Clever new petrol engine tech makes VWs new 1.5 as efficient as a diesel, the automaker claims

Clever new petrol engine tech makes VW's new 1.5 as efficient as a diesel, the automaker claims

Volkswagen claims its recently launched 1.5TSI ACT micro-hybrid 130PS petrol engine is as efficient as a diesel.

Features include temporary cylinder deactivation and complete engine shut-off plus variable turbine turbocharger technology.

The automaker describes the new engine, out first in Germany in the Golf as offering "diesel-like fuel economy but is significantly cheaper to buy".

This new 1.5 TSI ACT BlueMotion, to give it its full name, develops 96 kW/130 PS and is currently offered for the Golf hatchback and estate car with availability in other VW (and, likely, group) models coming later.

Combined fuel consumption is 4.8l/100km and the 'real world' NEDC test extra-urban is 4.0l/100 km/h (or, with DSG twin clutch automatic: 4.1 l/100 km).

"Every driver can achieve low fuel consumption figures with this 1.5 litre engine, as it is 10% more efficient than comparable petrol engines," the automaker said.

This is achieved using the Miller combustion process and turbocharger with variable turbine geometry (VTG). The new I4, within the partial load range, either deactivates some cylinders (Active Cylinder Management or ACT) or completely shuts itself off ('eco-coasting' micro-hybrid mode).

ACT, whenever possible, deactivates the two inner cylinders and always does this when power demand is low. Deactivation operates within the 1,400rpm to 4,000rpm range and at vehicle speeds up to 130 km/h (about 75mph). The driver is advised by a '2-cylinder mode' notification in the instrument display.

When combined with the seven-speed DSG dual clutch gearbox, this powertrain can be classified as a micro-hybrid: as soon as the driver releases the accelerator to coast, the management system switches the engine off completely and decouples it from the drivetrain by disengaging the clutch.

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

In parallel with the cylinder deactivation and complete engine shut off, the Miller cycle combustion process uses optimised valvetrain control (early closure of inlet valves) combined with higher compression and turbocharging. This controls combustion to improve efficiency by up to 10%.

This further reduces fuel consumption and emission levels plus delivers powerful torque, even at low engine speeds, according to VW.

Compared with other petrol engines, the inlet valves are open for a shorter time, a critical difference, because the inlet valves get 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 can be retarded for the sake of better efficiency while reducing the risk of uncontrolled combustion processes and compression ratio can be raised to 12.5:1 which also increases efficiency, reducing fuel consumption and CO2 emissions.

As the inlet valves are open for shorter periods in the Miller cycle, the cylinders must be filled with the cylinder charge at a boosted charge pressure using a turbocharger with variable turbine geometry. This makes it possible to adjust increases in turbine output.

VW claims chargers of this kind combined with petrol engines have to date been found only in high performance sports cars and it claims to be the first automaker to use a VTG charger in a mainstream petrol engine car.

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, losing energy in the 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.

Volkswagen plans to launch further derivatives this clever new 1.5 world wide.

It said: "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."