Optical absorption and luminescence experiments were used to study the photoconversion of neutral oxygen vacancies ͑F centers͒ in MgO single crystals thermochemically reduced at elevated temperatures. In crystals with an undetectable concentration of hydride ions and a moderate concentration of F centers (Ϸ10 17 cm Ϫ3), excitation with UV-light produces positively charged anion vacancies (F ϩ centers͒ and electrons which are subsequently trapped at impurities. Under continuous excitation, the F ϩ centers release holes which are trapped at cation vacancies charge compensated by impurities. In crystals with high concentrations of both hydride ions and F centers (Ϸ10 18 cm Ϫ3), the electrons from the F to F ϩ photoconversion are trapped mainly at the hydride ions to form H 2Ϫ ions, which are metastable at room temperature. ͓S0163-1829͑99͒12101-5͔
Both energy consumption and the ecology of mobile wood-chipping machines are important issues in forest management. One way of improving the efficiency of wood-chippers is to use innovative design solutions in drive unit control systems. This can result in a reduction in fuel consumption and quantitative exhaust gas emissions. This article presented the results of research conducted on the fuel consumption of a cylindrical wood chipper driven by a small engine. We carried out testing of the unmodified chipper (A), made two different chipper modifications (B) and (C), and tested the modified versions to achieve the indicated results. The process allowed analysis and comparison of recorded data. For this purpose, the engine was supplied with fuel in three different ways: carburetor (factory-made) (A), the injector (B), and injector with an adaptative drive control system (designed by the authors) (C). The construction of a maintenance-free and adaptive drive control system where its functioning depended on operating conditions was done following patent application P.423369. All three fuel supply systems: A, B, and C were tested experimentally in terms of fuel consumption. The research was conducted in both set exploitation conditions (idle work with high (1) and low (2) rotational speed, with a continuous chipping process (3)) and transient exploitation conditions (4) (resulting from the delivery time of wood waste). Thus, the first stage of research involved two constructions (A, B) for three different working conditions (1-3). The second stage consisted of three constructions (A-C) tested in the fourth working conditions (4). The tests showed that the injection system reduced fuel consumption by around 61% during the continuous chipping process in comparison with the carburetor system. The adaptive drive control system (C) reduced fuel consumption by 55-74% in comparison with the carburetor system (A), and by 24-60% in comparison with the injection system (B) without an adaptive drive control system. The level of energy consumption in these systems depended directly on the ratio of idle work time during the chipping process.
Branch chipping machines with low-power engines are distinguished with an intermittent operation due to a periodical supply of branches. A conventional drive speed control of these machines is not adapted to adjust the operating mode depending on frequency of material supply for shredding. This article discusses the issues related to the assessment of the application of adaptive systems similar in design to start–stop systems used in vehicles, as necessary in the driving of this type machine. During testing, an impact of a distance between a branch pile from the woodchipper, a number of operators on frequency of drive unit operating condition changes, and the mass and volume output (productivity) were considered. A percentage ratio of the active and passive (idle) operation in selected conditions of use was also determined. A low-power 9.5 kW engine-powered cylindrical-type woodchipper was used for testing. Material chopped in the chipper was freshly cut branches of oaks (Quercus L. Sp. Pl. 994. 1753) with a diameter in the largest cross-section ca. 80 mm and moisture content ca. 25%. Piles of branches were located at three different distances from the chipper, i.e., 3 m, 9 m and 15 m. Branches to the chipper were fed by one or two operators. It was demonstrated that the idle run time in tested conditions with one operator could be from 43% to 71% of the entire operating time. Frequency of operating condition changes when only one operator worked and fluctuated from ca. 6 to 2 times per minute. Increasing the number of operators from one to two had a slight impact on the frequency of operating condition changes (by ca. 7%) at the shortest distance from the chipper (3 m). However, at larger distances, the additional operator may increase the frequency of operating condition changes of the chipper by 77% for 9 m distance and 85% for 15 m distance. The mass and volumetric output of the cylindrical chipper in the most advantageous case is equal to 0.66 t/h and 3.5 m3/h, respectively. The increase of the branch pile distance from the chipper causes a drop in mass output by 32%, and volumetric output by 33.5%. The results of the tests confirmed the necessity for the development of low-power chipping machines designed for clearing operations rather than industrial production of biomass. A direction for development could be systems that adapt driving units to operating conditions, depending on a demand for the chipping process.
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