Study on the ignitability of a high-pressure direct-injected methane jet using a diesel pilot, a glow-plug, and a pre-chamber

Natural gas is a promising alternative fuel for internal combustion engines, it allows for a reduction of engine-out emissions without impairing high engine efficiencies. Although this approach is already utilized from small to large engine classes, it is almost exclusively based on the combustion of a premixed, homogeneous charge. For ignition, small engines use standard spark-plugs or pre-chambers, while large and lean-operated engines use pre-chambers and pilot injections. Direct high-pressure gas injection is a more recent, alternative way to operate gas engines which offers benefits compared to premixed operation such as high compression ratio, high combustion pressures, lean operation, quantity regulation, among others. However, in contrast to diesel direct injection, the compression temperatures are too low for the auto-ignition of the gas jets. Therefore, an additional ignition system is required, usually a pilot injection system is used. In this study, the usability and performance of three ignition strategies for direct injected high-pressure gas jets have been investigated in an optically accessible test-rig that is able to operate at engine-like conditions. The first type of ignition system is a pilot injection with a liquid fuel, the second is a glow-plug located near the main gas jet, and the third system is a pre-chamber with a nozzle hole aimed at the main gas jet. Results show that all three strategies are feasible options under the studied conditions. Ignition by a pilot fuel injection is a safe and reliable way to ignite the main fuel. The glow-plug is less reliable and leads to high cycle-to-cycle variation. The best option in the present study is the pre-chamber, it is very reliable, delivers the highest peak cylinder pressure and exhibits the lowest cyclic variability. The good performance is attributed to the intense mixing of the main gas jet with the hot jet exiting the pre-chamber.

GPU-Enabled High-Fidelity LES Simulations for Turbomachinery Flows

Engineers performing computational simulations of flow physics are often faced with a trade-off between turn-around time and accuracy. High-fidelity models that can accurately capture small details of flow, such as turbulent mixing, are typically too expensive and are therefore reserved for studying smaller, component level problems. Standard models, like Reynolds-Averaged Navier-Stokes (RANS) and Unsteady-RANS, are used to predict larger interactions without the ability to accurately compute the small scales, at a lower computational cost than high-fidelity models. However, with specific algorithmic choices and access to large-scale GPU systems, we can demonstrate high-fidelity simulations of large engine sections that can be completed within engineering design cycle turn-around times, instead of the typical weeks to months required for high fidelity simulations. In this paper we present the high-order GENESIS code, employed in the simulation of complex turbulent flows inside the high-pressure turbine of a jet engine. The code efficiently exploits GPU accelerators to execute high-fidelity simulations, while also demonstrating extraordinary accuracy validated by experimental data and previous RANS model predictions. This is demonstrated for a three-dimensional high-pressure turbine stator domain, for which the LES is able to accurately predict wake mixing and temperature distribution, factors that are critical for designing durable turbine components. The new capability allows for computational studies of phenomena such as laminar to turbulent transition and wake mixing, all applied to relevant three-dimensional geometries present in the high-pressure turbine, all within the time scale of a typical engineering design cycle.

A Generalized Control Model and Its Digital Algorithm for Aerospace Electrohydraulic Actuators

It is difficult to describe precisely, and thus control satisfactorily, the dynamics of an electrohydraulic actuator to drive a high thrust liquid launcher engine, whose structural resonant frequency is usually low due to its heavy inertia and its complicated mass distribution. A generalized model is therefore put forward for maximum simplification and sufficient approximation, where a second-order transfer function is used to model the heavy mass-spring nature of the large engine body outside of the rod position loop, another second-order transfer function with two zeros and two poles representing the hydro-mechanical composite resonance effect in the closed rod position loop. A combined control strategy is applied to meet the stringent specification of static and dynamic performances, including a notch filter, a piecewise or nonlinear proportional, integral and differential (PID) controller and a feed-forward compensation. The control algorithm is implemented in digital signal processors with the same software structure but different parameters for different aerospace actuators. Compared to other approaches, this one makes it easier to grasp the system resonance nature, and, most importantly, the traditional dynamic pressure feedback (DPF) is replaced with the convenient digital algorithm, bringing prominent benefits such as a simplified design, reduced hardware cost and inherent higher reliability. The approach has been validated by simulation, experiments and successful flights.

Attribution of the Tibetan Plateau to Northern Drought

<p>The Tibetan Plateau (TP), which is located in Asia and has an average elevation of over 4000 m, acts as a raised source of heat and an isolated region of humidity in the atmosphere. The TP serves as a “world water tower” because it stores large amounts of water as glaciers, lakes, and rivers. Furthermore, previous studies have found that the easterly outflow of water vapor and clouds away from the TP contributes significantly to precipitation over downstream regions. However, the dynamic mechanism behind these observations is still unclear. It is known that the key driver in the transportation of air and water resources from the TP is the wind field. Under global warming, the pole ward expansion of the Hadley circulation and the thermal effect of the terrain over the TP forces the mid-latitude subtropical westerly jet(SWJ) to shift. However, the true effects of the SWJ are unclear.</p><p>Here, we propose a dynamic mechanism of the northern drought attributable to the TP in summer. The TP, similar to a very large engine, drives the nearby movement of water vapor, clouds, and aerosols. This “engine effect” controls precipitation near the TP and can trigger flooding or droughts in downstream regions. The northern drought is driven by the collocation of the subtropical westerly jet (SWJ) position and the TP engine effect. The meridional shift in the SWJ is the determining factor of the northern drought in summer. When the SWJ shifts northward, the upper-level westerly wind is weakened; thus, the water vapor, clouds or dusty clouds over the TP are transported to north less often, reducing precipitation and causing more frequent droughts. In contrast, when the SWJ shifts southward, the northern area of China experiences increased precipitation in summer.</p><p> </p>

Model Based IAS Analysis for Fault Detection and Diagnosis of IC Engine Powertrains

Internal combustion (IC) engine based powertrains are one of the most commonly used transmission systems in various industries such as train, ship and power generation industries. The powertrains, acting as the cores of machinery, dominate the performance of the systems; however, the powertrain systems are inevitably degraded in service. Consequently, it is essential to monitor the health of the powertrains, which can secure the high efficiency and pronounced reliability of the machines. Conventional vibration based monitoring approaches often require a considerable number of transducers due to large layout of the systems, which results in a cost-intensive, difficultly-deployed and not-robust monitoring scheme. This study aims to develop an efficient and cost-effective approach for monitoring large engine powertrains. Our model based investigation showed that a single measurement at the position of coupling is optimal for monitoring deployment. By using the instantaneous angular speed (IAS) obtained at the coupling, a novel fault indicator and polar representation showed the effective and efficient fault diagnosis for the misfire faults in different cylinders under wide working conditions of engines; we also verified that by experimental studies. Based on the simulation and experimental investigation, it can be seen that single IAS channel is effective and efficient at monitoring the misfire faults in large powertrain systems.

Benchmarking of two flexible multibody dynamic simulation software in engine simulations

In this paper, two different commercial multibody dynamic (MBD) simulation software cases are studied. Due to the restrictions determined in the conditions of contract, the names of the software are not revealed, instead being called Software S and Software E. The central purpose of this research was to investigate the abilities of Software S in the simulation of a large engine, as a part of the strength analysis process. The abilities were studied by comparing the program with another, here called Software E, which is designed primarily for engine simulations. The capabilities of Software E have been proven after years of usage at Wärtsilä, resulting in its essential role in the strength analysis process today. The aim was to find the shortcomings and restrictions of Software S but also advantages it could bring to the strength analysis process for Wärtsilä. Similar simulation models were also built using both programs during this research. A 16-cylinder V-engine was selected as the subject because of its size in order to obtain further information about the behavior of the program when working with extensive model files. The components of the engine were flexible and were reduced FE models, also called super elements. The forces and contact situations that occur inside the engine were modeled using elements provided by the MBD programs. Different levels of detail of the modeling elements were used to obtain information about the flexibility of the program. The results obtained from time integrations were compared to ensure the similarity of both modeling elements used. Also, this paper reports the calculation times. In addition, a small-scale study was performed for Software S to clarify the effect of the modes used in time integrations towards results accuracy and calculation times. Simulation models were built successfully in both programs, and the results obtained correlated with each other on an adequate level. Significant differences appeared in the features and usability of the programs in general. The GUI of Software S is advanced and user-friendly, whereas Software E is not focused on these features. On the other hand, the modeling element library of Software E covers all of the required features related to large engine simulations, some of which Software S is lacking. This work can be used in assistance when considering buying new software for a company as well as when investigating new development areas that could be improved with new software.

Model-based calibration of reaction-based diesel combustion dynamics

This paper presents a control-oriented, reaction-based diesel combustion model that predicts the time-based rate of combustion, in-cylinder gas temperature, and pressure over one engine cycle. The model, based on the assumption of a homogeneous thermodynamic combustion process, uses a two-step chemical reaction mechanism that consists of six species: diesel fuel (C10.8H18.7), oxygen (O2), carbon dioxide (CO2), water (H2O), nitrogen (N2), and carbon monoxide (CO). The temperature variation rate is calculated based on the rate of change of species concentrations; the heat loss correlation is also used to study the model performance. The accuracy of the model is evaluated using test data from a GM 6.6 L, eight-cylinder Duramax engine. The main contribution is the model calibration under different key operational conditions over a large engine speed and load range as well as different injection timings and exhaust gas recirculation rates by solving the optimal calibration problem. The calibrated reaction-based model accurately predicts the indicated mean effective pressure, while keeping the errors of in-cylinder pressure and temperature small, and, at the same time, significantly reduces the calibration effort, especially when the engine is operated under multiple fuel injection operations compared with Wiebe-based combustion models. The calibrated model parameters have a strong correlation to engine speed, load, and injection timings, and, as a result, a universal parameter calibration structure is proposed for entire operational conditions.