The Environmentally-friendly Engine Research Group is conducting researches for analysis of phenomena by simulations and spray combustion measurement. We are aiming at reducing the fuel consumption of the engine and harmful substances in exhaust gas emitted from it by understanding the physical phenomena related to the engine.
The rapid compression machine (RCM, Fig.1) is mainly used for the experiment. This device can control the temperature, pressure of the intake gas. In addition, the intake gas compositions can be controlled (components of gas fuel premixture and their concentrations, CO2 and O2 concentrations, and so on). In other words, RCM can support various atmospheric conditions. Furthermore, with RCM, we can measure various items; visualization image of premixed combustion and diesel spray combustion (shadow graph, two-color method, radical spectroscopic measurement, etc.); exhaust gas concentrations (NOx, O2, CO2, CO, soot, HC, etc.), and wall heat flux.
Fig.1 Rapid Compression Machine
(◎: Head of the Group)
In a diesel engine, liquid fuel is injected into a high-temperature, high-pressure cylinder, atomized and evaporated, then ignited and burned to be converted into power. Therefore, burning the fuel at an appropriate time is important for reducing the fuel consumption rate and harmful exhaust emissions. However, the ignitability differs depending on the fuel used. And since ignition is a factor that determines the subsequent combustion state, indices for ignitability evaluation have been studied and proposed so far. Currently, definitions and test methods for fuel ignitability are standardized by ISO (International Organization for Standardization) and JIS (Japanese Industrial Standards), and various performance evaluations regarding engines are made on the premise of the fuel standards.
Considering the future, it can be expected that the fuel properties will change due to future regulations and changes in domestic and global supply and demand. At that time, if the theoretical relationship between fuel properties and ignitability is known, it will be possible to evaluate the fuel that will be supplied in the future to some extent. We are conducting experiments and creating computational models focusing on the aromatic components contained in fuels.
In addition, part of this research is being conducted with the fuel supply and advice from fuel suppliers.
Fig.2 Conceptual Diagram of Fuel Properties Estimation Model
Fig.3 Radicals in Fuel Spray Combustion
(Upper: C2 Radical, Lower: OH Radical)
The ignition method, in which a small amount of liquid fuel is injected into a premixed gas (gas in which fuel and air are premixed) and used as the ignition source, is called the micropilot method and is often used in gas engines. This method is more complicated than ignition by a spark plug used in a gasoline engine, because of the additional processes of liquid fuel atomization/evaporation and two-fuel mixture of liquid and gas fuel. This phenomenon is still unclear. Unlike diesel spray combustion, which ignites and burns during the fuel injection period, the minutely injected fuel moves while diffusing and mixing in the cylinder between the end of injection and ignition. For this reason, the behavior of the fine fuel spray is important, and we are investigating the relationship with the ignition process.
Fig.4 Conceptual Diagram of 1-D Fuel Injection Model
Fig.5 Effect of Pilot Fuel Injection Pressure on Ignition and Flame Propagation
(Upper: 50MPa, Middle: 90MPa, Lower: 130MPa)
The three-dimensional spray combustion simulation for diesel and gasoline engines has become available for practical design and theoretical analysis if its applications are limited. Among them, the numerical simulation of gas-liquid two-phase flow related to fuel spray requires various models for each phenomenon (droplet motion, droplet breakup/coalescence, droplet evaporation, etc.). In general, they are not available under any conditions. They can only be used under limited conditions. Therefore, comparison and verification with experiments are always required, and a model with a wider range of application is required.
So far, we have been in charge of development and implementation of the spray part of 3D-CFD software "HINOCA" in "Innovative Combustion Technology" (2014-2018) project under Cross-ministerial Strategic Innovation Promotion Program (SIP). Currently, we are elucidating the cause of the different spray shape each time fuel injection is performed, which is seen in the experimental results, and we are proposing and improving the model to reproduce it.
Part of this research is carried out in collaboration with universities and research institutions through joint research with engine manufacturers. (Click here for a book that describes the details of the spray model.)
Fig.6 Physical Image of Spray Process and their Models
Fig.7 Fuel Spray Simulation without Evapolation