Hybrid Drives Turbocharging Reimagined: Boost Without Compromise

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SIMTOS (Seoul International Machine Tool Show)

An integrated electric motor on the turbocharger shaft provides more power and fewer emissions without the classic turbo dilemma. How it works and why the technology developed by BorgWarner opens up entirely new opportunities for hybrid manufacturers.

Every drivetrain engineer knows the problem: designing a turbocharger for maximum rated power comes at the cost of sluggish response at low speeds. Conversely, optimizing for fast boost pressure buildup limits peak performance. The industry managed this trade-off with compromises for decades, until an electric drive directly on the turbocharger shaft decoupled both sides of the equation.

The eTurbo combines an exhaust turbocharger and a permanently excited synchronous motor into a highly integrated core assembly. The result: the compressor can be accelerated even when exhaust energy is not yet sufficient. This enables boost pressure to build up at least twice as fast as without electric assistance—measurable in around 1.5 seconds of full electrification until reaching peak torque.

λ = 1 Up to the Rated Power

For hybrid engines with a stoichiometric air-fuel ratio (λ = 1), this is particularly relevant: enriching to achieve rated power is no longer permitted by regulations. The solution lies in a deliberately oversized turbocharger that does not require enrichment at the rated power point, with its sluggish response offset by the integrated electric motor. This allows for specific power densities of around 100 kW/l (approx. 2.18 hp/cu in) without compromising dynamic driving performance. A side effect: the larger turbocharger reduces exhaust back pressure, improves gas exchange in the consumption-relevant operating range, and thereby reduces fuel consumption.

In combination with an additional traction motor—as implemented with BorgWarner technology at a major European manufacturer—the transient torque build-up increases four to five times compared to a conventional combustion engine. This is not a theoretical value: the 400-V version has been in series production since 2024.

Recover Energy Instead of Wasting It

Normally, a wastegate bypasses excess exhaust energy without use. The eTurbo turns it into a resource: instead of limiting turbine power, it converts the excess energy into electricity—with up to 20 kW of continuous electrical power directly into the high-voltage battery. This protects the battery, relieves the drivetrain, and improves the overall efficiency of the hybrid system.

The benefit is complemented by the secondary air function during cold start: the eTurbo acts as an electric air pump, accelerating the catalyst light-off—a direct contribution to NOₓ reduction without additional components.

High Integration under Extreme Conditions

The technical challenge lies in the spatial constraints: turbine housings with exhaust temperatures of up to 1,000 °C (approx. 1,832 °F), high-voltage insulation systems, and power electronics are housed within just a few centimeters. The thermal management combines high-temperature water cooling for the housing and stator with targeted oil spray cooling in the rotor area. The stator's insulation strength is permanently ensured through a vacuum full encapsulation with high dielectric strength and good thermal conductivity.

For the power electronics, silicon carbide (SiC) power switches are used—with switching times of around 10 ns and high switching frequency to minimize rotor losses. The power board is directly bonded to a water-cooled aluminum cover, thermally decoupled from the turbine side. Low-profile samarium-cobalt magnets enable a compact rotor design with high temperature stability despite maximum rotational speeds of 150,000 rpm. To withstand the extreme centrifugal forces generated, carbon-fiber-reinforced polymer banding secures the permanent magnets, axially enclosed by metallic end plates. The motor control operates sensorlessly with field-oriented control and sinusoidal commutation, which reduces harmonic distortion in the phase current and further lowers rotor losses.

The bearings are positioned as spring-preloaded rolling bearings with the smallest possible distance to the stator winding heads—this shortens the shaft overhang, improves rotor dynamics, and enhances smooth running at high speeds. An integrated water cooling system for the bearing assembly ensures thermal stability under full load even with minimal oil lubrication.

An integrated temperature model calculates the conditions in critical areas in real time—from the rotor and stator to the SiC power switches. If limits are consistently exceeded, an automatic derating intervenes: the power is gradually limited before components reach the thermal threshold. The system remains operational instead of shutting down abruptly.

Secure by Design

As an emissions-relevant component, the eTurbo is subject to strict safety requirements. Core functions such as voltage and current measurement are designed in accordance with ASIL-B; additional OBD-relevant signals like speed and torque enhance operational safety. Self-diagnostics and plausibility checks run in real time in the background; a self-test during system start ensures integrity with the HV on-board network. For electromagnetic compatibility, a patented choke is used directly at the high-voltage input—a further development of the concept proven in the eBooster, which operates without additional cooling.

Scalable in Both Directions

The 400-V platform is the current series base. The portfolio can be expanded from a 48-V version for mild hybrid applications to an 800-V version for high-performance high-voltage architectures—and includes an intermediate solution without an exhaust turbine with the eBoosterPlus, available as a 400- or 800-V variant with up to 10 kW continuous and 15 kW peak power. Higher power classes are in development for the commercial vehicle sector.