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The turbulent in-cylinder air flow and the unsteady high-pressure fuel injection lead to a highly transient air fuel mixing process in spark-ignition direct-injection (SIDI) engines, which is the leading cause for combustion cycle-to-cycle variation (CCV) and requires further investigation. In this study, crank-angle resolution particle image velocimetry (PIV) was employed to simultaneously measure the air flow and fuel spray structure at 1300 rpm in an optically accessible single-cylinder SIDI engine. The measurement was conducted at the center tumble plane of the four-valve pent-roof engine, bisecting the spark plug and fuel injector. 84 consecutive cycles were recorded for three engine conditions, i.e. (1) none-fueled motored condition, (2) homogeneous-charge mode with start of injection (SOI) during intake (50 crank-angle degree (CAD) after top dead center exhaust, aTDCexh), and (3) stratified-charge mode with SOI during mid compression (270 aTDCexh).

A primary goal of large eddy simulation, LES, is to capture in-cylinder cycle-to-cycle variability, CCV. This is a first step to assess the efficacy of 35 consecutive computed motored cycles to capture the kinetic energy in the TCC-III engine. This includes both the intra-cycle production and dissipation as well as the kinetic energy CCV. The approach is to sample and compare the simulated three-dimensional velocity equivalently to the available two-component two-dimensional PIV velocity measurements. The volume-averaged scale-resolved kinetic energy from the LES is sampled in three slabs, which are volumes equal to the two axial and one azimuthal PIV fields-of-view and laser sheet thickness. Prior to the comparison, the effects of sampling a cutting plane versus a slab and slabs of different thicknesses are assessed. The effects of sampling only two components and three discrete planar regions is assessed.

A four-valve-pentroof, direct-injection, optical engine fueled with n-heptane has been operated at four different steady-state HCCI operating conditions including 10% and 65% residuals, both at low and high swirl conditions. Both, planar toluene LIF and volume chemiluminescence show large scale inhomogeneity in the ensemble averaged images. The interpretation of the toluene-tracer LIF signals (when premixed with the fresh-air charge) as a marker for reaction homogeneity is discussed. A binarization scheme and a statistical analysis of the LIF images were applied to the per-cycle planar-LIF images revealing inhomogeneities both from cycle-to-cycle and within the regions of individual cycles that track with the average heat release rate. Comparison of these two homogeneity metrics between the four operating conditions reveals weak but discernable differences.

In previous work, Reuss [1] studied the cycle-to-cycle variation in the large-scale velocity structures of high and low-swirl in-cylinder flows of an IC engine. The vector flow fields were obtained from PIV measurements in a two-valve, pancake-shaped, Transparent Combustion Chamber (TCC) engine. In this study, the Reynolds-decomposed turbulence properties such as kinetic energy, length scales, and dissipation rate were directly measured for the two cases. The results demonstrate that, at TDC compression, the low-swirl flow is dominated by turbulence at the largest scales, whereas the high-swirl flow has a considerably lower turbulence Reynolds number. The dissipation rate and length scale calculated from mixing-length theory greatly exceeded the dissipation computed from the 2-D velocity-gradients and integral-length scales computed from the autocorrelation, respectively.

Two-dimensional PIV was used to measure the cycle to cycle variability of large-scale flow-structures at TDC in a motored, two-valve, four-stroke engine. Over two hundred velocity distributions were measured for both a highly directed flow using a shrouded valve and a relatively undirected flow using a standard valve. Each cycle of the directed flow had the appearance of the single large swirl structure seen in the ensemble mean. Cyclic variability of the large-scale flow structure was manifest as variations in swirl ratio (rotational speed). Generally the variability was limited to scales smaller than 10 mm in size. For the undirected flow, none of cycles had the appearance of the ensemble mean. The flow appeared to be multimodal in that large-scale flow-structure patterns could be classified into three types based on flow-pattern recognition.

Two-dimensional in-cylinder velocity distributions measured with Particle Image Velocimetry were compared with computed results from Computational Fluid Dynamics codes. A high-swirl, two-valve, four-stroke transparent-combustion-chamber research engine was used. Comparisons were made of mean-flow velocity distributions, swirl-ratio evolution during the intake and compression strokes, and turbulence distributions at top-dead-center compression. This comparison with the measured flows led to more accurate calculations by identifying code improvements including swirl in the residual gas, modeling of the gas exchange during the valve overlap, and improved numerical accuracy.

Particle Image Velocimetry (PIV) was used to make instantaneous velocity measurements over a 24 mm by 32 mm area in a fired two-stroke cycle engine. The unburned-gas regions of the photographs were successfully interrogated adjacent to the flames and with sufficient resolution to resolve the velocity integral-length scales. A highpass filtering algorithm, different from that used in a previous motored-engine study, was implemented to allow for the arbitrary flame boundary. The large-scale vorticity in this study was considerably higher than in a previous study where a different engine was used. The large-scale normal and shear strain-rates distributions revealed only a small increase over those in the previous study, and the magnitude of the vorticity and shearstrain-rate appeared to be larger near the flame. However, the data are too limited to offer general conclusions about the flow.

Particle-image velocimetry (PIV) has been used in an engine to produce a virtually continuous two-dimensional velocity-vector map over a 12 × 32 mm area. The particle-seeded flow field in the clearance volume of a motored engine (600 r/min, 8:1 compression) was illuminated by a double-pulsed sheet of laser light (20-40μs pulse separation) oriented parallel to the piston. The illuminated particles (<1μm) were photographed at 78deg BTDC compression and 12deg ATDC with 1 × magnification, resulting in paired particle images separated by distances ∼200μm. The two-dimensional velocity distribution was determined by interrogating 0.9-mm square spots on a 0.5-mm grid spacing. The average particle image-pair displacement within each interrogation spot was determined by performing a spatial correlation, and thus the magnitude and direction of the average velocity within the interrogation spot was inferred from the light-pulse separation.