Seldom is any road vehicle’s powertrain required to work at its peak power and torque capacity all the time. Light cruising typically requires a fraction of a powertrain’s full capability, so Tula’s Dynamic Skip Fire (DSF—a.k.a. Dynamic Fuel Management, or DFM) engine strategy boosts combustion engine efficiency in these conditions by working a few cylinders hard (and efficiently) while the others nap, known as cylinder deactivation. As we outlined in Feb. 2021, Tula’s new Dynamic Motor Drive (DMD) is similar, but instead a software solution for electric motor powertrains that sends higher, more efficient bursts of electric current to a motor that are interspersed with zero-current “rest periods” to achieve the required torque delivery with improved efficiency.
Testing NVH, Not Efficiency
Tula’s combustion DSF/DFM systems have proven smooth and reliable in over a million V-8 Chevy and GMC trucks so far. To find out how seamlessly Tula’s new DMD works in a zero-cylinder electric car, we sampled the tech in a Chevrolet Bolt EV.
The Bolt EV is powered by a permanent-magnet motor. These are the most efficient motors currently on the market, and the Dynamic Motor Drive programming admittedly struggles to improve the motor’s efficiency by only about a half-percent; Tesla’s slightly less efficient AC induction-type motors see an improvement closer to 2 percent. But the control strategy in terms of pulsing the power in dense bursts is essentially the same for any motor type, so in this test car we drove, we’re assessing noise, vibration, and harshness (NVH) only—we’re not counting how many yards of extra range DMD is granting us. On a short journey it would be tough to draw conclusions.
DMD can modulate the torque pulsation independent of vehicle or motor speed and hence can (and does) strive to avoid bothersome or resonant frequencies. For example, humans typically find vibration frequencies between 0.2 and 20 Hz noticeable and bothersome, while the resonant frequencies of steering wheels and seat tracks falls in the 63-77 Hz range and around 87 Hz respectively.
High-energy pulsing of the motor will likely be more noticeable to occupants while driving versus a lower-energy, steadier flow, so we’re curious how NVH is impacted. In theory, the lower the pulsing frequency of the motor, the higher the efficiency gain; however, the higher the pulsing frequency, NVH should improve. There are some tradeoffs and development decisions Tula has to make in its programming. So in our Bolt, Tula programs the DMD to function mostly between 20 and 60 Hz.
How Does It Feel?
On rare patches of smooth Michigan pavement, traveling 25-40 mph at very light load, a slight buzz can be felt, which quickly goes away upon acceleration. The vibes are perhaps a bit more noticeable than the comparable combustion DSF/DFM system, because a V-8 that selects which of eight cylinders to fire every 90 degrees of crankshaft rotation can randomize the torque pulsing somewhat better than DMD can.
That said, someone who wasn’t straining to sense vibration on our drive may never have done so, especially as road vibrations easily cover them up on most surfaces (this suggests that vehicles incorporating road-sensing suspensions could perhaps order more DMD on rough surfaces, less on the smoothest ones). It’s also worth noting that DMD also increases efficiency during regenerative braking to optimize energy recovery, and that vibration was far less noticeable during deceleration. And this prototype vehicle hasn’t undergone the extensive algorithm tuning a production fitment would get.
Where Does the Efficiency Come From?
DMD helps an electric motor and the inverter that runs it operate more efficiently by eliminating “switching losses” that generate heat as the system controls the frequencies of the electromagnetic fields that power a motor. The biggest improvements are found in reduced heat generation in the inverter and the motor core, which more than make up for slight increases in heat generated in the rotor and stator during DMD operation. The results shown here reflect testing on the U.S. Multi-Cycle Test (which generate EPA numbers—results on the generally less rigorous WLTP test cycle indicated a 19-percent improvement for DMD).
When we covered this technology last year, we touted its potential ability to make the crude, industrial, synchronous-reluctance motor viable for EV applications. But Tula reps now say those motors are unlikely to make the jump to EVs, being better suited to wind turbines and other big applications (DMD can still improve their performance, but cost-reduction is less of a priority).
When Might You Drive DMD?
Where it really makes sense is in externally excited synchronous motors (EESM). That’s industry speak for the permanent-magnet-free motors that use an electromagnet rotor, like the type of motor BMW recently released under the name “current-excited synchronous motor.” By eliminating permanent magnets, such motors can save between $250-$300, but at some cost to efficiency.
The easiest way to improve efficiency of any electric motor is by switching to a silicon-carbide (SiC) inverter, but these add cost. DMD is a software solution that incurs primarily development (and intellectual property) costs, while still capable of improving the efficiency of a cheap externally excited synchronous motor, or EESM, by up to 2 percent relative to a permanent-magnet motor. That nearly matches the efficiency advantage of adding the pricey silicon-carbide inverter to a magnet-machine at a cost savings of between $350-$400 relative to the baseline permanent magnet motor. Adding the silicon-carbide inverter on top of these gains can take that efficiency up almost another percentage point, as you can see in the chart featured above (labeled EESM + SiC + DMD).
Tula Technology is currently demonstrating its Bolt EV prototypes to OEMs and suppliers, and we know none has signed as yet, because on September 21, DMD was awarded PACEpilot innovation award Automotive News. That category is only open to early-stage innovations not yet sold to an OEM customer. After one signs, it’ll take a few years of development to reach the market.
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