The role of spray-enhanced swirl flow for combustion stabilization in a stratified-charge DISI engine

Abstract

Implementation of spray-guided stratified-charge direct-injection spark-ignited (DISI) engines is inhibited by the occurrence of misfire and partial burns. Engine-performance tests demonstrate that increasing engine speed induces combustion instability, but this deterioration can be prevented by generating swirling flow during the intake stroke. In-cylinder pressure-based heat-release analysis reveals that the appearance of poor-burn cycles is not solely dependent on the variability of early flame-kernel growth. Cycles can experience burning-rate regression during later combustion stages and may or may not recover before the end of the cycle. Thermodynamic analysis and optical diagnostics are used here to clarify why swirl improves the combustion repeatability from cycle to cycle.The fluid dynamics of swirl/spray interaction was previously demonstrated using high-speed PIV measurements of in-cylinder motored flow. It was found that the sprays of the multi-hole injector redistribute the intake-generated swirl flow momentum, thereby creating a better-centered higher angularmomentum vortex with reduced variability. The engine operation with high swirl was found to have significant improvement in cycle-to-cycle variations of both flow pattern and flow momentum. This paper is an extension of the previous work. Here, PIV measurements and flame imaging are applied to fired operation for studying how the swirl flow affects variability of ignition and subsequent combustion phases. PIV results for fired operation are consistent with the measurements made of motored flow. They demonstrate that the spark-plasma motion is highly correlated with the direction of the gas flow in the vicinity of the spark-plug gap. Without swirl, the plasma is randomly stretched towards either side of the spark plug, causing variability in the ignition of the two spray plumes that are straddling the spark plug. In contrast, swirl flow always convects the spark plasma towards one spray plume, causing a more repeatable ignition. The swirl decreases local RMS velocity, consistent with an observed reduction of early-burn variability. Broadband flame imaging demonstrates that with swirl, the flame consistently propagates in multiple directions to consume fuel-air mixtures within the piston bowl. In contrast, operation without swirl displays higher variability of flame-spread patterns, occasionally causing the appearance of partial-burn cycles.

Quantitative metrics for comparison of in-cylinder velocity fields using particle image velocimetry

“…For incylinder air motion, particle image velocimetry (PIV) is a common experimental technique (Böhm et al 2011) due to its non-intrusive nature and ability to provide multiple velocity components in two or three dimensions (Prasad 2000;Adrian and Westerweel 2011;Peterson et al 2017;Raffel et al 2018). Accordingly it is the technique of choice for many engine related velocimetry studies (Justham et al 2006;Zeng et al 2014Zeng et al , 2016Zhang et al 2014;Stiehl et al 2016) . In-depth validation of CFD using PIV flow fields requires the application of quantitative metrics for comparisons between simulated and experimental vector fields.…”

High-speed PIV, spray, combustion luminosity, and infrared fuel-vapor imaging for probing tumble-flow-induced asymmetry of gasoline distribution in a spray-guided stratified-charge DISI engine

“…However, practical implementation of SC DISI engines has been inhibited by combustion instability; in particular the occurrence of misfire and partial-burn cycles [2]. Specifically, unfavorable flow direction and fuel distribution during early kernel growth can cause complete misfires or partial burns [3,4]. Hence, there is a need for managing flow and fuel-air mixing to reduce cycle-to-cycle variation of the main combustion.…”

“…Hence, there is a need for managing flow and fuel-air mixing to reduce cycle-to-cycle variation of the main combustion. Intake-generated swirl flow has been incorporated in SC DISI engines to reduce flow variability during the early flame growth period and subsequent flame propagation [4,5].…”

“…The engine has a centrally located 8-hole fuel injector with a nominal included angle of 60°. A single injection during the latter part of the compression stroke delivers 12.6 mg of certification gasoline per cycle [4,5], producing a gross indicated mean effective pressure (IMEP g ) of 380 kPa. The spark-plug gap is located 13 mm away from the injector tip.…”

“…Differences in both fuel distribution and flame spread can potentially explain this. Based on recent results in Ref [4],. flame-spread differences are discussed first.…”