Kernel Growth Research Articles

Numerical and Experimental Characterization of a Natural Gas Engine With Partially Stratified Charge Spark Ignition

This paper outlines the development of a comprehensive numerical framework for the partially stratified charge (PSC) lean-burn natural gas engine. A 3D model of the engine was implemented to represent fluid motion and combustion. The spark ignition model was based on the works of Herweg and Maly (1992, “A Fundamental Model for Flame Kernel Formation in SI Engines,” SAE Technical Publication, Paper No. 922243) and Tan and Reitz (2006, “An Ignition and Combustion Model Based on the Level-Set Method for Spark Ignition Engine Multidimensional Modeling,” Combust. Flame, 145, pp. 1–15). The EDC model (Ertesvåg and Magnussen, 2000, “The Eddy Dissipation Turbulence Energy Cascade Model,” Combust. Sci. Technol., 159, pp. 213–235) with a two-step mechanism was used to model natural gas turbulent combustion process. An open geometry simulation strategy was adopted to account for intake-exhaust gas and valve movements. Each simulation was executed for multiple cycles to produce a representative residual gas fraction. The numerical results were compared with the experimental data obtained on the Ricardo Hydra single cylinder research engine for both homogeneous and PSC cases and they were found to be in excellent agreement in pressure trace and heat release rate. The detailed investigation of the numerical data showed the development of an ignitable mixture under PSC cases, allowing stable kernel growth well beyond the lean misfire limit of the bulk mixture. Furthermore, limits on successful ignition can be identified using the ignition model, which exhibited self-similar behavior in terms of flame speed and turbulent fluctuation. It can also be shown that, at ultralean air-fuel ratios, the PSC plume helps replicate the ignition conditions that can be found under stoichiometric operation.

A spark ignition model for large eddy simulation based on an FSD transport equation (ISSIM-LES)

The aim of this study is to develop a spark ignition model for LES of aero-engines and piston engines. This model called ISSIM-LES (Imposed Stretch Spark Ignition Model) is based on the same electrical circuit description as AKTIM [1] and flame kernel initialisation as AKTIMeuler [2]. The main difference with AKTIMeuler resides in the description of the flame kernel growth which is now solely modelled thanks to a modified flame surface density (FSD) equation. This new model allows multi-ignition description without any ad hoc adaptation since the different ignitions are directly controlled by the flame surface density (FSD) equation, which is modified during early flame ignition. This model is applied to flame kernel growth in isotropic homogeneous turbulence in a fan-stirred bomb for various equivalence ratios and turbulence levels [3]. Discussions and analysis focus on the sensitivity of the results to the transition from the SGS ignition kernel to the fully resolved turbulent flame, on the comparison between ISSIM-LES and AKTIMeuler, on the effect of stretch on the flame kernel growth and finally, on the sensitivity of the early flame kernel development to the initial turbulent velocity field.

A forced ignition probability analysis method using LES and Lagrangian particle monitoring

A large eddy simulations-based methodology is developed for predicting the probability of igniting a turbulent non-premixed jet flame with a localized spark. The method couples the LES to a Lagrangian monitoring of fluid particles and explores, through a sub-grid turbulent flame speed model, the local turbulent flow properties on upstream and downstream flame propagation in the axial direction following radial kernel growth. The velocity and mixing fields are validated by using experimental measurements. The predicted region of possible upstream flame propagation, characterized by near-stoichiometric mixture and low velocity flow, compares well with the available experimental data regarding its form and upstream flame speed. The probabilistic generation of a kernel, its convection and the subsequent flame propagation are investigated and the conditional statistics of the mixture at the ignition location, and the convection and transition to upstream propagating flame are determined. The conditional probability of formation, convection, growth and stabilization of flame kernels results in a prediction of the overall flame ignition probability, which agrees well with experiment.

LES study of cycle-to-cycle variations in a spark ignition engine

Multi-cycle Large Eddy Simulation (LES) of flow and combustion in a single cylinder engine set-up are presented in order to validate the ability of this technique to reproduce cyclic combustion variability in a spark ignition engine. The simulated engine operating point is part of a larger experimental database acquired at IFP and specifically designed to validate LES techniques in terms of cycle-to-cycle prediction. To characterize individual cycles, the database combines acquisitions of pressure and temperature in different locations of the engine set-up with optical diagnostics in the intake pipe and the cylinder. This new experimental database combined with LES is a powerful tool to study cycle-to-cycle variations (CCV). The operating points include points with low and high CCV levels. The LES includes the whole engine set-up, and covers 25 consecutive four-stroke engine cycles for a reference operating point with low CCV. The results show that LES is able to reproduce the experimental findings, and illustrate how LES can help explain the sources of CCV. In the present case it is shown that CCV are essentially due to velocity fluctuations at the spark plug, which induce variations of the early flame kernel growth and of the overall combustion duration.

Adequate zinc nutrition alleviates the adverse effects of heat stress in bread wheat

Zinc (Zn) deficiency and heat stress affect the productivity of cereal crops in many semi arid regions of the world. Zinc nutrition affects membrane integrity and activity of superoxide dismutase (SOD) enzymes which are also important to a plant’s ability to cope with high temperatures. Two experiments were conducted to examine the interaction between Zn nutrition and heat stress at two growth stages. Wheat was grown at deficient and adequate levels of Zn at 22/16°C before being exposed to a short period of high temperature (40/20°C). One experiment examined the response to a 3 day heat stress treatment at 10 days after anthesis and the second experiment examined the response to 6 days of heat stress in 30-day old seedlings. Varieties differing in thermotolerance and sensitivity to Zn deficiency were compared. Zinc deficiency and heat stress reduced kernel growth rates, kernel weight and grain yield and the greatest reductions were measured in Zn-deficient, heat stressed plants. The greater reduction in kernel growth rates from heat stress in Zn deficient plants occurred in both thermotolerant and sensitive varieties and was negatively associated with grain Zn concentration. Genetic differences in tolerance to Zn deficiency did not have consistent effects on the response to heat stress. Chlorophyll fluorescence and chlorophyll content were reduced and chloroplast ultrastructure was disrupted by heat stress and the effect was exacerbated by low supplies of Zn. While the responses to heat stress and low Zn were largely additive effects, the experiments demonstrated that low levels of Zn nutrition can exacerbate the adverse effects of short periods of heat stress on kernel growth and chloroplast function.

On flame kernel formation and propagation in premixed gases

Flame kernel formation and propagation in premixed gases have been studied experimentally and theoretically. The experiments have been carried out at constant pressure and temperature in a constant volume vessel located in a high speed shadowgraph system. The formation and propagation of the hot plasma kernel has been simulated for inert gas mixtures using a thermodynamic model. The effects of various parameters including the discharge energy, radiation losses, initial temperature and initial volume of the plasma have been studied in detail. The experiments have been extended to flame kernel formation and propagation of methane/air mixtures. The effect of energy terms including spark energy, chemical energy and energy losses on flame kernel formation and propagation have been investigated. The inputs for this model are the initial conditions of the mixture and experimental data for flame radii. It is concluded that these are the most important parameters effecting plasma kernel growth. The results of laminar burning speeds have been compared with previously published results and are in good agreement.

LES-pdf simulation of a spark ignited turbulent methane jet

A spark ignited turbulent jet flame is simulated using Large Eddy Simulation (LES) in conjunction with the filtered probability density function (pdf) equation approach, which is solved using the Eulerian stochastic field method. The spark energy deposition is mimicked by a gaussian distributed source term in the enthalpy equation. A dynamic model for the sub-grid stresses together with a simple gradient diffusion approximation for the scalar fluxes is adopted. The chemistry is represented by a global 4-step chemistry involving seven species. The different stages of the ignition sequence, namely the flame kernel growth, the triple flame propagation against the flow and the stabilisation were in good agreement with the experimental data.

Calcium regulation of carbohydrate modification in sorghum

Two improved Nigeria sorghum cultivars (KSV 8 and ICSV 400) were used to evaluate the effects of steep water Ca2+ treatment on carbohydrate modification in sorghum. The response of all the carbohydrate mobilization indicators evaluated [
- and - amylases, diastatic activity (DP), extract and cold water soluble carbohydrates (CWS-carbohydrates)], to steep water Ca2+ treatment was highly significantly (p  0.001) cultivar and steep water Ca2+ treatment dependent. In contrast to KSV 8, Ca2+ treatment generally caused significant repression of
-amylase development in ICSV 400. Development of -amylolytic activity in KSV 8 was however, significantly repressed by Ca2+ treatment. Interestingly, - amylase activity constituting well over 80% of total diastatic activity was attained in ICSV 400 grains subjected to 100 ppm Ca2+ treatment. Hot water extract (HWE) showed statistically insignificant (p  0.1) linear variation with Ca2+ treatment. Although Ca2+ treatment significantly (p  .001) repressed CWS carbohydrates in both cultivars, significantly higher CWS-carbohydrates and HWE were released in ICSV 400 for each DP unit than the corresponding DP in KSV 8 malts would permit. Thus, suggesting important roles for factors other than DP, possibly proteolysis, in determining HWE and CWS carbohydrates. The benefits of reduced kernel growth and malting loss were neutralized by the general repression of carbohydrate modification indices for both cultivars.

A Thermodynamic Model for Argon Plasma Kernel Formation

Plasma kernel formation of argon is studied experimentally and theoretically. The experiments have been performed in a constant volume cylindrical vessel located in a shadowgraph system. The experiments have been done in constant pressure. The energy of plasma is supplied by an ignition system through two electrodes located in the vessel. The experiments have been done with two different spark energies to study the effect of input energy on kernel growth and its properties. A thermodynamic model employing mass and energy balance was developed to predict the experimental data. The agreement between experiments and model prediction is very good. The effect of various parameters such as initial temperature, initial radius of the kernel, and the radiation energy loss have been investigated and it has been concluded that initial condition is very important on formation and expansion of the kernel.

Trait dissection of maize kernel weight: Towards integrating hierarchical scales using a plant growth approach

Maize (Zea mays L.) yield is a function of the number harvested kernels per unit land area and the individual kernel weight (KW). Kernel weight and its development show a wide variability due to the genotype, the environment, the crop management, and all possible interactions. Commercial maize hybrids differ markedly in the patterns (rate and duration of kernel growth) behind differences in final KW. The same can be observed when public or elite proprietary maize inbred lines are analyzed. To progress in our understanding of KW variability, we reviewed and discussed current knowledge for analyzing kernel growth as an integrated system, modulated by processes linking different levels of organization (the different kernel tissues, the whole kernel, the plant, the canopy). Ideas on how to integrate this knowledge towards the development of a multi-hierarchical scale framework for predicting KW under different growth environments are currently needed, as they have high relevance for dissecting the genetic basis of kernel growth and maize yield definition at the canopy level. At the kernel and tissue level, we highlight the need of focusing and studying traits like assimilate movement into developing kernels, endosperm cell division, endosperm cell death, kernel internal homeostasis and kernel water relations. These specific processes need to be connected affecting the rate or duration of grain filling at the whole kernel level for studying kernel size variability. At the plant and canopy levels, the dynamic response of both kernel number and potential KW to plant growth around flowering appears critical for understanding KW variations, as maize final KW is highly associated with the potential KW determined during the first stages of grain filling. Focusing on the period around flowering in this species is important for yield improvement through both yield components, kernel number and size. At present it seems that maize potential yield can only be improved by modifying the sink capacity established at the end of the lag phase and increasing the source strength during grain filling so as to fulfill this potential.

Genetic Variation in Potential Kernel Size Affects Kernel Growth and Yield of Sorghum

Large‐seededness can increase grain yield in sorghum [Sorghum bicolor (L.) Moench] if larger kernel size more than compensates for the associated reduction in kernel number. The aim of this research was to investigate kernel filling characteristics associated with the high kernel weight of KS115 germplasm and consider implications for grain yield improvement. Germplasm with a twofold range in potential kernel size was grown in two field experiments. In both experiments, a large‐seeded hybrid based on KS115 yielded significantly more grain than a normal‐seeded hybrid with the same female parent. Genotypic differences in final kernel weight were associated with both an increased rate and extended duration of kernel filling. A high kernel filling rate was associated with a high rate of increase in kernel water content and kernel volume, which resulted in large maximum water content and volume per kernel. The long kernel filling duration of KS115‐based germplasm was associated with an extended duration of the period of rapid water uptake into the kernel following the onset of rapid dry mass accumulation in the grains. For this particular germplasm, this was likely due to the presence of large ovaries before fertilization. Because of a trade‐off between potential kernel weight and number, prolonged duration, rather than an increased rate of kernel filling, appeared to be the main mechanism through which large‐seededness of KS115 could increase grain yield.

Maize Kernel Oil and Episodes of Shading during the Grain‐Filling Period

ABSTRACTPrevious studies documented the stability of maize (Zea mays L.) kernel oil concentration for a wide range of kernel weights promoted by contrasting post‐flowering assimilate availabilities per kernel (i.e., source–sink ratios). These studies mainly modified the sink size with a low impact on the source size. In this study, we focused on kernel oil concentration response to source–sink ratio alterations promoted by different timings and intensities of shading during the effective grain‐filling period. Two crosses with contrasting kernel oil concentration (‘DK752’ × DK752 and DK752 × ‘5MG’) were tested. Kernel oil concentration was positively related to the embryo–kernel ratio (r = 0.96, P < 0.001) and embryo oil concentration (r = 0.94, P < 0.001) and crosses differed in both traits. Severe shading (85% reduction of incident solar radiation) at early stages of kernel growth reduced the final embryo–kernel ratio and the embryo oil concentration of both crosses. Contrarily, moderate shading (45% reduction of incident solar radiation) did not modify the kernel oil determinants. Kernel oil concentration and kernel weight declined when severe shading shortened the kernel growth period. Our results collectively sustain the hypothesis that maize kernel oil concentration is commonly sink‐limited. We established that kernel oil concentration of both crosses was reduced when post‐flowering plant growth rate per kernel was less than 50% of kernel growth rate.

The Martin Kernel for Unbounded Domains

The unbounded domains under consideration include the parabolic-type regions and shrinking one-sided horns that have appeared in the literature. We show the Martin boundary at ∞ is a singleton and we determine the growth of the corresponding Martin kernel. For a parabolic-type region expanding at a power law rate, the growth is subexponential and for a horn shrinking at a power law rate it is superexponential.

Effect of Spark-Ignition on Combustion Periods during Spark-Assisted Compression Ignition

Previous works employed pressure-based heat release analysis, spark-discharge current/voltage measurements, and cycle-resolved combustion imaging to identify four periods of stratified-charge spark-assisted compression ignition: spark discharge, early kernel growth (EKG), flame propagation, and compression ignition. While it was found that variability in the EKG, and not the spark discharge, influenced combustion phasing, the influence of the spark discharge on the EKG could not be assessed because the imaging system was incapable of recording both the spark discharge and the flame development from the same cycle. This limitation has been overcome here by recording images of both the spark discharge and combustion chemiluminescence during each engine cycle. Additionally, the spark discharge plasmas were recorded from two nearly-orthogonal views to assure that out-of-plane excursions were measured; such are not available from a single view. The results in this study demonstrate that the spark discharge event was not deterministic of the EKG. Circumstantial evidence is given to suggest that the fuel/air distribution and velocity at the spark plug dominate the EKG period variability, though this could not be confirmed definitively.

Characterization of Grain‐Filling Patterns in Diverse Maize Germplasm

Information regarding genotypic variability for maize (Zea mays L.) grain‐filling patterns is scarce, especially at the inbred level. We characterized a large set of public and elite proprietary inbred lines for kernel growth traits. Lines were selected to sample divergent kernel size, genetic diversity, and lines released from breeding programs. The traits characterized were final kernel weight (KW), kernel growth rate (KGR), grain‐filling duration (GFD), maximum water content and moisture concentration (fresh weight basis) at physiological maturity. We evaluated 32 inbred lines in 2006 and 35 inbred lines in 2007. Kernel weight ranged from 104 to 317 mg per kernel in 2006 and from 96 to 327 mg per kernel in 2007. Variation in KW was achieved through different combinations of KGR (0.14 to 0.44 mg °Cd−1 kernel−1) and GFD (610 to 1137 °Cd). There was no correlation between KGR and GFD. Moisture concentration at physiological maturity showed significant genotypic differences (p < 0.001), ranging from 280 to 600 g kg−1 A previously described framework for predicting kernel growth patterns based on kernel water accumulation was tested for accuracy and differences were observed (p < 0.01), suggesting that a general pattern cannot be used to describe all the genotypic variability available. Results demonstrate the high variability in KW across elite and public maize inbreds, reflecting a wide range of KGRs and GFDs.

Modeling ignition phenomena in spray-guided spark-ignited engines

An ignition model for spray-guided (SG) direct-injection gasoline engines called SparkCIMM—Spark Channel Ignition Monitoring Model—is presented in this paper. The model concept is motivated by high-speed imaging data showing complex processes for spark (formation, turbulent stretching and wrinkling, and multiple restrikes) and ignition (localized flame kernel formation and growth). Turbulent fluctuations occurring on the scale of turbulent spark-channel wrinkling (∼0.05–0.1 mm) and local ignition (∼0.1–0.5 mm) are analyzed using expressions for the sub-grid turbulent kinetic energy, eddy turnover velocity and mixture-fraction variance. Computational particles are introduced along the spark channel to monitor its motion and ignitability. Ignition is determined by a local Karlovitz-number criterion that incorporates effects of turbulence, detailed chemical kinetics, and continuous energy deposition from the spark. Wherever suitable ignition conditions are found along the elongated spark channel, a small flame kernel is launched and tracked as it grows and merges with other flame kernels to form the turbulent flame surface. When the flame surface is sufficiently large, a G-equation flamelet combustion model tracks the turbulent flame front. Both the early flame-kernel-growth and G-equation combustion models include effects of local mixture-fraction variance. Results from the SparkCIMM ignition model and G-equation combustion model are compared with experiments in an SG direct-injection engine. The model reproduces the general experimental features of spark-channel stretching, corrugation, and localized ignition; and flame front location probabilities are in good agreement.

Experimental metrics for identifying origins of combustion variability during spark-assisted compression ignition

Spark-assisted compression ignition, SACI, can be used to control the combustion phasing of compression-ignition gasoline engines. However, implementation of this technique can be confounded by cyclic variability. The purpose of this paper is to define experimental metrics that describe the SACI process and to demonstrate the use of these metrics for identifying the source(s) of cyclic variability during the SACI process. This study focused on a light load condition (7 mg/cycle, 200 kPa i.m.e.p.), where spray-guided direct fuel injection with spark ignition and an exhaust-rebreathing strategy was employed to achieve flame propagation, which led to compression ignition. This study employed a combination of measurements including pressure-based heat-release analysis, spark-discharge voltage/current measurements, and cycle-resolved combustion imaging. Based on these measurements, four distinct combustion periods were identified; namely, the spark discharge, the early kernel growth (EKG), flame propagation, and the compression ignition periods. Metrics were defined to characterize each period and used to identify the contribution of each period to the cyclic variability of the main heat release. For the light load condition studied here, the EKG period had the largest effect on the crank angle (CA) position of 50 per cent mass burned, CA50. The spark-discharge event may affect CA50 indirectly through its influence on EKG. However, this could not be definitively assessed here since the camera was incapable of recording both the spark-discharge event and the flame images during cycles of the same tests.

Prediction of Combustion Parameters, Performance, and Emissions in Compressed Natural Gas and Gasoline SI Engines

The simulation of heat release, flame propagation speeds, and pollutant formation was carried out in both a turbocharged compressed natural gas (CNG) engine and a multivalve naturally aspirated bifuel engine running on either CNG or gasoline. The predictive tool used for investigation is based on an enhanced fractal geometry concept of the flame front, which is able to capture the modulation of turbulent to laminar burning speed ratio throughout the overall combustion phase without introducing flame kernel growth or burnout submodels. The prediction model was applied to a wide range of engine speeds, loads, relative air-fuel ratios, and spark advances, and the obtained results were compared to experimental data. These latter were extracted from measured in-cylinder pressure by an advanced diagnostics technique that was previously developed by the authors. The results confirmed a quite accurate prediction of burning speed even without any kind of tuning, with respect to different currently available fractal as well as nonfractal approaches for the simulation of flame-turbulence interaction. Furthermore, the computational code proved to be capable of capturing the effects of fuel composition, different combustion-chamber concepts, and operating conditions on engine performance and emissions.

Maize kernel growth and kernel water relations as affected by nitrogen supply

Improving maize ( Zea mays L.) growing conditions near flowering by applying nitrogen (N) could affect both kernel number per unit area (KN) and potential kernel weight (KW). Potential kernel weight can be estimated with maximum kernel water content (MKWC), as final kernel weight and kernel water relations are strongly associated in maize. At the crop level, the product of KN per unit area and MKWC could provide an appropriate index of potential sink capacity. The main objective of this study was to determine if the decrease in potential sink capacity (i.e. the product of MKWC and KN), under N deficit and with a late planting date, is due to MKWC or KN reductions. Additionally, we evaluated sink growth rate per unit area (i.e. the product of KN and kernel growth rate) during grain fill period as related to potential sink capacity. Three N rates under optimal and late planting dates and two hybrids were evaluated in experiments carried out at Paraná, Argentina (31°50′S; 60°31′W) during 2002–2003 and 2003–2004 growing seasons (seasons 1 and 2, respectively). Except for the late planting date during season 1, there was a significant positive response on grain yield, KN, KW, and MKWC to N supply. Experiments explored a broad range of KN, from 1645 to 5066 kernel m −2. MKWC and KN were positively correlated for DK682. Nitrogen increased the potential sink capacity and sink growth rate only as KN increases from a particular threshold in both hybrids. The sink growth rate was largely related to potential sink capacity, as MKWC was highly correlated with KGR across all treatments. Our analytical approach, considering potential sink capacity as a product of KN and MKWC, is integrative of sink demand both for individual kernel and crop levels. This work highlights the role of early establishment of potential sink capacity on yield determination under a wide range of N conditions.

Deficit irrigation without reducing yield or nut splitting in pistachio ( Pistacia vera cv Kerman on Pistacia terebinthus L.)

An irrigation experiment involving pistachio ( Pistacia vera cv Kerman on Pistacia terebinthus L. rootstocks) was performed over a four-year period in central Spain to determine the effect of regulated deficit irrigation (RDI) on nut yield and quality. The growth season was divided into three phenological stages: stage I – from sprouting until the end of rapid nut growth; stage II – from maximum nut size until the beginning of kernel growth; and stage III – from the beginning of kernel growth until harvest. Control trees were irrigated to supply their full water needs throughout the growth season, except for the post-harvest period. Sustained deficit irrigation at 65% (DI 65) and 50% (DI 50) of control irrigation was provided to two other groups of trees. The RDI provided to a further group was designed to provide a stress period during stages I and II but no water stress during stage III; the aim was to reduce water use and increase the percentage of split nuts. A fifth group of trees was maintained under rain fed conditions. Water potential and leaf conductance were affected in the DI 65, DI 50 and rain fed treatments mainly during stages II and III, with midday water potentials below −2.0 MPa. The RDI trees were only significantly water stressed during stage II, showing midday water potentials of around −1.4 MPa. On most days, leaf conductance was not significantly affected in any of the irrigation treatments. The nuts of the DI 65 and DI 50 trees were smaller in diameter and their total yield was reduced compared to the controls. However, no significant differences in kernel dry weight were observed. The RDI trees showed a total yield and percentage of split nuts similar to those of the controls, even though they received around 20% less water. The split nut yield showed a linear relationship with crop evapotranspiration. However, since the percentage of split nuts was similar in all treatments this variation was likely related to the total yield. The RDI trees did not show the normal alternate bearing pattern (which was clearly maintained in the control trees). Early splitting, a process that decreases the quality of the yield, was not related to water status but to temperatures lower than 13 °C. The results suggest that P. terebinthus L. rootstocks confer P. vera scions a degree of drought-resistance, reducing the likelihood of water stress and, therefore, allowing more severe RDI scheduling.