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The new plug-in hybrid XC90 powertrain and chassis underpinnings, including the ERAD, as seen using Volvo's newly adopted Microsoft HoloLens untethered holographic design system. Volvo and Microsoft are co-developing HoloLens for automotive applications. 

Volvo's new plug-in XC90 is really a tri-motor

Unusually for an OEM, Volvo Cars is being a bit modest about its new XC90 plug-in hybrid, describing it as the T8 Twin Engine. But technically the 2016 PHEV is, in aerospace parlance, a tri-motor, having a 2.0-L gasoline engine, an electric motor, and a starter-generator.

“We could say triple," admitted Lars Lagström, the vehicle's Project Leader and Product Manager, "because we have a 34-kw motor between the engine and main electric motor. But we have sufficient power without mentioning that!”

In fact the extra 34 kW (45 hp) is used to help fill torque gaps and smooth shift changes when the IC engine is engaged, and to give an extra boost when required.

The new hybrid, part of the XC90 model plan from its inception and based on its Scalable Product Architecture (SPA), arrives eight months after the conventional models entered production. Volvo decided that time would be well spent maturing the plug-in car’s software to provide those ultra-smooth shifts.

The hybrid was conceived at a difficult time for Volvo, explained Lagström: “It was in 2008 and we knew we were no longer wanted by Ford [which owned Volvo at the time] and we had to create our own future. We needed to be independent. We had our own engine plant and we did not want to pay any licensing fees or be part of a collaborative project with another producer.”

The company became part of Geely Group in 2010 but Volvo has retained a high degree of independence within the Chinese OEM, further broadening its engineering expertise and application.

Seven drive modes

The hybrid T8 called for a wholly new electrical system, which was a challenge, said Lagström. “We knew what we needed but not how it was to be achieved – there were so many standards to consider and of course, we were looking to the future,” he said. The electrics were essentially an in-house development with specialist input from both Siemens and LG Chem.

Key decisions for the PHEV included developing an energy dense 2.0-L 4 cylinder engine (all XC90s now have 2.0-L units) driving the front wheels through an Aisin 8-speed auto gearbox and an Electrical Rear Axle Drive (ERAD), which comprises the electric motor, single-speed gearbox, clutch, differential and driveshafts.

Deciding not to use an off-the-shelf electric motor, Volvo engineers chose Siemens to design the T8’s 64-kW (86-hp) unit. It provides regenerative braking capability to help add power to the car’s 400-V battery, supplied by LG Chem. The battery is tucked into the spine of the car, between the front seats rearwards, in what would be – for a conventional vehicle – the prop shaft. Plug-in recharging via a 16-A fast charge cable can be achieved in 2.5 h; using a domestic plug takes between 3.5 h at 10A, or six hours at 6A.

Extracting maximum power from the ICE resulted in a supercharged and turbocharged solution with a target output of 210 kW (281 hp). This was upped by a further 26 kW (35 hp) during development. Total twin engine output is 299 kW (401 hp) but that little starter generator boosts this to a somewhat theoretical 333 kW (446 hp). The engine, with claimed peak torque of 400 N·m (295 lb·ft), was developed in-house and is manufactured at Volvo's Skövde plant.

The new 4-cylinder range is 30 to 90 kg lighter (66 to 199 lb) than predecessor units. Friction losses have been minimized, including use of roller-bearing camshafts, and Lagström is particularly proud that crankshafts of the 2.0-L engines can be turned by hand; quite a feature.

The engine gives the XC90 hybrid a very rapid performance of 0-100 km/h (0-62 mph) in a claimed 5.6 s. Vmax is 230 km/h (143 mph). Official combined fuel consumption calculated on the NEDC basis is 2.1 L/100 km for emissions of 49 g/km. Pure electric range is up to 43 km (27 mi).

The T8 has seven drive modes: Pure, Hybrid, Power, Save, AWD, Off-road, and Individual. Pure mode uses electric as much as possible at speeds up to 125 km/h (78 mph). Hybrid is the default mode. The car can operate in front-, rear- or all-wheel drive.

“The challenge has been achieving the smoothness needed when switching from pure electric into ICE,” stated Lagström. “Occupants should not sense engagement of disengagement of the system”—a claim that the author confirmed during a test drive. Added Lagström: “Our test program was as thorough for the Twin Engine as it is for all our models but with a much wider scope.”

GKN Automotive has launched what it describes as "the world's first disconnecting eAxle on a premium SUV" for the T8. The eAxle is tucked into a 263 x 310 x 293-mm (10.3 x 12.2 x 11.5-in) package weighing 15 kg (33 lb). GKN engineers noted that the design simplifies integration into global PHEV vehicle platforms.

Volvo’s V60 PHEV, which preceded the XC90, uses a diesel engine but Volvo decided the SUV T8 needed a gasoline ICE to meet the needs of global markets, notably the U.S. and China.

The PHEV mass burden

Volvo’s electrification program is to strengthen further, said Lagström. “In 2019 we plan to have a pure electric model based on the SPA platform that will have a range of 500 km,” he said. The SPA took four years to develop and is described by Volvo as a “cornerstone” of its ongoing $11 billion product development plan.

Battery weight remains a problem for all hybrid vehicle producers. The XC90 PHEV is 225 kg (496 lb) heavier than a comparable conventional version of the car. The battery pack alone weighs 115 kg (253 lb), and associated controls and electrical system weigh 100 kg (220 lb). According to Lagström the new car has a “favorable for handling” rear:front bias of 47:53. Weight of the conventional XC90 was reduced by up to 125 kg (275 lb) compared to the previous XC90, which helps to offset the PHEV's additional mass.

Hot-formed steel comprises some 40% of the car’s total body weight, claimed by Volvo as representing the highest percentage of this type of alloy used in the industry.

Lagström foresees improvements in electric motor efficiency and power output, provided higher temperatures can be efficiently controlled. In a related development, Volvo is working with Microsoft to jointly develop advanced design technology. Microsoft HoloLens, described by the company as the world’s first fully untethered holographic computer, could be used by customers to configure cars in 3D. Holograms are mixed into the physical world. Other auto applications are also possible.

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