Optimal Speed Profiles for Passenger Cars
Tutor / director / evaluatorEriksson, Lars
Document typeMaster thesis (pre-Bologna period)
Rights accessRestricted access - author's decision
Fuel consumption in passenger vehicles has become more important due to the significant increase of fuel cost and the awareness of the environmental impact. Thus finding optimal speed profiles and gear shift strategies to cover a distance in order to decrease fuel consumption is an important issue. The optimization problem is presented and solved by means of dynamic programming and using a quasistatic vehicle model approach. Different models are presented in order to solve the problem with different assumptions. For one model it is assumed an instantaneous gear shift maneuver and for another model the gear shift maneuver requires a certain time to be done. For this second model, the influence of taking into account a realistic way to compute the losses during a gear shift maneuver is also developed and studied. Comparing the results provided by the two models, this method to compute the gear shift losses in the optimization problem has influence in the optimal speed profiles compared to the models that do not take into account losses during the gear shift. There are two engine models used in this project, a Willans approximation engine model and a map based engine model. The differences between the results from each engine model are explained and illustrated. Moreover, the influence of the trip length is studied, stating that without a time constraint it has no influence on the results. This investigation highlights the importance of selecting a suitable discretization. A smart choice of the discretization is explained in order to have the coasting solution achievable in the deceleration phase. In order to study the results in different road situations, the optimization problem is solved by changing the road topology. It is stated like in previous research papers that keeping constant speed to drive through a hill is optimal, if no braking is required to maintain constant speed. Finally some vehicle parameters are changed in order to observe its sensitivity on the optimization results. Results for several vehicle masses and engine sizes are illustrated and compared between them. Regarding the engine size it is shown that it has a significant influence on the fuel consumption as well as on the driving mission.
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