I acually built some test equipment for a company that converted gas engines to run on propane. these were small engines, mainly used on lawn mowers. The company I worked for went broke, when the price of propane went up. This was 2012-2013 IIRC.
Same guy was building a kit to fog the intake on taxi cabs with propane, this was the 5.0 liter ford motors found in old crown Vics. The controller was a simple ardunio. It simply read the TPS and PWM a single propane fuel injector, which wasn't a proper injector at all, just a 12v air valve (
https://www.amazon.com/Yukon-Gear-YZ.../dp/B0078UA1F4). You can get them cheaper, that's just first one I found. I bought them off ebay for around 10 bucks. They have other uses also.
I don't have access to the programing, but I'm sure it was a simple table. The car still used gas as the primary fuel. at the time gas was over 3.50 and propane was 1.90. Economics changed, gas came down, propane went up some reason, I can't recall. Whole deal went went under.
I still have about 5 K of circuit boards and parts in my garage for the test equipment I was building. it was a wide band 02 sensor, pressure gauge for the propane( it was regulated down to 4 PSI) all wrapped up in a package and controlled by an ardunio pro. It was designed for engines converted to run entirley on propane. The device I built allowed an idiot to tune the conversion kit. you just turned the regulator screw until the lights on the tester turned green.
I'm not sure how the hybrid gas-propane engines were tuned. I'm guess he monitored the exhaust with a wide band and kept upping the propane until it ran rich. We never thought about raw propane going out the exhaust. Sometimes it pays to be stupid because you don't over think these things.