Automakers charge more for hybrid-electric vehicles than standard cars because hybrids need costly batteries as well as motor/generators and power electronics to cut fuel consumption. But what if car manufacturers could achieve the same gain more cheaply by using mechanical flywheels instead?

The concept of a hybrid car that can tap the kinetic energy of a rotating flywheel, has long been a popular topic for study by automotive engineers. The simplicity, relatively light weight, and high power density of "mechanical batteries" attracts their interest, as does the fact that they avoid losses from the need to convert between chemical, electrical, and mechanical energy forms. But despite the potential advantages, few flywheel hybrids have ever hit the road.

That began to change a couple of years ago when Porsche and Jaguar fielded high-performance flywheel hybrid prototypes, and then Audi Sport’s flywheel hybrid racecar won the 2012 Le Mans 24-hour endurance race. Volvo recently announced that it is teaming with Flybrid Automotive to develop an S60 sedan with a 60-kg (132-lb) flywheel KERS (kinetic energy recovery system) that can boost power by 80 hp (60 kW) as it reduces fuel use. The flywheel unit, which the companies claim could cut costs by a quarter to a third, could be ready in four or five years.

But perhaps the flywheel hybrid’s best chance for widespread adoption lies in a long-term effort by engineers at Punch Powertrain, the Belgium-based transmission builder, to develop a cost-effective flywheel hybrid drivetrain for the Chinese and emerging Asian markets, where the heavy, stop-and-go nature of urban traffic favors hybrid use.

Mechanical hybrid

Punch Powertrain's bare-bones mecHybrid drivetrain concept uses a steel flywheel for energy storage and a push-belt CVT (continuously variable transmission) for power transmission, according to Alex Serrarens, manager of business development. The company, which is located in Sint-Truiden, was once part of ZF Getriebe.

In the mecHybrid system, a CVT push-belt "charges" a flywheel—which essentially is a rotor that spins inside a partial vacuum—with recovered braking energy or engine power when it is operating at low load. The engine would shut down at stops and the stored rotational energy would then be siphoned off to accelerate the vehicle from stationary. Once the car is rolling, the restarted engine would take over. Thus the flywheel can take on the propulsion load when the engine is least efficient—at low speeds, for example.

This simple mild-hybrid configuration, which Serrarens said could yield fuel savings of around 15% depending on the drive cycle, contains only low-cost mechanical components including the steel rotor, gear sets, and compact clutches. A commercial product might cost a fifth that of a typical hybrid system.

Long collaboration

The mecHybrid-drivetrain stems from a research project that Serrarens and two research colleagues, Bas Vroemen and Roëll van Druten, began in the late 1990s when the trio were doctoral mechanical engineering students at the Technical University of Eindhoven. The work was part of the EcoDrive project, a partly government-funded cooperative research project conducted by project-owner Van Doorne’s Transmissie (now Bosch Transmission Technology); the Netherlands Organization for Applied Scientific Research (TNO), a nonprofit R&D company; and the university.

Batteries have considerable storage capacity (high energy density), but comparatively little power, so it takes many batteries to overcome inertia and accelerate a car from a static position. The reverse is true for flywheels, which have a lot of power (high power density), but little capacity.

“Flywheels had always been a topic of interest at Eindhoven,” Serrarens recalled. “But they had always been too complex and costly, and there were persistent performance issues.”

With incentives from Bosch, which wanted to expand push-belt applications, he said, “we designed a new flywheel combination that used a push belt, which was becoming a mature technology—low-cost and efficient."

Zero-inertia powertrain

The three doctoral students developed what they called a “zero inertia powertrain” that addressed the principal performance problem with CVT-based flywheel systems: hesitation and response lag. By combining a flywheel and CVT with a power-splitting planetary gear set and clutches that could disconnect the CVT from the drivetrain as needed, they achieved much better response as well as improved fuel economy, said Serrarens. “The flywheels of the time tended to be elaborate, carbon-fiber systems,” he noted. “Instead, we focused on simple, solid steel flywheels—as small as possible.”

The current cylindrical flywheel unit, which is 150 mm (5.90 in) in diameter and weighs about 20 kg (44 lb), “spins at 35,000 rpm on almost catalog [off-the-shelf] bearings,” he explained. Beyond producing the new design, “we learned a lot about flywheels, which helped as we continued to collaborate with Punch in 2009, 2010, and afterward.”

The joint R&D project eventually included Punch Powertrain (CVT and hydraulics design), Bosch (advanced hydraulics), SKF (bearings, seals, lubrication), CCM (rotor dynamics) and the team at Eindhoven (energy management and control). In the meantime, the trio established a start-up company called Drivetrain Innovations (DTI). In 2003, DTI bought back the "zero inertia" patents from Bosch and began working on packaging, functionality, and cost issues. Then last year, DTI was acquired by Punch Powertrain.

The relatively straightforward mechanical hybrid system poses some challenges regarding energy management, he said, in terms of optimally directing the power flows at the system level. Other challenges include hydraulic design, control strategies for CVT slip control and optimal operation, and exhaust emissions during cold-engine start-ups.

A 'hybrid hybrid'

The Punch Powertrain team has also developed several other potentially cost-effective flywheel hybrid combinations, and among them Serrarens thinks that “a less exotic electro-mechanical KERS unit could be the most popular.”

The emKERS module is an “e-axle technology” and would be installed on the rear axle of a front-wheel-drive car. It would create what is in some sense “a hybrid in a hybrid” because it can tap energy from both a 50-kW (67-hp) steel flywheel as well as a lithium-ion battery and 10-kW switched-reluctance electric motor. The power of the flywheel and electrical machine are combined using the power-split planetary gear arrangement.

“With the flywheel, we can raise the power by three to five times, to 60-kW peak power,” Serrarens said. Meanwhile, the electric motor is used “to steer the torque.” The drivetrain system, which was developed for 48-V operation, offers high boost power using low electrical power and low voltage.

“We’re at the sample stage with emKERS,” Serrarens said, noting that it could be on the market before 2020. The resulting through-the-road hybrid vehicle could improve fuel consumption 15%, plus more from the stop-start system.

Although Serrarens is hopeful of success, he is also realistic: “The market still has to get used to flywheels. No one has had any experience with this technology.”