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This work employs a combination of pressure trace analysis, high-speed optical measurements and laser-based techniques for the assessment of the effects of various post-injection schemes on the soot reduction potential in an optical single-cylinder light-duty diesel engine. The engine was operated under a multiple injection scheme of two pilot and one main injection, typical of a partially premixed combustion mode, at the lower end of the load and engine speed range (ca 2.0 bar IMEP at 1200 r/min). Experiments considering the influence of the post-injection fuel amount (up to 15% of the total fuel quantity per cycle) and the post-injection timing within the expansion stroke (5, 10 and 15 CAD aTDC), under a constant total fuel mass per cycle, have been conducted. Findings were analysed via means of pressure trace and apparent rate of heat transfer analyses, as well as a series of optical diagnostic techniques, namely, high-speed flame natural luminosity imaging, CH*, C∗2 and OH* line-of-sight chemiluminescence, as well as planar laser-induced incandescence measurements at 31 and 50 CAD aTDC. The combination of post-injection fuel amount and timing has substantial effects on charge reactivity and soot oxidation potential. The analysis reveals that an amount of fuel (7% of the total fuel mass per cycle) injected more than 10 CAD after the main combustion event leads to higher levels of soot emissions, while a larger amount of fuel (15% of the total fuel mass) injected 5 CAD after the main combustion event appears to have a beneficial effect on the soot oxidation processes. Overall, results indicate that a post-injection scheme close to the main combustion phasing could reduce soot levels and improve engine performance, that is, higher IMEP levels at the same fuel consumption rates, although it could increase engine noise.

This study reports on the use of fractal grids as a new type of turbulence generators in premixed combustion applications. Fractal grids produce turbulence fields which differ from those formed by regular turbulence generators such as perforated plates or meshes. Fractal grids generate high turbulence intensities over an extended region some distance downstream of the grid with a comparatively small pressure drop. Additionally, the integral scale of the flow does not change downstream of the grid. The extended region of high turbulence can also be optimized for the specific application at hand by changing certain parameters of the grid which makes it possible to design the downstream development of the turbulence field. Four space-filling fractal square grids were designed to independently vary the resulting turbulent field and a regular square mesh grid with similar turbulent intensity acted as a reference case. The structure of the resulting premixed V-shaped flames was investigated using Conditioned Particle Image Velocimetry (CPIV). At the same downstream position, flames in the turbulence field of fractal grids showed larger turbulent burning velocity compared to flames in regular grid generated turbulence. However, when compared for the same turbulence intensity, flames in fractal grid generated turbulence produced similar turbulent burning velocities compared to flames in regular grid generated turbulence. In particular, it could be shown that theories such as Taylor’s theory of turbulent diffusivity and Damkohler’s theory of premixed flame propagation, which were deduced from regular turbulence fields, adequately described the increase of effective flame surface area due to the increase in turbulence intensity. Using fractal grids allows the independent variation of the turbulent fluctuations, the integral length scale and the turbulent Reynolds number. An unexpected finding was that the burning velocity ratio, s t / s l was negligible influenced by the integral length scale. A correlation between the burning velocity ratio, s t / s l , and the normalized velocity fluctuations of the flow, u ′/ s l , showed a negligible influence of the integral scale on the turbulent burning velocity. A literature review revealed that the influence of the integral scale on the turbulent burning velocity is still unclear and further research is required. In this context, fractal grids are particularly helpful as they cover a wider range of integral length scales for sufficiently turbulent flows, u ′ ⩾ s l , compared to regular grids.

The experimental evaluation of the topological characteristics of the turbulent flow in a ‘box’ of homogeneous and isotropic turbulence (HIT) with zero mean velocity is presented. This requires an initial evaluation of the effect of signal noise on measurement of velocity invariants. The joint probability distribution functions (pdfs) of experimentally evaluated, noise contaminated, velocity invariants have a different shape than the corresponding noise-free joint pdfs obtained from the DNS data of the Johns Hopkins University (JHU) open resource HIT database. A noise model, based on Gaussian and impulsive Salt and Pepper noise, is established and added artificially to the DNS velocity vector field of the JHU database. Digital filtering methods, based on Median and Wiener Filters, are chosen to eliminate the modeled noise source and their capacity to restore the joint pdfs of velocity invariants to that of the noise-free DNS data is examined. The remaining errors after filtering are quantified by evaluating the global mean velocity, turbulent kinetic energy and global turbulent homogeneity, assessed through the behavior of the ratio of the standard deviation of the velocity fluctuations in two directions, the energy spectrum of the velocity fluctuations and the eigenvalues of the rate-of-strain tensor. A method of data filtering, based on median filtered velocity using different median filter window size, is used to quantify the clustering of zero velocity points of the turbulent field using the radial distribution function (RDF) and Voronoi analysis to analyze the 2D time-resolved particle image velocimetry (TR-PIV) velocity measurements. It was found that a median filter with window size 3 × 3 vector spacing is the effective and efficient approach to eliminate the experimental noise from PIV measured velocity images to a satisfactory level and extract the statistical two-dimensional topological turbulent flow patterns.

The work investigates the effect of various post-injection strategies on the flame patterns in a Ricardo Hydra optical single-cylinder, light-duty diesel engine, operated in a partially premixed combustion mode, under low load (IMEP: ca. 2.3 bar), low speed (1200 rpm) conditions. The effect of post-injection fuel amount (12% and 24% of the total fuel quantity per cycle) and post-injection timing (0, 5, 10 deg aTDC) are investigated via pressure trace analysis and optical measurements. Flame propagation is captured by means of high-speed flame natural luminosity imaging and of CH*, C2*, and OH* line-of-sight chemiluminescence measurements. Results suggest that post-injections suppress mixture reactivity but enhance oxidation, and that a larger amount of fuel and/or later post-injection, leads to higher levels of natural luminosity, indicating possible higher soot-out emissions, while post-injection close to the main combustion event appears to have a beneficial effect on the soot oxidation processes.

We quantify the temporal development of the mixing field of a starting jet by measuring the mixture fraction and the scalar dissipation rate and their statistics in an isothermal, impulsively started, gaseous jet. The scalar measurements are performed using planar laser induced fluorescence and, with appropriate processing of the resulting images, allow scalar dissipation rate measurements within 20%. The probability density functions of the mixture fraction, measured within a region of the order of 3 times the Batchelor length scale of the flow, are bimodal and skewed around a well-mixed radial location, which depends on the downstream distance and the time after the start of injection. The instantaneous distributions of the scalar dissipation rate reveal regions of high mixing at the jet periphery and at the developing vortex ring. The normalised probability density function (pdf) of the scalar dissipation rate at various flow positions and times after the start of injection has the same characteristic shape but differs from the usually suggested lognormal distribution at both low and high dissipation values; the same, also, holds true for the pdf conditioned on different values of the mixture fraction. The mean of the scalar dissipation rate conditional on mixture fraction shows a variation across the mixture fraction range, which differs between flow locations and times after the start of injection; however, at later times and for larger downstream distances the conditional mean between flow locations has similar distributions. Implications of the measurements for the auto-ignition of gaseous jets are examined and demonstrate that near the nozzle exit or at earlier times conditions are un-favourable for auto-ignition.

Measured filtered density functions (FDFs) as well as assumed beta distribution model of mixture fraction and “subgrid” scale (SGS) scalar variance z′′2⎯⎯⎯⎯⎯⎯⎯, used typically in large eddy simulations, were studied by analysing experimental data, obtained from two-dimensional planar, laser induced fluorescence measurements in isothermal swirling turbulent flows at a constant Reynolds number of 29 000 for different swirl numbers (0.3, 0.58, and 1.07). Two-dimensional spatial filtering, by using a box filter, was performed in order to obtain the filtered variables, namely, resolved mean and “subgrid” scale scalar variance. These were used as inputs for assumed beta distribution of mixture fraction and top-hat FDF shape estimates. The presumed beta distribution model, top-hat FDF, and the measured filtered density functions were used to integrate a laminar flamelet solution in order to calculate the corresponding resolved temperature. The experimentally measured FDFs varied with the flow swirl number and both axial and radial positions in the flow. The FDFs were unimodal at flow regions with low SGS scalar variance, z′′2⎯⎯⎯⎯⎯⎯⎯< 0.01, and bimodal at regions with high SGS variance, z′′2⎯⎯⎯⎯⎯⎯⎯> 0.02. Bimodal FDF could be observed for a filter size of approximately 1.5-2 times the Batchelor scale. Unimodal FDF could be observed for a filter size as large as four times the Batchelor scale under well-mixed conditions. In addition, two common computational models (a gradient assumption and a scale similarity model) for the SGS scalar variance were used with the aim to evaluate their validity through comparison with the experimental data. It was found that the gradient assumption model performed generally better than the scale similarity one.

Schlieren [Schlieren and Shadowgraphy Techniques (McGraw-Hill, 2001); Optics of Flames (Butterworths, 1963)] is a non-intrusive technique that can be used to detect density variations in a medium, and thus, under constant pressure and mixture concentration conditions, measure whole-field temperature distributions. The objective of the current work was to design a schlieren system to measure line-of-sight (LOS)-averaged temperature distribution with the final aim to determine the temperature distribution inside the cylinder of internal combustion (IC) engines. In a preliminary step, we assess theoretically the errors arising from the data reduction used to determine temperature from a schlieren measurement and find that the total error, random and systematic, is less than 3% for typical conditions encountered in the present experiments. A Z-type, curved-mirror schlieren system was used to measure the temperature distribution from a hot air jet in an open air environment in order to evaluate the method. Using the Abel transform, the radial distribution of the temperature was reconstructed from the LOS measurements. There was good agreement in the peak temperature between the reconstructed schlieren and thermocouple measurements. Experiments were then conducted in a four-stroke, single-cylinder, optical spark ignition engine with a four-valve, pentroof-type cylinder head to measure the temperature distribution of the reaction zone of an iso-octane–air mixture. The engine optical windows were designed to produce parallel rays and allow accurate application of the technique. The feasibility of the method to measure temperature distributions in IC engines was evaluated with simulations of the deflection angle combined with equilibrium chemistry calculations that estimated the temperature of the reaction zone at the position of maximum ray deflection as recorded in a schlieren image. Further simulations showed that the effects of exhaust gas recirculation and air-to-fuel ratio on the schlieren images were minimal under engine conditions compared to the temperature effect. At 20 crank angle degrees before top dead center (i.e., 20 crank angle degrees after ignition timing), the measured temperature of the flame front was in agreement with the simulations (730–1320 K depending on the shape of the flame front). Furthermore, the schlieren images identified the presence of hot gases ahead of the reaction zone due to diffusion and showed that there were no hot spots in the unburned mixture.

The operating range of lean-burn spark-ignition (SI) engines is limited by the cycle-to-cycle variability of the fuel concentration at or near the spark plug at ignition timing. An experimental investigation was undertaken to measure the temporal and spatial distribution of hydrocarbon (HC) concentration in a spark-ignition engine, using the infrared (IR) laser absorption at 3392 nm. The purposes were to establish whether there is a correlation between time-resolved HC measurements for a range of global air-to-fuel (A/F) ratios (A/F = 15.5–23) with the strength of the firing stroke and to establish how this varies with fuel port-injection strategies against either open (injection timing 30° crank angle (CA) after intake top dead center) or closed-valves (injection timing 180°CA after intake top dead center), respectively resulting in stratified and near-homogeneous charge distributions. The results showed that IR line-of-sight (LOS) averaged A/F ratio measurements yielded a good agreement with the global A/F ratio readings obtained by the linear air-to-fuel (LAF) zirconia-based sensor. Furthermore, the cyclic variability of the measurements of the fuel concentration increased with increasing A/F ratio. At A/F = 23, closed-valve injection strategy resulted in small spatial stratification of the fuel charge with an ensemble-averaged correlation coefficient of fluctuations of the IR LOS A/F ratios with fluctuations of peak in-cylinder pressure of 0.37. For open-valve injection strategy, which resulted in axial fuel mixture charge-stratification but no radial charge-stratification, a maximum correlation coefficient of the IR LOS A/F ratios with fluctuations of peak pressure of 0.34 was measured (at the measurement location closest to the spark). This correlation was reduced to 0.17 at locations furthest from the spark plug. Finally, results showed that at A/F = 23, fuel stratification can be used to control lean-burn SI combustion, while at A/F = 15.5, no significant difference was observed between the two injection timings. The consequences of these findings on the fuel distribution in the pentroof and the control of the lean limits of SI engines are discussed.

Measurements of the scalar dissipation rate are performed in an impulsively started gas jet, using planar laser induced fluorescence. The measurements are well resolved spatially. The deteriorating effect of experimental noise on this experiment is treated with a Wiener filter, which is shown to be applicable to this large-scale inhomogeneous flow. The accuracy of the scalar dissipation rate is within $20\%$, as determined from an explicit calculation of the filtering errors. The residual fields that remain after the filtering are analysed in detail and their statistical properties show that these resemble white noise to a good approximation. The level of corrections is minimal for the scalar field but it is of the order of $40\%$ for the scalar dissipation rate. An examination of the filtering operation using modeled spectra and the measured spatial resolution shows that the Wiener filter produces errors in the estimate of the scalar dissipation rate $\sim30\%$, for Taylor-scale Reynolds number up to 1000. The implications of this modelling are discussed with respect to common experimental situations and point out the relative merits of improving the spatial resolution as compared to improvements in the signal to noise ratio.

A space-filling, low blockage fractal grid is used as a novel turbulence generator in a premixed turbulent flame stabilized by a rod. The study compares the flame behaviour with a fractal grid to the behaviour when a standard square mesh grid with the same effective mesh size and solidity as the fractal grid is used. The isothermal gas flow turbulence characteristics, including mean flow velocity and rms of velocity fluctuations and Taylor length, were evaluated from hot-wire measurements. The behaviour of the flames was assessed with direct chemiluminescence emission from the flame and high-speed OH-laser-induced fluorescence. The characteristics of the two flames are considered in terms of turbulent flame thickness, local flame curvature and turbulent flame speed. It is found that, for the same flow rate and stoichiometry and at the same distance downstream of the location of the grid, fractal-grid-generated turbulence leads to a more turbulent flame with enhanced burning rate and increased flame surface area.