Seamless is a much used, and abused, word in the automotive industry—whether it refers to the gearshift quality of an automatic transmission, the deactivation and reactivation of selected engine cylinders in the cruise, or even the handover of a company from one CEO to another.
But it is a word that is particularly apposite for hybrid cars because it should describe the transition events between electric and internal combustion propulsion, and the union of the two. With its new 2015 Golf GTE plug-in hybrid, Volkswagen shows how it can be done.
The GTE complements the pure-electric e-Golf (http://articles.sae.org/13065/) and shares the powertrain of the Audi A3 e-tron: 1.4-L 110-kW (147 hp) turbocharged gasoline direct-injection engine (TSI) plus a synchronous traction motor that provides 75 kW (100 hp) driving through a 6-speed dual-clutch gearbox. With 150 kW (201 hp) of combined system power, the GTE goes some way to providing GTI levels of performance. But it also offers a claimed 50 km (31 mi) pure electric range.
Average fuel consumption on the NEDC (New European Driving Cycle) is rated at 1.5 L/100km with 11.4 kW·h/100 km electricity, which equates to CO2 emissions of 35 g/km.
Hanno Jelden, Head of Drivetrain Electronics for Hybrid Development, explained the fundamental challenge of achieving the required level of seamlessness: “Two power units, each with its own rotational energy to be tamed, mean more challenges to smooth progress," he said. The Golf GTE employs a modified version of Volkswagen’s proven DSG gearbox, which has always had to provide refinement without the benefit of a torque converter. This new DQ400e transmission designed for transverse-engine hybrid vehicle applications contains three wet clutches—two drive clutches and an engine-disconnect clutch located between the gasoline engine and the electric motor. The clutches feature purpose-designed Borg-Warner-supplied wet-friction surfaces.
The powertrain management system controls the speed and torque of both motors and the three clutches to make progress smooth under all conditions, even when a rapid re-start of the gasoline engine is required on the move when the driver requests maximum power. "Under this condition the electric motor shortens any delay by instantly adding power during the brief period when the gasoline engine is re-started,” he noted.
When appropriate, the disengagement clutch decouples the TSI from the driven front axle and shuts if off. This occurs when the car is coasting.
City car range on battery alone
As Automotive Engineering experienced during testing of the new GTE in and near Zurich, Switzerland, the transitional phases are indeed smooth, and barely noticeable by the car’s occupants and by the driver via dashboard instruments.
The GTE’s electric motor is integrated into the gearbox housing. An electro-mechanical brake servo and an electric air conditioning compressor are fitted. The TSI is placed on the left of the engine compartment (seen from the front), the electric motor in the center (in the dual-clutch housing), and the gearbox itself on the right.
As always with any car that incorporates electric motive power, weight is a significant factor. Breaking down the powertrain, the TSI engine weighs 103 kg (227 lb), the electric motor 34 kg (75 lb), the DSG 98.5 kg (217 lb) and the power electronics 12 kg (26 lb). The 120-kg (265-lb) liquid-cooled battery is tucked beneath the floor just ahead of the rear seat. Unladen weight A (EU including 75 kg driver) is 1599 kg (3525 lb); DIN curb weight is 1524 kg (3360 lb).
The GTE loses just over 100 liters (3.5 ft3) of trunk space, compared with 9.6 ft3 in non-hybrid Golf models, to package all necessary hybrid components. Because of this, the fuel tank is repositioned and is 10 liters smaller than that of a conventional Golf.
In pure electric mode, selectable via a button, the car has sufficient practical range to fulfill a city car role. With maximum whole system torque of 350 N·m (258 lb·ft)—the ICE's peak torque is 250 N·m (184 lb·ft) from 1600 to 3500 rpm—throttle tip-in is brisk and the car is capable of reaching a governed 130 km/h (80 mph).
Recharge time of its 8.7-kW·h lithium-ion battery is a claimed 3 h 45 min via a standard 240-V, 10-A European household socket or 2 h 15 min from a 240-V, 16-A wallbox or public charging station. The car's charging socket is configured for SAE's J1772 "combo connector" and is neatly hidden beneath the VW emblem in the grille.
Fighting hybrid system cost
The GTE has five operating modes. The car always starts in E (electric drive unless battery charge is low or ambient temperature is also very low). The other four modes are: GTE, Battery Hold, Battery Charge, and Hybrid Auto.
GTE mode brings into play the car’s claim to provide near-GTI dynamics, the ICE and electric motor combining to give a 0-100 km/h time of 7.6 s and a top speed of 222 km/h (138 mph; these figures are at curb weight plus 200 kg).
A problem though for the GTE is its cost. In Germany the retail price is €36,900—the equivalent of nearly $48,500 at current exchange rates and almost $23,500 more than the U.S. Golf GTI. In the U.K. it is priced at about £28,000, which includes a £5000 Government subsidy for vehicles of this type. While VW has announced that it will bring the Audi A3 e-tron to the U.S., it has not yet announced when, or if, it will import the GTE. A Passat GTE debuted at the 2014 Paris Motor Show. Audi models in line to get PHEV powertrains include the Q7, A6, and A8.
Said Jelden: “We looked a lot at reaction to cost. But it is very difficult to get a cheap powertrain plus all the electronics required and the charger. Of course, volume production will be helpful in this respect, so we start with the plug-in hybrid Golf GTE and do everything we can to reduce costs and optimize production and component design.”
Battery costs remain very high but Jelden is confident that these will fall, reducing from some €500 per kW·h three years ago to possibly €200 by 2016, he estimates: “It is a matter of being patient,” he stated.
Lithium-ion battery capacity will also improve, he believes, possibly by a factor of around 1.5 to 1.7 in about five years: “But it, too, is an evolutionary process.”
Battery technology is a continuing challenge: “Everything that is possible to have in series production in 10 years’ time we need to know about today," Jelden said. "It takes a decade from research to market for complex batteries.”
Software and control systems are both developed in-house by Volkswagen and their modular application to other hybrids in the VW Group makes a significant contribution to economies of scale.
Higher-torque-density e-motor coming
But some of this may be offset by the need for more powerful electric motors. “We will try to get higher torque in the same package – the same length. Or we can make a smaller electric motor with the same 75-kW power of the type used for the GTE.” Like aerodynamics, advances are expected to be achieved via attention to detail: “We have plans to achieve a high torque density in the package,” he noted.
Use of a diesel engine would be a contributor to higher system torque but that would put the vehicle unit cost even higher, Jelden noted. And the DQ400e DSG has an input-torque limitation of 350 N·m (258 lb·ft). It would be possible to increase this, but that might make the transmission physically bigger and heavier to create a packaging challenge, and add cost.
As for the vexed issue of fuel cell cars, Jelden observed cautiously: “Possible time to market has been constant for the last 20 years: 10-15 years.” But he believes that if battery technology continues to improve, the fuel cell solution may not be necessary.
Meanwhile, though, the VW Group continues its fuel cell program—just in case.