Engine-relevant Pressures Research Articles

Imaging of flame propagation and temperature distribution in an all-metal gasoline engine with endoscopic access via anisole fluorescence

Laser-based optical diagnostic tools are widely used to investigate in-cylinder processes in internal combustion engines. For laser-induced fluorescence (LIF), many tracers have been used in the past. Recently, anisole has been characterized spectroscopically for engine-relevant pressures and temperatures and emerged as a potentially advantageous alternative to more commonly used tracers in the past due to its photo-physical properties. Its high fluorescence quantum yield and large absorption cross section result in high signal intensity. This is particularly beneficial for endoscopic imaging systems, which typically have worse light collection efficiency than traditional imaging systems in fully optically accessible engines with transparent liners. In this work, we exploited the strong anisole LIF signal in two single-shot experiments: to image flame propagation, and the instantaneous gas-phase temperature during compression stroke and gas exchange process. Measurements were performed in a production gasoline engine modified for endoscopic optical access via an advanced UV endoscope system. First, LIF of anisole was compared to that of toluene during compression stroke. Anisole LIF yields a much higher signal-to-noise ratio and better image quality with lower tracer concentrations. Due to the higher signal of anisole LIF, small structures of the turbulent flame burning into the anisole/isooctane mixture were well visible after ignition. Second, the red-shift of the anisole fluorescence spectrum with increasing temperature and oxygen partial pressure was exploited for ratiometric temperature measurements based on single-shot images. The available spectroscopic data were used to develop several signal ratio models, which were calibrated in situ using a heated tracer/bath gas mixture introduced inside the combustion chamber. The calibrated signal ratio models were then extrapolated to the engine-relevant ranges. Models with two-step exponential interpolation show better agreement with the adiabatic temperature than linear or 3D surface exponential interpolation. The gas-phase temperature images based on single shots were obtained using one selected model, showing a near uniform and a stratified temperature distribution during the compression stroke and gas exchange process, respectively.

Sooting characteristics of ethanol-ethylene blends in laminar coflow diffusion flames up to 10 bar

Ethanol, being a renewable product, has been perceived for a long time as a fuel that burns clean and more complete than conventional hydrocarbons as well as being thought as a soot suppressant in blends with gasoline and diesel fuel. Some of the results from a wide-range of studies investigating soot-suppression potential of ethanol at atmospheric pressure, including Smoke Point measurements, have yielded occasionally contradictory conclusions. Further, scaling of soot studies conducted under atmospheric conditions to engine-relevant pressures has always been problematic because of the non-monotonic response of combustion events to pressure. Ethanol’s soot inhibiting characteristics at elevated pressures under tractable conditions have not been previously documented and may not be inferred easily from atmospheric pressure measurements. Information from benchmark investigations with ethanol-hydrocarbon blends at elevated pressures could help in understanding the soot aerosol emissions from practical engines fueled with ethanol blends. Soot production in ethanol/ethylene laminar diffusion flames at elevated pressures was investigated on a 3 mm diameter coflow burner mounted in a high-pressure combustion vessel. Ethanol was added to ethylene, which is diluted with nitrogen, in various quantities from 0% to 40% in terms of total carbon of the fuel stream keeping the carbon flow rate constant at 0.41 mg/s. Experiments were conducted at pressures from 3 to 10 bar, and in addition to keeping the mass flow of carbon constant, the ratio of mass flow of nitrogen to mass flow of carbon was also kept fixed at 6 at the fuel nozzle. The soot spectral emission spectroscopy technique was used to obtain line-of-sight intensities of soot radiation which were then converted to radial soot temperature and volume fraction distributions at a given axial height within the flame. Soot production increased by a small but measurable amount with the fuel mixture containing 10% carbon from ethanol at 5 and 8 bar pressure flames indicating synergistic interactions; while at 3 and 10 bar, no measurable change was observed. When the ethanol’s carbon contribution was increased to 30% and 40%, soot formation was suppressed and the soot yields decreased below the soot level of neat nitrogen-diluted ethylene at 3 to 10 bar pressures. Sensitivity of soot production to pressure in ethylene/ethanol flames is found to be similar to that of ethylene up to 8 bar.

Quantifying the Effect of Kinetic Uncertainties on NO Predictions at Engine-Relevant Pressures in Premixed Methane–Air Flames

Abstract Accurate and robust thermochemical models are required to identify future low-NOx technologies that can meet the increasingly stringent emissions regulations in the gas turbine industry. These mechanisms are generally optimized and validated for specific ranges of operating conditions, which result in an abundance of models offering accurate nominal solutions over different parameter ranges. Under atmospheric conditions, and for methane combustion, relatively good agreement between models and experiments is currently observed. At engine-relevant pressures, however, a large variability in predictions is obtained as the models are often used outside their validation region. The high levels of uncertainty found in chemical kinetic rates enable such discrepancies between models, even if the reactions are within recommended rate values. This work investigates the effect of such kinetic uncertainties in NO predictions by propagating the uncertainties of 30 reactions that are both uncertain and important to NO formation, through the combustion model at engine-relevant pressures. Understanding the uncertainty sources in model predictions and their effect on emissions at these pressures is key in developing accurate thermochemical models to design future combustion chambers with any confidence. Lean adiabatic, freely propagating, laminar flames are therefore chosen to study the effect of parametric kinetic uncertainties. A nonintrusive, level 2, nested sparse-grid approach is used to obtain accurate surrogate models to quantify NO prediction intervals at various pressures. The forward analysis is carried up to 32 atm to quantify the uncertainty in emissions predictions to pressures relevant to the gas turbine community, which reveals that the NO prediction uncertainty decreases with pressure. After performing a reaction pathway analysis (RPA), this reduction is attributed to the decreasing contribution of the prompt-NO pathway to total emissions, as the peak CH concentration and the CH layer thickness decrease with pressure. In the studied lean condition, the contribution of the pressure-dependent N2O production route increases rapidly up to 10 atm before stabilizing toward engine-relevant pressures. The uncertain prediction ranges provide insight into the accuracy and precision of simulations at high pressures and warrant further research to constrain the uncertainty limits of kinetic rates to capture NO concentrations with confidence in early design phases.

A numerical study on combustion mode characterization for locally stratified dual-fuel mixtures

Combustion modes in locally stratified dual-fuel (DF) mixtures are numerically investigated for methanol/n-dodecane blends under engine-relevant pressures. In the studied constant-volume numerical setup, methanol acts as a background low-reactivity fuel (LRF) while n-dodecane serves as high-reactivity fuel (HRF), controlling local ignition delay time. The spatial distribution of n-dodecane is modeled as a sinusoidal function parametrized by stratification amplitude (Y′) and wavelength (0.01 mm<λ<15 mm). In contrast, methanol is assumed to be fully premixed with air at equivalence ratio 0.8. First, one-dimensional setup is investigated by hundreds of chemical kinetics simulations in (Y′, λ) parameter space. Further, the concepts by Sankaran et al. (2005, Proceedings of the Combustion Institute) and Zeldovich (1980, Combustion and Flame) on ignition front propagation speed are applied to develop a theoretical analysis of the time-dependent diffusion–reaction problem. The theoretical analysis predicts two combustion modes, (1) spontaneous ignition and (2) deflagrative propagation, and leads to an analytical expression for the border curve called β-curve herein. One-dimensional chemical kinetics simulations confirm the presence of two combustion modes in (Y′, λ) parameter space while the β-curve explains consistently the position of phase border observed in the simulations. Finally, the role of convective mixing is incorporated to the theoretical expression for the β-curve. The effect of convection on combustion mode is assessed by carrying out two-dimensional fully-resolved simulations with different turbulence levels. Two-dimensional numerical simulation results give evidence on combustion mode switching, which is consistent with predictions of the modified β-curve for turbulent cases. The practical output of the paper is the β-curve which is proposed as a predictive tool to estimate combustion modes for various fuels or fuel combinations.

Experimental study of pre-chamber jet ignition in a rapid compression machine and single-cylinder natural gas engine

Pre-chamber jet ignition is a promising combustion technology to achieve fast combustion in natural gas engines. First, the ignition and combustion characteristics of mixtures in a pre-chamber system with different diameter orifices were studied under engine-relevant pressures and temperatures in a rapid compression machine. The tested fuels, CH4/air stoichiometric mixtures, were diluted by different proportions of CO2/N2 to simulate the corresponding exhaust gas recirculation conditions in engines. High-speed photography was applied to visualize the jet ignition and combustion processes. The experimental results revealed that two ignition patterns existed in the pre-chambers depending on the diameter of orifices. Pre-chamber jet flame ignition pattern appeared when the orifice diameter of the pre-chamber exceeded a critical value, which produced jet flame and ignited the mixtures in the main chamber directly. Pre-chamber jet auto-ignition pattern produced jet which promoted the auto-ignition of mixture in the main chamber when the orifice diameter was smaller and presented much shorter combustion durations. Based on the experimental results in the rapid compression machine, a practical pre-chamber system was designed in a single-cylinder natural gas engine to investigate the combustion performance and emission characteristics. The experimental results indicated appropriate 0.8%–1.4% increases of indicated thermal efficiency were achieved by pre-chamber jet ignition due to the higher combustion efficiency and shorter combustion duration compared to conventional spark ignition. Lower total hydrocarbon and CO emissions but higher NO x emissions were produced by pre-chamber jet ignition due to the faster burning velocity and higher combustion temperature.

Numerical investigation of strained extinction at engine-relevant pressures: Pressure dependence and sensitivity to chemical and physical parameters for methane-based flames

Resistance to extinction by stretch and and laminar flame speed are important properties of any combustible mixture. Recent work has shown that extinction by stretch controls the overall structure of several important types of methane-based turbulent flames. The parameter used to quantify this phenomena, Extinction Strain Rate (ESR), is numerically studied here for methane-based flames across a range of pressures relevant to gas turbines and internal combustion engines, 1-40 atm. The pressure trends are compared with those of laminar flame speed which is historically better studied. Current kinetic models agree that ESR of lean flames is a non-monotonic function of pressure and that ESR of rich flames increases significantly with pressure, but are found to differ significantly in their numerical predictions of ESR, particularly at higher pressures. To better identify the source of model prediction differences and what governs the overall accuracy of the ESR predictions, various model sensitivity analyses were conducted. Pressure-dependent kinetics are shown to be vital to determining ESR pressure trends as are molecular collision efficiencies. Yet, reactions sensitivities for ESR largely mirror those for laminar flame speed calculations. Sensitivity to the transport parameter, Lennard Jones diameter, significantly exceeds reaction sensitivities for the fuel, oxidizer and bath gas. Thermodynamic parameter ESR sensitivities vary widely with pressure, but at least for enthalpy, appear insignificant when uncertainties are considered. This study informs and motivates further efforts to understand the phenomena of flame extinction by stretch at elevated pressures.

Kinetic modeling study of surrogate components for gasoline, jet and diesel fuels: C7-C11 methylated aromatics

Abstract Kinetic mechanisms for aromatics are needed to successfully simulate the autoignition of transportation fuels using the surrogate fuel approach. An aromatic detailed kinetic mechanism that describes kinetics of C7-C11 methylated aromatics, including toluene, o-xylene, p-xylene, 1,2,4-trimethylbenzene, 1,3,5, trimethylbenzene and α−methylnaphthalene has been developed in the current study. The kinetic mechanism was built hierarchically using similar set of reaction pathways and reaction rate rules. In the mechanism developed, special emphasis is put on describing the detailed low-temperature ignition chemistry of o-xylene and 1,2,4-trimethylbenzene and, to our knowledge, this is the first attempt to do so in a detailed kinetic mechanism. In addition to kinetic modeling, new experimental data were acquired for toluene, o-xylene, and 1,2,4-trimethylbenzene using a rapid compression machine at low-to-intermediate temperatures and engine relevant pressures. In addition, the mechanism has been compared against data sets from the literature covering ignition delay times, flame speeds, and speciation profiles measured in a jet-stirred reactor and flow reactor. Good agreement is observed between the mechanism predictions and the experimental data. Kinetic analysis demonstrated the importance of including the low temperature chemistry of the benzylperoxy radicals to accurately capture the ignition propensity of o-xylene and 1,2,4-trimethylbenzene at low-to-intermediate temperatures and high pressures. The kinetic mechanism developed in the current study can be used for surrogate modeling of gasoline, jet and diesel fuels.

Reduced chemical model for low and high-temperature oxidation of fuel blends relevant to internal combustion engines

A hybrid approach is proposed to develop reduced kinetic models for complex engine-relevant fuels. The reduced mechanism is composed of a core submechanism including C 0 –C 4 chemistry, ethanol chemistry, and NO x chemistry and a fuel-dependent submechanism. The fuel-dependent submechanism consists of three species and ten reactions describing both the low and high-temperature fuel decomposition pathways. Calibrations for these reactions can be made for single component and multicomponent mixtures using experimental targets or results from detailed kinetic mechanisms. In the present study, the methodology is applied to the combustion of gasoline surrogates. Reduced mechanisms are calibrated for primary reference fuels and multicomponent mixtures at engine-relevant pressures. The reduced mechanisms capture the low-temperature heat release and negative temperature coefficient (NTC) behavior, which are important to simulations of internal combustion engine performance. The mechanisms are very compact in size and can be easily calibrated for multicomponent mixtures without any detailed knowledge of the chemistry of the fuel components, making them well suited for CFD simulations of new fuel blends.

Understanding the ignition mechanism of high-pressure spray flames

A conceptual model for turbulent ignition in high-pressure spray flames is presented. The model is motivated by first-principles simulations and optical diagnostics applied to the Sandia n-dodecane experiment. The Lagrangian flamelet equations are combined with full LLNL kinetics (2755 species; 11,173 reactions) to resolve all time and length scales and chemical pathways of the ignition process at engine-relevant pressures and turbulence intensities unattainable using classic DNS. The first-principles value of the flamelet equations is established by a novel chemical explosive mode-diffusion time scale analysis of the fully-coupled chemical and turbulent time scales. Contrary to conventional wisdom, this analysis reveals that the high Damköhler number limit, a key requirement for the validity of the flamelet derivation from the reactive Navier–Stokes equations, applies during the entire ignition process. Corroborating Rayleigh-scattering and formaldehyde PLIF with simultaneous schlieren imaging of mixing and combustion are presented. Our combined analysis establishes a characteristic temporal evolution of the ignition process. First, a localized first-stage ignition event consistently occurs in highest temperature mixture regions. This initiates, owed to the intense scalar dissipation, a turbulent cool flame wave propagating from this ignition spot through the entire flow field. This wave significantly decreases the ignition delay of lower temperature mixture regions in comparison to their homogeneous reference. This explains the experimentally observed formaldehyde formation across the entire spray head prior to high-temperature ignition which consistently occurs first in a broad range of rich mixture regions. There, the combination of first-stage ignition delay, shortened by the cool flame wave, and the subsequent delay until second-stage ignition becomes minimal. A turbulent flame subsequently propagates rapidly through the entire mixture over time scales consistent with experimental observations. We demonstrate that the neglect of turbulence-chemistry-interactions fundamentally fails to capture the key features of this ignition process.

A high pressure ignition delay time study of 2-methylfuran and tetrahydrofuran in shock tubes

Ignition delay time studies for tetrahydrofuran (THF) and 2-methylfuran (2MF) as well as optical investigations of combustion for 2MF have been carried out using two shock tubes. The experiments with undiluted THF/air mixtures were performed at 20 and 40bar in a high pressure shock tube (HPST) at an equivalence ratio of Ф=1 covering an overall temperature range of 780–1100K and 691–1006K, respectively. Undiluted 2MF/air mixtures (Ф=1) were also investigated in the HPST at 40bar in the temperature range of 820–1215K. The experimental data of 2MF obtained at 40bar were supported with kinetic simulations of existing models from literature. Additionally, sensitivity analyses of 2MF at several temperatures were performed for finding out the most sensitive reactions. Schlieren imaging was employed in a rectangular shock tube (RST) utilizing a high speed video camera through which the ignition process was captured for a stoichiometric 2MF/O2/Ar mixture at pressures of about 10bar and in the temperature range of 871–1098K.The pressure signals of THF and 2MF at 40bar indicate two types of pre-ignition at low temperatures: a short two-stage ignition for THF and a relatively long and smooth increase in pressure before main ignition for 2MF. Furthermore, in case of 2MF at 40bar, far-wall ignitions at low temperatures could be observed. The deviation between simulation and experiment as well as the presence of pre-ignitions in the low temperature regime were the main reasons for undertaking optical investigations of 2MF. The Schlieren images show that the ignition process at low temperatures (T⩽940K) begins as a deflagrative phase in the form of flame kernels and ends in a strong ignition (explosion in explosion).The current study analyzes the auto-ignition of THF and 2MF at engine relevant pressures and temperatures. The optical investigations have been conducted to analyze the ignition behavior of 2MF.