Experimental Characterization of Pilot-Fuel Ignition, Combustion, and Soot Formation in Dual-Fuel Combustion Systems

In recent years, many natural gas-fueled engine concepts have emerged for transportation, as well as stationary applications. They seem as an attractive solution to meet current and upcoming emission legislations at uncompromised efficiency. Furthermore, the application of natural gas is advantageous regarding the associated CO2 reduction, high resistance to auto-ignition (knock), and low sooting propensity. Especially in the large engine applications ranging from heavy-duty to marine and stationary, the lean burn combustion systems with a potential for lower pollutant emission at simultaneously higher than diesel-engine efficiencies are employed. Advanced ignition systems like pilot ignition dual-fuel or pre-chamber spark ignition are utilized to ensure stable and fast combustion. The dual-fuel combustion process is highly complex, involving short transient pilot-fuel injection into the premixed gaseous fuel charge, autoignition, and combustion mode transition into premixed flame propagation. Despite many advantages, the dual-fuel engines in gas operation often exhibit a tradeoff of either (a) higher NOx and soot emissions or (b) high unburnt hydrocarbon emissions at deteriorated thermal efficiency. This clearly elucidates the need for an improved understanding of the combustion process to support the development of advanced engine configurations and control systems, in order to achieve the design goals of fulfilling the present and future emission standards at highest possible thermal efficiency. However, fundamental investigations of the dual-fuel combustion process are scarce. The aim of this thesis was to advance the fundamental understanding of the dual-fuel combustion process. Characteristics of ignition, combustion, and the sooting propensity of a short pilot injection in compressed methane/air charge were investigated. Experiments were performed in an optically accessible Rapid Compression Expansion Machine, featuring quiescent charge throughout the cycle. A single-hole coaxial diesel injector mounted at the cylinder periphery was employed to admit the pilot fuel. The comprehensive measurement matrix includes variations of premixed charge (methane/air) equivalence ratio, pilot-fuel injection pressure and duration, as well as investigations at reduced oxygen charge content. Several cross-variations were performed. The optical diagnostics setup includes advanced laser-based as well as passive optical methods. Pilot-fuel mixing was characterized under non-reactive conditions using simultaneous quantitative high-speed tracer-PLIF, schlieren, and Mie-scattering imaging techniques. In reactive cases, the diagnostic setup featured simultaneous high-speed CH2O-PLIF imaging, OH* chemiluminescence and schlieren imaging for the detection of low-temperature and high-temperature ignition events, as well as to observe the transition into premixed flame propagation. Finally, combustion sooting propensity, as well as the combustion spectral footprint were investigated using the diffuse back-illumination and 1D-spectrography approach. To aid the evaluation of optical results homogeneous reactor, laminar flame speed, and 1D-flamelet simulations with detailed kinetics were employed. The pilot-fuel auto-ignition was shown to be governed by a complex interplay between micro-mixing, two-stage autoignition chemistry, and air entrainment. Methane strongly inhibits the low-temperature pilot-fuel reactivity, leading to a considerably delayed pilot-fuel autoignition. This chemical influence is amplified under less reactive conditions, like lower charge temperature or reduced charge oxygen content. The influence of prolonged ignition delay was found to propagate through several combustion stages. First, the overall reactivity of the pilot-fuel mixture is reduced. This leads to a prolonged burning time of the pilot-fuel, a shift of the ignition location to the spray tip, and a later onset of the flame propagation. Again, this is more pronounced in cases with a reduced charge oxygen content. Finally, the longer ignition delay results in a leaner pilot-fuel mixture at ignition, reducing the soot formation for sufficiently short injections. Nevertheless, for longer injections with pilot-fuel richer conditions at ignition, methane moderately increases the combustion sooting propensity and strongly inhibits the oxidation of soot. This potentially causes higher engine-out soot emissions. The investigations of this study advanced the fundamental understanding of this complex combustion process. Furthermore, these findings are seen as a valuable input for the design of 0D/1D dual-fuel combustion models and provide an understanding of the governing mechanisms. Also, 3D-CRFD researchers can use the presented experimental results for the purposes of model tuning and validation