A Comparative Study of Mechanical and Electrical Variable Transmissions

  • Omar Hegazy (Jury)
  • Florian Verbelen ((PhD) Student)
  • Kurt Stockman (Promotor)
  • Peter Sergeant (Co-promotor)
  • Gert De Cooman (Jury)
  • wim De Waele (Jury)
  • Mia Loccufier (Jury)
  • Hendrik Vansompel (Jury)
  • Walter lHOMME (Jury)

Scriptie/masterproef: Doctoral Thesis


In drive trains, a gearbox is used to match the required torque and speed of the
load with the operating range of the source. This source could be anything, from
an electrical machine to an Internal Combustion Engine (ICE). In most of the industrial
applications, the gearbox is used to increase the speed towards the source.
Therefore, the torque that has to be delivered by the source is smaller, resulting
in a smaller sized electrical machine (or other source). Classical vehicles with a
manual transmission go one step further as the driver can select multiple gear ratios
(in a vehicle with automatic transmission, shifting is done by a controller). At
standstill, a small gear ratio is chosen resulting in a low speed of the wheels but
with a high starting torque. During acceleration, a higher gear is selected to reduce
the required speed of the ICE. This is important as at high engine speed, the ICE is
operated at a lower efficiency. Shifting gears is thus done, among other things, to
reduce the fuel consumption. The problem with the manual transmission is that it only contains a fixed number of gear ratios. Although it could be interesting to use a gear between first and second, the transmission obviously does not allow that. To solve this, the variable
transmission has been developed, also known as a Continuously Variable Transmission (CVT). A variable transmission is basically a gearbox of which the speed
ratio can be varied continuously between 2 finite values. Consequently, any gear
ratio between the limits can be selected. Therefore, the ICE can be operated on
its optimal operating line, which results, in theory, in a lower fuel consumption
compared to the manual transmission. Besides adding a variable transmission, another possibility to reduce the fuel consumption is to change the topology of the drive train. An example of such an enhanced drive train is a Hybrid Electrical Vehicle (HEV). In these HEVs, the ICE and the wheels are decoupled. Therefore, the operating point of the ICE can be chosen independently of the wheels and that significantly reduces the fuel consumption.
A well-known HEV drive train is the Toyota Hybrid System (THS). This
drive train consists of a planetary gear that is used as power split device. The planetary gear is able to distribute the power to 2 electrical machines. As a result, one
extra degree of freedom is obtained. That explains why the ICE can be operated independently of the requested power of the wheels. Despite the many advantages,
the planetary gear introduces losses due to friction and valuable space is taken by
the 2 electrical machines.
Datum Prijs8 okt 2019

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