2. Supervariator
Supervariator is a multi-mode compound-split infinitely variable
electromechanical transmission with deep power splitting. Supervariator consists
of the differential unit, matching gearbox with range shifting mechanism, and two
electric motor-generators with dual inverter.
As a unit in a vehicle powertrain, supervariator performs multiple functions:
- IVT mode (moving-off, crawling)
- CVT driving modes
- Fixed gears modes
- Engine start function
- Power supply function
- EV traction mode
The supervariator may be applied in the following areas:
- Land vehicles with conventional powertrain
- Land vehicles with electric hybrid powertrain
- Land vehicles with flywheel hybrid powertrain
- Industrial variable transmissions
7. Prototype test plan
1. Supervariator running-in, control system adjustments.
Input torque limitation: 135 Nm
Input speed limitation: 800 rpm
Output torque limitation: 550 Nm
Control system powering: self-powered
Testing duration: 30 hours
Status: completed
2. Steady operation test, part 1 (with input speed limitation).
Modes: R, I, I+II, II, II+III, III, III+IV, IV
Input torque limitation: 270 Nm
Input speed limitation: 1600 rpm
Output torque limitation: 1100 Nm
Control system powering: self-powered
Test points number: 1000
Testing duration: 50 hours
Status: completed
8. Prototype test plan
3. Engine start-up simulation
Input torque limitation: -300 Nm
Input speed limitation: 800 rpm
Control system powering: from 12 V/75 Ah battery using 12/600 V 3 kW DC/DC converter.
Test points number: 50
Testing duration: 3 hours
Status: completed
4. Pure electric traction simulation
Mode: I
Output torque limitation: 1100 Nm
Output speed limitation: 1100 rpm
Control system powering: from 12 V/75 Ah battery using 12/600 V 3 kW DC/DC converter.
Test points number: 100
Testing duration: 5 hours
Status: processing
5. Simultaneous engine start-up and electric traction simulation
Mode: I
Input torque limitation: -150 Nm
Output torque limitation: 550 Nm
Output speed limitation: 1100 rpm
Control system powering: from 12 V/75 Ah battery using 12/600 V 3 kW DC/DC converter.
Test points number: 200
Testing duration: 10 hours
9. Prototype test plan
6. Steady operation test, part 12 (rated input speed).
Modes: R, I, I+II, II, II+III, III, III+IV, IV
Input torque limitation: 270 Nm
Input speed limitation: 3200 rpm
Output torque limitation: 1100 Nm
Control system powering: self-powered
Test points number: 700
Testing duration: 35 hours
7. Hybrid mode simulation (on mixed mechanical and electrical power). Tests with overload.
Modes: R, I, I+II, II, II+III, III, III+IV, IV
Input torque limitation: 540 Nm
Input speed limitation: 3200 rpm
Output torque limitation: 1100 Nm
Mechanical input limitation: 90 kW
Electrical input limitation: 30 kW
Control system powering: 600 V DC
Test points number: 2000
Testing duration: 100 hours
10. Supervariator structure diagram
Engineers can correlate experimental results with theoretical study. For this purpose, structure
diagrams for all modes are shown.
Mode I (IVT, i = ∞..2.93)
Mode II (CVT, i = 2.93..1.71)
Mode III (CVT, i = 1.71..1)
Mode IV (CVT, i = 1..0.4)
Mode I+II (fixed gear, i = 2.93)
Mode II+III (fixed gear, i = 1.71)
Mode III+IV (fixed gear, i = 1)
Legend: D1, D2, D3 – three-link differentials. 1 – sun gear, 2 – ring gear, 0 – carrier. RE2, R1, R2, R3, R4 – reducers.
E1, E2 – electric motor-generators.
Parameters: i12D1 = -1.92;
i12D2 = -2.66;
iR1 = 3.42;
iR2 = 2.36;
i12D3 = -2.14;
iRE2 = 1.92;
iR3 = 1.17;
iR4 = 0.80.
11. Test results examples
n1 = 800, 1600 rpm (25%, 50% of rated speed)
T1max = 270 Nm (100% of rated torque)
i = ∞...0.4
12. Test results: high efficiency of the supervariator
The experimentally gained efficiency charts show up the superiority of the
power-split transmission over series electrical drivetrain. The efficiency of
the small power series electric transmission, that forms electric path in the
supervariator (red curves), doesn’t exceed 70%. At the same time, overall
efficiency of Combarco power-split transmission (90..97% within whole
ratio spread) is higher than that for most known continuously variable
transmissions, despites this is a very first prototype.
At fixed gears electrical drivetrain is deactivated. Electrical losses are
eliminated, that’s why the efficiency raises by few percents. At straight
fixed gear (III+IV) all the mechanical path is locked, and the efficiency of up
to 99.5% was measured.
A great potential for further efficiency improvement has been revealed
during the research work. The final target efficiency is 94..98% within the
main CVT ratio spread, what is typical for multispeed mechanical
gearboxes.
15. Test results: low power flow via the electrical path
The outstanding efficiency of the supervariator is caused by extra deep power splitting. The
electric power flow doesn’t exceed 14% of the total transmitted power within the whole
ratio spread. This is the best result among all known power-split transmissions. Due to low
installed power of the electric torque converter, the supervariator combines high efficiency
with small package and weight. In commercial vehicles applications a final target specific
weight of the supervariator is 0.5..1 kg/kW.
The electric power flow of the 90 kW supervariator doesn’t exceed 11 kW at highest load.
Low installed power of electric torque converter gives high potential for cost and weight
reduction.
Two low-cost industrial AC induction motors are used in the supervariator prototype.
Custom AC motors are going to be used in commercial products to reduce weight by 60%
while their efficiency keeps at the same level.
Taking overload capacity into consideration, two 15 kW inverters are enough for a 90 kW
supervariator. Comparing to single-mode power-split transmission (for example, Toyota
Prius eCVT), the installed power of the electric components can be reduced by 3..4 times.
18. Test results: low power flow via the electrical path
Phase voltage
Phase current
19. Test results: large torque spread
In 4-mode design, torque spread of the supervariator is 7..9. These values are
usually enough for passenger cars, LCVs and agricultural machinery. Higher
number of modes (5..6) assures torque spread of 13..20 and even more, thus
meeting the requirements for heavy duty vehicles.
22. Test results: synchronous shifting of the modes
All
engaging
parts
are
synchronized before shifting the
modes (“soft shifting”). The
shifting process is shock-free,
that’s why compact and reliable
dog-type clutches are used
instead of multi-plate wet
clutches. Unlike wet friction
clutches, dog-type clutches do
not reduce overall transmission
efficiency.
23. Test results: efficiency maps
a) at n1 = 800 rpm
At low input rpm, the infinitely highest kinematic ratio can be reached for moving-off
the vehicle. Comparing to mechanical gearboxes, energy losses during moving-off are
reduced by 50%.
The lowest ratio at n1=800 rpm may reach 0.4, thus making possible keeping low rpm
of the engine at high vehicle speeds. At higher crankshaft speeds kinematic and torque
spread are reduced within the range of 5.05..0.58.
24. Test results: efficiency maps
b) at n1 = 1131 rpm
At shifting points between the modes (i = 2.93, 1.71 and 1) fixed gears can be
engaged. In these modes the electrical drivetrain may be deactivated, thus increasing
the efficiency of the supervariator by 1..5%. At i = 1 straight connection is engaged
between input and output shafts and all the gearings get locked. In this mode, the
efficiency reaches 99.5%
25. Test results: efficiency maps
c) at n1 = 1600 rpm
The efficiency of the supervariator prototype exceeds the efficiency of most known
CVTs. It remains high at wide spread of load and speed. Highest efficiency values (up to
96%) have been gained at CVT modes III and IV, the most demanded for automotive
transport (about 80% of total operation time). The efficiency tends to grow at higher
loads and speeds.
26. Test results: engine start-up and power
supply emulation
Due to compound split structure, loads and speeds of the electric motors are
equalized during engine start-up operation of the stopped vehicle. Thanks to that,
the efficiency of the electric drive reaches 58..60% at crankshaft speeds of 100 rpm
and crankshaft power of 3 kW. Electronic control eliminates inrush battery overload.
At zero crankshaft speed the measured current was about 100 A. At speeds over 100
rpm the measured current was 480 A.
If high-power high-voltage battery of a hybrid powertrain is used for start-up
function, the efficiency of the system may be improved up to 80%. Peak shaft power
at engine start-up mode may be raised up to 22 kW. During the tests, shaft power
was limited by 14 kW.
Power supply function was also tested. Electrical power of up to 12 kW was taken off
at engine speed of 800 rpm. Measured electrical efficiency was up to 71%.
28. Test results: engine start-up emulation
12 V/480 Amps start-up system steady-state operation charts
29. Conclusions
1. Supervariator prototype was operable in all modes.
2. Control system was stable in all tested modes (with input shaft speed of up to
1600 rpm)
3. Experimental results correlate theoretical study well.
4. Average CVT mode efficiency at high loads was 94%. At fixed gear modes the
efficiency reached 93..99.5%.
5. The area of highest efficiency was estimated at higher loads and speeds. These
modes should be probed during further tests.
6. Heat dissipation of the supervariator was moderate. Steady-state value of
temperature rise was 40..50 centigrade at input torque of 190..270 Nm and
input speed of 1600 rpm.
7. Starter function works well. The parameters of starter modes were improved
comparing to conventional starter system.
8. Alternator function also works well. Higher efficiency and capacity were also
verified comparing to conventional belt-driven alternator.
