The top graph in Fig。 9 shows the evaporationprocess between the working fluid and the three heat sources。 This figure confirms how the critical point is again the pinch-point (PP3)。 Fig。 9(bottom graph) shows all the high temperature heat sources used in the Rankine cycle。 Only a small part of the power of exhaust gases and aftercooler cannot be used in the cycle, due to their low temperatures。

This configuration with high temperature heat sources produces an increase in the output power about 46 kW (as can seen in the bottom graph in Fig。 9) instead of the 59 kW that was achieved using the configuration with all heat sources。 Although the heat dissipated in the configuration with high temperature heat sources is lower than in the configuration with all heat sources, this still needs a heat exchange larger than the exchanger used in the reference engine。 However, it would allow the installation of the configuration with high temperature heat sources in an HDD engine since space requirements are not so large。

7。Summary

Table 4 summarizes the main results obtained in the configu- rations described in this paper。 As show the table, the high power increment is produced by the configuration with all the heat sources with a binary cycle。 But the important increase in total heat transfer could be a problem to design the necessary heat exchangers。 Thus, the best solution with lower heat transfer rates is the configuration with only the high temperature heat  sources。

8。Conclusions

A method to analyze different possibilities to use waste energies in a Diesel engine is described in the present paper。 This method involves estimating wasted energy values and uses this information to analyze the application of these energy sources in a bottoming Rankine cycle。 Mechanical energy of a Diesel engine is about half of the total wasted energy。 An important part of this wasted energy is used as thermal energy in intercoolers, radiators and exhaust gases expelled to the atmosphere。 However, an important problem to recover these wasted thermal energies is the low temperature values of the available sources。 Thus, it is difficult to achieve an acceptable efficiency using these sources。

The configuration with all heat sources includes waste energy recovery in two different cycles (binary cycle)。 The main problem of this solution is the big size of heat exchanger surface  necessary。

The configuration with high temperature heat sources uses only high temperature waste energy sources in a water Rankine cycle。 This solution is more realistic, but reduces the energy recovery in comparison with the configuration with all heat  sources。

The external irreversibilities of this Rankine cycle have been extensively studied in the present work。 The most important conclusion in the studied cases, with ideal processes and when the engine dissipates more heat energy is that it can only recover between 8% and 9% of the total energy dissipated by the engine once internal irreverisibilities are also considered。 Thus, because the characteristics of the residual heat in the studied engine operating point (maximum speed and maximum load), the resulting optimal working fluid is water。 However, this type of engine, working in partial loads, have different operating condi- tions, meaning that the organic fluids (ORC) are more optimal for

energy recovery。 Therefore, the working fluid used will depend on engine operating conditions where the energy of these residual sources will be recovered。 This paper is a study of maximum and these problems have not been addressed, leaving them for a future work。

Acknowledgements

This work was partially funded by “Programa de Apoyo a la Investigación y Desarrollo de la Universidad Politécnica de Valen- cia” and “Programa de Formación de Profesorado  Universitario”。