The present work concerns the study of the potentialities of high-boost small-displacement C.R. (Common Rail) diesel engines where the compressor and the expander are mechanically disengaged for the purpose of power cogeneration from the exhaust gas. This objective can be achieved by means of advanced concept electrical devices capable of delivering the energy produced by the expander either to the drivetrain transmission or to the even more power-demanding auxiliary equipment of both the engine and the vehicle.
The performance of a small-displacement boosted diesel engine with a common-rail injection system has been predicted by means of a computational code obtained by integrating different in-house non-commercial codes that simulate the intake, combustion and exhaust processes.
The model validation has been carried out by means of the experimental data obtained at Fiat Research Center on a commercial small-displacement C.R. turbocharged diesel engine. The data related to: in-cylinder pressure time-histories, injection-rate distributions, air- and fuel-flow rates, EGR (Exhaust Gas Recirculation) mass fraction, discharge flow coefficients of the intake and exhaust valves, intake- and exhaust-gas pressures as well as engine performance at different speeds and loads. The computational code requires the heat-release distribution and the heat-transfer coefficients as inputs for the simulation of the engine cycle. Therefore, a heat release analysis of the measured in-cylinder pressure was performed and the heat-transfer coefficients in the model were adjusted so that the computed in-cylinder pressure matched the measured one.
After calibration, the model was applied to the analysis of the engine performance at A/F1 of approximately 19 for different boosting and exhaust pressures as well as for different engine speeds and loads. The compressor and the expander were taken as mechanically disengaged and the maximum boost pressure was chosen so as to comply with an in-cylinder pressure limit of ≈ 180 bar. The engine performance were characterized in terms of the following main parameters: engine torque, power and fuel consumption, expander power output and overall system efficiency. This latter was evaluated as the ratio between the available net power of the whole system, including the supercharger, and the energy supplied by the fuel. It was ascertained that an overall efficiency of the system higher than 45% can be achieved. Moreover, an insight was given into the CO2 and pollutant exhaust emissions.