|Comparison of optimized start of injection of advanced diesel combustion for brake thermal efficiency, NOx, PM, THC, and CO of diesel (open bar); HTFT (gray bar); and LTFT (black bar) fuels. Credit: ACS, Lilik and Boehman. Click to enlarge.|
A high ignition quality (high cetane number) fuel is well suited for operation under an advanced diesel combustion mode (High Efficiency Clean Combustion, HECC) and leads to reductions in all primary pollutant emissions—i.e., THC and CO as well as NOx and PM—according to research by a team from The Pennsylvania State University.
HECC mode, a premixed charge compression ignition (PCCI) process (a Low Temperature Combustion, LTC, process), is achievable in current production diesel engines by modifying several engine operation parameters via the electronic control unit (ECU). Increasing the rate of introduction of exhaust gas recirculation (EGR) into the intake air enhances premixing before the onset of combustion by retarding the start of combustion and reducing the intensity of the ignition process, note Gregory Lilik and André Boehman in a paper published recently in the ACS journal Energy & Fuels.
HECC produces a decrease in both NOx and PM emissions while maintaining, or even increasing, fuel efficiency. However, operation in the HECC mode, or any other PCCI mode, typically results in increased total hydrocarbons (THC) and carbon monoxide (CO) emissions and is limited to low and medium load operation.
Conventional diesel fuels are optimized for conventional diesel operation. Modifications of fuel composition and, thus, fuel properties have the potential to optimize PCCI operation processes, such as HECC, and eliminate undesirable effects arising from increasing the fraction of premixed combustion. Fuel properties are directly dictated by the molecular structure of the hydrocarbons in the fuel. Normal alkanes, branched alkanes, cyloalkanes, alkenes, and aromatics account for the major species that comprise conventional liquid hydrocarbon fuels.
The ignition delay for a fuel is the time between SOI [start of injection] of the fuel and a significant in-cylinder pressure increase due to ignition, which is characterized by the fuel property cetane number. Ignition delay is composed of a physical ignition delay, in which fuel and air mix, liquid fuel is atomized, and fuel droplets evaporate, and a chemical ignition delay, which is kinetically controlled. The molecular structure of a fuel directly affects the ignition quality and thus the cetane number. In general, the cetane number of compounds with a similar number of carbon atoms follows the order of n-alkane > alkene > cycloalkane > alkyl aromatic. Furthermore, as chain length increases with the addition of carbons, cetane number increases. The standard for diesel fuel oil is a minimum cetane number of 40; however Heywood indicates that a normal diesel fuel has a cetane number range of 40 to 55.
...In a recent study, fuel samples from the Fuels for Advanced Combustion Engines (FACE) working group were combusted in a diesel engine operating in HECC. The FACE fuels are a set of nine laboratory test fuels designed to vary in cetane number, volatility, and aromatic content. Cetane number was shown to be the fuel property having the largest effect on emissions. Specifically it was shown that the set of fuels having a target cetane number of 30 produced nearly twice as much THC and CO emissions as the fuels having a target cetane number of 55.—Lilik and Boehman
In their study, Lilik and Boehman operated a light-duty 2.5L turbodiesel engine in HECC modes using three different fuels including a conventional ultralow sulfur diesel fuel (diesel), a synthetic fuel produced in a high temperature Fischer-Tropsch (HTFT) process, and a synthetic fuel produced in a low temperature Fischer-Tropsch (LTFT) process with cetane numbers of 45, 51 and 81, respectively. They swept SOI timing from –8° ATDC to 0° ATDC to find the optimized injection timing for each fuel.
They found that the HTFT fuel (51 DCN) decreased THC and CO emissions by 32% and 31%, respectively, compared to the conventional diesel fuel (45 DCN). They found that the higher ignition quality of the HTFT fuel reduced emission from incomplete combustion by presumably consuming more of the fuel charge before it reached a region of the cylinder where it was too lean to effectively burn. However, with the HTFT fuel, NOx and PM emissions increased relative to the diesel baseline due to a higher peak heat release rate, presumably caused by 2% less EGR during the HTFT fuel’s operation.
In contrast, the LTFT fuel (81 DCN) enabled an 80% reduction in THC emissions and a 74% reduction in CO emissions compared to the diesel fuel. The LTFT fuel, though having a very short ignition delay, did not increase NOx and PM emissions apparently due to the fuel burning in a shorter, less intense premixed combustion phase followed by a prominent mixing controlled combustion phase.
Use of the LTFT fuel produced the other following results at optimized injection timing of –4° ATDC:
- brake thermal efficiency increased by ~1.5%;
- NOx reduced by ~17% versus standard diesel fuel in PCCI; and
- PM reduced by ~63% versus the diesel fuel in PCCI mode.
Gregory K. Lilik, André L. Boehman (2011) Advanced Diesel Combustion of a High Cetane Number Fuel with Low Hydrocarbon and Carbon Monoxide Emissions. Energy & Fuels 25 (4), 1444-1456 doi: 10.1021/ef101653h