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Mixture Preparation Effects on Gaseous Fuel Combustion in SI Engines
Abstract:
This paper presents a comparison of the influence of different mixture preparation strategies on gaseous fuel combustion in SI engines. Three mixture preparation strategies are presented for a dedicated LPG fuelled engine, showing varying results - gaseous phase port injection (PFI-G), liquid-phase port injection (PFI-L) and gaseous-phase throttle-body injection (TBI-G). Previous work by the authors has shown considerable differences in emissions and thermal efficiency between different fuelling strategies. This paper extends this work to the area of combustion characteristics and lean limit operation and closer analyses the differences between these systems.
A dedicated LPG in-line six cylinder engine with compression ratio increased to 11.7:1 (up from the standard 9.65:1) was tested over a range of speed/torque conditions representing most of the steady-state parts of the Euro drive-cycle for light duty-vehicles. The air-fuel ratio was varied from lambda 1.0 to the lean limit. Cylinder-to-cylinder measurements of air/fuel ratio and emissions were conducted. Additionally, sweeps of lambda at five important engine operating points were undertaken for each configuration and results of COV of IMEP, thermal efficiency, HC, NOx and spark timing compared. Comparisons were also made at two world wide mapping points. Furthermore, in-cylinder pressure measurements were processed to compare combustion characteristics - 0-2.5% and 0-90% mass fraction burn durations - at important operating points.
It is shown that location of injectors as well as fuel state (liquid or gaseous) can have a considerable affect on combustion characteristics and engine stability. PFI-G is shown to have very slow burns, attributed to poor mixing as evidenced by emissions variations at a given lambda.
However, TBI-G using the same injectors as PFI-G is shown to approach PFI-L efficiency due to superior mixing, and to extend the lean limit to lambda 1.6 at which NOx emissions are very low. The three injection systems are shown to deliver significantly different cylinder-to-cylinder air/fuel ratios and mixture homogeneity which along with temperature effects contribute to significantly different overall performance.