Combustion System Development of an Opposed Piston 2-Stroke Diesel Engine

Today, the interest towards 2-stroke, opposed-piston compression-ignition engines is higher than ever, after the announcement of imminent production of a 2.7L 3-cylinder light truck engine by Achates Powers. In comparison to other 2-stroke designs, the advantages in terms of scavenge and thermal efficiency are indisputable: a perfect "uniflow" scavenge mode can be achieved with inexpensive and efficient piston controlled ports, while heat losses are strongly reduced by the relatively small transfer area. Unfortunately, the design of the combustion system is completely different from a 4-stroke DI Diesel engine, since the injectors must be installed on the cylinder liners: however, this challenge can be converted into a further opportunity to improve fuel efficiency, adopting advanced combustion concepts. This paper is based on a previous study, where the main geometric parameters of an opposed piston engine rated at 270 kW (3200 rpm) were defined with the support of CFD 1D-3D simulations. The current work will focus on the influence of an innovative combustion system, developed by the authors by means of further CFD-3D analyses, holding constant the boundary conditions of the scavenging process. The numerical study eventually demonstrates that an optimized 2-S OP Diesel engine can achieve a 10% improvement on brake efficiency at full load, in comparison to an equivalent conventional 4-stroke engine, while reducing in-cylinder peak pressures and turbine inlet temperatures

On the application of Large-Eddy simulations in engine-related problems

In internal combustion engines the combustion process and the pollutants formation are strongly influenced by the fuel-air mixing process. The modeling of the mixing and the underlying turbulent flow field is classically tackled using the Reynolds Averaged Navier Stokes (RANS) modeling method. With the increase of computational power and the development of sophisticated numerical methods the Large Eddy Simulation (LES) method becomes within reach. In LES the turbulent flow is locally filtered in space, rather than fully averaged, as in RANS. This thesis reports on a study where the LES technique is applied to model flow and combustion problems related to engines. Globally, three subjects have been described: the turbulent flow in an engine-like geometry, the turbulent mixing of a gas jet systemand the application of flamelet-basedmethods to LES of two turbulent diffusion flames. Because of our goal to study engine-related flow problems, two relatively practical flow solvers have been selected for the simulations. This choice was motivated by their ability to cope with complex geometries as encountered in realistic, engine-like geometries. A series of simulations of the complex turbulent, swirling and tumbling flow in an engine cylinder, that is induced by the inlet manifold, has been performed with two different LES codes. Additionally one Unsteady RANS simulation has been performed. The flow field statistics from the Large-Eddy simulations deviated substantially between one case and the next. Only global flow features could be captured appropriately. This is due to the impact of the under-resolved shear layer and the dissipative numerical scheme. Their effects have been examined on a square duct flow simulation. An additional sensitivity that was observed concerned the definition of the inflow conditions. Any uncertainty in the mass flow rates at the two runners, that are connected to the cylinder head, greatly influences the remaining flow patterns. To circumvent this problem, a larger part of the upstream flow geometry was included into the computational domain. Nevertheless, the Large-Eddy simulations do give an indication of the unsteady, turbulent processes that take place in an engine, whereas in the URANS simulations all mean flow structures are very weak and the turbulence intensities are predicted relatively low in the complete domain. The turbulent mixing process in gaseous jets has been studied for three different fuel-to-air density ratios. This mimicked the injection of (heavy) fuel into a pressurized chamber. It is shown that the three jets follow well the similarity theory that 152 Abstract was developed for turbulent gas jets. A virtual Schlieren postprocessingmethod has been developed in order to analyze the results similarly as can be done experimentally. By defining the penetration depth based on this method, problems as typically in Schlieren experiments, related to the definition of the cutoff signal intensity have been studied. Additionally it was shown that gaseous jet models can be used to simulate liquid fuel jets, especially at larger penetration depths. This is because the penetration rate from liquid sprays is governed by the entrainment rate, which is similar as for gaseous jets. However, it remains questionable if gas jet models can in all cases replace the model for fuel sprays. The cone angle for gas jets can deviate strongly from those observed in spray experiments. Only when corrected for this effect, the penetration behavior was similar. Two turbulent diffusion flames have been investigated with a focus on the modeling of finite rate chemistry effects. Concerning the first flame, the well known Sandia flame D, two methods are compared to each other for the modeling of the main combustion products and heat release. These methods are described by the classical flamelet method where the non-premixed chemistry is parameterized using a mixture fraction and the scalar dissipation rate, and a relatively new method, where a progress variable is used in non-premixed combustion problems. In the progress variable method two different databases have been compared: one based on non-premixed flamelets and one based premixed flamelets. It is found that the mixture fraction field in the Large-Eddy simulation of Sandia flame D is best predicted by both the classical flamelet method and the progress variable method that is based on premixed chemistry. In these cases the flame solution was mostly located close to its equilibrium value. However, when correcting for the prediction of the mixture fraction in the spatial coordinates, it is shown that the progress variable method based on non-premixed chemistry is better, compared to experiments. Especially at locations where a flame solution near chemical equilibrium is not adequate this model is more appropriate. Additionally a sooting turbulent benzene diffusion flame has been investigated. Therefore a steady laminar flamelet library has been applied which is based on a very detailed reaction mechanism for premixed benzene flames. In the Large-Eddy simulations the total PAH/soot mass and mole fractions have been computed explicitly, while the source terms for these variables are based on a classical flamelet parametrization. The regions of PAH/soot formation have been identified, showing distributed parcels where PAH/soot formation takes place. The results show a growth of PAH/soot volume fraction up to levels of about 4 ppm. The average particle size increases steadily in this flame, up to about 30 nm

Optimization and development of low environmental impact propulsion systems

The aim of the Ph.D. research project was to explore Dual Fuel combustion and hybridization. Natural gas-diesel Dual Fuel combustion was experimentally investigated on a 4-Stroke, 2.8 L, turbocharged, light-duty Diesel engine, considering four operating points in the range between low to medium-high loads at 3000 rpm. Then, a numerical analysis was carried out using a customized version of the KIVA-3V code, in order to optimize the diesel injection strategy of the highest investigated load. A second KIVA-3V model was used to analyse the interchangeability between natural gas and biogas on an intermediate operating point. Since natural gas-diesel Dual Fuel combustion suffers from poor combustion efficiency at low loads, the effects of hydrogen enriched natural gas on Dual Fuel combustion were investigated using a validated Ansys Forte model, followed by an optimization of the diesel injection strategy and a sensitivity analysis to the swirl ratio, on the lowest investigated load. Since one of the main issues of Low Temperature Combustion engines is the low power density, 2-Stroke engines, thanks to the double frequency compared to 4-Stroke engines, may be more suitable to operate in Dual Fuel mode. Therefore, the application of gasoline-diesel Dual Fuel combustion to a modern 2-Stroke Diesel engine was analysed, starting from the investigation of gasoline injection and mixture formation. As far as hybridization is concerned, a MATLAB-Simulink model was built to compare a conventional (combustion) and a parallel-hybrid powertrain applied to a Formula SAE race car

A comprehensive CFD methodology for the simulation of Spark Ignited Engines

In this work, a Computational Fluid Dynamic methodology for the simulation of the charge formation process in Gasoline Direct Injection engines is presented. The aim of the work is to develop a methodology suitable in an industrial environment to drive and support the development process of modern GDI engines. A big emphasis is placed on the comparison of the proposed CFD models with experimental data obtained using a single-cylinder optical engine. Chapter 1 describes the working context and sets the aim of the work. After a brief recall of the theoretical background of CFD in chapter 2, an overview of the optical techniques interesting for Internal Combustion Engine applications is presented in chapter 3, and the basic principles of spray atomization theory are reviewed in chapter 4. In chapter 5 the CFD simulations for the charge motion in-cylinder are described. Two different engines were investigated, and the effect of different turbulence models and numerical schemes are analyzed, comparing the results with optical experimental data. The standard k-eps model, together with the MARS numerical scheme, showed the better capability to reproduce the charge motion and turbulence pattern in-cylinder, and therefore they were used for the remaining part of the work. In chapter 7 the injection model used is discussed. Despite a traditional Lagrangian-Eulerian approach, the model presents an innovative procedure capable to reproduce also the liquid core. After that the effects of the use of the liquid core and a bi-component fuel are analyzed, the in-cylinder injection results for the two investigated engines are presented. The injection model shows its capability to correctly reproduce the spray shape and penetration in different operating conditions and for different injector types, using a reduced amount of calibration parameters. Finally, chapter 8 presents some "diagnostic indexes" capable to resume the results of the CFD simulations in a reduced number of parameters. In particular, some indexes to assess the quality of the mixture and the wall impingement tendency are proposed, allowing to use the CFD simulations to address these crucial aspects in the choice of injector targeting and actuation strategy. The proposed methodology allows to use CFD simulations to support the engine development process, and was successfully applied to many different spark ignited engine

NUMERICAL INVESTIGATION OF COMBUSTION PROCESS IN REACTIVITY CONTROLLED COMPRESSION IGNITION (RCCI) ENGINE

Ph.DDOCTOR OF PHILOSOPH

Sparse approximate inverse preconditioners on high performance GPU platforms

Simulation with models based on partial differential equations often requires the solution of (sequences of) large and sparse algebraic linear systems. In multidimensional domains, preconditioned Krylov iterative solvers are often appropriate for these duties. Therefore, the search for efficient preconditioners for Krylov subspace methods is a crucial theme. Recent developments, especially in computing hardware, have renewed the interest in approximate inverse preconditioners in factorized form, because their application during the solution process can be more efficient. We present here some experiences focused on the approximate inverse preconditioners proposed by Benzi and Tůma from 1996 and the sparsification and inversion proposed by van Duin in 1999. Computational costs, reorderings and implementation issues are considered both on conventional and innovative computing architectures like Graphics Programming Units (GPUs)

Large eddy simulation of fuel injection and spray combustion in an engine environment

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Large eddy simulation of spray combustion in an HSDI engine is carried out in this thesis. The implementation of the code was performed in logical steps that allowed both assessment of the performance of the existing KIVA-LES and later development. The analysis of the liquid annular jet confirmed existence of typical, annular jet exclusive structures like head vortices, stagnation point and recirculation in the inner zone. The influence of the swirl in the ambient domain was found to have profound impact on the development, penetration and radial spreading of the jet. Detailed results were reported in Jagus et al. (2008). The code was further validated by performing an extensive study of large eddy simulation of diesel fuel mixing in an engine environment. The reaction models were switched off in order to isolate the effects of both swirl and the different numerical treatment of LES. Reference RANS simulation was performed and significant differences were found. LES was found to capture much better the influence of the swirl on the liquid and vapour jets, a feature essentially absent in RANS results. Moreover, the predicted penetration of the liquid was much higher for the LES case and more in accordance with experimental measurements. Liquid penetration and subsequent evaporation are very important for prediction of heat release rates and encouraging results formed a good basis to performing a full simulation with models for ignition and combustion employed. The findings were analyzed in the paper by Jagus et al. (2009). Further modifications were introduced into the LES code, among them changes to the combustion model that was originally developed for RANS and calculation of the filter width. A new way of estimating the turbulent timescale (eddy turnover time) assured that the incompatibilities in the numerical treatment were eliminated and benefits of LES maximized. The new combustion model proved to give a very good agreement with experimental data, especially with regard to pressure and accumulated heat release rates. Both qualitative and quantitative results presented a significant improvement with respect to RANS results and old LES formulation. The new LES model was proved to give a very good performance on a spectrum of mesh resolutions. Encouraging results were obtained on a coarse mesh sets therefore proving that the new LES code is able to give good prediction even on mesh sizes more suitable for RANS. Overall, LES was found to be a worthy alternative to the well established RANS methods, surpassing it in many areas, such as liquid penetration prediction, temperature-turbulence coupling and prediction of volume-averaged data. It was also discovered that the improved LES code is capable of producing very good results on under-resolved mesh resolutions, a feature that is especially important in industrial applications and on serial code structure

Theoretical and experimental investigation of a CDI injection system operating on neat rapeseed oil - feasibility and operational studies

This thesis presents the work done within the PhD research project focusing on the utilisation of plant oils in Common Rail (CR) diesel engines. The work scope included fundamental experimental studies of rapeseed oil (RSO) in comparison to diesel fuel, the feasibility analysis of diesel substitution with various plant oils, the definition and implementation of modifications of a common rail injection system and future work recommendations of possible changes to the injection system. It was recognised that neat plant oils can be considered as an alternative substitute for diesel fuel offering a natural way to balance the CO2 emissions. However, due to the differences between diesel and plant oils, such as density, viscosity and surface tension, the direct application of plant oils in common rail diesel engines could cause degradation of the injection process and in turn adversely affect the diesel engine’s performance. RSO was chosen to perform the spray characterisation studies at various injection pressures and oil temperatures under conditions similar to the operation of the common rail engine. High speed camera, Phase Doppler Anemometry and Malvern laser techniques were used to study spray penetration length and cone angle of RSO in comparison to diesel. To study the internal flow inside the CR injector the acoustic emission technique was applied. It was found that for oil temperatures below 40°C the RSO viscosity, density and surface tension are higher in comparison to diesel, therefore at injection pressures around 37.50 MPa the RSO spray is not fully developed. The spray penetration and cone angle at these spray conditions exhibit significant spray deterioration. In addition to the lab experiments, KIVA code simulated RSO sprays under CR conditions. The KH-RT and RD breakup models were successfully applied to simulate the non-evaporating sprays corresponding to the experimental spray tests and finally to predict i real in-cylinder injection conditions. Numerical results showed acceptable agreement with the experimental data of RSO penetration. Based on experimental and numerical results it was concluded that elevated temperature and injection pressure could be the efficient measures to overcome operational obstacles when using RSO in the CR diesel engine. A series of modifications of low- and highpressure loops was performed and experimentally assessed throughout the engine tests. The results revealed that the modifications allowed to run the engine at the power and emission outputs very close to diesel operation. However, more fundamental changes were suggested as future work to ensure efficient and trouble-free long-term operation. It is believed that these changed should be applied to meet Euro IV and V requirements

Numerical investigation on the in-cylinder flow with SI and CAI valve timings

The principle of controlled auto-ignition (CAI) is to mix fuel and air homogeneously before compressing the mixture to the point of auto-ignition. As ignition occurs simultaneously, CAI engines operate with lean mixtures preventing high cylinder pressures. CAI engines produce small amounts of nitrogen oxides (NOx) due to low combustion temperatures while maintaining high compression ratios and engine efficiencies. Due to simultaneous combustion and lean mixtures, CAI engines are restricted between low and mid load operations. Various strategies have been studied to improve the load limit of CAI engines. The scope of the project is to investigate the consequences of varying valve timing, as a method to control the mixture temperature within the combustion chamber and therefore, controlling the mixture auto-ignition point. This study presents computational fluid dynamics (CFD) modelling results of transient flow, inside a 0.45 litre Lotus single cylinder engine. After a validation process, a chemical kinetics model is combined with the CFD code, in order to study in-cylinder temperatures, the mixture distribution during compression and to predict the auto-ignition timing. The first part of the study focuses on validating the calculated in-cylinder velocities. A mesh sensitivity study is performed as well as a comparison of different turbulence models. A method to reduce computational time of the calculations is presented. The effects of engine speed on charge delay and charge amount inside the cylinder, the development of the in-cylinder flow field and the variation of turbulence parameters during the intake and compression stroke, are studied. The second part of the study focuses on the gasoline mixture and the variation of the valve timing, to retain different ratios of residual gases within the cylinder. After validation of the model, a final set of CFD calculations is performed, to investigate the effects of valve timing on flow and the engine parameters. The results are then compared to a fully homogeneous mixture model to study the benefits of varying valve duration. New key findings and contributions to CAI knowledge were found in this investigation. Reducing the intake and exhaust valve durations created a mixture temperature stratification and a fuel concentration distribution, prior to auto-ignition. It resulted in extending the heat release rate duration, improving combustion. However, shorter valve timing durations also showed an increase in heat transfer, pumping work and friction power, with a decrease of cylinder indicated efficiency. Valve timing, as a method to control auto-ignition, should only be used when the load limit of CAI engines, is to be improved