Combustion processes in compression ignition engines lead to the inevitable generation of nitrogen oxides, which cannot be limited to the currently desired levels just by optimising the in-cylinder processes. Therefore, simulation-based engine development needs to include all engine-related aspects which contribute to tailpipe emissions. Among them, the SCR (selective catalytic reduction) aftertreatment-related processes, such as urea–water solution injection, urea decomposition, mixing, NOx catalytic reduction, and deposits’ formation, are the most challenging, and require as much attention as the processes taking place inside the cylinder. Over the last decade, the urea-SCR aftertreatment systems have evolved from underfloor designs to close-coupled (to the engine) architecture, characterised by the short mixing length. Therefore, they need to be tailor-made for each application. This study presents the CFD-based development of a multi-platform SCR system with a short mixing length for mobile non-road applications, compliant with Stage V NRE-v/c-5 emission standard. It combines multiphase dispersed flow, including wall wetting and urea decomposition kinetic reaction modelling to account for the critical aspects of the SCR system operation. The baseline system’s design was characterised by the severe deposit formation near the mixer’s outlet, which was attributed to the intensive cooling in the mounting area. Moreover, as the simulations suggested, the spray was not appropriately mixed with the surrounding gas in its primary zone. The proposed measures to reduce the wall film formation needed to account for the multi-platform application (ranging from 56 to 130 kW) and large-scale production capability. The performed simulations led to the system design, providing excellent UWS–exhaust gas mixing without a solid deposit formation. The developed system was designed to be manufactured and implemented in large-scale series production.

The aim of this study is to investigate the effect of flash boiling on global structures of n-heptane sprays formed by a multi-hole injector in a wide range of fuel temperatures and ambient pressures at different injection pressures to provide a solid foundation for a new sub-model capable of reproducing spray evolution under flashboiling conditions using a Lagrangian Discrete Droplet Method (DDM). The experiments were focused on qualitative analysis of the spray structures and quantitative parameters such as axial spray penetration and spray angle. Based on the experimental and literature data, a new sub-model for half of the spray-cone angle (HOCA) was proposed, and the bubble number density was calibrated to simulate the evolution and collapse of flashboiling sprays accurately. A commercial six-hole injector operating at three injection pressures (5, 10 and 15 MPa) was used to inject n-heptane at three initial temperatures (60, 90 and 120 degrees C) into a constant volume chamber filled with air at a pressure ranging from 0.01 to 0.1 MPa (absolute). It was observed that regardless of the injection pressure, the effect of flash boiling on the global parameters of the spray was similar. However, due to a stronger axial momentum, the global spray angle was lower for higher injection pressures. Observed changes in the spray parameters were used to create a new simple spray angle formula and calibrate the bubble number density submodel, which was implemented into the AVL FIRETM solver. The model predicted the widening of the near-nozzle spray angle and accurately reproduced the spray collapse. However, it failed to capture an enhanced propagation of the centre part of the collapsed spray front, which was associated with the lack of vapour condensation, which wasn't included in the model.

Combustion knock is one of the key factors limiting the performance of spark-ignition engines, and thus great emphasis is placed on developing methods for effective and efficient knock suppression. The majority of works in this area are focused on the influence of these methods on the engine parameters, while maintaining the same "knock level" represented by a certain statistical parameter of knock combustion. The knock level defined as the mean or other statistical parameter of a stochastic process does not represent its characteristics. Thus, it is not evaluated on a sufficient level to show how different solutions for the knock reduction affect the knock probability distribution; nor is it known whether a universal treatment to characterise this influence can be developed. In this work we aim to fill this gap, and by the analysis of the knock combustion in a single-cylinder directinjection gasoline engine with co-injection of water, methanol, and their blends, conclude on the effects of different additives on the probability distribution of in-cylinder pressure-related parameters. We also applied a method to normalise the knock phenomenon by a model representation of its probability distribution. The analysis was performed for knock-limited spark advance conditions for different amounts of co-injected water, methanol, and their blends. The method to normalise the knock distribution was based on the log-normal probability distribution, and it was successfully applied to obtain the knock characteristics at a quantified characteristic limit of knock, which was impossible to precisely meet directly in the tests. The results revealed that depending on the co-injected additive and its amount, the knock distribution undergoes slight but systematic changes, in terms of distribution skewness and kurtosis. At the selected knock limit, the injection of methanol resulted in knock characteristics more skewed to the knock limit comparing to the baseline case. Water addition resulted in opposite trends. Blending water and methanol gave a similar knock peak-to-peak distribution as in the gasoline-only case.

In this study, we apply flash boiling to low-pressure injection of a urea-water solution (UWS) and investigate its influence on the selective catalytic reduction (SCR) system performance using numerical simulations. The research was done in two steps: first, the spray models for both subcooled and flash-boiling conditions were calibrated in a stationary condition based on experimental measurements. Second, the calibrated models of flashing and nonflashing sprays were used to simulate the whole SCR system to evaluate the influence of flash boiling on its performance. The aim of this evaluation was to include all important effects of flash boiling that were previously reported. Therefore, much emphasis has been put on the first part of the study, where different approaches to flash boiling spray modeling were validated. From the result of the study, a simple approach was proposed to model a low-pressure flashing spray based on a conventional evaporation model, with modified input parameters to include the flash-boiling effects on droplet size, spray penetration and angle, and the spatial droplet distribution. The full exhaust system simulations, in turn, have shown that the flash-boiling effect can be considered as an effective way to promote smaller droplets' formation while avoiding air assistance and a pressure increase in low-pressure UWS injection systems; furthermore, it will improve the performance of SCR systems. However, a direct increase of the UWS's temperature, without any modifications to the SCR system initially designed for nonflashing sprays, may lead to the opposite effect.

Successive diesel engine emissions' regulations impose increasingly tighter limits on the automotive industry. A well-established method to meet nitrogen oxides emissions' requirements is selective catalytic reduction with a urea-water solution injection. State-of-the-art designs are based on close-coupled exhaust architecture which suffers from a lack of space for a proper decomposition of a urea-water solution. The optimization of such systems is very challenging and is always intensively supported by numerical simulations. Within this work a two-zone spray representation for a urea-water solution injection is proposed. The two-zone approach captures a non-uniform droplet distribution inside the spray cone while maintaining a reasonable calculation time, since it is still based on a Lagrangian method. The proposed model was applied into exhaust system simulations and tested against a standard spray representation. The results revealed that uniform spray representation led to a smaller wall film formation and different mass balance. Ammonia distribution at the catalyst inlet was not directly affected significantly; however, in real systems one could expect catalyst clogging due to wall film formation, followed by a decrease of ammonia distribution uniformity. The results clearly show that uniform spray representation may lead to misestimation of the selective catalytic reduction system's performance and the novel two-zone approach should be used instead.

One of the limiting factors improving the efficiency of gasoline engines is engine knock. Various techniques including using fuels that result in charge cooling are employed to mitigate knock and improve efficiency. Water and methanol have higher heat of vaporization than gasoline. When water or methanol is injected into the intake manifold, it evaporates by exchanging energy with the charge mixture resulting in charge cooling. This allows the engine to be run with advanced spark timing without engine knock. With this motive, the impact of water methanol injection on the engine performance of a gasoline direct injection engine was investigated. Experimental studies were conducted on a single-cylinder 0.55L engine with a compression ratio of 10.9:1 at 800 kPa net indicated mean effective pressure and 1500 revolutions per minute. Baseline tests without water injection were conducted by direct injection of gasoline fuel blended with 10% ethanol (E10). Four mixtures: 100% water, 75% water + 25% methanol, 50% water + 50% methanol and 100% methanol were used with port injection. Spark ignition timing, flow rate of the fuel and the four mixtures were varied to be within the controlled knock limit while maintaining an excess air ratio of 1.0. Comparisons on the effectiveness of these mixtures indicate that higher methanol content in the mixture helped in reaching the maximum brake torque condition at lower mixture fuel ratios. Combustion stability of the engine was improved with the addition of water and water-methanol blends due to the sensitivity of combustion phasing at advanced spark timings reducing the variation in indicated mean effective pressure. Exhaust gas temperatures decrease with the addition of water and water-methanol blends due to the combined effect of increased charge cooling and improved combustion phasing.

The flash-boiling effect in sprays has been shown to have the capability to decrease droplet diameters and spray penetration, as well as improve the vaporization process in gasoline direct injection systems. It has also been shown that in certain conditions, the effect on spray penetration can be the opposite. Under a high degree of overheating the spray collapse effect may appear and lead to increased spray penetration - flare flash boiling. Moreover, when the boiling is already strongly present in the nozzle it may lead to flow choking. Thus, the process is difficult to be controlled. While most studies so far have focused on high-pressure direct injection systems, we investigate the process in a low-pressure system to verify if the effects observed in high-pressure systems will be similar. The study was performed on two liquids, water and a urea-water solution, to limit any individual substance’s effects on the results and to improve the practical importance of the study. The results showed no increased penetration in any of the flash-boiling cases, although at the highest considered temperatures the two plumes tend to form a single spray cloud. Although the characteristics were similar for both studied liquids, in certain measurement conditions the differences in almost all analysed spray parameters were considerable. Moreover, the study has shown that the flash-boiling effect can be considered as an effective way to improve jet and droplet break-up while avoiding a pressure increase in low-pressure injection systems.

In spray studies related to selective catalytic reduction (SCR) systems a common approach is to replace the urea–water solution (UWS) with pure water, even though there is very limited detailed information on the spray properties for these two liquids obtained under the same conditions using the same experimental equipment. Neither is it known how the possible differences in spray properties influence computational fluid dynamics (CFD) simulations. In this study, besides the flow characteristics, we compare both global and local spray parameters measured for UWS and pure water in the same conditions. To our knowledge, this is the first study which examines the influence on the injection process of replacing UWS with water over such a wide range. Moreover, the influence of different spray properties on CFD simulations is also examined. The experimental studies showed differences in almost all considered spray parameters. Moreover, different spray behaviour was noticed in terms of primary break-up. One important finding is that water and UWS sprays do have a similar Sauter mean diameter, but at the same time the droplet size distribution is considerably different. The simulation results indicated noticeable differences in terms of wall film formation; nevertheless, the overall mixing performance was not significantly affected.

Abstract Selective Catalytic Reduction (SCR) is well known method for reducing NOx emission in diesel engine exhaust gas. Urea-water solution (UWS) injected into hot stream decomposes due to thermolysis into ammonia and isocyanic acid which hydrolyses further into more ammonia and carbon dioxide. Resultant ammonia is the NOx reductor, producing water vapour and carbon dioxide from the reduction reaction. To provide sufficient NOx reduction efficiency, UWS needs to be properly atomized and mixed with exhaust gas. However, due to more and more restrictive emissions regulations provided by European Union and Close Coupled trend of aftertreatment systems in vehicles the design process is very complex and demanding. Computational Fluid Dynamics (CFD) simulations are integral part of product development, allowing save time and reduce costs of preparing prototypes for further tests. However, it is necessary to understand all the processes and problems connected with NOx reduction in SCR system. Strong turbulent flow of hot stream gas, urea-water solution spray injection, droplets interaction with wall, wallfilm generation are included. The objective of this work is to investigate the impact of heat transfer modelling inside mixing elements of SCR system on urea mixing uniformity and wallfilm deposit on the walls of the system. Simplified and more complex approach is compared with no heat transfer cases. All the simulations were conducted using AVL FIRETM software. Results showed that wall heat transfer might have an impact on mixing efficiency and wallfilm formulation. It is necessary to take into account the effect of mixing elements heat conduction in CFD simulations during the aftertreatment design process.

This study presents the influence of the UWS injection frequency on a close coupled SCR systems performance. The investigation was performed with the CFD tool AVL Fire. In the paper the analysis of four different UWS injection frequencies in the three different operating points of diesel engine was shown. The assessments of the system performance was referred to the ammonia distribution at catalyst intake and wall film formation inside the investigated geometry, as these are considered as crucial in such a configuration. The results showed that injection frequency affects both factors on different level depending from the flow conditions. In addition, the wall film crystallization risk was discussed basing on the obtained wall film characteristics.

The automotive industry is facing increasingly stringent requirements in aspect of pollutants emission, which is considered as one of the main reasons of global warming and climate changes. The most demanding limits relate to the nitrogen oxides (NOx) emission for diesel engines. Until now, it has been reduced over 85% and 95%, respectively by the successive European Emission Standards and U.S Emission Standards. What is more, further restrictions are just a matter of time. Hence, there is a high need to gain more knowledge in Computational Fluid Dynamics (CFD) modelling and measurement methodology for exhaust systems investigation purposes.