ENGINE PRINCPLES Chapter 5. Exhaust System

ENGINE PRINCPLES

Chapter 5.

Exhaust System

1. Exhaust System

By opening exhaust port of the cylinder head, the combusted gas is exhausted through the exhaust manifold, the exhaust pipe gathering the combusted gas from each cylinder, the catalyst converter purifying the used gas and the silencer (muffler) reducing combustion noise.

The most important thing in the exhaust system is the smooth path also. The exhausted gases from each cylinder are bumped at the exhaust pipe gathering the each exhaust manifolds. So, it is important to prevent from hindering each flow or to increase the exhaust efficiency using the exhaust inertia effect having the same principle in the intake inertia effect. However, it is hard to balance between the increasing the exhaust efficiency and the enhancing the engine performance, because there are many weak points from the engine to the muffler.

The exhaust manifold is made of the casting iron having high heat resistance, or the aluminum alloy. Also the exhaust gas temperature is high. So, it is need to make the exhaust system with the material having better heat resistance or to cool the exhaust system with the wind.

The catalyst converter is used for purification of the exhausted gases. There are a manifold catalyst installed near the manifold and a under catalyst installed under the floor. The manifold catalyst is more effective because the exhaust temperature is higher than the other system. However, it can be easily aged by the high temperature so usually two pieces are used. The under catalyst is not easily degraded, however it has high performance of purification.

The muffler is equipped for reducing the temperature and pressure of the exhausted gas as well as the combustion and exhausting sound. Generally, the muffler has many sector walls to make the inside space into maze path for flowing the exhaust gas, so it called as the maze type. There is also a straight muffler type in which a tube having many holes on the surface and silencer such as glass wool inside the tube. The maze type muffler has more effective sound absorption ability but has larger flow resistance. The straight muffler has more enhanced output but has louder sound.

In some cases, these two muffler types are simultaneously equipped using separated two pipes so that the maze type muffler is only used when the engine is working in low speed, and the straight type muffler will be additionally used when the engine is working in high speed and load.

2. Exhaust Inertia Effect and Pulsation Effect

The most important thing determining the performance of the exhaust manifold is the smoothness in the exhausting. The exhaust interference is the main problem hindering the smoothing exhaust. From the each cylinder, the combusted gas is exhausted according to the order of ignition. As they are merged into the manifold, if the exhaust system is not well arranged and combined, then one exhausted gas passing through the manifold can be collided with another exhausted gas from another cylinder or the pressure in the manifold will be increased, so that the exhaust gas can not be exhausted well.

The exhaust interference can be overcome by elongating the distance between the exhaust valves to the collection portion of each cylinder gas or be making the angle of collecting portion into an obtuse angle to ensure the well flow. As increasing the number of cylinder, it is hard to ensure the well exhaust flow by combining the increased manifolds.

In the serial 6-cylinder turbo engine, with not being fully closed the first exhaust valve, the fifth exhaust valve is opened, and with not being fully closed the fifth exhaust valve, the third valve is opened. So, if the all 6 exhaust manifolds are gathered into one collecting pipe, then the sequentially exhausted gases can be collided each other. Therefore, the manifolds are divided into two groups; the one includes the first, the second and the third manifolds and the other includes the fourth, the fifth and the sixth manifolds. Each group has own turbo charger to eliminate the exhaust interference and to enhance the engine output.

The inertia effect and pulsation effect are utilized to exhaust the remained combustion gases in the combustion chamber. As being the same with the intake system, when the exhaust valve is closed, the gas density around the valve shall be reduced to accelerate the exhausting gases from the chamber effectively.

When the exhaust valve is open, the combusted gas having the high pressure is exhausted through the exhaust valve and the remained gases will be exhausted by the following compressing pressure of the piston at the exhaust stroke and then the exhaust valve is closed. Therefore, the exhausted gas flow has high density portion and low density portion in the manifold.

As we mentioned in above chapter, when a gas flow has a difference in density, then pressure wave is generated. Therefore, this difference is transmitted during the manifold with sound velocity. This is called the exhaust pulsation.

Just before the exhaust valve is closed, if it is possible that the density around the valve is lower than other portions, then it is accelerated that the remained gas in the chamber shall be took out as well as the fuel mixture shall be inhaled from the intake valve.

3. Component of the Exhaust Gas

The exhaust gas from engine and fuel system to atmosphere comprises of the burnt gas from the exhaust pipe, the blow by gas from the crank room, and the vapor gas from the fuel tank by the hot weather and hot temperature of the working engine. As these gases include harmful materials contaminating the atmosphere, a purification system should be equipped.

Especially, the exhaust gas is the most important gas.

If the fuel is combusted perfectly, then any harmful material is not included in the exhaust gases. The fuel, gasoline, consists of hydrocarbon, the compound of the carbon and the hydrogen. In the chamber, the fuel is changed into the carbon dioxide (CO₂) and water (H₂O) with making heat energy.

In actual chemical reaction, the hydrocarbon and the oxygen are not changed at a moment into the carbon gas and water vapor. This chemical reaction is very complicated. For example, the hydrocarbon would be divided into a small unstable material by the heat and reacts with the oxygen, or the results particles are reacting each other, and so on.

Among the gases generated during this complicated reaction, the carbon monoxide, hydrocarbon gas and the nitrogen oxide are the major harmful materials. The carbon monoxide (CO) is the unstable material having one carbon and one oxygen so that it can be easily changed into the carbon dioxide, the stable and harmless material, if additional oxygen and heat are supplied. If we breath the carbon monoxide, then it will catch the oxygen delivered by the hemoglobin in the blood to be carbon dioxide, more stable material. So, our body is lag of oxygen.

The hydrocarbon gas (Hm Cn : here m, n are integer number) is come from the fuel not combusted or the intermediated material during the chemical process of combustion. It is come from the blow by gas or the vaporized fuel from the fuel tank.

If this gas is revealed at the atmosphere, then it will react with the oxygen and hydrogen and change into the aldehyde, the harmful material having strong stimulus.

The nitrogen oxide (NOx) is come from the reaction between the nitrogen (78% of the air) and the oxygen in the air by the high temperature of 2000℃ in the chamber. As the nitrogen oxide is made by the difference mechanism with the carbon monoxide or hydrocarbon gas, it will be increased when the carbon monoxide and hydrocarbon are reduced by almost perfect combustion. When the combustion temperature is low, the nitrogen oxide will be less; however, the combustion efficiency will be worse. So, the nitrogen oxide shall be treated at the exhaust system.

4. Air-Fuel Ratio and Exhaust Composition

As the harmful material in the exhaust gas is the product from the combustion, one of chemical reactions, the amount of it is decided by the air-fuel ratio, e.g. the ratio between the amount of air and the gasoline, the combusting temperature and the gas flow status. When the air-fuel ratio is lower than theoretical value (Stoichiometric), that is rich fuel, the combustion is not completely performed, so that more hydrocarbon gas and carbon are made.

Contrarily, if the air-fuel ratio is higher than the theoretical value (Stoichiometric), that is lean fuel, then the gasoline shall be combusted completely. So, the amount of the carbon monoxide and the hydrocarbon gas will be less, however, the amount of the nitrogen oxide will be increase because the combustion temperature is high. Especially, when the temperature is over 2000℃, the nitrogen oxide will be abruptly increase. Even the nitrogen and the oxygen are not react in the room temperature, in the high temperature, they are changed into the nitrogen monoxide and then changed into nitrogen dioxide After exited from the exhaust system.

The amount of the nitrogen oxide will be maximized at about 16 air-fuel ratio higher than the theoretical ratio (14.7). With lower ratios than 16, the nitrogen oxide amount will be reduced because the combustion temperature is lowered. When the air-fuel ratio is over 18 with less fuel, the fuel can not combusted properly so that the hydrocarbon will be increased.

To reduce the harmful material, it is important how to decide the air-fuel ratio. In some cases, the air-fuel ratio can be controlled by inhaling the combusted gas into the mixtures; it is called the exhaust gas recirculation device (EGR).

The exhaust gas recirculation device is called as EGR as an abbreviation. It is the device for returning some amount of the exhaust gas back to the cylinder. Doing so, the actual amount of fuel is reduced and the combustion speed is slow, and then the maximum temperature of the combusting chamber will be lowered and the amount of the nitrogen oxide will be also reduced. But, if the amount of the re-circulated exhaust gas is too much, then the engine output and fuel efficiency will be worse, so it is important to control the amount of EGR.

In the carburetor system, the amount of the re-circulated exhaust gas is controlled by the reverse pressure of the intake manifold. In the ECM system, the amount will be determined in optimizing the vehicle status by the sensing and calculating with the temperature of mixture and cooling water, the vehicle speed, and the load.

5. Exhaust Purification System

Devices for reducing the harmful material from the exhaust gas are the exhaust oxidation device combusting the carbon monoxide and carbon and the 3way catalysts device treating the exhaust gas using oxidation and de-oxidation reaction by the three catalysts for carbon monoxide, hydrocarbon, and nitrogen oxide.

As the carbon monoxide and the hydrocarbon gas are come from the incomplete combustion of the hydrocarbon and oxygen, the oxidation device supply additional air to the exhaust port to make oxidation the incomplete combusted gas included in the exhaust gas. So it is called as the secondary air device. In some system, in the middle of the exhaust pipe, a oxidation catalyst coveter, the vessel including the oxidation catalyst may be equipped for converting the carbon monoxide and the hydrocarbon into the carbon dioxide and water, respectively.

The electronic control engine uses the 3way catalysts device, generally.

In the nitrogen oxidation, there are nitrogen monoxide consisting of one nitrogen and one oxygen, and nitrogen dioxide consisting of one more oxygen. If the oxygen is removed from the nitrogen oxidation, e.g. de-oxidation reaction, then it becomes nitrogen gas. If the acquired oxygen from de-oxidation reaction of the nitrogen oxidation is supplied to the carbon monoxide and hydrocarbon to oxide them, then the three harmful gases can be simultaneously purified.

From this idea, the chamical action is performed the de-oxidation reaction on the nitrogen oxide and the oxidation reaction on the carbon monoxide and the hydrocarbon by controlling air-fuel ratio to eliminate the oxygen in the combusted gas completely. The catalyst is the material accelerating certain chemical reaction. The catalyst used in this purification is called the 3way catalysts. There are the pellet type covering a film of platinum and rhodium on the particle alumina and the honey comb type.

As the theoretical air-fuel ratio, the ratio of complete combustion is 14.7. Because the 3way catalyst is not work properly when the oxygen is remained, it is necessary to maintain the air-fuel ratio with the theoretical value. To do so, an oxygen sensor is used for detecting the oxygen. If oxygen is detected in the exhaust gas, then the computer will calculate the amount of intake air and EGR gas to maintain the fuel ratio to the theoretical value.

6. Blow-by Gas Recirculation Device

The blow-by gas is the leaked gas to the crankcase through the end gap of piston ring at the combustion stroke. It includes vaporized engine oil, too. The old type engine or some racing engine exhausts this gas into the atmosphere. When you stand near the racing car, you can smell like some oil burning result from the blow-by gas.

The components of this gas are the flammable gas of 75∼80% and the combusted gas of 20∼25%. As they are the main causes of air pollution, the perfect combustion system by recirculation of this gas should be equipped by law. This device is called as the blow-by gas recirculation device or the positive crankcase ventilation, PCV in abbreviation.

In the 1ℓ of blow-by gas, 0.04∼0.05g of the strong acid moistures are included, so that the inside of the engine can be easily corroded and the engine oil can be easily oxdized. Therefore, it is important to treat the blow-by gas for maintenance the engine.

The blow-by gas amount will be increased as the pressure difference between the cylinder and the crankcase is large. The pressure inside the crankcase is not changed so much even the engine is running with high speed. So, when the engine speed and load is increased, the blow-by gas will be increased. The blow-by gas treating should be performed in two steps according to the engine load.

The blow-by gas recirculation device comprise of hoses, one is connected between the rocker cover and the surge tank, the other is connected between the rocker cover and the intake duct before throttle body for fresh air. Also there is an air passage between the crankcase and the rocker cover.

When the engine is working, the pressure in the intake manifold is always negative pressure so that the blow-by gas will be flow from the crankcase to the manifold.

The blow-by gas in the intake manifold is inhaled into the cylinder. The blow-by gas will be treated by these method.