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Journal Article

Experiments and Modeling of Adaptive Injection Strategies (AIS) in Low Emissions Diesel Engines

Homogeneous Charge Compression Ignition (HCCI) has been shown as a promising technique for simultaneous NOx and soot reduction while maintaining diesel-like efficiency. Although HCCI has been shown to yield very low emissions levels, spray-wall impingement and high pressure rise rates can be problematic due to the early injection timings necessary for certain HCCI operations. To address spray-wall impingement, an Adaptive Injection Strategy (AIS) was employed. This strategy uses multiple pulses at both low and high injection pressures to prepare an optimal in-cylinder mixture. A unique Variable Pressure Pulse (VPP) was developed to investigate the AIS concept experimentally. The VPP has the capability of delivering multiple injections at both low and high injection pressures (∼100 bar and ∼1000 bar respectively) through a single injector in the same engine cycle. Comparisons were made between model predictions and engine experiments using the VPP system.
Journal Article

Development of an Improved NOx Reaction Mechanism for Low Temperature Diesel Combustion Modeling

The development of a new Nitric Oxide (NOx) reaction mechanism has been conducted by adding species, including hydrogen cyanide (HCN) and the CH radical to a reduced chemistry diesel combustion model. The additional chemical reactions were added to the ERC's reduced 12-step NOx mechanism, which consists of N, NO, N2O, and NO2. The new NOx mechanism was implemented into the KIVA/ERC-CHEMKIN code and was found to be able to predict the experimentally observed trend that the amount of engine-out NOx decreases as engine load is increased, which is not reproduced by the current reduced NOx mechanism. HCN and CH were found to be species that bridge CxHy products and N radicals via the reaction CH+N2→HCN+N under high equivalence ratio conditions, and Zeldovich NO formation is suppressed by the formation of HCN, a species in the Fenimore NO formation pathway. The additional species and reactions were also found to influence the prediction of engine-out soot emissions.
Technical Paper

A Numerical Investigation of Nozzle Geometry and Injection Condition Effects on Diesel Fuel Injector Flow Physics

A three-dimensional homogeneous equilibrium model (HEM) has been developed and implemented into an engine computational fluid dynamics (CFD) code KIVA-3V. The model was applied to simulate cavitating flow within injector nozzle passages. The effects of nozzle passage geometry and injection conditions on the development of cavitation zones and the nozzle discharge coefficient were investigated. Specifically, the effects of nozzle length (L/D ratio), nozzle inlet radius (R/D ratio) and K or KS factor (nozzle passage convergence) were simulated, and the effects of injection and chamber pressures, and time-varying injection pressure were also investigated. These effects are well captured by the nozzle flow model, and the predicted trends are consistent with those from experimental observations and theoretical analyses.
Technical Paper

An Experimental Investigation of Partially Premixed Combustion Strategies Using Multiple Injections in a Heavy-Duty Diesel Engine

Optimizations were performed on a single-cylinder heavy-duty Caterpillar SCOTE 3401E engine for NOx, PM and BSFC reductions. The engine was equipped with a Caterpillar 300B HEUI fuel injection system capable of up to four injections with timings from 90 BTDC to 90 ATDC. The engine was operated at a medium load (57%), high speed (1737 rev/min) operation point. A micro-genetic algorithm was utilized to optimize a hybrid, double-injection strategy, which incorporated an early, premixed pilot injection with a late main injection. The fuel injection parameters, intake boost pressure, and EGR were considered in the optimization. The optimization produced a parameter set that met the 2007 and 2010 PM emissions mandate of 0.0134 g/kW-hr, and was within the 1.5x not to exceed NOx + HC mandate of 2.694 g/kW-hr. Following the optimization exercise, further parametric interaction studies were performed to reveal the underlying interactions and phenomena.
Technical Paper

Soot Structure in a Conventional Non-Premixed Diesel Flame

An analysis of the soot formation and oxidation process in a conventional direct-injection (DI) diesel flame was conducted using numerical simulations. An improved multi-step phenomenological soot model that includes particle inception, particle coagulation, surface growth and oxidation was used to describe the soot formation and oxidation process. The soot model has been implemented into the KIVA-3V code. Other model Improvements include a piston-ring crevice model, a KH/RT spray breakup model, a droplet wall impingement model, a wall-temperature heat transfer model, and the RNG k-ε turbulence model. The Shell model was used to simulate the ignition process, and a laminar-and-turbulent characteristic time combustion model was used for the post-ignition combustion process. Experimental data from a heavy-duty, Cummins N14, research DI diesel engine operated with conventional injection under low-load conditions were selected as a benchmark.
Technical Paper

Modeling the Effects of EGR and Injection Pressure on Soot Formation in a High-Speed Direct-Injection (HSDI) Diesel Engine Using a Multi-Step Phenomenological Soot Model

Low-temperature combustion concepts that utilize cooled EGR, early/retarded injection, high swirl ratios, and modest compression ratios have recently received considerable attention. To understand the combustion and, in particular, the soot formation process under these operating conditions, a modeling study was carried out using the KIVA-3V code with an improved phenomenological soot model. This multi-step soot model includes particle inception, surface growth, surface oxidation, and particle coagulation. Additional models include a piston-ring crevice model, the KH/RT spray breakup model, a droplet wall impingement model, a wall heat transfer model, and the RNG k-ε turbulence model. The Shell model was used to simulate the ignition process, and a laminar-and-turbulent characteristic time combustion model was used for the post-ignition combustion process.
Technical Paper

Optimizing HSDI Diesel Combustion and Emissions Using Multiple Injection Strategies

Multiple injection strategies have been experimentally and computationally studied for simultaneously reducing diesel engine NOx and particulate emissions. However, injection strategies featuring three or more pulses per engine cycle have not been sufficiently studied previously. The large number of parameters to be considered, in addition to the complicated interactions among them, challenge the capability of experimental hardware, computational models, and optimization methods. In the present work, multiple injection strategies including up to five pulses per engine cycle, are computationally investigated to optimize High Speed Direct Injection (HSDI) diesel engine combustion and emissions at a single part-load operating condition. The KIVA-3V code coupled with a Genetic Algorithm were used as the modeling and optimization tools, respectively. It was found that widely separated injection with two-stage combustion appears to provide optimal HSDI diesel performance at part load.