Abstract
A homogeneous charge compression ignition (HCCI) engine has been known to have high thermal efficiency and low nitrogen oxide emission. However, the control of ignition and its combustion period over wide range of engine speed and load is one of the barriers of the realization of the engine. In the lean side of equivalence ratio, control of ignition is difficult for its long delay of ignition and there are knock-like problems in the high range of load. Regarding the extension of the range of load, the onset of hot flame and its combustion period were examined for a wide range of fuel input by Chemkin II computation and experiment. In the computation, detailed reaction scheme of dimethyl ether proposed by P. Dagaut et al. in 1998 was used in the study. Also, the computational results were compared to the experimental results from a HCCI engine. The intake temperature and the amount of CO_2 mixing were changed for the control of hot flame onset angle. The onset angles of cool flame from computation corresponded well with experimental results. However, the pressure rise in this stage was well higher than that in experimental results. Therefore, the onset angle of hot flame was more advanced than that was in experiment. From the computational results, a flat region of hot flame onset angle where hot flame onset angle hardly changed with increase in fuel input could be found for a given intake temperature and CO_2 mixing rate in the rich side, which was caused by opposing effects of the raised specific heat of the mixture and advance in ignition delay between cool and hot flame. However, the confirmation of flat region was not realized in the experiment for the progressive advance of cool flame onset angle and knock-like combustion near rich side. In the experiment, the operable range (the possible range of fuel input from just ignitable to knock-occurring fuel input) shifted to rich side with decrease in intake temperature while the range of fuel input was reduced, as the hot flame onset angle advanced more quickly than it did under high intake temperature. And the progressive advance of cool flame due to the increased temperature of residual gas and cylinder wall by raised fuel input is considered to take an important role in the fast advance of hot flame onset angle and generation of knock-like combustion. However, the mixing of CO_2 made the operable range shift to the rich side with keeping up the width of it as shown in Fig. 1, possibly due to the relatively small portion of heat generation in cool flame. The duration of hot flame was examined too, and it was confirmed that the duration was mostly dependent on fuel input and its onset angle, in spite of any variation of intake temperature or rate of CO_2 mixing as shown in Fig. 2.
