UNDERSTANDING THE DPF SYSTEM

Emissions requirements for new vehicles require Particulate Filters to be fitted to the exhaust of diesel vehicles since the ‘Euro 5’ standards were enforced in 2009. Prior to this many vehicles also have a DPF fitted in anticipation of the change to regulations. This ‘Euro 5’ and beyond standards aim to deliver reductions of >80% in soot (diesel particulates). However, these filtration systems are not without problems 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 so a sensor transmits data to an engine control unit that then calculates the precise requirement for post-injection fuel to increase the temperature during the regeneration phase. As with any filter, the DPF must be regularly emptied otherwise, over a period of time, it will 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?

Passive regeneration:

The DPF system is designed in such a way that when the engine is consistently running at approximately 2,500 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.

Active Regeneration:

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 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:- ‘Driver profile’ – whereby the ECU records the pattern of exhaust gas and oxygen temperature sensor readings and then calculate the degree of saturation, and DPF flow resistance’ – whereby the ECU records signals from sensor regarding flow, temperature and pressure and then calculates of the degree of saturation.

The 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 post-injection following the principal injection in order to increase the combustion temperature.
  • Enforcing a second post-injection which leads to unburnt fuel evaporating inside the combustion chamber. The unburnt 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.
  • Forced regeneration:

    When the DPF system has exceeded a set level of saturation it becomes impossible for the vehicle to perform a passive or active regeneration. A mechanic may at this stage attempt what is known as a forced regeneration using a computer to instruct the DPF system to heat up (similar to the active regeneration process) and burn off the excess soot. Depending on the vehicle this will either be done whilst driving or possibly stationary with the engine running and the ecu controlling the RPMS.


    Unfortunately all too often forced regenerations will fail or only be beneficial for a short period of time. Why? Simply because either the saturation is too high or the operator has not diagnosed and repaired the fault causing the DPF system to fail the process.


    If a forced regeneration fails its common practice you’ll be told by a mechanic that a new DPF is required and at huge expense. In most instances this advice, even when it’s from the manufacturer dealers, is inaccurate!


    What it more than likely means is the mechanic has reached the limit of their understanding of a very complicated system and therefore assumes the DPF has completely failed. Or in the instance of it being a manufacturer dealer giving such advice, they are simply a business looking to make a profit. In our experience, this diagnosis is rarely correct. For most replacing the DPF will only resolve the problem short-term, with the fault still present it’s only a matter of time before the new expensive DPF also becomes saturated.

    If your vehicle has a DPF issue, or has even failed a DPF regeneration by a professional, and therefore requires professional intervention to diagnose and resolve the problem there is no better alternative than a skilled UKCC technician.

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