Smoke Opacity

W. Addy Majewski, Hannu Jääskeläinen

Abstract: Smoke opacity instruments measure optical properties of diesel smoke, providing an indirect way of measuring of diesel particulate emissions. There are two groups of instruments: opacity meters, which evaluate smoke in the exhaust gas, and smoke number meters, which optically evaluate soot collected on paper filters. Correlations have been developed to estimate PM mass emissions based on opacity measurement. Second generation opacity meters based on laser light scattering are much more sensitive and appear to hold promise for application to newer engines with much lower particulate emissions.

Introduction
Conventional Smoke Meters
Opacity and PM Mass Fraction
Second Generation Opacity Meters

Introduction

Smoke and smoke opacity meters are instruments measuring the optical properties of diesel exhaust. These instruments have been designed to quantify the visible black smoke emission utilizing such physical phenomena as the extinction of a light beam by scattering and absorption. In general, smoke and opacity meters are much simpler (some of them very simple) and less costly in comparison to most other instruments used for PM measurement. They are often used to evaluate smoke emissions in locations outside the laboratory, such as in maintenance shops or in the field. In fact, the smoke opacity measurement is the only relatively low-cost and widely available method to measure a PM-related emission parameter in the field. For this reason, opacity limits are used in most inspection and maintenance (I&M) or periodic technical inspection (PTI) programs for diesel engines. Smoke opacity limits may be also included as auxiliary limits in new engine emission standards.

In view of the demands of advanced, low emission diesel engines, the following areas of concern can be identified in conventional smoke opacity meter technology:

Insufficient resolution: Smoke levels in low emission, smokeless diesel engines (e.g., Euro IV and later) are near the resolution of a conventional opacity meter. Resolution, stability, and noise have to be improved to allow opacity measurements in modern diesel engines.
Cross sensitivity to nitrogen dioxide: Catalytic exhaust aftertreatment devices, such as oxidation catalysts or particulate filters, can increase the percentage of NO₂ in the total NOx from less than 10% to as much as 40% and more. Nitrogen dioxide absorbs green light, which is used in conventional opacity meters. Opacity meters read between 0.00016 and 0.00024 m^-1 per ppm NO₂, depending on the sensor bandwidth [853]. A 300 ppm NO₂ concentration in a diesel engine—not an unrealistic assumption—would cause a 0.06 m^-1 opacity reading, which is equivalent to 40% of the Euro III EEV pass level, intended to be for soot emissions. For engines fitted with catalytic particulate filters, nearly all opacity signal can be caused by nitrogen dioxide.
Insensitivity to small particles: The majority of diesel particles have diameters below 200 nm. Particles of 200 nm diameter or greater block green light in proportion to their cross-section surface area. Particles of 50 nm diameter, however, block only about 15% of their surface area [853]. This means that opacity readings depend on particle sizes and will be underestimated if smaller particles are measured.

There have been various attempts to improve the sensitivity of diesel opacity meters, for instance by using multiple light path systems with mirrors [648]. The NO₂ cross-sensitivity could be eliminated by switching to a different light wavelength, e.g., to red light. Red, however, is less sensitive to small particles than green. A switch from green to ultraviolet, in turn, could improve the “visibility” of small particles [853]. In view of these conflicting solutions, a simple switch to a different wavelength would not be sufficient.

However, as no satisfactory solution has yet been found, conventional opacity meters based on light extinction measurement may become replaced with devices utilizing other measurement principles. “Second generation” opacity meters based on laser light scattering are much more sensitive. They appear to hold promise for application to newer engines with much lower particulate emissions, including engines fitted with particulate filters.

Smoke opacity readings generally do not correlate well with other PM measurement parameters. Numerous correlations between opacity or smoke readings and PM mass that have been developed can provide only approximate results. Since opacity readings may be affected by sulfates, HCs, water vapor, as well as by PM composition or physical conditions (e.g., coagulation), no accurate correlation is possible.

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