Multi-Objective Optimisation of Vehicle Drivetrains
Vehicle drivetrains are complex integrated systems, which need to be designed for numerous thermodynamic, economic and environmental factors. As part of a project of the Alliance for Global Sustainability between the MIT, the SFITs and the University of Tokyo, a new evolutionary multi-objective optimisation algorithm has been developed and applied to various problems including vehicle drivetrain configurations. The simulation of a potential vehicle is complex and calculation intensive. The new algorithm, the Clustering Pareto Evolustionary Algorithm (CPEA) is attractive in this kind of problems because it allows an efficient use of each vehicle simulation - gathering more information about the solution domain for the same effort as a single point optimisation. In addition it allows optimisation of disparate objectives, for example, costs and emissions to be considered separately. Conventional, series electric hybrid and parallel electric hybrid have been evaluated over the ECE-EUDC and US06HWY drive cycles considering performance in terms of CO2, NOx, fuel economy, estimated investment, operating and pollution costs. Although some own technology models have been developed and used, the major part of the optimisation results are based on models from Advisor which are available on the web. The parallel hybrids were shown to be preferable to series hybrids in all cases studied, but were frequently beaten by conventional vehicles. In this first study, pollution costs considerations were limited to NOx (13.8 Frs/kg) and CO2, (0.03 Frs/kg) and diesel engine vehicles proved difficult to beat in terms of operating costs, and even overall investment and operating cost. Indeed it was found that a five fold increase in pollution costs traditionaly considered in the literature, would be needed before any difference would be observed. Of course the situation looks different if investment and operating cost, without any pollution costs, are evaluated only in comparison with the NOx emission levels or if a pollution cost of particulates is added. Among marketed hybrids the “Insight” was found to be highly attractive (keeping however in mind its limited seating capacity), although its exceptional performance is undoubtedly linked to the ultra light weight structure, which, in spite of the braking energy recovery of hybrids, is still shown to have a major effect. It is interesting to note that the pollution costs were found to be of low significance compared to the operating and investment costs and an average vehicle, mainly due to the significant level of tax already applied to fuel in the road transport domain. Politically it would seem that applying a uniform CO2 cost accross the board of all energy domains will have a much more limited effect on the road vehicle technologis, than an other technologies such as those used for building heating and air conditioning.