Automotive Engineer

The pace of powertrain electrification at Volkswagen seems to be lagging behind some competitors. Its only showroom hybrid is the V6 Touareg SUV and its first electric vehicle, the A-segment Up, won’t appear until 2013.

But the engineers working on the firm’s concepts have demonstrated some very advanced thinking. The diesel micro hybrid from 2002 showed the potential of the lowest possible weight, frontal area, aerodynamic drag and motive power – just 6.5kW – to achieve fuel consumption of only 1 litre/100km. The L1 followed in 2009. The body retained the original tandem seating configuration but housed a much more sophisticated powertrain. An 800cc two-cylinder commonrail diesel developing 29kW/100Nm was coupled to a 10kW electric motor, powering the rear wheels through a seven-speed dry dual-clutch transmission (DCT).

Now there’s the XL1. Its powertrain is similar but improved in every way. The depth of development suggests that it will appear in VW’s next-generation passenger cars, offering remarkably low CO₂ emissions.

“Optimisation of the driveline and the vehicle has made it possible to achieve fuel consumption on the NEDC of only 0.9 litres/100km,” said Dr Michael Zillmer, VW’s head of hybrid concept development. “The insight we’ve gained from this project is certainly something we’ll use for the development of mass-production drivelines.”

The XL1 looks like it belongs in a megacity of the future but VW knows that concepts such as this have to connect with consumers as well as function as technology carriers. 

So it has a conventional seating arrangement and a dashboard that, unlike the L1’s, will be familiar to anybody who’s ever driven a Golf. With the exception of the DCT, though, most of the moving parts will not.

The combustion engine is an all-aluminium 830cc diesel developing 35kW/120Nm. For packaging and weight saving it uses plasma-sprayed bore coatings. The two cylinders run in parallel with 360° ignition intervals so a balancer shaft was essential. It’s driven from the crankshaft, and also powers the switchable water pump.

To improve mixture formation the engine has four valves per cylinder, operated by twin camshafts via a belt running in an oil bath. “This is friction-optimised and lighter and quieter than a comparable chain drive,” said Zillmer.

Zillmer alluded to the fact that variable valvetrains might be useful in meeting future emissions standards but this engine is already Euro 6 compliant without such technology.

Instead, NOx is handled by high- and low-pressure exhaust gas recirculation (EGR). The system architecture is similar to that used in the OEM’s latest four-cylinder for the US: ports for the high-pressure EGR are formed in the cylinder head casting, which also mounts the EGR valve. “This compact design means that we have very low thermal losses,” said Zillmer.

At the initial phase of the XL1’s development the most important target was the 1 litre/100km fuel consumption, but how best to hybridise the diesel engine was open to question.

The first option to be ruled-out was a micro hybrid: a simple stop-start system paired with an automated-manual transmission. This combination is light, simple and low-cost but limited in consumption reduction potential and driving comfort.