This paper reports a numerical and experimental analysis on a twin-cylinder turbocharged Spark Ignition engine carried out to investigate the cylinder-to-cylinder variability in terms of performance, combustion evolution and exhaust emissions.
The engine was tested at 3000 rpm in 20 different steady-state operating conditions, selected with the purpose of observing the influence of cylinder-by-cylinder A/F ratio variations and the EGR effects on the combustion process and exhaust emissions for low to medium/high loads. The experimental outcomes showed relevant differences in the combustion evolution (characteristic combustion angles) between cylinders and not negligible variations in the emissions of the single cylinder exhaust and the overall engine one. This misalignment resulted to be due to differences in the injected fuel amount by the port injectors in the two cylinders, mainly deriving from the specific fuel rail geometry.
The experimental data were then used to validate a 1D engine model, integrated with refined sub-models of turbulence, combustion, heat transfer 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. A satisfactory accuracy was reached in reproducing the overall engine performance and the combustion process in the two cylinders. In particular, the emission sub-models confirmed that the variations of the cylinder-out exhaust emissions (NOx, HC and CO) were mainly due to the non-uniform effective in-cylinder A/F 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 at different engine operating conditions up to the entire domain, thus assisting the engine calibration phase and reducing the experimental efforts.