Formation and Characterization of Carbon Deposits from Thermal Stressing of Gasoline and Diesel Fuels Dionne Hallett, Research Scholar Faculty Mentor: Dr. Semih Eser, Associate Professor The Pennsylvania State University Abstract All commercial gasoline and diesel fuels build up deposits on the components of an engine’s fuel injector and combustion chamber, which significantly lowers the engine performance over time. In this research study, three samples of commercial gasoline with different octane numbers (87,89,93) and a sample of diesel fuel were stressed at 350˚C and 30 atm (500 psi) in a flow reactor in the presence of SS304, SS416 and Inconel 718 alloys for 5h with a 4ml/min flow rate. Carbon deposit accumulation on the metal alloy substrates was quantified and characterized by using a LECO-Multiphase Carbon Analyzer and Scanning Electron Microscopy (SEM). In addition, hydrocarbon and sulfur composition of the fuels before and after thermal stressing were identified and compared by using Gas Chromatography/Mass Spectroscopy (GC/MS) and Pulsed Flame Photometric Detector (PFPD) analysis. The results show that carbon deposit formation depends on the fuel composition and on the nature of the metal substrate. It appears, for example, that the deposit formation on SS416 is inversely proportional to the sulfur content of the fuel. 1. Introduction The formation of carbonaceous deposits on metallic surfaces from fuel significantly lowers the efficiency of internal combustion engines, including spark-ignition (gasoline) and compression-ignition (diesel) engines. The areas in an engine that are affected the most are the fuel injector and the combustion chamber. The fuel injector plays a critical role in the function of an engine. Originally used in diesel engines, because of diesel fuel’s higher viscosity and the need to overcome high pressure in the compressed air in the cylinders, the fuel injector system results in improved fuel economy and engine performance as well as a reduction in polluting exhaust emissions. In a diesel engine the injector sprays a regular, timed, and metered amount of fuel into a cylinder, distributing the fuel throughout the air within. This process is different in a gasoline engine. In a gasoline engine the fuel injector delivers fuel or a fuel air mixture to the cylinders of the engine by way of pressure from a pump [1]. Over time, a deposit forms on the small opening of the fuel injector. This affects the combustion process as a whole because the engine power depends on the quantity of fuel that is injected and the method of introducing this fuel to the combustion chamber, where the air-fuel mixture is burned [2]. If the fuel spray pattern is negatively affected by the deposits, the decrease in combustion quality leads to loss in power, decrease in fuel economy, as well as increased exhaust emissions [3]. The amount of deposit found in an engine’s combustion chamber could depend on the engine type. There are three major differences between a gasoline and a diesel engine. In a gasoline engine, a mixture of gas and air, provided by the fuel injector, is compressed and ignited with a
Transcript
Formation and Characterization of Carbon Deposits from Thermal Stressing of Gasoline and Diesel Fuels
Dionne Hallett, Research Scholar
Faculty Mentor: Dr. Semih Eser, Associate Professor The Pennsylvania State University
Abstract
All commercial gasoline and diesel fuels build up deposits on the components of an engine’s fuel injector and combustion chamber, which significantly lowers the engine performance over time. In this research study, three samples of commercial gasoline with different octane numbers (87,89,93) and a sample of diesel fuel were stressed at 350˚C and 30 atm (500 psi) in a flow reactor in the presence of SS304, SS416 and Inconel 718 alloys for 5h with a 4ml/min flow rate. Carbon deposit accumulation on the metal alloy substrates was quantified and characterized by using a LECO-Multiphase Carbon Analyzer and Scanning Electron Microscopy (SEM). In addition, hydrocarbon and sulfur composition of the fuels before and after thermal stressing were identified and compared by using Gas Chromatography/Mass Spectroscopy (GC/MS) and Pulsed Flame Photometric Detector (PFPD) analysis. The results show that carbon deposit formation depends on the fuel composition and on the nature of the metal substrate. It appears, for example, that the deposit formation on SS416 is inversely proportional to the sulfur content of the fuel. 1. Introduction The formation of carbonaceous deposits on metallic surfaces from fuel significantly lowers the efficiency of internal combustion engines, including spark-ignition (gasoline) and compression-ignition (diesel) engines. The areas in an engine that are affected the most are the fuel injector and the combustion chamber. The fuel injector plays a critical role in the function of an engine. Originally used in diesel engines, because of diesel fuel’s higher viscosity and the need to overcome high pressure in the compressed air in the cylinders, the fuel injector system results in improved fuel economy and engine performance as well as a reduction in polluting exhaust emissions. In a diesel engine the injector sprays a regular, timed, and metered amount of fuel into a cylinder, distributing the fuel throughout the air within. This process is different in a gasoline engine. In a gasoline engine the fuel injector delivers fuel or a fuel air mixture to the cylinders of the engine by way of pressure from a pump [1].
Over time, a deposit forms on the small opening of the fuel injector. This affects the combustion process as a whole because the engine power depends on the quantity of fuel that is injected and the method of introducing this fuel to the combustion chamber, where the air-fuel mixture is burned [2]. If the fuel spray pattern is negatively affected by the deposits, the decrease in combustion quality leads to loss in power, decrease in fuel economy, as well as increased exhaust emissions [3]. The amount of deposit found in an engine’s combustion chamber could depend on the engine type.
There are three major differences between a gasoline and a diesel engine. In a gasoline engine, a mixture of gas and air, provided by the fuel injector, is compressed and ignited with a
spark. A diesel engine takes just air, compresses it and then injects the fuel into the compressed air. The heat of the compressed air ignites the fuel spontaneously. The method of fuel injection in a diesel engine is different as well. A gasoline engine uses a port fuel injection, where fuel is injected just prior to the intake stroke. In contrast, a diesel engine uses direct fuel injection where the diesel fuel is injected directly into the cylinder. The compression ratios of the gasoline and diesel engines are also different. A gasoline engine piston-cyclinder compresses at a ratio of 8:1 to 12:1, while a diesel engine compresses at a ratio of 14:1 to as high as 25:1, which gives the diesel engine more power and a higher efficiency [4].
One similarity of gasoline and diesel engines is that they both have a combustion chamber. For gasoline engines, combustion chamber deposits can lead to an increase in fuel octane requirement because of an increase in effective compression ratio. The maximum performance of the engine may also be reduced due to significantly retarded ignition required to avoid detonation [5]. The presence of combustion chamber deposits is the single most important operational variable affecting octane requirement increase (ORI) [6]. The octane number of a fuel is a measure of its anti-knock performance. The more deposits formed in an engine the more you have to increase the octane number of the fuel for the engine not to knock.
The presence of sulfur in gasoline and diesel fuel also has a role in the formation of carbon deposits in the gasoline and diesel engine. The sulfidation of metals at high temperatures has become a matter of increasing concern in industrial applications, particularly in energy conversion systems using fossil fuels. Because of the high diffusivity and formation of eutectic melts with metals, sulfur reacts with metals at relatively low temperatures forming severe localized corrosion on heat-resistant alloys and superalloys [7].
In this study, metal foils of different alloys were thermally stressed with three samples of gasoline and a sample of diesel to see how much deposit was formed on the surface of the metal foils. The working hypothesis was that different fuels will produce different amounts of deposits on metal substrates. 2. Experimental Section 2.1. Thermal Stressing Experiments. Thermal stressing of different commercial octane number gasoline (87,89,93) and diesel fuel in the presence of different alloy foils were performed in a 1/4-in. outer diameter (o.d.) flow-through reactor. Figure 1 shows a schematic diagram of the flow reactor system. Metal foils, 15 x 0.3 x 0.015 cm length, width, and thickness, respectively, were placed at the bottom of a 20-cm-long reactor. The reactor containing the foil samples was heated to 350˚C and maintained at that temperature for 10 min in flowing argon at 34 psi before the fuel was introduced. The argon was introduced to carry the heat through out the system. The fuel was preheated to 200˚C in a valve oven before entering the reactor. Throughout the experiments, the reactor wall temperature, fuel pressure, and liquid fuel flow rate were kept at 350˚C, 34 atm, and 4 ml/min, respectively. The stainless steel preheating section was 2 mm in inner diameter (i.d.) (1/8-in. o.d.) and 2 m in length. The fuel residence time in this preheating zone is 1.57 min at a liquid fuel flow rate of 4 ml/min. The fuel residence time in the reactor (4-mm i.d., 1/4-in. o.d., and 20-cm length) was 0.42 min at the same fuel flow rate. At the end of the reaction period (12 h), the foils were cooled under an argon flow in the reactor.
2.2 Materials. For thermal stressing, foils of three different metal alloys were used: Stainless Steel 304, Stainless Steel 416, and Inconel 718. SS 304 is a member of austenitic type stainless steels which are iron based alloys that have heat resistant strength up to 650˚C. SS 416 is a member of ferritic steels which show good oxidation resistance compared to that of austenitic grades. As seen in Table 1, this alloy contains only Fe and Cr alloying elements. Superalloys, especially Fe-Ni based formulations, are extensions of stainless steel technology. Inconel 718 alloy is Fe-Ni based superalloy, which contains Ti, Al, Nb+Ta in addition to Ni, Fe, and Cr elements. The addition of minor alloying elements form different crystalline structures in the alloy, which increases its corrosion resistance and strength at high temperatures up to 900˚C for a longer period of time. As different fuels, three samples of gasoline with octane numbers of 87, 89, and 93 and a sample of diesel fuel were obtained form commercial vendors. All gasoline samples came from the same vendor. 3. Results 3.1. Hydrocarbon Composition. Gasoline is a complex mixture of hundreds of hydrocarbons. The hydrocarbons vary by class-paraffins, olefins, , and aromatics- and within each class, by size. The mixture of hydrocarbons in a gasoline determines its physical property and engine characteristics. The carbon number distribution of a typical gasoline is in the range of C4 to C12 and the average is C6.8. Generally, three grades of gasoline fuels with different antiknock index or octane number are available in US-regular (gasoline 87), midgrade (gasoline 89), and premium (Gasoline 91, 93, and 94). All three gasolines contain detergents and possibly other additives against carbon deposit formation, but the premium type fuel has the highest additive level.
Diesel fuel is heavier than gasoline and it derived from a fraction of petroleum called middle distillates. Diesel fuel is a very complex mixture of thousands of individual compounds, most with carbon numbers between 10 and 22.
During thermal stressing, because of thermal reactions and/or catalytic effect of the metal substrate the hydrocarbon composition of the fuels changes. Both lighter and heavier compounds are produced by cracking, polymerization, and dehydrogenation reactions. Cycloalkanes go through dehydrogenation reactions to become aromatic compounds. For example, cyclohexane becomes benzene through the loss of hydrogens. Benzene is more stable than cyclo-hexane. Upon subsequent polymerization and dehydrogenation reactions, the lighter compounds become heavier, eventually carbonaceous solids are formed since hydrogen content gets lower and lower, leaving behind carbon. As can be seen in Figure 2 the higher the octane number of the gasoline the lighter the gasoline is. The GC/MS analysis of diesel fuel is very different than that of the gasoline fuels (Figure 2). This is because diesel fuel is heavier than the other fuels.
3.2. Fuel Effect. The amount of carbon deposit produced from different fuels on metallic
foils was measured by using a LECO-Multiphase Carbon Analyzer. Temperature-programmed oxidation used in this analysis also gives the evolution of carbon dioxide peaks from the carbon
deposit as a function of temperature-called a TPO profile. Figure 3 shows a photograph of the foils used in thermal stressing experiments. A description of the results according to fuel type follows.
• Gasoline 87: As you can see from Figure 4, gasoline 87 deposit on SS304 has the
highest carbon signal with a temperature program oxidation (TPO) peak at 800°C. On SS416 deposition from gasoline 87 gave three TPO peaks at 350°C, 400°C and 450°C. The lowest carbon signal for gasoline 87 deposit was found on Inconel 718 with a TPO peak at 400°C. Overall gasoline 87 produced the highest amount of carbon deposits.
• Gasoline 89: Gasoline 89 deposits on SS304 gave two TPO peaks at 400°C and 450°C
as can be seen in Figure 4. Gasoline 89 deposit on SS416 also gave two TPO peaks, one at 350°C and the other at 500 °C. The lowest amount of carbon deposit formed from gasoline 89 was found on Inconel 718 with a TPO peak at 500°C.
• Gasoline 93: As can be seen on Figure 4, gasoline 93 gave overall the lowest amount of
carbon deposit out of the three gasoline samples. Gasoline 93 deposit on SS304 produced a TPO peak at 400°C. Gasoline 93 deposits on SS416 have the second highest carbon signal with a TPO peak at 450°C. Out of the three metals tested with gasoline 93, Inconel 718 collected the lowest amount of carbon deposit with a TPO peak at 450°C.
• Diesel fuel: The deposit from this fuel gave the lowest carbon signal out of all the fuels
that were tested. With all three metallic surfaces that were tested there are barely any TPO peaks. Overall the diesel fuel sample leaves the least amount of carbon deposit on all the surfaces tested.
3.3. Material Effect. The composition of the materials used in the stressing experiments appeared to be a key factor in the production of carbon deposits. The gasoline and diesel fuel samples reacted differently on different metal surfaces. A description of the results according to the metal type follows.
• SS304
o Gasoline 87: As seen in Figure 5, the thermal stressing of gasoline 87 appears to produce four different types of carbon deposits oxidized at 300°C, 350°C, 400°C, and 450°C, respectively. This result may be attributed to the formation of large amounts of metal sulfide crystals creating an attractive surface trapping the fuel for a longer stressing period. The sulfide crystals are known to be mostly Fe1-XS, Pyrhotite, from previous studies [8]. Gasoline 87 has a lower carbon deposit amount than gasoline 89 most likely because of corrosion of the metal surface. The deposit formed on the material was so great that it began to corrode the surface of the metal resulting in the scale like island formation as seen in Figure 6-a. As can be see in Table 2, the total carbon amount found in this fuel after thermal stressing is 36.4 µgC/cm2.
o Gasoline 89: Fuel 89 produced a high temperature TPO peak at 500°C in
addition to the peak evolved at a lower temperature, 350°C. The high-
temperature peak results most probably from the oxidation of filamentous type carbon deposit on the SS304 surface after 12 h of thermal stressing at 350°C. The total carbon amount found in this fuel is the highest at 63µgC/cm2.
o Gasoline 93: This fuel produced a very low amount of carbon deposit as seen in
Figure 5. There are two peaks at 350°C and 450°C. Although, very low carbon deposits from TPO profiles were obtained, Scanning Electron Microscope (SEM) images (Figure 6-c) show popcorn like formations. At this point it is not known whether they are carbon deposits, or some type of metal sulfide formations. The total carbon deposit amount is 32.5µgC/cm2.
o Diesel: There were no significant metal sulfide formations. White deposits on
SEM images, Figure 6-d, are carbon deposits. The total carbon deposit amount for this fuel is the lowest out of all the fuels tested at 2.2µgC/cm2.
• Inconel 718
o Gasoline 87: The surface is very clean as seen in Figure 7-a. There are also two
TPO peaks in figure 8 at 325°C and 400°C with lesser extent of carbon signal compared to gasoline 89 and 93. From Table 2 you can see that the total carbon deposit amount found in this fuel after thermal stressing is the lowest at 4µgC/cm2.
o Gasoline 89: Fuel 89 formed less amount of carbon deposit compared to fuel 93
on the Inconel 718 surface. The deposits are in the form of small particles mixed with metal sulfides. The TPO peak at 475°C indicates the deposits structure can be attributed to more ordered deposits than amorphous. The total carbon deposit amount for this fuel is 10.7µgC/cm2.
o Gasoline 93: Two TPO peaks at 300°C and 450°C can be seen in Figure 8,
indicating amorphous and more ordered carbon deposits as seen in Figure 7-c. There is metal sulfide formation on Inconel 718 in the form of semi-spherical particles less then 0.3µm in diameter. From Table 2 you can see that the total carbon deposit amount for this fuel is the highest at 14.7µgC/cm2.
o Diesel: There are five TPO peaks at 200°C, 300°C, 400°C, and 500°C, 650°C,
which can be seen in Figure 8. According to Figure 7-d, no significant metal sulfide formations were detected. White deposits on SEM images are carbon deposits. Diesel has a total carbon deposit amount of 14.7µgC/cm2.
• SS 416
o Gasoline 87: Gasoline 87 had the highest carbon signal on this material. From
Figure 9 you can see that it has one TPO peak at 400°C. There is what is most likely metal sulfide on the surface on the metal, which can be seen in Figure 10-a.
The total carbon deposit for gasoline 87 on SS 416 is 125.4µgC/cm2. This is the highest out of all the fuels tested on this material.
o Gasoline 89: Fuel 89 had the second highest carbon signal out of the four fuels.
It has three TPO peaks at 390°C, 400°C, 450°C, which can be see in Figure 9. As can be seen in Figure 10-b there is less carbonaceous deposits found on this surface with this fuel than with gasoline 87. The total carbon deposit amount for gasoline 89 on SS 416 is73.9µgC/cm2.
o Gasoline 93: Gasoline 93 gives the third highest carbon signal out of the fuels
tested on this metallic surface. As can be seen in figure 9 it has a TPO peak at 400°C. As you can see in Figure 10-c the surface is fairly clean with little metal sulfide and carbonaceous deposits. The total carbon deposit amount is 37.4µgC/cm2.
o Diesel: Diesel fuel has the lowest carbon signal out of the four fuels tested. It has
a TPO peak at 450°C. According to Figure 10-d there is no significant metal sulfide formation. The total carbon deposit amount was the smallest out of all the fuels tested on this material at 11.2µgC/cm2.
3.4. Sulfur Content of Fuel Samples. The amount of sulfur in the fuel samples varied
among the different octane numbered samples and the diesel fuel. Table 3 gives the total sulfur content of each fuel before and after thermal stressing. As you can see the amount of sulfur does not differ that much between stressing. This analysis suggests that only a small fraction of the sulfur compounds reacts possibly because of the short residence time of the fuel in the reactor. The formation of metal sulfides on the substrates shows clearly that certain sulfur compounds in the fuels react with metals under the experimental conditions used. A description of the sulfur effect based on fuel type follows.
• Diesel fuel: Table 3 shows that diesel fuel has the highest sulfur content of the four fuels
tested with 155ppm. The large amount of sulfur in diesel fuel contributes to air pollution through the formation of particulates and sulfur dioxide. Figure 11 shows that there are more peaks in the chromatogram for the diesel fuel then any of the other fuels. One can also see that the sulfur compounds in diesel fuel start to appear after a retention time of 10 minutes, compared to the 2 minute retention time before the first appearance of sulfur compound peaks in the gasoline samples. This is because the compounds (hydrocarbons and sulfur species) in diesel fuel have higher boiling points than the compounds found in the gasoline samples.
• Gasoline 87: As seen in Table 3, gasoline 87 has the highest amount of sulfur out of the
three gasoline fuels, 109 ppm. You can also see this in Figure 11 with gasoline 87 having higher peaks than that of any of the gasoline fuels.
• Gasoline 89: Gasoline 89 has a total sulfur content of 102 ppm as can be seen in Table 3.
The PFPD chromatogram of this fuel, which can be seen in Figure 11, shows that it has the second highest concentration of sulfur out of the three gasoline samples.
• Gasoline 93: Gasoline 93 has the lowest amount of sulfur out of all the fuels. As you
can see in Table 3 the total sulfur content of this fuel is 54ppm, much lower than that of the other fuel samples. This fuel also has the least concentration of peaks, also indicating the low amount of sulfur in this fuel.
4. Conclusions
The combined use of LECO-Multiphase Carbon Analyzer, Scanning Electron Microscopy, and Gas Chromatography/Mass Spectroscopy and Gas Chromatography with Pulsed Flame Photo Detector examination of products from thermal stressing of fuels with metallic foils provides useful information for the characterization and source of carbon deposits on SS304, SS416, and Inconel 718. Carbon deposit formation appears to be inversely proportional to the sulfur content of the fuel on SS 416. This relationship can be seen in Figure 12. It is concluded that the reaction between the sulfur and the metal alloy, to some extent, controls hydrocarbon decomposition which results in low carbon deposit formation. This is concluded from the GC/MS analysis. It seems that during thermal stressing lighter compounds are produced which leads to a loss of hydrogen and an increase in carbon.
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Acknowledgements I thank Dr. Orhan Altin and Dr. Semih Esser for all their help during my research. I would also like to thank the Student Research Opportunities Program for giving me the opportunity to do this research.
Table 1. Elemental Composition of AlloysNi Fe Cr Mo Co Al Ti Nb Cu Mn Si C S W Y
