Speed of sound in pure fatty acid methyl esters and biodiesel fuels André F.G. Lopes a , Maria del Carmen Talavera-Prieto b , Abel G.M. Ferreira b,⇑ , Jaime B. Santos a , Mário J. Santos a , António T.G. Portugal b a Department of Electrical and Computers Engineering, University of Coimbra, Polo II, Rua Silvio Lima, 3030-970 Coimbra, Portugal b Department of Chemical Engineering, University of Coimbra, Polo II, Rua Silvio Lima, 3030-970 Coimbra, Portugal highlights  New data on sound speed is reported for fatty acid methyl esters.  New data on speed of sound is reported for biodiesel fuels.  Molar compressibility was calculated for fatty acid methyl esters and biodiesel.  Predictive models for speed of sound and molar compressibility are proposed. article info Article history: Received 7 June 2013 Received in revised form 12 July 2013 Accepted 16 July 2013 Available online 31 July 2013 Keywords: Biodiesel Speed of sound Compressibility Correlation abstract The property changes associated with the differences in chemical composition of biodiesel may change the fuel injection timing which in turn cause different exhaust emissions and performance of engines. The property that has an important effect on the fuel injection timing is the speed of sound (related with isentropic bulk modulus). Despite the speed of sound of pure fatty acid (methyl and ethyl) esters being reasonably known in a wide range of temperature the experimental data for biodiesel are very scarce in the literature. In this work the speed of sound of six fatty acid methyl esters (FAME = laurate (MeC12:0), myristate (MeC14:0), palmitate (MeC16:0), stearate (MeC18:0), oleate (MeC18:1), linoleate (MeC18:2)) and six biodiesel fuel samples were measured using a non-intrusive ultrasonic methodology. The mea- surements for FAMEs were made at atmospheric pressure from a minimum of 288.15 K to a maximum of 353.15 K, and in the temperature range 298.15–353.15 K for biodiesel samples. The uncertainty of the measurements was estimated as less than ±1 m s 1 . The speed of sound data combined with available density data from literature was used to calculate the isentropic compressibility and the molecular com- pressibility for the FAMEs and for the biodiesel samples. The results for molecular compressibility evi- denced that this property is almost independent of the temperature in the temperature range of calculations both for FAMES and biodiesel. Linear relationships were established between the molar com- pressibility and the molecular weight for FAMES and biodiesel. The before mentioned behavior of molar compressibility face to temperature and molecular weight make it possible to develop prediction meth- ods for the calculation of the speed of sound. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction The conventional fossil fuels (petrofuels) are non-renewable, increasingly scarce, with growing emissions of combustion result- ing pollutants, and with increasing costs of production. On the other hand, fuel reserves are concentrated in certain planet regions and most of them are reaching the production peak. All these cir- cumstances make biomass sources more attractive in particular the biodiesel. Unlike petrodiesel, biodiesel is a renewable fuel offering important benefits including reduction of green-house emissions, biodegradability, and non-toxicity. Biodiesel shows to- tal miscibility with petrodiesel and compatibility with modern en- gines [1,2]. Nowadays, biodiesel production has important economic and social impacts at the regional development level especially to developing countries [3]. Technically, biodiesel is a fuel formed by long chain of fatty acid esters produced from a large variety of feedstocks including vegetable oils and animal fats, with designation of B100, meeting the property and quality require- ments of the American Society Testing (ASTM) D6751 standard. Biodiesel can be produced through transesterification chemical reaction along which the raw material reacts with alcohol (usually methanol or ethanol) in the presence of a catalyst that can be metal alkoxide [4], ionic liquids [5], or others [6]. The resulting products 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.07.044 ⇑ Corresponding author. Tel.: +351 239 798 729; fax: +351 239 798 703. E-mail address: abel@eq.uc.pt (A.G.M. Ferreira). Fuel 116 (2014) 242–254 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel