Measurement of Emissions

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Abstract: Engine emission measurements are performed on an engine or vehicle dynamometer, over a standardized emission test cycle. Emission test cycles are repeatable sequences of engine operating conditions, designed to simulate real-world operation in the laboratory. In the sampling system, exhaust gases are most commonly diluted with air using the CVS method. Emission tests have also been developed using PEMS analyzers and other methods that do not require testing in the laboratory and that can measure vehicle emissions during real-world operation.

Introduction

Emissions can be characterized, regulated, or controlled only if they can be accurately measured. The health and environmental concerns about engine emissions resulted in the development of a wide range of measurement techniques of different levels of sophistication, equipment cost, and accuracy to suit a variety of applications. Many techniques, especially those used for regulatory purposes, are highly standardized to produce comparable results even if performed at different testing laboratories. The main types of engine emission measurements can be grouped as follows:

Laboratory emission testing methods generally use very complex and sophisticated equipment to deliver the highest possible accuracy and repeatability. Test methods used for regulatory purposes, such as engine/vehicle emission certification or compliance testing, are highly standardized. Detailed description of the measurement setup, type of equipment, and test procedures are an important part of every emission regulation. These standard methods cover the measurement of regulated pollutants, which traditionally include CO, HC, NOx, and PM, all of which are measured in units of mass. Some regulations—beginning with Euro 5b—also include a solid particle number (PN) limit. Due to the standardized methods, measurements of regulated pollutants taken at different laboratories usually correlate very well. Emission research laboratories also perform measurements of a number of unregulated emissions, using techniques that may be subject to various non-regulatory standards and recommended procedures, or else using non-standard methods. However, if non-standard methods are used, results from different laboratories may not be comparable. This is especially true in measurements intended to characterize particulate matter which, being a complex mix of many chemical compounds, is actually defined by its measurement method.

Measurement equipment and methods used for measuring engine emissions in the field vary widely, from “mobile laboratories”, with capabilities comparable to those of stationary emission laboratories, to simple, low-budget tools, which can offer only very approximate results. Examples of high-end mobile laboratories include the diesel aerosol sampling rig built by the University of Minnesota [794], the transportable heavy-duty vehicle chassis dynamometer and emission test facility developed and operated by the University of West Virginia [795], the mobile laboratory for “chasing” of city traffic by Helsinki Polytechnic [1102] or the mobile pollutant laboratory operated by PSI in Switzerland [2356].

A newer class of vehicle emission test equipment, introduced in the late 1990s, are portable emission measurement systems—high accuracy on-board emission analyzers that can be installed on a vehicle to measure real-world emissions with a near-laboratory level of accuracy [796]. In some jurisdictions, regulatory PEMS emission testing requirements have been implemented in addition to laboratory dynamometer testing. The first such PEMS program was introduced by the US EPA as an in-use emission compliance measure for heavy-duty trucks, in the wake of the 1998 Consent Decrees with US engine manufacturers over dual-mapping of heavy-duty engines.

In the European Union, various PEMS testing requirements were implemented during 2010s, as a mandatory component of new engine certification testing, as an in-use compliance measure, or else as an in-service monitoring program. These PEMS testing requirements are known as Real Driving Emissions (RDE) testing for light-duty vehicles, and in-service conformity (ISC) PEMS testing for heavy-duty vehicles. RDE testing requirements have also been adopted by other countries, for instance China.

Another area of emission measurement is vehicle maintenance. Several US states operate mandatory vehicle inspection and maintenance (I&M) programs, where vehicles must pass a periodic emission check. In other countries, an emission check may be a part of mandatory periodic technical inspection (PTI) programs. The test methods vary from relatively sophisticated chassis dynamometer tests (IM240 in the USA) to a simple emission measurement at engine idle condition. In the case of heavy-duty diesel engines, the most common procedure had been to measure smoke opacity during engine acceleration. This test is no longer effective for modern low emission engines, in particular those equipped with particulate filters, because their emission levels are below the detection limits of opacity meters [793]. To address this problem, some countries adopted PN measurements as part of their PTI emission checks. These PN-PTI tests are conducted at idle, using a portable particle number counter. New technologies—such as remote exhaust emission sensing—may also be used in future I&M programs. However, with the growing emission sensing capabilities of vehicle’s onboard diagnostic (OBD) systems, there is a trend to rely on vehicle’s OBD information for I&M purposes in lieu of emission testing.

Emission-assisted equipment maintenance practices have been developed in some engine applications, particularly in underground mining [797]. These practices are based on the fact that an improperly maintained diesel engine can emit higher emissions compared to a well-maintained diesel engine, and on-site emission tests can be employed to determine whether there is a need for maintenance.

The areas of emission testing mentioned so far were concerned with the measurement of tailpipe emissions from an engine or vehicle. There are several applications, such as the occupational health practice, where the focus is on the ambient air exposure to pollutants, including diesel exhaust pollutants. While certain principles of emission sampling and analysis have general validity, the measurement of ambient diesel aerosol has evolved into a separate area of expertise.