With this blog I should like to make another contribution to the discussion of pressure sensing and control in the exhaust system.
Today`s topic: Pressure sensors before and after the particulate filter.
As stated in my last post, the exhaust after treatment for combustion engines is largely determined by thermodynamic processes. Temperature and pressure conditions play a major role. The monitoring of various pressures with special sensors in the exhaust pipe, among other locations, can give information about the load condition* of the particulate filter.
Euro 6 legislation recommends a particulate filter
Following the introduction of the latest emission standard “Euro 6”, all manufacturers of diesel engines are required to install particulate filters in their exhaust systems. With modern diesel particulate filters (DPF), up to 99% of these fine dust particles can be eliminated from the diesel exhaust. (Ref: see footnote.) The filter principle is based on catching and collecting the particles; so clearly, such a filter will eventually reach its capacity limit. The filter is then filled with carbon particles and must be regenerated by cleaning.
The cleaning of the filter is carried out by oxidizing (burning off) the collected soot particles. There are various procedures for this regeneration. In stationary engines, these range from replacing loaded filter cartridges, to passive, more or less continuous soot combustion.
In all cases special pressure sensors are needed for precise operating of this process.
Active systems are most common in mobile applications
Most common for mobile applications, however, are active systems. Here, the regeneration process is actively initiated in various ways. Either the internal combustion engine increases the exhaust gas temperature to over 600 °C, or by deliberately adding additives to the fuel, the ignition temperature of the soot coating is reduced to below 450 °C. (See also my last note on temperature monitoring.)
Pressure in the exhaust pipe (relative to the environment pressure) is the trigger for the regeneration signal of the pressure sensors
In both the cases described, regeneration is actively initiated. The trigger comes from (of course) a pressure sensor that registers how strong the filter load actually is. An indicator of this loading is the back pressure built up in front of the particulate filter in the exhaust pipe. The higher the load condition in the filter, the greater the back pressure. A simple pressure sensor detects the dynamic pressure relative to the environment as long as there are comparable pressure conditions in the exhaust pipe after the particle filter (as with Euro 5). If a certain dynamic pressure value is reached, and the sensor has transmitted the information to the control unit, the active regeneration is started.
This kind of sensor must fulfill special requirements. The mounting position must be very close to the exhaust pipe – especially in trucks – as the sensor is not protected from the environment. This requires high robustness. At the same time, the internal pressure of the system is very low – which means that a very sensitive internal measurement is required – yet the media inside this system is one of the most aggressive we can find in automotive applications.
Specific pressure sensor for sophisticated demands in the exhaust pipe
Our Type 96724 sensor has been designed specifically for this application. A very sensitive semiconductor pressure measuring cell (MEMS – micro electro mechanical system) converts the pressure signal into an electrical voltage. This very low measurement signal is amplified in a specially developed amplifier unit (ASIC) in the sensor to a usable level, and at the same time conditioned against environmental influences.
The AB special design takes account of these requirements. Extensive optimization at the connection and joining mechanism, and close cooperation with users have enabled development of a sturdy sensor that is externally very robust, yet extremely sensitive internally.
The next level: Diesel Euro 6 engines get another catalyst
Differential Pressure Sensors for Euro 6 enable systems with much higher accuracy
What has most recently discredited the diesel engine is the emission of NO and NO2, the nitrogen oxides. To effectively reduce emission of these nitrogen oxides, an additional catalytic converter is being installed in Euro 6 vehicles after the particle filter. I shall discuss this topic in a later post.
You can´t wait for the next post? Here is the link to a short video about the functionality of the differential-pressure-sensor.
*The soot generated during fuel combustion in the engine is perceived as particulate matter, and are found especially in diesel engines. Particulate Matter consists of microscopically small, suspended dust grains with a particle size of at most 10 micrometers (= 10 thousandths of a millimeter). The particles are so small that they can be deposited in the lungs during inhalation, and even pass through the bloodstream into the entire human body, including the brain. (Source: AGVS Training Center Bernese Oberland: “Exhaust-relevant components”, p.14).
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