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The effects of the inert components of exhaust gas recirculation (EGR) gas on reducing the pressure rise rate of homogeneous charge compression ignition engine combustion were investigated numerically by utilizing the CHEMKIN II package and its SENKIN code, as well as Curran's dimethyl ether reaction scheme. Calculations were conducted under constant volume combustion and engine combustion (one compression and one expansion only, respectively) conditions. Results show that with constant fuel amount and initial temperature and pressure, as EGR ratio increases, combustion timings are retarded and the duration of thermal ignition preparation extends non-linearly; peak values of pressure, pressure rising rate (PRR), and temperature decrease; and peak values of heat release rate in both low temperature heat release (LTHR) and high temperature heat release decrease. Moreover, maximum PRR decreases as CA50 is retarded. With constant fuel amount, mixtures with different EGR ratios can obtain the same CA50 by adjusting the initial temperature. Under the same CA50, as EGR ratio increases, the LTHR timing is advanced and the duration of thermal ignition preparation is extended. Maximum PRR is almost constant with the fixed CA50 despite the change in EGR ratio, indicating that the influence of EGR dilution on chemical reaction rate is offset by other factors. Further investigation on the mechanism of this phenomenon is needed.

The laminar combustion characteristics of DME–hydrogen–air premixed mixture were studied at different equivalence ratios, ratios of hydrogen to DME plus hydrogen, and initial pressures by using a constant volume combustion bomb. The influences of the equivalence ratio, the hydrogen addition, and the initial pressure on flame speed and combustion characteristics are analyzed. The results show that the flame speed, the laminar burning velocity, and the mass burning rate increase with the increase of hydrogen addition. Increasing the ratio of hydrogen to DME plus hydrogen will shorten the combustion duration. In the case of small hydrogen addition, the Markstein length decreases with the increase of equivalence ratio, and this reveals that lean mixture has better flame front stability compared with that of the rich mixture. In the case of large hydrogen addition, the Markstein length increases with the increase of equivalence ratio, and this indicates that the rich mixture has better flame front stability than the lean mixture. Maximum combustion pressure increases with the increase of initial pressure, and initial pressure has larger influence on combustion pressure than that from hydrogen addition. The flame development duration decreases with the increase of hydrogen addition and the increase of equivalence ratio.

In this paper, the effect of cetane number (CN) improver on performance and emissions, including particulate number concentration and size distribution, of a turbocharged, common-rail diesel engine fueled with biodiesel-methanol were studied. Two volume fractions (0.3% and 0.6%) of CN improver were added to BM30 (30% of methanol in the biodiesel-methanol blend) in the experiment. The results show that, compared with those of biodiesel-methanol blend, the peak value of cylinder pressure increases, the second peak of heat release rate decreases, the start of second heat release are advanced, and the fuel economy and thermal efficiency are improved when CN improver is added to biodiesel-methanol blend. Besides, CO and HC emissions decrease, NO x emission varies little and smoke emissions increase slightly. Moreover, exhaust particles of BM30 mainly distribute in nanosize range. Furthermore, particle number concentration decreases and peak of size distribution profile shifts toward large size direction.

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