Comparative study on air dilution and hydrogen-enriched air dilution employed in a SI engine fueled with iso-butanol-gasoline
Introduction
Fossil fuel is the main source of energy in the world; it continues to be excessively consumed despite its limited resources. Transportation accounts for about 25% of fossil fuel consumed [1]. Automobile exhaust introduces a significant amount of pollutants into the air [2,3]. The renewable nature of biofuels makes them the “middle link” of the carbon cycle. The carbon emissions of biofuels are substantially lower than those of fossil fuels. Among them, bio-alcohol fuels have become a particularly popular research subject including methanol, ethanol and butanol [[4], [5], [6], [7], [8]]. Bioethanol is already commercially available in the United States, China, Europe, and elsewhere [9,10]. Butanol has also garnered a great deal of attention recently. It can be produced biologically and its combustion characteristics are closer to gasoline than those of ethanol or methanol [11,12]. Butanol, which has been identified by the U.S. Department of Energy as a notable potential biofuel under investigation, has higher energy density than ethanol or methanol, is minimally corrosive to engine components and fuel supply systems, and is not prone to containing excess water [13,14].
Iso-butanol is an alcohol with four carbon atoms and oxygen-containing properties that contribute to combustion. Butanol addition has positive influence on engine-out emissions [15]. During the acetone-butanol-ethanol fermentation process of starch or glucose, biobutanol is produced [16]. According to Tier 3 evaluations, iso-butanol is a candidate fuel for extant automobile engines [17]. Butanol is less corrosive and has energy density closer to gasoline than ethanol or methanol [18,19]. Butanol dissolves in gasoline, and is less volatile than ethanol or gasoline, reducing the likelihood of cavitation and vapor lock [20]. Wallner et al. [21] investigated blend fuel emissions under stoichiometric mixture conditions to find that butanol addition has little effect on engine HC, CO, or NOx emissions. Rice et al. [22] compared the emissions of iso-butanol-gasoline, ethanol-gasoline, and methanol-gasoline to find that CO and NOx of alcohol-gasoline blends are lower than those of gasoline. Dernotte et al. [23] assessed quantified the influence of butanol blending rate (0, 20%, 40%, 60%, 80% vol.) in a port fuel injection SI engine, the improved combustion stability was observed, NOx emissions were not affected significantly until the butanol rate exceeded 80%, the specific fuel consumption (SFC) increased with butanol blending rate while the SFC of B40 rose no more than 10% relative to pure gasoline. Some researchers studied the effect of butanol addition on particle emissions in SI engines, the accumulation particle number reduced significantly at rich-fuel mixture, the particle number concentration can be reduced with n-butanol additive relative to that of gasoline [24,25].
Despite its numerous advantages, iso-butanol does result in increased UHC emissions and decreased thermal efficiency with low blending ratio compared to gasoline. Gu et al. [25] studied the emissions of an n-butanol engine to find that butanol addition adversely affects HC emissions; pure butanol combustion results in higher HC emissions than gasoline, primarily because combustion temperature decreases with butanol addition. The use of exhaust gas recirculation (EGR) further lowers the combustion temperature, resulting in a nearly two-fold increase in HC emissions [26,27]. Butanol has higher latent heat of vaporization, which leads to lower mixture temperature at the end of compression stroke. Further, the higher viscosity of butanol compared to gasoline is not conducive to the formation of a high-quality mixture and adversely affects the evaporation and atomization of fuel droplets [28,29]. For these reasons, as the temperature in the combustion chamber decreases due to lean burn, the addition of butanol has an increasingly adverse effect on HC emissions [[30], [31], [32]]. To date, the cost of butanol is higher than ethanol and methanol, but which will decrease in near further with increasing mass production. The U.S. Department of Energy has proposed further biofuels production plan including iso-butanol [17]. Therefore, in the future, the cost of butanol is expected to be significantly reduced as the production scale and efficiency increase.
The dilution of air and EGR can enhance engine efficiency and cut down fuel use [33]. Dilution results in higher ratio of the specific heats of the working fluid resulting in more useful work in the expansion stroke [34]. The oxygen content in the cylinder is higher after air dilution, which is beneficial to complete combustion, improves engine fuel economy, and reduces CO, HC and particle emissions. Air dilution also reduces throttling loss and decreases combustion temperature to reduce heat transfer loss to the chamber surfaces [35]. However, combustion stability tends to decrease as the dilution level increases due to a reduction in flame propagation velocity; the narrow flammability of butanol and gasoline also causes a mixture ignitability drop accompanied by increased likelihood of partial combustion and misfires [36]. The knock tendency is reduced with lean mixture which enable higher compress rate to improve engine thermal efficiency [37,38]. The conversion efficiency of three-way catalyst drops as the mixture concentration deviates from stoichiometric, especially for the NOx catalysis, the NOx issue can be solved through lean NOx trap (LNT) technology whose cost is gradually decreasing with increasing LNT applications. In addition to combustion instability, the lean burn is accompanied by low reaction rates, high sensitivity to mixing, flame extinction, and complex NOx emissions [39]. Targeting these problems may improve engine performance under air dilution conditions.
Hydrogen is an ideal alternative, renewable, clean fuel for internal combustion engines [40,41]. The addition of hydrogen can improve the combustion of alcohol-gasoline, increase combustion temperatures under air dilution conditions, and reduce unburnt hydrocarbon emissions [42,43]. According to reports from Su and Ji, combustion speed rates decreased as air dilution increased, but could be increased by adding hydrogen [[44], [45], [46]]. Hydrogen has a wider ignition limit and lower ignition energy than other fuels, which enables it to be ignited in leaner mixtures than gasoline or butanol [47,48]. In addition, the laminar flame velocity (LFV) of hydrogen is much higher than that of gasoline or butanol, which is also favorable under lean mixture combustion conditions and can thus mitigate the air dilution problem [49]. A combination of hydrogen and lean burn can ensure appropriate combustion speed rates without excessively high or low pressure rise rate [50,51].
Previous studies have shown that adding hydrogen to spark ignition (SI) engines can improve combustion stability, which mitigates lean burn-induced issues [52]. Huang et al. [53] studied the effect of hydrogen addition on the natural gas combustion in a constant volume vessel and found that the burning speed was increased due to enhance chemical reaction activity with hydrogen addition. It was reported that hydrogen resulted in an increase in mole fractions of OH and O. However, when hydrogen is used as the sole fuel component, problems such as premature combustion, reduced charge efficiency, knock, and NOx emissions are likely to occur [54,55]. Verma et al. [56] studied the effect of hydrogen addition on the combustion of HCNG engines to find that thermal efficiency increases as H/C ratio increases; the lean burn limit is broadened and the combustion temperature increases as well. Optimal performance for hydrogen-enriched natural gas was observed with H/C ratio of 4.5. Amrouche et al. [57] studied the effect of hydrogen addition on an ethanol rotary engine under wide open throttle (WOT) and lean burn conditions. Zhang and Ji et al. reported the influence of hydrogen additive on nbutanol. The research works confirmed that the BTE was improved and lean burn limit was extended with hydrogen introduction [58]. Raviteja and Kumar investigated the hydrogen additive in butanol engine with 10% hydrogen addition, the results indicated that NO emissions approximately doubled, but CO and HC reduced by almost half [59]. Hydrogen appeared to enhance the burning process by truncating the combustion duration, reducing cycle variations, improving thermal efficiency, and reducing unburned HC emissions.
Although some researchers have investigated the performance and emissions of butanol blends in SI engines, most of which focused on nbutanol instead of iso-butanol. The research works concerning lean burn for iso-butanol blends in SI engines were seldom reported. The effect of hydrogen on iso-butanol-gasoline in SI engines was rarely investigated. In particular, for hydrogen-enrich butanol-gasoline blends, very few studies concerned PM emissions; the number concentration and size distribution under different air-fuel rates are not available.
Since iso-butanol-gasoline is an important potential fuel, but its performance and emissions in hydrogen-added mixtures are yet unknown. In this study, the effects of hydrogen-iso-butanol-gasoline were investigated as per the thermal efficiency and emissions of the spark ignition (SI) engine under air dilution conditions. PM emissions from blended fuels under lean mixture was measured and analyzed. The paper gave quantitative results, which proved that the results of this scheme are favorable and provide a reference for researchers.
Section snippets
Test engine setup
A schematic of the test SI engine bench used in this study is provided in Fig. 1. The test was carried out in a gasoline direct injection (GDI) engine. The main specifications of the test engine are listed in Table 1. Cooling water temperature was regulated at 85 °C ± 5 °C throughout the test and oil temperature was 90 °C ± 5 °C. An air flow meter (HFM5, BOSCH) was installed in the intake pipe of the engine to measure the intake flow. Fuel and hydrogen flow measurements were performed on a
Results and discussion
The brake thermal efficiency (BTE) of hydrogen-enriched iso-butanol-gasoline in the SI engine was assessed first in this study. The BTE of B0, B33 and hydrogen-B33 under various lambda values is shown in Fig. 2. The BTE of B33 increased in the lean mixture, which reached its maximum at lambda of 1.2–1.3. When 3% hydrogen was added, BTE approximately increased 3% as lambda ranged from 1.2 to 1.4. Hydrogen addition also appears to have expanded the lean burn limit in the test engine. Its laminar
Conclusions
The influence of hydrogen 3% v/v in an iso-butanol-gasoline (B33) SI engine on combustion behaviors and emissions were investigated in this study. Hydrogen addition was found to enhance both the BTE and BMEP of B33 fuel, especially at high air dilution ratios. Hydrogen addition also resulted in overall improvement of combustion behavior, reduction in burning duration, and reduced flame propagation under air dilution condition. The combustion of hydrogen-enriched B33 was more stable owing to the
Acknowledgments
The work discussed in this paper is supported by the National Key Research and Development Program of China (NO. 2018YFC0604402), the Fundamental Research Funds for the Central Universities, China (No. 2302015 FRF-BR-15-052A) and the China Postdoctoral Science Foundation, China (No. 2013M540251).
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