Simulation of Combustion and Emission Characteristics in a Dual Fuelled Diesel Engine

In this paper, Computational Fluid Dynamics (CFD) approach is adopted to study the combustion and emission progression in a single cylinder four stroke diesel engine, operated in both diesel and dual-fuel modes. The study of dual-fuel mode is performed by using synthesis gas (syngas) with 75:25 and 50:50 volumetric combinations of hydrogen and carbon monoxide, respectively. The modeling and meshing of the constant volume combustion chamber is carried out by using GAMBIT tool. The meshing of the combustion chamber is performed using tetrahedral elements and the k–ε turbulence model is introduced along with non-premixed combustion modeling. The modeled hemispherical-piston-top combustion chamber is then simulated in FLUENT solver across the experimental boundary conditions at 40%, 60%, 80% and 100% of full load for both diesel and dual-fuel. The results of simulation incorporate the study of maximum combustion temperature, maximum combustion velocity and H2O mole fraction subsequent to combustion. Further, the concentrations of emissions have also been investigated for both diesel and dual-fuel modes. The results of simulations show a good agreement with the corresponding experimental data.

A General Rezoning Technique for KIVA3V Internal Combustion Engines CFD Simulations

A computational mesh is required when performing CFD-combustion modeling of internal combustion engines. For combustion chambers with moving pistons and valves, like those in typical cars and trucks, the combustion chamber shape changes continually in response to piston and valve motion. The combustion chamber mesh must then also change at each time step to reflect that change in geometry. The method of changing the mesh from one computational time step to the next is called rezoning. This paper introduces a new method of mesh rezoning for the KIVA3V CFD-combustion program. The standard KIVA3V code from Los Alamos National Laboratory comes with standard rezoners that very nicely handle mesh motion for combustion chambers whose mesh does not include valves and for those with flat heads employing vertical valves. For pent-roof and wedge-roof designs KIVA3V offers three rezoners to choose from, the choice depending on how similar a combustion chamber is to the sample combustion chambers that come with KIVA3V. Often, the rezoners must be modified for meshes of new combustion chamber geometries to allow the mesh to successfully capture change in geometry during the full engine cycle without errors. There is no formal way to approach these modifications; typically this requires a long trial and error process to get a mesh to work for a full engine cycle. The benefit of the new rezoner is that it replaces the three existing rezoners for canted valve configurations with a single rezoner and has much greater stability, so the need for ad hoc modifications of the rezoner is greatly reduced. This paper explains how the new rezoner works and gives examples of its use.

Gas Quality Sensor to Improve Biogas-Fueled CHP/DG

Today, renewable fuels such as biogas are being used to fuel combined heat and power (CHP) and distributed generation (DG) systems. The composition of biogas delivered to power generation equipment varies depending upon the origin of the anaerobic digestion process and site-specific factors. For improved process control and optimum utilization of CHP/DG systems, the biogas composition needs to be monitored. A new apparatus has been developed for characterization of hydrocarbon fuel mixtures. The method utilizes near infrared absorption spectroscopy to monitor composition and heating value of landfill gas, natural gas, and other hydrocarbon fuel gases. The measurement is virtually instantaneous. A commercialized version of this sensor is expected to cost less than half the price of gas chromatographs, which are widely used in the gas industry today.

Improving Efficiency, Extending the Maximum Load Limit and Characterizing the Control-Related Problems Associated With Higher Loads in a 6-Cylinder Heavy-Duty Natural Gas Engine

High EGR rates combined with turbocharging has been identified as a promising way to increase the maximum load and efficiency of heavy duty spark ignition Natural Gas engines. With stoichiometric conditions a three way catalyst can be used which means that regulated emissions can be kept at very low levels. Most of the heavy duty NG engines are diesel engines which are converted for SI operation. These engine’s components are in common with the diesel-engine which put limits on higher exhaust gas temperature. The engines have lower maximum load level than the corresponding diesel engines. This is mainly due to the lower density of NG, lower compression ratio and limits on knocking and also high exhaust gas temperature. They also have lower efficiency due to mainly the lower compression ratio and the throttling losses. However performing some modifications on the engines such as redesigning the engine’s piston in a way to achieve higher compression ratio and more turbulence, modifying EGR system and optimizing the turbocharging system will result in improving the overall efficiency and the maximum load limit of the engine. This paper presents the detailed information about the engine modifications which result in improving the overall efficiency and extending the maximum load of the engine. Control-related problems associated with the higher loads are also identified and appropriate solutions are suggested.

Effect of Biodiesel, JP-8 and Ultra Low Sulfur Diesel Fuel on Autoignition, Combustion, Performance and Emissions in a Single Cylinder Diesel Engine

Concern about the depletion of petroleum reserves, rising prices of conventional fuels, security of supply and global warming have driven research toward the development of renewable fuels for use in diesel engines. These fuels have different physical and chemical properties that affect the diesel combustion process. This paper compares between the autoignition, combustion, performance and emissions of soybean derived biodiesel, JP-8 and ultra low sulfur diesel (ULSD) in a high speed single-cylinder research diesel engine equipped with a common rail injection system. Tests were conducted at steady state conditions at different injection pressures ranging from 600 bar to 1200 bar. The ‘rate of heat release’ traces are analyzed to determine the effect of fuel properties on the ignition delay, premixed combustion fraction and mixing and diffusion controlled combustion fractions. Biodiesel produced the largest diffusion controlled combustion fraction at all injection pressures compared to ULSD and JP-8. At 600 bar injection pressure, the diffusion controlled combustion fraction for biodiesel was 53% whereas both JP-8 and ULSD produced 39%. In addition, the effect of fuel properties on engine performance, fuel economy, and engine-out emissions is determined. On an average JP-8 produced 3% higher thermal efficiency than ULSD. Special attention is given to the NOx emissions and particulate matter characteristics. On an average biodiesel produced 37% less NOx emissions compared to ULSD and JP-8.

End-of-Injection Behavior of Diesel Sprays Measured With X-Ray Radiography

The behavior of diesel fuel sprays at the end of injection is poorly understood, yet has important implications regarding diesel engine emissions. Recent research has shown that at the end of injection, an entrainment wave is created, causing the fuel spray to rapidly entrain ambient gas. This rapid entrainment creates a dilute mixture of fuel that may be a source of unburned fuel emissions. In this study, x-ray radiography is used to examine the end-of-injection behavior of diesel sprays. X-ray radiography permits quantitative mass distribution measurements in dense sprays, providing data that cannot be obtained with optical techniques. Analysis of the spray velocity at steady-state suggests an entrainment wave speed of several hundred m/s, which is supported by the appearance of a travelling entrainment wave at low ambient density. The spray density declines most rapidly near the nozzle, behavior that matches the expected entrainment wave behavior. In several cases, the spray distribution in a cross-section across the nozzle axis becomes smoother at the end of injection. Three-dimensional reconstructions of the spray density at the end of injection show that the spray plume widens considerably, enhancing the dilution caused by the reduction in spray mass in the flowfield. Measurements of injector needle motion with x-ray phase contrast imaging show that throttling across the needle seat may cause a smearing of the ideally sharp entrainment wave.

Soot Modeling for Advanced Control of Diesel Engine Aftertreatment

Diesel Particulate Filters (DPFs) are well assessed aftertreatment devices, equipping almost every modern diesel engine on the market to comply with today’s stringent emission standards. However, an accurate estimation of soot loading, which is instrumental to ensuring optimal performance of the whole engine-after-treatment assembly is still a major challenge. In fact, several highly coupled physical-chemical phenomena occur at the same time, and a vast number of engine and exhaust dependent parameters make this task even more daunting. This challenge may be solved with models characterized by different degrees of detail (0-D to 3-D) depending on the specific application. However, the use of real-time, but accurate enough models, may be of primary importance to face with advanced control challenges, such as the integration of the DPF with the engine or other critical aftertreatment components (Selective Catalytic Reduction (SCR) or other NOx control components), or to properly develop model-based OBD sensors. This paper aims at addressing real time DPF modeling issues with special regard to key parameter settings, by using the 1D code ExhAUST (Exhaust Aftertreatment Unified Simulation Tool), developed jointly by the University of Rome Tor Vergata and West Virginia University. ExhAUST is characterized by a novel and unique full analytical treatment of the wall that allows faithful representation with high degree of detail the evolution of soot loading inside the porous matrix. Numerical results are compared with experimental data gathered at West Virginia University (WVU) engine laboratory using a Mack heavy-duty diesel engine coupled to a Johnson Matthey CCRT (DOC, Diesel Oxidation Catalyst+CDPF, Catalyzed DPF) aftertreatment system. To that aim, the engine test bench has been equipped with a DPF weighing setup to track soot load over a specifically developed engine operating procedure. Obtained results indicate that the model is accurate enough to capture soot loading and back pressure histories with regard to different steady state engine operating points, without needing any tuning procedure of the key parameters. Thus, the use of ExhAUST for application to advanced after-treatment control appears promising at this stage.

Modeling the Effects of Intake Plenum Volume on the Performance of a Small Naturally Aspirated Restricted Engine

Intake tuning is a significant method of boosting performance by enhancing volumetric efficiency in a naturally aspirated engine. Elements of intake tuning can involve varying intake runner length, geometry and plenum shape and volume. Previous research has demonstrated the beneficial effects of increasing plenum volume on engine torque. This objective of this study was to evaluate the ability of analytical and two computer based models (simple and complex) to accurately predict the effects of varying plenum volume on steady state and transient engine performance for a small restricted spark ignition engine. The computer models were only moderately successful in characterizing steady state performance. The simple model matched torque peak locations but failed to adequately predict the advantageous effect of plenum volume on torque. The complex model more effectively simulated torque effects of plenum volume increase but did not adequately capture torque peak locations. Both models underestimated mid-range torque by up to 20%. Transient manifold filling was estimated well with both the complex computer model and analytical methods. Transient torque response differed by only 1–2 engine cycles and was also well predicted by the computer simulation.

Evaluation of Diesel Low Temperature Combustion Fuel-Injection Strategies at Different Engine Loads

High hydrocarbon levels in the exhaust, increased cycle-to-cycle variation and reduced energy-efficiency are typical problems associated with diesel LTC operation. To overcome these challenges, three different fuel injection strategies (late single-injection, early multiple-injections and split-injections) have been investigated on a modified single cylinder common-rail diesel engine. The effects of EGR, boost and injection pressure on the emissions and combustion efficiency have been analyzed. The effect of heavy EGR has been quantified in terms of a trade-off between the combustion phasing and the combustion efficiency. To minimize fuel condensation and wall-wetting with early injections, a criterion for selecting the earliest timing for injection during the compression stroke has also been evaluated. This research is concluded with the formulation of a load management strategy to enable energy-efficient diesel LTC up to 10 bar IMEP.

Quasi-Two-Zone Modeling of Diesel Ignition Delay in Pilot-Ignited Partially Premixed Low Temperature Natural Gas Combustion

This paper presents simulated ignition delay (ID) results for diesel ignition in a pilot-ignited partially premixed, low temperature natural gas (NG) combustion engine. Lean premixed low temperature NG combustion was achieved using small pilot diesel sprays (2–3% of total fuel energy) injected over a range of injection timings (BOIs ∼ 20°–60° BTDC). Modeling IDs at advanced BOIs (50°–60° BTDC) presented unique challenges. In this study a single-component droplet evaporation model was used in conjunction with a modified version of the Shell autoignition (SAI) model to obtain ID predictions of pilot diesel over the range of BOIs (20°-60° BTDC). A detailed uncertainty analysis of several model parameters revealed that Aq and Eq, which affect chain initiation reactions, were the most important parameters (among a few others) for predicting IDs at very lean equivalence ratios. The ID model was validated (within ± 10 percent error) against experimentally measured IDs from a single-cylinder engine at 1700 rpm, BMEP = 6 bar, and intake manifold temperature (Tin) of 75°C. For BOIs close to TDC (e.g., 20° BTDC), the contribution of diesel evaporation times (Δθevap) and droplet diameters to predicted IDs were more significant compared to advanced BOIs (e.g., 60° BTDC). Increasing Tin (the most sensitive experimental input variable affecting predicted IDs), led to a reduction in both the physical and chemical components of ID. Hot EGR led to shorter predicted and measured IDs over the range of BOIs, except 20° BTDC. In general, the thermal effects of hot EGR were found to be more pronounced than either dilution or chemical effects for most BOIs. Finally, uncertainty analysis results also indicated that ID predictions were most sensitive to model parameters AP3, Aq, and Af1, and Eq, which affected chain initiation and propagation reactions and also contributed the most to overall uncertainties in IDs.