Diesel Engine Exhaust Gas Properties

Engine exhaust gas properties are very important matter to be considered. Because it becomes one of the main reason for environmental pollution, green house effect, air pollution etc. Both petrol and diesel engine have two methods to measure their exhaust gas properties. Opacity of the exhaust gas is checked in diesel engines

Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radioactive transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc. An opaque object is neither transparent (allowing all light to pass through) nor translucent (allowing some light to pass through). When light strikes an interface between two substances, in general some may be reflected, some absorbed, some scattered, and the rest transmitted (also see refraction). Reflection can be diffuse, for example light reflecting off a white wall, or specular, for example light reflecting off a mirror. An opaque substance transmits no light, and therefore reflects, scatters, or absorbs all of it. Both mirrors and carbon black are opaque. Opacity depends on the frequency of the light being considered. For instance, some kinds of glass, while transparent in the visual range, are largely opaque to ultraviolet light. More extreme frequency-dependence is visible in the absorption lines of cold gases. 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.

Diesel soot is by nature oily thus quickly clogging the filters and sample piping of a "normal" petrol exhaust gas analyzer. Those maintenance items are fairly expensive to be changed after every single test. That's why the smoke metering method for diesel engines has been implemented by all governments. A modern Smoke meter is a almost no-maintenance device.

Absorption factor (K factor ) is a more important factor to be considered when considering the opacity of an engine. This factor is also known as “Light Extinction Coefficient” and “Light Absorption Coefficient. K factor  is expressed on a per meter basis . The smoke density is a function of the number of smoke particles per unit gas volume, the size distribution of the smoke particles, and the light absorption and scattering properties of the particles. In the absence of blue or white smoke, the size distribution and the light absorption, scattering properties are similar for all diesel exhaust gas samples and the smoke density is primarily a function of the smoke particle density. Large K factor values indicate that air pollution is high, so there are standards for each type of vehicle  to minimize the environmental effect.

                                           

    Figure no 01 : How to use the instruments 

Figure no 02 : How to calculate the Opacity of the engine