Stage V engines in construction28 January 2021

A major change in the legislation on mobile plant engines is coming this year. The EU Stage V emissions requirements will mandate dramatic reductions in pollutants such as particulates and oxides of nitrogen (NOx), for plant of almost any size which uses an internal combustion engine, whether diesel or spark-ignition (SI). This category of Non-Road Mobile Machinery (NRMM) includes everything from off-road construction equipment to electrical generator sets, and has now all been brought under the same rules. By Toby Clark

The Stage V regulation (EU) 2016/1628 was due to come into force (for most categories of engines) in 2020, but last-minute lobbying by European trade bodies such as the Committee for European Construction Equipment (CECE) and the UK’s Construction Equipment Association (CEA) led to an extension of a year. The reason for the change was the economic slowdown due to COVID-19, which meant that existing pre-Stage V engines would not be able to be incorporated into machinery before the deadline.

Now the relevant deadline for building machines installed with pre-Stage V engines has moved to 30 June 2021, and they must be placed on the market by 31 December 2021. These were already the dates in place for engines in the band from 56-130kW, which – for the moment, at least – remain unchanged. The same dates will be enforced under UK law.

Stage V relates to engines classified by the EU in the following categories:

  • Category NRE–Engines for mobile non-road machinery, suited to move or to be moved, that are not included in any of the points below
  • Category NRG–Engines above 560kW used in generating sets
  • Category NRSh–SI (spark-ignition; that is, not diesel) engines below 19kW exclusively for use in hand-held machinery
  • Category NRS–SI engines below 56kW that are not included in category NRSh

NRE is the category of greatest interest, but it is worth pointing out that plant such as diesel generators that are permanently installed in one location – and not intended to be moved – are not in scope of Stage V.

NRE engines are split into seven bands of net power output, each with slightly different emissions requirements:

  • <8kW (diesel only)
  • 8-19kW (diesel only)
  • 19-37kW (diesel only)
  • 37-56kW (diesel only)
  • 56-130kW (diesel or spark ignition)
  • 130-560kW (diesel or spark ignition)
  • >560kW (diesel or spark ignition)

(Incidentally, the apparently arbitrary output power bands in these standards relate to older horsepower ratings: 37kW is around 50hp, 19kW around 25hp and so on.)

Some manufacturers cover the whole range of NRE engines; Perkins, for example, offers engines from 6kW to over 600kW, and at some of the ‘nodes’ between power bands offers as many as four different models.


The new requirements are being brought in because of concerns about the risk to human health, and the environment, of engine fumes. Oxides of nitrogen – in particular nitric oxide (NO) and nitrogen dioxide (NO2) – are considered significant pollutants, as they tend to combine with water to form acids, and with organic compounds to create ozone. These products in turn can cause or exacerbate respiratory problems, particularly among those with asthma.

The other pollutants that give health concerns are particulates – tiny particles of unburnt hydrocarbons or combustion products, almost always produced by diesel engines. The fine particulates known as PM2.5, measuring 2.5 micrometres in diameter or less, are implicated in some of the worst health effects. For this reason, the measurement of particulates has also changed: now there is both the mass-based figure known as PM (here a limit of 0.015g/kWh) and a number-based figure called PN – in this case, 1x1012/kWh.

The most dramatic change at Stage V comes for small engines of between 19kW and 37kW output: for these, PM must fall from 0.6g/kWh to just 0.015g/kWh. This is a difficult area, as there is no existing base of road vehicle engines to rely on. Duncan Riding, former senior technical steward at Perkins, says that these engines will go from being ‘simple diesel engines’ to incorporating turbocharging, electronic common-rail fuel injection, diesel oxidation catalysts (DOC) and diesel particulate filters (DPF). Some of these technologies have been Incorporated Into automotive and lorry drivetrains to meet Euro 6/VI emissions requirements introduced in 2014 for road-going vehicles.

Other pollutants, such as unburnt hydrocarbon vapour (HC) and carbon monoxide (CO), are also in the scope of Stage V, while carbon dioxide (CO2)levels are pretty directly related to the fuel efficiency of the engine.

In the past couple of decades, common-rail fuel injection, or more rarely unit injectors, have become standard fitments to diesel engines for road use. Either system gives highly controllable injection of fuel at extremely high pressures, improving power output and fuel efficiency. But the extremely high temperatures and pressures involved tend to produce more NOx emissions. These have been cut using exhaust gas recirculation (EGR), which reduces NOx emissions at source, but at the cost of increased fuel consumption and particulate levels.

The other emissions reduction measures are ‘aftertreatment’ systems, although some are increasingly integrated into the engine assembly.

Diesel particulate filters (DPF) reduce the PM output, and the latest ‘wall flow’ filters are said to be highly efficient, but they need to be regularly regenerated, In whichh some of the captured soot is burnt off at intervals. Perkins, which says it has logged 1.3 billion hours with DPFs, has managed to reduce the regeneration temperature from 700degC to 300degC for Stage V.

But DPFs also pick up non-carbon particles that cannot be burnt off. There are two approaches to this: either factor in occasional servicing, with downtime while the filter is refurbished or (more likely) exchanged; or specify a larger DPF, which will not need servicing for the life of the engine. Perkins has stated that it follows the latter approach.

Meanwhile, HC and CO are dealt with by a diesel oxidation catalyst (DOC), which converts them to relatively inert carbon dioxide and water.

The large reduction in automotive emissions levels that came with Euro VI brought selective catalytic reduction (SCR) to the fore – it reduces NOx emissions dramatically with little or no increase in fuel consumption.

SCR systems rely on injecting an aqueous urea solution into an exhaust catalyst. This product needs to be topped up at regular intervals, and is generally known as DEF (for diesel emission fluid or diesel exhaust fluid) or AdBlue; the products are completely interchangeable. The engine can run without DEF present, but NOx emissions will be unacceptably high so the engine management system will either shut it down or restrict its power drastically.

A number of firms offer retrofit exhaust treatment systems, primarily aimed at reducing particulate emissions. Among these is BISAF, which can fit a Johnson Matthey active DPF to an existing engine; these and other reputable aftermarket kits are accredited by the Energy Saving Trust. BISAF recently fitted such a kit to a number of crawler cranes operated by BAM Nuttall, enabling them to meet Stage IIIb so that they could be operated within the M25.

Emissions technology firm Eminox has developed a Non-Road Mobile Machinery (NRMM) retrofit emissions control system for construction equipment. The system was originally developed as a retrofit to a Junttan PM20 piling rig currently deployed on the HS2 project through Balfour Beatty. This was tested and validated by Imperial College London (Centre for Low Emission Construction) and Emission Analytics.


Generator hire company Aggreko produces its own hardware, and its latest 600kVA canopy generator employs a DPF, a DOC and SCR to achieve compliance with Stage V. This allows it to be used in Clean Air Zones such as parts of central London, where constant-speed machinery (of 37kW or above) such as generators must now meet Stage V – although a temporary delay in the local legislation due to COVID-19 means that this is not being enforced until 28 February 2021. Some other NRMM with variable-speed engines such as construction equipment is only required to meet the older Stage IV or IIIb standards.

Toby Clark

Related Companies
Aggreko UK Ltd
Johnson Matthey plc
Perkins Engines Co Ltd

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