Eric Kurtz

Modeling of Diesel Combustion, Soot and NO Emissions Based on a Modified Eddy Dissipation Concept

A three-dimensional reacting flow modeling approach is presented for predictions of compression ignition, combustion, NOx and soot emissions over a wide range of operating conditions in a diesel engine. The ignition/combustion model is based on a modified eddy dissipation concept (EDC) which has been implemented into the KIVA-3 V engine simulation code. The modified EDC model is used to represent the thin sub-grid level reaction zone and the small scale molecular mixing processes. In addition, a realistic transition model based on the local normalized fuel mass fraction is implemented to shift from ignition to combustion. The modified EDC model is combined with skeletal n-heptane chemistry and a soot dynamics model, which includes nucleation, surface growth and oxidation and coagulation processes. The NO formation and destruction processes are based on the extended Zeldovich reaction mechanism. The modeling results are calibrated against experimental engine data taken at benchmark conditions. The model is subsequently used to conduct parametric studies of the effects of injection timing and exhaust gas recirculation (EGR) on engine combustion and emissions. Predictions of cylinder pressure traces and heat release rates are in very good agreement with the experimental data (e.g., pressure predictions within 3 bar of the experimental data) for a range of injection timings, EGR rates and speeds. The experimental trends observed for the soot and NO emissions are also reproduced by the modeling results. Overall, the modeling approach demonstrates promising predictive capabilities at reasonable computational costs.

IDENTIFYING A CRITICAL TIME FOR MIXING IN A DIRECT INJECTION DIESEL ENGINE THROUGH THE STUDY OF INCREASED IN-CYLINDER MIXING AND ITS EFFECT ON EMISSIONS

Abstract An experimental study aimed at understanding the effect of in-cylinder mixing on direct injection diesel combustion was conducted in which an auxiliary gas injector was installed on to a single-cylinder diesel engine. The gas injector was used to inject air and nitrogen, thereby enhancing in-cylinder mixing at a specific time in the combustion cycle. The phasing of that gas injection event with respect to the fuel injection event was changed. The results suggest that it is primarily the fuel injected at the tail end of the fuel injection event that has difficulty mixing. Prior to the end of fuel injection, the kinetic energy input during fuel injection is sufficient to mix the fuel with air. However, near the end of fuel injection the kinetic energy is low and the fuel is not followed by any kinetic energy inputs that could help bring about mixing, which causes that fuel to produce large amounts of soot that ultimately have trouble oxidizing. It was also found that in-cylinder turbulence had no effect on NOx emissions when it occurred after +20° ATDC (after top dead centre), just after the end of fuel injection in these experiments. It was believed that, by that time, the NOx chemistry was frozen. However, NOx emissions were affected by early gas injection such that injecting air brought about an increase in NOx emissions while injecting nitrogen reduced NOx emissions. This finding suggests that early and prolonged injection of diluents could be used as a soot-friendly form of exhaust gas recirculation, provided that the injection overlaps the end of fuel injection.

Correlation of cylinder pressure–based engine noise metrics to measured microphone data

Peak pressure rise rate, ringing intensity, and combustion noise analysis are typically cited measures of the noise generated by combustion in an internal combustion engine. In this publication, peak pressure rise rate, ringing intensity, and the combustion noise calculation results are compared to microphone data over a wide range of engine operating conditions for one gasoline and one diesel engine. Multiple methods for calculation of peak pressure rise rate are explored, including the use of different time steps for gradient calculation, calculation of weighted gradients, application of various digital filters, and ensemble averaging of the pressure signal. The value of peak pressure rise rate was found to vary substantially depending on the data processing method used. Similar sensitivity to the data processing method was observed with ringing intensity. Combustion noise analysis provided the best correlation to microphone noise, especially at lower speeds where engine noise is dominated by combustion noise rather than mechanical noise. No correlation between crank angle based peak pressure rise rate and microphone data was found. The correlation between ringing intensity and microphone noise was very similar to the correlation between time-based peak pressure rise rate and microphone data. The correlation between ringing intensity and microphone noise (and between time-based peak pressure rise rate and microphone noise) was highly dependent on the method used to calculate the peak pressure rise rate. A method and a MATLAB function for calculating combustion noise for simulation data are provided in this study.

A Numerical Study on Diesel Engine Size-Scaling in Low Temperature Combustion Operation

A numerical study was conducted for diesel engine size-scaling in the low temperature combustion regime. Conventional scaling models were found to be inadequate for the low temperature combustion regime where chemical time scales become very important. Equal-time scaling was suggested to ensure the same chemical time scale for the scaled engines. With this approach, combustion phasing was well matched. However, peak in-cylinder pressure was higher in the upscaled engine, which is attributed to more homogeneous mixture conditions due to smaller surface to volume ratio (i.e., less wall friction). The overly enhanced mixing can be compensated with lower injection velocity.

Computational Optimization of a Down-Scaled Diesel Engine Operating in the Conventional Diffusion Combustion Regime Using a Multi-Objective Genetic Algorithm

Computational optimization of a high-speed diesel engine, combined with diesel engine size-scaling, is presented. A multi-objective genetic algorithm was employed to simultaneously optimize fuel consumption and engine-out emissions of the down-scaled version of a previously optimized baseline engine. By separating the design parameters into hardware parameters (e.g., the piston bowl geometry) and controllable parameters (e.g., injection pressure and timings), multiple operating conditions were optimized simultaneously. A new variable was introduced to evaluate the convergence of the optimization, defined as the ratio of the number of Pareto designs and the number of valid designs in each generation. Particular interest was placed on the effect of injection pressure on the optimization of the engine and whether the previously optimized baseline engine design holds for different engine sizes. For 32 generations, totaling 1024 designs, no better design than the initial optimum, which was generated for the baseline engine, was found. This indicates that the current engine size-scaling model works well.

Piston geometry effects in a light-duty, swirl-supported diesel engine: Flow structure characterization:

This work studied how in-cylinder flow structure is affected in a light-duty, swirl-supported diesel engine when equipped with three different piston geometries: the first two featuring a conventional re-entrant bowl, either with or without valve cut-outs on the piston surface and the third featuring a stepped-lip bowl. Particle image velocimetry experiments were conducted inside an optical engine to measure swirl vortex intensity and structure during the intake and compression strokes. A full computational model of the optical diesel engine was built using the FRESCO code, a recently developed object-oriented parallel computational fluid dynamics platform for engine simulations. The model was first validated against the measured swirl-plane velocity fields, and the simulation convergence for multiple cycles was assessed. Flow topology was studied by addressing bulk flow and turbulence quantities, including swirl structure, squish flux, plus geometric and operating parameters, such as the presence of valve cut-outs on the piston surface, compression ratio and engine speed. The results demonstrated that conventional re-entrant bowls have stronger flow separation at intake, hampering bowl swirl, but higher global swirl than for stepped-lip bowls thanks to a stronger and more axisymmetric squish mechanism and less tilted swirl. Stepped-lip bowls have larger inhomogeneities (tilt and axisymmetry) and higher turbulence levels, but also faster turbulence dissipation toward top dead center. They have weaker squish flux but larger squish inversion momentum as a result of the smaller inertia.

A comparison of computational fluid dynamics predicted initial liquid penetration using rate of injection profiles generated using two different measurement techniques

The rate of injection profile is a key parameter describing the fuel injection process for diesel injection. It is also an essential input parameter for computational fluid dynamics simulations of spray flows. In the present work, rate of injection profiles of a multi-hole diesel injector were measured using the Zeuch method and the momentum flux method. The rate of injection profiles measured by the momentum flux method had a faster rise in rate of injection during the initial ramp-up phase than with the Zeuch method. The measured rate of injection profiles were applied in three-dimensional computational fluid dynamics simulations of diesel sprays under non-vaporizing and vaporizing conditions with sweeps in injection pressure, bulk charge gas density, and bulk charge gas temperature. Analytical results were compared against experimental data for liquid penetration generated under those conditions. Computational fluid dynamics results with the rate of injection profile measured by the Zeuch method under-predict liquid penetration during the initial ramp-up phase, while computational fluid dynamics results with the rate of injection profiles measured by the momentum flux method showed much better agreement with the experimental data of liquid length and penetration. This suggests that current computational fluid dynamics spray models may be able to more accurately model transient liquid penetration when using the velocity profile developed from momentum flux measurements. Further study is needed to evaluate how computational fluid dynamics predictions of combustion and emissions of affected when using these two rate of injection profiles.

Development of a reduced tri-propylene glycol monomethyl ether– n -hexadecane–poly-aromatic hydrocarbon mechanism and its application for soot prediction

A reduced chemical kinetic mechanism for tri-propylene glycol monomethyl ether has been developed and applied to computational fluid dynamics calculations for predicting combustion and soot formation processes. The reduced tri-propylene glycol monomethyl ether mechanism was combined with a reduced n-hexadecane mechanism and a poly-aromatic hydrocarbon mechanism to investigate the effect of fuel oxygenation on combustion and soot emissions. The final version of the tri-propylene glycol monomethyl ether–n-hexadecane–poly-aromatic hydrocarbon mechanism consists of 144 species and 730 reactions and was validated with experiments in shock tubes as well as in a constant-volume spray combustion vessel from the Engine Combustion Network. The effects of ambient temperature, varying oxygen content in the tested fuels on ignition delay, spray lift-off length and soot formation under diesel-like conditions were analyzed and addressed using multidimensional reacting flow simulations and the reduced mechanism. The results show that the present reduced mechanism gives reliable predictions of the combustion characteristics and soot formation processes. In the constant-volume spray combustion vessel simulations, two important trends were identified. First, increasing the initial temperature in the constant-volume spray combustion vessel shortens the ignition delay and lift-off length and reduces the fuel–air mixing, thereby increasing the soot levels. Second, fuel oxygenation introduces more oxygen into the central region of a fuel jet and reduces residence times of fuel-rich area in active soot-forming regions, thereby reducing soot levels.

An evaluation of cetane sensitivity in low-temperature combustion and options to compensate for market cetane variation

A single-cylinder engine was used to assess the sensitivity of emissions, noise and fuel consumption to variation in cetane number when operated in low temperature combustion both with and without the application of compensation strategies. Without compensation, changes in cetane caused greater variability in all of the parameters studied when operated in low temperature combustion as compared to conventional diesel combustion. Correcting combustion phasing was explored by adjusting three different parameters: start of injection timing, burnt gas fraction and intake manifold temperature. None of these methods were able to achieve substantially equivalent combustion and emissions across all fuels evaluated. Large variations in hydrocarbon and particulate emissions were observed when start of injection was adjusted to correct combustion phasing. Adjusting the burnt gas fraction caused large differences in nitrogen oxides and noise, particularly with low cetane fuels. Finally, intake manifold temperatures low enough to reach the target combustion phasing with the higher cetane fuel were unachievable, suggesting that this method would not be viable as a production solution. It was concluded that the most effective way to ensure robust combustion and emissions in low temperature combustion is to reduce ignition delay variability by refining the fuel specification, either through tighter control over the cetane number range or by shifting the fuel specification toward higher cetane number, where ignition delay is less sensitive to changes in cetane rating.