In official tests, cars are gently accelerated to 30mph (50kph) and then slowed down several times on rollers in a laboratory. This is then repeated to faster speeds. All diesel cars passed. The UK testers then reversed the order, starting with the faster part.
The VW group car did not recognise this as a test and failed, the other cars still passed. This is good news, but when driven in the same way outside, nitrogen oxides in the exhaust doubled in 33 of the 38 cars and some emitted over five times more. It seemed that the colder the weather, the more exhaust pollution.
To examine this, the German investigation included the standard laboratory test, but at 10C instead of the normal 20-30C. Less than 20% of the cars passed. This temperature anomaly had also been noticed in tests on seven cars in Norway last year. The extra pollution was not connected with the engine starting in the cold.
Manufacturers responded that the exhaust clean-up was restricted when driving in colder weather to protect the engine from condensation, but the International Council on Clean Transport concluded that this is unnecessary until temperatures drop below 5C.
The UK’s health burden from nitrogen dioxide is estimated as 23,000 deaths annually, but we now learn that exhaust clean-up on most diesel cars made after 2009 is only optimised for driving on warm, sunny days.