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The non-premixed turbulent combustion and emission formation in a modern DI diesel engine relies mostly on the mixture formation process induced by the diesel fuel spray. Therefore the numerical simulation of this process has to incorporate accurate spray modeling which captures the physics of the spray formation, propagation and vaporization. A widely used framework for spray modeling is the Discrete Droplet Model (DDM) which also is applied in the present work. In the DDM framework, separate submodels account for droplet breakup, droplet-droplet interaction and evaporation. Due to the empirical nature of these submodels (particularly droplet breakup and collision) necessitated by an incomplete representation of the physics, and by the inability to isolate each process under diesel engine relevant conditions, some of the constants controlling the outcomes of these submodels require calibration.

Superior fuel economy was achieved for a small-displacement spark-ignition direct-injection (SIDI) engine by optimizing the stratified combustion operation. The optimization was performed using computational analyses and subsequently testing the most promising configurations experimentally. The fuel economy savings are achieved by the use of a multihole injector with novel spray shape, which allows ultra-lean stratification for a wide range of part-load operating conditions without compromising smoke and hydrocarbon emissions. In this regard, a key challenge for wall-controlled SIDI engines is the minimization of wall wetting to prevent smoke, which may require advanced injection timings, while at the same time minimizing hydrocarbon emissions, which may require retarding injection and thereby preventing over-mixing of the fuel vapor.

Full-load operation of a small-displacement spark-ignition direct-injection (SIDI) engine was thoroughly investigated by means of computational analysis and engine measurements. The performance is affected by many different factors, which can be grouped as those pertaining to volumetric efficiency, to mixing and stratification, and to system issues, respectively. Volumetric efficiency is affected by flow losses, tuning and charge cooling. Charge cooling due to spray vaporization is often touted as the most significant benefit of direct-injection on full-load performance. However, if wall wetting occurs, this benefit may be completely negated or even reversed. The fuel-air mixing is strongly affected by the injection timing and characteristics at lower engine speeds, while at higher engine speeds the intake flow dominates the transport of fuel particles and resultant vapor distribution. A higher injector flow rate enhances mixing especially at higher engine speeds.

A comprehensive modeling and visualization study of flow and combustion is reported for a production four-valve-per-cylinder homogeneous-charge four-stroke-cycle spark-ignited engine. Coupled port and in-cylinder computations are presented for five combinations of valve deactivation, valve shrouding, and cam profile. Motored (induction and compression) results are compared with transient-water-analog flow-structure visualizations. A new flamelet model for homogeneous-charge turbulent premixed combustion has been implemented for fired engine simulations. Several issues in the application of CFD to flow and combustion modeling in practical port-and-cylinder systems are addressed. These include numerical inaccuracy, elucidation of the role of induction-generated flow structure and turbulence, and new insights into premixed flame propagation.

A multidimensional model of scavenging in a loop-scavenged two-stroke-cycle engine has been validated through detailed comparisons between computed and measured mean and rms velocities in a commercially available engine under motored operating conditions. The laser-Doppler velocimetry (LDV) measurement database constitutes one of the most complete characterizations of in-cylinder flows currently available. A novel feature of the calculations is that flow in the coupled intake-port/in-cylinder/exhaust-port system including a simplified crankcase is calculated. Through systematic variations in initial conditions and geometric configuration, the modeling suggests plausible explanations for some of the (at first sight) unexpected features found in the LDV measurements.