Air pollution from Automobile Engines

The automobiles play an important role in the transport system of world. With an increase in population and living standard, the transport vehicles as well as car population is increasing day by day. In addition to this, there is steep increase in the number of two wheelers during the last two decades. All these are increasing exhaust pollution and particularly in metros as density of these vehicles in metros is very high. Delhi which had only 11000 registered vehicles in 1348 have at least 2.2 million vehicles today. The Mumbai and Calcutta also count in the same range. In addition to this, nearly 700 new vehicles are added every day in Delhi which counts 0.25 million a year. Possessing less than 1% national population, Delhi has more than 10% of the total number of vehicles. Other major metro-cities are in the same range. The rate of growth of vehicles in the metropolitan cities in the coming years could be even faster.

By the year 2000, the roads in Delhi and other metros would be almost choked. At the present rate, by the year 2000, there may be 150-200 vehicles per road kilometer in Delhi. The situation in Calcutta and Mumbai could be even more critical because of inadequate road space.

The daily emissions by the vehicles in Delhi is 872 tones and same figures for Mumbai and Calcutta are 548 tones and 283 tones respectively as per data available in 1979. Because of the large emissions in cities in a limited area, the large number of people are affected that include pedestrians, office employees, road side vendors, shopkeepers and residents of road side houses.

Presently, India is producing 25 lakh vehicles per year and Bombay alone gets 1730 tones of pollutants per day which counts for 61.5% of the total in the city.

There are 30 lakh vehicles is Delhi and expected to be 60 lakh in 2010. Delhi emits 3000 tones of emission per day and causes a death of 7500 people per year out of respiratory problems. This death rate is 12 times higher than national average.

Air pollution can be defined as an addition of any material which will have a deteterious effect on life upon the earth. The main pollutants contributed by I.C. engines are CO, NOx unburned hydro-carbons (HC) and other particulate emissions. Other sources such as Electric power stations industrial and domestic fuel consumers also add pollution like NOx, SO, and particulate matters In addition to this, all fuel burning systems emit CO2 in large quantities and this is more concerned with the Green House Effect which is going to decide the health of Earth. A noted mateorologist recently predicted that polluted air could put an end to life on the earth within a century,

In advanced countries like USA, the air pollution by vehicles is about 50 to 60% of the total air pollution in the country. The pollution from automobiles is concerned is small in urban areas.

 C.I. Engine Emissions

The diesel engine is used more than any other type of engine for transportation the generation and many other industrial and agricultural applications. The exhaust combustion in diesel engine are no different from those of combustion processes in the difference being only in the level of concentration of individual pollutants. The sam exhaust may be free from smoke, odor and HC or may be heavily smoke laden highly and can have heavy concentration of unburned HC.

The pollutants from diesel engine can be classified into two types as visible and emissions. Visible emission is the smoke which is objected more by the public. The emissions include, CO, HC, NOx, SO2, partially oxidized organics (as aldehydes and keto odors. An unpleasant odor is also heavily objected by the public. The smoke and odor not harmful to the public health but they are objectionable because of its unsightliness, un smell and possible reduction in visibility. Other invisible emissions mentioned above have si effect on health as they are also emitted by petrol engines. The concentrations of these constituents are considerably low compared to petrol engine, however as the specific air consumption in diesel engine is considerably high because of excess air, the total amount of pollutants are nearly same in diesel and petrol engine exhausts.

(a) Effect of Engine Type on Diesel Emission. The type of the engine and speed of the engine are two main factors which influence the exhaust emission from a diesel engine. Table 23.3 shows the emissions for four different types of engines at full load and rated speed. It can be seen from the table, there is a significant difference in emission levels from different engines except the odor level.

The following observations can be summarized:

 1. A two stroke air-scavenged engine produces high HC and intermediate NOx. The smoke level remains low at all load conditions.

2. A 4-stroke medium speed engine has lowest emissions of all constituents except high smoke intensity.

 3. A 4-stroke high speed engine has high HC emissions.

 4. A turbo-charged 4-stroke engine is notably low in HC but high in NOx. The smoke level is also considerably low compared with other engines.

Modifications in the Engine Design

 Engine modifications improve the emission quality. A few parameters which improve an emission are listed below:

1. Combustion Chamber Configuration. Modification in the combustion chamber as reducing surface/volume ratio can reduce quenching zone and can reduce HC emission. This can also be achieved by reducing dead space around piston ring.

2. Lower Compression Ratio. Lower compression ratio also reduces the quenching area and thus reduces HC emission. Lower compression ratio also reduces NOx emission due to lower maximum temperature. But lowering compression ratio reduces thermal efficiency and increases fuel consumption. But using petrol of lower Octane number, it is possible to phase the lead out of petrol, i.e., use of unleaded petrol.

3. Induction System. The supply of designed A: F ratio mixture to multi-cylinder engine is always difficult under all operating conditions of load and power. This can be achieved by proper design of induction system or using high velocity or multi-choke carburettors.

4. Ignition Timing. Retarding spark ignition allows increased time for the fuel to burn. Retarding the spark reduces NOx formation by decreasing NOx emission. It also reduces HC emission by causing higher exhaust temperature. However, retarding the ignition results in loss of power and consumption of fuel. The controls are designed to retard the spark timing during idling and providing normal spark advance during acceleration.

5. Reduced Value Overlap. Increased overlap carries fresh mixture with the exhaust and increases emission level. This can be avoided by reducing the value overlap.

A new variable value timing (VVT) system allows for controlled scheduling of value timing events. Many vehicle manufacturers are developing this system and many will go into production shortly. An incorporation of VVT-system improves engine performance and it will also help the engine to meet harsher emission requirements imposed by the Government. It is also claimed that VVT system will work best with petrol injection. VVT system is also applicable to petrol as well as to diesel engines.