Chemical mechanistic analysis of additive effects in homogeneous charge compression ignition of dimethyl ether

Controlling ignition timing in the homogeneous charge compression ignition (HCCI) of dimethyl ether (DME) by adding methanol and ozone has been studied in a motored engine. To obtain chemical mechanistic information relevant to the ignition behavior, a composition analysis under moderate, single cool flame conditions, along with standard pressure profile measurement, was conducted. Methanol was confirmed to retard ignition timing LIP to 15 degrees crank angle (CA) at an 8% addition to DME. In addition, simultaneous reductions of fuel consumption and HCHO formation were recognized. Associated reduction of cool flame heat release is the main cause Of the final thermal ignition delay. These observations were well reproduced by the model of Curran et al. A simple formulation accounting for the retarding effect was established, ill which HCHO + OH and methanol + OH reactions are responsible for the termination of DME chain reaction system in low temperature oxidation. In contrast, ozone addition advanced the ignition timing up to 20 degrees CA at an estimated addition of only 0.015% to DME. This acceleration is caused by an increase of heat release in the cool flame. The cool flame composition analysis showed increases of fuel consumption and HCHO formation, whereas the HCHO 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 flame onset temperature by 70 K, reducing the tentative HCHO formation owing to the slow decomposition of the most stable intermediate, inducing the longer chain duration before termination. (c) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.