Some vehicles use extra fuel to keep the engine warm enough for emissions to be low. A mid-80s vw 1.6 turbo diesel won't ever warm up to running temp if left id

Some vehicles use extra fuel to keep the engine warm enough for emissions to be low. A mid-80s vw 1.6 turbo diesel won't ever warm up to running temp if left idling, they are so efficient there is very little waste heat generated. (The Diesel and Brayton cycles both 'consume' heat as part of the process.) Still, there is a need for high exhaust temps to reduce noxious gasses, so having a recovery method for that would result in some efficiency gains. This is a study on the application of heat recovery systems on IC engines: http://journals.sagepub.com/doi/pdf/10.1177/1687814015590314 http://journals.sagepub.com/doi/pdf/10.1177/1687814015590314 "The maximum recuperated power for non-ideal cycle was 1515W at the point where reference engine power was 26,550W. That improves the effective engine efficiency by 5.7% from 33.97% to 35.91%."