|Powertrain elements of the new Cross Coupé diesel plug-in hybrid concept. Click to enlarge.|
Volkswagen is revealing a diesel-engined version of its Cross Coupé concept plug-in hybrid electric vehicle at the Geneva Motor Show. (Earlier post.) The original Cross Coupé concept introduced last December as driven by two electric motors and the turbocharged gasoline direct-injection engine (TSI) and offered fuel consumption of 2.7 l/100 km (87 mpg US) equivalent to 62 g/km CO2.
The new Cross Coupé is powered by a turbodiesel direct-injection engine (TDI) and two electric motors. Combined fuel consumption in the New European Driving Cycle (NEDC) is 1.8 l/100 km (131 mpg US), equivalent to CO2 emissions of 46 g/km. Technically, the SUV is based on the new Modular Transverse Matrix (MQB) from Volkswagen AG.
Within the MQB, there are certain design parameters that are standardized across model series and brands—such as the distance between the accelerator pedal and the front wheel axis and the mounting position for all engines. There are also variable parameters that make it possible to optimally and individually develop a wide variety of models. They include such parameters as the wheelbase, track widths and wheel sizes, for example.
In the new Cross Coupé, MQB elements for a future generation of SUVs include the MQB front and rear suspensions, 140 kW / 188 hp TDI of the new EA288 engine series and a 6-speed DSG. They were merged with electrical components including the lithium-ion battery in the center tunnel, front E-motor with 40 kW and rear E-motor with 85 kW.
The Cross Coupé also demonstrates the range of variations that can be implemented based on the MQB. The modular systems used in the concept car could theoretically have been used, just as easily, to produce a full hybrid (without external interface) or a vehicle with only an internal combustion engine or only an electric drive instead of the plug-in hybrid (battery with external interface for charging) that was actually implemented.
The Cross Coupé being shown in Geneva features a top speed of 220 km/h (137 mph); 225 kW / 302 hp of power (system power) and a curb weight of 1,858 kg. It accelerates to 100 km/h in 6.5 seconds, and its TDI develops a torque of up to 400 N·m (295 lb-ft) from approximately 1,600 rpm. The two E-motors deliver 180 N·m (133 lb-ft) of torque in front and 270 N·m (199 lb-ft) at the rear. During boosting—which involves full use of both motors and the engine—the drive system supplies a maximum system torque of 700 N·m (516 lb-ft).
In pure electric driving, the turbodiesel is decoupled from the drivetrain by disengaging the clutch, and it is shut off. The clutch on the gearbox side remains closed, and the 6-speed DSG is engaged. The E mode is activated by pressing the EV mode button next to the gear shift lever.
In pure electric mode, which limits top speed to 120 km/h (75 mph), the concept car can cover up to 45 km (28 miles). In pure E-mode, the driver can choose whether the focus should be on optimizing electric driving range or dynamic performance. This is implemented by the combination of E-mode and CITY mode and the combination of E-mode and SPORT mode. In the electric driving range maximizing E-mode (CITY), only the E-motor on the rear axle powers the car. In the dynamic E-mode (SPORT), the electric drive system powers both the rear and front wheels.
Even during fast driving, the internal combustion engine is not engaged, provided that the battery is sufficiently charged. As soon as the power of the TDI is needed again, either because of the battery’s charge state or other parameters, the engine engages and is actively coupled to the drivetrain within fractions of a second.
The lithium-ion battery consists of eight modules. The battery has an energy capacity of 9.8 kWh and is located in the centre tunnel of the Cross Coupé. Power electronics, operating at a voltage of approximately 370V, are integrated in the front engine compartment, and manage the flow of high-voltage energy to and from the battery and the electric motors. Meanwhile, the 12 Volt electrical system is supplied with power via a DC/DC converter. The electric drive system does not impose any space limitations on the interior. The battery can be charged either from external power sources (230V connection) or while driving.
The driver can intentionally switch to CHARGE mode (by pressing another button next to the gear shift lever) to charge the battery via the TDI engine while driving. This mode might make sense, for example, if the driver wishes to drive in certain designated environmental zones that might be toll-free for electric vehicles operating with zero emissions.
Electric driving range and total range of the Cross Coupé are continually displayed in the instruments. An engine controller regulates the overall energy and drive management system under consideration of the specific load demanded by the driver. The driver can choose from a total of five driving modes: CITY (an eco mode with minimal fuel consumption), SPORT (high dynamic performance), OFFROAD (continuous all-wheel drive), E-mode (pure electric driving) as EV-CITY or EV-SPORT, or CHARGE (via the TDI). In parallel, the Cross Coupé utilizes different operating states, in which the motors/engine and drive axles are coupled or decoupled as a function of driving needs:
Zero emissions by ‘sailing’. As soon as the driver lifts his or her foot from the accelerator, both electric motors and the engine are shut off and decoupled from the drivetrain—provided that the battery is sufficiently charged. This is referred to as ‘sailing’. Zero emissions are generated in this case as well.
Zero emissions in battery regeneration mode. If the driver takes his or her foot off the accelerator pedal or brakes, and the battery is not sufficiently charged, the two electric motors are operated as generators to feed energy recovered from braking into the lithium-ion battery. The TDI is also shut off and decoupled in this operating state.
Boosting. When driving should be especially sporty in nature, the E-motors assist the TDI. The Cross Coupé is driven by all four wheels in this mode.
Offroad with ‘electric propshaft’. As soon as the driver chooses to activate OFFROAD mode, all four wheels are once again driven. However, in this mode the front electric motor, now supplied with energy by the TDI, is the sole source of electrical power for its counterpart at the rear axle. Since the energy for the rear electric drive is by wire and does not flow mechanically, this is referred to as an ‘electric propshaft’. Since the TDI powers the rear electric motor via the front electric motor in OFFROAD mode, the all-wheel drive system is fully functional, even if the lithium-ion battery is in a low charge state.
Driving with the TDI. If the TDI is powering the vehicle by itself, the Cross Coupé is a pure front-wheel drive vehicle. In driving states in which greater fuel efficiency is possible by load boosting, the E-motors act as generators to charge the battery. When all of the on-board energy reserves are utilized, the vehicle’s combined fuel consumption value is 1.8 l/100 km; the vehicle’s 55 litre fuel tank thus enables a theoretical driving range of 1,287 km (800 miles).
Engines of the MQB. The TDI of the Cross Coupé is an engine developed especially for use in the Modular Transverse Matrix (MQB). A key characteristic of the MQB is a uniform mounting position for all engines. Therefore, Volkswagen developed two new engine series—the EA288 series was created for the diesels, and the EA211 series for the new gasoline engines. (Earlier post.) Within the MQB, these two engine series and relevant electric motors make it possible to implement any classic, alternative or hybrid drive system.
The new TDI engines of the MQB will be available in versions with 1.6 and 2.0 liter displacement and will span a power range from 66 kW / 89 hp to the 140 kW / 188 hp of the concept car in Geneva. The diesel engines are also refined and smooth running. The 2.0 TDI of the Cross Coupé uses two balancer shafts are used. They eliminate system-induced free inertial forces that naturally occur in a reciprocating engine. Meanwhile, the toothed belt drive for the oil and vacuum pump, which operates in the sump oil, and the engine’s encapsulated injection nozzles also have a positive effect on acoustic comfort.