Modelling of Sequentially Turbocharged Marine Diesel Engines

Abstract: In the marine field, the request of high performance diesel engines rises from the wish of improving the ship transport capacity and speed. Nowadays high supercharged diesel engines are developed, characterised by increasing values of b.m.e.p. (brake mean effective pressure), not only at design conditions, but also at part load conditions. To this aim, a promising solution is the sequential turbocharging technique, proposed by some diesel engine manufacturers as a convenient alternative to the adoption of a variable geometry turbine. The sequential turbocharging solution, however, needs an accurate study and development, in particular as regards the sequential turbocharging connection/disconnection phases. This can be done by combining to the necessary experimentation a theoretical analysis based on a complete and validated dynamic simulation model of the diesel engine and his turbochargers. In this paper, a computer code for the dynamic simulation of a sequentially turbocharged high performance four stroke marine diesel engine is presented. A two zone, non adiabatic, actual cycle model is employed for the chemical and thermodynamic phenomena simulation in the cylinder. Fluid mass and energy accumulation in the engine volumes are considered. The model is applied to the simulation of the Wartsila 18V 26X engine, a high performance, recently developed, sequentially turbocharged marine diesel engine, when the load is varying according to a cubic power function (propeller load). The transient variations of the most important engine variables during a typical acceleration/deceleration manoeuvre are presented and analysed both in the time domain and with reference to the engine and compressor performance maps. Experimental results are employed for the steady state and transient validation of the simulation code including sequential turbocharging connection/disconnection phases.