Diesel particulate filter
On passenger cars with a diesel engine, a diesel particulate filter has been used as part of the exhaust system since 2000. With the introduction of Euro-5 standards in January 2011, the use of a particulate filter in passenger cars with a diesel engine is mandatory.
In a diesel engine, soot is formed during incomplete combustion of fuel. Soot particles have a size of 10 nm to 1 μm. Each particle consists of a carbon core with which hydrocarbons, metal oxides, sulphur and water are connected. The specific composition of soot is determined by the engine operating mode and fuel composition.
In the exhaust system, the particulate filter is located behind the catalytic converter. In a number of designs, the particulate filter is combined with an oxidation-type catalytic converter and is located immediately behind the exhaust manifold where the temperature of the exhaust gases is maximum. It is called a catalytic coated particulate filter.
The main structural element of the particulate filter is the matrix, which is made of ceramic (silicon carbide). The matrix is placed in a metal case. The ceramic matrix has a cellular structure consisting of channels of small cross-section, alternately closed on one and the other side. The sidewalls of the channels have a porous structure and play the role of a filter.
The cells of the matrix are square in cross-section. More perfect are the input cells of an octagonal shape. They have a larger surface area (compared to the outlet cells), allow more exhaust gases to pass through and provide a longer diesel particulate filter life.
In the operation of a particulate filter, two successive stages are distinguished: filtration and soot regeneration. During filtration, soot particles are captured and deposited on the filter walls. The greatest difficulty for retention is represented by soot particles of small size (from 0.1 to 1 μm). Their share is small (up to 5%), but these are the most dangerous emissions for humans. Modern particulate filters trap these particles as well.
Soot particles accumulated during filtration create an obstacle for exhaust gases, which leads to a decrease in engine power. Therefore, periodically cleaning the filter from accumulated soot or regeneration is required. Distinguish between passive and active regeneration of the particulate filter. In modern filters, as a rule, passive and active regeneration is used.
Passive regeneration of the particulate filter is carried out due to the high temperature of the exhaust gases (about 600 ° C), which is achieved when the engine is running at maximum load. Another way of passive regeneration is the addition of special additives to the fuel, which provide soot combustion at a lower temperature (450-500 ° C).
Under certain engine operating conditions (light load, traffic in the city, etc.), the exhaust gas temperature is not high enough and passive regeneration cannot occur. In this case, the active (forced) regeneration of the particulate filter is carried out.
Active regeneration of the particulate filter is carried out by forcibly increasing the temperature of the exhaust gases for a certain period of time. The soot accumulated in this process is oxidized (burned out). There are several ways to increase the temperature of the exhaust gases during active regeneration:
- late fuel injection;
- additional fuel injection on the exhaust stroke;
- the use of an electric heater in front of the particulate filter;
- injection of a portion of fuel immediately in front of the particulate filter;
- heating of exhaust gases by microwaves.
The design of the particulate filter and the systems ensuring its operation are constantly being improved. Currently, a particulate filter with a catalytic coating and a particulate filter with a system for introducing additives into the fuel are most in demand.
Catalytic coated diesel particulate filter
The catalytic coated diesel particulate filter is used on Volkswagen vehicles and a number of other manufacturers. The catalytic coated particulate filter distinguishes between active and passive regeneration.
With passive regeneration, the soot is continuously oxidized due to the action of the catalyst (platinum) and the high temperature of the exhaust gases (350-500 ° C). The chain of chemical transformations during passive regeneration has the following form:
- nitrogen oxides react with oxygen in the presence of a catalyst to form nitrogen dioxide;
- nitrogen dioxide reacts with soot particles (carbon) to form nitric oxide and carbon monoxide;
- nitric oxide and carbon monoxide react with oxygen to form nitrogen dioxide and carbon dioxide.
Active regeneration occurs at a temperature of 600-650 ° C, which is created using a diesel control system. The need for active regeneration is determined on the basis of an estimate of the throughput of the particulate filter, which is carried out using the following sensors of the diesel control system: air flow meter ; exhaust gas temperature to the particulate filter; exhaust gas temperature after the particulate filter; differential pressure in the particulate filter.
Based on the electrical signals of the sensors, the electronic control unit additionally injects fuel into the combustion chamber, and also reduces the air supply to the engine and stops the exhaust gas recirculation. In this case, the temperature of the exhaust gas rises to the value required for recirculation.
Diesel particulate filter with a fuel injection system
The diesel particulate filter with a fuel injection system is a development of the PSA concern (Peugeot-Citroen). Since the pioneers in the use of additives for regeneration are the French, the name FAP filter (from the French Filtre a Particules) was assigned to the filter. A similar approach is implemented in the particulate filters of other automakers (Ford, Toyota).
The system uses an additive containing cerium, which is added to the fuel and ensures the burning of soot at a lower temperature (450-500 ° C). But even this temperature of the exhaust gases cannot always be reached, therefore, active regeneration of the diesel particulate filter is periodically performed in the system. The diesel particulate filter is usually installed separately behind the catalytic converter.
The additive is stored in a separate tank with a capacity of 3-5 litres, which is enough for 80-120 thousand kilometres (filter life). Structurally, the tank may be in the fuel tank or outside it. The additive level in the tank is monitored using a float type sensor. The additive is supplied to the fuel tank using an electric pump. The additive is supplied at each refuelling of the fuel tank in proportion to the refuelling volume of fuel. The start and duration of the additive supply is regulated by the engine control unit (in some designs, by a separate electronic unit).
A side effect of the use of the additive is that during combustion it settles in the form of ash on the walls of the filter and is not removed from it, which reduces the resource of the device. The service life of a modern particulate filter is 120,000 km. Manufacturers declare the release of a filter with a resource of 250,000 km in the near future.
Due to the high cost, the particulate filters that have exhausted the resource are usually not replaced by car owners but removed, followed by a flashing of the engine management system.
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