Alok Warey

A One-Dimensional Model for Particulate Deposition and Hydrocarbon Condensation in Exhaust Gas Recirculation Coolers

Cooled exhaust gas recirculation (EGR) is widely used in diesel engines to control engine out NOx (oxides of nitrogen) emissions. A portion of the exhaust gases is recirculated into the intake manifold of the engine after cooling it through a heat exchanger. EGR cooler heat exchangers, however, tend to lose efficiency and have increased pressure drop as deposit forms on the heat exchanger surface. This adversely affects the combustion process, engine durability, and emissions. In this study, a 1-D model was developed to simulate soot deposition, soot removal, and condensation of several hydrocarbon (HC) species in a circular tube with turbulent gas flow at constant wall temperature. The circular tube, which makes up the computational domain in the model, represents a single channel from any EGR cooler geometry. The model takes into account soot particle deposition due to thermophoresis, diffusion, turbulent impaction, and gravitational drift. However, thermophoresis was found to be the most dominant deposition mechanism for boundary conditions at which EGR coolers typically operate. Soot removal was modeled by considering a force balance between the drag and van der Waals forces. A lognormal distribution of particles was assumed at the tube inlet. The evolution of the particle distribution in the bulk flow along the tube as well as the mass distribution in the deposit layer on the tube walls is predicted by the model. Condensation of six HC species between C15-C24 alkanes was also modeled. Predictions made by the model are in reasonably good agreement with experimental data obtained from a laboratory reactor under the same boundary conditions. There are several assumptions and simplifications built into the model, which can be refined further to improve it. Copyright 2012 American Association for Aerosol Research

Characterization of Soot Deposition and Particle Nucleation in Exhaust Gas Recirculation Coolers

Cooled exhaust gas recirculation (EGR) is used to control engine out NOx (oxides of nitrogen) emissions from modern diesel engines by re-circulating a portion of the exhaust gases into the intake manifold of an engine after cooling it through a heat exchanger commonly referred to as an EGR cooler. However, EGR cooler fouling due to presence of soot particles and hydrocarbons (HC) in engine exhaust leads to a decrease in cooler efficiency and increased pressure drop across the cooler. This can adversely affect the combustion process, engine durability, and emissions. In this study, a multicylinder diesel engine was used to produce a range of engine out HC and soot concentrations to investigate soot deposition and particle nucleation in an EGR cooler. A portion of the engine exhaust was passed through an EGR cooler, while particle size and HC concentration measurements were made at the cooler inlet and outlet. Tests were conducted over a range of EGR cooler coolant temperatures and engine out soot and HC concentrations to determine the impact on the nucleation and accumulation modes of the exhaust particle size distributions. A reduction in the accumulation mode particle concentration at the EGR cooler outlet was observed for high soot concentrations indicating soot deposition within the EGR cooler. As the EGR coolant temperature was reduced, the outlet accumulation mode particle concentration was reduced further, indicating increased soot deposition in the cooler due to increased thermophoresis. There were no signs of diffusiophoresis due to HC diffusion within the cooler over the range of conditions used in the study. A significant increase in outlet nucleation mode particle concentration was observed for the low soot concentrations. This mode increased with either increasing HC concentration or decreasing coolant temperature, indicating the saturation ratio (SR) dependence of the nucleation mode formation. However, as the soot concentration was increased, the nucleation mode disappeared because of HC adsorption onto the increased soot surface area. Copyright 2012 American Association for Aerosol Research

Deposition of Nanosized Soot Particles in Various EGR Coolers Under Thermophoretic and Isothermal Conditions

This study investigates the performance of various types of exhaust gas recirculation (EGR) coolers, that is, smooth tube, corrugated tube, and plate–fin, when subjected to particulate fouling by soot particles. Experiments were carried out for different temperature gradients of 170 and 320°C (thermophoretic) and 0°C (isothermal). Soot particles with an average diameter of 130 nm were produced by a soot generator. Experimental results showed that generally soot deposition under isothermal conditions is negligible compared to thermophoresis for any given cooler geometry, but is not universal. It may become appreciable when complex coolers with extended surfaces, that is, plate–fin type, are used due to impaction and settlement of soot particles onto the extended surfaces, which act as barrier to the flow. Contrariwise, under thermophoretic conditions, the plate–fin cooler performed best, followed by the corrugated tube and smooth tube cooler. Coolers with larger heat transfer surface area are also found to be less sensitive to the loss in effectiveness, but show a higher pressure drop.

A Combination of Swirl Ratio and Injection Strategy to Increase Engine Efficiency

The support of GM Global R&D and the Spanish Ministry of Economy and Competitiveness (TRA2014-58870-R,) is greatly acknowledged.

Onset of unsteady flow in wavy walled channels at low Reynolds number

Using computational modeling, we examine the development of an unsteady laminar flow of a Newtonian fluid in a channel with sinusoidal walls. The flow is driven by a constant pressure gradient. The simulations reveal two types of unsteady flows occurring in sinusoidal channels. When the amplitude of the wavy walls is relatively small, vortices forming in the channel furrows are shed downstream. For larger wall wave amplitudes, vortices remain inside the furrows and exhibit periodic oscillations and topological changes. We present a phase diagram in terms of wall amplitude and driving pressure gradient separating different flow regimes. Our simulations establish the optimum wall amplitude and period leading to an unsteady flow at the minimum pressure gradient. The results are important for designing laminar heat/mass exchangers utilizing unsteady flows for enhancing transport processes.

An Investigation of Radiation Heat Transfer in a Light-Duty Diesel Engine

In the last two decades engine research has been mainly focused on reducing pollutant emissions. This fact together with growing awareness about the impacts of climate change are leading to an increase in the importance of thermal efficiency over other criteria in the design of internal combustion engines (ICE). In this framework, the heat transfer to the combustion chamber walls can be considered as one of the main sources of indicated efficiency diminution. In particular, in modern direct-injection diesel engines, the radiation emission from soot particles can constitute a significant component of the efficiency losses. Thus, the main of objective of the current research was to evaluate the amount of energy lost to soot radiation relative to the input fuel chemical energy during the combustion event under several representative engine loads and speeds. Moreover, the current research characterized the impact of different engine operating conditions on radiation heat transfer. For this purpose, a combination of theoretical and experimental tools were used. In particular, soot radiation was quantified with a sensor that uses two-color thermometry along with its corresponding simplified radiation model. Experiments were conducted using a 4-cylinder direct-injection light-duty diesel engine fully instrumented with thermocouples. The goal was to calculate the energy balance of the input fuel chemical energy. Results provide a characterization of radiation heat transfer for different engine loads and speeds as well as radiation trends for different engine operating conditions.

Effect of in-cylinder swirl on engine efficiency and heat rejection in a light-duty diesel engine

During the last years, the growing awareness about the impacts of climate change lead to an increase in the importance of the efficiency over other criteria in the design of internal combustion engines. In this framework, the heat transfer to the combustion chamber walls can be considered as one of the main sources of indicated efficiency diminution. Hence, the main objective of this research is to thoroughly assess the effect of the swirl ratio on the heat rejection to the chamber walls, and thus on the efficiency, of a fully instrumented four-cylinder direct-injection diesel engine with variable swirl ratio (covering the range from 1.4 to 3). The analysis, based on the engine global energy balance, includes a combination of theoretical and experimental tools such as thermal flow measurement and dedicated thermocouples in the cylinder head and liner. Considering the results, it is shown that an increase in swirl ratio leads to a heat transfer enhancement, along with important changes on the combustion development. As a result of the combination of these two effects, it is shown that intermediate swirl ratios can slightly improve engine efficiency at low load, while increasing swirl worsens the combustion process and efficiency at high load. However, convective heat transfer increases about 3% of the fuel energy in the chamber when swirl ratio increases from 1.4 to 3. The heat rejection characterization is completed with the analysis of the wall temperatures. Despite the observed trends, heat transfer does not seem to be the only key issue for explaining the indicated and brake efficiencies, thus the pumping work plays an important role due to the effect of reducing the intake section to generate the swirl motion.

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.

Application of a zero-dimensional model to assess the effect of swirl on indicated efficiency

Increasing internal combustion engine efficiency continues being one of the main goals of engine research. To achieve this objective, different engine strategies are being developed continuously. However, the assessment of these techniques is not straightforward due to their influence on various intermediate phenomena inherent to the combustion process, which finally result in indicated efficiency trade-offs. During this work, a new methodology to assess these intermediate imperfections on gross indicated efficiency using a zero-dimensional model is developed. This methodology is applied to a swirl parametric study, where it has been concluded that the heat transfer and the rate of heat release are the single relevant changing phenomena. Results show that heat transfer always increases with swirl affecting negatively gross indicated efficiency (around −0.5%), while the impact of combustion velocity is not monotonous. It is enhanced up to a certain swirl ratio (it changes with engine speed) at low engine speed (resulting in an increment of +1.7% in gross indicated efficiency), but it is slowed down at high engine speed with the consequent worsening of gross indicated efficiency (−0.8%).

On the Reduction of Combustion Noise by a Close-Coupled Pilot Injection in a Small-Bore DI Diesel Engine

For a pilot-main injection strategy in a single cylinder light duty diesel engine, the dwell between the pilot- and main-injection events can significantly impact combustion noise. As the solenoid energizing dwell decreases below 200 μs, combustion noise decreases by approximately 3 dB and then increases again at shorter dwells. A zero-dimensional thermodynamic model has been developed to capture the combustion-noise reduction mechanism; heat-release profiles are the primary simulation input and approximating them as top-hat shapes preserves the noise-reduction effect. A decomposition of the terms of the underlying thermodynamic equation reveals that the direct influence of heat-release on the temporal variation of cylinder-pressure is primarily responsible for the trend in combustion noise. Fourier analyses reveal the mechanism responsible for the reduction in combustion noise as a destructive interference in the frequency range between approximately 1 kHz and 3 kHz. This interference is dependent on the timing of increases in cylinder-pressure during pilot heat-release relative to those during main heat-release. The mechanism by which combustion noise is attenuated is fundamentally different from the traditional noise reduction that occurs with the use of long-dwell pilot injections, for which noise is reduced primarily by shortening the ignition delay of the main injection. Band-pass filtering of measured cylinder-pressure traces provides evidence of this noise-reduction mechanism in the real engine.When this close-coupled pilot noise-reduction mechanism is active, metrics derived from cylinder-pressure such as the location of 50% heat-release, peak heat-release rates, and peak rates of pressure rise cannot be used reliably to predict trends in combustion noise. The quantity and peak value of the pilot heat-release affect the combustion noise reduction mechanism, and maximum noise reduction is achieved when the height and steepness of the pilot heat-release profile are similar to the initial rise of the main heat-release event. A variation of the initial rise-rate of the main heat-release event reveals trends in combustion noise that are the opposite of what would happen in the absence of a close-coupled pilot. The noise-reduction mechanism shown in this work may be a powerful tool to improve the tradeoffs among fuel efficiency, pollutant emissions, and combustion noise.Copyright