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Investigations from recent years have shown that at operating conditions characterized by long ignition delays and resulting large proportions of premixed combustion, the NOx emission trend does not correspond to the (usually) postulated correlation with an appropriately defined (adiabatic) burnt flame temperature. This correlation, however, is the cornerstone of most published NOx models for direct injection diesel engines. In this light, a new phenomenological NOx model has been developed in Brückner et al. (Part 1), which considers NOx formation from products of premixed and diffusion combustion and accounts for compression heating of post-flame gases, and describes NOx formation by thermal chemistry. In this study (Part 2), the model is applied to predict NOx emissions from two medium-speed direct injection diesel engines of different size and at various operating conditions. Single parameter variations comprising sweeps of injection pressure, start of injection, load, exhaust gas recirculation rate, number of injections, and end-ofcompression temperature are studied on a single-cylinder engine. In addition, different engine configurations (valve timing, turbocharger setup) and injection parameters of a marine diesel engine are investigated. For both engines and all parameter variations, the model prediction shows good agreement. Most notably, the model captures the turning point of the NOx emission trend with increasing ignition delay (first decreasing, then increasing NOx) for both engines. The differentiation in the physical treatment of the products of premixed and diffusion with increasing ignition delay showed to be essential for the model to capture the trend-reversal. Specifically, the model predicted that peak NOx formation rates in diffusion zones decrease with increasing ignition delay, whereas for the same change in ignition delay, peak formation rates in premixed zones increase. This is caused by the high energy release in short time, causing a strong compression of existing premixed combustion product zones that mix at a slower rate and have less time to mix, significantly increasing their temperature. In contrast, the model under-predicts NOx emissions for very low oxygen concentrations, in particular below 15 vol.%, which is attributed to the simple thermal NOx kinetic mechanism used. It is concluded that the new model is able to predict NOx emissions for conventional diesel combustion and for long ignition delay operating conditions, where a substantial amount of heat is released in premixed mode. [ABSTRACT FROM AUTHOR]

There exists a well-established correlation of exhaust NOx emissions arising from diesel engines with the adiabatic flame temperature, in particular for conventional (i.e. short ignition delay, diffusion combustion-dominated) operating conditions. Most published NOx emission models rely on this correlation. However, numerous experimental studies have identified operating conditions where this correlation fails to capture the exhaust NOx trend. In this work, a novel phenomenological NOx model concept is introduced, including a first successful validation against experimental data. The model development is based on experimental observations and is supported by three-dimensional computational fluid dynamics computations, strengthening the understanding of the underlying mechanisms leading to the discrepancy between the adiabatic flame temperature and exhaust NOx trend. For long ignition delay operating conditions, the improved mixture preparation before ignition leads to reduced mixing rates during and after combustion. Both the improved mixture preparation before ignition and the instantaneous increase of mass observed above 2000K after start of combustion are due to compression heating of the burned gases. Key features of the model are improved description of mixture distribution at start of combustion, NOx formed in products of premixed burn, different physical treatments of premixed and diffusion sourced products, and inherent consideration of burned gas compression heating. Model results capture the NOx emissions for conventional diesel combustion, as well as for operating conditions where the NOx emissions do not follow the adiabatic flame temperature trend. Moreover, the results show that the contribution of NOx from products from premixed burn and the consideration of compression heating effects on burned (postflame) gases are essential to capture the NOx emissions under the latter conditions. [ABSTRACT FROM AUTHOR]

Soot onset in n-dodecane sprays is investigated both experimentally, by means of high-speed imaging data from the Sandia spray combustion vessel, and numerically, using the conditional moment closure combustion model and an integrated two-equation soot model in a Reynolds-averaged Navier–Stokes framework. Five operating conditions representative of modern diesel engines are studied at constant density (22.8 kg/m3) with variations in ambient oxygen concentration and temperature. The reference case at 15% O2 and 900 K is compared with measurements in terms of the evolving soot mass distribution and spatiotemporal distributions of formaldehyde and polycyclic aromatic hydrocarbons obtained by 355-nm laser-induced fluorescence (polycyclic aromatic hydrocarbons represented by C2H2 in simulation) and soot optical thickness (KL) signal obtained by diffused back-illumination extinction imaging. All operating points are validated in terms of ignition delay and lift-off length, soot onset time and location, soot mass evolution, and peak location. Measurements show that time lag between ignition and soot onset is considerably increased by a reduction in ambient oxygen or temperature. The trend of this time lag is captured very well by the simulations, as is the evolving axial distribution of soot, despite the simple soot model employed. Building on the good agreement between spatiotemporal distributions in experiment and simulation, further results from the latter are extracted to provide insight into relevant processes. The advancing soot tip lags behind the fuel–vapor spray tip due to soot oxidation. Tracking the Lagrangian time history of notional fluid particles from the soot onset location back to the injector orifice reveals that their trajectories evolve along rich conditions (φ > 1.5) throughout the entire path. Overall, novel insights obtained from experiments with respect to soot and soot precursor evolutions are complemented by simulations using the integrated conditional moment closure/soot modeling approach, showing encouraging results for prediction and understanding of transient soot processes in high-pressure diesel sprays. [ABSTRACT FROM AUTHOR]