DPF Additive Systems (FAP)

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The most commonly fitted type of DPF has an integrated oxidizing catalytic converter and is located very close to the engine where exhaust gases will still be hot (600-650°C). This heat means that passive regeneration is more likely to be successful.

Some models, across a wide range of manufacturers, use a different type of DPF, namely FAP which relies on a fuel additive to lower the ignition temperature of the soot particles.

FAP is the brand name of the particulate filters fitting in cars made by Peugeot-Citroen-PSA. This type of filter was the first to be installed on standard production cars (the 2.2 HDI Peugeot 607 engine). Use was later extended to 2.0 HDI and gradually installed on an increasing number of cars, including those of the FIAT-PSA joint venture (Ulysse-Phedra). From a technical point of view, FAP belongs to the type of filters which require the use of additives (cerium oxides, iron oxide, etc.). Eolys is the brand name of an additive. These were the first filters to be installed on cars and therefore are also the one which are best known, in terms of problems, servicing methods and repair procedures.

As previously mentioned, the filter regeneration process consists of burning the particulate collected by the trap. The particulate is burnt at a temperature of approximately 600-650°C. In order to reach such temperatures, modern diesel engines carry out post-injections after TDC which burn on the oxidising catalyser arranged in front of the filter. The purpose is to increase the temperature of the exhaust gases.

  • Example of post injection strategy


On the FAP systsem the additive is appropriately added to the fuel to lower the regeneration threshold by reducing the particulate combustion temperature to approximately 450°C. The gas temperature reaches 450°C with the post-injections to that the particulate inside the filter is burnt and the filter is regenerated.

The additive is stored in a separate tank next to the fuel tank and is automatically mixed with the fuel whenever you fill up. Only very small quantities are used so a litre of additive should treat around 2800 litres of fuel – enough to cover 25,000 miles at 40mpg. It lasts about 70000 miles and is replenished during a service – at extra cost.


The FAP system consists of:

  • A silicon carbide anti-particulate filter in conjunction with a catalyzer (located upstream of the filter) and sensors for checking the temperature and pressure;
  • A control logic built into the engine management node, designed to operate the artificial regeneration of the filter and guarantee the system's self-diagnosis;
  • An additive system which injects an appropriate amount of cerium oxide-based additive (EOLYS DPX42 or EOLYS 176) into the fuel tank each time it is refilled. This additive lowers the combustion threshold for the particulate, from 550°C to 450°C.

  • Example of a FAP system layout


  • A. Engine
  • B. Common rail
  • C. High pressure pump
  • D. Engine ECU
  • E. Fuel pump
  • F. Level indicator
  • G. Injector and adjuster
  • H. Fuel tank
  • L. Additive
  • M. Muffler
  • N. Sensor
  • P. Particulate filter
  • R. Pre-catalyst


The body computer node allows the following operations to be carried out:

  • transmitting information concerning the volume of fuel supplied to the engine management control unit,
  • detecting any fuel supply (tank cap presence sensor),
  • Notifying the driver of the state of the pollutant particles filtering system,
  • Reinitializing the system after a maintenance operation.

The engine management control unit software includes

  • The injection and pollutant control strategies,
  • The control of the particle filter regeneration,
  • The management of the start and the timing of the injection of additive into the fuel tank,
  • The management of the total amount of additive injected starting from the initial activation of the filter,
  • The driving comfort strategies,
  • The recovery strategies,
  • The management of the fans and the signalling warning lights,
  • The fault diagnosis and memorizing of errors,
  • The dialogue with the automatic transmission control unit,
  • The dialogue with the stability control unit.

The engine management control unit guarantees the electrical control of the following elements:

  • Differential pressure sensor,
  • Exhaust gas temperature sensor (downstream of the catalyzer),
  • Additive injection pump.


During combustion, the additive combines with the particulate in order to:

  • Lower the combustion level of the particulate by around 100°C,
  • Facilitate the spread of combustion of particulate deposits.

The (EOLYS DPX42 or EOLYS 176) additive is a cerium oxide-based compound diluted in a solvent. It consists of:

  • Cerium oxide (about 4% mass),
  • A product that allows the dilution of the oxide,
  • Solvent (combustible hydrocarbons).

During combustion of the particulate (which takes place in the anti-particle filter), the additive (cerium oxide) does not burn, but is deposited on the inner walls of the filter.

The additive injected into the fuel tank mixes spontaneously through the movement of the fuel in the actual tank.ADDITIVE RESERVOIR The additive reservoir is located under the vehicle near the fuel tank; it has a capacity of 5 litres. This quantity of additive allows the vehicle to run for about 80,000 km with EOLYS DPX42 additive or around 120,000 km (or 180,000 km starting from 12/2004) with EOLYS 176 additive, for normal usage.

The reservoir includes:'

  • Injection pump: The additive injection pump provides the pressure (3 bar) and the flow rate (80 l/h) required for correct operation of the additive circuit. The pump is immersed in the tank and cannot be separated from it. The non-return valves prevent additive leaks in the event that the connectors are detached.

The pump is supplied at 12 V by the additive control (Body Computer) in the following cases:

  • For 5 seconds after the ignition is turned ON,
  • During the additive stage.

Minimum level sensor (built into the pump):

This sensor is incorporated in the pump assembly and is designed to notify the additive control (Body Computer) that the minimum level has been reached.

The sensor consists of a resistive element; the resistance value varies very quickly depending on whether it is immersed in the additive or exposed to the air (minimum level).

The minimum level is shown, starting from a quantity of additive equal to 0.3 litres, by the engine diagnostic warning light in the instrument panel coming on.

The quantity of additive corresponding to the reserve, guarantees six complete fuel refills; if the tank is refilled a greater number of times, the particulate filter will become clogged and the engine management node will halt the regeneration strategies under these circumstances, at the same time restricting the fuel flow rate (limiting the vehicle speed).

Minimum additive level

The engine management control unit informs the body computer node which activates the following events:

  • Diagnostic warning light coming on intermittently for 20 seconds after the contact is switched on,
  • Display of a message on the multifunction screen (minimum diesel additive level) after the igntion is switched on)

Following the refilling of the additive reservoir, the diagnostic warning light remains on until the additive counter in the BSI is zeroed.

Safety valve:

The safety valve has the function of:

  • Insulating the tank from the introduction of dirt and water,
  • Regulating the pressure inside the tank as the level of additive varies.

The valve consists of:

  • Vacuum valve (ensures that the correct pressure is maintained as the level of additive decreases),
  • Pressure relief valve (guarantees that the excess pressure due to the evaporation of the solvent is discharged).


The additive injector, located on the main tank, allows a pre-set amount of additive to be atomized in the fuel tank.

It is operated by the additive node and is an electro-magnetic type injector similar to the injectors used in petrol engines.

A pressure regulator keeps the pressure in the activation circuit at a constant 3 bar. The additive mixes naturally with the fuel thanks to the movement of the fuel in the tank.


The cap presence sensor informs the additive control (Body Computer) of whether the fuel tank is open or closed.It consists of a switch on which a magnet positioned on the tank cap acts, with the presence or absence of the cap causing a variation in the resistance measured at the sensor connector terminals.


  • 1. Tank cap
  • 2. Presence sensor
  • 3. Magnet

The magnet causes the switch to open when the tank cap is fitted


Diagnostics for tank cap

Figure A: Tank cap missing giving a 15ohm signal to the BSI

Figure B: Tank cap fitted giving a 150kohm signal to the BSI

Each time the fuel is refilled, the engine management node zeroes an integrated mileage counter that enables it to determine the distance covered since the last supply of additive.

Opening the tank cap followed, at least 5 seconds later, by closing it, is known as a "cap cycle".

This cycle allows the additive node to understand that the fuel level is about to be altered.


The Body Computer Node sends the Engine Management Control Unit the following information:

  • volume of the fuel supplied,
  • volume counter for the fuel supplied,
  • additive system status (additive system diagnosis).

The Body Computer Node receives the following information from the Engine Management Control Unit:

  • minimum fuel additive alarm level,
  • request to turn on MIL warning light,
  • risk of particle filter blockage.

This information makes it possible to update the driver on the status of the particle filter.


The engine management control unit manages the injection of fuel additive through:

  • The management of the start and the timing of the injection of additive into the fuel tank,
  • The management of the total amount of fuel additive injected from the start of the life of the particle filter.
  • Recovery strategies,
  • Memorizing of faults.

The engine management control unit ensures the electrical control of the fuel additive pump.

Don't be tempted to ignore a warning light showing that the additive tanks need refilling. It's absolutely essential this tank is refilled as without it regeneration is unlikely to be successful and a new DPF may be needed – at significant cost. Fuel consumption can increase as a result of failed regeneration's too.