Partikkelfilter i Trondheim
Partikkelfilteret i Trondheim – Oppbygning og virkemåte av DPF/FAP-filter
Også kjent som DPF, FAP, GPF, er et filter som renser eksos fra faste partikler som sot og aske. De første filtrene ble brukt allerede fra 1980, og i biler fra 1996 – franske HDI-motorer. I 2005 trådte Euro4-standardene i kraft, som tvang bruken av DPF, selv om noen produsenter klarte å unngå dette. Innføringen av Euro5-standarden i 2009 medførte at alle kjøretøy med dieselmotor som kjører på offentlige veier i Europa må være utstyrt med partikkelfilter i Trondheim.
Som navnet antyder, har partikkelfilteret til hensikt å fange opp faste partikler som er et biprodukt av forbrenning av drivstoff. For å forlenge levetiden utføres det periodisk en aktiv regenerering, også kjent som selvrensing ved forbrenning. Denne prosessen blir vanligvis initiert av motorstyringen basert på målt trykk i eksosstrømmen inne i filteret.
De mest populære forkortelsene er FAP (fransk: filtre à particules), DPF (engelsk: diesel particulate filter eller tysk: diesel partikel filter), og disse er filtre beregnet på dieselmotorer. Generelt sett er det også akseptert at forkortelsen FAP tilskrives våte filtre som hovedsakelig brukes i franske motorer, mens tørre DPF-filtre brukes andre steder. For bensinmotorer kalles partikkelfilteret GPF (engelsk: gasoline particulate filter), og det må monteres obligatorisk fra juli 2018.
Partikkelfilter i Trondheim - Oppbygning
The structure of a DPF filter is not complicated; it consists of a ceramic core and a metal casing. The DPF particulate filter somewhat resembles a catalytic converter, and its construction resembles a honeycomb, but the number of channels is larger, they are partially blocked and porous. Soot particles are larger than the pores, so they are trapped in the filter and removed later during the regeneration process. The exhaust flows through open channels where small particles pass through the pores and exit the exhaust system, while the large ones (soot particles) are trapped inside the filter.
Types of DPF Filters
Dry DPF Filters: In a dry DPF filter, there are particles of aluminum oxide and cerium oxide as well as platinum on the porous walls (they oxidize CO to CO2, NO to NOx).
Wet DPF Filters: These filters use an additive to the fuel (called catalytic fluid).
Regeneration of Particulate Filters
There are three main modes of regeneration/cleaning of DPF:
Passive Regeneration
In the case of passive regeneration, the soot is automatically neutralized (thermochemical reaction).
Active Regeneration
Aktiv regenerering blir utløst av motorstyringen, som øker eksostemperaturen ved å injisere ekstra drivstoff under eksosfasen, ECU forsinkelse av injeksjonsvinkelen og lukker EGR-ventilen. Aktiv regenerering kan identifiseres ved endret motorlyd og økt drivstofforbruk som vises midlertidig på bilens instrumentpanel. Trykkforskjellssensoren overvåker trykkforskjellen mellom inntaket og utløpet av filteret, slik at ECU kan estimere graden av sotfylling i filteret. Jo mer sot, desto vanskeligere er det for eksosen å passere gjennom. Dette fører til økt trykkforskjell – som et resultat er trykket foran filteret høyere enn trykket bak. Avhengig av hvor stor denne forskjellen er, endres sensorens motstand.
Forced Regeneration
There are several steps indicating how full the DPF filter is. Exceeding level 1 starts active regeneration. Level 2 is a transitional state between active and forced regeneration. This filling step gives the user a last chance to create the conditions to burn the filter, if not, and the amount of soot increases, it moves on to level three. Level 3 blocks the possibility of active regeneration, and the only way to remove excess soot is to start forced regeneration. This regeneration cannot be performed by the user alone. To perform it, you need to start a service procedure to burn the DPF using service equipment.
Sensors for Monitoring the Operation of Solid Particle Filters FAP/DPF
Sensors
The operation of the particulate filters is monitored by several independent sensors. The type and location depend on the vehicle's brand. An example of the (VW 1.9/2.0TDI) placement of sensors is shown in the drawing on the left.
Pressure Sensor (G450)
These sensors are the most important in the entire exhaust cleaning system. We can distinguish between two types of sensors; in the first case, they measure only the pressure before the filter, comparing it with the standard pressure stored in the engine control unit, while in the second case, they check the pressure before and after the DPF filter and compare them. These pressure-sensing sensors operate using a strain gauge measurement of membrane deformation.
DPF Pressure Difference Sensor
The pressure from the DPF filter is transferred to the sensors via metal-rubber hoses, it acts on a piezoelectric coating, and the value is converted into an electrical signal. An active system that measures pressure changes is a small plate containing micro-piezoresistors of small size that respond to pressure changes. Another part of the system is a small vacuum chamber that acts as a membrane, which bends under pressure. The chamber is placed on the sensor wall and covered with a silicone protective surface. On the other side, the chamber is covered with a glass plate. Changes in pressure in the DPF filter lead to changes in the resistance of the piezoelectric element, which changes the voltage in the circuit, detected by the engine control unit. The sensor works in the same way as a strain gauge that measures the voltage in deformed elements, which is a measure of the pressure difference before the DPF and the vacuum in the reference chamber.
For sensors where the pressure comes from both in front of and behind the filter, the protective layer reacts to membrane deformation caused by the pressures delivered on both sides of it. Depending on the membrane's deformation, the value of the signals from piezoelectric sensors (0.5 – 5 V) changes, based on which the ECU identifies the degree of filter filling. If the circuit is broken or the sensor is damaged, active regeneration is blocked. The sensor is usually mounted in the engine near the DPF to ensure that the elastic pressure hoses are as short as possible, reducing the possibility of errors and pressure fluctuations, or the possibility of pressure differences in the wire itself. It should be remembered that after each sensor replacement, it should be adapted using a service tool so that the engine control unit "knows" that the sensor is new and uses a different algorithm to calculate the filter filling. This is because the ECU (algorithm) takes into account the sensor's wear, which is calculated based on mileage and expected sensor wear (membrane deformation).
Air Mass Meter G70
The air mass meter is located in the intake system, after the air filter, and before the turbocharger. The sensor measures the amount of air drawn into the engine. The air mass meter consists of several elements:
- Measurement tunnel – the diameter of the intake pipe or slightly smaller.
- Measuring sensor – made of a glass plate, heating element, and gauges.
Construction of the air mass meter G70
In the absence of airflow, air with the same temperature from the heating element is transferred to both resistances. During the airflow, the air flows around the plate and takes heat from the first resistance. The temperature of the second resistance is constant (varies within 1-2%). The change in airflow direction is automatically detected by the system, as the air flowing will cool down the second resistance at a constant temperature on the first one.
Exhaust Gas Temperature Sensors
Exhaust gas temperature sensors in exhaust cleaning systems are tasked with controlling the exhaust temperature during DPF filter regeneration processes. The engine control unit collects information from the sensors and controls the regeneration actions of the filter, determining the regeneration mode, the amount of injected fuel based on the data from the sensors. Usually, two temperature sensors are used, one in front and one behind the particulate filter. There are also more complex systems where 3 sensors are installed (e.g., Mazda 2.0 143HP), while in simple systems, only one temperature sensor is used (e.g., Ford Focus 1.6TDCI).
Construction of the temperature sensor for exhaust gases (1 – electrical connection, 2 – housing, 3 – resistance).
The sensor is a relatively simple device; inside the housing, there are one of two types of thermistors:
- NTC (Negative Temperature Coefficient), a semiconductor component where the resistance decreases with increasing temperature (e.g., Opel Vectra 1.9CDTI),
- PTC (Positive Temperature Coefficient), a semiconductor component where the resistance increases with increasing temperature (e.g., Honda Accord 2.2CTDI).
From 1 to 4 temperature sensors are installed in the system, usually 2, one in front and one behind the particulate filter. If one of them fails, the possibility of active regeneration is blocked.
