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A series of experiments were performed to investigate the effect of ignition energy (Eig) and hydrogen addition on the laminar burning velocity (Su0), ignition delay time (tdelay), and flame rising time (trising) of lean methane−air mixtures. The mixtures at three different equivalence ratios (ϕ) of 0.6, 0.7, and 0.8 with varying hydrogen volume fractions from 0 to 50% were centrally ignited in a constant volume combustion chamber by a pair of pin-to-pin electrodes at a spark gap of 2.0 mm. In situ ignition energy (Eig ∼2.4 mJ ÷ 58 mJ) was calculated by integration of the product of current and voltage between positive and negative electrodes. The result revealed that the Su0 value increases non-linearly with increasing hydrogen fraction at three equivalence ratios of 0.6, 0.7, and 0.8, by which the increasing slope of Su0 changes from gradual to drastic when the hydrogen fraction is greater than 20%. tdelay and trising decrease quickly with increasing hydrogen fraction; however, trising drops faster than tdelay at ϕ = 0.6 and 0.7, and the reverse is true at ϕ = 0.8. Furthermore, tdelay transition is observed when Eig > Eig,critical, by which tdelay drastically drops in the pre-transition and gradually decreases in the post-transition. These results may be relevant to spark ignition engines operated under lean-burn conditions.

Combustion characteristics and harmful emissions with emphasized soot emission in the new concept of a biogas-dimethyl ether (DME) hybrid dual-fuel engine were analyzed. The effects of DME content, biogas compositions and diesel injection were examined. At any biogas composition, a rise in DME content in the fuel mixture leads to an increase in indicative engine cycle work (Wi) and NOx but a decrease in CO and soot volume fraction (fv). The effects of DME on Wi and soot volume fraction are more significant for poor biogas than for rich biogas, contrary to its effect tendency on CO and NOx concentrations. With a given operating condition and DME content, the biogas compositions slightly affect the performance and emission of a biogas-DME hybrid dual-fuel engine. At a fixed global equivalence ratio, the reduction of diesel injection leads to an increase in Wi and NOx concentration but a decrease in CO and soot volume fraction. The lower the diesel injection is, the more significant the effects of DME content on the combustion properties and pollutant emissions are. At a given operating condition and the same global equivalence ratio, the biogas-DME PCCI combustion mode is more advantageous than biogas-DME dual-fuel combustion mode. The substitution of diesel pilot ignition by DME pilot ignition in a biogas-DME hybrid dual engine is the optimal solution for both performance improvement and pollution emissions reduction.

Water reservoirs play a significant role in agricultural irrigation systems in Vietnam. It is necessary to quantify and monitor the water storage levels in reservoirs on a timely basis in order to properly forecast the water supply for the local agriculture. For this reason, the amount of water loss due to evaporation at the open surface should be properly quantified. This study evaluated different evaporation models in the literature using published experimental data to select an appropriate model for calculating the evaporation rate at the free water surface under the local weather conditions in Da Nang city and Quang Nam province of Vietnam. Water loss calculations were made for a number of major reservoirs in these two regions, which are located in a tropical monsoon climate region of Vietnam. The results show that water loss for a typical dry day is 11.31 mm/day, whereas it is 6.82 mm/day for a typical rainy day. Estimations of the water loss per day calculated for different reservoirs that may be useful for the agricultural irrigation system management board. The results reported in this study may also benefit the future designs of the irrigation reservoirs.

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