Hybrid transmission: design of the electric oil pump

Abstract: Hybrid vehicles are a step towards more environmentally friendly way of
transportation. This type of vehicles will play an important role in the
foreseeable future to decrease the overall dependence on oil reserves but
also to reduce the combustion engine's negative impact on e.g. pollution
and noise in urban areas. Hybrid propulsion systems can be accomplished in
several different ways, but the common idea is to add an electrical motor
for additional tractive power. This also gives possibility to recuperate
the kinetic energy which is otherwise lost when braking.

The hybrid transmission under development uses pressurized oil for gear
shifting operations, clutch activation and cooling. Conventional automatic
transmissions usually have a hydraulic pump driven by the combustion engine
for pressurizing the oil. In the hybrid transmission, oil pressure is
necessary also when the combustion engine is not running. In this Master's
Thesis a design proposal for an electric oil pump for the transmission are
developed. The goal has been to design a pump concept able to operate in
all possible driving modes for a hybrid vehicle. The concept was designed
considering capacity and pressure requirements, winter temperature
performance, efficiency and package space.

Three different hydraulic pump concepts has been developed and analyzed. A
current transmission oil pump has also been analyzed for reference
purposes. The different concepts have been extensively evaluated by methods
described by Pahl & Beitz [5].

The chosen concept is a single stage spur gear pump driven by an electric
motor. The volume displaced at each revolution is 5cm3 and the output flow
can be varied by altering the rotational speed of the electric machine. The
output produced by the pump can by an intelligent control system be
precisely matched to the flow demanded by the hydraulic system in the
transmission. Compared to a conventional transmission pump where the output
is constrained to the engine speed, less power is needed because of the
demand oriented flow production. A reduction in driving power of up to 46%
for a highway cruise cycle can be achieved by incorporating the electro-
hydraulic pump concept into a transmission. This corresponds to a total
fuel saving of approximately 1.8%. The overall efficiency for the pump
concept developed is 85% under optimal operation conditions, for the
reference pump, overall efficiency is 64% under the same conditions.

Packing size of the proposed design is strongly dependent on electric motor
size since the motor is 5 times larger than the actual pump unit. This is
due to the cold operation performance of the pump where low temperatures
increase viscous friction dramatically and increases torque loss. If the
cold operation performance was irrelevant and the motor could be designed
for optimum operation conditions, the size of the motor could be reduced by
40%. The complete package including pump and electric motor is however
substantially larger than the reference pump design, even if designing for
optimum operation conditions.