Net Acceleration Research Articles

Full utilization of conjugate symmetry: combining virtual conjugate coil reconstruction with partial Fourier imaging for g-factor reduction in accelerated MRI.

In this study we propose a method to combine the parallel virtual conjugate coil (VCC) reconstruction with partial Fourier (PF) acquisition to improve reconstruction conditioning and reduce noise amplification in accelerated MRI where PF is used. Accelerated measurements are reconstructed in k-space by GRAPPA, with a VCC reconstruction kernel trained and applied in the central, symmetrically sampled part of k-space, while standard reconstruction is performed on the asymmetrically sampled periphery. The two reconstructed regions are merged to form a full reconstructed dataset, followed by PF reconstruction. The method is tested in vivo using T1-weighted spin-echo and T2*-weighted gradient-echo echo planar imaging (EPI) sequences, using both in-plane and simultaneous multislice (SMS) acceleration, at 1.5T and 3T field strengths. Noise amplification is estimated with theoretical calculations and pseudo-multiple-replica computations, for different PF factors, using zero-filling, homodyne, and projection onto convex sets (POCS) PF reconstruction. Depending on the PF algorithm and the inherent benefit of VCC reconstruction without PF, approximately 35% to 80%, 15% to 60%, and 5% to 30% of that intrinsic SNR gain can be retained for PF factors 7/8, 6/8, and 5/8, respectively, by including the VCC signals in the reconstruction. Compared with VCC-reconstructed acquisitions of higher acceleration, without PF, but having the same net acceleration, the combined method can provide a higher SNR if the inherent benefit of VCC is low or moderate. The proposed technique enables the partial application of VCC reconstruction to measurements with PF using either in-plane or SMS acceleration, and therefore can reduce the noise amplification of such acquisitions.

Design of a plasmonic metasurface laser accelerator with a tapered phase velocity for subrelativistic particles

A metallic metasurface-based laser-driven particle accelerator for subrelativistic particles is proposed and studied theoretically. The metasurface consists of a nonperiodic array of nanoslits which focuses the field of the driving laser, utilizing the phenomenon of extraordinary plasmonic transmission, to maximize the acceleration gradient. In order to account for the actual change in the particles' velocity during their propagation through the structure, the separation between successive slits is not constant but rather optimized according to the expected trajectory of the particles. The metasurface laser accelerator (MLA) is designed for an ultrafast driving laser source operating at $2\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ wavelength. An approximate analytical model verified by particle tracking simulations predicts a net average acceleration with a normalized acceleration gradient of 1.34 times the incident laser field. Compared to other laser-driven accelerator designs, the MLA provides substantially higher efficiency, due to the field enhancement associated with nanoantennas, and relaxed fabrication challenges (especially for subrelativistic particles). It is found that the output particle beam is microbunched, suggesting the possibility of using a short MLA structure as a prebuncher to improve the initial capture efficiency in a subsequent longer MLA device. The impact of space-charge effects is also studied, and the loaded gradient and optimal bunch charge are estimated.

Correlations between the CME acceleration, other CME parameters and flare energy

While the Coronal Mass Ejections (CMEs) without associated flares are known to accelerate and decelerate being moderated by the Lorentz force, gravity and the drag force due to solar wind, the flare association is known to prolong the acceleration. However, with or without the flare association, a significant proportion of slow CMEs decelerate and a similar proportion of fast CMEs accelerate. In the case of accelerating CMEs, various parameters of CMEs such as the mass, angular width etc. show good correlation and this improves with flare association. When the flares and CMEs are associated, there is apparently a division of energy between the flares and the CMEs. It is also known that the magnetic flux spanned by the flare arcade and ribbons after flare maximum, roughly equals the magnetic flux content of the CME and their ratio could be between one and two. The magnetic flux content of the CME can be estimated from the final angular width of the CME. Hence, we suggest that the CMEs experience net acceleration when the different parameters of CMEs such as angular width, mass etc. are correlated and when the CME parameters are correlated with flare duration and peak flux. The absence of the same may lead to CMEs experience net deceleration.

Simultaneous multislice cardiac magnetic resonance fingerprinting using low rank reconstruction.

This study introduces a technique for simultaneous multislice (SMS) cardiac magnetic resonance fingerprinting (cMRF), which improves the slice coverage when quantifying myocardial T1, T2 , and M0 . The single-slice cMRF pulse sequence was modified to use multiband (MB) RF pulses for SMS imaging. Different RF phase schedules were used to excite each slice, similar to POMP or CAIPIRINHA, which imparts tissues with a distinguishable and slice-specific magnetization evolution over time. Because of the high net acceleration factor (R=48 in plane combined with the slice acceleration), images were first reconstructed with a low rank technique before matching data to a dictionary of signal timecourses generated by a Bloch equation simulation. The proposed method was tested in simulations with a numerical relaxation phantom. Phantom and in vivo cardiac scans of 10 healthy volunteers were also performed at 3T. With single-slice acquisitions, the mean relaxation times obtained using the low rank cMRF reconstruction agree with reference values. The low rank method improves the precision in T1 and T2 for both single-slice and SMS cMRF, and it enables the acquisition of maps with fewer artifacts when using SMS cMRF at higher MB factors. With this technique, in vivo cardiac maps were acquired from three slices simultaneously during a breathhold lasting 16 heartbeats. SMS cMRF improves the efficiency and slice coverage of myocardial T1 and T2 mapping compared with both single-slice cMRF and conventional cardiac mapping sequences. Thus, this technique is a first step toward whole-heart simultaneous T1 and T2 quantification with cMRF.

Numerical study of dynamical friction with thermal effects – I. Comparison to linear theory

We investigate by means of numerical simulations with nested Cartesian meshes the force exerted on a massive and luminous perturber moving at constant speed through a homogeneous and opaque gas, taking into account thermal diffusion in the gas and the radiative feedback from the perturber. The force arising from the release of energy into the ambient medium by the perturber, or heating force, is directed along the direction of motion and induces an acceleration of the perturber. Its value is compared to analytic estimates in the low and high Mach number limits, and found to match those accurately. In addition, the drag exerted on a non-luminous perturber significantly departs from the adiabatic expression when thermal diffusion is taken into account. In the limit of a vanishing velocity, this drag tends to a finite value which we determine using linear perturbation theory and corroborate with numerical simulations. The drag on a non-luminous perturber in a non-adiabatic gas therefore behaves like dry or solid friction. We work out the luminosity threshold to get a net acceleration of the perturber and find it to be generally much smaller than the luminosity of accreting low-mass planetary embryos embedded in a gaseous protoplanetary disc at a few astronomical units. We also present in some detail our implementation of nested meshes, which runs in parallel over several \emph{Graphics Processing Units} (GPUs).

Distinction in the Interplanetary Characteristics of Accelerated and Decelerated CMEs/Shocks

A set of 58 Coronal Mass Ejections (CMEs) with different kinematics near the sun in LASCO Field of view (FOV) is classified into two groups (i) CMEs which are accelerating (group-I) and (ii) CMEs which are decelerating (group-II). We analyze their interplanetary propagation characteristics to study the distinction between these two groups of events. Some of the following deviations are noted between the two groups as: (i) While group-II events have greater mean values of Standoff distance, Standoff time than the group-I events, the mean transit times of ICMEs and IP shocks are relatively lower for them. (ii) Group-II events are more (30%) radio-rich than the group-I (10%) and they are associated with type II solar radio burst in lower corona, (iii) The possibility of having excess magnetic energy that supports the propagation of CMEs to some extent is studied using estimated speed (VEST) and it is found that a slightly more number of events in group-I (48%) has VEST > VLASCO than group-II (33%). (iv) Net interplanetary acceleration is positive for 35% and 19% in group-I and group-II events respectively. (v) It is also found that ICME/IP shock characteristics of the two groups depend strongly on the CME acceleration.

Acoustic response of a lean premixed flame propagating upward in a tube

An analysis is carried out of the response of a curved flame front rising in a vertical tube to acoustic perturbations of frequency small compared to the ratio of the speed of sound to the radius of the tube. The gas around the flame front, whose characteristic size is the radius of the tube, is then subjected to a oscillatory, nearly uniform acceleration that adds to the acceleration of gravity. As in the better studied case of a downward propagating flame, the response of the flame front to this oscillatory acceleration leads to fluctuations of the front area and the global thermal expansion of the gas, which may inject energy into the acoustic field and thus induce a thermoacoustic instability of the flame plus tube system. Acoustic temperature perturbations associated to adiabatic compression and expansion of the gas are assumed to be much smaller that the Frank–Kamenetskii temperature of the flame and are left out. The transfer function relating thermal expansion fluctuations to infinitesimal acoustic velocity perturbations is computed as a function of the frequency for conditions typical of a very lean flame in the standard flammability tube. The response of the flame front to finite amplitude acoustic perturbations has been also analyzed. At frequencies small compared to the inverse of the residence time of the gas around the flame front, the response is quasi-stationary until the oscillatory acceleration overcomes the acceleration of gravity during a part of the cycle sufficiently long for the front to develop wrinkles and eventually break down under a net upward acceleration. At higher frequencies, the curvature of the front decreases when the amplitude of the acoustic perturbation increases. A secondary instability of the oscillating flame that develops over many oscillation cycles and leads to breakdown appears at a certain value of the amplitude of the perturbation. The onset of this instability seems to be related to the oscillatory motion of the tube wall relative to the flame tip, which induces a flow that carries upwards a part of the front and causes negative velocities of the tip relative to the far fresh gas. This instability features period doubling in a certain range of frequencies.

Deviations from hydrostatic equilibrium in the circumgalactic medium: spinning hot haloes and accelerating flows

Hydrostatic equilibrium (HSE), where the thermal pressure gradient balances the force of gravity, is tested across a range of simulated EAGLE haloes from Milky Way L* haloes (M200|$\approx$| 1012M⊙) to cluster scales. Clusters (M200|$\approx$|1014M⊙) reproduce previous results with thermal pressure responsible for |$\sim$|90 per cent of the support against gravity, but this fraction drops for group-sized haloes (M200|$\approx$| 1013M⊙) and is even lower (40–70 per cent) for L* haloes between 0.1 and 0.3R200. Energy from feedback grows relative to the binding energy of a halo towards lower mass resulting in greater deviations from HSE. Tangential motions comprise the largest deviation from HSE in L* haloes indicating that the hot circumgalactic medium (CGM) has significant subcentrifugal rotation and angular momentum spin parameters 2−3 × higher than the dark matter spin parameters. Thermal feedback can buoyantly rise to the outer CGM of M200 ≲ 1012M⊙ haloes, both moving baryons beyond R200 and feeding uncorrelated tangential motions. The resulting hot halo density and rotation profiles show promising agreement with X-ray observations of the inner Milky Way halo, and we discuss future observational prospects to detect spinning hot haloes around other galaxies. Acceleration and radial streaming motions also comprise significant deviations from HSE, especially net outward accelerations seen in L* and group haloes indicating active feedback. Black hole feedback acts in a preventative manner during the later growth of group haloes, applying significant accelerations via shocks that do not feed tangential motions. We argue that HSE is a poor assumption for the CGM, especially in the inner regions, and rotating baryonic hot haloes are a critical consideration for analytic models of the CGM.

The Acceleration Process of a Solar Quiescent Filament in the Inner Corona

Abstract Coronal mass ejections (CMEs) are frequently associated with filament eruptions. Theoretical studies propose that both magnetic reconnection and ideal magnetohydrodynamic instability of magnetic flux ropes can convert coronal magnetic energy into the filament/CME kinetic energy. Numerical simulations and analytical considerations demonstrate that both mechanisms can have significant contributions to the filament/CME acceleration. Many observational studies support that reconnection plays an important role during the acceleration, while it remains open how to resolve observationally the contribution of the ideal instability to the acceleration. On the other hand, it is difficult to separate and compare their contributions through observations as both mechanisms often work in a close time sequence. In this Letter, the above issues are addressed by analyzing the eruption process of a quiescent filament. The filament started to rise from ∼00:00 UT on 2011 December 25, 20 minutes earlier than the starting time of the flare impulsive phase (∼00:20 UT), and reached the maximum velocity at the flare peak time (∼00:50 UT). We divide the acceleration process into two stages, corresponding to the pre-flare and flare impulsive phases, respectively. The analysis indicates that an ideal flux-rope instability is dominant in the first stage, while reconnection below the flux rope becomes important during the second stage, and both mechanisms may have comparable contributions to the net acceleration of the filament.

The Sun–Earth saddle point: characterization and opportunities to test general relativity

The saddle points are locations where the net gravitational accelerations balance. These regions are gathering more attention within the astrophysics community. Regions about the saddle points present clean, close-to-zero background acceleration environments where possible deviations from General Relativity can be tested and quantified. Their location suggests that flying through a saddle point can be accomplished by leveraging highly nonlinear orbits. In this paper, the geometrical and dynamical properties of the Sun–Earth saddle point are characterized. A systematic approach is devised to find ballistic orbits that experience one or multiple passages through this point. A parametric analysis is performed to consider spacecraft initially on $$L_{1,2}$$ Lagrange point orbits. Sun–Earth saddle point ballistic fly-through trajectories are evaluated and classified for potential use. Results indicate an abundance of short-duration, regular solutions with a variety of characteristics.

Boost-Phase Trajectory Inference From Space-Borne LOS Measurements With a Sieve Method

Boost-phase trajectory inference is one of the major objectives of the space-borne missile early-warning system. Traditional methods can be classified as either profile-based or profile-free methods. The profile-based methods are accurate but inadaptable to the types of missiles, whereas the profile-free methods are adaptable but inaccurate. To integrate the strengths of the profile-based and profile-free methods, a multimodel trajectory inference approach is proposed. First, a general net acceleration model (GNAM) containing only type-free prior information is constructed by the method of sieves. Then, a new kind of net acceleration profile is proposed by incorporating type-dependent prior information into the GNAM. After that, the multimodel approach is proposed following the Bayesian framework. Simulations indicate that the approach is accurate in estimation and capable for type identification.

Realistic Simulations of Non-Linear Acceleration of the Rocket in the Air

The motion of the object in the medium has always been a hot research topic, and it is closely connected with many applications in our life. The acceleration of the object with multiple forces becomes very complicated, especially when these forces depend on the motion of the object. The exact formula for the object motion is a differential-integral equation and is very difficult to be solved analytically. One example of this kind of motions is the rocket launch. With sufficient thrust, the rocket can obtain an acceleration large enough to escape from the gravity of the earth. With the increasing height, the gravity from the earth becomes smaller, which affects the net acceleration of the rocket. Meanwhile, the air resistance becomes more and more important when the velocity of the rocket increases. It even plays the main role in the middle stage of the launch. Also, as the air resistance depends on both the velocity of the rocket and the air density (there is no air resistance in vacuum), the air resistance will decrease when the air density becomes small enough at the large height. In this article, a model that includes all of the factors mentioned above is established, and how these forces change the velocity of the rocket is analyzed. Two scenarios, one with air resistance and one without, are described. The velocity of the rocket in each scenario is represented by graphs, which are compared. With justification, the Taylor series is used to solve the differential-integral equation, and it is found that the fuel thrust and the gravity become important in the rocket launch at the beginning stage. In the middle stage, the air resistance begins to have a significant effect and reduces the acceleration of the rocket. In the final stage, there is virtually no gravity or air resistance, and only the fuel thrust contributes to the acceleration of the rocket.

Ageing-related changes in the cortical processing of otolith information in humans.

Acoustic short tone bursts (STB) trigger ocular and cervical vestibular-evoked myogenic potentials (oVEMPs/cVEMPs) by activating irregular otolith afferents. Simultaneously, STBs introduce an artificial net acceleration signal of otolith origin into the vestibular network. VEMP parameters as diagnostic otolith processing markers have been shown to decline after the age of thirty. To delineate the differential effects of healthy ageing on the cortical vestibular subnetwork processing otolith information, we measured cVEMPs and the differential effects of unilateral STB in three age groups (20-40, 40-60 and 60+; n=42) using functional neuroimaging. STB evoked responses in the main vestibular hubs in the parieto-opercular cortex. Whereas cVEMP amplitudes declined linearly with age, analysis of the BOLD response size depicted a u-shaped curve. Vestibular perception of the otolith stimulus on the other hand remained unchanged with age. Therefore, we propose that the comparably larger BOLD responses past the age of sixty could reflect a mechanism of central sensitisation for otolith perception to counterbalance the concurrent peripheral vestibular and somatosensory function decline.

Gaseous Dynamical Friction in Presence of Black Hole Radiative Feedback

Abstract Dynamical friction is thought to be a principal mechanism responsible for orbital evolution of massive black holes (MBHs) in the aftermath of galactic mergers and an important channel for formation of gravitationally bound MBH binaries. We use 2D radiative hydrodynamic simulations to investigate the efficiency of dynamical friction in the presence of radiative feedback from an MBH moving through a uniform density gas. We find that ionizing radiation that emerges from the innermost parts of the MBH’s accretion flow strongly affects the dynamical friction wake and renders dynamical friction inefficient for a range of physical scenarios. MBHs in this regime tend to experience positive net acceleration, meaning that they speed up, contrary to the expectations for gaseous dynamical friction in absence of radiative feedback. The magnitude of this acceleration is however negligibly small and should not significantly alter the velocity of MBHs over relevant physical timescales. Our results suggest that suppression of dynamical friction is more severe at the lower mass end of the MBH spectrum which, compounded with inefficiency of the gas drag for lower mass objects in general, implies that <107 MBHs have fewer means to reach the centers of merged galaxies. These findings provide formulation for a sub-resolution model of dynamical friction in presence of MBH radiative feedback that can be easily implemented in large scale simulations.

Direct diffusion tensor estimation using a model-based method with spatial and parametric constraints.

To develop a new model-based method with spatial and parametric constraints (MB-SPC) aimed at accelerating diffusion tensor imaging (DTI) by directly estimating the diffusion tensor from highly undersampled k-space data. The MB-SPC method effectively incorporates the prior information on the joint sparsity of different diffusion-weighted images using an L1-L2 norm and the smoothness of the diffusion tensor using a total variation seminorm. The undersampled k-space datasets were obtained from fully sampled DTI datasets of a simulated phantom and an ex-vivo experimental rat heart with acceleration factors ranging from 2 to 4. The diffusion tensor was directly reconstructed by solving a minimization problem with a nonlinear conjugate gradient descent algorithm. The reconstruction performance was quantitatively assessed using the normalized root mean square error (nRMSE) of the DTI indices. The MB-SPC method achieves acceptable DTI measures at an acceleration factor up to 4. Experimental results demonstrate that the proposed method can estimate the diffusion tensor more accurately than most existing methods operating at higher net acceleration factors. The proposed method can significantly reduce artifact, particularly at higher acceleration factors or lower SNRs. This method can easily be adapted to MR relaxometry parameter mapping and is thus useful in the characterization of biological tissue such as nerves, muscle, and heart tissue.

Real-time cardiac magnetic resonance cine imaging with sparse sampling and iterative reconstruction for left-ventricular measures: Comparison with gold-standard segmented steady-state free precession

BackgroundSegmented cine imaging with a steady-state free-precession sequence (Cine-SSFP) is currently the gold standard technique for measuring ventricular volumes and mass, but due to multi breath-hold (BH) requirements, it is prone to misalignment of consecutive slices, time consuming and dependent on respiratory capacity. Real-time cine avoids those limitations, but poor spatial and temporal resolution of conventional sequences has prevented its routine application. We sought to examine the accuracy and feasibility of a newly developed real-time sequence with aggressive under-sampling of k-space using sparse sampling and iterative reconstruction (Cine-RT). MethodsStacks of short-axis cines were acquired covering both ventricles in a 1.5T system using gold standard Cine-SSFP and Cine-RT. Acquisition parameters for Cine-SSFP were: acquisition matrix of 224×196, temporal resolution of 39ms, retrospective gating, with an average of 8 heartbeats per slice and 1–2 slices/BH. For Cine-RT: acquisition matrix of 224×196, sparse sampling net acceleration factor of 11.3, temporal resolution of 41ms, prospective gating, real-time acquisition of 1 heart-beat/slice and all slices in one BH. LV contours were drawn at end diastole and systole to derive LV volumes and mass. ResultsForty-one consecutive patients (15 male; 41±17years) in sinus rhythm were successfully included. All images from Cine-SSFP and Cine-RT were considered to have excellent quality. Cine-RT-derived LV volumes and mass were slightly underestimated but strongly correlated with gold standard Cine-SSFP. Inter- and intra-observer analysis presented similar results between both sequences. ConclusionsCine-RT featuring sparse sampling and iterative reconstruction can achieve spatial and temporal resolution equivalent to Cine-SSFP, providing excellent image quality, with similar precision measurements and highly correlated and only slightly underestimated volume and mass values.

Higher resolution cine imaging with compressed sensing for accelerated clinical left ventricular evaluation

To assess the clinical feasibility of a compressed sensing cine magnetic resonance imaging (MRI) sequence of both high temporal and spatial resolution (CS_bSSFP) in comparison to a balanced steady-state free precession cine (bSSFP) sequence for reliable quantification of left ventricular (LV) volumes and mass. Segmented MRI cine images were acquired on a 1.5T scanner in 50 patients in the LV short-axis stack orientation using a retrospectively gated conventional bSSFP sequence (generalized autocalibrating partially parallel acquisition [GRAPPA] acceleration factor 2), followed by a prospectively triggered CS_bSSFP sequence with net acceleration factor of 8. Image quality was assessed by published criteria. Comparison of sequences was made in LV volumes and mass, image quality score, quantitative regional myocardial wall motion, and imaging time using Pearson's correlation, Bland-Altman and paired 2-tailed Student's t-test. Differences (bSSFP minus CS_bSSFP, mean ± SD) and Pearson's correlations were 14.8 ± 16.3 (P = 0.31) and r = 0.98 (P < 0.0001) for end-diastolic volume (EDV), 8.4 ± 11.3 (P = 0.54) and r = 0.99 (P < 0.0001) for end-systolic volume (ESV), -0.4 ± 2.5 (P = 0.87) and r = 0.97 (P < 0.0001) for EF, and -0.9 ± 11.8 (P = 0.92) and r = 0.97 (P < 0.0001) for LV mass. Bland-Altman analyses [bias and (limits of agreement)] revealed strong agreement in LVEDV [8.7 ml, (-12.1, 29.6)], LVESV [4.3 ml, (-11.9, 20.6)], LVEF [-0.02%, (-5.37, 5.33)], and myocardial mass [-6.1 g, (-14.7, 26.9)]. Image quality was comparable with a similar mean score (P = 0.42), with a good correlation in image quality observed (r = 0.68, P < 0.0001). Quantitative regional myocardial wall motion demonstrated strong correlation between the sequences (r = 0.87, P < 0.0001). Imaging time was significantly shorter for the CS_bSSFP sequence (1.1 ± 0.5 versus 5.6 ± 1.6 min, P < 0.0001). The novel high-resolution cine CS_bSSFP accurately and reliably quantitates LV volumes and mass, shortens acquisition times, and is clinically feasible. 1 Technical Efficacy: Stage 2 J. MAGN. RESON. IMAGING 2017;45:1693-1699.

Simulation-based performance assessment: an innovative approach to exploring understanding of physical science concepts

ABSTRACTThis paper illustrates the use of simulation-based performance assessment (PA) methodology in a recent study of eighth-grade students’ understanding of physical science concepts. A set of four simulation-based PA tasks were iteratively developed to assess student understanding of an array of physical science concepts, including net force, acceleration, velocity, and inertia. Illustrative examples drawn from the administration of these simulation-based PA tasks to a sample of eighth-grade students (n = 74) highlight the potential of simulation-based tasks for revealing conceptual understanding of science concepts and for informing curriculum development within the context of design-based implementation research. The paper also describes a variety of challenges encountered over the course of the development, administration, and analysis of the PA data that may be informative for researchers and educators as they consider implementing simulation-based PAs.

Exploring Student Understanding of Force and Motion Using a Simulation-Based Performance Assessment

Performance assessment (PA) has been increasingly advocated as a method for measuring students’ conceptual understanding of scientific phenomena. In this study, we describe preliminary findings of a simulation- based PA utilized to measure 8th grade students’ understanding of physical science concepts taught via an experimental problem-based curriculum, SLIDER (Science Learning Integrating Design Engineering and Robotics). In SLIDER, students use LEGO robotics to complete a series of investigations and engineering design challenges designed to deepen their understanding of key force and motion concepts (net force, acceleration, friction, balanced forces, and inertia). The simulation-based performance assessment consisted of 4 tasks in which students engaged with video simulations illustrating physical science concepts aligned to the SLIDER curriculum. The performance assessment was administered to a stratified sample of 8th grade students (N=24) in one school prior to and following implementation of the SLIDER curriculum. In addition to providing an illustration of the use of simulation- based performance assessment in the context of design-based implementation research (DBIR), the results of the study indicate preliminary evidence of student learning over the course of curriculum implementation.

Improvement on the linear and nonlinear auto-regressive model for predicting the NOx emission of diesel engine

Given the increasingly stringent emission regulations, an accurate model of emission prediction is required for the aftertreatment systems of diesel engines. For example, the selective catalytic reduction system can realize higher accuracy emission control if the mass of nitrogen oxides (NOx) is known. Given its simplicity, convenience, and effectiveness, the method of data-driven modeling has been widely researched and considered a primary method to estimate the NOx mass before it reaches the aftertreatment device of a diesel engine. To fully use the known engine operating data and therefore improve the prediction accuracy, this study proposes and develops a general linear and nonlinear auto-regressive model with exogenous inputs (GNARX) for NOx prediction. A recursive least square algorithm with forgetting factor is given to estimate the model parameters, and a new simulated annealing based pruning algorithm is developed to identify the model structure. The proposed methods are first used to model the simulation and engineering data to validate their effectiveness and superiority in comparison to the conventional methods. Based on gray relational analysis, the main factors that influence NOx formation, such as the net engine torque, turbo speed, and accelerator pedal position, are then determined as the inputs for modeling the NOx emission of the diesel engine. The results show that the modeling and prediction accuracy of the GNARX model are higher than those of other models, which indicates that the GNARX model is feasible to predict NOx emission.