Study on performance and emissions of a passenger-car diesel engine fueled with butanol–diesel blends

Highlights

• Effects of butanol addition are investigated experimentally on a high-speed diesel engine.

• The studied butanol blending ratio is up to 40% by volume.

• Engine maximum power output cannot be affected as fueled by the 40% butanol blend.

• Effects of butanol addition on exhaust emissions are varied with loads.

• Butanol could be used conveniently up to high blending ratio with diesel fuel in diesel engines.

Abstract

In this work, the commercially available diesel fuel (Bu00), butanol(20%)–diesel(80%) (by vol.) (Bu20), butanol(30%)–diesel(70%) (Bu30), and butanol(40%)–diesel (60%) (Bu40) fuels were tested. Experiments were conducted on a high-speed direct injection (DI) diesel engine for passenger-car application for varied loads at two representative engine speeds. The results showed that butanol–diesel blends slightly increased combustion pressure and accelerated burning rate. As fueled by the Bu40 blend, the maximum power output of the engine cannot be affected. Besides, butanol–diesel blends increased BSFC and brake thermal efficiency. Moreover, the effects of butanol addition on diesel engine exhaust emissions were varied with loads. Under low-load conditions, CO emissions obviously increased while NO_x emissions decreased as butanol blending ratio increased. Under high-load conditions, on the contrary, CO emissions decreased but NO_x emission increased. For the 40% butanol–diesel blend, in addition, HC emissions were higher than the neat diesel and lower percentage of butanol–diesel blends, especially at low-load. It is interesting that smoke decreased significantly at all conditions with the use of butanol–diesel blends, and the more butanol blending ratio the less smoke. Overall butanol is a potentially promising biofuel, which could be used conveniently up to high blending ratio with diesel fuel in diesel engines.

Introduction

With the increasing seriousness of shortage fossil oil supplying, more and more attentions have been paid to high-efficiency engine applications and alternative fuel developments. As is well known, diesel engines, which have higher efficiency than gasoline engines, are the most common internal combustion engine at present. Diesel engines emit larger volumes of nitrogen oxides (NO_x) and smoke (soot) emissions, which are hard to be decreased or eliminated simultaneously due to the nonuniform of fuel distribution [1]. Some efforts have been attempted to search for clean alternative fuels like alcohols [2], [3], DME [4], [5], LPG [6], [7], CNG [8], etc. for the diesel engines.

Indeed, alcohol has been used as fuels throughout history [2], [3], [9], [10], [11], [12], [13]. The first four aliphatic alcohols (methanol, ethanol, propanol and butanol) are of interest as fuels because they can be synthesized biologically, and they have characteristics such as less carbon, sulfur content, and contain more oxygen than traditional fossil-based fuels, which allow them to be applied in current engines. As one of the primary alcohol kinds, butanol has more advantages than methanol and ethanol as an alternative fuel for internal combustion engines. Butanol has a lower auto-ignition temperature than methanol and ethanol, so it can be ignited easier when burned in diesel engines. Besides, butanol has lower volatility and higher energy density than ethanol and methanol [14]. In addition, butanol is less corrosive and can be blended with diesel fuel without phase separation [15].

Fortunately, butanol can be produced by fermentation of biomass, such as algae, corn, and other plant materials containing cellulose that could not be used for food and would otherwise go to waste [15], [16]. There are a number of biotechnology companies, such as Butyl Fuel, Cathay Industrial Biotech, Cobalt Biofuels, Green Biologics, Metabolic Explorer, Tetravitae Bioscience, and others around the world dedicated to develop biological fermentation technologies of butanol production [16].

Due to the high octane number of butanol like gasoline fuel, a majority of attentions are paid in the past to butanol utilization in gasoline engines [17], [18], [19], [20], [21], [22], [23], [24]. These research activities demonstrated that the concentrations of 20–40% butanol in gasoline enabled to run an engine at a leaner mixture than gasoline for a fixed performance. Moreover, butanol–gasoline blend burned faster than pure gasoline at the same conditions, making higher the indicated efficiency of the engine work cycle. By adding butanol, the coefficient of variation of indicated mean effective pressure (COV_IMEP) was reduced, particularly with lean mixtures. The study performed by Broustail et al. showed that butanol had a similar or higher laminar flame speed than gasoline [25].

In consideration of the effect of biofuel (oxygenated by nature) on soot emission of diesel engines, recently, butanol application in diesel engines has rapidly emerged. A limited number of experimental studies are reported to evaluate the effects of butanol blends with conventional diesel fuel on the performance, exhaust emissions, and combustion behavior [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37]. These works point to the exciting possibility of diesel–butanol combustion.

Sukjit et al. investigated the effects of ethanol and butanol addition to diesel fuel and showed that comparison between the alcohols, butanol rather than ethanol produced lower CO, THC and soot emissions [26]. The studies by Rakopoulos et al. [27], [28] and Karabektas et al. [29] showed that compared to diesel fuel, HC emissions increased, NO_x, CO, and smoke emissions decreased with the use of n-butanol and isobutanol fuel blends. Dogan also suggested the similar emission results [15]. While the research by Yao et al. [30] indicated that n-butanol addition can significantly reduce soot and CO emissions without a serious impact on BSFC and NO_x emissions. According to some studies [29], [31], isobutanol/diesel fuel blends showed a trend to reduce the engine power, brake thermal efficiency (BTE), and exhaust gas temperature, as increased the brake specific fuel consumption (BSFC) with increasing isobutanol content in the fuel blends. However, it was reported that n-butanol/diesel fuel blends increased BTE and BSFC, and decreased exhaust gas temperature [15], [27]. The test results of butanol/diesel fuel blends and their effects on engine performance and exhaust emissions in the above literatures are different from each other due to the properties and technologies of the test engines.

To investigate the formation mechanism of NO (nitric oxide) and smoke under various accelerating conditions for the turbocharged diesel engines, three fuel blends (30% by vol. biodiesel, 25% n-butanol and diesel fuel) were tested, and it was reported that a blend of diesel fuel with 25% n-butanol led to serious smoke reductions but also NO increased compared with the baseline operation of the diesel engine [32].

A drive cycle analysis in a light-duty turbo-diesel vehicle was carried out with two blends of n-butanol (20% and 40%, by vol.) and compared with diesel fuel [33]. The results showed that both HC and CO emissions increased for the urban drive cycle, when larger quantities of n-butanol were added to the diesel fuel. NO_x emissions were not significantly affected by the 20% isobutanol blend and decreased with the 40% n-butanol blend. In the same study, HC and CO emissions were not significantly impacted, but NO_x emissions showed a slight increase for the highway drive cycle, when n-butanol percentage increased in the fuel blends. In addition, an 80% reduction in filter smoke number was observed for the 40% butanol blend.

In the above study [33], furthermore, it was reported that the drivability of vehicle was considered decreased noticeably for 40% n-butanol–diesel blend, especially for cold-start urban drive cycle, due to a large reduction of approximately 30% in fuel economy and a significant increase in HC and CO emissions. Similarly, Sakda et al. [34] also found that the 20% butanol–diesel blend could be used in common rail engine without any modification to the engine since ECM (electronic control module) of the engine could maintain set lambda value, but higher than it could deteriorate engine performance and fuel consumption. A recent work by the authors [35] studied n-butanol addition on performance and emissions with diesel low temperature combustion. With the increase of n-butanol fraction, ignition delay prolonged, pressure rise rate increased and indicated thermal efficiency (ITE) had a slight increase. In addition, it was said in the Ref [36] that the rather low cetane number of the n-butanol addition to diesel engine may promote cyclic (combustion) variability, but it was found that the addition of n-butanol in the blend, at least for up to 24% n-butanol in the blend, did not practically affect the cyclic variability.

From the above, it is obvious that a gap exists concerning the performance and emission behavior of this promising biofuel when fueling diesel engines, with the relevant information being incomplete and limited mainly on finite operation conditions and low butanol/diesel fuel blend ratios, while many aspects are unexplored especially concerning the higher butanol/diesel fuel blend ratios where engines were reported to show a loss in fuel economy or power [33], [34]. In order to fill this gap, the present work reports the results of complete experimental investigation conducted on a high-speed direct injected diesel engine for passenger-car application. The investigation evaluates the effects of using various blends of diesel fuel with 20%, 30% and 40% (by vol.) of n-butanol on the performance and the regulated emissions, i.e. smoke, NO_x, HC and CO at various loads.