
Originally Posted by
Karsus
Here's an extremely simplified explanation of why the system you describe would not work. Someone please kick me if I've said something completely wrong...
Say for instance that burning 1 unit of fuel gives out 100 units of energy. We'll say our engine is 27% efficient as you state above, so 27 units, out of the 100 total, is usable by the car to do work (things like move, power electrics, that stuff). 3 units of energy goes to waste heating up your engine block. The remaining 70 units exist as heat in the exhaust gas. In order to compress that gas back down into a liquid, or at least into a small volume, you need to take away those 70 units of heat.
You can either... 1. Dump the exhaust gas into the atmosphere. The exhaust gas will lose those 70 units of heat pretty quick as it expands into the atmosphere, but you haven't captured it.
Or you can... 2. Actively cool and compress the gas to capture it. To do this, you will need to spend energy. In a 100% efficient process, you need to spend 1 unit of energy in order to extract 1 unit of heat from the gas.
We'll be generous and say you have a 100% efficient refrigeration unit on board. But where are you going to get the energy to run it? You could take some of the energy you gained from burning the fuel in the first place, but the more you take, the slower the car will go. Even if you spend all 27 units available to the car, you'll still have 43 units of heat in the exhaust gas.
What about using an independent power supply to run the refrigerator? Great! So, you've burnt 1 unit of fuel, the car has gained 27 units of energy, the gas has gained 70 units of heat, your independent power supply has spent 70 units to cool and compress that gas back down... Gained 27, spent 70, you've just wasted 43 units of energy. Oops... You could have just spent 27 from the independent supply in order to move the car.
Unfortunately, no such power supply exists, which is why we burn the fuel in the first place...