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Dimethyl ether has high cetane number and low temperature reaction characteristics. These mean that blending small amount of dimethyl ether to the spark-ignited engine would be helpful for improving the performance under lean conditions. Spark timing is one of the important factors influencing the engine combustion. Thus, it is necessary to investigate performance of the dimethyl ether-blended gasoline engine under various spark timings. The engine was run at 1400 rpm, a manifolds absolute pressure of 60 kPa and a constant excess air ratio of 1.20. Test results showed that the addition of dimethyl ether resulted in the raised indicated mean effective pressure for the gasoline engine. Over increased and decreased spark timing tended to cause the dropped indicated mean effective pressure. The coefficient of variation in indicated mean effective pressure was diminished with the spark timing advances and dimethyl ether addition. NOx and HC emissions were dropped with the spark timing decrease. NOx emissions from the dimethyl ether-mixed gasoline engine are decreased with the decrease of spark angle.

Adding dissociated ammonia (NH3) is an effective way to improve the combustion properties of NH3. Research on the laminar burning velocity of partially dissociated NH3 mixtures was shown in part I. In order to further explore the feasibility of enhancing ammonia combustion by adding dissociated ammonia, several essential combustion properties were numerically studied in part Ⅱ of this study. The burned gas Markstein length was obtained by the experiment and theoretical calculation, and the relevant combustion parameters such as adiabatic flame temperature, heat release rate and NO formation were calculated by planar flame simulation. Results show that the Markstein length increases with the increasing dissociating degree on the lean side, but decreases under rich conditions, which is mainly due to the determination of flame thickness, effective Lewis number and Zel’dovich number. The addition of dissociated NH3 increases the mixture heating value, but the combustion efficiency decreases slightly. The primary exothermic elementary reactions in the combustion are gradually replaced by the reactions in the H2 system, and the exothermic process shifts to a lower temperature with the increase of dissociating degree. NO is the main NOx emission, the formation of NO is determined by the concentration of the O/H radicals and NH3, its mole fraction first increases and then decreases with the increase of dissociating degree. Moreover, the NO emission on the rich side only decreases slightly under high dissociating degree conditions, which is different from the significant plummet under the low and medium dissociating degree conditions.

A computational fluid dynamics model was established and validated to numerically investigate the effects of different injection positions and injection angles on the mixture formation and combustion process in a gasoline direct injection rotary engine. The results show that as the injection position approaches the top dead center and the increasing injection angle, the rich zone of fuel changes from the rear to the front of the combustion chamber. The local equivalence ratio near the two spark plugs shows an increasing trend with the injection position upward and the increasing injection angle. Compared with the lower and middle injection positions, the upper injection positions can make fuel more concentrated near the leading spark plug and the trailing spark plug at ignition timing. The injection strategies corresponding to the upper injection position are conducive to complete combustion, in which the mass fraction of fuel burned all beyond 99%. Besides, with the increasing injection angle, the oil film formation position changes from the rotor recess to the middle cylinder block, which further affects the mixture formation and combustion process. Especially, the highest peak pressure is obtained at 0° injection angle corresponding to the upper injection position and is 12.39% higher than the in-cylinder peak pressure of intake port injection.

This work aims to improve the idle performance of the n-butanol rotary engine by blending hydrogen and reducing idle speed. Hydrogen volume percentage (βH2) changes from 0 to 7.94% and the idle speed decreases from 2600 to 2400 rpm. The test results show that the engine can achieve better stability and economy due to the coupling effect of mixing hydrogen and reducing the idling. The total fuel flow rate reduces from 24.57 MJ/h at 2600 rpm when pure n-butanol is fueled to 19.35 MJ/h at 2400 rpm when βH2 equals to 7.94%. The period of flame development and propagation are observably decreased by blending hydrogen even if reducing idle speed has a few negative effects. Hydrogen enrichment is an effective way to reduce the emissions of CO and HC. Besides, the negative effects of reducing idle speed on HC and CO emissions are decreased to a negligible level when βH2 passes about 7%. Besides, NOx emission is maintained at an extremely low value in the testing range. Also, enrichment hydrogen can compensate for the negative effects on combustion and emissions of reducing idle speed.

Ammonia is considered as a carbon-free alternative fuel and hydrogen carrier to realize green and renewable energy conversion, but the oxidation mechanism of NH3 still needs to be improved and validated by more experimental data. In this study, the laminar burning velocity (LBV) was measured in a constant volume vessel at various equivalence ratios, initial temperatures, and oxygen fractions up to undiluted condition. Thirteen latest NH3 kinetics mechanisms were collected and compared with each other and the experiments. Results showed that large discrepancies of predictions presented between different mechanisms. The maximum LBV of undiluted NH3/O2 mixtures could reach 125.05 cm/s. The maximum predicted LBV of NH3/O2 is nearly two times higher than the minimum one for NH3/O2 mixtures. The reduced Okafor (2019) and Zhang (2017) mechanism gave the best agreement with experiments for lean and rich mixtures, respectively. Sensitivity analysis of rate constant, species thermal, and transport parameters were adopted to identify the critical reactions and species in the premixed flame. It was found that because of the coupled nature of the properties mentioned above in the premixed flame, the species involved in the essential reactions also exhibit substantial thermal and transport sensitivities.

This study aims to investigate the effects of data points number on the accuracy of flame speed extrapolation using expanding spherical flame, which is a practical issue in high flame speed experimental measurement. Monte Carlo simulations were conducted based on the flame radius data obtained from one-dimensional spherical flame simulation. Given number of points were randomly picked from 70 points to obtain the unstretched flame speed by linear and nonlinear extrapolation under different Lewis number, fuel type, and initial pressures. The results were statistically analyzed and show that the least number for flame speed extrapolation is related to the Lewis number of mixtures. Mixtures with Lewis number smaller than unity need more data points to ensure accurate extrapolating results. In addition, the accuracy of flame speed determination is very sensitive to the error in flame radius when data points are sparse. When 0.1% of Gauss noise was added to flame radius data, additional dozens of points are required to keep the same accuracy. At least 30 data points were suggested to be adopted in the extrapolation of flame speed to reduce the uncertainty from data points number to less than 1%. The effects of the flame radius range on the determination of flame speed again validated by the current study, indicating a wide range of flame radius should be used to avoid uncertainty. It was also shown that the nonlinear extrapolation method needs fewer data points than the linear method to achieve the same accuracy for mixtures.

In this study, the laminar premixed CH4/H2/air combustion was investigated by the global reaction pathway (GP) analysis and sensitivity analysis of physical parameters for individual species under various hydrogen fractions (α) and pressures to further understand the blended fuel combustion and explore two questions: (1) whether extra GPs exist in the blended fuel combustion beyond the pure fuel reacting system; (2) whether there are insignificant species in pure CH4 and H2 combustion showing large influence in binary fuel combustion. Results show that different GPs dominate fuel oxidation under different α. The increasing radicals are responsible for the linearly increasing laminar burning velocity (LBV) at small α, and the transition from CH4 chemistry to H2 chemistry is responsible for the nonlinearly increasing LBV at large α. Extra GPs are shown and play a role in the binary fuel combustion, but they are still within the CH4 chemistry. For most species, the sensitivity of transport and thermal parameters to laminar burning velocity were found has a positive relationship with their maximum mole fraction at different α. No species were found only important to the binary fuel, however, some species such as H2O show higher sensitivity for binary fuel combustion than that of pure fuel combustion. The changes of GPs with increasing pressure verifies that CH4 chemistry is dominant for small α (20%) combustion while H2 chemistry dominant is for large α (80%) combustion. Most physical parameter sensitivity will increase with increasing combustion pressure, which should be paid more attention in high-pressure combustion.