Browse Publications Technical Papers 2019-24-0075
2019-09-09

Performance and Emissions of an Advanced Multi-Cylinder SI Engine Operating in Ultra-Lean Conditions 2019-24-0075

Along the design process of a new engine, the calibration phase at the test bench usually involves a relevant percentage of the overall time-to-market. Each control variable, in fact, needs to be properly selected to optimize the performance and emissions, complying with thermal and mechanical stresses limits of the engine. This issue is still more critical for advanced engine architectures, which include additional control variables, such as valve phasing, turbocharger control, EGR level, etc. The aim of this work is the development of a numerically performed calibration procedure, applied to a prototype multi-cylinder Spark Ignition (SI) engine, designed to operate at very lean mixtures. To this aim, an active Pre-Chamber ignition system is considered. The required air flow rate is indeed provided by a Low-Pressure (LP) variable geometry turbocharger group, coupled to a high-pressure e-compressor. A Variable Valve Timing (VVT) device is also selected to reduce pumping losses at low load and for knock control at high-load. For the above engine, seven control variables have to be fixed in each operating condition, namely the Air/Fuel ratio in both pre-chamber (PC) and main-chamber (MC), the rack position for the LP compressor, the electrical power required to drive the e-compressor, the VVT position, the spark timing and the throttle opening. Firstly, a 1D engine model of the described architecture is built up in GT-Power environment. The modeling framework utilizes in-house developed phenomenological sub-models for the prediction of turbulence, combustion, heat transfer, knock and NOx emission. In particular, the combustion and turbulence sub-models were recently extended to handle the burning process in both the PC and the MC. The model accuracy was validated for a single cylinder research engine in a previous authors’ work, denoting a good agreement with the measured data. In a second step, various PIDs are included in the 1D model to select the values of each control variable and to compute the overall engine operating plane. To validate the PID-based calibration, the model is integrated in a general-purpose commercial optimizer to set the control variables which minimize the fuel consumption in a limited number of operating points. Performance and emission levels, together with the values of control variables, given by the optimizer, are compared with the PID-based results, and a good agreement is obtained. The overall numerical procedure underlines the benefits of a multi-cylinder pre-chamber engine in terms of brake thermal efficiency and nitrogen oxides emission, as well. The proposed methodology proves to be a useful tool to realize a “virtual” engine calibration, achieving a good compromise between high accuracy and limited computational efforts. It can be utilized in an industrial environment to support and drive the engine development phase, so reducing the related time and costs.

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