A coupled thermodynamic and dynamic model of a three cylinder diesel engine: A novel approach for gas exchange process

In this study, a combined thermodynamic and dynamic analysis of diesel engines has been conducted and a simulation program has been prepared for a three-cylinder conceptual engine having 3 L swept volume. The dynamic model used in the analysis consists of motion equations of pistons, connecting rods and the crankshaft. The dynamic model involves the hydrodynamic and asperity frictions as well as the gas forces and moments. The thermodynamic aspect of the analysis has been modelled by replacing the idealized thermodynamic processes with more realistic processes. In this content, the gas pressure in the cylinder during compression and expansion periods was calculated at polytrophic conditions by means of the first law of the thermodynamics which comprises the heat generation in the cylinder and the heat loss to the cylinder walls. The gas pressures in the cylinder during the intake and exhaust periods have been mathematically modeled by setting a relation between mass variation of the gas in the cylinder and pressure difference in and out of the cylinder. Pressure, temperature and mass variations in the cylinder were found to be compatible with expectations. The numerical results of the mathematical model of intake and exhaust processes have also been compared with experimental data obtained from a test engine and found to be compatible as well. Via the prepared simulation program, the performance of the engine was tested at a constant throttling condition (constant heat input) and at a constant speed. Results obtained from simulation tests were found to be compatible with physical and practical situations. At constant heat input test conducted at 1460 J/cycle heat input, the optimum torque, power and total thermal efficiency of the conceptual engine were determined as 105 N m, 25.7 kW, and 30.15% respectively while the engine speed and intake manifold pressure were 244.5 rad/s and 1.3 bar. At constant speed testing conducted at about 250 rad/s, the optimum torque, power and the effective thermal efficiency of the conceptual engine were determined as 158 N m, 39.7 kW and 34% respectively while the intake manifold pressure is 1.6 bar.