1. General Motors Global Propulsion Systems, Corso Castelfidardo 36 – 10129, Torino, Italy
2. Universidad Nacional de Colombia – Sede Medellín, Facultad de Ciencias, Escuela de Física, Calle 59 A, N 63-20, Medellín 050034, Colombia
3. College of Engineering, Mathematics and Physical Sciences, University of Exeter, Harrison Building, North Park Road, Exeter, Devon, EX4 4QF, UK