Exhaust system - What will the car's exhaust system do?

The story of the exhaust system is as complicated (or simple, as you like) as much as the story of the car's intake. Thus, in this chapter we will study all parts of the exhaust system, which include the exhaust manifold (or exhaust manifold), the exhaust muffler (muffler), the catalyst, the exhaust particle filter and the oxygen sensor (O2 sensor or Lambda probe), and we will talk about the unusual ones. mystical liquids that are injected into the exhaust due to additional environmental effects.

Under the common denominator, we can say that the task of the exhaust system (in addition to the exhaust) is to collect, cool, silence (conditionally speaking) and purify the exhaust gases. But let’s move on (in some) order.

Exhaust manifold - the place where the story begins

After combustion, the hot gases go through the open exhaust valve into the exhaust manifold, pass by the oxygen sensor (lamp-probe), enter the catalyst and then pass through one or more mufflers. In the case of a diesel engine, there may be an exhaust particulate filter somewhere in the whole story, commercially commonly referred to as a DPF (Diesel Particulate Filter). But in the end, all that's left goes through the exhaust pipe into the environment, so we breathe it in nicely, and that's it

The exhaust manifold, or exhaust manifold, is similar in appearance to the intake manifold. But, these are pipes that continue to the exhaust openings of the cylinders. The exhaust manifold in "ordinary" cars is usually made of metal cast in a mold, but there are also some that are made of welded rolled pipes, which is usually the case with higher performance engines or sports cars. There is not much to talk about the exhaust branch, except that we will mention that (while the engine is running) it should not be touched because, as we know, the exhaust gases that pass through it are extremely high temperatures.

But what is technically more significant, however, is the construction of the exhaust manifold, of which we have presented the four most common. On the left in the picture above, the exhaust manifold of an ordinary and nothing but exciting 4-cylinder engine, usually made of cast iron, is shown. Exhaust gases are supplied here from all four cylinders by pipes that connect in one place from where everything goes to the catalyst and damping pots.

But the next collector shown is already a little "smarter". With it, the exhaust pipes are connected gradually, which reduces the internal resistance of the exhaust system, thanks to which their flow is accelerated. The third version of the story is basically the same as the second, while the most complicated one shows the exhaust manifold of engines that are installed on sports cars, ie. high performance engines.

In the fourth, most complicated example, the basic goal of the design (all pipes are approximately equal in length) is to reduce the internal resistance of the exhaust system as much as possible so that the exhaust gases flow as quickly as possible, thus facilitating the "job" of the engine. speed numbers. This type of exhaust manifold has pipes whose length is adjusted so as to enable the maintenance of high speeds of movement of exhaust gases, which, on the other hand, improves their discharge from the cylinder (when the exhaust valve is open).

Let's say, once again, that the exhaust manifold is one of the exposed parts of the car that is most heated (besides, perhaps, the turbocharger housing). temperatures between 250 and 500 ° C are quite common, which is why most cars today are equipped with some sort of insulation and / or exhaust manifold protection.

The term "oxygen sensor" means nothing to anyone, but when we say Lambda…

Indispensable in modern cars, part of all engines is the O2 sensor or Lambda probe. This unusual device with a Greek name is actually a sensor that reads the amount of oxygen in the exhaust. As we have learned before, for complete combustion of gasoline, it is necessary to achieve a ratio of fuel and air of 14,7: 1 (in favor of air, of course).

The lambda, which is placed at the point where all the pipes of the exhaust manifold connect into one, measures the amount of oxygen in the exhaust gases and "compares" it with the amount of its percentage in the atmosphere. The probe itself, the size of an average spark plug, is an electrical device that reacts to a change in the amount of O2 in the exhaust by changing the voltage at its electrical connection (the range is usually between 0,15 and 1,30 V).

When the ratio of fuel and air injected into the cylinder is close to the stoichiometric ideal (14,7: 1) Lambda gives a voltage of approx. 0,45 V (450 mV). When the oxygen content falls below that value, the voltage at the output connector of the Lambda increases, which indicates a rich mixture. Certainly, with a poor mix, the opposite is true.

This voltage signal from the Lambda sensor travels to a central computer that adjusts the amount of fuel injected into the cylinders. In such a case, we can talk about a closed-circuit fuel injection control system. Namely, unlike the open circuit system, the first method of injection control does not determine the required amount of fuel exclusively according to the map stored in the memory of the electronic control unit of the engine but is primarily based on data obtained from the Lambda probe, in real time. The goal, of course, is to achieve the most complete combustion of fuel, which, in addition to reducing consumption, also achieves more efficient engine operation and reduces harmful emissions.

It should also be mentioned that the first Lambda probe for car engines was introduced by the German factory Robert Bosch GmbH in 1976, and the first manufacturer to start installing it (along with a 3-stage catalytic converter) was Volvo.

Finally, it is interesting to note that the Lambda probe can also be used to tune older engines (without catalysts and similar "wonders"), which is sometimes done by competitors with limited budgets. For this purpose, a hole is drilled in the exhaust branch (where all the pipes are connected into one) into which the oxygen sensor is screwed. An ordinary voltmeter then measures the voltage at the sensor connection, so it is possible to set the ideal mixture, either on the carburetor or the injection system.

What does a catalyst actually catalyze?

The nightmare of all "racers" on today's cars is certainly a catalytic converter or, more popularly, a catalyst. This is, in fact, a metal box containing a honeycomb ceramic monolith, most often, coated with platinum (which is actually the catalyst itself, in a chemical sense). The role of the catalyst, which is used in engines run on unleaded gasoline, is to reduce emissions. Three-stage catalysts act on exhaust gases to cause oxidation of carbon monoxide (CO) and hydrocarbon (HC) and reduction of nitric oxide (NOx). However, in order for these processes to take place on the porous surface of the catalyst interior, it is necessary to bring it to operating temperature (300 - 800 ° C). The biggest problem here occurs when starting a cold engine when the exhaust gases pass through an unheated catalyst. In order to bring the catalytic converter to operating temperature as quickly as possible, various solutions are used in the exhaust system of today's engines. Sometimes a system of subsequent injection of fresh air into the exhaust (in front of the catalyst) is applied, which, by adding oxygen, raises the temperature of the exhaust gases.

Another version of heating is the one with electric heaters inside the catalyst, but the simplest solution is to place the catalyst as close as possible to the exhaust branch, in a place where the temperature of the exhaust gases is high. The nightmare mentioned at the beginning of this section actually stems from the fact that the internal structure of the catalytic converter creates a great resistance to the flow of exhaust gases which reduces engine power (in some, allegedly up to 20%!), So the catalysts today so do diesel engines) a rather complicated topic if we are talking about sports cars. Certainly, some question the purpose of reducing harmful gases by such "blocking" if, then, to achieve the same power as without a catalyst, you need to use a more powerful engine that burns more fuel. But let's leave that for another, philosophical discussion…

Earlier catalysts were 2-stage or so-called. oxidation catalysts. This means that they were able to perform two tasks simultaneously: the oxidation of carbon monoxide to produce carbon dioxide and the oxidation of carbon to hydrogen (unburned and / or partially burned fuel) to produce carbon dioxide and water. Two-stage catalysts were used in earlier gasoline engines where they were replaced by 3-speed, but in diesel engines today are used exactly 2-speed, or. oxidation catalysts.

Three-stage catalytic converters or catalysts have the added ability to control nitrous oxide. Thus, in addition to the functions performed simultaneously by 2-step catalysts, such catalysts also have the function of reducing nitric oxide to nitrogen and oxygen. And, this is precisely what achieves a significant reduction in greenhouse gas emissions.

 

So, get acquainted with DEF (Diesel exhaust fluid) systems or urea solution injection systems. What it is? A solution containing 32,5% high purity urea and 67,5% deionized water is used in the Selective Catalytic Reduction (SCR) process to reduce the concentration of nitric oxide (NOx) in the exhaust gases of diesel engines. Namely, since diesel engines use a relatively poor mixture in their work (in order to reduce the formation of soot and burn possibly unburned fuel), excess oxygen causes the formation of nitric oxide in the exhaust gases.

To overcome this problem, some manufacturers have started using DEF systems that inject the said solution into the exhaust gas stream, which is thus converted into steam and decomposed into ammonia and carbon dioxide. At the end of the exhaust gas path, in the SCR catalyst, with the help of ammonia, nitrogen oxides are reduced to harmless components - water and nitrogen. Finally, let us add that one of the most well-known commercial names for the solution used for SCR processes is AdBlue. Depending on the car, this liquid can be topped up by the owner at the pumps offered by AdBlue or refilled exclusively in a workshop.

Every pot (needs) a lid

At the end of the exhaust comes, probably the most famous part, the exhaust pot which, precisely because of its position, we left for the end of the story about this system. More correctly, a "muffler", this part of the exhaust system is, again, a kind of metal box that tries to reduce noise. Namely, the gases produced by the combustion of a mixture of fuel and air expand very quickly coming out of the cylinder under high pressure (and supersonic speed). As a result, in the exhaust system, these gases cause very strong vibrations (frequencies of several thousand per minute) which, without adequate damping, would create a huge noise (you probably heard, once, heard the engine of a race car that did not have a muffler on the exhaust, Harleys not to mention…). In order to calm these vibrations, mufflers are placed at the end (after the catalyst) of the exhaust system.

The cheapest, and most common, construction of the muffler is the so-called. a reflective damper that significantly slows down the flow of gases, but thus creates a relatively large back pressure in the exhaust system, thus reducing the usable power of the engine. The best solution to this problem is an absorption damper. In it, the gases pass through a perforated tube wrapped in absorption (damping) material. But, with such mufflers, the speed of gas flow does not decrease significantly, which results in significantly higher engine noise (in addition, only well-calculated dimensions of the exhaust will give really more power, whatever muffler we put). As always, car manufacturers offer a compromise solution. One such is the absorption-reflection muffler, which, according to the created back pressure and noise at the outlet, is, of course, somewhere "halfway" between the previous two solutions.

However, it should be emphasized that the permissible amount of noise is also regulated by law. Therefore, it will be difficult to evoke the cheerful smiles of the police with a makeup exhaust pot.