Last reviewed 14 January 2016

Richard Smith looks at the heavy duty diesel engine’s role in poor air quality and how operators can tackle the problem.

The VW emissions scandal will not have concerned operators of large goods and passenger vehicles but it has again focused attention on an issue that is very much relevant to them as users of diesel engines: air quality in our towns and cities, particularly the levels of oxides of nitrogen (NOx). On 5 January 2015 it was announced that London’s Oxford Street had already exceeded the national allowance for NOx for the whole year, four days before Mayor Boris Johnson’s consultation on an ultra-low emission zone for central London closed.

Just one week into 2016, London has already breached annual pollution limits, only weeks after the Government published its plans to clean up the UK’s air. On 8 January 2016, Putney High Street in West London breached annual limits for nitrogen dioxide (NO2), a toxic gas produced by diesel vehicles that has been linked to respiratory and heart problems.

Emissions components

Total exhaust emissions from both petrol and diesel engines consist of a number of different substances, almost all of them having a potentially harmful effect on humans.

  • Carbon monoxide (CO) has the effect of preventing oxygen from reaching the body tissue and can quickly lead to death.

  • Unburnt hydrocarbons (HC) have a narcotic effect and irritate the mucous membranes but are also a main contributor to photochemical smog.

  • • NOx also produce irritation of the respiratory system and damage to lung tissue.

  • • Particulate matter (PM) can lodge deep in the lungs and result in emphysema and cancer.

Of these, CO and HC are particularly associated with the petrol engine and have been dramatically reduced since the adoption of the catalytic converter from 1991. These pollutants are not normally a problem associated with the diesel engine because of the more efficient nature of its operation.

NOx is most predominantly a problem for diesel engines. NOx is also produced by petrol engines but is rendered harmless by the catalytic converter. PM is exclusively a pollutant associated with diesel engines (unless a petrol engine is in very poor condition). It was the NOx measurement that the VW defeat software was intended to cheat, though the tactic adopted by VW could not have worked on a heavy duty diesel engine because the test is carried out in a different way.

History of emissions standards

EU standards for exhaust emissions have been applied to light vehicles since 1970 and involved limits for only CO and HC until 1977 when NOx was also included. In these early years the limits were easily met by refinement of existing technology but a watershed came in 1991 with what became the Euro 1 regulations (see Note below) which cut the limits so drastically and with such a short timescale for implementation that the only practicable solution was exhaust gas after-treatment by means of the three-way catalytic converter. This effectively stalled the development of lean-burn technology which would have cut fuel consumption and reduced emissions of all exhaust products including, critically, carbon dioxide.

EU limits for heavy duty engines were first introduced much later, in 1988, followed by a set of Euro I regulations in 1992 and Euro II in 1996. Euro III followed in 1999 and manufacturers managed to meet the limits set by all of these regulations by developing the existing technology.

Euro IV and V limits followed in 2005 and 2008. These dramatically reduced the maximum limit for PM and it was expected that meeting these limits would require the use of a Diesel Particulate Filter (DPF), as became common practice on light duty diesel engines under Euro 4 and 5 regulations. Instead manufacturers achieved the limits for PM by tuning engines for low fuel consumption with the unfortunate side effect of increasing NOx. Unhappily, while this solution worked for the larger particles it has far less effect on the smaller, more damaging, ones. To meet stricter limits on NOx after-treatment of the exhaust gas using selective catalytic reduction (SCR) was then required.

Carbon dioxide (CO2)

CO2 has, until now, not been regulated and is not harmful to humans except in great concentrations. Indeed, CO2 is essential to plant growth and green plants absorb large quantities of it, giving out oxygen. Unfortunately, CO2 became implicated as the cause of supposed global warming and in response to environmental lobby groups politicians pursued a policy of encouraging a switch from petrol to diesel for light vehicles. While this did not affect operators of trucks and buses who were almost exclusively already using diesel it was all too successful in dramatically increasing the number of diesel-powered cars on the road, with a consequent increase in total NOx and PM.

Options for operators

Faced with strict low emissions zones (LEZs) in town and city centres, or at least heavy financial charges, what can operators do? The problem is one that faces goods and passenger operators equally since both have to operate in urban environments and the London LEZ in particular is very widely drawn. The Oxford Street problem quoted earlier, we should note, is exclusively a passenger operator problem since the road is closed to all but buses and taxis.

Modified diesel engines

Upgrading the entire fleet to new vehicles to the latest Euro VI standards is an obvious solution, though not necessarily an economic one given the long service life of heavy goods and passenger vehicles — and Euro VII will be along soon enough. In the meantime, thoughtful scheduling will help, using only the latest specification vehicles on journeys where there is likely to be an air quality problem and keeping the older ones for other work.

Even if a new vehicle is not bought, it is still possible to fit a later-specification engine to an existing one and this may prove cost-efficient. If the right conditions are met the vehicle can then be issued with a Reduced Pollution Certificate (RPC) which will not only allow it to operate in the LEZ but also achieve a reduction in Vehicle Excise Duty (VED).

A cheaper, less comprehensive, option is to retro-fit an approved particulate filter to the existing engine. While this will not qualify it for an RPC, it may enable a Low Emission Certificate (LEC) to be issued that will allow it to operate in the LEZ — though without the reduction in VED that the RPC brings.

Particulate filters solve the problem of particulate matter emissions for the current London LEZ, but from 2020 it is proposed that the LEZ becomes an ultra-low emission zone (ULEZ) with restrictions on NOx as well – requiring a minimum of Euro VI. It is possible to retro-fit a reduction catalyst to limit NOx emission but there is no indication that there will be an equivalent to the LEC for NOx in the ULEZ.

However, the basic problem comes from the nature of the combustion in the diesel engine so perhaps the best solution is to look for an alternative source of power.

Electric power

Many vehicles, both light and heavy are already using hybrid power trains — smaller internal combustion engines augmented by electric motors, the batteries of which are charged by kinetic energy recovered during braking. The smaller engine goes a long way to reducing emissions but does not solve the problem completely. In some cases, the vehicle can be propelled by the electric motor alone for zero-emission running but this is unlikely to be practical for heavy vehicles.

Full electric cars are gaining popularity as new battery technology allows an acceptable range between full recharges. New flash charging technology, which allows a bus to be recharged in 15 seconds every time it is at a bus stop where the permanent infrastructure can be installed, makes full-electric urban bus services a practical possibility but cannot apply to goods vehicles.

While electric traction is usually referred to as “zero-emission”, that only applies to the actual point of use. Somewhere there is a power station generating the electricity for recharging and that may not be so clean. This is not the only problem: old, dependable fossil-fuel power stations are rapidly being decommissioned in the hope that sun and wind will be able to provide sufficient capacity. There are already warnings that the margin between demand and supply is unacceptably thin, but add in many more electric vehicles recharging, not to mention electrification of the Great Western Railway and HS2 and HS3 and it is difficult to see where the power will come from. It is already being said that even when the over-schedule and over-cost GWR project is finally complete the trains will not be able to run at their design speed because of power shortages.

Alternative fuels

Perhaps the best short to medium term (or even long-term) option is to be found in the use of compressed natural gas (CNG). Liquefied petroleum gas (LPG) conversions have long been available but CNG offers the attraction of a permanent and renewable supply of fuel from waste products that is not merely carbon-neutral but may even be carbon-negative. The CNG engine uses spark-ignition and requires little in the way of exhaust treatment to meet the most stringent limits. It is also notably quieter.


It is certain that operators will be driven constantly towards ever-stricter limits on exhaust emissions and the adoption of new power sources. Careful fleet planning and vehicle replacement policies will be required if costs are not to run out of control and vehicle utilisation is to be maximised. Operators making long-term investment decision will also want to know that government has a consistent and coherent policy.

Note: Successive EU emissions regulations are identified by Arabic numerals for light duty engines (both petrol and diesel), eg Euro 6; and by Roman ones for heavy duty engines, eg Euro VI.