Abstract
This paper offers new insights into a partial fuel stratification (PFS) combustion strategy that has proven to be effective at stabilizing overall lean combustion in direct injection spark ignition engines. To this aim, high spatial and temporal resolution optical diagnostics were applied in an optically accessible engine working in PFS mode for two fuels and two different durations of pilot injection at the time of spark: 210 µs and 330 µs for E30 (gasoline blended with ethanol by 30% volume fraction) and gasoline, respectively. In both conditions, early injections during the intake stroke were used to generate a well-mixed lean background. The results were compared to rich, stoichiometric and lean well-mixed combustion with different spark timings. In the PFS combustion process, it was possible to detect a non-spherical and highly wrinkled blue flame, coupled with yellow diffusive flames due to the combustion of rich zones near the spark plug. The initial flame spread for both PFS cases was faster compared to any of the well-mixed cases (lean, stoichiometric and rich), suggesting that the flame propagation for PFS is enhanced by both enrichment and enhanced local turbulence caused by the pilot injection. Different spray evolutions for the two pilot injection durations were found to strongly influence the flame kernel inception and propagation. PFS with pilot durations of 210 µs and 330 µs showed some differences in terms of shapes of the flame front and in terms of extension of diffusive flames. Yet, both cases were highly repeatable.
Highlights
The need for a reduction in CO2 and pollutant emissions from mobile sources, has in the last few decades led to the development of high efficiency engines for the automotive sector through the use of several different technologies [1]
This paper offers new insights into a partial fuel stratification (PFS) combustion strategy that has proven to be effective at stabilizing overall lean combustion in direct injection spark ignition engines
High spatial and temporal resolution optical diagnostics were applied in an optically accessible engine working in PFS mode for two fuels and two different durations of pilot injection at the time of spark: 210 μs and 330 μs for E30 and gasoline, respectively
Summary
The need for a reduction in CO2 and pollutant emissions from mobile sources, has in the last few decades led to the development of high efficiency engines for the automotive sector through the use of several different technologies [1]. With respect to spark ignition (SI) engines, gasoline direct injection (GDI) has been extensively adopted since it allows considerable gains in fuel economy, which together with exhaust emissions represent the most crucial points in terms of spark ignition engine regulations. It is easier to achieve an exact control of the amount of fuel to control the air–fuel ratio, especially for transient operation. With this configuration, less fuel is wasted through the air exchange processes [2]. GDI engines can potentially use fuel injection during the compression stroke to create a fully or partially stratified charge
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