A plug-in hybrid is a plug-in hybrid, right? Things aren’t quite that simple. Although Volkswagen’s XL1 concept does abide by the same concept behind the Chevrolet Volt -- arguably one of the most publicized plug-in hybrids to hit the market -- the execution does differ notably between the two.
At the heart of the Volt lies a unique powertrain configuration that blends aspects of parallel and series hybrid systems together. Power is split through an electrically variable transmission, but the electric motor -- even when the battery’s charge is almost depleted -- primarily drives the front wheels. The engine typically serves as a generator, sending electricity to that motor. However, in certain conditions (i.e. hard launches, high-speed cruising, etc), it too can help provide motive force to the drive wheels.
The XL1, however, uses a pure parallel system, similar to that used in the Touareg Hybrid. The package sandwiches an electric motor between a 0.8-liter, two-cylinder diesel engine, and a seven-speed DSG dual clutch transmission, which is then coupled to the drive axle.
Volkswagen believes this configuration is more efficient because of the mechanical link between the power source and the drive wheels. When functioning as an EV, the motor has seven different forward gears to move the car -- in contrast, the Volt only has two ratios. Additionally, when the XL1’s battery pack is depleted, the TDI engine is directly connected to the wheels. VW says the engine is more efficient performing in this manner than it is functioning as a generator.
Interestingly, this powertrain configuration may change in the near future. Jürgen Leohold, VW’s director of research & development revealed to us that VW does plan on building a small batch of XL1s by 2013, but it is unlikely the vehicles will actually be fitted with the small diesel two-cylinder shown here. Instead, the quasi-production models will likely use a turbocharged, direct-injection gasoline two-cylinder.
Not only does such an engine already exist within VW’s engineering labs (it was once rumored to be used in the Up! family of small cars), but Leohold says there are a number of advantages to going gasoline instead of diesel. Although the TSI is 10 percent more thirsty than the comparable TDI, it needs less equipment to meet emissions regulations. Ironically, that makes the engine less expensive to install, although the TDI’s design is reportedly mechanically simpler.