Rejection of hot gases

Site last update April 29th_2021

The combustion engines of road vehicles or boats with diesel as well as gasoline have a fairly low efficiency, of the order of 30%. This low efficiency is explained, in part, by the release into the atmosphere of a large amount of heat at the engine exhaust. The temperature at the outlet of the combustion chambers would be of the order of 100 ° C.

This heat could be exploited via a second engine cycle, the maximum efficiency of which would be determined by the ratio of the absolute temperatures (degree Kelvin) corresponding to that at the combustion outlet and that of the atmosphere. For example, for a combustion outlet temperature of 100 ° C and an ambient temperature of 10 ° C, the Carnot efficiency would be 24% representing the maximum efficiency attainable by the engine cycle. This efficiency is higher for a lower ambient temperature and lower for a higher ambient temperature (27% at 0 ° C and 21% at 20 ° C). However, it appears that significant energy could be extracted from the combustion gases.

The engine cycle would be carried out on the basis of a refrigerant fluid (with gas – liquid phase change) circulating in a closed cycle through four pieces of equipment or four steps. First, a heat exchanger (evaporator) extracts the heat from the combustion gases. The refrigerant carried at high pressure in the exchanger is expanded through an gas expander (a kind of radial turbine similar to that used by turbo expanders of compressed engines) delivering part of the energy contained into the gas. At the end of the expansion, the refrigerant is cooled by ambient air so as to liquefy it through a second heat exchanger (the condenser). After liquefaction, a pump raises the pressure of the liquid to the operating pressure of the evaporator. The fluid is then sent to the evaporator to carry out a new cycle.

This cycle is very similar to the heat pump cycle described in the section on aquathermy except that it works in reverse. The energy supplied by the expander (the enthalpy) is substantially equal to the difference in heat taken from the evaporator and released by the condenser. Thus, the lower the expansion ratio of the expander, the more the values of heat exchanged by the evaporator and the condenser equalized and the lower the energy delivered by the expander and the lower the efficiency of the cycle.

To obtain a good cycle efficiency, a refrigerant must be chosen which allows the evaporator to operate at high pressure and a short phase transition (from gas to liquid) at the condenser.

The release of hot gases into the atmosphere concerns, combustion gases from motor vehicles (cars, trucks), boats, gas turbines and boilers. It is obvious that is will make more sense when it is applied to large engines. Applying this technology to small engines of road vehicles would require strong efforts in the miniaturisation in order not to penalize these vehicles with limited access.

This motor cycle mounted at engine exhaust would be fully justified when associated with a carbon dioxide capture feature.