an emission control system for an internal combustion engine cools the exhaust gases sufficiently to condense a substantial portion of the pollutants as a separable liquid or solid phase . the engine exhaust gases are initially cooled and then compressed , again cooled , and work - expanded in an open reversed brayton cycle to obtain a significant temperature decrease . the resulting cooled gases from this the pollutants are readily separated may be employed to provide refrigeration or air - conditioning for moving vehicles .