Never ran across this before- thanks. Looking quickly through the Henri Michelet biography of the guy, he seems to have suffered the normal kind of abuse inventors have come to expect- too little money of his own to develop an invention, too little ability to get it from others, too much dismissal by important people, not enough of an immediate market for the device.

The engine in some ways resembles an atmospheric steam engine of the early type, in that there's a single stroke done by the engine, and a weight is involved. The exploding gas pushed a weight up, and then the weight dropped down to drive a rack-and-pinion arrangement connected to the wheels.

Getting past the dubious practicality of a rack-and-pinion ratcheting drive system, the problem that's immediately obvious with both of his designs ( one hydrogen, the other coal-gas powered) is the fuel. Hydrogen would have been quite difficult to make, let alone store, in quantities big enough to run a vehicle. Coal gas was rather new, having been developed by William Murdoch not many years before, and , again, would have been rather hard for someone to carry around, or store on board the vehicle ( it was mostly used for street lights, and so generated in a plant and distributed through pipes). A liquid fuel was much easier to transport and store; this simply pre-dates the petroleum industry which would produce that. (Still, you'd think he would have tried alcohol)

In some ways his last vehicle resembled Cugnot's, who also seemed to think of a powered vehicle as a tractor for doing heavy pulling or transport. Thus he may have thought of it as never going very far from a fuel source.