# Exhaust liquefaction in internal combustion engines

Does an internal combustion engine provide enough efficiency to still deliver mechanical power, if it is forced to liquify it's own exhaust gases by compression?

If gas containers would be added to the exhaust, the exhaust stroke of an four-stroke diesel or otto engine would theoretically compress the gas into the containers, and at some point it would liquify I guess. This way the engine would only consume air from the athmosphere but not release any exhaust to the athmosphere. The containers would fill up with liquified nitrogen, carbon dioxide, water and other combustion residuals.

How much power of the engine would be sacrified to this process? Would it still run?

• The exhaust of an internal combustion engine is mostly nitrogen and nitrogen won't liquify above a critical temperature of -146.9 degrees Celsius. For CO2 it's 31 degrees C, so even that would need a pretty beefy pre-cooler. The required partial-pressure for CO2 will be 72bar, which means that without separation the entire exhaust gas would have to be compressed to hundreds of bar. In other words, since you don't need the liquid nitrogen, anyway, you would need a gas separator, first, a pre-cooler second, a gas dryer to get rid of the water and then a pretty strong compressor for the CO2. – CuriousOne Feb 9 '16 at 10:24
• While this is a cool theoretical question, I'd suggest that dumping $N_2$ back into the atmosphere is just fine, environmentally speaking. Capturing $CO_2$ would be helpful , but even then where are you going to store the dry ice? If you don't convert it into an inert solid compound, sooner or later it's going to return to its gaseous state. – Carl Witthoft Feb 9 '16 at 13:17
• @CuriuosOne your comment should be an answer fully expanded. – hyportnex Feb 9 '16 at 23:23
• But would it theoretically be possible to liquify the entire exhaust? The container would have to be quite cool I guess. – dronus Feb 20 '16 at 14:22

a diesel engine produces 2.68 kg CO2 per liter diesel. The stoichiometric mixture is 14.5 by mass. Air is 0.78 N2 (28 gr/mol) and 0.209 O2 (32 gr/mol) -> 11.2 kg N2 and 3.3 kg O2 per kg diesel.

energy needed per kg liquid N2: currently about 0.5 kWh (according to this) -> total 5.6 kWh

liquefaction of CO2: 0.09 to 0.14 kWh per kg. -> total energy needed 0.37 kWh (at most).

TOTAL: 6.0 kWh per kg diesel.

Energy content diesel: 86 gr per kWh. Most efficient Diesel engine uses 0.16 kg diesel per kWh -> 6.2 kWh per kg.

A 3.3% margin, not enough to compensate for incomplete combustion at the given air-fuel ratio. But it's close, and there's still room for improvement (liquefaction: ideal cycle is 0.233 kWh per kg). Would be a supertanker rather than a car though...

• So the engine can do this, but there is no energy left for driving a car anymore. It could still be used as liquification plant and for room heating this way... – dronus Jul 6 '16 at 14:18