Research Paper Assessment of the efficiency of tractor transmissions using acceleration tests Marco Bietresato*, Dario Friso, Luigi Sartori University of Padova, Department of Land, Environment, Agriculture and Forestry, T.e.S.A.F., Section of Mechanization, Construction and Plant Engineering, viale dell’Universita ` 16, I-35020 Legnaro (PD), Italy article info Article history: Received 7 July 2011 Received in revised form 20 March 2012 Accepted 26 March 2012 Published online 25 April 2012 A theoretical model that could be useful for investigating the performance of ground vehicles using acceleration tests was developed. The model takes into account all of the forces acting on a moving vehicle to give a description of its kinematics: the rolling friction of the propulsion devices, their slip on the soil, aerodynamic resistance, road slope and traction developed by the engine. In the methodology of applying the model, the experimental data collected during acceleration tests on a flat and sloping road from a standing start to reach a given distance or a stated speed are used to determine the average efficiency of the transmission of three tractors with different equipment (a mechanical full-powershift drive and two different hydro-mechanical continuously- variable ratio transmissions e CVT). The same model was also used to predict tractor performance during other acceleration tests. The results of the CVT tests suggest that there probably is an influence of the transmission architecture that takes precedence over the general type of the transmission. ª 2012 IAgrE. Published by Elsevier Ltd. All rights reserved. 1. Introduction 1.1. General considerations and problem description The efficiency of a transmission (TE) can be defined as the ratio between the energy transmitted to the wheels and the energy provided by the engine at the crankshaft over an equal reference time period or, alternatively, as the ratio between the correspondent powers. Due to the losses (e.g. sliding friction in the mechanical members in contact and viscous friction for members submerged in lubricating oil), its value is less than unity. The value of TE is influenced by many factors, primarily the transmission architecture, that derives from design choices (e.g. number of shafts, gears and electro-driven frictions, number of teeth of each cogwheel, presence of a torque converter, and presence of a hydrostatic section). In addition, there are sources of friction dependent in a complex way on the speed of individual moving parts and the level of lubrication. Also, not all parts of a transmission are utilised at any given time during vehicle operation. Further- more, in hydro-mechanical power-split CVTs, a variable distri- bution of transmitted power is present between the hydraulic and mechanical parts, and this depends on vehicle speed. Knowledge of the TE of an agricultural tractor permits the energy losses to be estimated while the tractor is executing some operations and, to evaluate the goodness of different tractor/technical solutions. Assessing the transmission effi- ciency, given as a set of values related to different speeds and/ or transmitted powers or as a continuous function throughout the operating range of the vehicle engine, and considering the construction of any type of transmission (e.g. traditional, full- powershift, CVT), is always very difficult both experimentally * Corresponding author. Tel.: þ39 049 8272723; fax: þ39 049 8272774. E-mail addresses: marco.bietresato@unipd.it (M. Bietresato), dario.friso@unipd.it (D. Friso), luigi.sartori@unipd.it (L. Sartori). Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/issn/15375110 biosystems engineering 112 (2012) 171 e180 1537-5110/$ e see front matter ª 2012 IAgrE. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.biosystemseng.2012.03.009