Effect of biodiesel fuels on diesel engine emissions
Introduction
The fuels used in road transportation are subject to increasingly stringent regulations (EN-590 in Europe [1], ASTM D 975 in USA [2]). In recent years, the reduction in sulfur content is the most notable restriction (50 ppm currently, 10 ppm by 2009 in Europe), and it has had economic consequences on oil company investments and on the final fuel price. This, together with the oscillating increase in the oil price per barrel and with the total or partial detaxation of biofuels, depending on the country, has opened the way for the commercialization of biodiesel and bioethanol and has provided a useful tool to fight against the impact of transportation (considered as a diffuse source of emissions) on climate change. In the case of biodiesel, such an impact could be even higher in European countries where the “dieselization” process has sharply increased in the last decade, leading to an unbalanced fuel production in oil refineries.
The term biodiesel commonly refers to fatty acid methyl or ethyl esters made from vegetable oils or animal fats, whose properties are good enough to be used in diesel engines. The regulations limiting such properties are EN-14214 in Europe [3] and ASTM D-6751-03 in USA [4], although ethyl esters are not yet acknowledged as biodiesel in Europe [5]. Research papers presenting results of diesel engine emissions from biodiesel often ignore some of the basic properties of the biodiesel used [6], which makes it difficult to determine whether its quality has some effect or not.
The call for the use of biofuels, and particularly of biodiesel, which is being made by many governments following international energy policies is presently finding some resistance from car and components manufacturing companies, private users and local administrations. This opposition makes it more difficult to reach the targets of increased use of biofuels in internal combustion engines. One of the reasons for this resistance is a certain lack of knowledge about the effect of biodiesel on diesel engines. There are four issues related to biodiesel where public knowledge is still low:
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Automotive fuels are delivered in petrol stations by volume, and their price is correspondingly established per unit volume. However, it is not the volume but the energy which moves vehicles. Both volume and energy are related through fuel density and its heating value, or in summary, through the heating value in energy basis (MJ/l). It should be kept in mind that biodiesel has around 9% less heating value in volume than conventional diesel fuel. Thus, if engine efficiency is the same, engine fuel consumption should be proportionally higher, and consequently vehicle autonomy proportionally lower, when using biodiesel. However, differences in efficiency have occasionally been found in the literature comparing diesel and biodiesel fuels, along with claims of increases in consumption different to those expected, as shown below.
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Biodiesel fuels have higher lubricity than conventional fuels, but they can contribute to the formation of deposits, the degradation of materials or the plugging of filters, depending mainly on their degradability, their glycerol (and other impurities) content, their cold flow properties, and on other quality specifications [7]. The long-term effects of biodiesel are currently one of the least explored issues and a very small number of experimental studies have been published about this.
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Biodiesel is 100% renewable only when the alcohol used in the transesterification process is also renewable, but this proportion is reduced to around 90% (if the balance is made in mass) or 95% (if the balance is made in carbon mass) when fossil alcohol (usually methanol) is used. This high renewable proportion justifies the nil consideration of CO2 emissions from biodiesel combustion in European directives. However, life-cycle analyses of CO2 emissions should be accounted for in order to evaluate the impact of biodiesel on the global greenhouse effect. Results of well-to-wheel CO2 emissions are very variable [8], [9], [10], [11], [12], locally dependent and often unreliable, but a saving of between 50% and 80% (and even more in the case of waste-oil biodiesel [9], [10], [11]) with respect to petroleum diesel emissions could be accepted as a high confidence range. In any case, this makes biodiesel a powerful tool to reduce CO2 emissions from transportation, which is considered responsible for 23% of greenhouse emissions in the Annex I countries of the Kyoto Protocol, as published by the United Nations Framework Convention for Climate Change [13].
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Biodiesel fuels also have an interesting potential to reduce chemical emissions. However, the effect of biodiesel is specific for each of the different pollutant species, and depends on the type of engine, on the engine speed and load conditions, on the ambient conditions, on the origin and quality of biodiesel, etc.
The main objective of this paper is to analyze the latter issue by means of a literature review. Although a previous review was published by Graboski and McCormick [14] in 1998, the increasing interest in the use of biodiesel calls for a new revision of the state-of-the-art, since many experiments have been carried out in the last years to clarify some of the effects of biodiesel on diesel emissions. Out of all emissions, oxides of nitrogen and particulate matter (PM) are the most significant in diesel engines due to the high flame temperature and diffusive combustion in the combustion chamber. Since nitric oxides (NOx) and PM emissions from current diesel technologies are close to the limits permitted by regulations and both limits will be even more stringent in the near future, these two emissions will be critical factors in the development of new diesel engines. For example, Euro 5 will reduce NOx and PM emission limits for passenger cars from 0.25 and 0.025 g/km to 0.18 and 0.005 g/km, respectively (emissions tested over the NEDC chassis dynamometer procedure [15]). Moreover, Euro 5 will consider both mass and number based PM emission limits, although the measurement method for particle number must previously be established [16]. For the other regulated emissions, carbon monoxide (CO) and total hydrocarbon (THC), no further development in engines seems to be necessary to meet future limits.
An improved knowledge of the potential to reduce these types of emissions could help (a) engine manufacturers to adapt their engines to the use of biodiesel and to optimize them, readjusting the compromise between efficiency, costs (mainly due to aftertreatment systems) and emissions within the regulation limits, (b) national administrations to design their energy policies and to define measures to externalize environmental costs, (c) local administrations to promote its use in urban areas, especially in countries with extreme dieselization, where particle concentrations in the air are reaching alarming levels, and (d) private users, to encourage them to use biodiesel, attesting to their environmental concern.
The literature reviewed was selective and critical. Highly rated journals in scientific indexes were the preferred choice, although other non-indexed publications, such as SAE technical papers or some internal reports from highly reputed organizations (National Renewable Energy Laboratory, National Biodiesel Board, Environmental Protection Agency) have also been cited. Some papers have been excluded as they do not mention the instrumentation or methodology used. Finally a bibliometric study showed (Fig. 1) that the number of publications related to both biodiesel and biodiesel emissions has increased exponentially in the last 15 years, which reveals the increasing interest of this alternative and of its environmental benefits.
A wide range of diesel engine sizes and types was tested in the reviewed literature. The most frequently used engines were direct injection, turbocharged, and 4-cylinder diesel engines. Since engine characteristics might have some influence on the effects of biodiesel, this information has been considered useful to this study. However, in order to avoid awkward reading, this information is only specified in the following sections if the tested engine was different to the above mentioned type. To the same end, the biodiesel fuels used in the reviewed studies were composed of methyl esters produced from different oils, unless otherwise specified. However, although the original vegetable oils are usually mentioned in the reviewed studies, many of the quality specifications (i.e. glycerin content, ester content, etc.) of biodiesel fuels are often missing, which makes it difficult to discuss the results provided. When indicated, the specifications belong to the ranges shown in Table 1, unless otherwise specified. For example, the following sections only specify the cases where the sulfur content of the diesel fuel used for comparisons is ultra low (below 15 ppm, ULS hereinafter) or high (above 500 ppm).
Section snippets
Brake effective power
Nowadays automotive engines are usually oversized and the power output when using biodiesel fuels is usually the same as with diesel fuel, as the accelerator is not fully pressed down in most cases. Drivers unconsciously overpress the accelerator with respect to how they used to drive with diesel fuel, in order to compensate for the reduced heating value of biodiesel. When testing an engine in a test bench, equivalent performance requires attaining the same engine speed and torque, regardless
Effect of biodiesel
Although most of the literature reviewed shows a slight increase in NOx emissions when using biodiesel fuel (these works will be classified as Group I hereinafter), some works showing different effects have been found. Some of them found NOx increases only in certain operating conditions (Group II), some others did not find differences between diesel and biodiesel fuels (III), and others still found decreases in NOx emissions when using biodiesel (IV).
I. An experimental work carried out in a 7.3
Effect of biodiesel on PM and soot emissions and on smoke opacity
Although some authors have occasionally reported some increases in PM emissions when substituting diesel fuel by biodiesel [72], [106], [107], a noticeable decrease in PM emissions with the biodiesel content can be considered as an almost unanimous trend [14], [21], [53], [67], [75], [80].
PM emissions data collected from a number of laboratory studies were used by EPA [46] to adjust the following equation, statistically significant with a 95% confidence level:
This equation
Effect of biodiesel on THC emissions
Most authors’ results show a sharp decrease in THC emissions when substituting conventional diesel fuel with biodiesel fuels [53], [67], [87], [140], [141], [142]. The EPA review [46], already mentioned, shows a 70% mean reduction with pure biodiesel with respect to conventional diesel, according to Eq. (4) and Fig. 7:
However, a few studies may be found in the literature showing no significant differences [47], [60], [66], [134] or increases [57] in THC emissions when
Effect of biodiesel on CO emissions
With regard to most of the literature reviewed, a decrease in CO emissions when substituting diesel fuel with biodiesel can be considered as the general trend [32], [46], [48], [140], [147]. Nevertheless, a few authors found no differences between diesel and biodiesel [66], and even noticeable increases when using biodiesel [57]. After revising several works, EPA [46] proposed Eq. (5) for the general trend, leading to mean CO reductions of almost 50% with biodiesel with respect to conventional
Conclusion
A wide disparity of results has been found in general concerning emissions from biodiesel. Although a dominant trend has been found in most cases, there have always been opposing trends proposed elsewhere by contrast. One reason for this is the large number of different engine technologies tested, the varying operating conditions or driving cycles followed, the different biodiesel fuels used (from different feedstocks and with different qualities), and the various measurement techniques and
Acknowledgments
The authors wish to acknowledge the Spanish Association of Renewable Energy Producers (APPA) for suggesting and financing this review study, and to the Spanish Ministries of Education and Science, and of Environment for the financial support of many of the experimental works performed by the authors’ research group in the field of engine emissions with different biodiesel fuels (Projects EDIBIO, ref: ENE2004-07776-C03-01 and CEBIOMA, ref: 231/2006/2-13.3, respectively). Although these studies
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