The paper illustrates the interdisciplinary matching of different numerical and experimental techniques, aimed to characterize the performances, emissions and combustion noise radiated from a small-size DI diesel engine. The main objective is the development of proper models to be included within an optimization procedure, able to define an optimal injection strategy for a common-rail engine. The injection strategy is selected to simultaneously reduce the fuel consumption, the pollutant emissions and the combustion noise.
The engine considered is a naturally aspirated, four strokes, two valves, single-cylinder engine (505 cm3 displacement), to be equipped with a prototype common-rail fuel injection system. A preliminary experimental investigation is carried out on the above engine, equipped, however, with a standard mechanical injection system (base engine). Both performance and radiated noise are measured on the base engine to provide reference experimental data in different operating conditions. Moreover, the prototype common-rail fuel injection system is characterized on a flow-meter to measure the instantaneous fuel flow rate corresponding to a pilot-plus-main injection strategy.
A deep 3D CFD analysis of the combustion period is firstly carried out on the base engine, employing realistic initial conditions derived from a 1D simulation of the whole engine. The CFD analysis is effected by accounting for the fuel spray dynamics and for the subsequent chemical reactions, leading to the prediction of the rate of heat release, pollutants formation and in-cylinder pressure cycle. 1D and 3D analyses are validated with reference to overall engine performance and experimental in-cylinder pressure data, respectively. In addition, the 3D computed pressure cycle is post-processed to estimate the combustion related noise. The acoustic methodology is based on an innovative study performing a decomposition of the pressure signal, and allowing to quantify the noise emission of the chosen engine operating point. The numerical results are compared to sound pressure level measurements, taken at 1m from the engine.
Once validated, both the CFD and the acoustic procedures are applied to the simulation of the common-rail prototype engine and are finally coupled to an external optimizer (ModeFRONTIER®). The experimental pilot-plus-main injection strategy is properly parameterized, for a constant overall mass of injected fuel, in terms of three degrees of freedom: the start of injection, the dwell-time and pilot injection duration. The above parameters are continuously varied by the optimizer to the aim of simultaneously minimize the fuel consumption, the pollutant emissions and the radiated noise.
The results clearly highlight a trade-off among the various objectives and the need to select a compromise solution among them. The study has the major result of proposing a technique for indicating paths towards an automatic engine optimization, characterized by the highest power output and the lowest environmental impact in terms of both pollutants and noise emitted.