It is widely accepted that the plug-in hybrid vehicle is the
most economically viable near-term solution to the petroleum and
CO₂ problems associated with passenger vehicles. The battery and
electrics costs of gas/electric plug-in hybrid vehicles can be
significantly reduced with the use of a hydraulic transmission and
the implementation of hydraulic regenerative braking. This has not
been done to date due to the unavailability of a low cost,
reliable, compact, and efficient hydraulic transmission. This paper
describes a conceptual design of such a transmission and evaluates
its performance using the attributes of existing components.
The main causes of the lack of past success with hydraulic
transmissions are the many drawbacks of variable-displacement
hydraulic pumps and motors. This transmission is based on fixed
displacement pumps and motors to avoid these drawbacks. The
efficiency of the transmission is evaluated under various driving
conditions and is equal to or better than present day automatic
transmissions due in part to the use of an all-mechanical bypass
when cruising. The transmission is configured to allow a
continuously variable level of braking torque. The design,
construction and performance of this digital hydraulic transmission
are detailed. The use of internal gear type pump/motors results in
a small size that will allow the transmission to fit within the
outline of a typical SUV transmission, offering a low cost way to
convert existing vehicles to hybrids. The required accumulator size
is discussed.
The design eliminates the need for either a friction clutch or
torque converter and is scalable such that it is applicable from
compact cars to large trucks and buses. A variation is described
that is suitable for adding hydraulic regenerative braking to an
all-electric vehicle.
The technical trend in hydraulic pumps and motors is to higher
operating pressures and speeds. Both these trends result in
proportional reductions in the size and cost of this style of
transmission and increased power handling capability.
