Detailed Hydrocarbon Species and Particulate Emissions from a HCCI Engine as a Function of Air-Fuel Ratio

Concentrations of individual species in the engine-out exhaust gas from a gasoline-fueled (101.5 or 91.5 RON), direct-injection, compression-ignition (HCCI) engine have been measured by gas chromatography over the A/F range 50 to 230 for both stratified and nearly homogeneous fuel-air mixtures. The species identified include hydrocarbons, oxygenated organic species, CO, and CO 2 . A single-cylinder HCCI engine (CR = 15.5) with heated intake charge was used. Measurements of the mass and size distribution of particulate emissions were also performed. The 101.5 RON fuel consisted primarily of five species, simplifying interpretation of the exhaust species data: iso-pentane (24%), iso-octane (22%), toluene (17%), xylenes (10%), and trimethylbenzenes (9%). The thermal oxidation of iso-pentane and iso-octane were studied individually during brief experiments in a Pyrex reactor in order to ascertain the major primary and secondary products formed during oxidation of these fuel components at 765 K. A sharp increase occurs in the emissions indices of the organics formed by the combustion of the major fuel components beginning near A/F = 70. These product mole fractions increase by factors of 15 to 40 as the A/F ratio becomes leaner while unburned fuel species mole fractions increase by factors of 2 to 3. As observed previously in this engine and by others, the CO mole fraction increases sharply beginning near A/F = 70. The size distribution of the particulate emissions for A/F 70, the distribution changes sharply to that characteristic of semi-volatile particles. The body of data, including detailed examination of the cylinder pressure traces, indicate that a significant change occurs in the nature of the combustion process near A/F =70.