2018-11-01

Real Driving Emissions: Meaning and Monitoring

Road traffic is a key contributor to ambient air pollution in many parts of the world today. Throughout the years, different legislative efforts have been made to reduce pollutant emissions, both at national and international levels. One of the recent steps taken, is to introduce limits on emissions when driving in actual traffic, also called Real Driving Emissions, RDE. Accordingly, such exhausts have to be monitored with on-board measurement equipment.

When vehicle emissions first came in focus, most efforts were spent on improving fuel economy, thereby reducing CO₂ emissions as well as reducing the levels of carbon monoxide (CO), nitrogen oxides (NO_X), and unburnt hydrocarbons (HC), the latter sometimes expressed as total hydrocarbons (THC) or non-methane hydrocarbons (NMHC). The solution was introduction of exhaust gas recycling (EGR) in combination with three-way catalysts. This has been standard in vehicles in most parts of the world since the 1980s or 1990s. Later on, regulations were also introduced resulting in further reductions of emissions of NO_X and particles (PM) by means of filters and NO_X traps, and more recently (in the case of NO_X) catalytic converters (SCR) based on urea (“AdBlue”) injection. These additional pollutant reduction methods have been widely introduced in vehicles during the most recent decade.

However, emission limits are meaningless unless it is also specified under which conditions they are to be met. Initially, the legislators therefore also devised precisely regulated simulations of driving patterns to be executed in laboratories. An example of this was the “New European Driving Cycle” (NEDC) for passenger cars, first introduced in the 1980s. However, as the emission limits over the years became tougher to meet, it became evident that some car manufacturers optimized the cleaning methods for the laboratory tests rather than actual driving conditions. As a result, in particular NO_X and PM pollution levels often increased in urban environments. Part of this owed to a general increase in number of vehicles, but it was also clear that the individual vehicles, also the new ones, emitted pollutants way above the limits met at the tests.

To address the discrepancies between tests and real life driving, a new test cycle called WLTP (Worldwide harmonised Light vehicle Test Procedure) was designed in the 2000s, and it was supplemented with a standard to measure emissions during real driving conditions (the “RDE” standard) in 2011. The standards were further refined and clarified in subsequent years. Within the European Union, starting 2017, all new light vehicle type approvals for pollutant emissions are conditioned on compliance with emission limits obtained under the WLTP and RDE driving standards.

Hence, reduction of emissions of air pollutants from vehicles is achieved both by lowered emission limits and by more demanding test cycles such as the WLTP and RDE standards, under which the limits apply. By example, in the European Union, the emission limits were lowered by replacing the Euro 5 with the Euro 6 standard in 2014. Initially, the Euro 6 standard was to be met under the NEDC test procedure, but it was effectively tightened further in 2017 by the introduction of the WLTP and RDE standards. Meeting the current limits using the WLTP and RDE standards is referred to as “Euro 6d temp”. "Temp" owes to the RDE part of the tests currently having a higher limit than the WLTP part. The RDE limit will be lowered in 2020. Vehicles meeting this lower limit will be said to comply with “Euro 6d”.

To measure emissions during real driving is more of a challenge than doing so in a laboratory. The measurement equipment has to be ruggedized, light-weight and compact enough to fit in or on the vehicle, its power consumption must be low enough to allow self-powering during several hours of testing, it must be quick and easy to mount and dismount, and it must be safe and simple to operate. OPSIS has utilized its decades of experience from static emissions monitoring in the industry to develop a portable emissions monitoring system (PEMS) for vehicles, meeting these requirements. It is based on the proven components utilized in industrial environments. The system, called RD100, fulfils the monitoring needs of Real Driving Emissions tests.

Author

Bengt Löfstedt
OPSIS AB

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