Analysing hybrid drive system topologies

Abstract: In this thesis a simulation model is presented that enables a comparison of different hybrid topologies, with respect to fuel consumption, emissions and performance. The obtained results stress the properties of the different topologies and form a foundation for the choice of hybrid topology. The simulation models included in this thesis are the result of collaboration with Petter Strandh at the Division of Combustion Engines, Department of Heat and Power Engineering, Lund University. The studied hybrid electric vehicles (HEVs) are restricted to those with an internal combustion engine (ICE), an electrical energy storage and at least one electric machine in the driveline.

The goal with the work presented in this thesis is to:
# 1. Model hybrid topologies as comparable as possible regarding motor models, battery models, control laws, major energy converters and relevant limitations due to speed and torque.
# 2. Compare the number of possible topologies, which increase drastically when adding more clutches, electric machines and other components.

Therefore only four, however essential, topologies are chosen for the comparison. The four topologies that have been selected are series, parallel, strigear and power split hybrid. The hybrid topologies have been simulated in two different cycles, the demanding highway cycle US06 and the slower urban cycle ECE15 with its much smoother accelerations.

The reference vehicle in the simulations has been a Toyota Prius, an electric hybrid family car, which is available on the market today. As input for the ICE, measured values from a SAAB naturally aspirated gasoline engine has been used, but scaled to better correspond to the ICE in the real Prius. There are many possible parameters in the simulation models, that are adjustable; vehicle chassis parameters, engine, electric machine(s) and battery size and types, losses models, charging strategies and driver behaviour etc. To investigate all of them is possibly interesting but not realistic in this survey. It is not the aim and the result flow would be overwhelming. Therefore six key parameters are chosen and thereafter adjusted one by one. The chosen ones are ICE dynamic response time constant, battery inner resistance, ICE charging gain, engine and motor sizes and finally maximum vehicle speed. The work presents a limited number of results. The results presented have been chosen to illustrate the impact the individual parameter has to the behaviour of the single topology.

The results of the simulated topologies have been compared with measurements made by MTC in Sweden and EPA in USA on a Toyota Prius.

The received results can be evaluated with the help of different criteria. Two different criteria are shown as an example of how the chosen criteria influence the results; the price of fuel consumption and produced emissions respectively a mutual comparison using weight factors.

The results from the simulations made, show that the parallel topology is the most efficient alternative. It is also the topology with the lowest complexity. It should be observed that the differences between the strigear and the parallel hybrid do sometimes not exist or turn to be of advantage to the strigear. The parallel topology however turns to be the preferred choice due to its lower complexity.