Motortrades Insight Magazine Online Subscribe About Us Contact Us Motortrades Insight - Motoring News, Articles & Interviews Home Finance Reviews Under The Hood Motorsport Business Profiles Archive Jobs I am looking for in Get your business listed Related Tags: mot test, MOT, petrol mot test, diesel mot test, redex Petrol cars are twice as likely to fail MOT tests as diesels By: Danny Hewitt Ref:A0783 Published: 5 November 2013

THE MOST recent MOT test data shows that excess exhaust emissions levels account for more than a quarter of all MOT failures. 

Significantly, the proportion of petrol cars tested that failed to meet the required emissions standards (9.7 per cent) was more than double the proportion of diesel-engined cars tested (3.9 per cent).

The figures have been released by Redex, which produces additive products that minimise engine emissions by breaking down the carbon deposits that can build up over time, impeding the efficient, clean running of vehicle fuel systems. Redex evaluated the data that is submitted to the Vehicle Operator and Services Agency (VOSA) by every MOT test facility in the UK. 

The latest data available – covering the full year of 2010 and the first nine months of 2011 – shows that 47.97 million MOT tests were taken over that period, and that 14.43 million MOT test failure reasons were recorded. Of these failure reasons, more than a quarter, 3.78 million, were attributed to emissions that exceeded permitted levels.

Bruce Ellis, research and development manager at Redex, said: “There are a number of possible explanations for the marked difference in MOT emissions failure rates between petrol and diesel cars. 

“These include the different driving cycles that traditionally are undertaken. Namely, petrol cars are more likely than diesels to be chosen for frequent shorter, urban trips, whereas diesels are more likely to be chosen if the vehicle is to be commonly used for longer, motorway-based journeys. Also, diesels operate at lower engine speeds and are therefore a little less stressed than higher revving petrol motors.

“But the most obvious explanation is likely to be the MOT test itself. Petrol cars are assessed for a high number of gases and hydrocarbons in the exhaust emissions, while diesels are tested only for smoke levels. This means that relatively speaking, diesels probably have an easier emissions test than petrol-engined cars.

“For modern as well as older cars – petrol or diesel – regular use of a quality fuel system additive reduces carbonaceous deposits that can restrict the flow of fuel through an engine’s injectors. Blocked injectors can reduce the efficiency of combustion, leading to increased exhaust emissions.  Obstructed fuel systems can also increase fuel consumption.”

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