Measurement of Exhaust Emission

        Exhaust gas emissions are monitored satisfy the legislation on pollution and also because of the insights the measurement provide into engine performance. The emissions governed by legislation on pollution are carbon monoxide, nitrogen oxides, unburnt hydrocarbons and particulates. Carbon dioxide and oxygen level in exhaust will help to calculate the air/fuel ratio.

CO Measurement:

Infrared radiation is absorbed by a wide range of gas molecules including CO and each of which has a characteristic absorption spectrum. Fig it shows the component in a non-dispersive infrared gas analyser. The detector cells are fitted with the gas that is to be measured such as carbon monoxide. They absorb the radiation in the wavelength band associated with that gas. The energy absorbed in the detector cells causes the cell pressure to rise. The reference cell is filled with air and the gas to be analysed flows through the sample cell. If carbon monoxide is preset in the sample, than infrared will be absorbed in the sample cell and less infrared will be absorbed in the detector cell. This leads to the carbon monoxide concentration. The calibration is determined by passing gases of known composition through the sample cell.

NO measurement:

Nitric oxide and nitrogen dioxide exist in the exhaust of an engine and NO is used to refer to the sum of the nitrogen oxides. Nitrogen dioxide can be measured by passing the sample through a catalyst that converts the nitrogen dioxide to nitric oxide.

It shows the arrangement of the NO analyser. The vacuum pump controls the pressure in the reaction chamber by drawing in the ozone and exhaust sample. Ozone is generated by electrical discharge in oxygen at low pressure. Flow of ozone is controlled by the oxygen pressure and the orifice. The sample can either bypass or flow through the nitrogen dioxide converter. The sample flow rate is regulated by two orifices. The bypass flow is drawn through by a sample pump. This arrangement ensures a high flow rate of sample gas, so as to minimize the instrument response to to a change in NO concentration in the sample. The flow of sample into the reactor is controlled by the pressure differential across the orifice upstream of the NO converter and controlled by the regulator.

Oxygen and air/fuel ratio analysers:

Oxygen measurement in exhaust emission is useful in evaluating the air/fuel ratio and the oxygen analysers are usually based around a galvanic cell. A galvanic cell comprises a PTFE memberane with a gold coating that acts as the cathodeas shown (fig 4.15). A silver or lead anode is immersed in the electrolyte. A potential is applied across the electrodes and the oxygen diffuses through the memberane. Oxygen is reduced electrochemically and a current flows proportional to the partial pressure of the oxygen in the sample.

Particulates and smoke emission:

The most widely used system is the Bosch Smokemeter. A controlled volume of exhaust is drawn through a filter paper and the change in the reflectance of the paper corresponds to the smoke level. A value of zero is assigned to a clean filter paper and a value of 10 is assigned to a piece of paper that reflects no light. The calibration of intermediate values can be checked by placing a perforated piece of non-reflecting paper over a filter paper.

Exhaust particulates are defined as material that can be collected on a filter paper maintained at 325K. It is impractical to pass the whole of the exhaust stream through the filter. A sample of the exhaust is drawn off and cooled by dilution with air. The filter is weighed before and after use and the mass of the particulates is evaluated. The particulates consist of particles and high molecular mass hydrocarbons.

It is not the mass of particulate matter that is significant, but it's size or number of particles is quite significant. The smallest particles have the ability to travel furthest into human lungs. Particulate matter above 10µm  is mostly filtered by the nasal passages but some of the particles below this size will penetrat into the pulmonary and bronchial systems and be deposited there. It is generally accepted that particles below 2.5µm are capable of penetration deep into the lungs and can thus pose a health hazard. Particles of this size are invisible and are in fact emitted by both spark ignition and diesel engines.