How to measure vehicle emissions

New vehicles are traditionally measured in the laboratory for checking their emissions. Official laboratory testing is performed on fixed trips and fixed conditions. Historically the advantage of this was the repeatability of testing across vehicles. However, it has been shown that these testing conditions poorly reflect real-world driving, where emissions were much higher and fuel economy much worse. In an effort to address this problem, official testing procedures have become more extensive over the years with a shift to on-road testing. The aim of this being to better reflect real-world emissions performance during type approval emission testing.

Laboratory testing

When testing in the laboratory, the vehicle is placed on a chassis dynamometer or ‘roller bench’. A chassis dynamometer is designed to operate on a stationary platform specifically designed for this purpose and to simulate real-world vehicle operations. The vehicle is driven at a pre-defined speed-time pattern (driving cycle), while the dynamometer simulates the inertia (mass) and driving resistances (rolling resistance and air drag) of the vehicle. For every vehicle tested, the level of resistance is adjusted in the dynamometer in order to simulate the level of resistance would be produced if the vehicle operated on the road. During the vehicle driving operation on the roller bench, all emissions from the vehicle tailpipe are collected in sealed bags and subsequently analysed. The emission results, measured in grams of pollutant per kilometre driven, are then determined (EEA, 2016).

The New European Driving Cycle (NEDC) was established in 1980 to simulate typical driving conditions and to be used for the official type approval of the vehicles. Originally, the NEDC was developed when vehicles were lighter and less powerful than those available today. As such, it didn't reflect how modern cars are driven, and produced unrealistically low emission measurements and high fuel economy figures. Hence, the NEDC became outdated.

As of September 2019, the NEDC was replaced by the Worldwide Harmonized Light Vehicles Test Procedure (WLTP). The WLTP has introduced much more realistic testing conditions, including a more realistic driving involving harder acceleration and more time at high speeds, a stricter car set-up and measurement conditions. The following figure shows a comparison of the main characteristics of NEDC and WLTP.

Comparison of NEDC and WLTP characteristics. Source: (Pollard, 2019)

On-road testing with PEMS

The latest Euro 6 standard brought more stringent requirements in vehicle emission control in Europe including the RDE test. The most widely known method for RDE testing, involves the use of certified mobile measurement equipment known as Portable Emissions Measurement System (PEMS). PEMS contains a variety of instruments, for the monitoring of real-time emissions and for specific pollutants. It can be installed inside the vehicle, either in the trunk or in the passenger compartment, as illustrated in the following picture. If there is no available space, installation is also possible on a rack outside the vehicle.

PEMS mounted on vehicles to be tested on the road

One of the advantages of PEMS is that it is rather simple and inexpensive for purchase and maintenance comparing to the chassis dynamometer. Its main limitations are its relative heavy and bulky equipment that adds extra weight to the vehicle mass, as well as the limited range of pollutants that are measured during the test, compared to the laboratory test. Furthermore, PEMS instrumentation is subject to measurement uncertainties and affected by ambient conditions (e.g. temperature, altitude, vibrations etc), during real-world driving, hence repeatability of the testing results may be difficult to ensure. However, technology advancements will enable the measurement of additional pollutants and the reduction of measurement uncertainties in the near future.

On-road testing with SEMS

Whereas the use of PEMS is required by the EU legislation for checking emissions, they are not very practical for long-term monitoring of emission performance under a greater range of driving conditions. Therefore, the Smart Emissions Measurement System (SEMS) has been developed more recently to cover this gap. SEMS is a highly compact sensor-based system that measures emissions and can be easily built into a vehicle. Because it is so compact, the vehicle can be used in the normal way while the measurements are being taken. This means that measurements can be taken over a longer period of time, making it possible to gather large quantities of practical data. SEMS offers a cost-effective alternative that makes large-scale monitoring possible (Kadijk et al., 2016).

Smart Emissions Measuring System (SEMS)

Roadside testing

Another means of measuring motor vehicle emissions is Remote Emission Sensing (RES), which enables measuring a large sample of vehicles over a short period of time. A light source and detector are placed in a way that the pollutants produced by the vehicles interact with the light and finally, the vehicle exhaust emissions can be determined. The latest remote sensing systems incorporate additional equipment and sensors in order to acquire more information about the vehicle. This includes cameras which record vehicle specifications (e.g., model, fuel type, make, engine size), devices that measures speed and rate of acceleration and provides information regarding the engine load at the time of the measurement, as well as sensors that can measure ambient conditions such as temperature, relative humidity and pressure (Dallmann, 2018).

Remote sensing measurements can be used to measure a large number of vehicles in a relatively short period of time. However, each remote sensing measurement lasts less than one second and therefore does not provide detailed insights into the emissions performance of a vehicle. This means that RES emissions data have limited utility, since they cannot be linked with significant information with regards to the vehicle characteristics, or how the vehicle is being operated during the measurement. RES can however be used to detect high emitters and can serve as a screening for the selection of vehicles for a more dedicated measurement, for example in relation to the enforcement of emission limits. Different RES set-ups are illustrated in the schematic below.

Illustration of horizontal and vertical remote sensing. Source: (ICCT, 2017)

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