With the bold claim of inventing the “world’s most power-dense variable speed electric motor,” U.K. specialist company Aeristech believes it can help solve a significant challenge facing the use of downsized engines in pursuit of low CO2 emissions.
Said CEO Bryn Richards: “For extreme engine downsizing neither multi-stage nor mechanical superchargers are the answer, as an engine using them would lack power at low speeds. To support the next generation of downsized engines requires an electric supercharger with enough power to supply all the low speed boosting needs of the engine combined with sufficiently effective cooling to enable continuous operation."
Thus the development of the Aeristech eSupercharger, designed to provide the low-speed torque and instant throttle response that customers expect but can be challenging to achieve in turbocharged downsized and "right-sized" engines.
Richards explains that multi-stage turbocharging, combining large and small turbos, makes downsized engines easier to drive across a wider speed range, but introduces other issues such as cost, complexity, thermal management, and catalyst performance. The latter, he noted, is likely to be of growing significance as tailpipe emissions are further restricted: "The thermal mass of a two-stage turbocharger system located between the engine and the catalyst makes light-off more difficult to maintain during periods of light load or low engine speed,” he explained.
The obvious alternative is to use a supercharger, but conventional mechanical superchargers don’t deliver at very low engine speeds. And Richards believes that electrically-driven units “have so far only been able to provide the briefest transient boost."
He regards the achievement of a satisfactory electric supercharger with sufficient power to meet, convincingly, all low speed boosting needs could be achieved by the industry’s present likely move towards 48-V systems.
“As for meeting cooling capability requirements, that would be a must for continuous operation," he told Automotive Engineering, pointing out that Aeristech's proprietary electric motor technology delivers the full load air requirements of the engine in under 1 s without the cooling issues associated with conventional motors.
New motor-control strategy
The much-anticipated move towards mild hybrids with 48-V architecture would provide the power necessary for an electric supercharger to deliver the boost levels required by a downsized engine in the lower half of its speed range. With this arrangement, the mechanical turbo could be optimized purely for higher speeds, allowing greater specific power and less exhaust restriction.
Richards explained that to unlock the potential of 48-V systems to provide electric supercharging, Aeristech uses permanent magnet technology, providing a faster response without the cooling challenges arising from the alternative switched reluctance (SR) motors. He believes that SR motors are the only alternative to Aeristech’s patented technology and he expects that when mass produced, the cost of the two systems would be similar.
“Our motor control strategy separates commutation and power control, which means the electrical switching frequency need be no higher than running speed," he said. This is a fundamentally new approach that dramatically reduces the cost of many key switching components while ensuring exceptionally accurate high-transient speed control."
The strategy would make permanent magnet technology cost-effective, he claimed, "meaning that for the first time an eSupercharger can run continuously at boost levels of 2.5 bar or more.”
The technology is applicable to both diesel and gasoline engines. “In the face of increasingly restrictive legislation, diesels are relying on filter systems to remove particulates from the exhaust, increasing the back pressure," Richards noted. "It makes more sense to convert the particulates to useful energy in the combustion chamber, rather than filtering them out of the exhaust. The ability to match the air supply to the fuel input at low engine speeds by using an eSupercharger with lag-free response to transient changes would both cure the emission problem and improve fuel economy.”
Significant BMEP gains
Aeristech claims industry-leading power density and low inertia for the motor in its eSupercharger, which accelerates to 150,000 rpm with a transient response of idle-to-target speed in under 0.4 s. The company has successfully subjected its technology to independent evaluation by Ricardo and Mahle Powertrain U.K.
Richards said that Ricardo had carried out detailed modeling of a 221-kW 2.0-L gasoline engine with a single-stage turbocharger using Ricardo’s WAVE simulation software. Adding a 48-V Aeristech eSupercharger enabled the turbine to be increased in size by 80%, "improving BSFC [Brake Specific Fuel Consumption], torque and transient response throughout the engine speed range," he said.
Ricardo concluded that without the eSupercharger only a larger engine could have met the requirements, if relying on a conventional single-stage turbocharger, according to Richards. Mahle demonstrated the 48-V eSupercharger in a D-segment appraisal vehicle using its downsized 1.2-L 3-cylinder gasoline engine. Originally configured with 2-stage turbocharging, the engine achieved 144 kW (193 hp) and just over 30 bar BMEP.
"When the smaller turbo was replaced by the eSupercharger, allowing the larger turbo to be re-optimized, the engine achieved a maximum power output of 193 kW and a remarkable 33-bar peak BMEP at 2000 rpm," Richards reported. He said torque was increased over the entire speed range with a maximum value of 313 N·m (231 lb·ft); BMEP at 1200 rpm was increased from 20 bar to almost 29 bar.
"Mahle Powertrain U.K. has said that this is a higher torque per unit of cylinder capacity than any series production engine,” he noted. The eSupercharger directly influences the high torque figure, Richards claimed, and is also indirectly responsible for the increased power output. By replacing the smaller turbo in a two-stage turbocharging arrangement with the eSupercharger, the Mahle engineers were able to increase the size of the main turbo with satisfactory drivability and transient response.
For a future C-segment vehicle aiming at CO2 emissions of 95 g/km, Richards predicts greater levels of downsizing than even the most ambitious of today’s engines. He said a highly boosted 800cm3 engine with an eSupercharger can develop the power and torque of a conventional 2.2-L naturally aspirated unit, "providing equally satisfying throttle response but with much lower fuel consumption."