Exhaust gas is a gas that internal combustion engine discharge, namely carbon dioxide, carbon monoxide, hydrocarbons, nitrogen oxides, sulfur dioxide, hydrogen sulfide, ammonia, ozone, and oxidants (also called discharge gas). The internal combustion engine discharges cover gas in Cylinder and air mixture, and burn carbon quickly. After burning, it lets the gas out and sucks fresh gas. At that time, this gas is exhausted gas. The thing that has discharges gas most is gasoline engine of car. In the exhausted gas of car, it has various harmful elements and makes the earth dirty. In 1950s, this has been problem with pollution in United States. For this, Muskie Act was activates for reducing the pollution in September, 1970. 
How to observe exhaust gas
① Get a car for observation from near by.
② Record basic information such as a kind of car, year, displacement, and kind of fuel.
③ Cover the car's vent pipe with a white sock and tie it with a rubber band.
④ Turn on the car, set it to park, and let it idle for 5 minutes.
⑤ After 5 minutes, turn off the car.
⑥ Put on a glove and take out the sock.(the temperature of the bent pipe is high)
⑦ Check the pollution level on the sock.
⑧ Fill out half of water in good standing heat plastic bag, and put the pH paper in.
⑨ Tie the plastic bag by rubber band on the bent pipe.
⑩ Do same thing as number ④.
⑪ Take out the plastic bag.
⑫Observe the pH paper.
⑬ Record pH result.
Some important things
The exhausted gas has so much hydrocarbons, nitrogen oxides, carbon monoxide, and carbon dioxide. They are a cause of Acid rain and photochemical smog. When these cars run for one hour, it reduce our oxygen of 15 days. When you guys want to check the pollution level, use the white sock method.
Engine exhaust gases are cleared into the environment through the exhaust system. The exhaust system includes several specialized works. The designer of the exhaust system works must know a number of exhaust gas properties. Compared to the composition of air, the diesel exhaust gas contains increased concentrations of water vapor and carbon dioxide. The concentrations of both H2O and CO2 can vary from a few percent, up to about 12% in diesel exhaust. These burning products displace oxygen, the concentration of which can vary from a few percent, up to about 17%. According to comparison, the concentrations of diesel exhaust pollutants are very small. The properties of air can be used for diesel exhaust gas calculations. The error linked with ignoring the burning products is usually no more than about 2%. The exhaust system designer must know certain other exhaust gas parameters. This include this exhaust gas temperature, which is of special importance for the design of catalytic after treatment devices. Another important parameter is the maximum pressure drop through the exhaust system, caused by the hydraulic resistance of exhaust system components. 
Car pollution is a major cause of damage to the environment. Air pollution caused by cars is a serious problem. The exhausted gases of car contain toxic substances that cause car pollution. When fuel is burnt in the car engines, the main gases produced are carbon dioxide, carbon monoxide hydrocarbons, nitrogen oxides, and water. The carbon dioxide released from car exhausts is one of the major green house gases. With large amounts of carbon dioxide being released into the atmosphere everyday, the carbon cycle goes out of balance, and the carbon dioxide gas remains in the earth’s atmosphere. This situation is a cause of trapping heat, global warming and climate change. Nitrous oxides add to car pollution. Nitrogen oxide is a major cause of acid rain. Scientists have searched for better techniques that reduce the amount of nitrous oxide that is released from cars. Another harmful and poisonous gas from car exhaust is carbon monoxide. It is produced when there is unfinished burning of the fuel. Our cars give about 2/3 of the world’s carbon monoxide gases in the atmosphere. The exhausted gases of car also contain harmful air pollutants like soot, sulfur dioxide, benzene and aldehyde. Acidic gases from the exhausted gases of car dissolve in rain to form acid rain. Acid rain can in turn acidify water bodies. It dissolves in soil and then the soil can effect the plants. Acid rain also makes the problem of buildings and monuments. Cars will continue to be a significant problem to air pollution. Although there are new technologies and cleaner fuel that can help make better productions of pollutants into the atmosphere, if the amount of moving cars are not reduced, it will not be helpful. 
EGR Exhaust Gas Recirculation