N-heptane micro pilot assisted methane combustion in a Rapid Compression Expansion Machine

The autoignition and combustion behaviour of an n-heptane spray in a premixed methane/air charge was investigated. A Rapid Compression Expansion Machine (RCEM) with a free floating piston was employed to reach engine relevant conditions at Start of Injection (SOI) of the micro n-heptane pilot. The methane content in the ambient gas mixture was varied by injecting different amounts of methane directly into the combustion chamber; the ambient equivalence ratio for the methane content ranged from 0.0 to 0.99. Filtered OH∗ chemiluminescence images of the combustion were taken with a UV intensified camera at a rate of 20 kHz through the optical access in the piston and schlieren imaging was performed at 20 kHz for the detection of density gradients arising from pilot injection, ignition and flame propagation in the unburned mixture. Filtered photomultiplier signals of the total emitted light for OH∗, CH∗ and C2∗ radicals as well as the pressure signal were simultaneously recorded and the effect of methane content, charge temperature and ambient oxygen concentration on ignition locations, ignition timing and combustion behaviour was analysed. It was found that increasing methane contents in the ambient mixture considerably prolongs the ignition delays of the pilot spray due to inhibiting effects of the premixed methane on the reactions leading to high temperature ignition triggered by n-heptane. Longer ignition delays due to lower ambient temperatures or dilution of the ambient oxidizer cause higher heat release during the combustion of the pilot spray, since more methane/air is entrained and mixed with the pilot spray and oxidized simultaneously. This behaviour is reversed with increasing ignition delays arising from increased methane addition. There, lower heat release rates during pilot combustion were observed due to decreased reactivity in the n-heptane spray area. Comparison of different strategies with respect to the determination of high temperature ignition delay highlighted shortcomings as well as the potential in the usage of spectrally filtered chemiluminescence data, since changes in the spectrum of the emitted chemiluminescence are expected to change over time in the case of pilot ignition due to the transition between combustion modes. An assessment of the methane content influence on the ignition reactions by means of homogeneous, adiabatic Perfectly Stirred Reactor (PSR) simulations with a suitable reaction mechanism is further presented which allows for separation of influences and supports the experimental findings.