Experimental and 1D Numerical investigations on the exhaust emissions of a small Spark Ignition engine considering the cylinder-by-cylinder variability 2020-01-0578
The stringent legislations on pollutant and CO2 emissions require relevant efforts to improve both the combustion efficiency and the exhaust emissions of internal combustion engines. In the case of spark ignition (SI) engines, various techniques have been tested and implemented in the last generation SI engine architectures. On the other hand, a reduced emphasis has been posed on the analysis of individual cylinder behavior, since a systematic sub-optimal operation may occur, due to cylinder-by-cylinder non-uniformities.
The main purpose of this work is to accurately forecast the combustion and the exhaust emissions of a twin-cylinder turbocharged SI engine, taking into account the overall performance and individual cylinder-by-cylinder operation, with particular attention to volumetric efficiency, injected fuel quantity, and residuals content.
To this aim, a dedicated experimental activity is performed on the engine under investigation. Preliminary measurements have shown relevant differences in combustion evolution in the two cylinders, mainly ascribed to variations in the injected fuel quantities, which in turn depend on the fuel rail geometry . As a consequence, cylinder out emissions are also quite different in the two cylinders.
To better characterize this phenomenon, the test bench is modified to measure the cylinder-by-cylinder exhaust emissions at different engine loads and various EGR rates. These measurements are then employed to validate a 1D engine model, integrated with refined sub-models of turbulence, combustion and emissions. The model takes into account the in-cylinder production of noxious species, and their propagation in the exhaust system, up to the three-way catalytic converter (TWC).
A satisfactory accuracy is reached in reproducing the overall engine performance, the combustion process in the two cylinders and the engine behavior upstream the TWC, as well. In particular, the emission sub-model confirms that the variations of the cylinder-out exhaust emissions (NOx, HC and CO), are mainly due to the non-uniform effective in-cylinder air-fuel ratio.
The proposed numerical methodology has the potential to highlight unexpected combustion non-uniformities among different cylinders and represents a powerful support to the engine design and development. It also allows for the prediction of the overall exhaust emissions in whole engine operating plane, thus assisting the engine calibration phase and reducing the experimental efforts.