If you are looking for a solution to a car exhaust system problem, you have landed at the right place. And this means that it’s time to get acquainted with how to identify and recognize a gas leak and how to organize a repair exhaust leaks. Of course, a gas leak is a troublesome business, but in order to somehow ease our fate, let’s try to understand the problem at its root. For me, for example, and for my favorite, this is a really pressing and annoying problem, seemingly minor, but can have major consequences. The car growls, swears, blows smoke... It’s impossible to endure.
So, the exhaust system of a car is a so-called protective mechanism that serves to remove sound (muffling) and gas contaminants that are produced from the combustion chamber of a car.
Functions and design of a car exhaust system
This vehicle system has been developed and modified to better filter and clean the exhaust gases released by the internal combustion engine. It consists of an exhaust manifold connected to the exhaust pipe, terminated by a catalytic converter, a muffler and a final exhaust pipe.
The manifold collects the exhaust gases released by the various combustible cylinders and delivers them to the exhaust structure for purification. The catalytic converter cleans exhaust gases like carbon monoxide and unburnable hydrocarbons and turns them into less harmful gases like carbon dioxide and other mixtures. The muffler muffles the sound created by the car engine while the exhaust pipe completes releasing purified exhaust gases.
Exhaust gas recirculation (EGR) system
Exhaust gas recirculation system for ZMZ-4025, 4026 engines Exhaust gas recirculation system for ZMZ-4061, 4063 engines
Ecology and the car…. Until the 60s of the last century, few people thought about this. The number of cars was insignificant, the damage from their exhaust was invisible. Everything changed with the beginning of the automobile boom. In some countries earlier, in others later, but people physically could not breathe! Remember Japan - vending machines selling clean air began to appear on the streets. Or smog over large cities - when only the tops of skyscrapers are visible because of it...
Environmentalists sounded the alarm, and legislation established requirements for equipping cars with systems that reduce harmful emissions. The very first was the crankcase ventilation system. The law on its implementation was issued in 1961, and can rightfully be considered the founder of the environmental movement in the automotive industry. Next came the OBD on-board self-diagnosis system, which monitors the health of components and assemblies that affect increased emissions, the catalyst system and much more.
Let's begin our consideration of the car's environmental systems with the exhaust gas recirculation system (EGR - Exhaust Gas Recirculation). Despite the abundance of information on the operation of this system, there are still quite a lot of ambiguities about its operation and how to check it.
Figure 1 shows the components formed during fuel combustion.
Rice. 1. Exhaust components
The first 4 components are described in sufficient detail in the literature and repair manuals. Therefore, we will take a closer look at the last component, the mechanisms of its formation and ways to reduce it.
At high temperatures and pressure in the combustion chamber, air nitrogen N is able to combine with air oxygen O to form NO2 and NO3, briefly called nitrogen oxides NOx. The air begins to burn! By itself it is a brown poisonous gas, but in combination with water H2O forms nitric acid HN03. In the presence of sulfur (and our fuel is famous for its high content), part of the nitric acid is converted into sulfuric acid H2SO4. Thus, the exhaust of any car contains (in addition to the usual harmful components CH and CO) also a mixture of nitric and sulfuric acids. A good combination, isn't it? In the form of acid rain, this “bouquet” falls on the surface of our planet, killing all life around.
Rice. 2. Concentration of nitrogen oxides in the exhaust (taking into account the ignition timing)
That is why a real war on nitrogen oxides has been declared all over the world. In our countries, the environmental movement, unfortunately, is not very developed. But we remember - you and I will not have another planet, and we are obliged to leave it clean and comfortable for our children.
So, how is this “enemy number 1” of the modern automotive industry formed? Let's consider a graph of the formation of nitrogen oxides depending on the composition of the mixture and the ignition timing.
1. Rich mixtures (lambda coefficient less than 1). The formation of nitrogen oxides is extremely insignificant. The combustion temperature of nitrogen in the combustion chamber is about 1100 -1200 degrees, and the combustion temperature of gasoline is about 500 degrees. When there is a lack of oxygen, gasoline begins to burn first, and there is practically no oxygen left for nitrogen. Excess fuel also cools the combustion chamber, which helps reduce NOx emissions.
2. Stoichiometric mixtures (lambda coefficient is 1). Free oxygen appears. The pressure and temperature in the combustion chamber increase, and the engine efficiency increases. Emissions of nitrogen oxides are also increasing.
3. Lean mixtures (lambda coefficient greater than 1). There is excess oxygen. But if there is a shortage of fuel, the temperature and pressure in the combustion chamber drop, and the temperature at which nitrogen begins to burn is not reached. Nitrogen oxide emissions are falling.
4. Early ignition angle. The pressure and temperature in the combustion chamber increase, and the concentration of nitrogen oxides in the exhaust increases.
5. Late ignition angle. The pressure and temperature in the combustion chamber drop, and the concentration of nitrogen oxides in the exhaust decreases.
So, pay attention to the insidiousness of this component. Occurs only in the most efficient engine operating modes! How to reduce these emissions? There is only one way - to reduce the temperature and pressure in the combustion chamber. At the same time, power and efficiency are reduced (sacrificed to the environment). How to achieve environmental parameters without reducing engine efficiency? It is impossible to change the composition of the mixture - emissions of other components will increase while engine performance deteriorates. For the same reasons, it is impossible to change the ignition timing (injection timing on a diesel engine).
And this is where the EGR system - exhaust gas recirculation system - comes to the rescue. Fundamentally, there are 2 schemes for constructing these systems - external and internal.
Operating principles of external exhaust gas recirculation system
The general operating principle is shown in Fig.Z.
Rice. 3. Block diagram of the external EGR system
Part of the exhaust gases 1 is directed through the EGR valve 3 to the intake manifold. The degree of valve opening (and, accordingly, the amount of exhaust gases) is regulated by electro-pneumatic valve 2, controlled by electronic control unit 4. When they enter the combustion chamber, they displace part of the fresh mixture. There is no oxygen and no fuel in the exhaust gases. Therefore, the composition of the mixture in the combustion chamber does not change - just less of it enters the cylinder. Accordingly, pressure and temperature drop, and as a result, the concentration of nitrogen oxides decreases. Control over the amount of supplied exhaust gases was carried out by reducing the readings of the mass air flow sensor 5 (on early FORD models, control by a lambda probe was also used). At a constant position of the throttle valve 6, a decrease in the flow of fresh air led to a decrease in the opening time of the injector 7. The fuel-air ratio remained unchanged, the amount of harmful emissions of CO and CH did not increase. There was also no need to shift the ignition angle to the later side.
To increase the accuracy and control over the operation of this system, additional sensors began to appear in subsequent developments. The first of these was the valve lift sensor. It was a regular potentiometer located on the valve stem. It informed the control unit about the actual lift height, thereby monitoring the serviceability of the valve drive and the accuracy of its operation. The appearance of electronic components in the exhaust gas environment has sharply reduced the reliability of this unit.
Also, to improve the accuracy of operation, the pneumatic drive was replaced by an electric drive, which also did not improve reliability. At the moment, failures of valve electronic components are quite common. Further, to control the operation of this system, a temperature sensor was installed at the valve outlet, an absolute pressure sensor in the intake manifold (on diesel cars, for its operation it was necessary to install a Venturi tube in the exhaust gas supply channel) and a number of other sensors. Any deviations in the operation of the system caused error codes to appear and the vehicle to go into emergency mode.
Conditions for EGR error codes
Error code "EGR valve lift sensor"
1. The lift height sensor is faulty.
2. The drive does not process control unit commands correctly.
3. Errors in connecting vacuum tubes (pneumatic drive).
Error code "Ineffective operation of the EGR system"
When the valve is opened by a specified amount, the exhaust gas flow does not correspond to the calculated value. It is determined by deviations of the readings of mass air flow sensors, absolute pressure and other sensors from the calculated values when the valve opens. Possible reasons:
1. The EGR valve is coked or burnt out.
2. The flow area of the exhaust gas supply tube has decreased.
3. Incorrect signals from control sensors.
4. EGR valve is plugged.
Algorithm for operating the EGR system on gasoline engines
The operation of a gasoline engine is characterized by external mixture formation (the mixture is prepared in the intake manifold). The load on the engine is regulated by changing the amount
wa of the mixture entering the cylinder (cyclic filling of the cylinder) due to the opening or closing of the throttle valve. There are several modes of operation of a gasoline engine.
1. Cold start. The combustion chamber temperature is low and the mixture is rich. No nitrogen oxides are formed and there is no need for a recirculation system. The EGR valve is closed.
2. Warming up the engine. Enriched mixture. The valve is also closed.
3. Idling (zero load mode). The throttle is closed, little fresh mixture is supplied. The temperature and pressure in the combustion chamber are low, nitrogen oxides are formed in extremely small quantities. Opening the valve in this mode is not required, moreover, it is unacceptable. The entry of exhaust gases into the combustion chamber at idle speed (with low filling of the cylinder with fresh mixture due to a closed throttle valve) leads to catastrophic consequences in engine operation. For example, if the EGR valve burns out or does not close tightly, “extra” exhaust gases lead to disruptions in the combustion process. In this case, unstable engine operation is observed, and CH emissions increase sharply. The EGR valve must be closed in this mode. The exception is direct injection engines. When idling, they have excess oxygen, which requires the EGR valve to open slightly. For example, on GDI engines, the valve opening degree in this mode ranges from 4% to 19%.
4. Partial load mode (steady state). The throttle is slightly open, its position does not change. The mixture is stoichiometric (or slightly lean). The filling of the cylinder increases, the pressure and temperature in the combustion chamber are high. All conditions are created for the formation of nitrogen oxides. The degree of opening of the EGR valve is determined by the control unit depending on the degree of throttle opening, air flow and a number of other parameters. Since high engine power is not required in steady state, opening the valve does not affect the operation of the vehicle. Valve failures in this mode have little effect on vehicle operation.
5. Acceleration mode (transition mode). The mixture is enriched and the formation of nitrogen oxides is insignificant. Maximum performance is required from the engine. The valve must be closed. If it fails, sluggish dynamics and failures when accelerating are observed.
6. Power load (kick-down). The gas pedal is pressed all the way down. The mixture is rich, maximum performance is required from the engine. Opening the EGR valve leads to a drop in power and a deterioration in the dynamic characteristics of the car. Must be closed.
Let's summarize all of the above.
1. On a gasoline engine, the EGR valve should only be open at steady state (partial loads). All other modes are prohibitive conditions for the operation of this system.
2. Malfunctions of this system affect operation in all modes, with the exception of partial load mode.
But as always, there is one “but”. When the throttle is opened (acceleration mode and full load), the filling of the cylinder increases, the pressure and temperature in the combustion chamber increase. The likelihood of the formation of nitrogen oxides, although slightly, increases. Due to the enrichment of the mixture in these modes, CO and CH emissions also increase. Therefore, in order to comply with environmental standards on modern cars, the duration of these modes is limited, the EGR valve opens slightly, which negatively affects the power and dynamics of the engine, but this is a necessary measure. These parameters are sacrificed to the environment.
Algorithm for operating the EGR system on diesel engines
The operation of a diesel engine is characterized by internal mixture formation (the mixture is prepared directly in the cylinder). The load on the engine is regulated by changing the amount of fuel supplied per cycle (cyclic supply). The filling of the cylinder with air is maximum in all diesel operating modes. Those. in all modes, excluding full load mode (maximum cyclic supply), an excess of free oxygen is observed. The excess air coefficient at idle can reach 10, at full load - 1.5, and only on sports cars (for example, Paris - Dakar) it approaches 1. Taking into account higher compression ratios (higher temperatures and pressures) and the presence of free oxygen the likelihood of nitrogen oxide formation increases sharply. The EGR system is placed under greater pressure to suppress these emissions. The operating algorithm is fundamentally different from its gasoline counterpart, because aimed at limiting the air supply. Let's return to Fig.Z. On a diesel engine, devices appear that are completely uncharacteristic for it: mass air flow sensor (5), throttle valve (6).
The throttle valve assembly includes the throttle itself, its position sensor and an electronic drive. On earlier models (TOYOTA, NISSAN), a mechanical drive was used using a throttle cable. At the moment it is practically not used. Quite often, the EGR valve and throttle are structurally made into one unit.
1. Idling (zero load mode). Cyclic fuel supply is minimal. Based on its value, the control unit (to limit the air supply) opens the EGR valve and closes the throttle until the air flow sensor readings indicate that the desired fuel-air ratio has been achieved. The amount of incoming air fully corresponds to the fuel supply.
2. Partial load mode. The cyclic fuel supply is average, the EGR valve is slightly open, the throttle is in an intermediate position. The amount of air supplied is still fully consistent with the amount of fuel.
3. Full load mode. The cyclic fuel supply is maximum, the EGR valve is closed, the throttle is fully open. The fresh air supply is maximum, the fuel-air ratio remains unchanged.
All failures of the EGR system on a diesel engine lead to incorrect calculation of the air supply. If more of it is supplied than necessary, this does not affect the operation of the diesel engine, only emissions of nitrogen oxides increase. If less air is supplied than needed, the diesel engine begins to smoke black smoke. As a rule, this defect is very noticeable at full load. Main reasons for failures:
1. Poor closing or malfunction of the EGR valve.
2. Incorrect air flow sensor readings.
3. Malfunction of the throttle valve assembly.
If this system fails, the diesel engine goes into emergency mode. An error code is recorded and the malfunction indicator lamp (MIL) lights up. The EGR system is turned off, the cyclic fuel supply is limited to no more than 30%.
Troubleshooting the EGR system
The health of the system can be assessed by measuring engine parameters when the EGR valve is forced on or off.
1. Gasoline engine. When the valve is forced to close, engine operation does not change. When the valve is forced to open at idle, the pressure in the intake manifold sharply increases, the readings of the air flow sensor drop, and engine operation becomes extremely unstable. It is better to check with a scanner, viewing the data stream. We carefully compare the degree of valve opening, air flow readings and absolute pressure in the intake manifold. If more accurate results are needed, we output these signals to several beams of a USB oscilloscope. They all need to change synchronously. Any non-synchronization clearly indicates that the valve is jammed and (or) its malfunction.
2. Diesel engine. When the valve is forced to close, engine operation does not change. When the valve is forced to open, idling also remains unchanged, but when the gas is overloaded, the smoke increases sharply. The check should be carried out using a smoke meter.
On a working diesel engine, the smoke coefficient “K” during over-gassing should not exceed the established limits (1-1.5). Permissible limits are indicated on the engine compartment plate or in databases. When the valve is turned off, the smoke curve does not change. When the valve is forcibly turned on, the smoke curve during re-gassing rises to values K = 6-10. If the difference between the smoke coefficient with the valve forced off and on is small, this clearly indicates a malfunction of the EGR valve. If the smoke curve in normal mode during over-gassing is higher than when the EGR valve is forcibly turned off, we can judge that the control system for this system is faulty.
Rice. 4. Smoke curves when re-gasping a working engine
Figure 4 shows the opacity curves of a serviceable engine during throttling with the EGR valve forcibly turned on and off. To increase the accuracy of measurements, you should take several measurements (shown in different colors on the screen).
Note. If the vehicle is equipped with a particulate filter, the smoke curves may differ significantly from the stated values.
Operating principles of internal exhaust gas recirculation system
A distinctive feature of this system is the absence of a tube connecting the exhaust and intake manifolds. Instead, a throttle valve is installed in the exhaust gas exhaust tract. The block diagram is shown in Fig. 5.
Rice. 5. Block diagram of internal exhaust gas recirculation
The exhaust gases do not enter the cylinder again, but simply do not leave it. The amount of remaining exhaust gases is controlled by the opening angle of the exhaust damper. The drive of this damper can be either pneumatic or electric and is controlled from the control unit. The exception is a number of Chinese-made cars. For them, this damper is manually actuated by a cable from the driver’s cab.
This scheme is rarely used on passenger cars. Installed on a number of BMW models and some other manufacturers. They are more common on medium and heavy trucks. In addition to recirculation functions, it can be an element of the “mountain brake” system, accelerated turbine stop, etc. The advantage of this scheme is that the exhaust gases remaining in the cylinder have a higher temperature. The engine warms up faster and its bottom-out performance increases. But an increase in temperature in the combustion chamber leads to an increase in emissions of nitrogen oxides - and this is what this system is designed to combat! Therefore, on highly accelerated engines, the use of an internal recirculation system (despite all its advantages) is inappropriate. Many manufacturers use a combined EGR system. A reasonable combination of internal and external exhaust gas recirculation systems makes it possible to achieve acceptable environmental performance that does not worsen (and sometimes improve) engine performance parameters.
Fedor Ryazanov AvtoMaster
Exhaust system malfunctions and signs of leakage
Damage to any part of the exhaust system can cause harmful gases to leak into the vehicle before they can be cleared. Such damage is usually caused by the accumulation of moisture in it, which leads to corrosion of parts.
Another cause of damage is direct physical impact. And it's good if the problem is detected immediately. Otherwise, the exhaust gases may act like carbon monoxide in the passenger compartment and may lead to serious and dangerous consequences.
Signs and identification of leaks:
Loud, roaring, or shooting NOISE
This nasty symptom occurs when you start your car. The device screams for attention.
Rust
There is only one way to check this - by taking a flashlight. Better wear gloves, a mask and safety glasses. Using a brush, you can clean the parts of the exhaust structure.
Smell of gas
A distinctive gas smell, along with a strong emission of smoke when starting the engine, indicates a clear problem.
Methods for eliminating gas leaks
One of the quick ways to eliminate leaks is to fill the holes in the exhaust device with special plaster and wrap it with dense material. However, this is only a temporary salvation until we get to the nearest service station, where specialists must qualitatively weld parts of the exhaust system and completely replace it, thereby eliminating exhaust leaks .
So, you have learned the causes and signs of a leak in a car and, I hope, you will not delay for too long in solving this problem. Good luck!
[edit] Why should you replace a faulty oxygen sensor?
If the oxygen sensor is “broken” (Read How to check the functionality of the oxygen sensor on a Subaru). In this case, the ECU begins to operate according to the average parameters recorded in its memory: in this case, the composition of the resulting fuel-air mixture will differ from the ideal one. As a result, there will be increased fuel consumption, unstable engine idling, an increase in CO content in the exhaust gases, a decrease in dynamic characteristics, but the car remains on the move. In some car models, the ECU reacts to the failure of the lambda probe very seriously and begins to so zealously increase the amount of fuel supplied to the cylinders that the fuel supply in the tank “melts” before our eyes, black smoke comes out of the chimney, CO “goes off scale,” and the engine “goes dull.” "and you will most likely have to get to the nearest service station in tow.
The list of possible malfunctions of the lambda probe is quite large and some of them (loss of sensitivity, decreased performance) are not detected by the car’s self-diagnosis. Therefore, the final decision to replace the sensor can only be made after a thorough check, which is best entrusted to specialists. It should be especially noted that attempts to replace a faulty lambda probe with a simulator will lead to nothing - the ECU does not recognize “foreign” signals and does not use them to correct the composition of the prepared combustible mixture, i.e. simply “ignores”.
When the lambda probe is burned out or disconnected, the CO content in the exhaust increases by an order of magnitude: from 0.1 - 0.3% to 3 - 7%, and it is not always possible to reduce its value, since the power reserve of the mixture quality screw may not be enough. In cars whose L-correction system has two oxygen sensors, the situation is even more complicated. If the second lambda probe fails (or the catalyst section is “punched”), it is almost impossible to achieve normal engine operation.
In general, the lambda probe is the most vulnerable sensor in a car with an injection system. Its service life is 40 – 80 thousand km, depending on operating conditions and engine serviceability. Poor condition of oil scraper rings, antifreeze getting into the cylinders and exhaust pipes, enriched fuel-air mixture, and malfunctions in the ignition system greatly reduce its service life. The use of leaded gasoline is strictly unacceptable - lead “poisons” the platinum electrodes of the lambda probe after several uncontrolled refills.
Replacing a faulty oxygen sensor with a new sensor allows you to save fuel, improve vehicle dynamics, reduce exhaust toxicity, and prevent premature failure of an expensive catalyst.
