Comparative environmental behavior of bus engine operating on blends of diesel fuel with four straight vegetable oils of Greek origin: Sunflower, cottonseed, corn and olive D.C. Rakopoulos, C.D. Rakopoulos ⇑ , E.G. Giakoumis, A.M. Dimaratos, M.A. Founti Internal Combustion Engines Laboratory, Department of Thermal Engineering, School of Mechanical Engineering, National Technical University of Athens, Zografou Campus, 9 Heroon Polytechniou St., 15780 Athens, Greece article info Article history: Received 14 April 2011 Received in revised form 5 June 2011 Accepted 8 June 2011 Available online 7 July 2011 Keywords: Bus diesel engine Emissions Diesel fuel blends Straight vegetable oils abstract An experimental study is conducted to evaluate the use of sunflower, cottonseed, corn and olive straight vegetable oils (SVO) of Greek origin, in blends with diesel fuel at proportions of 10 vol.% and 20 vol.%, in a fully instrumented, six-cylinder, turbocharged and after-cooled, heavy duty (HD), direct injection (DI), ‘Mercedes-Benz’, mini-bus engine installed at the authors’ laboratory. The series of tests are conducted using each of the above blends, with the engine working at two speeds and three loads. Fuel consump- tion, exhaust smokiness and exhaust regulated gas emissions such as nitrogen oxides (NO x ), carbon mon- oxide (CO) and total unburned hydrocarbons (HC) are measured. With reference to the corresponding neat diesel fuel operation, the vegetable oil blends show reduction of emitted smoke with slight increase of NO x and effectively unaffected thermal efficiency. Theoretical aspects of diesel engine combustion, combined with the very widely differing physical and chemical properties of the vegetable oils against those for the diesel fuel, aid to the correct interpretation of the observed engine behavior. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Apart from engine-related techniques to meet stringent imposed emissions regulations [1–4], engine researchers have focused their interest on the domain of fuel-related techniques, such as for example alternative gaseous fuels of renewable nature or oxygenated fuels that are able to reduce particulate emissions [5–10]. Considerable attention has been paid in the development of alternative fuel sources in various countries, with emphasis on bio-fuels that possess the added advantage of being renewable [11–13]. Bio-fuels made from agricultural products (oxygenated by nature) reduce the world’s dependence on oil imports, support local agricultural industries and enhance farming incomes and, moreover, offer benefits in terms of usually reduced exhaust emis- sions. Among those, vegetable oils, their derived bio-diesels (methyl or ethyl esters) and bio-alcohols are considered as very promising fuels. Bio-fuel production is a rapidly growing industry in many parts of the world. Bio-ethanol is the primary alternative at present to gasoline for spark-ignition engines. For diesel engines, vegetable oils and their derived bio-diesels are favored, as well as bio-ethanol mixed in small proportions with diesel fuel (having poor miscibility with diesel fuel) [14–17]. However, other bio-fuels such as bio-butanol, biomass-derived hydrocarbon fuels and hydrogen are being researched at present, being regarded as the next generation bio-fuels [18–21]. The advantages of vegetable oils as diesel fuel, apart from renewability, are the minimal sulfur and aromatic contents, the higher flash point, the higher lubricity, and the higher biodegrad- ability and non-toxicity. On the other hand, their disadvantages include the very high viscosity, the higher pour point, the lower ce- tane number, the lower calorific value and the lower volatility. Their major problem is associated with highly increased viscosity, 10–20 times greater than normal diesel fuel. Thus, although short- term tests using neat vegetable oils showed promising results, problems appeared after the engine had been operated for longer periods. These included: injector coking with trumpet formation, more carbon deposits, piston oil-ring sticking, as well as thickening and gelling of the engine lubricating oil. To solve the problem asso- ciated with their very high viscosity, the following usual methods are adopted: blending in small blend ratios with diesel fuel, micro- emulsification with methanol or ethanol, preheating, cracking, and conversion into bio-diesels mainly through the transesterification process [22–25]. The advantages of bio-diesels as diesel fuel, apart from renew- ability, are the minimal sulfur and aromatic content, the higher flash point, the higher lubricity, the higher cetane number, and the higher biodegradability and non-toxicity. On the other hand, their disadvantages include the higher viscosity (though much 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.06.009 ⇑ Corresponding author. Tel.: +30 210 7723529; fax: +30 210 7723531. E-mail address: cdrakops@central.ntua.gr (C.D. Rakopoulos). Fuel 90 (2011) 3439–3446 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel