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A. Peter, S. Riess, M. Wensing, J. Fruehhaber, T. Lauer:
"Investigation of Mixture Formation and Flammability of Natural Gas and Diesel under Dual Fuel Operating Conditions in the Limits of Flame-quenching and Knocking";
Talk: The 9th International Conference on Modeling and Diagnostics for Advanced Engine Systems [ COMODIA 2017 ], Okayama, Japan (invited); 2017-07-25 - 2017-07-28.

Dual Fuel engines combine both the efficiency and torque characteristics of diesel engines and the reduced emissions of gas engines. Combustion of natural gas is free of particulate matter (PM) and produces less C02 than petrol fuel. Combined with the application of modern heavy diese] engines, the dual fuel combustion process faces latest exhaust gas legislations.
The substitution of air with natural gas leads to a density change of the ambient gas which influences the propagation of direct-injected fuel sprays. In this work, first we investigate this influence on mixture formation with schlieren measurements in a high-pressure high-temperature combustion vessel. In a density range from 11kg/m3 to 29kg/m3 and with an injection pressure variation between 80-160MPa, the air-entrainment is investigated using total momentum conservation. Few millimeters upstream the injector orifice significant amounts of environmental gas are already entrained.
Second, the influence of the reduced oxygen content and the presence of natural gas on ignition and combustion is analyzed in an optically accessible rapid compression machine in matters of an injection pressure variation. Homogenous, lean premixed natural gas-air charge is introduced in the combustion chamber and is intlamed by a diesel injection. Between the limits of early flame-quenching and the irregular self-ignition of the natural gas (knocking) the gas-air-diesel composition is varied at same total energy level. The measurements encompass the determination of the combustion behavior via cylinder pressure signal and via high-speed chemoluminescence imaging of the OH-radical and natural Iuminosity of the flame, respectively. This study shows, the ignition timing remains unaffected at increasing injection pressure. The ignition delay is general increased by natural gas-air atmosphere. A higher injection pressure means that more fuel penetrates faster into the combustion chamber which leads to faster energy release but not too often correlated better mixture formation.