(Wolfsburg, Germany, October 7, 2009) In mid-2008, Volkswagen unveiled the first prototype of its Golf TwinDrive plug-in hybrid based on the MK V Golf. The TwinDrive configuration uses an innovative new hybrid powertrain architecture and Green Fuels Forecast had a chance to drive it this week near the company's headquarters in Wolfsburg Germany.

Volkswagen has followed a different path with the TwinDrive to create a system with elements of both series (such as the Chevrolet Volt) and parallel (Toyota Prius, Ford Fusion) hybrid configurations. A 1.5-liter TDI diesel engine is coupled to a 30 kW / 250 Nm motor/generator. This engine-generator combination can is connected to the front axle through a single speed reduction gear set and electronically controlled clutch.

A second larger 85 kW / 650 Nm traction motor is directly coupled to the same reduction gear and the front axle in the front wheel drive variant. An all wheel drive variant is also available which uses a 30 kW motor on the front axle and a pair of 30 kW hub motors at the rear wheels.

A 12 kWh lithium ion battery pack is located beneath the rear cargo floor of the five door hatchback. At speeds up to approximately 35 mph the TwinDrive Golf is electrically propelled with drive torque coming from the motors. If the battery has sufficient charge, the engine/generator can stay off and decoupled from the axle. A full charge of the battery can provide up to 30 miles of urban speed zero emissions driving.

If the driver's power demand as determined by the accelerator position is too high or the battery state of charge is too low, the engine-generator will start up. In this charge sustaining mode, the engine generator can maintain the battery level and provide electrical energy to the traction motor. In this mode, the TwinDrive functions as a Chevrolet Volt-like series hybrid.

As the vehicle speed climbs above 35 mph, the clutch engages, coupling the engine-generator to the axle allowing it to drive the vehicle directly. In this mode the system runs like a typical parallel hybrid. The traction motors can also provide a propulsion boost to the internal combustion engine.

Typical power-split systems use a continuously variable transmission to blend the power delivery from the electric and internal combustion sources. The Twin-Drive dispenses with this more expensive hardware, taking advantage of the high torque characteristics of both the diesel and electric drives.

Since the torque multiplication provided by lower gears is unnecessary in this case and propulsion from a standing start is executed by the electric drive a simple reduction gear is used. This is the same approach used in a pure electric vehicle. The single gear ratio is approximately equivalent to the sixth gear in a conventional Golf.

Once the vehicle is in motion and the engine is running, the combination of the engine torque and electric boost provides adequate acceleration for pretty much any driving needs.

Green Fuels Forecast had the opportunity to sample the TwinDrive in Germany. The car starts silently like most hybrids and all electric cars. The information display in the center console provides the usual power-flow diagram that can be found in most hybrid vehicles. This display shows a schematic of the vehicle powertrain that indicates whether the power is flowing from the motor or engine to the wheels or back to the battery for regeneration. 

The display also provides some additional pages with other helpful information, to help maximize efficiency. One page shows a graphical map of power output vs speed with all of the operating modes. This allows the driver to see where the powertrain is operating and then adjust behavior to reduce fuel consumption.

Another page shows a virtual control dial that gives the driver some control over when and where the electrical power is used. For example, in some European city centers, there are congestion charges which are waived for zero emissions vehicles. For example someone driving into the center of London could use the dial to preserve 15 miles of electric range for the end of the trip. Once the zero emissions zone is reached, the E-mode button can be pressed to force electric operation and the driver can have confidence that their is enough energy in the battery to complete the drive.

When the E-mode button is not enabled, the engine only stays off if the driver uses a very light foot on the accelerator. This is very similar to other conventional hybrids. A more aggressive apply will cause the engine to start up driving the generator and sustaining the battery charge. The E-mode will prevent this for drivers that are staying in an urban environment as long as the battery has sufficient charge.

Aside from the quieter operation, the TwinDrive Golf drives much like a standard Golf. The engine start-up could be detected only from the sound of the engine. There was no perceptible shudder when the engine started or stopped and the engagement of the engine drive was also seamless. Acceleration felt comparable to what could be expected from a Golf with a 1.6-liter internal combustion engine.

All of the additional hardware is packaged in the car without compromising passenger volume. Volkswagen representatives expect future development of the lithium ion battery pack to reduce its size and mass so that it fits under the rear seat rather than using the space under the cargo bay floor. Volkswagen will also be testing the TwinDrive system with a gasoline fueled TSI (turbocharged and direct injected) four cylinder engine. That configuration would avoid the extra cost of the diesel engine.

While Volkswagen's claimed 30 mile electric range does fit within the needs of the vast majority of drivers, it does not incorporate highway speed electric driving as the Chevrolet Volt does. On the other hand, the TwinDrive architecture could potentially be more efficient at higher speeds than the Volt especially when the extended range EV is in charge sustaining mode.

Because the Volt is always electrically driven, the charge sustaining mode requires two energy conversions, mechanical to electrical in the engine-generator and electrical to mechanical in the traction motor. Both conversions involve losses. However, GM officials are publicly confident that the Volt will have excellent efficiency in charge sustaining mode. Only time will tell which manufacturer is correct. Volkswagen will soon begin a four year field test of twenty TwinDrive Golfs with its utility and government partners.