双燃料燃烧系统中先导燃料点火，燃烧和碳烟形成的实验表征

      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 eciency. 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 eciencies 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 tradeo of either (a) higher NOx and soot emissions or (b) high unburnt hydrocarbon emissions at deteriorated thermal eciency. This clearly elucidates the need for an improved understanding of the combustion process to support the development of advanced engine con-gurations and control systems, in order to achieve the design goals of fulfilling the present and future emission standards at highest possible thermal eciency. 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.