Abstract
Autoignition behaviors of unsteady spray in a constant volume spray combustion system were investigated with primary reference fuels (PRFs), n-heptane and iso-octane, and diesel fuel at a range of low temperature conditions. The ignition quality tester (IQT) was adopted to simulate unsteady spray combustion in a diesel engine. The temperature of fuel-air mixture ranged from 720 to 850 K with pressures of 1.3, 1.6, and 2.1 MPa pertinent to homogeneous charge compression ignition (HCCI) or low temperature combustion (LTC) conditions. In order to compensate for low temperature, heat release-induced pressure increases and to more accurately determine start of ignition (SoIgn), a more effective method known as the differential method is introduced and applied in the present study. Comparisons of the present method with other existing definitions for SoIgn are also made. In this paper an attempt to scale the effect of pressure on total ignition delay is undertaken and the pressure exponent and activation energy for the Arrhenius expression for the fuels used are also calculated. Just in terms of autoignition behaviors of temperature and pressure, the diesel fuel is more similar to n-heptane than iso-octane. Thus, for direct injection and unsteady spray auto-ignition applications with low temperature diesel engine conditions, the well-controlled conditions of IQT for n-heptane make it a potential attractive chemical mechanism validation platform.
KEYWORDS – autoignition, unsteady spray, reference fuel, low temperature, pressure scaling