Controlled Power Technologies (CPT) and Austria’s Technische Universität Wien (TU Wien) recently investigated 48-V mild-hybrid technology. Mild hybrids are cars that pair a traditional internal-combustion engine with an electric motor and generator in parallel to let the former turn off during vehicle stopping or coasting. Now, such setups are easier than ever with low-voltage electric powertrains — and fast-acting starter-generators reduce residual NOx emissions of Euro 6 diesel engines.

The study verified 9% reduction of the pollutant in raw emissions while maintaining the efficiency and CO₂ benefits of the diesel engine. The setup’s cost effectiveness is underscored by its impact on lean NOx trap (LNT) and selective catalytic reduction after-treatment systems, which have less raw NOx emissions to process … potentially allowing for simpler exhausts with longer service life.

“Our research (incorporating e-motor simulation and engine-emission dynamometer testing) shows clear NOx reduction from a premium car with a three-liter V6 engine,” said CPT’s Paul Bloore, manager of applications engineering. “The decrease in NOx emissions was with almost 5% fuel-economy improvement and corresponding CO₂ reduction delivered simultaneously by the SpeedStart starter-generator.”

Belt-integrated starter-generator (BISG) systems are successfully applied to reduce CO₂emissions by the simple expedient of stop-start. So the same low-voltage technology can modify the load on the engine and optimize its performance to minimize NOx and particulate emissions. The challenge is having response quick enough for real-world driving. Here, CPT’s switched-reluctance technology is fast and controllable — so suitable for the application. The switched-reluctance machine (SWM) lowers raw emissions of NOx — and that in turn reduces the load on the selective catalytic reduction system … reducing consumption of reducing agent ammonia (AdBlue) not to mention the active catalytic components, which are usually precious metals.

In fact, a switched-reluctance machine might even allow use of LNT setups instead of costlier selective catalytic reduction. Diesel exhaust systems can also include a diesel particulate filter (DPF) which may also require less frequent purging.

This research shows that the transportation industry can reduce air pollutants in a cost-effective way, said CPT’s co-founder and chief executive Nick Pascoe.

“The economy of 48-V technology compared with full hybrid and plug-in solutions gives the automotive industry breathing space while pursuing more affordable fully electric vehicles. The necessary reduction in transient emissions refocuses the driveline electrification task to engine mounted ‘P0’ motor-generator configurations.”

According to Bloore, the cost of batteries must fall before EVs are as cost effective as internal-combustion engines running on petrol or diesel, and most don’t expect this for another decade. “Meanwhile, mild hybrid technology is cost efficient and adds immediate value by optimizing both air quality and carbon emission performance without compromising fuel consumption.”

“Motorists and transport operators want to be friendly to the environment, but must be mindful of the current pricing of high voltage electric vehicles,” added Pascoe.

The CPT-TUW study evaluated the switched reluctance technology against the Worldwide Harmonised Light Vehicles TestProcedure (WLTP) — the latest compliance requirement for any new cars sold into the EU in or after September 2017. WLTP defines a globally harmonized standard for determining emissions and energy consumption.

“Being closer to real-world driving conditions, WLTP offers more dynamic test profiles,” said Bloore. “So it’s more effective for demonstrating how SRMs very rapidly supply torque to the engine — delivered to the crankshaft via the front pulley belt system to adapt electric-assist levels to the traffic conditions and driving style.”

The CPT-TUW study considered the belt ratio and thermal efficiency of the water-cooled BISG as well as vehicle operation patterns over additional test cycles. BISG-induced improvements came from energy recovered during recuperation events with a balanced battery state-of-charge and a constant vehicle electrical load applied of 300 W.

“Real-world driving conditions put more load on an engine (and come with a corresponding increase in emissions) than in laboratory tests,” said Bloore. “But the precision and repeatability of laboratory test cycles makes them invaluable for back-to-back and technology comparisons.”

That said, CPT is validating its technology through more realistic test cycles — which could further validate use of SRMs.

“SRMs offers consistent high power and high efficiency over a wide speed range, and unlike some alternative motor technologies, energy sapping electromagnetic field weakening is not an issue,” says Bloore. “The precise torque control enables a swift response within milliseconds to changes of load on the machine — and that’s one of our unique advantages against other types of electrical machine.”

Real-world conditions demand millisecond response times

Like mild hybrid systems generally, CPT’s SpeedStart technology harvests kinetic energy during deceleration, which is then reapplied as torque by the switched reluctance machine during acceleration. But sophisticated SRM electronic control can further influence the engine operating point by absorbing or supplying electrical energy to move the main engine into more optimal operation. That lets the SRM respond swiftly to transient conditions.

CPT’s switched-reluctance technology works in passenger cars and is currently being tested in trucks and off-highway vehicles. Controls CPT are refining are based on this study and other R&D. Ultimately, the aim will be to coordinate interrelated systems (including battery management, exhaust after-treatment and complementary thermal energy recovery) while providing a valuable computer-aided engineering to identify more opportunities to boost performance through 48-V electrification.