Diesel Spray and Premixed Methane-Air Ignition Behind a Reflected Shock Wave

Abstract

This study investigates single-event ignition of diesel fuel injected into methane-air that is applicable to compression ignition dual fuel engine conditions. Post compression cylinder temperature and pressure conditions were created behind a reflected shock wave in a 76 mm square-channel shock tube. A common-rail piezoelectric diesel injector with 8 tip orifices was mounted in the centre of the shock tube end wall to replicate in-cylinder placement. A single pass 150 mm diameter z-type high-speed schlieren system was used to capture ignition in a 250 mm long optical section. Additionally, direct photography was used in some tests to identify the location of ignition via the first emission of visible light. Tests were carried out at a nominal reflected pressure of 10 bar and temperatures in the range of 880 – 1500 K, with the aim of studying ignition delay within and beyond the negative temperature coefficient regime. The test gas was composed of synthetic air, with argon replacing nitrogen. Both schlieren video and pressure-history were used to measure ignition delay time. The high-speed photography showed that premature ignition was promoted by metal particles in the bottom half of the channel. By filtering out the premature ignition events it was found that the ignition delay time, for both diesel spray and premixed methane-air, correlated well with an Arrhenius temperature dependency. Diesel experiments were carried out with two injection durations (0.15 and 0.5 ms), and it was found to play an important role in the ignition location but not the ignition delay time. For a 0.15 ms injection time no ignition was observed for temperatures below 1050 K because of overmixing. Methane-air tests were carried out primarily at equivalence ratios of 0.25 and 0.5, a limited number of tests were carried out at stoichiometric conditions. Two-stage ignition (mild and strong) was observed in methane-air for temperatures above 1000 K and 1100 K for equivalence ratios of 0.25 and 0.5, respectively. Constant volume model predictions using two reaction mechanisms from the literature compared unfavorably with the measured ignition delay time. The methane-air ignition delay time showed some dependency on equivalence ratio.