It's common in AU (Oz). It's added into the airstream as a percentage of the diesel injected, and it ignites at a different time than the diesel during combustion. It leads to a more complete combustion, as the lpg and diesel complement each other. It leads to more power, more torq over a wider rpm range, lower cost and much lower pollution. It's a win-win-win. And, I forgot, as the combustion is more even, it doesn't increase wear, and diesel knocking is reduced. Used to be state sponsored in Oz, to install such a system on diesel cars and lorrys.