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How Exhausts and the CAT work It is by understanding how the exhaust works you can see by playing around with the various components can change the sound, This is how the fitters by selecting different boxes can achieve different noises in your new system. You will also see after reading about the CAT why a decat system can provide more gains in performance as they are less restrictive in practice. Inside an Exhaust Box Located inside the Exhaust Box is a set of tubes. These tubes are designed to create reflected waves that interfere with each other or cancel each other out. Take a look at the inside of this Exhaust Box:
The exhaust gases and the sound waves enter through the center tube. They bounce off the back wall of the Exhaust Box and are reflected through a hole into the main body of the Exhaust Box. They pass through a set of holes into another chamber, where they turn and go out the last pipe and leave the Exhaust Box. A chamber called a resonator is connected to the first chamber by a hole. The resonator contains a specific volume of air and has a specific length that is calculated to produce a wave that cancels out a certain frequency of sound. How does this happen? Let's take a closer look The Resonator When a wave hits the hole, part of it continues into the chamber and part of it is reflected. The wave travels through the chamber, hits the back wall of the Exhaust Box and bounces back out of the hole. The length of this chamber is calculated so that this wave leaves the resonator chamber just after the next wave reflects off the outside of the chamber. Ideally, the high-pressure part of the wave that came from the chamber will line up with the low-pressure part of the wave that was reflected off the outside of the chamber wall, and the two waves will cancel each other out. Waves canceling inside a simplified Exhaust Box In reality, the sound coming from the engine is a mixture of many different frequencies of sound, and since many of those frequencies depend on the engine speed, the sound is almost never at exactly the right frequency for this to happen. The resonator is designed to work best in the frequency range where the engine makes the most noise; but even if the frequency is not exactly what the resonator was tuned for, it will still produce some destructive interference. There are other features inside this Exhaust Box that help it reduce the sound level in different ways. The body of the Exhaust Box is constructed in three layers: Two thin layers of metal with a thicker, slightly insulated layer between them. This allows the body of the Exhaust Box to absorb some of the pressure pulses. Also, the inlet and outlet pipes going into the main chamber are perforated with holes. This allows thousands of tiny pressure pulses to bounce around in the main chamber, canceling each other out to some extent in addition to being absorbed by the Exhaust Box's housing. Backpressure and Other Types of Exhaust Boxes One important characteristic of Exhaust Boxes is how much backpressure they produce. Because of all of the turns and holes the exhaust has to go through, Exhaust Boxes like those in the previous section produce a fairly high backpressure. This subtracts a little from the power of the engine. The exhaust from a NASCAR race car for instance has no Exhaust Boxes hence reducing back pressures. There are other types of Exhaust Boxes that can reduce backpressure. One type, sometimes called a cherry bomb, uses only absorption to reduce the sound. On a Exhaust Box like this, the exhaust goes straight through a pipe that is perforated with holes. Surrounding this pipe is a layer of glass insulation that absorbs some of the pressure pulses. A steel housing surrounds the insulation.
Diagram of Cherry Bomb Exhaust Box These Exhaust Boxes produce much less restriction, but don't reduce the sound level as much as conventional Exhaust Boxes. The Powerflow boxes are designed to minimise back pressure but also produce an acceptable level of sound.
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How Catalytic Converters Work There are millions of cars on the road in the United Kingdom, and each one is potentially a source of air pollution. Especially in large cities, the amount of pollution that all the cars produce together can create big problems. To solve those problems, cities, states and the government create clean-air laws, and many laws have been enacted that restrict the amount of pollution that cars can produce. To keep up with these laws, automakers have made many refinements to car engines and fuel systems. To help reduce the emissions further, they have developed an interesting device called a catalytic converter, which treats the exhaust before it leaves the car and removes a lot of the pollution.
Location of catalytic converter in car In this article, you will learn which pollutants are produced by an engine and why, and how a catalytic converter deals with each of these pollutants. Catalytic converters are amazingly simple devices, so it is incredible to see how big an impact they have! Pollutants Produced by a Car Engine In order to reduce emissions, modern car engines carefully control the amount of fuel they burn. They try to keep the air-to-fuel ratio very close to the stoichiometric point, which is the calculated ideal ratio of air to fuel. Theoretically, at this ratio, all of the fuel will be burned using all of the oxygen in the air. For Petrol, the stoichiometric ratio is about 14.7:1, meaning that for each pound of Petrol, 14.7 pounds of air will be burned. The fuel mixture actually varies from the ideal ratio quite a bit during driving. Sometimes the mixture can be lean (an air-to-fuel ratio higher than 14.7), and other times the mixture can be rich (an air-to-fuel ratio lower than 14.7). The main emissions of a car engine are: · Nitrogen gas (N2) - Air is 78-percent nitrogen gas, and most of this passes right through the car engine. · Carbon dioxide (CO2) - This is one product of combustion. The carbon in the fuel bonds with the oxygen in the air. · Water vapor (H2O) - This is another product of combustion. The hydrogen in the fuel bonds with the oxygen in the air. These emissions are mostly benign (although carbon dioxide emissions are believed to contribute to global warming). But because the combustion process is never perfect, some smaller amounts of more harmful emissions are also produced in car engines: · Carbon monoxide (CO) - a poisonous gas that is colorless and odorless · Hydrocarbons or volatile organic compounds (VOCs) - produced mostly from unburned fuel that evaporates Sunlight breaks these down to form oxidants, which react with oxides of nitrogen to cause ground level ozone (O3), a major component of smog. · Nitrogen oxides (NO and NO2, together called NOx) - contributes to smog and acid rain, and Three main regulated emissions, and also the ones that catalytic converters are designed to reduce. How Catalytic Converters Reduce Pollution Most modern cars are equipped with three-way catalytic converters. "Three-way" refers to the three regulated emissions it helps to reduce -- carbon monoxide, VOCs and NOx molecules. The converter uses two different types of catalysts, a reduction catalyst and an oxidization catalyst. Both types consist of a ceramic structure coated with a metal catalyst, usually platinum, rhodium and/or palladium. The idea is to create a structure that exposes the maximum surface area of catalyst to the exhaust stream, while also minimizing the amount of catalyst required (they are very expensive).
A three-way catalytic converter: Note the two separate catalysts. There are two main types of structures used in catalytic converters -- honeycomb and ceramic beads. Most cars today use a honeycomb structure.
Ceramic honeycomb catalyst structure The Reduction Catalyst The reduction catalyst is the first stage of the catalytic converter. It uses platinum and rhodium to help reduce the NOx emissions. When an NO or NO2 molecule contacts the catalyst, the catalyst rips the nitrogen atom out of the molecule and holds on to it, freeing the oxygen in the form of O2. The nitrogen atoms bond with other nitrogen atoms that are also stuck to the catalyst, forming N2. For example: 2NO => N2 + O2 or 2NO2 => N2 + 2O2 The Oxidization Catalyst The oxidation catalyst is the second stage of the catalytic converter. It reduces the unburned hydrocarbons and carbon monoxide by burning (oxidizing) them over a platinum and palladium catalyst. This catalyst aids the reaction of the CO and hydrocarbons with the remaining oxygen in the exhaust gas. For example: 2CO + O2 => 2CO2 But where did this oxygen come from? The Control System The third stage is a control system that monitors the exhaust stream, and uses this information to control the fuel injection system. There is an oxygen sensor mounted upstream of the catalytic converter, meaning it is closer to the engine than the converter is. This sensor tells the engine computer how much oxygen is in the exhaust. The engine computer can increase or decrease the amount of oxygen in the exhaust by adjusting the air-to-fuel ratio. This control scheme allows the engine computer to make sure that the engine is running at close to the stoichiometric point, and also to make sure that there is enough oxygen in the exhaust to allow the oxidization catalyst to burn the unburned hydrocarbons and CO The details contained on this page are for general information only different vehicles may have different setups to those shown. |
