[thereisnospork]

The short answer is that while that reaction works in a beaker, it is too slow, inefficient and fragile. Plus there's also the thermodynamic perpetual motion machine in using the energy from a combustion reaction to reverse that combustion reaction.

Basically you'd need a second car worth of engine to generate the electricity to convert 1/3ish of the co2 from the first engine to ethylene (the rest winds up as methane, ethane, and CO.). Plus storage, maintenance and misc.

There are a few reviews by Hori that are more or less the gold standard on the chemistry if you want to read more. Unfortunately the literature is full of fud though.