A Numerical Study of the Effects of FAME Blends on Diesel Combustion and Emissions Characteristics Using a 3-D CFD Code Combined with Detailed Kinetics and Phenomenological Soot Formation Models

摘要

The objective of the present research is to analyze the effects of using oxygenated fuels (FAMEs) on diesel engine combustion and emission (NO_x and soot). We studied methyl oleate (MO), which is an oxygenated fuel representative of major constituents of many types of biodiesels. Engine tests and numerical simulations were performed for 100% MO (MO100), 40% MO blended with JIS#2 diesel (MO40) and JIS#2 diesel (D100). The effects of MO on diesel combustion and emission characteristics were studied under engine operating conditions typically encountered in passenger car diesel engines, focusing on important parameters such as pilot injection, injection pressure and exhaust gas recirculation (EGR) rate. We used a diesel engine complying with the EURO4 emissions regulation, having a displacement of 2.2 L for passenger car applications. In engine tests comparing MO with diesel fuel, no effect on engine combustion pressure was observed for all conditions tested. However, combustion was enhanced by using MO under low temperature and high EGR rate (high equivalence ratio) conditions. Using MO, soot emission was significantly reduced without a concomitant increase in NO_x emission, but the apparent brake specific fuel consumption (BSFC) was worsened. However, the brake specific energy consumption (BSEC), defined on an energy basis, did not changed significantly between the tested fuels. In addition, CO and THC emissions were reduced by using MO. These effects were due to the enhancement of low temperature reactions with oxygenated fuels like MO. We also conducted a 3-D numerical study using the KIVA-3V code with modified chemical and physical models. To predict soot emission, a model dealing with the formation of precursors including polycyclic aromatic hydrocarbons (PAHs) was coupled with a detailed phenomenological particle formation model, taking into account soot nucleation from the precursors, surface growth/oxidation and particle coagulation. We adopted an engine condition of 25% load and 1500 rpm because it is typically encountered in the NEDC mode. The calculated in-cylinder pressure traces and heat release rates (HRRs) for all the fuels were in close agreement with the measured engine data and the results on soot emission also agree with analyzed data. Further, the numerical results suggested that the oxygenated fuel did not greatly affect soot oxidation rates and rates of oxidation by OH radicals. Instead, soot nucleation from the precursors and surface growth were found to be major factors influencing soot emission for the oxygenated fuel.