Combustion Prediction by a Low-Throughput Model in Modern Diesel Engines

A new predictive zero-dimensional low-throughput combustion model has been applied to both PCCI (Premixed Charge Compression Ignition) and conventional diesel engines to simulate HRR (Heat Release Rate) and in-cylinder pressure traces on the basis of the injection rate.

The model enables one to estimate the injection rate profile by means of the injection parameters that are available from the engine ECU (Electronic Control Unit), i.e., SOI (Start Of main Injection), ET (Energizing Time), DT (Dwell Time) and injected fuel quantities, taking the injector NOD (Nozzle Opening Delay) and NCD (Nozzle Closure Delay) into account. An accumulated fuel mass approach has been applied to estimate Q_ch (released chemical energy), from which the main combustion parameters that are of interest for combustion control in IC engines, such as, SOC (Start Of Combustion), MFB50 (50% of Mass Fraction Burned) have been derived. Qⁿ_et (charge net energy) has been evaluated and the in-chamber pressure during the combustion phase has been determined through the inversion of a single-zone combustion model. Optimized values of the isentropic coefficient have been identified and the compression and expansion phases have been approximated by proper polytropic evolutions. The calculated in-cylinder pressure time-history can be used to estimate PFP (Peak Firing Pressure), IMEP (Indicated Mean Effective Pressure) and CN (Combustion Noise).

The generality of the developed approach has been investigated in order to identify the model parameters that require specific tuning for different engine application. To this end, the model has been applied to PCCI diesel engines with different CR (Compression Ratio) values, to a twin-stage EURO V diesel engine equipped with piezoelectric injectors, and to three different diesel engines, with displacements of 2.9 dm₃, 1.7 dm₃ and 1.3 dm₃, respectively, equipped with solenoid injectors. The very accurate results that have been obtained highlight the robustness of the proposed approach.

The results have also substantiated the potential of the model to realize a real-time cycle-to-cycle combustion control, which can be applied to improve IC engine emissions, fuel consumption and combustion noise.