Observed Differences in Low-Temperature Heat Release and Their Possible Effect on Efficiency between Petroleum Diesel and Soybean Biodiesel Operating in Low-Temperature Combustion Mode

Abstract

Low-temperature combustion (LTC) in diesel engines has emerged as an enabling technology to simultaneously reduce oxides of nitrogen and smoke emissions. Combustion development continues to face challenges, however, with high emissions of carbon monoxide and unburned hydrocarbons and lower efficiencies than conventional combustion. Study of alternative fuels, such as biodiesel, with LTC shows varied promise of improving both combustion and fuel conversion efficiencies. The results are inconsistent, depending on the type of biodiesel (e.g., palm-based biodiesel relative to soy-based biodiesel). It was originally believed that differences in low-temperature heat release (LTHR) influence the phasing of main heat release (high-temperature heat release, or HTHR), thus creating differences in fuel conversion efficiencies among petroleum diesel and different types of biodiesels. This study attempts to address the issue of seemingly different LTHR behavior for different types of petroleum and biodiesel fuels between a baseline and LTC modes. Further, the study attempts to identify the effect, if any, that LTHR has on main heat release timing and rate. The study suggests, based on experimental analysis and reliance on observed behavior in literature, the roles that liquid and flame lift-off lengths and fuel chemical composition have on the appearance of LTHR in heat release profiles. The study further shows a disconnect between the extent of LTHR and the subsequent timing and rate of HTHR. Consequently, other factors seem to drive the phasing and extent of HTHR (which likely include the aforementioned parameters of liquid and flame lift-off lengths), which has a stronger influence on engine efficiency than the rate and timing of LTHR. ■ INTRODUCTIONEfficiency improvements of energy-converting devices, such as internal combustion engines, continue to be an important effort of research, particularly with the increased need to reduce atmospheric carbon emissions. Diesel engines, which typically offer higher fuel conversion efficiencies than other combustionoriented energy conversion technologies, could offer an opportunity to decrease such carbon emissions. There are other species present in diesel engine exhaust, however, which take a toll on human health and degrade the environment, including oxides of nitrogen (NO x ) and particulate matter (PM).Simultaneous reduction of NO x and PM in diesel engines is clearly desired. Conventional diesel combustion, however, does not permit this due to a complex temperature dependency of NO and soot formation and soot oxidation; 1 soot is a major component of PM. The simultaneous reduction can be obtained with use of low-temperature combustion (LTC). 2−7 One method to achieve LTC in a diesel engine phases combustion into the expansion stroke, where the two-stage ignition feature of some fuels can become visible 8−11 in the apparent rate of the heat release profile, calculated from the measured in-cylinder pressure. Two-stage ignition is a general term that recognizes the steady oxidat...

Combustion Characteristics of Bluff-Body Turbulent Swirling Flames with Coaxial Air Microjet

“…Increasing the combustion efficiency and reducing emissions are the goals being pursued by researchers. With the advancement of technology, more and more advanced combustion technologies have been proposed, such as flameless combustion, , low temperature combustion, , hydrogen enriched combustion, , and microjet assisted flames, − etc. Air microjet is defined as air injection from a small diameter nozzle .…”

Advancements in In-Cylinder and After-Treatment Strategies for Mitigating Pollutants from Diesel Engines: A Critical Review

“…Several low-temperature combustion strategies have been investigated extensively in the past decade to eliminate the NO x -soot trade-off and achieve ultralow values of NO x and soot by decoupling the fuel-injection event from the start of combustion . This approach ensures sufficient fuel-air premixing time by maintaining a positive ignition dwell (time elapsed between the end of fuel injection and the start of combustion), forming a lean, homogeneous fuel-air mixture . The lean and homogeneous nature of the charge eliminates the scope for fuel-rich regions that favor soot formation.…”

“…28 This approach ensures sufficient fuel-air premixing time by maintaining a positive ignition dwell (time elapsed between the end of fuel injection and the start of combustion), forming a lean, homogeneous fuel-air mixture. 112 The lean and homogeneous nature of the charge eliminates the scope for fuel-rich regions that favor soot formation. It simultaneously reduces NO x emissions by lowering the combustion temperatures to below 1800 K. 113 The most commonly investigated LTC strategies include HCCI, PCCI, and RCCI.…”

Numerical Study of Diesel Lift-Off Flame and Soot Formation under Low-Temperature Combustion

“…The conventional spray combustion under high exhaust gas recirculation (EGR) is the most effective way to achieve low-temperature combustion (LTC), which can avoid the high combustion temperature range associated with NO x formation. − LTC can also be extended to high load without an excessive heat release rate as a result of the overall low chemical reaction rate. Although a lack of oxygen may make the partial mixture too dense, the exhaust gas with a higher specific heat and more inert species can evidently slow the temperature rising of the mixture, so that the temperature of the mixture, especially within the range of ϕ > 2.0, may later reach and even avoid the soot formation range at the distribution map of soot yields and NO x for different equivalence ratio and temperature conditions. , However, only increasing EGR could usually increase the soot finally, with too high EGR inevitably bringing low thermal efficiency as well as high emissions of CO and hydrocarbons.…”