Suppose a fossil fuel is used in some kind of engine in which the temperature of the exhaust is kept at, say, a few degrees above the critical point for liquid CO2. Using a little more energy, we might reason that the CO2 could be captured in the vehicle as a liquid (or solid) and cheaply recovered for sequestration while refueling. Without digging into this, it seems intuitively to me that there should be some energy left to power the vehicle, since even liquid exhaust products are lower in energy content than the fuel. But since (as far as I know) no one has invented such an engine, I'm left to assume that it's impossible. What do you think?