Volkswagen's emissions-cheating device: how it works and how to fix it

Volkswagen is expected to begin recalling millions of diesel vehicles in coming months to remove a device that is aimed at cheating emissions tests, and to bring them into compliance with anti-pollution rules in each affected country.

Some 11 million cars are affected worldwide and the German auto giant is still drawing up a timetable for the massive repairs.

What's wrong with the vehicles?

The company has admitted to fitting the diesel vehicles with software that turns on emission controls when the car is being tested and then turns off when it is on the road.

There are two kinds of affected vehicles.

One group is fitted with a catalytic converter that reduces the amount of nitrogen oxide emitted by treating it with aqueous urea.

Another type is fitted with a system that stores nitrogen oxide temporarily when the car is undergoing tests, before releasing the harmful levels of the gas when the car is back on the road.

How will the cars be fixed?

Some vehicles will only require a software fix, but others will need total overhauls including installing new catalytic converters and injector systems.

Injectors are a key structural component of the engine, and play a big part in the formation of exhaust gas. Modern high-performance systems can significantly reduce the generation of nitrogen oxide.

In their search for solutions, engineers will not only have to worry about the availability of spare parts but also the amount of space available in the vehicle in cases, for instance, which involve major reequipping of injector systems.

VW chief Matthias Mueller has said: "We need not just three solutions, but thousands" to fix all 11 million vehicles.

Will the car's performance be affected?

Mueller says performance would not necessarily be compromised by the repairs, although he concedes that the cars may lose "three to five kilometres per hour" of their maximum speed.

Among issues VW is looking at is compensation for customers, if there are discrepancies with the car's advertised performance following the repairs, said the group's US chief Michael Horn.

How long could it take?

Mueller expects to begin recalling European cars in January and for all cars to be fixed by the end of 2016.

But Horn gave a longer timeline for US cars, saying the process could take "one, two years minimum" to fix the 430,000 vehicles affected.

Repairs were likely to take between five to 10 hours per car, he said, but there were other factors to be taken into account, including the availability of the car parts.

How much will it cost?

Customers will not have to pay for the repairs.

The bill for Volkswagen, however, could not be estimated at this stage.

The company has set aside 6.5 billion euros as a preliminary estimate for the repairs alone, but also faces fines in several countries and potential damages arising from lawsuits.

Mueller has conceded that the final cost would be far higher and said the group was reexamining all investments.