Frank Markus
NEWS I OPINION I GOSSIP I STUFFTechnologue
22 MOTORTREND.COM APRIL 2021G
eneral Motors has now sold more than a million
engines with Dynamic Fuel Management. This riff
on cylinder deactivation can shut off any and all
cylinders in the V-8 engines powering everything
from Chevy Silverado pickups to Cadillac Escalade SUVs.
This concept, developed by Tula Technologies and
covered here in May 2014, intriguingly can be applied
to electric motors—which use neither spark plugs nor
valves—and could unlock the potential of cheaper, more
sustainable rare earth–free motors.
The concept for both DFM and Tula’s new Dynamic
Motor Drive is that whenever a powertrain operates at
lower efficiency—say, eight cylinders doing the work of
two while loafing along—you’re better off making two
cylinders work harder and giving the rest a break. With
two cylinders at wide-open throttle and the rest bouncing
along as air springs, the engine needn’t struggle to suck
two cylinders’ worth of air past a closed throttle plate,
and it also loses less energy to the exhaust and coolant.
Electric motors are naturally more efficient than
combustion engines, so when you map load (torque)
versus speed for both, the area of highest efficiency is
much larger for the electric motor. But there are still
many lower-torque/higher-speed operating conditions
that lie below this peak efficiency region. It’s
under these operating conditions where DMD
can boost efficiency.
The Dynamic Motor Drive equivalent of DFM
shutting down the valves, fuel, and combus-
tion in some of the cylinders is to pulse brief
intervals of high-current torque generation
followed by periods of no current to achieve the
net torque requirement. Instead of reducing pumping
and combustion heat losses, DMD reduces heat buildup
in the electric motor’s rotor core and heat generated in
the power inverter.
Of course, this pulsing must be done judiciously to
prevent noticeable powertrain vibrations, especially
those that might align with a natural frequency of the
vehicle’s chassis or body structure. Tula has abundant
experience in this sort of programming.
Today’s most efficient, power-dense motors are those
employing rare earth–intensive permanent magnets,
which power most non-Tesla EVs. DMD can only improve
their efficiency by less than a percentage point. In an
inherently less efficient AC induction-type motor, like
those used in most Teslas, the gain is maybe 2 percent. But
that’s still worth pursuing, as there’s no material cost to
implementing DMD, just intellectual property royalties.
Where DMD really shines is with a third cheaper,
coarser, and less efficient motor type the EV world is just
beginning to consider: the synchronous reluctance motor.
Like an induction motor, it uses energized windings inthe outer housing (stator) to induce magnetic rotation
of the rotor. But instead of using unpowered electrical
windings (in which a magnetic field gets induced), the
SRM’s rotor is made of a cheap, soft magnetic material like
laminated steel with natural magnetic reluctance (oppo-
sition to magnetic flux). But its poles never change like
they do in an induction motor; to make it turn, successive
stator windings must be continuously and
variably switched to pull these fixed poles
around and around.
Typically employed in industrial applica-
tions, SRMs tend to be noisy, and they suffer
“torque ripple,” periodic dips in torque as
the shaft rotates. Advances in switching
electronics are helping combat these woes,
and when Tula adds its DMD logic to this more sophisti-
cated switching, efficiency improves by 9 percent.
That means a Tula-programmed SRM surpasses the
efficiency of an induction motor and approaches that of a
permanent-magnet motor while remaining dramatically
less expensive than either induc-
tion or permanent-magnet motors.
Adding cheap, abundant ferrite
permanent magnets to an SRM’s
rotor promises to further boost its
efficiency and performance at modest
additional cost.
It’s early days for both Dynamic
Motor Drive and EV implementation
of synchronous reluctance motors,
but if they arrive around the time
pricing comes down on the solid-state
batteries I covered last month, then
I can start to imagine inexpensive
long-range EVs selling to first-time
car buyers on price alone. QReluctant eMotor: Cylinder deactivation
to bring cheap industrial motors to EVs
In this graph,
DMD can’t help
with load/speed
conditions above
the white line, but
torque pulsing
brings potential
benefit when
operating below
it by providing, for
example, one-third
more torque than
required for one-
third of the time.SPEEDTORQUEEFFICIENCYTREND I 4.21Pulsed operation
at higher
efficiency ...... delivers that higher
efficiency at lower loads.Normalized^EnergyUsageat^ LowTorqueIMwithInverter^w/^DMDw/^DMDw/^DMDSynRMMotor TypeA SynRM with DMD
approaches the efficiency
of an IPM motorRare^EarthIPM