Search Results

Fast and accurate characterization of stability regions and operational range with respect to pull-in voltage and displacement is critical in the design and development of MEMS resonators and switches. This paper presents a mathematical and computational procedure for modeling and analysis of static and dynamic instabilities of capacitive microdevices employing resonant microbeams. The mathematical model consists of a nonlinear microbeam under distributed electrostatic actuation and squeeze film damping. The coupled system is described by the nonlinear beam equation and a modified compressible Reynolds equation to account for the rarefied gas in the narrow gap between the microbeam and substrate. The Differential Quadrature Method (DQM) is used to discretize partial differential equations of motion and solve for static deflection, natural frequencies, static pull-in voltage, and quality factors for various encapsulation air pressures and applied DC voltages.

Traumatic rupture of the aorta (TRA) is an important transportation-related injury. This study investigated TRA mechanisms using in situ human cadaver experiments. Four quasi-static tests and one dynamic test were performed. The quasi-static experiments were conducted by perturbing the mediastinal structures of the cadavers. The mechanisms investigated included anterior, superior, and lateral displacement of the heart and aortic arch. The resulting injuries ranged from partial tears to complete transections. All injuries occurred within the peri-isthmic region. Intimal tears were associated with the primary injuries. The average failure load and stretch were 148 N and 30 percent for the quasi-static tests. This study illustrates that TRA can result from appropriate application of nominal levels of longitudinal load and tension. The results demonstrate that intraluminal pressure and whole-body acceleration are not required for TRA to occur.

Gasoline Direct Injection (GDI) engine has a significant fuel economy improvement over the traditional port fuel injection engine. The tradeoff for this benefit is excessive exhaust emissions, especially NOx. Three-way-catalyst (TWC) is inefficient to treat NOx emission during lean operation. So Lean NOx Trap (LNT) is invented for NOx aftertreatment and it has both storage mode and purge mode. Research on modeling and control of LNT has been conducted, but it is still lack of the essential information on the temperature effect. This research focuses on the impact of trap temperature on LNT storage time, purge time and fuel economy. The mechanism of temperature effect on LNT is investigated at first. Then the temperature control strategy based on fuel economy improvement is proposed.

Misfiring or partial combustion during diesel engine operation results in the production of partial oxidation products such as ethylene (C₂H₄), carbon monoxide and aldehydes, in particular formaldehyde (HCHO). These compounds remain in the cylinder as residual gases to participate in the following engine cycle. Carbon monoxide and formaldehyde have been shown to exhibit a dual nature, retarding ignition in one temperature regime, yet decreasing ignition delay periods of hydrocarbon mixtures as temperatures exceed 1000°K. Largely unknown is the synergistic effects of such species. In this work, varying amounts of C₂H₄ and HCHO are added to the intake air of a naturally aspirated optical diesel engine and their combined effect on autoignition and subsequent combustion is examined. To observe the effect of these dopants on the low-temperature heat release (LTHR), ultraviolet chemiluminescent images are recorded using intensified CCD cameras.

Under the borderline autoignition conditions experienced during cold-starting of diesel engines, the amount and composition of residual gases may play a deterministic role. Among the intermediate species produced by misfiring and partially firing cycles, formaldehyde (HCHO) is produced in significant enough amounts and is sufficiently stable to persist through the exhaust and intake strokes to kinetically affect autoignition of the following engine cycle. In this work, the effect of HCHO addition at various phases of autoignition of n-heptane-air mixtures is kinetically modeled. Results show that HCHO has a retarding effect on the earliest low-temperature heat release (LTHR) phase, largely by competition for hydroxyl (OH) radicals which inhibits fuel decomposition. Conversely, post-LTHR, the presence of HCHO accelerates the occurrence of high-temperature ignition.

Advanced diesel engines developed for the commercial market need to be adapted to the military requirements by OPERAS (Optimizing the injection pressure P, the Exhaust gas recirculation E, injection events Retard and/or Advance and the swirl ratio S). The different after treatment devices, already used or expected to be applied to diesel engines, require feed gases of appropriate properties for their efficient operation. To produce these gases some OPERAS are needed to control the diesel combustion process. Since military vehicles do not need the after treatment devices, the OPERAS of the commercial engines should be modified to meet the military requirements for high power density, better fuel economy, reduction of parasitic losses caused by the cooled EGR system, and reduction of invisible black and white smoke in the field.