Volkswagen has developed and deployed a gasoline engine that offers diesel-like fuel economy but is significantly cheaper to buy. Currently applied in the new Golf and Golf Variant (earlier post), the 1.5 TSI ACT BlueMotion delivers 96 kW / 130 PS with combined fuel consumption of 4.8 l/100 km (49 mpg US). The NEDC extra-urban figure drops as low as 4.0 l/100 km/h (59 mpg US).

The 1.5-liter engine (earlier post) is 10% more efficient than comparable gasoline engines; Volkswagen achieves this through the new TSI Miller combustion process and the use of a high-tech turbocharger with variable turbine geometry (VTG).

Efficient TSI Miller cycle. The TSI Miller cycle combustion process entails the use of optimized valvetrain control (early intake valve closure) combined with higher compression and turbocharging. It innovatively controls combustion of the fuel and air mixture, which leads to the improvement in efficiency of up to 10%. This enables further reduced fuel consumption and emission levels plus powerful torque even at low engine speeds.

Compared with other gasoline engines, the intake valves of the 1.5 TSI ACT BlueMotion are open for a shorter time; the intake valves are closed during the intake stroke at a time when the piston still has a little way to travel and is moving further downward. This leads to a reduction in throttling losses under partial load.

Meanwhile, the mixture also benefits from increased expansion in the knock-limited range. This expansion reduces pressure and temperature within the mixture. The ignition timing thus can be retarded for the sake of better efficiency while reducing the risk of uncontrolled combustion processes. It now becomes possible to increase in the compression ratio to 12.5:1, with an accompanying increase in efficiency.

Variable turbine geometry. As the intake valves are open for shorter periods in the TSI Miller cycle, the cylinders must be filled with the cylinder charge at a boosted charge pressure. To do this Volkswagen utilizes a turbocharger with variable turbine geometry (VTG), which makes it possible to adjust increases in turbine output.

Chargers of this kind combined with gasoline engines are usually found only in high-performance cars. Volkswagen now becomes the first manufacturer to use a VTG charger in a mainstream gasoline engine car. By way of comparison: a conventional turbo system diverts part of the exhaust gas stream needed for the forced induction away from the turbine via a wastegate valve as a function of its operating state. Energy is lost in this process.

In the 1.5 TSI ACT BlueMotion, however, it is possible to route 100% of the exhaust gas stream via the turbine at any time by means of VTG. By doing this, the combustion chamber is always optimally supplied at high turbocharger efficiency levels. Fuel consumption and emission levels go down, and responsiveness improves.

The four-cylinder engine is particularly conservative in its use of fuel whenever the Golf is being driven with good anticipation and within the partial load range. Then the new engine either deactivates some cylinders (Active Cylinder Management - ACT) or completely shuts itself off (eco-coasting micro-hybrid mode).

• Active Cylinder Management - ACT. Whenever possible, Active Cylinder Management deactivates the two inner cylinders. It always does this at times when power demand is low. The deactivation function operates within an engine speed range of 1,400 to 4,000 rpm and at vehicle speeds up to 130 km/h (81 mph). The driver is made aware of the barely perceptible changeover from four to two-cylinder operation by the “2‑cylinder mode” notification in the instrument display.

  
  

• Eco-coasting. The 1.5 TSI ACT BlueMotion achieves an extra level of innovation when combined with the seven-speed DSG dual clutch gearbox. When so equipped, the drivetrain can be classified as a micro-hybrid system: as soon as the driver lets the Volkswagen coast—i.e., predictively releases the accelerator pedal for conditions ahead or is going down a slight slope—the engine management system switches off the TSI completely and decouples it from the drivetrain by disengaging the clutch.

  In this phase, the engine no longer needs to be “dragged along”; the use of the vehicle’s kinetic energy is thus optimized. When coasting, the Golf repeatedly switches over to a zero-emissions mode for brief periods of time. In practice, the temporary engine shut-off reduces fuel consumption by up to 0.4 l/100 km. The micro-hybrid system utilizes the 12-volt electrical system architecture and a compact lithium-ion battery to supply all relevant systems in the car with energy during the phases when the engine is inactive.

Futures. Volkswagen will launch further derivatives of the 1.5 TSI ACT BlueMotion in global markets. Fewer cylinders, smaller engine displacements and significantly greater power are all equally conceivable. Furthermore, engine developers have designed the basic technical matrix to enable mild-, full- and plug-in hybrid drive systems and natural gas engines as well.