Abstract
Controlling ignition timing in the homogeneous charge compression ignition (HCCI) of dymethyl ether (DME) by adding methanol and ozone has been studied in a motored engine. In the standard pressure, temperature and rate of heat release analyses in two-stage ignitions, reduction of the first stage (cool ignition) heat release with methanol addition and consequent retardation of the second stage (hot ignition) was confirmed. Composition analysis, conducted under moderate, single cool ignition conditions, exhibited liner reductions of fuel consumption and formaldehyde formation, while maintaining their ratio as 1:1. These observations were well reproduced by the detailed chemical kinetic model of Curran et.al. for DME oxidation at every examined equivalence ratio. A simple formulation accounting for the retarding effect was established, in which HCHO + OH and methanol + OH reactions are responsible for the termination of DME chain reaction system of low temperature oxidation. In contrast, acceleration with ozone addition was caused by increase of heat releases in the cool ignitions taking place at a lower temperature, while the thermal ignitions begin at a constant temperature. The cool ignition composition analysis showed increases of fuel consumption and formaldehyde formation, whereas the formaldehyde increase is less significant at a higher addition of ozone. Inclusions of ozone decomposition forming O + O_2 into the model enabled a good reproduction of these features. It was inferred that the early radical supply from ozone reduced the cool ignition onset temperature significantly, where a stable intermediate accumulates owing to slow decomposition, and that the resultant reduction of formaldehyde formation induced the longer chain duration.
