Design of a power loss model for vehicle drivetrains

Abstract: Lately the discussion on how big impact vehicles has on the environment has grown bigger and bigger. The vehicle manufacturers are building cars, trucks, wheel loaders etc. that are getting more and more fuel efficient for every year. But where do the engineers at the companies make the improvements? The drivetrain is one area of the vehicle where many different sources of losses are found. The gearbox is one example, where the losses are both dependent and independent on the torque transferred by the gear pairs. The gearbox consists of several components that all of them contribute to the total system power loss. It is therefore of great interest to understand the magnitude of the losses in different components for different operating conditions, and also to be able to determine how they change with, for instance, rotational speed and load.
There are currently some commercial software’s on the market that analyses how the losses in a drivetrain are distributed. These tools are useful for engineers when analyzing existing drivetrains, or when developing new products. The software’s shows how losses change depending on components, oils, engine speeds and load which helps the engineers concentrate on the right areas. Power loss calculations within these software’s are often based on simple and generalized methods. For instance could the user be needed to specify a contact friction coefficient of the system that will be held constant during the simulation. For starters a constant friction coefficient is a rather big simplification, and the task of assessing this value for the engineer is an intricate manner.
In this Master of Science thesis work a module based, user friendly software is developed. The model is taking the biggest loss sources, for example gearbox, differential and tires into account in a system with static conditions. No losses due to the dynamics in the system will be taken in consideration in this model. The final model is a good base containing most of the components of a person’s vehicle with rear wheel drive. All components from crankshaft to asphalt are modelled in simple modules built by Simulink blocks and Matlab m-code.
In the model created in this thesis all calculations are based on formulas from scientific experiments. This makes the loss calculations more advanced and accurate then the generalized formulas in most commercial software’s. It also gives the user bigger opportunities to for example use different methods to obtain the gear tooth contact friction coefficients. Two methods are currently implemented in the model. One method is to use an empirical formula, and the other is to use friction maps generated by experiments. As a result from the research program ProAct, lookup tables with friction coefficient depending on slide to roll ratio and entrainment speed can be implemented in the model. This makes it possible to show the difference in contact friction coefficients between the empirical formula and the friction maps. Ultimately, the purpose of the model is to be able to evaluate how a change in the system will affect the overall power loss in the drivetrain. A looped simulation between two mineral oils in this model shows that the gear frictional losses has the biggest influence on the total loss of the gearbox in a simulation of 31 investigated cases from 1000 rpm to 4000 rpm with an constant torque input of 100 Nm.