Power Electronics Progress Extends Automotive Efficiency

Component improvements and the advent of higher-voltage 48-V battery systems are boosting propulsive efficiency—not just in technology-rich platforms, such as electric vehicles, but across the spectrum, to rugged off-road types.

Rick DeMeis, Contributing Technical Editor

Texas Instruments

Developments in power electronics are boosting fuel efficiency in vehicles. Obviously, this impact is the greatest in those with electric powertrains, battery-electrics and plug-in hybrids. But the advantages of power-sipping systems, adroit power management, and the introduction of higher-voltage systems also benefit gasoline-powered vehicles—even the most basic and rugged off-road types.

Turning a new Leaf

Starting with the former, one example of progress in electric vehicles is the latest iteration of the Nissan Leaf battery-powered compact car. Introduced for the 2018 model year, improvements include a 40% range increase of 43 miles (69 km) to 150 miles (241 km). A greater capacity 40-kWh battery pack is largely responsible for that increase. (The original battery pack of roughly the same physical size was 24 kWh, improved to 30 kWh, giving those earlier models ranges of 85 and 107 miles (137 km and 172 km), respectively.)

 

Nissan Leaf image

Nissan: In addition to battery upgrades, the 2018 Nissan Leaf EV includes aerodynamic shaping improvements that lower its drag coefficient to 0.28—highly efficient for a short, compact vehicle.

The battery also charges quicker, lowering a driver’s wait time in “refueling.” For instance, one hour on a 240V charging station will increase range 22 miles (35 km). At a fast charging station often found along highways, a 30 minute charge provides an extra 88 miles (142 km). Conversely, when faster charging is reversed and power is applied to the electric motor, acceleration is increased.

Non-powertrain electronic advances on the Leaf include increased telematics connectivity, and “intelligent driving” features such as single-lane autonomous highway driving.

Keys to obtaining the aforementioned electric powertrain improvements are highlighted by Jonathan Ratliff, Nissan senior manager for EVs. Most important is the battery pack whose cell chemistry has been changed. “The Li-ion pack now uses NMC oxide (nickel manganese cobalt oxide) rather than the original manganese nickel chemistry. The polypropylene separators between the anodes and cathodes are thinner for reduced resistance and improved output.” [Ed. Note: The materials noted above are for the cathodes, with graphite-based anodes.]

The pack itself now consists of 24 modules of eight cells each. Previously 48 modules with four cells apiece formed the pack. The new configuration results in fewer connections between modules for lower resistance. This arrangement and having the thinner separators also means there is more space for energy storage materials within the same overall battery pack volume.

In addition, these changes facilitate the pack’s faster charging time. Ratliff says the battery’s improved “charge acceptance, utilizing more current for longer periods of time, allows recovering miles quicker [when charging].” Thus the charging current stays higher longer, shortening time on the cord. An analogy would be a garden hose with low backpressure for faster filling of a container.

Although faster charge transport contributes to the Leaf’s improved acceleration, Ratliff notes more significant is a new power inverter increasing power output from 80 to 110 kW. The inverter in turn drives a new electric motor with 37% more power, at 147 hp (up from 107 hp) and 26% more torque (236 lb-ft, versus 187 lb-ft).

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Further optimizing power and maximizing efficiency for greater range are battery and vehicle power control improvements. Nissan’s Ratliff highlights the combination of software and processor upgrades for quicker response, which also aids in faster charging.

Ratliff also emphasizes the reliability designed into the battery pack and its controls. A steel jacket surrounds the pack for safety in a collision. Each battery cell is tested, and then each pack is checked before installation in a Leaf. (The 300,000 Leaves produced so far result in extensive experience in battery reliability and testing.) In the vehicle, battery management includes relay control logic and detection-logic controls monitoring cell parameters. Recently, the company announced for the 2019 model year a slightly larger battery pack will be an option for buyers, giving the Leaf a 200 mile range.

Pedal to the Electrons

Finally, Ratliff mentions a noteworthy, but not obvious, clever feature on the Leaf boosting efficiency and range. The One Pedal driving mode allows a driver to let up on the “gas” pedal to activate aggressive regenerative braking to recharge the battery. Feathering the pedal is akin to coasting. Ratliff adds the feature even allows coming to a full stop and holding it without touching the brake pedal. Control is such that more energy is recovered than with manual braking, and eliminating even the short interval from throttle to brake pedal application adds significantly to energy recovery, he notes.

Efficiency hits the trail

One my not think of the off-road capable Jeep as an efficient vehicle, but power and other technology advances are saving more fuel in rugged trail use. The newly revised 2018 Wrangler comes with a 4-cylinder engine with a turbocharger, an 8-speed automatic transmission, and eTorque technology. The latter is a mild hybrid system using a motor/generator on a 48-V battery circuit (there is no 12-V battery) that allows stop/start functionality to save fuel, as well as regenerative braking and “intelligent” battery charging. The system also drives the electric power steering assist, controls fuel shutoff during stops, coasting, and deceleration, and transmission shifting. ETorque additionally supplies supplemental torque before the turbo comes up to speed, eliminating turbo lag.

 

Figure 2

Other features that improve fuel economy include more extensive use of aluminum and magnesium panels, lowering weight around 100 lbs (45 kg). A bend in the upper front grill and increased windshield rake helps bring about a 9% improvement in aerodynamic drag. While U.S. Environmental Protection Agency mileage numbers are not in as of this writing, Jeep expects approximately 3 mpg mileage improvement.

For the challenges in developing electronics for off-road vehicles, Fiat Chrysler’s Michael Duhaime, global director for electrified powertrain, provides some insights. “We design all of our electronics to withstand the environment that our customers put the Jeep through. The main difference is the battery. We fully seal the battery to operate fully submerged, taking care to insure we can operate and maintain all functionality.”

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As for the incorporation of both 12-V and 48-V electrical power systems, Duhaime offers, “We choose a motor generator unit [48-V] that can start and stop the engine it is placed on, as well as support regeneration to charge the battery and supply the [standard] 12-V system [and its components]. The 12-V system is managed through the high voltage battery. The hybrid controls optimize the system for maximum efficiency. A DC/DC converter takes the high voltage down to 12 V to support all those voltage loads of the vehicle—replacing the alternator. The battery is sized by the power needed to perform the functions throughout the temperature range.”

With the 4-cylinder Wrangler having twin-voltage electrical systems, system isolation and safety are foremost. “All of higher voltage electronics are isolated from the 12V systems by design, so the different voltages are always separated. There are controls in place monitoring this to insure that there is no ability to place high voltage on the network.” Duhaime notes.

He adds, “For service safety there are precautions such that, if a technician begins to open a high voltage component, it is automatically detected. The high voltage is disabled making the system safe for service.”

2.0-liter turbocharged inline 4-cylinder engine with eTorque technology

Efficiency keeps its upward trend

With automotive power electronics developments impacting vehicles from plug-ins to conventional internal combustion engine types (where mild hybrid systems are proliferating in use), engineers are still able to wring out more performance and efficiency from every type of powertrain design.

Additional Resources:

For more insight into automotive electrification trends for improved efficiency, read: Driving the green revolution in transportation.

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