DIFFERENCES IN RHEOLOGICAL PROFILE OF DIESEL AND BIODIESEL FUEL

ČUPERA, J., SEDLÁK, P., KARAFIÁT, P.: Diff erences in rheological profi le of regular diesel and bio-diesel fuel. Acta univ. agric. et silvic. Mendel. Brun. , 2010, LVIII, No. 5, pp. 67–74 Biodiesel represents a promising alternative to regular fossil diesel. Fuel viscosity markedly infl uences injection, spraying and combustion, viscosity is thus critical factor to be evaluated and monitored. This work is focused on quantifying the diff erences in temperature dependent kinematic viscosity regular diesel fuel and B30 biodiesel fuel. The samples were assumed to be Newtonian fl uids. Vis cosi ty was measured on a digital rotary viscometer in a range of 0 to 80 °C. More signifi cant diff erence between minimum and maximum values was found in case of diesel fuel in comparison with biodiesel fuel. Temperature dependence of both fuels was modeled using several mathematical models – polynomial, power and Gaussian equation. The Gaussian fi t off ers the best match between experimental and computed data. Description of viscosity behavior of fuels is critically important, e.g. when considering or calculating running effi ciency and performance of combustion engines. The models proposed in this work may be used as a tool for precise prediction of rheological behavior of dieseltype fuels. diesel fuel, biodiesel, kinematic viscosity, modeling Diesel fuel viscosity has a signifi cant impact on how well fuel (either regular or biodiesel) burns. As viscosity increases, fuel molecules cluster together. The molecules on the outside of the cluster mix well with oxygen during combustion and burn well. The molecules on the inside of the cluster do not mix as well and do not burn completely. Knowledge of fuel viscosity is important for several reasons, as it determines the temperature for handling, the size of the centrifuges and the temperature at which the fuel is injected into the engine. It is well known that as the temperature of the fuel is increased, the viscosity is reduced, but diff erences between individual blends or e.g. regular and biodiesel can be expected. Some fuel suppliers publish temperature/viscosity charts which are based on the average data of a large number of samples. However, estimations from the charts cannot be regarded as precise since the exact relationship depends on the source and composition of the fuel. The viscosity usually quoted is the kinematic viscosity expressed in centiStokes (cSt) at some reference temperature. Although ISO 8217 has a reference temperature of 100 °C, it is anticipated that this will be changed to 50 °C at the next revision. This is a value widely used throughout the industry. A few extra degrees of preheating can o en make a fuel suitable for injection equipment. Because of the viscosity/temperature relationship, a few degrees change could make a big diff erence to the injection viscosity. In practical terms, this means that if the actual fuel viscosity is greater than that ordered, it is likely that the fuel heater can accommodate this. Biodiesel refers to a vegetable oil or animal fatbased diesel fuel consisting of long-chain alkyl (methyl, propyl or ethyl) esters. Biodiesel is typically made by chemically reacting lipids (e.g., vegetable oil, animal fat (tallow)) with an alcohol. Biodiesel is meant to be used in standard diesel engines and is thus distinct from the vegetable and waste oils used to fuel converted diesel engines. Biodiesel can be used alone, or blended with petrodiesel. Historically, transesterifi cation of a vegetable oil was conducted as early as 1853 by scientists E. Duff y and J. Patrick, many years before the fi rst diesel engine became functional. Rudolf Diesel’s prime 68 J. Čupera, P. Sedlák, P. Karafi át model, a single 3 m iron cylinder with a fl ywheel at its base, ran on its own power for the fi rst time in Augsburg, Germany, on August 10, 1893 running on nothing but peanut oil. Despite the widespread use of fossil petroleumderived diesel fuels, interest in vegetable oils as fuels for internal combustion engines was reported in several countries during the 1920s and 1930s and later during World War II. Belgium, France, Ita ly, the United Kingdom, Portugal, Germany, Brazil, Argentina, Japan and China were reported to have tested and used vegetable oils as diesel fuels during this time. Throughout the 1990s, plants were opened in many European countries, including the Czech Republic, Germany and Sweden. France launched local production of biodiesel fuel (referred to as diester) from rapeseed oil, which is mixed into regular diesel fuel at a level of 5%, and into the diesel fuel used by some captive fl eets (e.g. public transportation) at a level of 30%. In 2009, ASTM published new Biodiesel Blend Specifi cations Standards (ASTM D6751). Blends of biodiesel and conventional hydrocarbon-based diesel are products most commonly distributed for use in the retail diesel fuel marketplace. Much of the world uses a system known as the “B” factor to state the amount of biodiesel in any fuel mix: fuel containing (100% biodiesel is referred to as B100, while 30% biodiesel is labeled B30, 20% biodie sel is labeled B20, 5% biodiesel is labeled B5, 2% biodiesel is labeled B2). Blends of 20 percent biodiesel with 80 percent petroleum diesel (B20) can generally be used in unmodifi ed diesel engines. Biodiesel can also be used in its pure form (B100), but may require certain engine modifi cations to avoid maintenance and performance problems. New ways and strategies of blending are broadly discussed e.g. in Joshi et al. (2010). But the biodiesel’s viscosity depends also on the other factors such as storing conditions (Tasyürek et al., 2010) Although fuel can be ordered to one of the grades in ISO 8217, frequently on delivery, only the vis cosi ty grade is stated. For example IF 180 – this means that the viscosity is a maximum of 180 cSt at 50 °C. The majority of engine installations are fi tted with fuel viscosity controllers so it is not normally necessary to estimate the injection temperature. Typically, a fuel with a viscosity up to 15–20% above that ordered can still be successfully used in the fuel treatment plant and engines. If the centrifuge is only marginally-sized, the slight increase in viscosity will reduce its performance. This may be overcome, if the piping confi guration permits, by operating two purifi ers in parallel with a clarifi er in series. There are several diff erent methods for measuring fuel’s viscosity. Except traditional methods (such us capillary, falling ball, rotary etc) – in detail described in Spearot (1989), Dawson (2000), or Troyer (2002), there are new approaches described eg. in Albertson et al. (2008). Biodiesel’s kinematic viscosity and its infl uence on diff erent operational characteristics are discussed and analyzed in Rahimi et al (2009) or Bhale et al. (2009). Fuel quality and its cha ra cte ris tics consequently aff ect the whole traction system and its performance (Chalupa, 2005; Chalupa et al. 2007). This study is focused on analyzing the problem of temperature dependent kinematic viscosity of regular diesel fuel and biodiesel and quantifi cation of diff erences in their behavior. This knowledge, as well as created mathematical models could improve understanding and description of processes running in the combustion engines. MATERIAL AND METHODS