Search Results

An integrated numerical model has been developed for diesel engine computations based on the KIVA-II code. The model incorporates a modified RNG k-ε, turbulence model, a ‘wave’ breakup spray model, the Shell ignition model, the laminar-and-turbulent characteristic-time combustion model, a crevice flow model, a spray/wall impingement model that includes rebounding and breaking-up drops, and other improved submodels in the KIVA code. The model was validated and applied to model successfully different types of diesel engines under various operating conditions. These engines include a Caterpillar engine with different injection pressures at different injection timings, a small Tacom engine at different loads, and a Cummins engine modified by Sandia for optical experiments. Good levels of agreement in cylinder pressures and heat release rate data were obtained using the same computer model for all engine cases.

Progress on the development and validation of a CFD model for diesel engine combustion and flow is described. A modified version of the KIVA code is used for the computations, with improved submodels for liquid breakup, drop distortion and drag, spray/wall impingement with rebounding, sliding and breaking-up drops, wall heat transfer with unsteadiness and compressibility, multistep kinetics ignition and laminar-turbulent characteristic time combustion models, Zeldovich NOx formation, and soot formation with Nagle Strickland-Constable oxidation. The code also considers piston-cylinder-liner crevice flows and allows computations of the intake flow process in the realistic engine geometry with two moving intake valves. Significant progress has been made using a modified RNG k-ε turbulence model, and a multicomponent fuel vaporization model and a flamelet combustion model have been implemented.

A computer model developed for describing multicomponent fuel vaporization, and ignition in diesel engines has been applied in this study to understand cold-starting and the parameters that are of significant influence on this phenomena. This research utilizes recent improvements in spray vaporization and combustion models that have been implemented in the KIVA-II CFD code. Typical engine fuels are blends of various fuels species, i.e., multicomponent. Thus, the original single component fuel vaporization model in KIVA-II was replaced by a multicomponent fuel vaporization model (based on the model suggested by Jin and Borman). The modelhas been extended to model diesel sprays under typical diesel conditions, including the effect of fuel cetane number variation. Necessary modifications were carried out in the atomization and collision sub-models. The ignition model was also modified to account for fuel composition effects by modifying the Shell ignition model.

Characterization of high pressure diesel sprays has been performed both experimentally and numerically. The experimental study was conducted using a fuel injection system which has a capability of producing multiple injection sprays. The fuel sprays were injected from a multi-hole nozzle into a pressurized cylindrical chamber with optical windows. In order to investigate the effects of a multiple injection strategy on spray characteristics, a double injection spray with the mass evenly distributed between the first and second sprays, and a 1 millisecond dwell between sprays was compared with a single injection spray. Both single and double injection cases had nominally the same injection pressure, injection delivery, and ambient gas density. Transient spray tip penetration lengths and spray angles were obtained from high speed photographic spray images. The spray droplet sizes were derived from the images by using a light extinction method.

An engine emissions and performance study was conducted in conjunction with a series of experiments using a constant volume cold spray chamber. The purpose of the study was to explore the effects of number of holes and hole size on the emissions and performance of a direct injection heavy duty diesel engine. The spray experiments provide insight into the spray parameters and their role in the engine's combustion processes. The fuel system used for both the engine and spray chamber experiments was an electronically controlled, common rail injector. The injector nozzle hole size and number combinations used in the experiments included 225X8 (225 gm diameter holes with 8 holes in the nozzle), 260X6, 260X8, and 30OX6. The engine tests were conducted on an instrumented single cylinder version of the Caterpillar 3400 series heavy duty diesel engine. Data were taken with the engine running at 1600 RPM, 75% load.

This research uses computational modeling to explore methods to increase diesel engine power density while maintaining low pollutant emission levels. Previous experimental studies have shown that injection-rate profiles and injector configurations play important roles on the performance and emissions of particulate and NOx in DI diesel engines. However, there is a lack of systematic studies and fundamental understanding of the mechanisms of spray atomization, mixture formation and distribution, and subsequently, the combustion processes in spray/spray and spray/swirl interaction and flow configurations. In this study, the effects of split injections and multiple injector configurations on diesel engine emissions are investigated numerically using a multidimensional computer code. In order to be able to explore the effects of enhanced fuel-air mixing, the use of multiple injectors with different injector locations, spray orientations and impingement angles was studied.

Multidimensional computations were carried out to simulate the in-cylinder fuel/air mixing process of a direct-injection spark-ignition engine using a modified version of the KIVA-3 code. A hollow cone spray was modeled using a Lagrangian stochastic approach with an empirical initial atomization treatment which is based on experimental data. Improved Spalding-type evaporation and drag models were used to calculate drop vaporization and drop dynamic drag. Spray/wall impingement hydrodynamics was accounted for by using a phenomenological model. Intake flows were computed using a simple approach in which a prescribed velocity profile is specified at the two intake valve openings. This allowed three intake flow patterns, namely, swirl, tumble and non-tumble, to be considered. It was shown that fuel vaporization was completed at the end of compression stroke with early injection timing under the chosen engine operating conditions.

A numerical study of air-fuel mixing in a direct-injection spark-ignition engine was carried out. In this paper, the numerical models are described and grid generation methods to represent a realistic port-valve-chamber geometry is discussed. To model a vaporizing hollow-cone spray resulting from an automotive pressure-swirl injector, a newly developed sheet spray atomization model was used to compute the processes of disintegration of the liquid sheet and breakup of the subsequent drops. Computations were performed of a particular 4-valve pent-roof engine configuration in which the intake process and an early fuel injection scheme were considered. After an analysis of the intake-generated flow structures in this engine configuration, the spray behavior and the spatial and temporal evolution of fuel liquid and vapor phases are characterized.

A, three-dimensional CFD code, based on the KIVA code, is used to explore alternatives to conventional DI diesel engine designs for reducing NOx and soot emissions without sacrificing engine performance. The effects of combustion chamber design and fuel spray orientation are investigated using a new proposed GAMMA engine concept, and two new multiple injector combustion system (MICS) designs which utilize multiple injectors to increase gas motion and enhance fuel/air mixing in the combustion chamber. From these computational studies, it is found that both soot and nitrous oxide emissions can be significantly reduced without the need for more conventional emission control strategies such as EGR or ultra high injection pressure. The results suggest that CFD models can be a useful tool not only for understanding combustion and emissions production, but also for investigating new design concepts.

Liquid fuel penetration was measured using an endoscopebased imaging system in an operating single-cylinder heavy-duty direct injection diesel engine with simulated turbocharging. Sprays were imaged via the elastic backscatter technique without significantly altering the engine geometry. Light loads (or pilot injections) were also studied because the spray breakup, mixing and vaporization processes can be isolated since they are less influenced by heat feedback from the flame than in a full injection case. The pilot injections included cases with three different fuel amounts (10%, 15% and 20% of the fuel injected in the baseline case, i.e., 75% load and 1600 rev/min) with different start-of-injection timings. Maximum liquid penetration lengths beyond which the fuel is completely vaporized were observed for all the cases studied. The maximum lengths varied from 23 mm to 28 mm for the different start-of-injection timings.

The paper presents the development of a novel approach to the solution of detailed chemistry in internal combustion engine simulations, which relies on the analytical computation of the ordinary differential equations (ODE) system Jacobian matrix in sparse form. Arbitrary reaction behaviors in either Arrhenius, third-body or fall-off formulations can be considered, and thermodynamic gas-phase mixture properties are evaluated according to the well-established 7-coefficient JANAF polynomial form. The current work presents a full validation of the new chemistry solver when coupled to the KIVA-4 code, through modeling of a single cylinder Caterpillar 3401 heavy-duty engine, running in two-stage combustion mode.

A numerical simulation and optimization study was conducted for a medium speed direct injection diesel engine. The engine's operating characteristics were first matched to available experimental data to test the validity of the numerical model. The KIVA-3V ERC CFD code was then modified to allow independent spray events from two rows of nozzle holes. The angular alignment, nozzle hole size, and injection pressure of each set of nozzle holes were optimized using a micro-genetic algorithm. The design fitness criteria were based on a multi-variable merit function with inputs of emissions of soot, NOx, unburned hydrocarbons, and fuel consumption targets. Penalties to the merit function value were used to limit the maximum in-cylinder pressure and the burned gas temperature at exhaust valve opening. The optimization produced a 28.4% decrease in NOx and a 40% decrease in soot from the baseline case, while giving a 3.1% improvement in fuel economy.

A study of statistical optimization of engine operating parameters was conducted. The objective of the study was to develop a strategy to efficiently optimize operating parameters of diesel engines with multiple injection and EGR capabilities. Previous studies have indicated that multiple injections with EGR can provide substantial simultaneous reductions in emissions of particulate and NOx from heavy-duty diesel engines, but careful optimization of the operating parameters is necessary in order to receive the full benefit of these combustion control techniques. The goal of the present study was to optimize the control parameters to reduce emissions and brake specific fuel consumption. An instrumented single-cylinder heavy-duty diesel engine was used with a prototype mechanically actuated (cam driven) fuel injection system.

An electronically controlled Caterpillar single-cylinder oil test engine (SCOTE) was used to study diesel combustion. The SCOTE retains the port, combustion chamber, and injection geometry of the production six cylinder, 373 kW (500 hp) 3406E heavy-duty truck engine. The engine was equipped with an electronic unit injector and an electronically controlled common rail injector that is capable of multiple injections. An emissions investigation was carried out using a six-mode cycle simulation of the EPA Federal Transient Test Procedure. The results show that the SCOTE meets current EPA mandated emissions levels, despite the higher internal friction imposed by the single-cylinder configuration. NOx versus particulate trade-off curves were generated over a range of injection timings for each mode and results of heat release calculations were examined, giving insight into combustion phenomena in current “state of the art” heavy-duty diesel engines.

A study was performed that takes into account both turbulence and chemical kinetic effects in the numerical simulation of diesel engine combustion in order to better understand the importance of their respective roles at changing operating conditions. An approach was developed which combines the simplicity and low computational and storage requests of the laminar-and-turbulent characteristic-time model with a detailed combustion chemistry model based on well-known simplified mechanisms. Assuming appropriate simplifications such as steady state or equilibrium for most of the radicals and intermediate species, the kinetics of hydrocarbons can be described by means of three overall steps. This approach was integrated in the KIVA-II code. The concept was validated and applied to a single-cylinder, heavy-duty engine. The simulation covers a wide range of operating conditions.