Feasible optimum design of a turbocompound diesel brayton cycle for diesel-turbo-fan aircraft propulsion

This paper is conceived to optimize the design of the thermodynamic cycle of a turbine (Brayton cycle) that uses a modern common rail diesel engine as an "active" combustion chamber. In this case the "active" combustion chamber produces the mechanical energy that drives the fan. The incoming air is compressed by the compressor, then is cooled (aftercooler) and inputted in the diesel engine. A high pressure common rail system optimizes the combustion in the diesel combustion chamber and the expansion begins inside the diesel engine. At the exhaust of the combustion chamber a turbine completes the expansion of the hot gases. A nozzle accelerates the exhaust from the turbine to increase the overall thrust. The mechanical energy from the diesel and from the turbine engizes the compressor and the fan. The system can be seen as a turbocharged diesel engine with the turbocharger that outputs energy to the turbofan, increasing the output power and or the efficiency. A diesel-turbine compound can be realized in this way. The coupling of the two system may be obtained in several different ways. The simplest is to put on the same shaft the compressor, the diesel crankshaft and the turbine. In front of the compressor a speed reducer drives the fan. A second example is to connect the turbine and the diesel on to electric generators. Electric engines are connected to the compressor and to the fan. The traditional turbo-diesel has the compressor coupled to the turbine, and the diesel engine that moves the fan. In this latter case, however, the turbine does not energize the fan. Many other hybrid and non hybrid solution are possible. The problem is to optimize temperatures, pressures and rpm to the different machines that form the compound. The availability of many experimental data for diesel and turbines makes it possible to obtain a design of a "true" feasible optimum Diesel-Brayton cycle. The high efficiency justifies the huge manufacturing and development costs of these turbocompound engines.