Computationally Efficient Simulation of Multi-Component Fuel Combustion Using a Sparse Analytical Jacobian Chemistry Solver and High-Dimensional Clustering

The need for more efficient and environmentally sustainable internal combustion engines is driving research towards the need to consider more realistic models for both fuel physics and chemistry. As far as compression ignition engines are concerned, phenomenological or lumped fuel models are unreliable to capture spray and combustion strategies outside of their validation domains — typically, high-pressure injection and high-temperature combustion. Furthermore, the development of variable-reactivity combustion strategies also creates the need to model comprehensively different hydrocarbon families even in single fuel surrogates. From the computational point of view, challenges to achieving practical simulation times arise from the dimensions of the reaction mechanism, that can be of hundreds species even if hydrocarbon families are lumped into representative compounds, and thus modeled with non-elementary, skeletal reaction pathways. In this case, it is also impossible to pursue further mechanism reductions to lower dimensions. CPU times for integrating chemical kinetics in internal combustion engine simulations ultimately scale with the number of cells in the grid, and with the cube number of species in the reaction mechanism. In the present work, two approaches to reduce the demands of engine simulations with detailed chemistry are presented. The first one addresses the demands due to the solution of the chemistry ODE system, and features the adoption of SpeedCHEM, a newly developed chemistry package that solves chemical kinetics using sparse analytical Jacobians. The second one aims to reduce the number of chemistry calculations by binning the CFD cells of the engine grid into a subset of clusters, where chemistry is solved and then mapped back to the original domain. In particular, a high-dimensional representation of the chemical state space is adopted for keeping track of the different fuel components, and a newly developed bounding-box-constrained k-means algorithm is used to subdivide the cells into reactively homogeneous clusters. The approaches have been tested on a number of simulations featuring multi-component diesel fuel surrogates, and different engine grids. The results show that significant CPU time reductions, of about one order of magnitude, can be achieved without loss of accuracy in both engine performance and emissions predictions, prompting for their applicability to more refined or full-sized engine grids.

Analysis and Modelling of the Transient Thermal Behaviour of Automotive Turbochargers

Turbochargers are a key technology to deliver fuel consumption reductions on future internal combustion engines. However, the current industry standard modeling approaches assume the turbine and compressor operate under adiabatic conditions. Although some state of the art modeling approaches have been presented for simulating the thermal behavior, these have focused on thermally stable conditions. In this work, an instrumented turbocharger was operated on a 2.2L Diesel engine and in parallel a one-dimensional lumped capacity thermal model was developed. For the first time this paper presents analysis of experimental and modeling results under dynamic engine operating conditions. Engine speed and load conditions were varied to induce thermal transients with turbine inlet temperatures ranging from 200–800°C; warm-up behavior from 25°C ambient was also studied. Following a model tuning process based on steady operating conditions, the model was used to predict turbine and compressor gas outlet temperatures, doing so with an RMSE of 8.4°C and 7.1°C respectively. On the turbine side, peak heat losses from the exhaust gases were observed to be up to double those observed under thermally stable conditions due to the heat accumulation in the structure. During warm-up, the model simplifications did not allow for accurate modeling of compressor, however on the turbine side gas temperature predictions errors were reduced from 150°C to around 40°C. The main benefits from the present modeling approach appear to be in turbine outlet temperature prediction, however modeling improvements are identified for future work.

Simulation of n-Heptane and Surrogate Fuels for Advanced Combustion Engines (FACE) in a Single-Cylinder Compression Ignition Engine

A CFD model of a HATZ diesel engine was developed for the purpose of simulating low temperature combustion (LTC) of surrogate diesel fuels for the Fuels for Advanced Combustion Engines (FACE). Initial validation of the model was performed using n-heptane data from a single cylinder HATZ diesel engine. Simulations were run with both a detailed n-heptane mechanism and several reduced mechanisms to determine the suitability of using a reduced mechanism to capture the main ignition characteristics and emissions. It was found that a 173 species n-heptane mechanism predicts start of combustion (SOC) within 0.5 crank angle degrees of the detailed 561 species mechanism. The 173 species mechanism required 27 hours of computational time to reach the end of the simulation whereas the 561 species detailed mechanism required 41 hours under the same conditions. Two additional reduced mechanisms, containing 85 and 35 species, were provided reasonable accuracy with a computational time of 8 hours and 2 hours, respectively. Due to the varying physical and chemical properties of the FACE surrogates, a sensitivity analysis of the effects of the physical properties was conducted by changing the n-heptane physical properties to those of n-hexadecane while keeping the chemistry the same. As expected, when the fuel properties of n-hexadecane (which is less volatile than n-heptane) were used with the n-heptane kinetics, SOC was delayed and the net heat release rate was reduced. The FACE fuels were developed to fulfill the need for research grade fuels that are able to represent common refinery stream fuels. Since the FACE fuels consist of hundreds of fuel components, it is not feasible to model the actual FACE fuels in a full-scale engine model. An alternative is to develop surrogates consisting of relatively few species for which detailed mechanisms are available. Even then this mechanism would need to be reduced to make the computation practical. For this work, a detailed diesel surrogate mechanism was reduced from 4016 species to 1046 species to match the characteristics for FACE fuels 1, 3, 5, 8, and 9. The surrogates only contain 4–7 species. Using the single chemical mechanism to represent five surrogate FACE fuels, it was found that ∼200°C of air preheat was required to achieve autoignition in the HATZ model compared to the 130°C of air preheat required experimentally. Initial runs have found that there were similar trends in SOC and heat release between the FACE fuel surrogate experiments and simulations for the respective fuels. Future work will require improvements on the single chemical mechanism to represent the five surrogate FACE fuels.

High-Performance Lead-Free Electroplated Composite Bearing Overlay for Heavy Duty Applications

Material selection for engine internal components, e.g. bearings, is becoming increasingly more complex and demanding as the operating environments become more aggressive with the introduction of new technologies for the reduction of CO2 emissions. Historically, engine bearings contained lead, which has excellent fundamental bearing properties such as compatibility (run satisfactorily under conditions of marginal lubrication), conformability (deform and accept small scale geometrical inaccuracies of the crankshaft), and embeddablity (tolerance to dirt and other foreign materials) whilst being readily alloyed to achieve good wear and fatigue resistance. However, facing new challenges, many Original Equipment Manufacturers have started development programs to replace lead-containing with lead-free engine components in order to comply with new end-of-life vehicle directives or anticipated future directives. For more than fifteen years, MAHLE has been successfully supplying the light, medium and heavy duty market, with premium electroplated leaded composite bearings, which are designed to improve wear resistance. Some of this market now demands a switch to lead-free materials, while maintaining or exceeding its aforementioned requirements on bearing material properties. Composites of hard particles in a softer metal matrix are in this context ideally suited bearing materials as they can be tailored to obtain the optimal mix between soft and hard properties for the individual application. Typical hard particles that are commonly used comprise of metal oxides, nitrides or carbides. In addition to higher load carrying capabilities and longer service life, new engine technology trends demand that bearings also must operate under mixed or boundary lubrication conditions without having any adverse effect on the performance and integrity of the engine system. Boundary lubrication is commonly observed upon starting the engine before the elastohydrodynamic oil film is fully established. In this state, load is carried by surface asperities rather than by the lubricant. So far, the incorporation and even distribution of the hard particles into an electroplated lead-free matrix was not achievable using conventional direct current electroplating techniques. MAHLE, therefore, has developed a patented pulse plating technique in order to incorporate hard particles into the overlay metal matrix. The refined and modified crystal structure of the resulting lead-free overlay, with incorporated hard particles, yields a premium electroplated bearing with superior wear and fatigue resistance. Corresponding rig and engine test results have been completed to support the material development.

Large Eddy Simulation of Vaporizing Sprays Considering Multi-Injection Averaging and Grid-Convergent Mesh Resolution

A state-of-the-art spray modeling methodology, recently presented by Senecal et al. [1, 2], is applied to Large Eddy Simulations (LES) of vaporizing sprays. Simulations of non-combusting Spray A (n-dodecane fuel) from the Engine Combustion Network are performed. An Adaptive Mesh Refinement (AMR) cell size of 0.0625 mm is utilized based on the accuracy/runtime tradeoff demonstrated by Senecal et al. [2]. In that work it was shown that grid convergence of key parameters for non-evaporating and evaporating sprays was achieved for cell sizes between 0.0625 and 0.125 mm using the Dynamic Structure LES model. The current work presents an extended and more thorough investigation of Spray A using multi-dimensional spray modeling and the Dynamic Structure LES model. Twenty different realizations are simulated by changing the random number seed used in the spray sub-models. Multi-realization (ensemble) averaging is shown to be necessary when comparing to local spray measurements of quantities such as mixture fraction and gas-phase velocity. Through a detailed analysis, recommendations are made regarding the minimum number of LES realizations required for accurate prediction of Diesel sprays. Finally, the effect of a spray primary breakup model constant on the results is assessed.

Experimental Investigation of Spray and Combustion Characteristics of Soybean Biodiesel in a Constant-Volume Combustion Chamber: The Effects of Fuel and Ambient Gas Temperature

Effects of fuel and ambient gas temperature on the spray and combustion characteristics of soybean biodiesel were studied in a constant-volume combustion chamber. Four different fuels or fuel blends including B0, B20, B50 and B100 were investigated experimentally. The soot mass data were obtained via a new technique called forward illumination light extinction (FILE). The ambient gas temperature was varied from 700 K to 1200 K. To simulate the engine operating conditions, the ambient oxygen concentration and its density were kept at 21 % and 15 kg/m3, respectively. A higher peak pressure is found as the biodiesel content decreases. B20, B50 and B100 have a shorter ignition delay than B0 and the ignition delay decreases with increasing biodiesel content. The liquid penetration decreases with decreasing biodiesel content. Moreover, the integrated natural flame luminosity (INFL) increases with decreasing biodiesel content. Shorter flame (i.e., soot luminosity) duration and a longer delay between start of combustion (SOC) and the appearance of flame are found as the biodiesel content increases. The flame duration also increases with increasing ambient gas temperature for all fuels. Soot is lower and appears later at a lower ambient gas temperature, while it is burned out at around the same time. Near-zero soot mass was observed for all tested fuels at 700 K. A shorter soot formation process is observed for biodiesel fuels. The soot reduction using B20 and B50 is not obvious compared to B0 at a low temperature. But under the ordinary diesel engine operating condition at 1000 K, the soot reduction is significant. It is also found that the soot can be reduced by 60% and above when B100 is used in this study.

Modular Bearing Designs to Cope With the New Engine Designs Demanding High Performance, Lead Free Solutions and Robustness

Engine development, driven by environmental considerations outlined in the different emission regulations, fuel economy and fuel availability in combination with economical boundary conditions, needs new approaches in bearing material and design. Since gas engines are gaining market share and firing pressures increase in Diesel engines in order to fulfill fuel economy a special focus has also been taken to tailor-made bearings for these applications. This complex task has to consider lining compound material strength and stability under different conditions like oil condition and dilution. Thin overlays with long term wear resistance and mixed friction capabilities as well as robust design for extraordinary events like dirt shock loading or adaptations at the engine start are necessary. To fulfill all these requirements different tasks have to be considered: 1. Bearing lining and steel shell compound to fulfill assembly requirements to combine a safe bearing seat with anti-fretting and high strength with base tribological characteristics 2. Design and use of different layers to compensate weakness of the one layer with the strength of another layer 3. Incorporation of special running conditions and cost reduction approaches in the layer design like polymer coatings for start stop and shaft designs with rougher surface finishes 4. Bearing design incorporating special shapes to cope better with deflections and geometric deficiencies of a special engine design or application In this publication existing and new lining compound approaches including lead free designs, a variety of different overlays from electroplated, polymer and sputtered ones are briefly described. Additionally it is explained how these layers are combined and how they work together to improve bearing performance. Testing of the bearing components and designs on bearing test rigs with new test conditions considering dirt shock and misalignment and their confirmation by engine running experiences are given for a gas engine and a high speed diesel engine applications. A special outlook on how this approach can be extended to other applications for the sake of robustness, cost reduction or performance increase will summarize the paper.

A Study on Measurement of Conformability of the Piston Oil Ring on the Cylinder Bore Under Engine Operating Condition by LIF Method Using Optical Fiber

Engine oil consumption must be reduced for reducing exhaust gas emissions. It is well known that a cylinder bore shape under engine operating condition affects oil consumption. This study aimed clarifying the conformability of an oil ring against the distorted cylinder bore. Oil film thickness at the sliding surface of oil ring upper and lower rails was successfully measured by LIF method using optical fiber, which was embedded in the oil ring. The piston motion was also measured and compared with measured oil film thickness. It was found that the piston tilting motion affected oil film thickness hence its conformability. It was also found that thicker oil film was found at the following rail than that at former rail, and it was suggested that oil was supplied to the following rail from not only the sliding surface of the former ring but also somewhere, for example, the oil ring groove.

Validation of a Three-Dimensional Internal Nozzle Flow Model Including Automatic Mesh Generation and Cavitation Effects

Fuel injectors often experience cavitation due to regions of extremely low pressure. In this work, a cavitation modeling method is implemented in the CONVERGE CFD code to model the flow in fuel injectors. CONVERGE includes a Cartesian mesh based flow solver. In this solver, a Volume Of Fluid (VOF) method is used to simulate the multiphase flow. The cavitation model is based on a flash-boiling method with rapid heat transfer between the liquid and vapor phases. In this method, a homogeneous relaxation model is used to describe the rate at which the instantaneous quality, the mass fraction of vapor in a two-phase mixture, will tend towards its equilibrium value. The model is first validated with the nozzle flow case of Winklhofer by comparing the mass flow rate with experimentally measured values at different outlet pressures. The cavitation contour shape is also compared with the experimental observations. Flow in the Engine Combustion Network Spray-A nozzle configuration is simulated. The mesh dependency is also studied in this work followed by validation against discharge coefficient data. Finally, calculations of a five-hole injector, including moving needle effects, are compared to experimental measurements.

Analysis and Resolution of Piston Pin Joint Tick Noise

A new measurement system and methodology has been developed which allows for direct measurement of the piston pin behavior of an internal combustion engine, particularly as it relates to pin joint ticking noise contributing to Noise Vibration and Harshness (NVH). Based on the measured dynamic of the piston pin during engine operation, the root cause of the pin joint ticking noise in this application was defined to be sticking of the pin at the end of the exhaust stroke which prevented pin lift. In order to better understand and mitigate this phenomenon, numerical simulation was conducted by means of Elasto Hydrodynamic Lubrication (EHL). The output of this simulation showed that modifying the shape of the connecting rod pin bore by ovality has a significant effect on the pin motion and pin acceleration which could eliminate the pin joint tick noise. This improved pin bore shape was tested in the engine and confirmed to eliminate the pin joint ticking noise by manipulating the pin motion and disallowing piston pin sticking. Finally, an analysis of the newly developed methodology to root cause and solve pin joint ticking is compared to the prior methodology.