In a gasoline engine, the converter efficiently converts toxic compounds of CO, CH, NOx into the harmless components of CO2, H2O and N2. With short-term deviations in the composition of the air-fuel mixture from the optimum or in violation of ignition, microparticles of unburned carbon may appear - in other words, soot. But while there are no serious malfunctions, its share in the exhaust gases is small and in the converter it is burned to carbon dioxide. Behind a car with a working black smoke converter does not happen.
With diesel exhaust is not so simple! When working with light loads, the temperature of the gases at the inlet to the converter is much lower than that of a gasoline engine, and soot simply does not have time to burn out. But throwing carcinogens into the atmosphere is a bad taste, so I got a diesel in addition to a neutralizer, a special, particulate filter. On modern engines, both nodes are located in a single housing - a collector located next to the engine. Nearby because the closer to the exhaust valves, the higher the temperature of the exhaust gases necessary for the effective operation of the cleaning system.
Diagram of the exhaust system of a diesel engine:
Diesel exhaust system diagram
1 - engine control unit
2 - mass air flow sensor
3 - gas temperature sensor at the inlet to the turbocharger
4 - turbocharger
5 and 9 - gas temperature sensors in front of the particulate filter and after it
6 - mixture composition sensor (broadband oxygen sensor)
7 - filter neutralizer
8 - gas differential pressure sensor
10 - silencer
But still, over time, the filter gradually fills with soot. So that she does not tightly clog the cells, she needs to be periodically disposed of, burned. Use two methods. In the first, the engine management system does not interfere in any way with the work process - this is the so-called passive regeneration. It occurs at a gas temperature at the inlet of the filter not lower than 350 degrees, in the presence of a catalyst - platinum deposited on its ceramic honeycomb. The latter are similar to the well-known cells of modern converters, but have a significant difference, shown in Fig. 2. Channels are divided into inlet and outlet. The first, open from the engine side, receive gases with a whole bunch of toxic substances, including soot. The second ones are open from the opposite side - of which the gases purified from soot go further into the neutralizer. The channels are staggered and separated by thin filter walls, impervious to soot (it remains in the inlet), but passing gases. Their material is porous silicon carbide coated with a mixture of aluminum and cerium oxides, and it serves as a supporting surface for the platinum layer.
The scheme of organization of the channels (cells) of the particulate filter:
The scheme of organization of the channels (cells) of the particulate filter. Soot accumulates in the inlet channels, and gases passing through the porous walls go into the outlet channels
Soot accumulates in the inlet channels, and gases passing through the porous walls go into the outlet channels
It is not always possible to ensure the exhaust gas temperature necessary for burning soot in a diesel engine - at low loads, a lot of air enters the cylinders, and there is little fuel! Enough heat is released only when operating at relatively high power - for example, at a speed of 60–80 km / h, or even higher. But often this is not feasible, especially in the city, and self-cleaning the particulate filter does not occur. If you rely only on it, then over time the soot will completely clog the intake channels (cells) and the working processes in the engine will be disrupted. To prevent this from happening, you need to get rid of soot, and to do this, keep the high temperature of the gases inside the particulate filter.
The second way to clean the filter is active regeneration. If necessary (see below), the engine control unit begins to supply a little extra fuel to the cylinders after the main dose, shortly before the exhaust valve opens. The "excess" diesel fuel does not have time to work and flies into the particulate filter, where it burns violently in the presence of platinum. The temperature of the gases rises, and soot actually burns out at the command of the control unit.
Back to fig. 1. The control unit 1 needs to decide when and for how long to activate the active filter regeneration mode. But how does he realize? Very simple: by the pressure drop of the gases on the converter. To this end, tubes are connected on both sides of the pipe, connected to a differential pressure sensor 8. When the delta exceeds the set value, the regeneration mode will be activated. Usually it lasts 10-15 minutes.
In fact, not everything is so simple. The pressure drop across the particulate filter is associated with the volumetric flow rate of the exhaust gases, which, in turn, depends on their temperature. Therefore, temperature sensors 5 and 9 are installed in front of and after the diesel particulate filter. And of course, for completeness of information, the control unit takes into account the mass air flow, sensor 2 of which is traditionally located in the intake pipe.
There is also a sensor 3 directly on the engine exhaust pipe. It monitors the temperature of the exhaust gases at the inlet to the turbocharger. If it approaches a limit beyond which overheating and destruction of a very expensive unit is possible, the control unit will limit the fuel supply - and the temperature will drop.