Thermal Engines

Abstract

The feasibility and limitations of thermodynamic cycles for the transformation of heat to work are considered in this chapter in relation to development scenarios for propulsion systems. As described in Sect. 1.3, these scenarios include thermal machines as direct propulsion systems, propulsion modules in hybrids, and on-board generators for electric energy. Thus, the efficiency of a thermal engine is considered in reference to its function as a propulsion system module, and not for a broad range of load and speed. The transformation of heat to work in a thermal engine-if related to the quantity (obtained work) or to the quality (achieved thermal efficiency)-basically depends on the maximum and minimum temperature during heat input and heat release during the cycle [1]. For a working fluid with known specific heat values at constant pressure and constant volume, c p (T) and c v (T) (kilojoules/kilogram Kel-vin), the extreme temperatures of the available heat sources (warm source and cold source) are T max and T min (Kelvin). Considering the utilization of a thermal engine for automobile propulsion, the cold source is given by the ambient air. The warm source within a thermodynamic cycle is generated, in most cases, by combustion of a fuel and air mixture. For comparable volumes of different types of thermal machines, the heat value of a fuel-air mixture H u (kilojoules/kilogram mixture) is decisive, but not the heat value of the fuel itself, H g (kilojoules/kilogram fuel). The mixture heat values for different fuels are listed in Table 3.1 in Chap. 3. It should be noted that, despite the strong differences in heat values for different fuels, the mixture heat values for all kinds of fuel-air mixtures are more or less similar (see Fig. 3.4 in Chap. 3). This fact is also determined by the strong differences between the stoichiometric air-to-fuel ratios for every fuel. The generation of a warm source by combustion of fuel with air is more efficient than any kind of heat transfer. On the other hand, the similar mixture heat values for gasoline, alcohol, oil, and hydrogen leads to the same domain of maximum temperature as a result of combustion. Therefore, the energy conversion from heat to work occurs in thermal