19/06/2002
The automotive landscape is continually evolving, with emissions regulations becoming increasingly stringent, particularly for diesel vehicles. Once hailed for their fuel efficiency, diesels now face intense scrutiny over their environmental impact. This article delves into independent testing results, revealing which diesel cars are leading the pack in cleaner emissions and, crucially, why so many others are failing the annual MOT test.
For years, official emissions figures were derived from laboratory tests, often failing to reflect real-world driving conditions. However, with the introduction of new regulations, the focus has shifted dramatically. Understanding these changes is vital for any diesel car owner in the UK.
The push for cleaner air has led to significant advancements in how vehicle emissions are measured. Historically, cars were tested under controlled laboratory conditions, which often produced results that were not representative of everyday driving. This discrepancy led to the development and implementation of the Real Driving Emissions (RDE) laws, which were set to come into force from 2017. These new regulations are designed to ensure that vehicles meet their emissions standards not just in a lab, but on actual roads, under varied driving conditions.
Independent nitrogen oxide (NOx) emissions tests conducted on hundreds of Euro5 and Euro6 diesel, petrol, and hybrid cars have painted a revealing picture. The findings indicated that a staggering just 30 per cent of the diesel cars assessed would pass the new RDE laws. This low pass rate highlights the significant challenge many manufacturers face in meeting these tougher real-world standards.
The newly published EQUA Air Quality Index by Emissions Analytics graded vehicles based on the grams per kilometre of NOx they emitted. The results were stark: out of 62 Euro6 diesels tested, only seven managed to achieve an 'A' grade. An 'A' grade signifies emissions of up to 0.08g/km of NOx, placing them well within the new legal limits. A further three vehicles were ranked 'B', emitting between 0.08 to 0.12g/km, which is still within the EU’s 2021 tolerances. Nine more scored 'C' grades (0.12-0.18g/km), which, while better than many, would only be compliant with the EU’s allowed limits up to 2019.
The Top Performers and Those That Struggled
Despite recent well-publicised issues concerning diesel emissions, the Volkswagen Group surprisingly emerged as one of the best performers in these independent tests. Several of their vehicles achieved an 'A' grade, showcasing their ability to meet stringent real-world emissions standards. These included the Audi A5, Skoda Superb, Volkswagen Passat, and Volkswagen Scirocco. BMW also demonstrated strong performance, with the BMW 3 Series earning an 'A' grade, and the BMW 2 Series achieving a 'B' grade.
Conversely, some vehicles performed exceptionally poorly, receiving 'H' grades. These included the Ssangyong Korando, Fiat 500X, and a 2013 Audi A8. An 'H' grade signifies emissions equivalent to over 12 times the legal limit, underscoring the vast disparity in emissions performance across different diesel models and manufacturers.
| Grade | NOx Emissions (g/km) | Compliance | Examples of Cars |
|---|---|---|---|
| A | Up to 0.08 | New legal limits (2017+) | Audi A5, Skoda Superb, VW Passat, VW Scirocco, BMW 3 Series |
| B | 0.08 to 0.12 | EU 2021 tolerances | BMW 2 Series |
| C | 0.12 to 0.18 | EU 2019 allowed limits | (Specific models not listed, but nine scored this) |
| H | Over 12x legal limit | Significantly non-compliant | Ssangyong Korando, Fiat 500X, 2013 Audi A8 |
The MOT Emissions Conundrum: Why Diesels Are Failing
Beyond the independent RDE tests, real-world data from the UK's annual MOT test further highlights the emissions challenges faced by diesel vehicles. BookMyGarage.com, a leading MOT and service price comparison site, conducted a Freedom of Information (FOI) request to the DVSA, revealing a significant trend: almost 1.3 million vehicles failed their MOT tests last year due to faults related to exhaust emissions.
The data unequivocally shows that diesel vehicles have experienced the most dramatic surge in failures linked to emissions. Compared to a 37% increase for petrol vehicles, diesel cars saw a colossal 240% rise in emissions-related MOT failures. This unprecedented increase occurred largely after the introduction of new regulations in 2017/18, making the last two years the highest on record for emissions failures.
Jessica Potts, head of marketing at BookMyGarage.com, commented on the findings: “The regulations have mostly impacted diesel cars, causing more than triple the number to fail, compared to petrol car failures which have only increased by a third.” This stark difference underscores the specific difficulties diesel technology is encountering under stricter scrutiny.
The Crucial Role of the Diesel Particulate Filter (DPF)
The primary driver behind this surge in diesel MOT failures is a crucial component known as the Diesel Particulate Filter (DPF). DPFs became standard equipment on all new diesel cars from 2009 to comply with Euro 5 emissions standards, although some older models might also be fitted with one. The DPF's purpose is to trap harmful soot particles from exhaust emissions before they are released into the atmosphere, thereby reducing their toxic impact on human health and the environment.
The new MOT regulations specifically target DPFs. A car equipped with a DPF will now automatically fail its MOT if there is clear evidence that the filter has been tampered with or removed. Furthermore, if any smoke of any colour – black, blue, or white – can be seen coming from the exhaust during the test, this also constitutes an immediate failure. This strict enforcement aims to ensure that these vital emissions control systems are functioning as intended.
DPFs work by collecting soot. Periodically, this collected soot needs to be burned off in a process called 'regeneration'. This usually happens automatically when the car is driven at sustained speeds, allowing the exhaust temperature to rise sufficiently. However, if a diesel car is predominantly used for short, stop-start journeys, the DPF may not get hot enough to regenerate, leading to a build-up of soot and potential blockages. A blocked DPF can significantly restrict exhaust flow, leading to reduced engine performance, increased fuel consumption, and ultimately, an emissions-related MOT failure.
The cost of repairing or replacing a DPF can be substantial, often running into hundreds or even thousands of pounds, which adds a significant financial burden for owners of affected vehicles. This highlights the importance of proper maintenance and understanding the operational requirements of your diesel car's emissions system.
Diesel vs. Petrol: A Closer Look at Emissions Failures
While the recent increase in MOT failures has disproportionately affected diesel vehicles, it's important to put these figures into perspective. Although diesels have seen a much larger *rate of increase* in failures in recent years, petrol cars are actually still *more likely* to fail their MOT due to emissions issues overall. Data shows that 4.5% of the total number of licensed petrol cars fail annually due to emissions, compared to 3.3% for diesels.
This difference can be attributed to several factors. Petrol cars, particularly older models, may struggle with emissions due to catalytic converter issues, worn spark plugs, or faulty oxygen sensors. Diesels, while having the DPF challenge, often have more robust emissions control systems when they are functioning correctly. The dramatic rise in diesel failures primarily reflects the new, stricter DPF-centric MOT rules and the fact that many existing diesel cars were not designed for constant urban, short-journey use, which can impede DPF regeneration.
Ensuring Your Diesel's Compliance: Tips for Owners
For diesel car owners, maintaining your vehicle's emissions system is crucial not only for environmental reasons but also to ensure it passes the annual MOT test and avoids costly repairs. Here are some essential tips:
- Regular Long Drives: If you primarily drive short distances, make an effort to take your diesel car for a longer run (e.g., 20-30 minutes at motorway speeds) at least once a week. This helps the DPF reach the necessary temperature for successful regeneration, burning off trapped soot.
- Monitor Warning Lights: Pay close attention to your dashboard warning lights. A DPF warning light indicates that the filter is partially blocked and needs regeneration. Ignoring this can lead to further blockage and potential damage.
- Quality Fuel and Additives: Using good quality diesel fuel can help reduce soot production. Some fuel additives are also designed to aid DPF regeneration and keep the system clean. Consult your mechanic or vehicle manual for recommendations.
- Professional Servicing: Ensure your car receives regular servicing from a qualified mechanic. They can check the health of your DPF and other emissions components, identifying potential issues before they become serious problems.
- Avoid Tampering: Never attempt to remove or tamper with your DPF. It is illegal, will result in an MOT failure, and can lead to significant fines.
Frequently Asked Questions (FAQs)
What are Nitrogen Oxides (NOx)?
Nitrogen Oxides (NOx) are a group of highly reactive gases that contain nitrogen and oxygen. They are produced during the combustion process in internal combustion engines, particularly diesel engines. NOx contributes to the formation of smog, acid rain, and can cause respiratory problems in humans. This is why strict limits are placed on their emission.
How do I know if my car has a DPF?
Most diesel cars manufactured after 2009 (to meet Euro 5 emissions standards) will be fitted with a DPF. If you are unsure, check your car's manual, look for a DPF warning light on your dashboard, or consult a qualified mechanic who can confirm its presence.
What happens if my DPF warning light comes on?
If your DPF warning light illuminates, it typically means the filter is partially blocked and requires regeneration. You should attempt a 'passive regeneration' by driving for 15-20 minutes at a consistent speed (e.g., 40-60 mph) to allow the exhaust system to heat up. If the light remains on or flashes, it may indicate a more serious blockage requiring professional attention, known as 'forced regeneration' or even replacement.
Can I remove my DPF?
No. Removing a DPF is illegal in the UK and will result in an immediate MOT failure. It is also an offence to drive a vehicle on the road that has had its DPF removed. Such modifications also increase harmful emissions and can invalidate your car insurance.
Are newer diesels better for emissions?
Generally, yes. Newer diesel cars, particularly those meeting Euro 6 standards and designed with Real Driving Emissions (RDE) in mind, incorporate more advanced emissions control technologies, including improved DPFs and Selective Catalytic Reduction (SCR) systems that use AdBlue to reduce NOx. As shown by the 'A' grade performers, these vehicles can achieve significantly lower real-world emissions.
In conclusion, the landscape for diesel vehicles is undoubtedly challenging, with stricter emissions regulations and MOT tests designed to ensure cleaner air. While some manufacturers are demonstrating impressive performance in real-world tests, many existing diesels are struggling to meet the new standards, largely due to issues with the Diesel Particulate Filter. Understanding these challenges and taking proactive steps to maintain your vehicle's emissions system are key to responsible diesel ownership and avoiding costly MOT failures.
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