In this paper, the cycle-by-cycle variations of a compressed-natural-gas (CNG) direct-injection (DI) engine were investigated. The results show that the CNG DI engine has a better lean burn capability, and misfire cycles and partial burn cycles exist when the engine operates at small equivalence ratio (>0.4). Meanwhile, the indicated mean effective pressure (IMEP) has a low value, and the high value of the coefficient of variation in the IMEP is presented in comparison with those operating at a high equivalence ratio. Cycles with a high maximum cylinder pressure correspond to the cycles of fast burning, and parameter interdependence is observed between the maximum cylinder gas pressure and its corresponding crank angle, between the maximum rate of pressure rise and its corresponding crank angle, and between the maximum cylinder pressure and the indicated mean effective pressure. Better parameter interdependence exists between the maximum cylinder pressure and the flame- developing period, between the maximum cylinder pressure and the rapid-burning period, and between the accumulated heat release amount per cycle and the indicated mean effective pressure. A small variation in the flame-developing duration will lead to a large variation in the rapid-burning duration under lean mixture combustion; the slow flame propagation speed of the lean mixture combustion is considered to lead to this phenomenon.

Terbium (Tb)-doped Zn3N2 thin films were prepared on glass and Si substrates by direct current magnetron sputtering in a mixture of pure argon and nitrogen gases. Glancing incidence x-ray diffraction patterns indicated that Zn3N2:Tb thin films were of cubic structure. Raman spectra showed only two Raman-active phonon modes located at 258 and 565 cm−1. The indirect optical band gap of Zn3N2:Tb was determined as 2.4 eV. The sharp characteristic emission lines corresponding to Tb3+ intra-4f shell transitions were resolved in the photoluminescence spectra at room temperature. Those results suggest that Tb-doped Zn3N2 may be a suitable material for visible optoelectronic devices.