Sensitivity analysis of fuel types and operational parameters on the particulate matter emissions from an aviation piston engine burning heavy fuels
Introduction
Aviation has grown faster than any other transportation modes worldwide [1]. Sooting in aircraft engines due to incomplete combustion indicates the decrease of combustion efficiency, and could lead to hardware fouling [2], [3]. Ultrafine particles with the size below one micron meter are typical of aviation emissions, and impose high risk on human health since they can penetrate deeper into the lungs, bloodstream and impair cardiovascular and nervous system [4], [5]. In terms of military aircrafts, the soot emissions increase infrared exhaust plume visibility, which severely affect and may ruin the training and combat missions [6].
Environmental Protection Agency (EPA), together with the Federal Aviation Administration (FAA), set the first PM standards in 1971, which were updated in 1987, 1997, 2006, and again in 2013 [7]. EPA now focuses on the regulation of the aviation particulate matter emissions with the size less than 1 μm, since ultrafine particles are deemed readily inhalable and thus more harmful than coarse particles [8]. Increasingly strict regulations for mandating vehicular PM emissions make it important to investigate emission mitigation strategies [9] such as clean fuel formulation [10], [11], [12], [13], engine calibration [14], [15], [16], and emission trap technologies [17], [18]. Compared with vehicular PM emissions, the studies on aviation emissions are much scarcer, but it is conceivable that aviation emissions will become a hot issue in the light of the upcoming aviation emission regulations established by International Civil Aviation Organization (ICAO) [19], [20]. The conventional smoke number (SN) metric does not address the current challenges to quantitatively measure the mass and number of PM emitted from aircraft engines. Therefore, Aerospace Information Repot (AIR) has been proposed as a primer by the pioneered PM research community to measure the aviation particles on mass and number basis, in order to meet the increasingly stringent regulations [21].
Heavy Fueled Engines (HFE), which have appealing characteristics, such as high economic feasibility, high torque and durability, are widely used in the military by using kerosene and diesel fuels [22], [23], [24]. This is in consistent with the ‘One Fuel Forward’ policy adopted by the US military [25], [26], derived from the logic of saving the big logistical cost, but also simplifying the fuel and pipeline systems [27]. Kerosene contains more volatile compositions than diesel and is thus more beneficial for fuel atomization, combustion efficiency and PM emissions [28]. Moreover, the development of derived substitutes for diesel fuel and kerosene, such as Fischer-Tropsch (FT) kerosene fuels, provides more promising solutions, which could improve the engine operations and emission characteristics [29]. The ‘iso-paraffinic kerosene’ FT fuels firstly produced by Sasol in South Africa from coal, biomass, and natural gas, were allowed for aviation use in blends up to 50% by volume in Jet A-1 in 1999 [30]. Recently, the FY 11, F-15, F-22 aircrafts in the U.S. military have been scheduled to be certified on the FT blends in the forthcoming future, approved by ASTM D7566 specification [31]. Nowadays, FT fuels are becoming commercially feasible in larger quantities and have been adopted in piston engines for aviation power generation, ground transportation, and ship propulsion by the United States Department of Defense, as a means to relieve the dependence on petroleum - based fuels [32], [33].
A number of researchers have been exploiting aviation piston engine fueled with diesel or kerosene for small aircrafts market [34], [35], [36], [37]. The relevant researches focused more on blends with JP kerosene series, while the literature is scarce to date on the investigation of RP3 kerosene fuels, which are widely used in Asian countries but have discrepancy with JP fuels [5]. Moreover, little research is available that has investigated the feasibility and the performance of neat FT fuel in aviation piston engines. In this study, the neat RP3 and alternative FT jet fuels were used in an aviation piston engine, and the effects of the fuel types and engine operational parameters were systematically evaluated by adopting Design of Expert (DoE), which has been widely used in various fields [38], [39], [40], [41]. DoE is a well-established method on the basis of mathematical statistics, for analyzing experimental data and exploring the cause-effect relationships. The Response Surface Method (RSM) in DoE utilizes face-centered composite design (FCCD) to evaluate how the variables affect the response [42]. The objective of this study is to provide a comprehensive analysis of the effects of fuel types, engine operational parameters and their interactions on the PM emissions. The factors influencing the accumulation mode particles have been quantitatively determined via the predictive model proposed by adopting the RSM in DoE.
Section snippets
Engine test system
The tests were operated utilizing a single-cylinder compression ignition engine modified from a four-cylinder common-rail diesel engine (specification was shown in Table S1). This engine was controlled by using the electronic control unit (ECU) via a program module. The injection parameters including the number of injections, injection pressure, injection quantity and injection timing could be adjusted manually by a PC terminator. The schematic of the engine system was illustrated in Fig. 1.
Engine load effect
The PM size distributions were analyzed under different engine loads (2, 4, 6, 8 bar IMEP) at fixed injection pressure (40, 60, 80 MPa) for the three test fuels. The typical results at the injection pressure of 60 MPa were chosen and shown in Fig. 2. The ultrafine particles smaller than 50 nm, namely nucleation mode particles, comprised of organic species and small fraction of soot inception species, were predominant at the low load of 2 bar IMEP (more than 2.2 × 107 #/cm3 for RP3; more than 1.3 × 107
Conclusion
The neat diesel, RP3 and FT fuels have been tested in a compression ignition engine. The particulate matter (PM) emissions were reduced effectively when RP3 and FT fuels were used. RP3 with high sulfur contents produced a large amount of nucleation mode PM at low engine loads while accumulation mode PM was favored at high engine loads for diesel. FT fuel demonstrated the lowest PM emissions under all the test conditions, primarily because of the absence of sulfur and aromatic contents. The
Acknowledgements
This research is supported by National Natural Science Foundation of China (51306011) and (91641119). Valuable guidance from Prof. Zhi Wang at Tsinghua University and Dr. Chongming Wang at University of Birmingham, and the FT fuel supply from Dr. Guozhu Liu at Tianjin University are gratefully acknowledged.
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