What is a Diesel Particulate Filter (DPF) and what does it do? Why does it get clogged up?
Emissions requirements for new vehicles require Particulate Filters to be fitted to the exhaust of diesel vehicles since the ‘Euro 5’ standards where enforced in 2009. Prior to this many vehicles also have a DPF fitted in anticipation of the change to regulations. This ‘Euro 5’ standard aims to deliver reductions of 80% in soot (diesel particulates). However these filtration systems are not without problems of clogging up or blockages causing DPF warning lights, engine limp mode safety activation, reduced efficiency / mpg and poor engine performance.
The diesel particulate filter is mounted as part of the exhaust system. In order for the filter to operate correctly it must regularly run a regeneration process to burn off the continuously accumulating soot. This process is completed by precisely regulating an increased temperature of the exhaust gas passing through the filter which then burns away the carbon deposits. To do this a sensor transmits data to an engine control unit that then calculates the precise requirement for post-injection fuel for increasing the temperature during the regeneration phase.
As with an filter the DPF must regularly emptied otherwise it will over a period of time start to clog up and will eventually become partially blocked.
Passive regeneration of the DPF can only occur at fast speeds on a motorway or A-road so with all the local driving and short journeys for many vehicles this process cannot occur often enough eventually leading to clogging of the DPF.
How does the DPF Regeneration process work?
There are many types of regeneration…
The DPF system is designed in such a way that when the engine is consistently running at approximately 3,000 rpms or more (e.g. on a motorway or fast A-road) the exhaust gas temperatures exceed 350°C soot particles are constantly being burned off without any engine management intervention
Where the vehicle is predominantly used for urban mileage the exhaust gas temperatures fail to get hot enough passive regeneration is no longer achieved. Therefore soot particles are no longer being burned off so start to accumulate inside the filter. This leads to clogging of the filter and when a 18 gram threshold is reached the engine’s control unit (ECU) triggers the active regeneration at its next opportunity.
The amount of soot accumulated inside the DPF is calculated by the ECU based on…
how_does_a_dpf_work.jpg (this has a larger pop up)
‘Driver profile’ whereby the ECU records the pattern of exhaust gas and oxygen temperature sensor readings and then calculate the degree of saturation
‘DPF flow resistance’ whereby the ECU records signals from sensor regarding flow, temperature and pressure and then calculates of the degree of saturation.
Pressure is measured at both the inlet and outlet of the DPF and the degree of saturation inside the filter is measured by the pressure sensor calculating the difference between the two readings. As the filter becomes clogged the voltage transmitted by the pressure sensor to the ECU rises. At a manufacture set voltage reading the ECU will proceed to raise the temperature of exhaust gas to approximately 600°C thus actively starting the regeneration process.
The ECU instructs this increase in temperature by:
Regulating air flow through the Electronic Throttle Control (ETC).
Switching the Exhaust Gas Regeneration (EGR) valve so that it recycling exhaust gases thus increasing engine combustion temperature.
Enforcing a first post-injection following the principal injection in order to increase the combustion temperature.
Enforcing a second post-injection which leads to un-burnt fuel evaporating inside the combustion chamber. The un-burnt hydrocarbons are oxidised by vapour inside the catalyst converter. During this stage temperatures inside the DPF can reach 620°C.
The sensor signal, upstream of the DPF, supplies the ECU data to determine the quantity of fuel to be injected during the second post-injection.
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