Shock-tube investigations on the self-ignition of hydrocarbon-air mixtures at high pressures

Abstract

The self-ignition behaviour of various fuel-air mixtures has been investigated without inert gas dilutionusing a high-pressure shock tube. In order to obtain data directly applicable to the modeling of engine combustion, the shock-tube facility was designed to handle fuel-air mixtures up to initial pressures of more than 40 bar and to achieve measuring times up to 10 ms. As typical representatives of engine fuel components, n-heptane, benzene, iso-octane, methanol, and methyl-tert-butylether (MTBE) were investigated. Two pressure levels for the investigation, 13 bar and nearly 40 bar, have been chosen. The ignition of n-heptane begins with a rapid pressure increase, especially at higher temperatures. Benzene, iso-octane, methanol, and MTBE show a slow initiation of the ignition without distinct pressure peak (mild ignition) at low temperatures, which at higher temperatures changes to a rapid pressure increase after a variable time lag (strong ignition). The strong ignition limit depending on temperature, pressure, and fuel was determined. An investigation was made of the dependence of the ignition delay times of iso-octane and benzene on equivalence ratio and temperature at nearly 40 bar. A comparison of the ignition delay times of all the fuels investigated is presented for stoichiometric mixtures and at two pressure levels.