Characterisation of Flow Structures in a Direct-Injection Spark-Ignition Engine Using PIV, LDV and CFD

Abstract

In-cylinder air flow structures are known to play a major role in mixture preparation and engine operating limits for DISI engines. In this paper PIV was undertaken on in-cylinder flow fields for three different planes of measurement in the intake and compression strokes of a DISI engine for a lowload engine operating condition at 1500 RPM, 0.5 bar inlet plenum pressure (World Wide Mapping Point). One of these planes was vertical, cutting through the centrally located spark plug (tumble plane); the other two planes were horizontal, one close to TDC (10 mm below fire face) and the other one close to mid stroke (50 mm below fire face). Statistical analysis was undertaken on the numbers of cycles needed to determine ensemble average flow-field and turbulent kinetic energy maps with up to 1200 cycles considered. The effect of engine head temperature was also examined by obtaining flow fields using PIV with the engine head coolant held at 20 °C and 80 °C. LDV measurements were also performed and compared to the data obtained by PIV. Finally comparisons were made between the experimental data and results from CFD simulations using two different turbulence models on a grid of 1 million cells.

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“…Apart from ensemble-average flow fields, the turbulence intensity u, v, w was also obtained as the RMS of the velocity measurements at each point. LDV measurements in the same engine [54] indicated that the air-flow during late compression was quite close to isotropy. Therefore, it was decided that the Turbulent Kinetic Energy (TKE) would be approximated by using the RMS of each velocity component at each point on that plane and estimating the 3 rd velocity component as the arithmetic average of the other two; this approach is addressed further in the results section.…”

“…Details about the engine and the test bed arrangement can be found in previous publications by the current authors, e.g. [13,20,54], hence only the most relevant features are described in this section.…”

“…This extended campaign and analysis solidified confidence to the conclusions of the subset of PIV data used in the current study. The complete set of data can be useful to the interested reader for broader interpretation of in-cylinder flow phenomena [54,55,62].…”

“…To bring this into context, the integral timescale in this engine measured by LDV at ignition timing was of the order 8-16 °CA (depending on velocity component) [54,55].…”

“…Two in-cylinder planes were considered, a horizontal plane (also called 'swirl' plane hereafter) located 1 mm below the spark plug's ground electrode (or 3 mm above the fire-face) and another one, vertical centrally located, cutting through the spark-plug from inlet to exhaust side (also called 'tumble' plane hereafter). Following extensive analysis on the minimum number of cycles required for representative mean and turbulent fluctuations data from PIV data [54], a minimum of 540 individual cycles were averaged for each experimental test point in batches of 60 cycles per run; this practice of batching was based on the data storage capabilities of the imaging system and image quality deterioration by fowling of the optical access with increased duration of test runs. Table 4 summarises the system's main settings.…”