mechanical governor to maintain RPMs
No OEM O2 sensor, open loop
Two fuel injectors and fuel pump are the only outputs from OEM ECU

[Automate]

Measured the voltage coming from the magneto primary coil with a scope and it measured up to negative 70 voltage to chassis ground.

Considering building a circuit to condition this signal down to 5V to feed intp the uaEFI or just add a HALL sensor to crankshaft pulley or flywheel.

[AndreyB]

Why? the job of existing VR circuitry is to handle exactly it?


VR sensors can generate high voltages (e.g., ±100V peak-to-peak). The MAX9924 handles this via Current Limiting.

Internal Protection: The chip has internal clamping diodes and matched resistors (typically ~100kΩ effective input resistance mentioned in system descriptions) that work to clamp the voltage at the silicon level.

External Resistors: To safely interface a high-voltage VR sensor, you must use external series resistors (typically 5kΩ to 10kΩ or higher, depending on the expected peak voltage) between the sensor and the IN+/IN- pins.

These resistors limit the current flowing into the chip's internal clamping diodes.

As long as the current is limited (typically to a few mA), the clamping diodes will safely hold the pin voltage within the -0.3V to 5.3V range, even if the sensor itself is producing 100V.

[Automate]

Why? the job of existing VR circuitry is to handle exactly it?

VR sensors can generate high voltages (e.g., ±100V peak-to-peak). The MAX9924 handles this via Current Limiting.

Internal Protection: The chip has internal clamping diodes and matched resistors (typically ~100kΩ effective input resistance mentioned in system descriptions) that work to clamp the voltage at the silicon level.

External Resistors: To safely interface a high-voltage VR sensor, you must use external series resistors (typically 5kΩ to 10kΩ or higher, depending on the expected peak voltage) between the sensor and the IN+/IN- pins.

These resistors limit the current flowing into the chip's internal clamping diodes.

As long as the current is limited (typically to a few mA), the clamping diodes will safely hold the pin voltage within the -0.3V to 5.3V range, even if the sensor itself is producing 100V.

@AndreyB Thanks for the reply.

I saw that spec of pin voltage within the -0.3V to 5.3V range and was concerned. The magneto has a very large coil that I'm sure can produce a lot more energy than a small VR. So the input conditioning needs to be able to dissipate this large quantity of energy. I did see the MAX9924 evaluation kit states the front-end can “evaluate … up to 300 V p-p”

I asked Chat-GPT and it recomended

image.png

This is similar to the uaEFI input of

image.png

Except the series resistance is much higher and they recommend a TVS instead of R775. BTW since R775 is a DNP I could not find the recomended resistance and size.