Measurement of Emissions

W. Addy Majewski, Heinz Burtscher

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Abstract: Diesel 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-life operation in the laboratory. In the sampling system, exhaust gases are most commonly diluted with air using the CVS method.

Introduction

Emissions can be characterized, regulated, or controlled only if they can be accurately measured. The increased health and environmental concerns about diesel 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 diesel 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—for instance Euro 5—also include a solid particle number (PN) limit. Thanks to the existence of standard 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. The focus in recent years has been the characterization of diesel particulate matter through the determination of such parameters as particle number concentrations and size distributions. Several laboratories developed highly accurate methods for these and other nonregulated emissions. However, if non-standard methods are used, results from different laboratories may not be comparable. This is especially true in measurements of particulate matter which, being a mix of many chemical compounds, is 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 new class of vehicle emission test equipment, first developed in the late 1990s, are high accuracy on-board emission analyzers [796]. Also referred to as portable emission measurement systems (PEMS), these units can be installed on a vehicle to measure real life emissions with a laboratory level of accuracy (Figure 1). PEMS systems include laboratory-class analyzers packaged in one or more portable units and a simplified sampling system. PEMS analyzers can provide results as both raw gas concentrations and mass-based emission factors, such as g/km or g/kWh. The exhaust gas flow rate and other parameters needed to convert concentrations into mass-based emissions can be obtained from the engine control module. On-vehicle testing opens several new possibilities for both engine developers and emission regulatory agencies. It can be used for compliance testing of new and aging vehicles (including NTE limits and “emission defeat” strategies), development of mobile source emission models, development of engine control strategies, aftertreatment systems, and on-board diagnostic (OBD) systems.

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Figure 1. PEMS Analyzers and Sampling System on a Passenger Car

(Source: Sensors Inc.)

Another area of emission measurement is vehicle maintenance. Several US states and a number of countries worldwide operate mandatory vehicle inspection and maintenance (I&M) programs, where vehicles must pass a periodic emission check. The methods vary greatly, 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 can no longer be used for modern low emission engines, in particular those equipped with particulate filters, because their emissions are below the detection limits of opacitymeters—possible alternatives are still under discussion. New technologies—such as remote exhaust emission sensing [793]—may be also used in future I&M programs. However, with the growing sophistication and emission sensing capabilities of vehicle’s onboard diagnostic (OBD) systems, there is a trend to increasingly rely on vehicle’s OBD information for I&M purposes in lieu of emission testing.

The areas of emission testing mentioned so far were concerned with the measurement of the tailpipe emissions from an engine or vehicle. There are several applications, including 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.

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