28-03-2014, 11:03 AM
Pushbelt CVT efficiency improvement potential of servo-electromechanical actuation and slip control
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ABSTRACT
By using the ability to shift to high overdrive ratios, a
CVT equipped vehicle can outperform the fuel econ-
omy of its counterpart with conventional automatic
transmission. A further signi cant reduction of fuel
consumption can be obtained by reducing transmis-
sion power loss. The main sources of power loss are
well known to be losses inside the V-belt variator and
losses caused by driving the hydraulic pump. Signi -
cant reduction of these losses will increase the attrac-
tiveness of the V-belt type CVT.
INTRODUCTION
Since its introduction over 15 years ago, the CVT has
contributed to the improvement of fuel economy of
passenger cars. It gives a better fuel economy than
the stepped automatic transmission (AT) due to its
wider ratio coverage and its ability to shift ratio with-
out compromising comfort. Another advantage is that
it allows for the torque converter to lock up at lower
vehicle speeds than when an AT is applied. Unfortu-
nately, the metal V-belt CVT does not have a much
better ef ciency than the AT. An important reason for
this is that it needs signi cantly higher hydraulic pres-
sure for variator actuation purposes, (typically up to
50 [bar]) than an AT does, which leads to a larger oil
pump drive torque. Since the oil pump usually rotates
at a speed proportional to engine speed, quick ratio
shifts at low engine speed dictate rather large pump
displacement. For a typical CVT, rated at 200 [Nm],
the pump displacement amounts to about 19 [cc/rev].
Making use of the double piston principle in combi-
nation with a two stage torque sensor, allows for a
more favorable pump displacement of 11 [cc/rev] at
310 [Nm] rated torque [1]. Additionally, the power loss
in the variator (de ned as belt, pulleys and shaft bear-
ings) is not small.
REFERENCE TRANSMISSION EFFICIENCY MOD-
ELLING
For benchmark purposes, the 2 liter class Jatco CK2,
manufactured by Jatco Transtechnology Co. Ltd. was
selected. This type of transmission will also be used
as a carrier for the servo-electromechanical actuation
system, that is currently in the realization phase. The
reference transmission comprises a torque converter
as drive-off element, a DNR set, a hypotrochoid gear
pump, a push-belt type variator with ratio coverage
of 5.4, followed by the na l reduction and differential
gear [3]. The majority of the presented data will be
limited to the over-drive ratio, since, this ratio is most
relevant for fuel economy evaluations.
EFFICIENCY ANALYSIS METHOD
In order to assess the ef ci ency improvement po-
tential of the proposed modi cations , rst the power
losses occurring in the reference transmission were
carefully modelled. The main power losses are
caused by the hydraulic pump and by the variator and
were obtained by means of measurements. Losses in
the nal reduction were calculated by means of litera-
ture data and models [4]. These loss components will
be treated separately in the following sections.
CONCLUSIONS
Both model calculations and test rig measurements
have been used to construct a detailed ef ciency
model of a commercially available CVT. By subtract-
ing pump loss and adding loss due to the EMPAct
system and by mapping variator loss as a function
of over-clamping, the effect of the new actuation
system and slip control on ef ciency could be esti-
mated and an indication could be given for the conse-
quences for fuel consumption. By applying a servo-
electromechanical actuation system and variator slip
control, signi ca nt reduction of power loss can be ex-
pected. This leads to a drastic improvement of ef -
ciency, especially at part load, which, in turn, leads
to a signi can t fuel economy potential, especially on
driving cycles where part load conditions prevail. Test
rig evaluations will be used to prove the predicted ef-
ciency potential. Vehicle evaluations will be carried
out in order to assess the practical use of both the
EMPAct system and slip control.