UK Emissions Testing: Prohibited Strategies Uncovered?

In an era where environmental responsibility is paramount, the Driver and Vehicle Standards Agency (DVSA) plays a crucial role in ensuring that vehicles on UK roads meet stringent safety and environmental standards. Through its dedicated market surveillance unit, the DVSA meticulously inspects vehicles, trailers, and equipment, providing transparency and accountability in the automotive industry. The 2024 report details comprehensive vehicle and component testing, with a keen eye on identifying any 'prohibited emission strategies' that could undermine environmental efforts. But what exactly do these tests involve, and what did the DVSA uncover?

The DVSA's Vigilance: Ensuring UK Road Compliance

The DVSA's market surveillance unit operates with a clear mandate: to safeguard public safety and environmental quality by verifying that vehicles adhere to established standards. This annual report is a testament to their commitment, offering a detailed insight into the robustness of modern vehicle emission control systems. Unlike initial type approval tests, these surveillance tests provide a real-world snapshot of vehicle performance after they have been sold and are in use. This proactive approach helps to ensure that manufacturers' claims align with actual vehicle behaviour on our roads, providing confidence to consumers and contributing to cleaner air.

How Vehicles Are Chosen for Testing

To ensure a representative overview of the UK's diverse vehicle fleet, the DVSA carefully selects a sample of the most popular vehicle types. In 2024, their testing encompassed a broad spectrum, including: diesel cars, petrol cars, light-duty vans, plug-in hybrid electric vehicle (PHEV) cars, hybrid cars (also known as self-charging hybrid cars), heavy goods vehicles (HGVs), and public service vehicles (PSVs). Vehicle selection was primarily based on UK sales figures, supplemented by additional models to guarantee a wide representation of manufacturers. Crucially, all vehicles were sourced from hire fleets and bus operators, meaning manufacturers had no opportunity to prepare or modify them before testing. This independence is key to the integrity of the surveillance process, ensuring that the vehicles tested are truly reflective of those driven by the public.

A Deep Dive into Emission Test Procedures

Every vehicle subjected to DVSA scrutiny underwent rigorous testing to confirm compliance with European emission standards, as adopted into GB legislation. Light-duty vehicles (cars and light vans) were assessed against Euro 6 standards, while heavy-duty vehicles (HGVs and PSVs) were held to Euro VI. The specific tests conducted varied depending on the vehicle type, but all were designed to provide a thorough understanding of emission performance under various conditions.

Legislative Standards: Euro 6 and Euro VI

Euro 6 and Euro VI are the latest European emission standards for light-duty and heavy-duty vehicles, respectively. These standards set strict limits on pollutants such as Carbon Monoxide (CO), Hydrocarbons (HC), Nitrogen Oxides (NOx), and Particulate Matter (PM) or Particulate Number (PN). Adhering to these standards is a legal requirement for all new vehicles sold in the UK, and the DVSA's market surveillance helps to verify ongoing compliance throughout a vehicle's lifespan.

Testing Light-Duty Vehicles: Cars and Vans

For petrol and diesel cars, hybrid vehicles, and light vans, a combination of laboratory and on-road tests was performed to assess their emission control systems and tailpipe emissions. Before any testing commenced, each vehicle underwent a thorough inspection for defects that might compromise the emission control system, and its fuel was drained and replaced with standard laboratory reference fuel to ensure consistency.

The WLTC Laboratory Test

The Worldwide Harmonised Light Vehicle Test Cycle (WLTC) is a legislative test conducted in a controlled laboratory environment. Each vehicle underwent a standard pre-conditioning test specific to its approval before being 'soaked' in a temperature-controlled room (20°C to 30°C) to ensure uniform temperature throughout, including engine oil and coolant. Emissions were then measured from a cold start. This test has defined legislative limits that a vehicle's emissions must not exceed. For Plug-in Hybrid Electric Vehicles (PHEVs) and hybrid vehicles, both 'charge sustaining' (battery state of charge maintained) and 'charge depleting' (battery charge reduced over multiple cycles) tests were performed. The final weighted result for PHEVs combined data from both cold laboratory tests, using utilisation factors defined in legislation.

The WLTP (Worldwide Harmonised Light Vehicle Test Procedure) replaced the older New European Drive Cycle (NEDC) in September 2017 for measuring official fuel consumption, CO2, and pollutant emissions. It became mandatory for all new internal combustion engine cars in September 2018. WLTP tests are designed to provide a more accurate indication of real-world fuel consumption and emissions, lasting 30 minutes, covering over 23 km, with an average speed of 45 km/h and a maximum of 131 km/h.

Real Driving Emissions (RDE) On-Road Tests

Complementing laboratory tests are the Real Driving Emissions (RDE) on-road tests. These are conducted on public roads using Portable Emissions Measuring System (PEMS) equipment, measuring pollutants as the vehicle is driven in real-world conditions. Each test typically lasts 1.5 to 2 hours, covering urban, rural, and motorway driving during the day and in normal traffic. For transparency, 'raw' RDE results in the report were not post-processed or adjusted for 'extended' conditions (e.g., temperatures below 3°C, which would typically apply a 1.6 factor). For PHEVs, two RDE tests were completed: a charge depleting test from a cold start, followed by a charge sustaining test from a hot start. It's worth noting that for some older NEDC-approved vehicles and early WLTP-approved vehicles (before September 2019), RDE was not a legislative test but an additional measure by DVSA to understand real-world performance.

Advanced Laboratory RDE Testing

A 'double RDE test' was also carried out in a laboratory setting using a dynamometer, with a consistent 'soak' and ambient test temperature of 0°C for all vehicles. For petrol cars and vans (including hybrids but excluding PHEVs), the first RDE cycle mirrored the road RDE drive pattern, while the second cycle was performed in reverse order (motorway, rural, urban). For diesel vehicles (including hybrids but excluding PHEVs), the first RDE cycle used a consistent speed profile to evaluate urban phase performance, with the second cycle also in reverse order. PHEV laboratory RDE tests involved a charge depleting condition from a cold start, followed by a charge sustaining condition from a hot start, both following the same profile as their road RDE counterparts. These tests were performed back-to-back, without switching off the ignition or engine, and the second test initiated within approximately 30 minutes of the first.

Special Considerations for PHEVs and Hybrids

PHEVs also underwent specific 0°C WLTP tests in the laboratory. Each vehicle was soaked to approximately 0°C and then subjected to testing in charge sustaining mode. The comprehensive testing regime for PHEVs and hybrids reflects their unique powertrain characteristics, ensuring that their emissions performance is accurately assessed across a range of electric and internal combustion engine operating conditions.

Testing Heavy-Duty Vehicles: HGVs and PSVs

HGVs and PSVs were subjected to on-road tests using PEMS equipment. Similar to light-duty vehicles, they underwent a pre-test check for defects and fuel replacement. Each heavy-duty vehicle completed two tests: one with a 10% payload and another with a 90% payload. These tests involved driving for approximately 2 hours 30 minutes over a public road route encompassing urban, rural, and motorway sections under normal traffic conditions. Emissions measured were normalised to the laboratory engine test using carbon dioxide as a measure of 'work done,' allowing calculation of mass emissions per unit of energy (kWh). The results were then compared against the legislative conformity factor (1.5 for NOx, HC, and CO), which accounts for the wider range of operating conditions and greater uncertainties in real-world measurements compared to dynamometer tests.

Independent Testing Locations

All emissions tests were conducted at commercial emission test laboratories across the UK. Crucially, the DVSA avoided using laboratories owned by vehicle manufacturers, ensuring the impartiality and independence of the testing process.

Engaging with Manufacturers

When vehicles exhibited poor performance, manufacturers were given the opportunity to explain the test results and describe the emission control strategies they employed. These discussions provide invaluable insight into why a vehicle might meet legislative limits under official test cycles but potentially show significantly higher emissions in other, non-legislated situations. This open dialogue is vital for continuous improvement in vehicle emission control.

Decoding the Results: Are Prohibited Strategies Present?

The central question addressed by the DVSA's market surveillance is whether non-regulatory tests indicate the use of prohibited emission strategies. A prohibited emission strategy would involve a device or software designed to reduce emissions during official tests but increase them during normal driving conditions, essentially 'cheating' the system. The good news from the 2024 report is clear: for every vehicle tested across all categories, the DVSA found no reason to believe that any vehicle was non-compliant with its legal emissions performance requirements, even when subjected to non-regulatory testing. All vehicles were compliant with required tailpipe pollutant emission limits under both WLTC and RDE legislative tests. The non-regulatory tests conducted aimed to understand if emissions behaviour changed significantly outside legislative conditions, which *could* indicate such strategies, but the comprehensive findings consistently showed compliance.

Diesel Car Performance Snapshot

The diesel cars tested, including models like the Audi A4, Citroen C4, Land Rover Defender, and Mazda CX60, consistently demonstrated compliance. For instance, the Audi A4, a 2022MY 1968cc diesel, showed strong performance across all tests.

As the table illustrates, the Audi A4's emissions were well below the legislative limits for both WLTC and RDE tests, a pattern consistent across all diesel vehicles examined.

Petrol Car Performance Snapshot

Petrol cars, such as the BMW 218 Gran Coupe, Hyundai i10, Hyundai Tucson, Kia Picanto, Skoda Karoq, SsangYong Korando, and Vauxhall Astra, also met all required emission limits. For example, the Hyundai i10, a 2022MY 998cc petrol, performed admirably.

Again, the results show significant headroom against the legislative thresholds, indicating robust emission control.

Light Van Performance Snapshot

Light vans, including the Ford Ranger (2021 & 2023 models), MAN TGE, Nissan Navara, Peugeot Boxer, Renault Kangoo, Vauxhall Vivaro, and Volkswagen Amarok, similarly passed all tests with flying colours. The Ford Ranger (2023MY) provides a good example:

The Ford Ranger's results consistently demonstrate compliance, reinforcing the effectiveness of its emission systems.

PHEV Performance Snapshot

Plug-in hybrid electric vehicles (PHEVs) like the Bentley Bentayga, Ford Kuga, MG HS, Skoda Octavia, Suzuki Across, and Vauxhall Astra also showed compliance. The Ford Kuga PHEV (2022MY) illustrates how these vehicles perform across different modes:

The PHEV results, particularly the low emissions in charge depleting mode, highlight the benefits of their electric capabilities, with overall weighted and charge sustaining results well within limits.

Hybrid Car Performance Snapshot

Hybrid cars, including the Lexus RX450, Renault Arkana, Renault Clio, and Suzuki Swift, also met their emission targets. The Lexus RX450 (2022MY) is a representative example:

Hybrid vehicles demonstrated their ability to maintain low emissions even with the internal combustion engine engaged, staying comfortably within legislative boundaries.

HGV and PSV Performance Snapshot

Heavy Goods Vehicles (HGVs) and Public Service Vehicles (PSVs) were also found to be compliant. The DAF LF260 HGV (2022MY) showed strong performance, with its conformity factors well below the 1.5 limit:

These results confirm that heavy-duty vehicles are also meeting the stringent Euro VI standards under real-world conditions, a critical achievement for urban air quality.

A Note on NOx Measurement Faults

Across various vehicle categories, the DVSA's testing partners identified a fault with the post-processing software that affected the integration of nitrogen dioxide (NO2) with nitrogen oxide (NO) when calculating the final bagged Nitrogen Oxides (NOx) result. To ensure accuracy and transparency, the NOx pollutant figure in this report was taken from the continuous dilute (conti-dilute) measurement, rather than the potentially flawed result from the sample bags post-test. This proactive correction demonstrates the DVSA's commitment to accurate reporting.

Understanding Emission Reduction Technologies

Modern vehicles employ sophisticated technologies to manage and reduce harmful exhaust emissions. Understanding these systems is key to appreciating how vehicles meet stringent legislative requirements.

Exhaust Gas Recirculation (EGR)

Exhaust Gas Recirculation (EGR) is a technique used to reduce Nitrogen Oxides (NOx) emissions by displacing intake air with a controlled amount of inert exhaust gas. This lowers combustion temperatures and reduces oxygen availability, thereby inhibiting NOx formation. While EGR can sometimes affect particulate emissions or fuel economy, well-designed systems minimise negative impacts. There are several types: internal EGR (controlled by exhaust valve timing) and external EGR (where exhaust gas is piped back to the inlet manifold, regulated by an EGR valve and the ECU). Cooled external EGR, where recirculated gases pass through a cooler, allows for wider application and further temperature reduction. Both high-pressure and low-pressure EGR systems are commonly used, sometimes in combination.

Diesel Oxidation Catalyst (DOC)

A Diesel Oxidation Catalyst (DOC) promotes the oxidation of various exhaust components, including carbon monoxide (forming carbon dioxide), hydrocarbons (oxidising to carbon dioxide and water), and the soluble organic fraction of particulate matter. Beyond regulated pollutants, DOCs also control non-regulated harmful species like aldehydes and polycyclic aromatic hydrocarbons (PAHs), and reduce exhaust odour. DOCs can also convert nitric oxide (NO) into nitrogen dioxide (NO2), which, while more harmful on its own, can be beneficial when used upstream of a Diesel Particulate Filter (DPF) or Selective Catalytic Reduction (SCR) system by aiding DPF regeneration and enhancing SCR performance.

Diesel Particulate Filter (DPF)

The Diesel Particulate Filter (DPF) is a critical component for trapping particulate matter (soot) from diesel engine exhaust. As soot accumulates, exhaust back pressure increases, necessitating a process called 'regeneration' to burn off the trapped particles. This can be achieved by increasing exhaust temperature (e.g., through late fuel injection to reach 600°C), by adding a fuel-borne catalyst to lower the soot combustion temperature to 350-450°C, or through passive regeneration where NO2 generated by a DOC continuously helps to burn off soot at lower temperatures.

Frequently Asked Questions (FAQs)

Q: What is a 'prohibited emission strategy'?
A: A prohibited emission strategy, often referred to as a 'defeat device', is any hardware or software that detects when a vehicle is being tested and alters its emission controls to perform better during the test than it would under normal driving conditions. The DVSA's non-regulatory tests are specifically designed to detect such discrepancies.

Q: Why does the DVSA conduct non-regulatory tests?
A: While legislative tests ensure that vehicles meet standards under specific, controlled conditions, non-regulatory tests simulate a wider range of real-world driving scenarios and environmental conditions (like cold temperatures). This helps the DVSA to identify if a vehicle's emission performance significantly degrades outside the official test cycle, which could suggest a prohibited strategy is in use.

Q: What is the difference between WLTC and RDE tests?
A: The WLTC (Worldwide Harmonised Light Vehicle Test Cycle) is a laboratory-based test performed under controlled conditions to measure emissions and fuel consumption accurately. The RDE (Real Driving Emissions) test, on the other hand, is conducted on public roads using portable equipment, measuring emissions under actual driving conditions, including varying speeds, loads, and terrains, providing a more realistic emission profile.

Q: What are Euro 6 and Euro VI standards?
A: Euro 6 is the latest European emission standard for light-duty vehicles (cars and light vans), setting limits for pollutants such as NOx, CO, HC, and particulate matter. Euro VI is the equivalent standard for heavy-duty vehicles (HGVs and PSVs). These standards are progressively tightened to reduce vehicle emissions and improve air quality.

Q: What is a 'conformity factor'?
A: A conformity factor is a multiplier applied to legislative emission limits during real-world driving emissions (RDE) tests for heavy-duty vehicles. It accounts for the inherent variability and greater measurement uncertainties associated with on-road testing compared to controlled laboratory tests, allowing for a realistic assessment of compliance in diverse operating conditions.

Conclusion: Upholding Emission Integrity

The DVSA's 2024 market surveillance report provides a reassuring picture of vehicle compliance on UK roads. Through a comprehensive and independent testing regime, encompassing both legislative and non-regulatory procedures, the agency has consistently found that tested vehicles meet their required tailpipe pollutant emission limits. Crucially, despite conducting a number of non-regulatory tests specifically to identify any significant changes in emissions behaviour that might indicate prohibited strategies, the DVSA has found no reason to believe that any of the examined vehicles were non-compliant with their legal emissions performance requirements. This rigorous oversight by the DVSA underscores a firm commitment to cleaner air and transparent automotive standards, providing confidence that vehicles sold in the UK are indeed performing as they should, contributing positively to environmental goals.

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