Abstract

Interest in hybrid electric vehicles (HEVs) and electric vehicles (EVs) has
increased rapidly over recent years from both industrial and academic viewpoints
due to increasing concerns about environmental pollution and global oil usage. In
the automotive sector, huge efforts have been invested in vehicle technology to
improve efficiency and reduce carbon emissions with, for example, hybrid and
electric vehicles. This thesis focuses on one design area of these vehicles – the
transmission – with the aim of investigating the potential benefits of improved
transmissions for HEVs and EVs.
For HEVs, a novel transmission developed by Nexxtdrive based on a twin
epicyclic design is analysed using a matrix method and its performance is
compared with the more common single epicyclic arrangement used successfully
in the Toyota Prius. Simulation models are then used to compare the performance
of a typical HEV passenger car fitted with these two transmissions over standard
driving cycles. The conclusion is that the twin epicyclic offers substantial
improvements of up to 20% reduction in energy consumption, though the benefits
are sensitive to the driving cycle used.
For EVs, most designs to date have used a single fixed ratio transmission, and
surprisingly little research has explored whether multi-geared transmissions offer
any benefits. The research challenge is whether it is possible to optimise the
usage of the electric motor in its region of high efficiency by controlling the
transmission. Simulation results of two EV examples confirm that energy
consumption benefits are indeed achievable – of between 7 and 14% depending
on the driving cycle.
Overall, the original aspects of this work – the analysis and modelling the twin
epicyclic gearbox; the analysis and modelling the twin epicyclic system in a vehicle
and a comparison of the results with single epicyclic system; and the analysis and
modelling of EVs with and without a transmission system of varying levels of
complexity – have shown that there are worthwhile performance benefits from
using improved transmission designs for low carbon vehicles.