Direct air carbon capture (CO2 from air)
In late game, when large scale hydrogen production becomes viable using FBRs, it becomes possible to replace crude oil with hydrogen + co2 -> fuel gas & fuel gas + o2 -> diesel conversions. However, this requires vast amounts of co2. Since vehicles use diesel without emitting exhaust that could be captured, this creates a large carbon deficit in the system.
I propose adding direct co2 capture from air as a late game tech, effectively closing the carbon cycle for vehicles. Such technology already exist in real world, and is currently considered to be used combined with nuclear power plants.
The easiest chemically correct way to implement it would be to create a building that would use steam and NaOH (caustic soda) to output co2. NaOH can be created from water and sodium, and sodium can be created with molten salt electrolysis, therefore the machine could just get steam and salt to output co2. The air separator could also be reused for this process. Picture from Wikipedia.