Combustion and emissions characteristics of high n-butanol/diesel ratio blend in a heavy-duty diesel engine and EGR impact

Highlights

• Effects of EGR on high n-butanol/diesel ratio blend (Bu40) were investigated and compared with neat diesel (Bu00).

• Bu40 has higher NOx due to wider combustion high-temperature region.

• Bu40 has lower soot due to local lower equivalence ratio distribution.

• Bu40 has higher CO due to lower gas temperature in the late expansion process.

• For Bu40, EGR reduces NOx emissions dramatically with no obvious influence on soot.

Abstract

In this work, the combustion and emission fundamentals of high n-butanol/diesel ratio blend with 40% butanol (i.e., Bu40) in a heavy-duty diesel engine were investigated by experiment and simulation at constant engine speed of 1400 rpm and an IMEP of 1.0 MPa. Additionally, the impact of EGR was evaluated experimentally and compared with neat diesel fuel (i.e., Bu00). The results show that Bu40 has higher cylinder pressure, longer ignition delay, and faster burning rate than Bu00. Compared with Bu00, moreover, Bu40 has higher NOx due to wider combustion high-temperature region, lower soot due to local lower equivalence ratio distribution, and higher CO due to lower gas temperature in the late expansion process. For Bu40, EGR reduces NOx emissions dramatically with no obvious influence on soot. Meanwhile, there is no significant change in HC and CO emissions and indicated thermal efficiency (ITE) with EGR until EGR threshold is reached. When EGR rate exceeds the threshold level, HC and CO emissions increase dramatically, and ITE decreases markedly. Compared with Bu00, the threshold of Bu40 appears at lower EGR rate. Consequently, combining high butanol/diesel ratio blend with medium EGR has the potential to achieve ultra-low NOx and soot emissions simultaneously while maintaining high thermal efficiency level.

Introduction

Butanol is a new generation substitute to conventional transportation fuels [1]. It can be produced from biomass (as “biobutanol”) as well as fossil fuels (as “petrobutanol”), but biobutanol and petrobutanol have the same chemical properties. Biobutanol is a 4-carbon alcohol (C₄H₁₀O) produced from the same feedstocks as ethanol including corn, sugar beets, and other biomass feedstocks containing cellulose that could not be used as food and would instead go to waste [2]. Compared with conventional fossil fuels – gasoline and diesel, biobutanol has more excellent fuel properties and environment performance. To begin with, biobutanol can be produced from a variety of feedstocks and increase energy security. Next, biobutanol contains more oxygen content, which leads to substantial reduction of soot. Also, its higher heat of evaporation contributes to the reduction of gas temperature in the cylinder. Furthermore, carbon dioxide captured by growing biomass feedstocks reduces overall greenhouse gas emissions by balancing carbon dioxide released from burning biobutanol [3]. Additionally, biobutanol also has more advantages than the widely used ethanol. It contains 25% more energy than ethanol and has better intersolubility, so it can be more easily blended with hydrocarbon fuels like gasoline and diesel, both in on-board transportation fuelling systems and in fuel pipelines.

Producing biobutanol via fermentation has been possible since the early 1900s. A clostridium acetobutylicum organism was found to be capable of converting large amounts of sugars into a mixture of acetone–butanol–ethanol (ABE) [4], which is one of the oldest known industrial fermentations. Since the 1950s, however, ABE fermentation declined continuously, and almost all butanol was produced via petrochemical routes. The production of butanol by fermentation declined mainly because the cost of biomass feedstock rose sharply and petroleum derived solvents became more cost effective at that time. Until the last decades, renewed interest in biobutanol as a sustainable vehicle fuel has spurred technological advances to ferment biobutanol. Some research groups and biotechnology companies have been developing new process to increase the butanol yield of fermentation production, such as Butyl Fuel, Cathay Industrial Biotech, Cobalt Biofuels, Green Biologics, Metabolic Explorer, and Tetravitae Bioscience [5]. With the use of new strains, economic potential of ABE fermentation has been found to be highly attractive. Research efforts in theory, engineering, and economics of ABE fermentation have brought biobutanol close to large-scale commercialization as liquid alternate fuel [6].

Butanol has four isomers based on the location of the OH and carbon chain structure. 1-Butanol, also better known as n-butanol, has a straight-chain structure with the OH at the terminal carbon. 2-Butanol (sec-butanol) is also a straight-chain alcohol but with the OH group at an internal carbon. Iso-butanol is a branched isomer with the OH group at the terminal carbon and tert-butanol refers to the branched isomer with the OH group at an internal carbon. Except for tert-butanol, other butanol isomers can be manufactured by fermentation. All contain about the same energy [4], but have different physical properties including density, octane number, boiling point, viscosity, etc., which are summarized in Ref. [1]. Thus, combustion characteristics on all four butanol isomers need to be investigated, since all of them can be used in engines either at present or in future.

Many previous combustion studies on the various butanol isomers have been conducted on different type of combustion devices as well as internal combustion engines. Experimental research on combustion properties of the butanol isomers at a more fundamental level have been carried out on such combustion devices like shock tube performed by Zhang et al. [7], [8] and Stranic et al. [9], rapid compression machine conducted by Weber et al. [10], [11], and constant volume combustion chamber done by Liu et al. [12], [13], [14]. Other intensive research on butanol also includes electrostatic atomization [15], [16], laminar flame [17], [18], [19], and chemical kinetic modeling [20], [21], [22].

Further, a certain amount of research on butanol isomers application in engines has been carried out. Due to its fuel properties like gasoline, butanol is generally used as a blending agent additive to gasoline to reduce the fossil part in fuel mixture and in this way to reduce life cycle CO₂ emissions. In earlier years, Alasfour [23], [24], [25], [26] has conducted a series of studies on a single-cylinder SI engine with focus on NOx emissions, fuel energy conversion efficiency, and HC emissions. Other some investigations also include cold start [27], H₂O addition [28], and direct-injection SI [29] and HCCI combustion [30].

Because butanol contains oxygen content and has good intersolubility with diesel without any cosolvents, in recent years, it has been getting plenty of attention as a substitute to the diesel. Rakopoulos et al. have conducted extensive studies to evaluate the effects of using blends of conventional diesel fuel with small volumes of n-butanol on performance and emissions in steady [31], [32], [33] and transient [34], [35] operation. Yao et al. [36] investigated the effects of diesel blends with low contents of n-butanol on a heavy-duty (HD) DI diesel engine with multi-injection and various EGR rates. Dogan [2] and Siwale et al. [37] also carried out the similar studies. All above those studies demonstrated that smoke can be greatly reduced by butanol addition. Moreover, Gu et al. [38], Al-Hasan and Al-Momany [39], and Karabektas and Hosoz [40] also investigated the effects of iso-butanol-diesel fuel blends on the engine performance and exhaust emissions. The result reported by Gu et al. [38] indicated that smoke emissions from iso-butanol–diesel blends were higher than those from n-butanol–diesel blends. Liu et al. [41] further studied the effects of fuel properties and oxygenated structures on engine combustion and emissions. Five different fuels, n-heptane, iso-octane, n-butanol, 2-butanol and methyl octynoate, were added into diesel fuel with the same blending ratio of 20% volume. Their result showed that effects of fuel and oxygenated structure on soot reduction depend on EGR rate, and the locations of OH between n-butanol and 2-butanol have small effect on soot.

The previously cited studies show that almost all of related experiments focus on low volume ratio of butanol addition in diesel engines. However, there is little intensive research concerning the effects of high butanol/diesel ratio on the combustion and emissions of diesel engines. To expand the range of butanol application in engines, therefore, the fundamentals of combustion and emissions formation of high butanol/diesel ratio blend were understood by experiment and simulation in this paper. Further, effects of EGR on combustion and emissions characteristics of high n-butanol/diesel ratio blend were also evaluated experimentally.