Detailed Investigation of Diesel-Gas Dual Fuel Combustion Phenomena by Means of Combustion Chamber Endoscopy and 3D-CFD Simulation

Abstract Fully flexible large diesel-gas dual fuel engines are known for their ability to be operated in pure diesel mode as well as in dual fuel mode for applications such as power generation and transportation. Dual fuel operation with these engines is characterized by a premixed gas-air cylinder charge and compression ignition via a pilot diesel injection into the combustion chamber. Proven benefits such as the flexibility to adapt the type of fuel to the market, fail-safe operation and relatively low engine-out nitrogen oxides emissions come at the price of downsides such as comparatively low efficiency and combustion stability. To overcome these detriments, an in-depth understanding of the fundamental processes and effects must be obtained. The aim of this paper is therefore to study specific diesel-gas combustion phenomena in detail. The focus is on investigating the influence of the diesel fuel injection timing on mixture formation, ignition and combustion of both diesel and natural gas-air mixture at nominal engine load and an energetic diesel fraction of only 1%. A combined approach consisting of single-cylinder research engine (SCE) testing and 3D-CFD simulation was followed. In the first step, experimental investigations were carried out on a large high-speed SCE with a displacement of 6.24 dm3 per cylinder and at a nominal IMEP of 24 bar. The combustion chamber of the SCE was optically instrumented with an endoscope and an illumination device. This provided detailed insight into the fuel injection process and combustion phenomena in the early combustion phase while being minimal invasive and withstanding the high mechanical and thermal loads at full load engine operation. To supplement the SCE measurement data and to enhance interpretation of the results, injection rate curves of the diesel injector were measured on a separate test rig. In the second step, the experimental data was used to calibrate and validate a corresponding 3D-CFD simulation model. Further insight into the entire combustion process was obtained from the results of subsequent simulations. The combined results comprehensively illustrate how the injection timing of the pilot diesel fuel affects mixture formation, ignition and the combustion process. Besides basic SCE performance and emission results, the spatially and temporally resolved results from 3D-CFD simulation provide key information about the processes in the combustion chamber. While it is already known from the existing literature that at very early pilot fuel injections, combustion phasing is not directly linked to injection timing, the 3D-CFD results allow an analysis and detailed explanation of this behavior. It has been found that varying the injection timing has a considerable impact on ignition delay due to the different in-cylinder conditions at the start of injection. Ignition delay in turn affects local mixture quality at start of combustion (and vice versa), which results in significantly different combustion processes and related performance indicators. This detailed understanding serves as the basis for future research that must investigate the impact of additional influencing factors such as combustion chamber and injection nozzle geometries as well as pilot fuel injection strategies (e.g., multiple injections per cycle) to holistically improve the diesel-gas combustion concept in a target-oriented way.