How To Reduce Carbon Emissions

How To Reduce Carbon EmissionsThe requirement to reduce exhaust emissions has touched almost every part of modern car development, meaning that DIY owners must consider their approaches carefully, as

Despite being a great liberator, the motorcar is also serial polluter. Whether powered by internal automobile has a negative environmental impact. As the majority of cars on our roads are ICE-powered, this article focuses on petrol and diesel tailpipe emissions of production cars, while providing an overview of the latest developments.

What do engines burn?

While ICEs can combust alternative fuels, petrol and diesel are a complex mix of chemicals, originating from not only crude oil but also bio-fuels. British diesel may contain up to 7% bio-diesel (hence the B7 pump labelling), while petrol can contain up to 5% bio-ethanol (E5), a percentage mooted to double at some point within the next twelve months. Added to this base fuel are additive packs, which differ between individual fuel brands. While you can conclude that the ‘base’ fuel is the same, regardless of the filling station, extra additive packs influence the combustion characteristics and, therefore, the resultant emissions. In the UK, fuel is supplied in both winter and summer grades, yet it is worth noting that petrol, especially, deteriorates if left for more than a few weeks. For these reasons (and more) the chemistry behind road fuels is immensely intricate.

The legislative situation

As governments realised the undesirable link between exhaust emissions and poor air quality, they drew-up statutory limits. While we do not intend to provide detailed legal analysis, knowledgeable DIYers must be aware of the situation, whether maintaining, repairing, or even modifying their cars. For specific legal advice, however, we advise that you consult a solicitor.

As an overview of today’s state of affairs in the UK, there are two main sources of legislation and two means of enforcement. The main one involves Whole Vehicle Type Approval, which assesses new car models, governed in Europe by the UNECE. The UK joined this organisation before the EEC, the forerunner to the European Union (EU). Therefore, the rules will continue post-Brexit.

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1. An illuminated engine malfunction indicator lamp (MIL) will fail the emissions on petrols used after July 2003 and post-July 2008 diesels. On certain models, faulty emissions equipment can cause other lights to illuminate. According to the RAC, these can also fail the test, because no dashboard warnings should be present.

Whole Vehicle Type Approval harmonises technical criteria between nations, including environmental standards. Besides, it classifies types of vehicle, which answers questions such as what distinguishes a motorcar from a quadricycle? The rules are incredibly detailed and all new car models must follow them so they can be sold. These include the limits that a model range must attain to achieve the relevant EURO emissions classes, which started with EURO 1 in 1992 and culminates with today’s EURO 6d. The range of gases assessed is quite detailed, many of which are not measured during an MOT Test but some of them are stated on the V5C registration certificate. Not every car is assessed for Whole Vehicle Type Approval, only certain test vehicles supplied by the carmaker to the authorities.

The second set of regulations is created by each member state, to ensure that owners of passenger cars do not modify cars in such a way that the Whole Vehicle Type Approval becomes voided. Perhaps the most relevant UK legislation for emissions is:

The Road vehicles (Construction and Use) Regulations (Regulation 61a(3)), which states that it is an offence to use a vehicle, which has been modified in such a way that it no longer complies with the air pollutant emissions standards it was designed to meet.

As an aside, this relates not to MOT standards (explained later) but those set by Whole Vehicle Type Approval. Additionally: Section 42 of the Road Traffic Act states that any person that fails to comply with a relevant construction and use requirement, or causes/permits a vehicle to be used, is guilty of an offence.

Practical implementation takes a two-pronged approach. Roadside spot-checks and random emissions measurements can occur but are more common for HGVs than passenger cars in the UK – for now. Rumours of EU representatives coming to the UK to assist with motorcar enforcement several years ago never materialised, fortunately. The second method is periodic technical inspections, such as our Ministry of Transport inspection.

The MOT Test

There are many practical reasons why an MOT cannot be a comprehensive Type Approval enforcement examination – test cost and time taken being two of them. From a legal perspective, the MOT does not define roadworthiness; Type Approval does. Therefore, consider that an MOT Test pass is not a watertight guarantee that your car is roadworthy.

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2. A huge range of additives are available to help lower emissions. Consider that they do not have to comply with any formal technical standard but this does not mean that they are useless. The main advice is to follow the dosing instructions carefully, or else you risk causing either mechanical damage, or even a vehicle fire.

For example, if your car has been modified and passes the MOT emissions standards, it might produce higher levels of other pollutants that the MOT does not measure (such as NOx), but Whole Vehicle Type Approval does. The same situation applies, if an MOT tester does not spot removed standard-fit emissions control equipment, such as a Diesel Particulate Filter (DPF), or an Exhaust Gas Recirculation Valve. You might gain a ‘pass’ but a roadside inspection may identify the illegal modification(s), which would give grounds for prosecution under breached Type Approval.

For MOT purposes, diesels have their particulate levels (‘smoke opacity’) measured, as opposed to NO and NO2 (NOx), for example. The test procedure on post-1980 cars involves the tester inserting a probe into the exhaust pipe and, once the engine has reached at least 80°C, the engine is accelerated from idle to its maximum speed, until the ‘governor’ (rev limiter) operates. This exercise is not especially mechanically sympathetic, the main risk being timing belt breakage. This is why Vehicle Test Stations display warnings to encourage the owner to replace a worn belt, which could fail during the test. If the tester believes engine damage is likely, he/she has the right to abandon the test.

The MOT Test procedures for petrol engines vary dependent on the age of the presented vehicle. Pre-August 1975 classics, pre-1988 rotary-engined and kit cars have a visual check alone for excessive smoke. Later cars without catalytic converters need a standard emissions test, whereas three-way catalysed vehicles from 1992 require a Basic Emissions Test procedure. If the car fails to pass the Basic Emissions Test, an Extended Emissions Test follows, one reason for which is to ensure that the catalytic converter has warmed sufficiently to be effective.

MOT databases and computerised equipment provide the data and procedures for the tester, which has become invaluable now that emissions requirements have become more vehiclespecific. If you wish to confirm the limits for a petrol car, you can download a 261-page-long document that explains the specifications and test procedures in greater detail: Aside from hydrocarbons (HC) and carbon monoxide (CO) emissions, Lambda (?) is also assessed on petrol engines, although the result originates from a calculation rather than a direct measurement. The figure ‘Lambda 1’ equates to 14.7 parts of air to one part fuel and is the ideal figure for perfect combustion, although this is not quite possible in actuality. A higher figure means a weak mixture; a lower one indicates a rich fuel/air ratio. While excessive fuel (or insufficient oxygen) increases pollutants, so too does a weak mixture, because the higher in-cylinder temperatures promote Nitrogen Oxides (NOx) production, although the test does not measure NOx. As Lambda does not involve the examiner interrogating the engine management, be wary that a small hole in the exhaust might not cause a major defect (and a theoretical failure) in itself – but it might draw oxygen into the exhaust system and prejudice the Lambda reading.

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3. The exhaust condition and its mountings are also inspected by the tester. Note that an exhaust in poor condition can also raise the emission levels.

Apart from ensuring that your vehicle is in good mechanical condition, the best way of ensuring a test pass is to give it a good run on its way to the station.

This ensures not only that the engine oil is warmed (which also saves the tester time) but also gets the catalytic converter(s) up to its peak working temperatures. Should your car not pass the test, inspect the emissions sheet and look for evidence that the oil temperature was measured. If that requirement was bypassed, question the tester.

To enforce UNECE Whole Vehicle Type Approval, any removed emissions control equipment counts as a major defect.

Additionally, most modern cars will not earn an MOT pass, if the Malfunction Indicator Lamp (MIL) is on with the engine running, even if tailpipe emissions are below the set limits. The DVSA, the government department that administers the MOT Test, justifies that a car with an engine management problem is likely to emit higher pollutants in real-world use. Assessing noise is a judgement call made by the tester on whether, or not, the exhaust system fitted is reasonably louder than it should be. We have also heard of some police forces carrying and using decibel meters to measure exhaust noise from the roadside.


Many people modify their cars to satisfy personal tastes. However, when owners and car enthusiasts cannot fathom out why manufacturers have designed modern cars in a certain way, emissions tend to be the reason. However, governments realise that there is little point in establishing high standards for new cars, when owners can simply remove anti-pollution equipment later for the sake of improving power, fuel consumption, and/or saving costs.

As it is illegal to modify a vehicle so that it does not comply with the emissions standards set by Type Approval, gutting a DPF and performing an ECU modification that removes it from the engine software is a typical modification that makes your vehicle unroadworthy.

Yet, this is not the only consideration. Very often, you will see aftermarket claims that a remap, or ECU chip, will increase power and decrease fuel consumption – so where is the catch? Very often, the cost is emissions. As mentioned earlier, sometimes extra fuel consumption helps to suppress toxic gas production. We spoke with several tuning firms about this topic and those companies that chose to engage with the question (rather than slamming down the telephone) stated firmly that a car, fitted with their modification, would still pass the MOT. However, considering that the legal emissions standards are set by Whole Vehicle Type Approval, and not the MOT, nobody would prove to us that their modification does not take the vehicle outside of those legal limits.

Perhaps the most obvious example is a tuning box company we contacted, which sells units for road cars that piggy-back onto the ECU, which emphasised that its boxes were road legal. Interestingly, the Vehicle Certification Agency (VCA) told us that, if a car were submitted with one of the company’s products fitted, it would fail its single vehicle Type Approval test (IVA), meaning that it is unroadworthy.

Therefore, should you wish to have your car tuned, or remapped, research your options very carefully and ask for assurances that your car would not breach its Type Approval limits, should you be stopped in the UK, or Europe, for a random roadside emissions check.

Maintenance & diagnosis

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4. Very minor exhaust leaks, such as through a failing gasket, might not be severe but they can cause a vehicle not to pass an MOT Test because of the resultant elevated emissions.

Regular servicing is key to keeping emissions suppressed. Therefore, when conducting a DIY MOT emissions failure post-mortem, ensuring that an engine is serviced and not excessively worn is a good starting point. Be wary that engine management systems can make automatic compensations to the fuel trim, which explains why certain modern cars require diagnostic intervention after routine maintenance operations are completed, not to just reset the dashboard panel. An obvious example is certain Mercedes-Benz models requiring diagnostic intervention after an air filter replacement.

The need to improve emissions was one reason for the introduction of fuel injection and complex engine management, which led to the birth of the universal On-Board Diagnostics (OBD) socket. In turn, this led to the standardisation of fault codes, so that a DIYer (and non-franchised workshop) does not always require manufacturer be spoke equipment. Diagnostics is a vast and ever-evolving topic, beyond the scope of this article. Never be led blindly by fault codes alone and do not forget the value of capturing live data as part of your fault-finding procedure.

It is also worth noting that analysing the exhaust emissions with a suitable gas analyser will help you to diagnose engine, exhaust and fuel injection problems. For reasons of space, we have not gone into further detail for now, but we plan to do so in a future article.


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5. Non-Type Approved replacement exhaust systems may not maintain the required backpressure needed to suppress harmful emissions. They may also not fit correctly. Old-stock, non-Type Approved catalysers and DPFs may also not contain sufficient expensive catalyst metals in order to work properly.

Modern cars rely on a host of sensors to not only assess the incoming air and outgoing gases but also monitor other factors that range from vehicle speed to throttle position. The precise layout varies between different models and CM’s regular Electronic Diagnostics series provides specific guidance. Should a fault code indicate a problem with a sensor, especially one that is easily-accessible, perform a live data reading to see if there is an obviously incorrect figure, prior to back-probing the sensor with a multimeter, and compare the Ohms reading against the resistance data for that sensor, if available.

The Lambda (or Oxygen/O2) sensor is, probably, one of the most important anti-pollution devices on modern petrol engines and some cars possess more than one of them. It detects the unburnt oxygen content of the exhaust gases and sends a voltage signal (typically between 0.1 and 0.9V) to the engine management system that adjusts the fuel mixture for optimum combustion. Due to the hostile environment in which they operate, Lambda sensors can deteriorate gradually but not excessively to trigger the MIL.

A typical expected lifespan for most modern Lambda sensors that incorporate a heater element is 100,000 miles. The main cause of failure is contamination by various deposits but consider that engine coolant is especially harmful, so factor for a replacement, if your car has suffered a head gasket failure and you suspect that antifreeze has entered the exhaust system. When installing a new sensor, use copper grease (or the supplied anti-seize compound) on the threads but never allow it to contact the sensor nose. Be equally careful of overtightening, which can crack the sensor.

Exhaust after-treatment systems: PETROL

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6. The modern motor car relies on a host of electrical sensors in order to keep its emissions within strict limits. Before renewing a sensor, back probe and test its resistance with a multimeter, to verify that it is really faulty and that other factors are not responsible, such as the wiring.

The Lambda sensor also protects the catalytic converter(s), located within the exhaust system, because its delicate honeycomb can melt, if exposed to an excessively-rich fuel mixture. A failed Lambda sensor, damaged injector(s), or even bump-starting might be responsible. Physical and thermal shock can also damage the converter internally, potentially rendering it useless. Once the converter has reached its ideal operating temperature (of around 400°C), it converts hydrocarbons (HC) and carbon monoxide (CO) to relatively harmless carbon dioxide (CO2) and water, which explains why the MOT emission limits for catalyst-equipped vehicles are significantly stricter than for those that did not have one fitted from new. The precious metals content (mainly platinum and rhodium), explains catalytic converters’ high scrap values and the rise in ‘cat thefts’, especially on hybrid petrols that use a greater quantity of these expensive elements.

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Exhaust after-treatment systems: DIESEL

Rather than utilising a three-way catalytic converter, complete with Lambda sensor, many diesels from the late 1990s use a two-way, unregulated Diesel Oxidisation Catalyst (DOC).

Although Diesel Particulate Filters (DPFs) became mandatory from 2009, when Euro 5 emissions standards came into force, DPFs can be found on far older cars. They work as soot traps, capturing harmful particulates before they enter the atmosphere. As soot is unburnt carbon, the DPF empties itself by super-heating the particulates until they vapourise, a process aided by the hot gases passing from the DOC into the DPF. This process is called regeneration, which occurs either naturally, during a sustained high-speed journey, or actively, by the engine management system allowing diesel to enter the exhaust to heat-up the filter artificially.

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7. As with all parts, service parts should be of at least Original Equipment (OE) quality. Spark plug condition and gap is critical to optimum emissions. While it tends to be unnecessary to adjust the gap periodically, verify that the gap of a new plug complies with the value provided in the car’s specifications.

Unless overloaded with soot, caused by a mechanical fault, or an excessive number of short journeys, the DOC and DPF are virtually maintenance-free. However, some systems require that a special additive ‘Eolys’ tank (or pouch) is replenished periodically. The cerium-based additive is injected automatically into the diesel tank, which reduces the temperatures required for successful regeneration, making the system more reliable. Never ignore dashboard level warnings; use the correct fluid and reset the relevant counters diagnostically afterwards.

To reduce NOx emissions, many Euro 6-compliant diesels use a Selective Catalytic Reduction system, a technology inherited from HGVs. This means that another oxidation catalyst is positioned between the DOC and DPF, which utilises an injector to coat it with a urea/distilled water solution (AdBlue). Its subsequent evaporation coats the catalyst with ammonia, leading to the conversion of NOx gases into harmless water and nitrogen, before they pass into the DPF.

A notable problem is caused by AdBlue being dosed directly into the main diesel tank and not into its separate reservoir. Break-downs have also been reported, where drivers ignore dashboard warnings of a depleted AdBlue tank, unaware that Type Approval law dictates that the engine cannot restart, until the fluid is replenished. For more detailed information on DPFs and SCR, consult CM’s September 2016 issue.

Exhaust Gas Recirculation (EGR)

Understandably, many technicallyminded motorists cannot fathom why rerouting dirty exhaust gases into the clean intake are advantageous in any way whatsoever. To an extent, they are correct. EGR is one cause of intake tract clogging, especially on Gasoline Direct Injection (GDI) petrol engines, which can restrict airflow and, ironically, raise emissions as a consequence. Yet, EGR reduces the temperatures within the combustion chamber, because no fuel can be burned with the gas depleted of oxygen. This reduces NOx emissions. In diesels particularly, the EGR system results in more particulates forming, a problem that the DPF must resolve.

The heart of the EGR system is a simple mechanical valve, linking the exhaust and inlet, operated either pneumatically, via vacuum pipes, or electronically, with a simple solenoid.

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8. Should any of the ignition system be in poor condition, the coils might be overloaded. Faulty coils can result in misfires that raise emission levels significantly.

Due to constant exposure to the hot exhaust gases, valve movement becomes restricted, which places extra strains on the actuating medium. While DIY cleaning is possible, it is not always effective and explains why 80,000 miles is seen by many professionals as a reasonable EGR valve life expectancy.

Even so, the EGR circuit brings certain running cost advantages. As the engine does not have to work as hard to suck fresh air into the cylinders via the air filter, because it simply opens the EGR valve in certain conditions, pumping losses reduce and fuel consumption reduces. Engine warming can also be hurried, by hot exhaust gases being rerouted into the intake, bringing further fuel efficiency advantages. Furthermore, the reduced cylinder temperatures place the turbocharger under less thermal stress. These advantages are not always appreciated by some motorists, who think that removing the EGR system (illegally) has no downside.

Even so, EGR is not perfect, although carmakers are addressing the disadvantages. One method sees separate high and low-pressure circuits, where most of the EGR’s function is provided by clean gases that enter the inlet after being processed through the catalytic converter/DPF and not before. More detailed information about EGR valves, including DIY repairs and replacement advice was covered in CM’s August 2019 back issue.

Buying parts

While Whole Vehicle Type Approval extends to new car models, certain safety critical replacement components must comply with separate but related Type Approval requirements, such as braking parts, tyres, glass, lighting and emissions equipment. Catalytic converters and diesel particulate filters had to satisfy mandatory technical standards, after inexpensive but inferior alternatives started to be sold on the aftermarket. These parts contained fewer precious metals and did not cleanse the gases effectively, compared with the Original Equipment (OE) part.

While replacement exhaust pipes and silencers are not required to comply with Type Approval in the UK, they must do so on the Continent. While you might think that non-Type Approved exhausts are a bargain, they could be a false economy. Due to potentially inferior materials and design, the backpressure might deviate beyond the OE requirements, resulting in either increased emissions (increasing the risk of an MOT Test fail), or the engine consuming more fuel. This is why certain suppliers promote their systems’ compliance as a positive marketing ploy.

Perhaps because they tend not to have to meet any legal technical standards, replacement electrical sensors can be hit-and-miss. CM recommends that you source replacement parts made by reputable brands and sold by reputable companies that have a solid returns policy. It is not a bad idea to test new sensors with a multimeter, presuming that you have access to the resistance specifications, before installation.

The future: Will emissions kill car DIY?

Aside from the significant inconvenience of having their ranges reassessed under WLTP, car manufacturers must achieve an average CO2 output figure of 95g/km per car, across their model ranges, from 2021. Should they fail to comply, manufacturers will be fined by the EU for every vehicle registered. The financial implications, which could run into many millions of Euros, are dire.

Achieving these challenging figures, while complying with the latest EURO pollution standards, as set by Whole Vehicle Type Approval, is virtually impossible with combustion engines alone, especially as some manufacturers with large sales figures outside the EU (such as Subaru) have ditched diesel. The motor industry, therefore, is focussing hard on hybrids and electrification but their technical development and profitability cannot be achieved overnight.

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9. Timing belts that run within the engine were developed to help manufacturers reach their emissions targets.

As the investments required are colossal, carmakers are widening their portfolios to bolster both revenue and profits. This includes supplying new parts for all makes to the independent motor trade, to even becoming involved with the circular economy, by acquiring large breakers yards and supplying parts from scrapped cars to repair roadworthy examples. However, franchised workshops have always been a good source of revenue but the profits tend to be retained by the main dealer, not the manufacturer. Carmakers, however, have tapped into data as a valuable revenue stream and it is getting harder for the non-repair professional to access it. For example, PSA, which owns Peugeot, Citroën, DS, Vauxhall, Opel and Chevrolet Europe has introduced charges to its Service Box system during March 2020 (starting at £5.50 per hour) for access to its replacement parts documentation, which was free beforehand.

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More worringly, certain manufacturers have been lobbying the EU to remove the OBD socket, so that independent repairers (including DIYers) cannot access emissions-related data any more. Manufacturers wish to monopolise the repair market, by having all requests for individual vehicle data to be processed wirelessly using their permissions, equipment and servers. Understandably, several bodies argue that this is anti-competitive.

Fortunately, the EU has decided that the socket will remain – for now. However, the war might not be over. It has been reported that discussions are ongoing in Brussels about how Euro 7 emissions standards can be enforced in the real-world from 2025. This could mean that each car’s exhaust emissions will be monitored continuously, meaning that an affected vehicle will stream live emissions data wirelessly all of the time.

While the consequential questions, including who should hold this data and what action might be taken against the driver of a non-compliant vehicle, remain unanswered, the issue of threatening privacy using the Trojan horse of air pollution is a serious concern. However, harming consumer choice is also a severe risk, because carmakers have a vested interest in restricting repair information. Should a vehicle break-down, the vehicle manufacturer can ‘assist’ the driver to a workshop of its choosing for repairs.

While no formal decisions have been made, at the time of writing, about who will be given permission to access an individual vehicle’s data, it is likely that car owners and DIYers will be excluded.

Worryingly, the current proposals indicate that the OBD socket will be killed off in the name of emissions, which will be a further hurdle for the DIY-orientated motorist to overcome.

EMISSIONS > where politics & engineering collide

A car manufacturer tunes engines to optimise cost, power, reliability, maintenance schedules and longevity, while meeting Whole Vehicle Type Approval targets. As emissions are set by legislators, perhaps it became inevitable that motor manufacturers and politicians became ever-closer. Post-’Dieselgate’, some quarters argued that the relationship had become too cosy.

Since the late 1990s, lawmakers decided to focus on carbon dioxide (CO2 – often called ‘carbon’, which is an inaccurate and misleading description) as a means to levy taxes. For the UK market, manufacturers scrambled to get their CO2 figures as low as they could to reduce the car owner’s tax burden, such as Vehicle Excise Duty rates. This policy advantaged diesel engines, because of their greater fuel efficiency and resultant lower CO2 advantages.

Naturally, car owners and fleets rejoiced in this win-win situation, because diesel engines tend to deliver superior miles to the gallon over petrol.

While reducing CO2 (and fuel consumption by association) is a virtuous exercise, it created a technical quandary. Pretty soon, automotive engineers had tuned their engines to achieve the optimum balance between CO2 and toxic gases.

Yet, pressure from politicians, marketing departments and customers demanded that CO2 be reduced further, which ventured beyond the technical tipping point. CO2 decreased but certain toxins skyrocketed. Effectively, political ambition overreached technical possibility, the result of which saw some car manufacturers tuning their engine management to recognise a laboratory Type Approval emissions test and operate in a way that would not be relevant to the real-world.

While most motorists did not notice that their cars were producing higher levels of toxins, they found it increasingly difficult to achieve advertised fuel consumption figures. Consequently, the controversial NEDC (New European Driving Cycle) tests, which were established during the 1980s and assessed fuel consumption/CO2/toxic emissions of new car models, were replaced by the WLTP (Worldwide Harmonised Light Vehicle Test Procedure) by September 2018. While the stricter WLTP protocol is still laboratory-based, it is designed to be more representative of the real world. Yet, its introduction caused car manufacturers a headache, because their current car range had to be re-evaluated from NEDC to WLTP standards by January 2019, which caused severe bottle-necks in new car deliveries and some car fleets had to revaluate their choices, because the CO2 figures (and, therefore, the taxation levied) were higher under the stricter regime.

Even so, when What Car? magazine tested 56 new car models, it found its fuel consumption results were, on average, 5.4% worst than the WLTP Combined fuel consumption data. CM’s advice is that you should still treat WLTP figures not only as a guide but also as a comparison tool. However, from an air quality standpoint, the Real Driving Emissions test (RDE) accompanies the WLTP as an integral part of Whole Vehicle Type Approval emissions testing, which ensures that certain limits are not breached during a driving exercise away from the laboratory.


Carbon Dioxide (CO2) Is a nonpoisonous greenhouse gas. Unlike many other tailpipe fumes, it is related directly to fuel consumption. Interestingly, burning more fuel (and increasing CO2 emissions) can reduce other poisonous gases. It is not assessed at MOT Test time but it is measured for Whole Vehicle Type Approval.

Carbon monoxide (CO) is a highly poisonous and odourless gas, caused by incomplete combustion. In petrol engines, excessive CO might be caused by the mixture being ignited but not burning completely. Diesel ICEs tend to produce less CO than petrols, because the combustion tends to take place with excess oxygen, which explains why diesel CO is not considered at MOT Test time.

Hydrocarbons (HC) While excessive HC can have the same causes as high CO, it tends to be caused by unburnt fuel. For petrols, an incorrect mixture, or a misfire can cause high HC readings.

Nitrogen Oxides (NOx) While both petrols and diesels produce NOx, the ‘dieselgate’ scandal associates it more with compression ignition engines. NOx refers primarily to gases that result from nitrogen and oxygen reacting together, the rate of which depends on heat and pressure. NOx contributes to acid rain and smog but is not measured at MOT Test time.

Particulates (PM) These carbon particles (soot) vary in size and can penetrate into the lungs and bloodstream and are carcinogenic. Direct-Injection petrol engines (GDI) produce significant quantities of PMs, which explains why the EURO 6 particulate limits for petrols and diesel were the same. Diesel engines have been fitted with particulate filters for over a decade but they are only starting to appear on petrols.

Nitrogen (N2), Oxygen (O) and Water (H20) can also exit the tailpipe but are of no environmental harm. Many DIYers confuse the natural condensing of water within the exhaust pipe as a cylinder-head gasket failure. With the exhaust system warmed after a run, most of the water escapes invisibly as steam.

Watch this video for How To Reduce Carbon Emissions

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