Han Lu; Tian Huayu; Li Bo; Guo Xin

(SINOPEC Research Institute of Petroleum Processing, Beijing 100083)

Abstract: In this paper the effect of gasoline formulations on fuel economy and emissions were studied, aiming at exploring the optimized fuel formulation that can alleviate energy crisis and greenhouse effect to some extent. Five gasoline blends with same research octane number (RON) were designed and tested on a calibrated gasoline direct injection (GDI)engine under the mapped characteristic conditions. Test results illustrate that the optimized fuel formulation shows good superiority in fuel economy, and reduces carbon dioxide (CO2) emissions at low engine speeds with medium loads. The brake-speci fic fuel consumption (BSFC) decreased by a maximum value of 3.26% mainly because of the improvement of combustion velocity and the optimization of low heating value. The optimized fuel formulation simultaneously increases total hydrocarbon (THC) emissions. Nevertheless, it also markedly reduces CO2 emissions, reaching the maximum value of 2.34%. The research results can be applied practically by re fineries to reduce the CO2 emissions and to alleviate the greenhouse effect.

Key words: gasoline formulations; aromatic hydrocarbons; fuel economy; GDI engine

1 Introduction

Nowadays transport is an essential part of modern society and nearly 40% of global transport energy is used in passenger cars[1]. The number of automobiles in China will continue to grow in the foreseeable future with the rapid move of urbanization[2]. Moreover, the CO2regulation and the limitation of tailpipe emissions are becoming increasingly stringent that keep providing pressure on fuel consumption of powertrain[3-5]. Therefore,it is urged to find solutions for fuel consumption and CO2reduction of spark ignition (SI) engines. Researchers have dedicated themselves to improving the engine combustion efficiency and reducing the fuel consumption by advanced engine technology and improved fuel property in recent years[6-7].

For SI engines, GDI engine has become popular due to its good performance in terms of thermal efficiency[8-10].However, the biggest obstacle for GDI engines to improve thermal efficiency is its knock tendency which is limited by the anti-knock quality of gasoline[11]. Therefore, the oxygenated compounds, especially ethers and alcohols are widely being investigated as alternative components due to their advantages in improving the octane number of gasoline in decades[12-14]. Generally, alcohols could potentially reduce soot, unburned hydrocarbons, and carbon monoxide (CO) emissions while increasing thermal efficiency when proper engine calibration was used[15-16]. Gasoline formulated with different ethers, such as ethyl-tert-butyl ether (ETBE) and dimethyl carbonate(DME), has been tested in engines with different combustion modes. The results illustrated that the brake thermal efficiency, cyclic variation, and hydrocarbon emissions could be improved with suitable strategies[17-19].Meanwhile, lots of progresses in the research field about emissions have been published concerning the impact of properties and molecules caused by different gasoline components[20-21]. However, there is less information about the correlation between the re finery-stream gasoline components and the fuel economy of engines. Besides the oxygenated compounds, gasoline is comprised of several blends with numerous molecules produced at re fineries[22].The proportion of hydrocarbon blends in gasoline reaches almost 95% in Chinese market. Therefore, studying the oxygenated compounds, one of the blends in gasoline,still can’t solve the fuel economy issues. Therefore,it is significant to study the effect of refinery-streams formulations of gasoline on engine performance.

Han[23]studiedn-pentane, methyltert-butyl ether (MTBE),and toluene as the surrogate compounds of alkanes,ethers and aromatics on a GDI engine and found that fuel economy could benefit from aromatics under all operating conditions. The conclusion was also verified on a GDI engine using the customized gasoline with different aromatic, alkane, and oxygen components[24].Nevertheless, in China, the Gasoline for Motor Vehicles Standard (China 6a) has come into effect on January 1,2019 and the content of aromatics in gasoline is strictly limited. Therefore, increasing the content of aromatics to improve fuel economy is scarcely possible due to the limitation of exhaust emissions. In actual production, the specific aromatic compounds of gasoline formulations are controllable. As a result, five re finery-stream gasoline blends with same RON and different distribution of aromatic compounds had been tested on a calibrated GDI engine in this study. Despite the same RON rating, some test fuels had different oxygen content. In this way, the optimal formulations of gasoline being produced easily by the re fineries can be implemented to alleviate the fossil energy crisis and greenhouse effect.

2 Experimental

2.1 Experimental setup

The representative tested GDI engine in market was equipped with four cylinders, four valves, four tracks,and a turbo-charged inlet gas system, with the detailed speci fications shown in Table 1. The engine was provided with an eddy current dynamometer (DynoRoad 202/12 Sx) to maintain the engine at a given constant speed(±1 r/min). The fuel consumption was measured by a fuel consumption meter (AVL 735S). A Kistler pressure transducer was used to monitor the in-cylinder pressure variations. The pressure value was collected via a charge amplifier (ETAS592). The real-time combustion indication and analysis were performed by a kiBox system (kiBox Cockpit C2.0). Moreover, the emissions including carbon monoxide (CO) and total hydrocarbon(THC) were measured by an AVL AMA i60 R2 gas analyzer. The measuring range and accuracy for these experimental apparatus are listed in Table 2. The engine test system is shown in Figure 1.

Table 1 The detailed speci fications for the tested engine

Table 2 The measuring range and accuracy of the experimental apparatus

2.2 Test fuels

To investigate the effect of gasoline formulations on fuel economy and emissions, five fuel blends were conducted using different components from selected re finery streams. The RON values of the test fuels were modulated around 99, which was higher than the market fuels, because the anti-knock quality was a significant contributing factor to the performance of GDI engines and high octane number gasoline could achieve high combustion efficiency potentially[25-26]. Figure 2 compares the group compositions of the five tested fuels, labeled as F1 to F5. The F1 to F5 fuels have exhibited the same octane level and similar content of group compositions.They mainly differ in the distributions of C7, C8and C9aromatics. It can be seen that the content of C7aromatic,namely toluene, decreases gradually from F1 to F5. The key properties such as distillation range, Reid vapor pressure (RVP), low heating value, and density of test fuels are listed in Table 3. In order to obtain the same RON value, F1 and F5 have relatively higher oxygen content compared with other test fuels. Owing to the various composition of aromatics, the disadvantage of low heating value caused by oxygenated compounds can be remedied. At the beginning of each test, the engine was warmed up with test fuels to make fuels evaporated rapidly in the cylinder and the evaporation properties of test fuels could be ignored. Therefore, the RVP of test fuels would scarcely have any impact on engine performance in this study.

Figure 1 The tested engine dynamometer

Figure 2 Group compositions of five test fuels

2.3 Test conditions

In this study, the tested GDI engine was mounted on the AVL test bench and ran according to the mapped characteristic conditions. Considering that the purpose of the test is to examine the acceleration given to the driver,the engine load is de fined by throttle angle. The operatingpoints selected included ten different throttle angles from 10% to 100% evenly coupled with ten speeds from 1 000 r/min to 5 500 r/min increasing through each step of 500 r/min. The status of engine is unstable at low speeds under high load operating conditions, which are prone to the phenomenon of low speed pre-ignition (LSPI)[27].Hence, the unstable operating conditions were not considered in this study. To avoid possible contaminations between test fuels, the engine was started and warmed up for a period of 30 minutes with the new fuel after the residual fuel was cleaned out. For each load point, the engine was maintained for 3 minutes to complete the steady-state measurements of performance and emissions.Upon considering the comparability and repeatability of the measurements, important operating parameters, such as intake air temperature, cooling water temperature, and oil temperature were controlled strictly and real-timely measured during each test. The intake air temperature,cooling water temperature, and oil temperature were automatically controlled at 20 ±5 °C, 80 ±5 °C, and 90±5 °C, respectively. All test parameters were acquired over a period of 20 seconds for 2 times and then were averaged. Herein, the parameter ΔXwas chosen to evaluate the comparative effect of test fuels on some engine performance.

Table 3 Properties of the test fuels

where the subscript “x” denotes the engine performance and emission parameters such as BSFC, CO2emissions,etc. ΔX(F1) and ΔX(test fuels) refer to the test results of F1 and other test fuels (F2, F3, F4, F5), respectively.

3 Results and Discussion

3.1 Fuel economy

Generally, BSFC was used as a critical index to measure the fuel economy of engines. BSFC results for tested engine fueled by F1 to F5 are depicted in Figure 3. It can be seen that the distinct economic BSFC are achieved mainly at medium loads and low speeds (1 000—3 000 r/min)conditions, which are also the commonly used operating conditions, and test fuels have various fuel economy in the same operating regions. The ΔBSFCof all loads at five speeds(1 000 r/min, 1 500 r/min, 2 000 r/min, 2 500 r/min, and 3 000 r/min) are depicted in Figure 4. ΔBSFC＞0 implies that less test fuel is consumed than F1 at the same engine loads and speeds conditions.

As illustrated in Figure 4, at the selected speeds, ΔBSFCfluctuates between 10% and -5% at low loads and changes to 5% — -3% at medium and heavy loads. In detail, the maximum ΔBSFCof F2 reaches 3.06% and for other test fuels of F3, F4, and F5, the maximum ΔBSFCreaches 3.45%,3.27%, and 3.60%, respectively, which indicates that F5 is likely to have the greatest potential for fuel saving.

Figure 3 BSFC characteristics maps for test fuels

Figure 4 ΔBSFC of all loads at five speeds for test fuels Speed, r/min ●—1000; ▼—1500; ▲—2000; ■—2500; ◆—3000

In order to determine the advantageous operating conditions for different test fuels, Figure 5 portrays the mean ΔBSFCvalues, which are concluded from Figure 3 and Figure 4, respectively. If test fuels have advantages under speci fic operating condistions, the mean ΔBSFCwill show a larger value than other test fuels. In general, F2, F3, F4,and F5 are able to optimize fuel economy under full operating conditions, especially F5. With regard to F2, F3,F4, and F5, the average values of mean ΔBSFCare 0.35%,0.93%, 1.25%, and 1.48%, respectively, which proclaim the general differences of BSFC. In detail, F5 presents the best fuel economy at low speeds conditions compared to medium and high speeds, while the maximum mean ΔBSFCvalue reaches 2.32%. This may be explained that at low speeds conditions, the turbulence and incylinder temperature of engine are relatively low and the difference of formulated fuels can be reflected on the engine performance[28]. Combustion in the cylinders has a direct impact on fuel economy. In order to better appreciate the differences of test fuels on fuel economy,the combustion characteristics of test fuels are compared in the next section.

3.2 Combustion characteristics

As mentioned above, the improvement of economic BSFC is mainly achieved at low speeds under low and medium loads. To better understand the changes of engine BSFC results, the in-cylinder combustion characteristics are adopted to interpret the combustion process. Upon considering the instability of combustion process under low loads, in this part the combustion parameters are selected to investigate the results of engine running at medium load(40%) and low speeds (1 000 r/min to 3 000 r/min), which are also in line with the improvement area of BSFC.

Figure 5 Mean ΔBSFC of test fuels at different speed (a) and load (b) ranges—F2;—F3;—F4;—F5

Figure 6 exhibits the peak combustion pressure (PCP)and the maximum pressure rise rate (PRR) of test fuels under 40% of load at low speeds. As indicated in Figure 6, under the same conditions, both the PCP and the maximum PRR of test fuels show obvious differences.At different speeds, the maximum values of ΔPCPand ΔPRRwhich are calculated from Figure 6 reach 4.62% and 6.63%, respectively, presenting the maximum differences of the test parameters. Moreover, the PCP and maximum PRR values of F5 are relatively larger compared with other test fuels operating under most conditions. It means that more fuel was burnt and more chemical energy was released in the vicinity of the top dead center (TDC)and the in-cylinder temperature and pressure were high,thereby resulting in high combustion pressure[29].

Figure 6 Peak combustion pressure (a) and maximum pressure rise rate (b) for test fuels under 40% load at low speeds—F1;—F2;—F3;—F4;—F5

Figure 7 and Figure 8 show initial combustion duration(ICD), combustion duration (CD) and their changes (ΔICDand ΔCD) for test fuels under 40% of load at low speeds(1 000—3 000 r/min). In this study, the 10%, 50%, and 90%mass fraction burned points are defined as CA10, CA50 and CA90. The ICD and CD are de fined as the difference of crank angle between CA50 and CA10, CA90 and CA10, respectively. In general, F5 has the shortest ICD and CD under the analyzed conditions. The mean ΔICDthat is calculated from Figure 7(a) of F2, F3, F4, and F5 is 0.15%,0.31%, 0.38%, and 2.26%, respectively. The shortened CD is one of the reasons that increases the maximum PRR because of the rapid combustion volecity of formulated fuel[30-31]. Figure 7 (b) exhibits that F5 produces the shortest CD compared to other test fules except for the case of 3 000 r/min, and veri fies the results in Figure 6(b) to some extent. The mean ΔCDof F2, F3, F4, and F5 except for the case of 3 000 r/min is 0.05%, 0.37%, 0.52%, and 1.80%,respectively. In conclusion, the in-cylinder combustion velocity can be correlated to the engine preformance closely.

Figure 7 ICD (a) and CD (b) for test fuels under 40% load at low speeds—F1;—F2;—F3;—F4;—F5

Figure 8 ΔICD (a) and ΔCD (b) for test fuels under 40% load at low speeds—F2;—F3;—F4;—F5

In SI engines, the engine flame generally exists in the wrinkled laminar flame regime at low and medium speeds and can be regarded as the stretched laminar flame, which implies that a thin laminar reactive structure is embedded into a turbulent flame[32-33]. Therefore, laminar burning velocity (LBV) of formulated gasoline has a stronger effect on combustion velocity at low speeds. The combustion velocity, as well as LBV, is a function of the instantaneous turbulence intensity and bulk flow within the combustion chamber[34]. Hydrocarbon molecules have different LBV according to structures and carbon numbers. The LBV of aromatics shows a strong dependence on the type and site of alkyl substitution. Toluene has a relatively lower LBV than ethyl benzene, i/n-propyl benzene, and tert-butyl benzene because of the formation of the resonantly stabilized benzyl radical during the oxidation process[35]. This process leads to a slower oxidative degradation of the toluene and a lower burning velocity. Thus, the increased content of toluene may reduce the LBV of formulated fuels. As mentioned before, F1 and F5 have relatively higher oxygen content than other test fuels. Generally, the LBV of oxygenated compounds are higher than typical gasoline molecules under some conditions[36]. It is likely to be the reason that F1 with high oxygen content presents shorter CD and ICD than other test fuels under some operating conditions.By comparing the in-cylinder pressure and combustion phase, it can be stated that the gasoline formulations have a direct relation with the combustion velocity.Consequently, in this study, the reduction of toluene content and the increase of oxygenated compounds content for the test fuels are advantageous to improving combustion velocity and the combustion process are suitable for the tested GDI engine. Nevertheless, compared with the hydrocarbon compounds, the oxygenated compounds with the same carbon number have a relatively lower energy content that will affect the value of BSFC[37]. Therefore, the value of BSFC is a function of many parameters and does not necessarily follow the ranking of the LBV.

3.3 Effect of fuel formulations on carbon emissions

To better appreciate the effect of fuel formulations on carbon emissions under real conditions, the emissions results after the three-way catalytic converter (TWC) have been plotted and analyzed. Figure 9 illustrates THC, NMHC,CO and NOx emissions for test fuels under 40% of load at low speeds. NMHC represents the level of detrimental hydrocarbon mainly including benzene compounds,which are carcinogenic. Therefore it is listed as one of the test parameters in China 6 standard. Although methane is inactivite in the atmosphere, it has a crucial impact on greenhouse effect which means THC also needs to be investigated. The lowest THC and NMHC emissions at different engine speeds were achieved by F1 as observed in Figure 9 (a) and (b). On the other hand, F5 shows a relatively high THC emissions result albeit without equally high NMHC emissions. The T90 of F1 and F5 are 140.8°C and 153.8 °C, respectively, as indicated in Table 3 and high T90 is one reason leading to the higher THC emissions[38]. T90 is related to the gasoline formulations especially the distribution of aromatic compounds. With the increase of light aromatics content, gasoline with similar total aromatic content will show a decrease in T90, which may lead to the reduction of THC and NMHC emissions.As illurstrated in Figure 9(c), the operating conditions have a greater in fluence on CO emissions than fuel compositions. The CO emission has increased to 36.20 μg/g when the engins was fuelled with F4 under 40% of load at 1000 r/min. CO emissions are mainly governed by the fuel-air ratio and kinetic equilibrium of CO to CO2, which is very much dependent on temperature[39]. Under the same load, with the increase of speeds, the cylinder tempreture increases rapidly which would certainly promote fuel oxidation and has a obvious decrease of CO emissions.

Figures 10 and 11 offer the results of CO2emissions and ΔCO2under 40% of load at low speeds and 30%—70% of loads at 2 000 r/min. It is observed that F5 has lower CO2emissions at medium loads and low speeds compared to other test fuels. The CO2emissions are by orders of magnitude higher than other gaseous pollutants. Therefore, the THC,NMHC, and CO emissions are oxidized to CO2, which will not affect the comparison results of CO2emissions according to the chemical equilibrium. As shown in Figure 11,test fuels F2, F3, F4, and F5 can reduce CO2emissions on an average by 0.09%, 0.47%, 0.16%, and 1.26%, respectively, at 40% of load and low speeds, and also by 0.44%,0.90%, 0.90%, and 1.35%, respectively, at medium loads and 2 000 r/min. In detail, F5 emits lower CO2emissions than F1, decreasing by 2.34% under 40% of load at 3000 r/min. The emissions of CO2and the components of fuels have a direct correlation with the carbon intensity of fuels.The lower values of CO2emission, fuel consumption and carbon fraction of fuels can reduce the carbon intensity of gasoline from tank to wheel[40]. Consequently, F5 has the lowest C/H mass ratio and CO2emissions, denoting that the carbon intensity of fuel can be reduced with the optimized formulation, which is beni ficial to the environment.

Figure 9 THC(a), NMHC(b), CO(c) and NOx(d) emissions of test fuels under 40% of load at low speeds—F1;—F2;—F3;—F4;—F5

Figure 10 CO2 emissions for test fuels under 40% of load at low speeds (a) and 40%—70% of load at 2 000 r/min (b)—F1;—F2;—F3;—F4;—F5

Figure 11 ΔCO2 for test fuels under 40% of load at low speeds (a) and under 40%—70% of load at 2 000 r/min (b)—F2;—F3;—F4;—F5

4 Conclusions

The comparative effects of refinery-stream-related gasoline formulations with different aromatic compounds on the fuel economy, combustion characteristics, and emissions from a modern GDI engine were investigated under mapping characteristics conditions. The primary conclusions are drawn as follows:

(1) The test fuels that have similar amount of aromatics and same octane number differ obviously in fuel economy due to the changes of aromatic distributions and the oxygenated compounds. The mean ΔBSFCof F5 with a lowest toluene content is by 3.60% higher compared with F1, which contains most toluene in this study.

(2) According to the combustion characteristics of five analyzed test fuels, the PCP and maximum PRR values of F5 are relatively larger than other test fuels. For the test fuels, the reduction of toluene content and the increase of oxygen content are advantageous to improving the combustion velocity, and the optimized combustion process is suitable for the test GDI engine under specific operationg conditions. Moreover, the disadvantage of the added oxygenated compounds due to their low heating value can be remedied by appropriate aromatics distribution.

(3) THC emissions have a direct relation with T90, which is mainly determined by the content and distribution of aromatics. F5 brings about a relatively higher THC emissions, which are barely showed up in NMHC emissions.Nevertheless, F5 fuel can reduce the CO2emissions by about 2% at medium loads and low speed, which is in line with the advantage of fuel economy compared with other test fuels. The carbon intensity of fuel can be reduced with the optimized fuel formulation.

In general, test fuels that have similar aromatics content,proper amount of oxygenated compounds and reduced toluene content are advantageous to BSFC, in a bid to raise peak pressure, and achieve maximum pressure rise rate and combustion velocity at medium loads and low speeds conditions. Moreover, the gasoline with low toluene content has an obvious influence in increasing THC emissions. Nevertheless, the CO2emissions demonstrate a downward trend with a reducing toluene content of test fuels. It shoud be noted that heavy aromatics have higher sooting tendencies as reported. Therefore, the advantages and disadvantages of high carbon number aromatics that are used in gasoline formulation need to be considered and balanced.

Acknowledgements: This work was supported by the National Key Research and Development Program of China(No.2017YFB0306505). The study is also supported by the Sinopec Group and the Research Institute of Petroleum Processing. The authors thank the staff workers for their specialization and time. The authors are indebted to the reviewers of this article for their invaluable suggestions.

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