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Premixed Charge Compression Ignition (PCCI), a Low Temperature Combustion (LTC) strategy for diesel engines is of increasing interest due to its potential to simultaneously reduce soot and NOx emissions. However, the influence of mixture preparation on combustion phasing and heat release rate in LTC is not fully understood. In the present study, the influence of injection timing on mixture preparation, combustion and emissions in PCCI mode is investigated by experimental and computational methods. A sequential coupling approach of 3D CFD with a Stochastic Reactor Model (SRM) is used to simulate the PCCI engine. The SRM accounts for detailed chemical kinetics, convective heat transfer and turbulent micro-mixing. In this integrated approach, the temperature-equivalence ratio statistics obtained using KIVA 3V are mapped onto the stochastic particle ensemble used in the SRM.

Although soot-formation processes in diesel engines have been well characterized during the mixing-controlled burn, little is known about the distribution of soot throughout the combustion chamber after the end of appreciable heat release during the expansion and exhaust strokes. Hence, the laser-induced incandescence (LII) diagnostic was developed to visualize the distribution of soot within an optically accessible single-cylinder direct-injection diesel engine during this period. The developed LII diagnostic is semi-quantitative; i.e., if certain conditions (listed in the Appendix) are true, it accurately captures spatial and temporal trends in the in-cylinder soot field. The diagnostic features a vertically oriented and vertically propagating laser sheet that can be translated across the combustion chamber, where “vertical” refers to a direction parallel to the axis of the cylinder bore.

Computational fluid dynamics of gas-fueled large-bore spark ignition engines with pre-chamber ignition can speed up the design process of these engines provided that 1) the reliability of the results is not affected by poor meshing and 2) the time cost of the meshing process does not negatively compensate for the advantages of running a computer simulation. In this work a flame propagation model that runs with arbitrary hybrid meshes was developed and coupled with the KIVA4-MHI CFD solver, in order to address these aims. The solver follows the G-Equation level-set method for turbulent flame propagation by Tan and Reitz, and employs improved numerics to handle meshes featuring different cell types such as hexahedra, tetrahedra, square pyramids and triangular prisms. Detailed reaction kinetics from the SpeedCHEM solver are used to compute the non-equilibrium composition evolution downstream and upstream of the flame surface, where chemical equilibrium is instead assumed.

Fundamental understanding of the sources of fuel-derived Unburned Hydrocarbon (UHC) emissions in heavy duty diesel engines is a key piece of knowledge that impacts engine combustion system development. Current emissions regulations for hydrocarbons can be difficult to meet in-cylinder and thus after treatment technologies such as oxidation catalysts are typically used, which can be costly. In this work, Computational Fluid Dynamics (CFD) simulations are combined with engine experiments in an effort to build an understanding of hydrocarbon sources. In the experiments, the combustion system design was varied through injector style, injector rate shape, combustion chamber geometry, and calibration, to study the impact on UHC emissions from mixing-controlled diesel combustion.

Gasoline compression ignition (GCI) technology shows the potential to obtain high thermal efficiencies while maintaining low soot and NOx emissions in light-duty engine applications. Recent experimental studies and numerical simulations have indicated that high reactivity gasoline-like fuels can further enable the benefits of GCI combustion. However, there is limited empirical data in the literature studying the gasoline compression ignition process at relevant in-cylinder conditions, which are required for further optimizing combustion system designs. This study investigates the temporal and spatial evolution of the compression ignition process of various high reactivity gasoline fuels with research octane numbers (RON) of 71, 74 and 82, as well as a conventional RON 97 E10 gasoline fuel. A ten-hole prototype gasoline injector specifically designed for GCI applications capable of injection pressures up to 450 bar was used.

High-efficiency, clean-combustion strategies for heavy-duty diesel engines are critical for meeting stringent emissions regulations and reducing the costs of aftertreatment systems that are currently required to meet these regulations. Results from previous constant-volume combustion-vessel experiments using a single jet of fuel under quiescent conditions have shown that mixing-controlled soot-free combustion (i.e., combustion where soot is not produced) is possible with #2 diesel fuel. These experiments employed small injector-orifice diameters (≺ 150 μm) and high fuel-injection pressures (≻ 200 MPa) at top-dead-center (TDC) temperatures and densities that could be achievable in modern heavy-duty diesel engines.

The lift-off length plays a significant role in spray combustion as it influences the air entrainment upstream of the lift-off location and hence the soot formation. Accurate prediction of lift-off length thus becomes a prerequisite for accurate soot prediction in lifted flames. In the present study, KIVA-3v coupled with CHEMKIN, as developed at the Engine Research Center (ERC), is used as the CFD model. Experimental data from the Sandia National Labs. is used for validating the model predictions of n-heptane lift-off lengths and soot formation details in a constant volume combustion chamber. It is seen that the model predictions, in terms of lift-off length and soot mass, agree well with the experimental results for low ambient density (14.8 kg/m3) cases with different EGR rates (21% O2 - 8% O2). However, for high density cases (30 kg/m3) with different EGR rates (15% O2 - 8% O2) disagreements were found.

Multi-zone CFD simulations with detailed kinetics were used to model iso-octane HCCI experiments performed on a single-cylinder research engine. The modeling goals were to validate the method (multi-zone combustion modeling) and the reaction mechanism (LLNL 857 species iso-octane) by comparing model results to detailed exhaust speciation data, which was obtained with gas chromatography. The model is compared to experiments run at 1200 RPM and 1.35 bar boost pressure over an equivalence ratio range from 0.08 to 0.28. Fuel was introduced far upstream to ensure fuel and air homogeneity prior to entering the 13.8:1 compression ratio, shallow-bowl combustion chamber of this 4-stroke engine. The CFD grid incorporated a very detailed representation of the crevices, including the top-land ring crevice and head-gasket crevice. The ring crevice is resolved all the way into the ring pocket volume. The detailed grid was required to capture regions where emission species are formed and retained.

As the incentive to produce cleaner and more efficient engines increases, diesel engines will become a primary, worldwide solution. Producing diesel engines with higher efficiency and lower emissions requires a fundamental understanding of the interaction of the injected fuel with air as well as with the surfaces inside the combustion chamber. One aspect of this interaction is spray impingement on the piston surface. Impingement on the piston can lead to decreased combustion efficiency, higher emissions, and piston damage due to thermal loading. Modern high-speed diesel engines utilize high pressure common-rail direct-injection systems to primarily improve efficiency and reduce emissions. However, the high injection pressures of these systems increase the likelihood that the injected fuel will impinge on the surface of the piston.

An engine cycle simulation code with detailed chemical kinetics has been developed to study Homogeneous Charge Compression Ignition (HCCI) combustion with hydrogen as the fuel. In order to attain adequate combustion duration, resulting from the self-accelerating nature of the chemical reaction, fuel and temperature inhomogeneities have been brought to the calculation by considering the combustion chamber to have various temperature and fuel distributions. Calculations have been done under various conditions including both perfectly homogeneous and inhomogeneous cases, changing the degree of inhomogeneity. The results show that intake gas temperature is more dominant on ignition timing of HCCI than equivalence ratio and that there is a possibility to control HCCI by introducing appropriate temperature inhomogeneity to in-cylinder mixture.

The Diesel engine is a commercially attractive powerplant, however it is noted to have significant specific output of harmful emissions under some operating conditions. One possible solution for reduction of the harmful emissions from the Diesel engine is greater control over the fuel injection event. To gain further understanding of liquid phase Diesel fuel injection spray characteristics, a 2.44 liter displacement, 4 stroke engine was modified for optical access and fitted with a Caterpillar Hydraulic Electronic Unit Injection (HEUI) system. The data collection system consisted of a high repetition rate diode pumped Nd:YAG laser frequency doubled to 532 nm for visible illumination and a Kodak High Speed Motion Analyzer for recording fuel spray images. The engine was motored under various inlet conditions to create an engine combustion chamber environment typical of those found in commercial engines of similar per cylinder displacement class.

In this work, linear, non-linear and a generalized renormalization group (RNG) two-equation RANS turbulence models of the k-epsilon form were compared for the prediction of turbulent compressible flows in diesel engines. The object-oriented, multidimensional parallel code FRESCO, developed at the University of Wisconsin, was used to test the alternative models versus the standard k-epsilon model. Test cases featured the academic backward facing step and the impinging gas jet in a quiescent chamber. Diesel engine flows featured high-pressure spray injection in a constant volume vessel from the Engine Combustion Network (ECN), as well as intake flows in a high-swirl diesel engine. For the engine intake flows, a model of the Sandia National Laboratories 1.9L light-duty single cylinder optical engine was used.

The characteristics of gasoline sprayed directly into combustion chambers are of critical importance to engine out emissions and combustion system development. The optimization of the spray characteristics to match the in-cylinder flow field, chamber geometry, and spark location is a vital tasks during the development of an engine combustion strategy. Furthermore, the presence of liquid fuel during combustion in Spark-Ignition (SI) engines causes increased hydro-carbon (HC) emissions. Euro 6, LEVIII, and US Tier 3 emissions regulations reduce the allowable particulate mass significantly from the previous standards. LEVIII standards reduce the acceptable particulate emission to 1 mg/mile. A good DISI strategy vaporizes the correct amount of fuel just in time for optimal power output with minimal emissions. The opening and closing phases of DISI injectors are crucial to this task as the spray produces larger droplets during both theses phases.

Intake port and cylinder flow have been modeled for a dual intake valve diesel engine. A block structured grid was used to represent the complex geometry of the intake port, valves, and cylinder. The calculations were made using a pre-release version of the KIVA-3 code developed at Los Alamos National Laboratories. Both steady flow-bench and unsteady intake calculations were made. In the flow bench configuration, the valves were stationary in a fully open position and pressure boundary conditions were implemented at the domain inlet and outlet. Detailed structure of the in-cylinder flow field set up by the intake flow was studied. Three dimensional particle trace streamlines reveal a complex flow structure that is not readily described by global parameters such as swirl or tumble. Streamlines constrained to lie in planes normal to the cylinder axis show dual vortical structures, which originated at the valves, merging into a single structure downstream.

The three-dimensional computer code, KIVA, is being modified to include state-of-the-art submodels for diesel engine flow and combustion. Improved and/or new submodels which have already been implemented are: wall heat transfer with unsteadiness and compressibility, laminar-turbulent characteristic time combustion with unburned HC and Zeldo'vich NOx, and spray/wall impingement with rebounding and sliding drops. Progress on the implementation of improved spray drop drag and drop breakup models, the formulation and testing of a multistep kinetics ignition model and preliminary soot modeling results are described. In addition, the use of a block structured version of KIVA to model the intake flow process is described. A grid generation scheme has been developed for modeling realistic (complex) engine geometries, and initial computations have been made of intake flow in the manifold and combustion chamber of a two-intake-valve engine.

This research focused on the effects of split injection on combustion in a diesel environment. It was done in a specially designed engine-fed combustion chamber (swirl ratio of 5) with full field optical access through a quartz window. The simulated engine combustion chamber used a special backwards spraying injector (105°). The electronically controlled injector could control the size and position of it's, two injections. Both injections were through the same nozzle and it produced very rapid injections (1.5 ms) with a maximum injection pressure of 130 MPa. Experimental data included: rate of injection, injector pressure, combustion chamber dumping (NO & NOx concentrations), flame temperature, KL factor (soot concentration) combustion pressure, and rate of pressure rise. Injection rates indicate that the UCORS injection system creates very rapid injections with the ability to produce controllable split injections.

A new technique which is based on optoacoustic phenomena has been developed for measuring in-cylinder gas temperature and turbulent diffusivity. In the experiments, a high energy Nd:YAG pulsed laser beam was focused to cause local ionization of air at a point in the combustion chamber. This initiates a shock wave and creates a hot spot. The local temperature and turbulent diffusivity are determined by monitoring the shock propagation and the hot spot growth, respectively, with a schlieren photography system. In order to assess the validity and accuracy of the measurements, the technique was also applied to a turbulent jet. The temperature measurements were found to be accurate to within 3%. Results from the turbulent jet measurements also showed that the growth rate of the hot spot diameter can be used to estimate the turbulent diffusivity. In-cylinder gas temperature measurements were made in a motored single cylinder Caterpillar diesel engine, modified for optical access.

This research focused on the effects of split injection on fuel spray behavior in a diesel environment. It was done in a special designed engine-fed combustion chamber (swirl ratio of 5) with full field optical access through a quartz window. The simulated engine combustion chamber used a special backwards spraying injector (105°). The electronically controlled injector could control the size and position of it's two injections. Both injections were through the same nozzle and it produced very rapid injections (1.5 ms) with a maximum injection pressure of 130 MPa. Experimental data included: rate of injection, injector pressure, spray plume images, tip penetration, liquid and vapor fuel distributions, combustion pressure, and rate of pressure rise. From 105° forward scatter images, tip penetration was observed to be very rapid and reached a plateau at 25 mm.

A three-dimensional computer code (KIVA) is being modified to include state-of-the-art submodels for diesel engine flow and combustion: spray atomization, drop breakup/coalescence, multi-component fuel vaporization, spray/wall interaction, ignition and combustion, wall heat transfer, unburned HC and NOx formation, soot and radiation and the intake flow process. Improved and/or new submodels which have been completed are: wall heat transfer with unsteadiness and compressibility, laminar-turbulent characteristic time combustion with unburned HC and Zeldo'vich NOx, and spray/wall impingement with rebounding and sliding drops.

An image acquisition-and-processing camera system was developed for in-cylinder diagnostics of a single-cylinder heavy duty diesel engine. The engine was equipped with an electronically-controlled common-rail fuel injection system that allowed both single and split (multiple) injections to be studied. The imaging system uses an endoscope to acquire luminous flame images from the combustion chamber and ensures minimum modification to the engine geometry. The system also includes an optical linkage, an image intensifier, a CID camera, a frame grabber, control circuitry and a computer. Experiments include both single and split injection cases at 90 MPa and 45 MPa injection pressures at 3/4 load and 1600 rev/min with simulated turbocharging. For the single injection at high injection pressure (90 MPa) the results show that the first luminous emissions from the ignition zone occur very close to the injector exit followed by rapid luminous flame spreading.