The Volkswagen Golf TwinDrive Plug-in hybrid prototype, introduced in mid-2008, has recently been made available to selected automotive journalists for short test-drives. It features a unique power train architecture which incorporates elements of both series and parallel hybrid configurations. The car's diesel engine is coupled to a 30 kW / 250 Nm mnotor/generator, and both connect to the front axle through a single speed reduction gear 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 (an all-wheel variant uses a smaller second motor in front and two direct-drive hub motors at the rear wheels).
At speeds up to 35 mph the TwinDrive Golf is propelled solely by the electric motors. If the battery has sufficient charge, the engine/generator stay off and remain decoupled from the axle. A full charge of the battery can provide up to 30 miles at sub-35mph speeds.
If the driver's control-input calls for greater speed or the battery charge falls beneath a pre-determined level, the engine-generator set will activate, feeding current to both the electric motors and the Lithium Ion battery pack. The car will then operate in series-hybrid mode, much as the Chevrolet Volt does.
At still higher speeds or a call for sharper acceleration, a clutch connects the diesel engine directly to the front axle, while the electric motors continue to provide added power as necessary. While most parallel hybrid systems like this one incorporate a continuously variable transmission to blend the output of the two types of motor, The TwinDrive diesel produces enough torque to dispense with this level of complication. A simple reduction gear, in concert with assistance from the electric motors, provides adequate power delivery across the entire range of speeds.
The dash-display toggles through three options, one of which shows a virtual control dial allowing the driver input concerning how and when electricity is used. An e-mode button can be used to keep the car operating in all-electric mode as long as the battery level allows.
Aside from the quieter operation, the TwinDrive Golf drives much like a standard Golf. The internal combustion engine ignition is barely audible, and there is no perceptible shudder when it starts or during direct-drive engagement. Acceleration is comparable to a standard Gulf with the 1.6l engine.
Passenger space is not compromised in any way, but the space that would otherwise be available beneath the cargo-bay floor is taken up by the lithium-Ion battery.
The plug-in hybrid system of the TwinDrive Golf makes an interesting contrast to that of the Chevrolet Volt. Volkswagen has chosen to implement a similar strategy of providing an initial period of battery-only travel, predicted to cover the typical commuting distance of most drivers. However, that EV operation will only take place at urban speeds; highway driving activates the diesel engine to drive the wheels directly. The Volt, by contrast, might travel some distance on the highway under battery power alone but at that speed it seems likely that it's projected 40 mile range would be cut drastically. At some unknown distance the range extending engine would be called on to begin generating electricity to power the car.
So at the point of ICE ignition the Golf will begin delivering power from the engine directly to the wheels mechanically while the Volt has to convert it to electricity then re-convert that electricity to mechanical work at the wheel. This would seem to favor the Volkswagen in terms of efficiency.
Or at least this is what Volkswagen is apparently betting, albeit in a cautious way; unlike the Volt which begins ramping up production in 2010, the TwinDrive Gulf will not be in show-rooms anytime soon. Instead, Volkswagen will begin a four-year field test of 20 vehicles before committing to any real production.
[SOURCE:green fuels forecast]
