An important condition for the operability of the converter is the correct composition of the working mixture in the cylinders. This mixture should be as close to stoichiometric as possible.
The poor mixture causing misfires is the most dangerous for the neutralizer - in this case, vapors of unburned fuel and unused oxygen enter it. In the presence of a catalyst - a thin film of platinum and rhodium deposited on the cells of the converter - the oxidation reaction proceeds so violently that the temperature rises above a critical temperature of 900–1000 ° С. The filler, if it is metal, is melted, the honeycomb is clogged. A ceramic filler is often completely destroyed. Both are shown in the pictures.
A rich mixture for the converter is not so dangerous, because the fuel that gets into it must be burned, and for this, oxygen is needed, which is not enough in the exhaust gas. Clogging the pores of the catalytic converter with soot is rather difficult.
And now about some of the subtleties of terminology. When talking about misfires in cylinders, it must be borne in mind that they are not necessarily caused by malfunctions in the ignition system - high-voltage wires, coils, candles, etc. No less important here are violations of the composition of the mixture in the cylinders (over-enrichment or over-depletion), low compression, incorrect valve timing, penetration of coolant into the cylinders, etc. Meanwhile, a mistake has long crept into some translations of technical literature from foreign languages: someone called the misfire ignition misfire, which, to put it mildly, is not the same thing, but the translator did not see the difference. What if in one of the cylinders the valve doesn’t fit snugly or does the nozzle not work? Alas, some unfortunate mechanics, when the engine is intermittent, immediately speak of “misfire”. However, do not forget the difference!
How does the controller “catch” the misfire? He does not know the cause of the malfunction, but only fixes the increased unevenness of rotation of the crankshaft, relying on the readings of his position sensor - DPKV. How? Suppose an engine with an order of flashes in cylinders 1-3-4-2 operates in steady state, with the first and third cylinders in order, and in the fourth there is no ignition. The half-revolution time of the first and third cylinders is the same, but for the fourth it is longer - the crankshaft slowed down. Then the second cylinder will accelerate it again. The controller detects a malfunction in the engine and marks it as a pass.
To calculate the passes, each cylinder has its own counter: SUM1, SUM2, SUM3, SUM4. Calculate the culprit controller helps the camshaft position sensor. Suppose a misfire is detected in the third cylinder, then the SUM3 value increases by one, etc. Counting continues for 1000 revolutions of the crankshaft (permissible if the meter accumulates five passes during this time), then the result is reset - and the counting resumes.
The self-diagnostic system in the Euro III configuration closely monitors the readings of the meters. If their sum exceeds the mark of 2.5%, a malfunction will be recorded and error code P0300 will be recorded. Codes P0301, P0302, P0303, P0304 indicate a specific cylinder malfunction.
Together with the SUM counter one more works - SUMKAT. Its task is to fix gaps in all cylinders that affect the performance of the converter. If one gap is detected, the counter reading does not change by one, as in the previous case, but by a large amount, depending on the engine operating mode (speed, load). The minimum jump is 30 units, and the maximum is 250. The calculation of passes is stopped every 200 revolutions of the crankshaft - and the reading is reset. If during such a cycle the SUMKAT reading exceeds 1000, then a malfunction will be recorded and codes P0300, P0301 … 304 will be written to the controller's memory. Then in the instrument cluster the control lamp will flash and after a short delay the nozzle in the faulty cylinder will turn off. With numerous omissions in two cylinders at once, the controller will turn off both of them - in any case, overheating of the converter is unacceptable.
In some cases, self-diagnosis may be mistaken for objective reasons. So, the movement of the car on an uneven surface means uneven rotation of the wheels, and with them the crankshaft. In order for the controller not to count the push of the wheel in the pit for ignition, in the engine compartment of some cars that meet Euro III standards (including the Kalina VAZ-1118), an “uneven road sensor” is installed next to the upper pillar support. This is simply a vibration sensor, whose signals are also calculated by the controller. Strong shocks affecting the uniformity of rotation of the crankshaft, the controller analyzes - and distinguishes from misfires.
According to European legislation (Euro III, Euro IV), on-board diagnostics must monitor the condition of the converter and, in the event of a malfunction, turn on the diagnostic lamp. To fulfill this condition, a second oxygen sensor was installed at the outlet of the converter. If the converter copes with its task, then in most modes at the output from it the amount of oxygen is negligible. This is indicated by the waveform of the second oxygen sensor - this is almost a straight line: the signal level fluctuations are very small, and it is quite high - about 0.7 V. If the converter has partially lost its effectiveness, the remaining oxygen goes to the corresponding sensor, its signal changes, and instead of a straight line on the monitor screen, we see a pronounced curve. It is similar to the signal of the first sensor, but with a smaller amplitude and a small phase shift. The latter is associated with the length of the converter and its partial operation.
But the role of the second oxygen sensor is not limited to this - it also participates in fine-tuning the composition of the air-fuel mixture, compensating for the error of the first sensor, which must be taken into account as it ages. The controllers of some companies, comparing the readings of both sensors, calculate the aging coefficient of the converter, on the basis of which the diagnostic experts build their forecasts.
According to the requirements of Euro III and Euro IV, the self-diagnosis system must register the misfire of the mixture. Because of them, the content of harmful substances in the exhaust gases - primarily unburned hydrocarbons (CH) - increases above the permissible norms. The afterburning of an excessive amount of hydrocarbons overheats the converter and can damage it. With the level of misfire in the engine of more than 4% (for every 100 operating cycles - more than 4 passes), the content of unburned fuel vapor in the exhaust gas becomes higher than allowed by modern standards. The warning lamp will blink, warning the driver about an emergency. To protect the converter from overheating, some automakers resort to turning off the fuel supply to the idle cylinder. But the Euro II standard did not require this, so on some, even not very old foreign cars, such a function may not be. AvtoVAZ, starting with cars satisfying Euro II, has included this feature in the list of mandatory. The owners of Chevrolet Niva with the MP7.0 controller know best how useful it is: because of defects in the ignition module, they had to replace an expensive converter assembly with a resonator. With the controller M7.9.7. the risk of damage to the converter is minimized.