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.     

Modeling of soot deposition in wavy-fin exhaust gas recirculator coolers

This work presents one of the first CFD studies carried out to understand the fouling of exhaust gas recirculator (EGR) cooler surfaces. The deposition of soot particles in wavy-fin EGR coolers is studied by way of simulations carried out in a periodic framework. In the presence of very high temperature gradients, usually prevalent in EGR flows, the particle deposition process is dominated by the thermophoretic force. Calculations are performed for 10 and 100 nm particles at various Reynolds numbers and wall temperature gradients ranging from 1.0 to 9.45 × 10⁶ K/m. It is seen that for the sub-micron particle sizes considered, the deposition process is independent of the particle size. Simulations in the wavy-fin geometry indicate the presence of preferential deposition patterns, corresponding to the regions of higher heat transfer. At lower Reynolds numbers, the amount of deposition increases considerably due to the higher particle residence times. Also, the amount of deposition exhibits a linear relationship with the applied wall temperature gradient, thus confirming the importance of thermophoresis in the soot deposition process.     

Influence of gas velocity on particulate fouling of exhaust gas recirculation coolers

The objective of this research is to study the influence of gas flow velocity on particulate fouling of exhaust gas recirculation (EGR) coolers. An experimental setup has been designed and constructed to simulate particulate fouling in EGR coolers in diesel engines. The setup consists of soot generator, gas/particle flow heater, testing section for EGR coolers and finally an exhaust system. Two sets of fouling experiments have been performed with and without water injection, and the gas velocity in each set has varied between 30, 70 and 120 m/s. The concentration of soot particles in the gas flow is 100 mg/m³, and the average diameter of the particles is 130 nm with a standard deviation of 55 nm. It has been found that the thermal resistance and thickness of the fouling layer and the fouling rate decrease as the gas velocity in the EGR cooler increases. If EGR coolers are operated with a gas velocity, which is just lower than the critical flow velocity for the largest particle in the flow, quick deterioration of the thermal performance of the heat exchanger will nevertheless occur. This strongly indicates that the gas velocity should exceed a certain critical flow velocity in order to prevent particulate fouling. In addition, the presence of water vapour in the gas flow improves the thermal performance of the cooler and decreases the fouling rate, and its influence decreases as the gas velocity increases.     

Fouling Problems in Exhaust Gas Recirculation Coolers in the Automotive Industry

Transportation is responsible for approximately 20% of global greenhouse gas emissions, such as CO₂, NO_x, and hydrocarbons that have not been burned completely in the engine. In particular, 55% of globally emitted NO_x, which is more harmful to the environment than CO₂, is produced by the automotive industry alone. Strict emission standards are now in place that set specific limits to the amount of pollutants that can be released into the environment. The widely used measure to reduce NO_x emissions in diesel engines is to return part of the exhaust gas to the intake of the engine. This is usually done through a heat exchanger known as an exhaust gas recirculation (EGR) cooler. However, EGR coolers are subject to severe fouling such that their thermal efficiency can drop by as much as 30% within a very short period of time. More importantly, the deposit layer is a blend of particulate matter and sticky heavy hydrocarbons that are very difficult to remove from the heat exchanger surfaces. The present study addresses this problem and provides a review on required research and development (R&D) activities to mitigate fouling of EGR coolers.     

Particulate Fouling and Challenges of Metal Foam Heat Exchangers

ABSTRACT

In recent years, open-cell metal foam has gained attention for utilization for exhaust gas recirculation coolers due to its large surface area and porous structure. Theoretically, the porous foam structure would have better transfer heat through conduction and convection processes. However, the exhaust gases that enter the cooler would carry particulate matter, which may deposit within the foam structure. The existing fouling studies cannot explain the underlying mechanisms of particulate deposition thoroughly within the foam structure. This study reviews the particulate fouling of heat exchangers, particularly in the exhaust gas recirculation system. Some past approaches to investigate fouling, particle transport, and deposition in the metal foam heat exchangers for many different applications are also included. In addition, this study also includes the challenges that lie ahead in implementing the metal foam heat exchangers in the industries.     

Effect of exhaust gas recirculation (EGR) temperature for various EGR rates on heavy duty DI diesel engine performance and emissions

DI diesel engines are well established today as the main powertrain solution for trucks and other relevant heavy duty vehicles. At the same time emission legislation (mainly for NO_x and particulate matter) becomes stricter, reducing their limit to extremely low values. One efficient method to control NO_x in order to achieve future emissions limits is the use of rather high exhaust gas recirculation (EGR) rates accompanied by increased boost pressure to avoid the negative impact on soot emissions. The method is based on the reduction of gas temperature level and O₂ availability inside the combustion chamber, but unfortunately it has usually an adverse effect on soot emissions and brake specific fuel consumption (bsfc). The use of high EGR rates creates the need for EGR gas cooling in order to minimize its negative impact on soot emissions especially at high engine load were the EGR flow rate and exhaust temperature are high. For this reason in the present paper it is examined, using a multi-zone combustion model, the effect of cooled EGR gas temperature level for various EGR percentages on performance and emissions of a turbocharged DI heavy duty diesel engine operating at full load. Results reveal that the decrease of EGR gas temperature has a positive effect on bsfc, soot (lower values) while it has only a small positive effect on NO. As revealed, the effect of low EGR temperature is stronger at high EGR rates.     

Effect of Exhaust Gas Recirculation (EGR) on performance,emissions, deposits and durability of a constant speed compression ignition engine

To meet stringent vehicular exhaust emission norms worldwide, several exhaust pre-treatment and post-treatment techniques have been employed in modern engines. Exhaust Gas Recirculation (EGR) is a pre-treatment technique, which is being used widely to reduce and control the oxides of nitrogen (NO_x) emission from diesel engines. EGR controls the NO_x because it lowers oxygen concentration and flame temperature of the working fluid in the combustion chamber. However, the use of EGR leads to a trade-off in terms of soot emissions. Higher soot generated by EGR leads to long-term usage problems inside the engines such as higher carbon deposits, lubricating oil degradation and enhanced engine wear. Present experimental study has been carried out to investigate the effect of EGR on soot deposits, and wear of vital engine parts, especially piston rings, apart from performance and emissions in a two cylinder, air cooled, constant speed direct injection diesel engine, which is typically used in agricultural farm machinery and decentralized captive power generation. Such engines are normally not operated with EGR. The experiments were carried out to experimentally evaluate the performance and emissions for different EGR rates of the engine. Emissions of hydrocarbons (HC), NO_x, carbon monoxide (CO), exhaust gas temperature, and smoke opacity of the exhaust gas etc. were measured. Performance parameters such as thermal efficiency, brake specific fuel consumption (BSFC) were calculated. Reduction in NO_x and exhaust gas temperature were observed but emissions of particulate matter (PM), HC, and CO were found to have increased with usage of EGR. The engine was operated for 96 h in normal running conditions and the deposits on vital engine parts were assessed. The engine was again operated for 96 h with EGR and similar observations were recorded. Higher carbon deposits were observed on the engine parts operating with EGR. Higher wear of piston rings was also observed for engine operated with EGR.     

Effects of biodiesel on a low heat loss diesel engine

In this study, the cylinder head, exhaust, and inlet valves of a diesel engine were coated with the ceramic material MgO–ZrO₂ by the plasma spray method, while the piston surface was coated with ZrO₂. Thus, a thermal barrier was provided for the elements of the combustion chamber with these coatings. Using identical coated and uncoated engines, the effects of canola methyl ester produced by the transesterification method, and ASTM No. 2D fuel on engine performance and exhaust emissions were studied. Tests were performed on the uncoated engine, and then repeated on the coated engine and the results were compared. An increase in engine power and decrease in specific fuel consumption, as well as significant improvements in exhaust gas emissions and smoke density, were observed for all test fuels used in the coated engine compared with that of the uncoated engine.     

The deconvolution of the thermal, dilution, and chemical effects of exhaust gas recirculation (EGR) on the reactivity of engine and flame soot

In a recent paper, we demonstrated that the exhaust gas recirculation (EGR) enhanced the oxidative reactivity of diesel engine soot. In this paper, we show that simulated EGR, via carbon dioxide (CO₂) addition to the intake air to an engine at concentrations of 0, 2, 4, and 8 vol.% and to the oxidizer stream of an ethylene diffusion flame at concentrations of 0, 5, and 10 vol.%, affects the reactivity of the soot in the same manner as actual EGR. Motivated by this fact, post-flame ethylene soot was produced from a co-flow laminar diffusion flame to better understand the mechanism by which the CO₂ affects soot reactivity. This objective was accomplished by successfully isolating and examining the thermal, dilution, and chemical effects of the CO₂ on soot reactivity. These three effects account for 45%, 35%, and 20% of the total reactivity of soot respectively, with the thermal effect being the most important factor governing the soot reactivity. The results showed that all of these effects account for a measurable increase in soot reactivity.     

Impact of exhaust gas recirculation (EGR) on the oxidative reactivity of diesel engine soot

This paper expands the consideration of the factors affecting the nanostructure and oxidative reactivity of diesel soot to include the impact of exhaust gas recirculation (EGR). Past work showed that soot derived from oxygenated fuels such as biodiesel carries some surface oxygen functionality and thereby possesses higher reactivity than soot from conventional diesel fuel. In this work, results show that EGR exerts a strong influence on the physical properties of the soot which leads to enhanced oxidation rate. HRTEM images showed a dramatic difference between the burning modes of the soot generated under 0 and 20% EGR. The soot produced under 0% EGR strictly followed an external burning mode with no evidence of internal burning. In contrast, soot generated under 20% EGR exhibited dual burning modes: slow external burning and rapid internal burning. The results demonstrate clearly that highly reactive soot can be achieved by manipulating the physical properties of the soot via EGR.     

Exhaust Gas Recirculation Cooler Fouling in Diesel Applications: Fundamental Studies of Deposit Properties and Microstructure

This article reports on the results of experimental efforts aimed at improving the understanding of the mechanisms and conditions at play in the fouling of exhaust gas recirculation coolers. An experimental apparatus was constructed to utilize simplified surrogate heat exchanger tubes in lieu of full-size heat exchangers. The use of these surrogate tubes allowed removal of the tubes after exposure to engine exhaust for study of the deposit layer and its properties. The exhaust used for fouling the surrogate tubes was produced using a modern medium-duty diesel engine fueled with both ultra-low-sulfur diesel and biodiesel blends. At long exposure times, no significant difference in the fouling rate was observed between fuel types and hydrocarbons levels. Surface coatings for the tubes were also evaluated to determine their impact on deposit growth. No surface treatment or coating produced a reduction in the fouling rate or any evidence of deposit removal. In addition, microstructural analysis of the fouling layers was performed using optical and electron microscopy in order to better understand the deposition mechanism. The experimental results are consistent with thermophoretic deposition for deposit formation, and van der Waals attraction between the deposit surface and exhaust-borne particulate.     

Analysis of reformed EGR on the performance of a diesel particulate filter

The use of a diesel particulate filter (DPF) in combination with an upstream diesel oxidation catalyst (DOC) has been successfully implemented and shown to reduce carbon monoxide (CO), hydrocarbon (HC) and Particulate Matter (PM) diesel exhaust gas emissions. However issues including cost, size and uncontrolled active regeneration under a low temperature window still require attention. This study therefore primarily focuses on the potential benefits of using a single catalytic coated DPF (cDPF) and a combined DOC-cDPF instead of the DOC-DPF aftertreatment system utilising a passive, low temperature regeneration method. Comparisons were made through monitoring exhaust gas compositions from an experimental single cylinder diesel engine as well as measuring the pressure drop across the filters to analyse the accumulation of soot particles. The influence of reformed EGR (REGR), enriched simulated hydrogen (H₂) and CO, on DPF and cDPF soot loading was of interest as H₂ promotes the NO to NO₂ oxidation. Similarly the addition of simulated reformate (added either directly into the engine intake or exhaust manifold) for optimal performance of the aftertreatment systems was examined.The effects of adding REGR resulted in a significant decrease in total engine-out NO_x emissions, as well as an increase in both NO₂ concentration and NO₂/NO_x ratio. This resulted in improved filter efficiency and overall loading, especially under a DOC-cDPF aftertreatment configuration system. As a whole, a simultaneous NO_x and PM reduction was achieved.     

发动机废气再循环冷却器性能计算仿真

发动机废气再循环是有效降低Nox的重要方法,废气再循环冷却器的设计开发显得尤其重要。本文,结合现有EGR冷却器结构参数,利用DOE的设计方法,通过对三种不同结构形式的EGR冷却器进行传热和阻力性能测试。同时通过多元回归分析,建立EGR冷却器热侧传热和流动实验关联式,并进一步利用Visual Baisc开发工具,建立EGR冷却器性能计算仿真平台。该仿真平台的开发,有效的提高了EGR冷却器设计计算精度和产品开发速度。     

The effects of Al2O3–TiO2 coating in a diesel engine on performance and emission of corn oil methyl ester

Today, as a result of increase in oil prices, limited fossil fuel resources, environmental consideration and global warming, the methyl ester fuels have been focused on alternative fuels. Methyl ester fuels can be used more efficiently in low heat rejection engines (LHR), in which the temperature of combustion chamber is increased by creating a thermal barrier. In this study, the piston, cylinder head, exhaust and inlet valves of a diesel engine were coated with the ceramic material Al₂O₃–TiO₂ by the plasma spray method. Thus, a thermal barrier was provided for the parts of the combustion chamber with these coatings. The effects of corn oil methyl ester that produced by the transesterification method, and No. D2 fuels’ performance and exhaust emissions’ rate were studied by using equal in every respect coated and uncoated engines. Tests were performed on the uncoated engine, and then repeated on the coated engine and the results were compared. A decrease in engine power and specific fuel consumption, as well as significant improvements in exhaust gas emissions (except NOx), were observed for all test fuels used in the coated engine compared with that of the uncoated engine.     

The evolution of the micro-morphology and micro-structure of particles from diesel engine in combination with exhaust gas recirculation

In this study, small angle X-ray scattering (SAXS), scanning electron microscopy (SEM), and atomic force microscopy (AFM) were used to investigate the microstructure, spatial structure, and structural rigidity of the particles in the particulate matter (PM) produced at different exhaust gas recirculation (EGR) rates, exhaust compositions and temperatures as well as the size and number of gaps in the aggregates. The results showed that with increasing EGR rate and exhaust temperature, the aggregate size of the PM and the number of primary carbon particles increased significantly, the electronic density difference in the PM decreased gradually, the statistical mean distance between the PM decreased, the size and number of gaps in the aggregates decreased significantly, the spatial structure gradually became tighter, the Young’s moduli of the powders increased gradually, the structural rigidity increased, the liquid bridge force and the van der Waals force increased gradually, the type of cohesive force changed from the liquid bridge force to a combination of the liquid bridge force and the van der Waals force, and the van der Waals force played a more prominent role. In contrast to the particles that formed due to the introduction of exhaust gas, the particles that formed due to the introduction of CO₂ exhibited a chain structure, and the cohesive force decreased significantly, which resulted in loose particle packing. The particles that formed due to the introduction of only N₂ mainly exhibited a clustered structure, the cohesive force did not change significantly, and the primary carbon particles were tightly packed. The particle gap sizes ranged from 4 to 6 nm, 3 to 4 nm and 11 to 13 nm when exhaust gas, N₂ and CO₂ were introduced, respectively. The N₂ in the exhaust gas was the main factor responsible for the aggregation of particles and the improvement of the structural rigidity, whereas CO₂ in the exhaust gas increased the statistical mean distance between the particles and decreased the packing density of the structure and the structural rigidity.     

Performance and emission characteristics of a diesel engine with Diesel Premixed Compression Ignition and exhaust gas recirculation

In the present work, diesel was used as a premixed fuel along with the conventional injection of diesel with a premixed ratio of 0.25. The premixed charge was burned in the cylinder along with the fuel directly injected into the cylinder by a conventional injection system. To control nitrogen oxide(s) (NO_x) emissions, Exhaust Gas Recirculation (EGR) was adopted and the exhaust gas was varied from 10% to 30% in steps of 10%. The performance and emission characteristics were compared with conventional 100% diesel injection in the main chamber. Based on the experiments conducted on a Compression Ignition Direct Injection (CIDI) engine, it was found that unburnt hydrocarbons, carbon monoxide, and soot emissions increase. Soot emission decreases with up to 20% EGR and increases when EGR was increased beyond 20%. Hence 20% EGR was found to be the optimum use for DPMCI mode with a premixed ratio of 0.25. Due to the lean operation, significant reduction in NO_x was achieved with the DPMCI combustion mode. Brake thermal efficiency was marginally decreased compared to CIDI mode.     

EGR冷却温度对重型柴油机性能及排放的影响

以一台配有废气再循环(exhaust gas recirculation,EGR)冷却系统和可变几何截面涡轮增压器的高压共轨重型柴油机作为研究对象,进行了EGR冷却温度对柴油机性能及排放影响的台架试验研究。结果表明:随着EGR冷却温度降低,柴油机燃油消耗率、烟度和NOx排放均持续降低。而EGR冷却温度每降低1℃,柴油机燃油消耗率、烟度和NOx排放在不同转速、负荷下降幅差异明显。燃油消耗率在中等转速、低负荷工况降幅最大,NOx排放和烟度在高转速、低负荷工况下降幅最大;在考虑到EGR冷却系统消耗的能量后,可以通过计算得到理论燃油消耗率。在兼顾燃油消耗率和排放性的原则下得到了各工况下EGR相对最优冷却温度,而所得到的相对最优EGR冷却温度正是各个试验工况下理论燃油消耗率最低的温度。     

Experimental study of various effects of exhaust gas recirculation (EGR) on combustion and emissions of an automotive direct injection diesel engine

Cooled exhaust gas recirculation (EGR) is a common way to control in-cylinder NO_x production and is used on most modern high-speed direct injection (HSDI) diesel engines. However EGR has different effects on combustion and emissions production that are difficult to distinguish (increase of intake temperature, delay of rate of heat release (ROHR), decrease of peak heat release, decrease in O₂ concentration (and thus of global air/fuel ratio (AFR)) and flame temperature, increase of lift-off length, etc.), and thus the influence of EGR on NO_x and particulate matter (PM) emissions is not perfectly understood, especially under high EGR rates. An experimental study has been conducted on a 2.0 l HSDI automotive diesel engine under low-load and part load conditions in order to distinguish and quantify some effects of EGR on combustion and NO_x/PM emissions. The increase of inlet temperature with EGR has contrary effects on combustion and emissions, thus sometimes giving opposite tendencies as traditionally observed, as, for example, the reduction of NO_x emissions with increased inlet temperature. For a purely diffusion combustion the ROHR is unchanged when the AFR is maintained when changing in-cylinder ambient gas properties (temperature or EGR rate). At low-load conditions, use of high EGR rates at constant boost pressure is a way to drastically reduce NO_x and PM emissions but with an increase of brake-specific fuel consumption (BSFC) and other emissions (CO and hydrocarbon), whereas EGR at constant AFR may drastically reduce NO_x emissions without important penalty on BSFC and soot emissions but is limited by the turbocharging system.     

Characterization and effect of using cotton methyl ester as fuel in a LHR diesel engine

In the present study, surfaces of cylinder head, piston, exhaust and inlet valve of a four-stroke, direct injection, single cylinder diesel engine were coated with molybdenum (Mo) by plasma spray method. Thus, thermal barrier characteristic was brought to these parts. Variances in performance and emission values of cotton methyl ester and 2D fuel mixtures were studied in the ceramic coated and uncoated engines under the same running conditions. Performance (6.0% for specific fuel consumption) and emission values (up to 18.0% for CO, 8.0% for smoke density) of the test fuel were improved in the coated engine compared with the uncoated engine. NO_x increase (4.5%) with the increased temperatures expected in the coated engine.     

Effect of exhaust gas recirculation on diesel engine nitrogen oxide reduction operating with jojoba methyl ester

Jojoba methyl ester (JME) has been used as a renewable fuel in numerous studies evaluating its potential use in diesel engines. These studies showed that this fuel is good gas oil substitute but an increase in the nitrogenous oxides emissions was observed at all operating conditions. The aim of this study mainly was to quantify the efficiency of exhaust gas recirculation (EGR) when using JME fuel in a fully instrumented, two-cylinder, naturally aspirated, four-stroke direct injection diesel engine. The tests were carried out in three sections. Firstly, the measured performance and exhaust emissions of the diesel engine operating with diesel fuel and JME at various speeds under full load are determined and compared. Secondly, tests were performed at constant speed with two loads to investigate the EGR effect on engine performance and exhaust emissions including nitrogenous oxides (NO_x), carbon monoxide (CO), unburned hydrocarbons (HC) and exhaust gas temperatures. Thirdly, the effect of cooled EGR with high ratio at full load on engine performance and emissions was examined. The results showed that EGR is an effective technique for reducing NO_x emissions with JME fuel especially in light-duty diesel engines. With the application of the EGR method, the CO and HC concentration in the engine-out emissions increased. For all operating conditions, a better trade-off between HC, CO and NO_x emissions can be attained within a limited EGR rate of 5–15% with very little economy penalty.