Effects of control parameters on performance and emissions of HSDI diesel engines: Investigation via two zone modelling

The paper deals with the development and experimental validation of a two zones combustion model of High Speed Direct Injection (HSDI) Diesel engines. The model is aimed to support the engine control design for common-rail Diesel engines with multiple injections, where the large number of control parameters (i.e. injection timing, injection dwell, rail pressure, etc.) makes the recourse to experiments extremely expensive, in terms of time and money. The modelling approach is based on a semi-empirical two-zone combustion model coupled with an intensive identification analysis, in order to implement a predictive tool for simulating the effects of control strategies on combustion and exhaust emissions. Fuel spray and combustion are simulated by dividing the combustion chamber into two control volumes, accounting for fuel jet and surrounding air. The fuel jet is further divided in two zones to separate liquid and vapour phases, while the surrounding air zone is composed of fresh air and residual gases. Fuel evaporation and combustion models are based on the semi-empirical formulation proposed by Whitehouse and Way, which accounts for the occurrence of premixed and diffusive regimes. The fuel spray model evaluates the spray motion into the cylinder, assuming an empirical formulation for the break time, and predicts the air entrainment by means of the conservation of momentum. NOx and soot exhaust emissions are predicted according to the well known mechanisms proposed by Zeldovich and Hiroyasu, respectively. Model accuracy has been successfully tested over a wide set of experimental data, composed of nearly 100 engine cycles measured on a commercial common rail multi-injection Diesel engine. Moreover, only 9 measured operating conditions were needed for model identification, thus confirming the limited recourse to experiments. Simulation results also evidenced that the model can predict the effects of different injection parameters, in case of single and multiple injection, in a short computational time. Therefore its implementation is suitable for the accomplishment of intensive simulations or optimization analyses, aimed to minimize consumption and/or emissions vs. injection strategy (number of injections, injection timing, rail pressure, etc.).