All-wheel-drive capability paired with hybrid-like fuel efficiency is a key selling point of a new propulsion system comprised of two independent 80-kW (107-hp) electric motors, two inverters, and a gearbox for each rear wheel. Associated pumps are contained in the bottom of the housing as are connection points for the cooling circuit, high-voltage battery pack, and low-voltage electronic drive controls.
“What’s unique with our Agilit-e design is that two electric motors are housed within the hybrid power unit, so that means that each rear wheel can independently apply torque,” Scott Maxwell, Linamar Corp.’s Director of Business Development & Marketing, told SAE Magazines.
With minimal changes to the existing vehicle architecture, the 27.7 x 17.5 x 12.3-in (705 x 446 x 314-mm) Hybrid Power Unit is designed to be fitted to front-wheel-drive vehicles to provide an all-wheel-drive hybrid. Linamar currently is demonstrating the patents-pending system to OEMs with an eye toward production in the 2018-19 timeframe.
Vehicle dynamics benefits
To create a technology demonstrator, the company converted a front-drive Cadillac SRX using the Agilit-e system. The conversion includes two 80-kW permanent magnet electric motors, each with its own gearbox and power inverter located coaxially between the rear suspension components, according to Kevin Ledford, Linamar’s Engineering Specialist-Advanced Engineering.
The prototype electric rear axle provides 160 kW (214.5 hp) of total power and 3000 N·m (2213 lb·ft)—1500 N·m available at each rear wheel, according to the company. Such prodigious torque is enabled by an 8.4:1 reduction-gear ratio, which greatly multiplies the electric motor torque. "Our Agilit-e hybrid power unit provides the same torque to the rear wheels as the OEM stock mechanical AWD system," Maxwell explained.
The electric axle drive is capable of speeds up to 85 mph (137 kph). The prototype unit weighs 264.5 lb (120 kg).
Said Maxwell: “If the vehicle is traveling faster than 85 mph—which is the limit of the electric motors—then the unit is decoupled. But we’re also developing a two-speed gearbox, so that the eAxle can be active at full vehicle speed.”
A 330-v/9-kW·h lithium-ion battery pack provides the demonstration crossover vehicle with EV power that is sufficient for approximately 20 mi (32 km) of operating range before switching to hybrid mode. Ledford noted that the batteries are charged in three ways: as a standard plug-in, through the integrated motor-generator unit, and using regenerative braking.
The combination of a conventional front-drive system and an electric rear wheel drive also nets handling benefits. “Since our patents-pending design uses a compact electric axle with two independent electric motors, we gain torque vectoring capability, which significantly improves traction and stability,” Ledford said. Response time for torque vectoring is less than 50 ms.
Micro-hybrid and PHEV/EV applications are prime application targets for the Agilit-e, but even AWD production vehicles are candidates. “The Agilit-e system will replace mechanical components, such as the PTU (power transfer unit), propshaft, coupler, and RDM (rear-drive module), while adding power and improving fuel economy,” Maxwell said.
On the SRX demonstrator, the Agilit-e powers the rear wheels in conjunction with the vehicle's front-drive powertrain. The combination offers four different drive modes: front-wheel drive, EV only (which is RWD powered by the battery and the electric motors), hybrid, and AWD.
Technical specialists built the demonstrator in nine months, while Linamar's engineers used data streams from virtual modeling, CAE modeling, and a full-vehicle chassis dynamometer at AVL’s California Technology Center.
According to Maxwell, “the majority of the Agilit-e’s calibrations were done by the time we finished with the dynamometer. And before we started on-road evaluations, we had looked at three different types of gearboxes and a small displacement engine, although our demonstration prototype retains the SRX’s standard 3.6-L V6.”
In a recent test drive evaluation, OEM engineers asked to experience specific vehicle handling responses.
“They wanted the Agilit-e demonstration vehicle to handle differently during hard cornering," Maxwell said. "Within two weeks our engineers were able to make software changes so that the vehicle handled the way the customer wanted. The customer was able to see how easy this system is to tune,” he added.
Another Agilit-e demonstration vehicle is being prepped and is expected to be ready in October 2013. “Our first demonstration SRX is booked through 2013 with events and customer test drives in North America, so the second SRX demonstration vehicle will be devoted to the European audience,” said Maxwell.
Having technologies in the portfolio that can help automakers meet the 54.5-mpg U.S. Corporate Average Fuel Economy requirements in 2025, as well as stringent European CO2 laws influenced Linamar’s development plan for the Agilit-e.
“We were very cognizant of the timing. We would normally have considered this type of system as a two- to three-year development effort," noted Maxwell. "But when the 2025 CAFE regulations went into effect, we said 2013 is when we need to have this technology in front of customers for evaluations."
He said initial testing shows that the Agilit-e system in a hybrid AWD configuration can improve city driving fuel economy by as much as 80%.
The Agilit-e Hybrid Power Unit will be in the design validation stage for several months as Linamar engineers work to refine the gearing for noise abatement as well as make modifications to strengthen system durability.
"We expect the first production application will be in the 2018-2019 timeframe,” Maxwell said.