VW emissions scandal: how Volkswagen's 'defeat device' works

The Environmental Protection Agency's (EPA) official notification issued to Volkswagen on 18 September sheds light on how the company managed to influence the emissions tests with some clever software.

“VW manufactured and installed software in the electronic control module (ECM) of these vehicles that sensed when the vehicle was being tested for compliance with EPA emission standards," the agency explained.

The EPA refers to what it calls a ‘switch’ that enabled the car’s ECM to know when it is being tested on a rolling road: “The ‘switch’ senses whether the vehicle is being tested or not based on various inputs including the position of the steering wheel, vehicle speed, the duration of the engine’s operation and barometric pressure. These inputs precisely track the parameters of the federal test procedure used for emission testing for EPA certification purposes.

“During emission testing, the vehicles’ ECM ran software which produced compliant emission results under an ECM calibration that VW referred to as the ‘dyno calibration’.

“At all other times during normal vehicle operation, the ‘switch’ was activated and the vehicle ECM software ran a separate ‘road calibration’ which reduced the effectiveness of the emission control system. As a result, emissions of NOx increased by a factor of 10 to 40 times above the EPA compliant levels, depending on the type of drive cycle (e.g. city, highway).”

All US vehicles must be issued with a certificate of conformity that proves they adhere to environmental standards.

To obtain a certificate of conformity, a manufacturer must submit a list of all auxiliary emission control devices (AECDs) present on the vehicle. The EPA’s definition of an AECD is “any element of design which senses temperature, vehicle speed, engine RPM, transmission gear, manifold vacuum or any other parameter for the purpose of activating, modulating, delaying or deactivating the operation of any part of the emission control system.”

The issue facing Volkswagen is the switchable system in the ECM was not declared.

“Due to the existence of the defeat devices in these vehicles, the affected Volkswagen vehicles do not conform in all material respects to the vehicle specifications described in the applications for the certificates of conformity that purportedly cover them,” said the EPA.

A defeat device is an AECD “that reduces the effectiveness of the emission control system under conditions which may reasonably be expected to be encountered in normal vehicle operation and use. Motor vehicles equipped with defeat devices cannot be certified.”

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