Determination of cycle number for real in-cylinder pressure cycle analysis in internal combustion engines

The in-cylinder pressure of internal combustion engines is one of the most important measurable parameter for analyzing the factors affecting performance characteristics of the engine. In many studies, in-cylinder pressure data are averaged over certain number of cycles at each crank angle in order to observe the effects of the parameters. If the number of cycles included is low, then the results may be misleading due to cyclic variations of in-cylinder pressure. The desired level of accuracy can only be obtained if the number of cycles is increased with increasing cyclic variations. The number of cycles used by researchers varies in the literature even for the same subject of study such as research and development, cyclic variations, cycle simulation, etc. There is no general agreement about how many cycle should be taken to obtain the average cycle to remove the effects of cyclic variations. The purpose of this study is therefore to determine the sufficient minimum cycle number at various engine operating conditions on a spark ignition engine by using statistical Levene’s test. The results showed that 50 cycles are enough to do accurate calculation of the average pressure cycle at various operation conditions of the engine.     

Markov Models of Systems with Demands of Two Types and Different Restocking Policies

Markov models of queuing-inventory systems with demands of two types are proposed. In these systems, two restocking policies are used: policies with fixed and variable demand size. High-priority demands have no access constraints while low-priority demands are accepted if the total number of demands in the system is less than a given threshold. Methods are developed to calculate performance measures of the systems under study and problems of their optimization are solved. Results of the numerical experiments are shown.     

Effect of Phonon Drag on the Thermopower in a Parabolic Quantum Well

The theory of phonon-drag thermopower resulting from a temperature gradient in the plane of a two-dimensional electron gas layer in a parabolic quantum well is developed. The interaction mechanisms between electrons and acoustic phonons are considered, taking into account potential screening of the interaction. It is found that the effect of electron drag by phonons makes a significant contribution to the thermopower of the two-dimensional electron gas. It is shown that the consideration of screening has a significant effect on the drag thermopower. For the temperature dependence of the thermopower in a parabolic GaAs/AlGaAs quantum well in the temperature range of 1–10 K, good agreement between the obtained theoretical results and experiments is shown.     

Energy spectrum of charge carriers in Ag2Te

Conductivity σ(T) and Hall constant R(B, T) are studied for Ag₂Te with excess 0.1% of Te. The change in the R sign from (?) to (+) is found in dependences R(B) at various temperatures. In the temperature dependences of R in a range of 1–3 kG, two extrema are found, namely, minimum at T ～ 60 and maximum at T ≈ 80 K, and at B ≥ 5 kG, the double change in sign of R from (?) to (+) and from (+) to (?) is found. Temperatures of sign inversion for R depend on the magnetic field. At B = 15 kG, the sign of R varies from (?) to (+) at T ≈ 38 K, and from (+) to (?) at T ～ 70 K. It is found approximately in the region of the change in the sign of R(T), the concentration n(T) and electrical conductivity pass through the minimum. It is established that the minima of n(T) and σ(T), extrema in R(T), and sign inversion for R(T) from (?) to (+) as well as the overestimated temperature dependence n∞T ⁴ are caused by localization of conduction electrons at acceptor levels entering the conduction band of Ag₂Te. The values of parameters of electrons (n, μ _n ) and holes (p, μ _p ) at the points of the change in the sign of R(T) from (?) to (+) and from (+) to (?) are determined.     

Modeling and design optimization of asynchronous flexible assembly systems with statistical process control and repair

This article presents the implementation of hybrid procedures involving the use of analytical performance evaluation techniques, discrete event simulation, and Monte Carlo optimization methods for the stochastic design optimization of asynchronous flexible assembly systems (AFASs) with statistical process control (SPC) and repair loops. AFASs are extremely complex and difficult to analyze in that such systems are subject to starvation and blocking effects, random jam occurrences at workstations, and splitting and merging of the assembly flow due to repair loops. Hence, an integrated approach simultaneously analyzing the interactions between product quality and optimal/near optimal system design is pursued. In the analytical analysis stage, a model based on GI/G/1 queueing network theory is used. In the Monte Carlo optimization stage, two alternative stochastic optimization approaches, namely, heuristic versions of stochastic quasigradient and simulated annealing algorithms, are implemented and compared in terms of their capabilities of solving complex AFAS design problems. The hybrid procedures presented appear to perform reasonably well in designing AFASs to reach a target production rate.     

The effect of thawing methods on the quality of eels (Anguilla anguilla)

Physical (colour), chemical (pH, total volatile basic nitrogen (TVB-N), thiobarbituric acid values (TBA)) and microbiological (total aerobic mesophylic bacteria, salmonella, coliform, yeast and mould counts) analyses were carried out on thawed European eel (Anguilla anguilla). Different thawing treatments were used (in a refrigerator, in water, in air at ambient temperature and in a microwave oven). The results obtained were compared statistically with those of fresh fish. pH, TBA and a^∗ values of thawed samples usually decreased significantly (P < 0.05) when compared to the fresh control. Salmonella was not detected in any of the samples. Coliform and mould counts of fresh control and thawed samples were <1 CFU/g. Total aerobic mesophylic bacteria count of all thawed fish decreased. However, the yeast count of the refrigerator–thawed samples increased. The lowest total aerobic mesophylic bacteria and yeast counts were determined in water–thawed samples. Water thawing is therefore suitable for frozen eel.