Interpretation of Ignition and Combustion in a Full-Optical High-Pressure-Dual-Fuel (HPDF) Engine using 3D-CFD Methods

In times of global climate change, reducing the CO2 and methane emissions is a main goal of internal combustion engine research. Because of the better C to H-atom ratio compared to diesel, the use of natural gas as main fuel seems a good way to lower the CO2 emissions. Mid-size marine engines therefore often operate in a dual fuel combustion mode, where the port fuel injected gas is ignited by a small amount of direct injected diesel. This classic dual fuel mode leads to extended methane emissions due to flame quenching effects in lean combustion. One way to avoid this is direct injection of the natural gas at high pressures and burn it in a diffusive combustion mode, similar to diesel combustion. Additionally, this operating mode allows compression ratios as high as used in diesel engines because there is no danger of knocking or glow ignition. In a 42-month research project, a high-pressure dual-fuel combustion was developed for high-speed marine engines using the new high-pressure gas-diesel injector of Woodward L'Orange. The investigations were conducted on a fully optical accessible 4.8l displacement single-cylinder research engine that was developed at the chair of internal combustion engines (LVK) of the Technical University of Munich. The engine has an optical access from the bottom of the combustion chamber through an elongated optical piston (Bowdich-design), as well as a lateral optical access in the upper part of the cylinder liner. In order to have the possibility to examine engine combustion processes under conditions relevant for modern combustion engines, the engine design enables high cylinder peak pressures. Until now, operating points with peak pressures above 190bar and indicated mean effective pressures above 20bar are possible. The engine uses natural gas as main fuel, injected just before top dead center at high pressures up to 350bar and ignites by a diesel pilot spray. The combustion development took place supported by experiments in a full-optical, one-cylinder engine. To minimize the experimental efforts a CFD-model was set up which was validated using measurements of the in-cylinder pressure, Particle-Image- Velocimetry (PIV) data of the flow field as well as high-speed-recordings of the soot luminescence. For the HPDF combustion, the influence of the diesel pilot injection timing relative to the gas injection was examined. With a highspeed camera, the soot luminescence of the diesel pilot as well as the diffusive gas combustion was recorded. Furthermore, the combustion chamber was illuminated trough the lateral optical access with a flashlamp in order to visualize the liquid phase of the diesel pilot injection. Through fine dispersed droplets of sealing oil (used to prevent gas leakage inside the injector) the gas jet is also visible through the illumination with the flashlamp. In order to describe the fuel injection and combustion in the simulation, the CFD-model uses a Lagrange-droplet approach to model the liquid spray injection of the Diesel-pilot and detailed chemistry to model the combustion on a 120-degree sector of the cylinder. The Chalmers-53 mechanism for N-Heptane is used to describe the ignition and combustion for both fuels. The natural gas enters the computational domain via mass flow boundary. The injection rate is the result of measurement of the Dual-Fuel injector. This work shows the influence of the diesel pilot injection timing relative to the gas injection in the experiment and how the validated CFD-model is used to interpret the high-speed images of the HPDF-Combustion in order to understand the mechanisms of the ignition of the gas jets and the following flame propagation. This is important at the edges of the optical measurement techniques, for example at a very early pilot ignition where no soot-light emissions are visible no more or at stratified charge combustion, where the gas burns almost premixed with very low production of soot.     «

In times of global climate change, reducing the CO2 and methane emissions is a main goal of internal combustion engine research. Because of the better C to H-atom ratio compared to diesel, the use of natural gas as main fuel seems a good way to lower the CO2 emissions. Mid-size marine engines therefore often operate in a dual fuel combustion mode, where the port fuel injected gas is ignited by a small amount of direct injected diesel. This classic dual fuel mode leads to extended methane emissio...     »