Petrol cars twice as likely to fail MOT on emissions

The latest MOT test data shows that 26.2% of all cars tested fail on excess exhaust emissions – and surprisingly petrol cars fail more than twice as often as diesels in this part of the annual roadworthiness test.

According to the official Vehicle Operator and Standard Agency (VOSA) figures, which cover all of 2010 and the first nine months of 2011, 9.7% of petrol cars failed to meet MOT emissions standards compared to just 3.9% of diesels.

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The analysis has been carried out by fuel additive “provider” Redex, which naturally has an interest in the findings, since its products are designed to breakdown engine deposits and improve efficiency. But never-the-less, its interpretation of the results is interesting.

According to research and development manager, Bruce Ellis, there are two main reasons why petrol cars may be more prone to MOT emissions failures.

Firstly, petrol cars tend to be used for shorter journeys than diesel cars. Diesels also generally operate at lower engine speeds, leading Ellis to suggest their components are under less stress (though they also produce more torque as a rule, so we’re not entirely convinced about that one).

More pertinently, the MOT emissions test is far more stringent for petrol cars than diesel cars. Says Ellis:

“Petrol cars are assessed for a high number of gases and hydrocarbons in the exhaust emissions, while diesels are only tested for smoke levels. This means, relatively speaking, diesels probably have an easier emissions test than petrol-engined cars.”

49.47 million MOT tests were carried out over the study’s 2010-2011 period, with 14.43 million MOT test failure reasons recorded. 3.78 million of these failures were emissions that exceeded permissible limits.

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