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Premixed charge compression ignition (PCCI) is an advanced combustion mode that has the aim of simultaneously reducing particulate matter and nitrogen oxide exhaust emissions, compared with conventional diesel combustion, thanks to a partially premixed charge and low temperature combustion. In this work, PCCI combustion has been implemented by means of an early single-injection strategy and large amounts of recirculated exhaust gas. Starting from a commercial Euro VI on-road engine, the engine hardware has been modified to optimize PCCI operations. This has involved adopting a smaller turbo group, a new combustion chamber and injectors, and a dedicated high-pressure exhaust gas recirculation system. The results, in terms of engine performance and exhaust emissions, under steady-state operation conditions, are presented in this work, where the original Euro VI calibration of the conventional engine has been compared with the PCCI calibration of the optimized hardware engine.

A precise estimation of the recirculated exhaust gas rate and oxygen concentration as well as a predictive evaluation of the possible EGR unbalance among cylinders are of paramount importance, especially if non-conventional combustion modes, which require high EGR flow-rates, are implemented. In the present paper, starting from the equation related to convergent nozzles, the EGR mass flow-rate is modeled considering the pressure and the temperature upstream of the EGR control valve, as well as the pressure downstream of it. The restricted flow-area at the valve-seat passage and the discharge coefficient are carefully assessed as functions of the valve lift. Other models were fitted using parameters describing the engine working conditions as inputs, following a semi-physical and a purely statistical approach. The resulting models are then applied to estimate EGR rates to both conventional and non-conventional combustion conditions.

In the present work, different combustion control strategies have been experimentally tested in a heavy-duty 3.0 L Euro VI diesel engine. In particular, closed-loop pressure-based and open-loop model-based techniques, able to perform a real-time control of the center of combustion (MFB50), have been compared with the standard map-based engine calibration in order to highlight their potentialities. In the pressure-based technique, the instantaneous measurement of in-cylinder pressure signal is performed by a pressure transducer, from which the MFB50 can be directly calculated and the start of the injection of the main pulse (SOImain) is set in a closed-loop control to reach the MFB50 target, while the model-based approach exploits a heat release rate predictive model to estimate the MFB50 value and sets the corresponding SOImain in an open-loop control. The experimental campaign involved both steady-state and transient tests.