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I hear many views of people who believe that DPF's and CDPF's fitted to their vehicles are a problem and many people resort to removing them, or more commonly cutting them open to remove the internal materials and then remapping the system. This comes across to me as being very expensive and can present its own problems when the mot test becomes due.
Spending a little time gaining some insight into what a DPF/CDPF actually is and what it does should really help people to understand how to prevent problems occurring in the first instance, this being rather than trying to solve the problems later when costs will increase.
So what is a DPF/CDPF anyway?
A DPF is a diesel particulate filter, which removes soot particles efficiency from the exhaust system. Some DPF's are constructed of porous ceramics, and some are manufactured from sintered metals.
Both types of filters can achieve a retention rate of around 95% for particles across the whole spectrum range between 10 nm to 1 micro m. Irrespective of the type of filter used the material must be freed from time to time from the deposited particles that build up internally. To carry this out the DPF/CDPF must be regenerated. As the driver drives the vehicle over time the soot particles build up in the DPF/CDPF's, and this build up of soot particles increases the exhaust back pressure, which impairs engine efficiency and acceleration power.
Regeneration can be a forced regeneration or can occur during driving cycles on the roads. The filter is regenerated by burning off the soot that has collected in the filter. The particle carbon component can be burned using the oxygen which is constantly present in the exhaust gas stream when the temperature is above 600 degrees C, which will then form non-toxic CO2. These high temperatures are not normally attained during normal town and country driving, which can be one reason why people suffer from problems as stated above. Because of this the manufacturers and designers have tried to take measures to lower the soot burn off temperature and try to raise the exhaust gas temperature.
Soot oxidizes at temperatures as low as 300 to 450 degrees C using NO2 as oxidizer. This is a method that is used in the continuously regenerating trap CRT system.
Additive system
The soot oxidation temperature of around 600 degrees C can be lowered to between 450 and 500 degrees C by using an additive. the additive is usually cerium or iron compounds in the diesel fuel, however this temperature is not always achieved in the exhaust gas system when the vehicle is in operation. The end result shows that the soot has not been fully burned off continuously. Active regeneration is therefore triggered above a specific level of soot saturation in the particulate filter. To achieve this, the combustion control in the engine is modified in that the exhaust gas temperature rises to the soot burn off temperature. One method of achieving this is to retard the diesel fuel injection point, which will raise the exhaust gas temperature. the additive in the fuel is retained as 'ash' after regeneration. This ash as well as ash from the engine oil is what gradually clogs the DPF/CDPF filters, therefore raising the exhaust gas back pressure. it is worth noting that the exhaust gas back pressure is lower in the sintered metal filter than the ceramic filter, with the same ash content.
Catalyzed Diesel Particulate Filter (CDPF)
Soot particle burn off can also be improved by coating the filter with noble metals such as platinum, however the desired effect is less than using an additive system. The CDPF system also requires further measures for regeneration to raise the exhaust gas temperature, similar to the measures taken with the additive system. By comparison with the additive system, the catalyzed coating has the advantage that no additive ash occurs in the filter.
CRT system
This system operates close to the maximum torque more frequently than everyday cars and light vans in use, which causes higher NOx emissions. In the commercial truck industry because these trucks are running hot continuously it is therefore possible to perform continuously the regeneration process of the particulate filter based on the CRT principle system.
I think up to this point I have explained enough about DPF's and CDPF's and their different types of vehicles in use and why problems occur.
Now what to do when problems occur?
This is a typical guide that I have written and used to diagnose and repair DPF/CDPF systems.
The technician should;
Testing the differential pressure sensor circuit
With reference to the manufacturer’s data and using an oscilloscope connect the scope to the output terminal of the DPF pressure sensor. This is normally the 0.5 V signal with the ignition on. Back probe each of the three terminals to establish which are the ECM voltage supply, the signal and the ground return.
Once the terminal wire has been located for the ground, using the scopes probe connect to the ground terminal of the DPF sensor, which is referred to as a floating ground.
Set the oscilloscope sample rate to read 4 MS. 1 ms, 100 ms, 5 s/div will show a 50 ms screen free run, no filtering should be used and no triggering should be set.
Scope setup
NOTE: Differential pressure in the exhaust stream should not increase at idle
With the KOEO engine not running and the vehicle standing, depress the throttle pedal to WOT and release immediately. The voltage should not increase more than 1.0 to 1.3 V; however reference to the vehicle manufacturer’s data for exact values should be sought. Ideally the voltage should have a maximum reading of 1 V.
Start the engine and allow the engine to idle, and then rev the engine to WOT to gain maximum revs and then return back to idle. Shut off the engine and stop the scope waveform.
First examination of the differential pressure waveform
The diesel particulate sensor (DPF) measures pressure before and after the DPF filter, the residual pressure, and atmospheric pressure.
A blocked DPF will have a positive pressure in front of the DPF filter.
A vacuum pump can be used on the rear section of the DPF differential pressure sensor (output of the DPF filter). The pump is used to measure the vacuum pressure and linear increase in the DPF sensor voltage, which should increase to 4.5 V approximately.
At this point I'd of liked to have included a line drawing of the results of testing the differential pressure sensor but the forum software seems to create text boxes for some reason?
NOTE: Always refer to the vehicle manufacturer recommendations for specific test data. The following is for guidance purposes only.
800/1013 = 0.8
Hence 800 mbar is 0.8 bar of pressure, and is equal to standard atmospheric pressure at 0.8 atmospheres.
How does a DPF vaporizer work?
Rather than being injected directly into the combustion chamber, fuel is directed to the Fuel Vaporizer. The Vaporizer then uses an integrated, electrically heated glow plug to evaporate the fuel before injecting it into the exhaust gas stream ahead of the catalytic converter/ diesel oxidation catalyst
How do you diagnose a DPF?
You will know whether active regeneration is taking place by the following symptoms:
Testing the fuel vaporizer
First read out any stored fault codes. Typical codes can be;
Spending a little time gaining some insight into what a DPF/CDPF actually is and what it does should really help people to understand how to prevent problems occurring in the first instance, this being rather than trying to solve the problems later when costs will increase.
So what is a DPF/CDPF anyway?
A DPF is a diesel particulate filter, which removes soot particles efficiency from the exhaust system. Some DPF's are constructed of porous ceramics, and some are manufactured from sintered metals.
Both types of filters can achieve a retention rate of around 95% for particles across the whole spectrum range between 10 nm to 1 micro m. Irrespective of the type of filter used the material must be freed from time to time from the deposited particles that build up internally. To carry this out the DPF/CDPF must be regenerated. As the driver drives the vehicle over time the soot particles build up in the DPF/CDPF's, and this build up of soot particles increases the exhaust back pressure, which impairs engine efficiency and acceleration power.
Regeneration can be a forced regeneration or can occur during driving cycles on the roads. The filter is regenerated by burning off the soot that has collected in the filter. The particle carbon component can be burned using the oxygen which is constantly present in the exhaust gas stream when the temperature is above 600 degrees C, which will then form non-toxic CO2. These high temperatures are not normally attained during normal town and country driving, which can be one reason why people suffer from problems as stated above. Because of this the manufacturers and designers have tried to take measures to lower the soot burn off temperature and try to raise the exhaust gas temperature.
Soot oxidizes at temperatures as low as 300 to 450 degrees C using NO2 as oxidizer. This is a method that is used in the continuously regenerating trap CRT system.
Additive system
The soot oxidation temperature of around 600 degrees C can be lowered to between 450 and 500 degrees C by using an additive. the additive is usually cerium or iron compounds in the diesel fuel, however this temperature is not always achieved in the exhaust gas system when the vehicle is in operation. The end result shows that the soot has not been fully burned off continuously. Active regeneration is therefore triggered above a specific level of soot saturation in the particulate filter. To achieve this, the combustion control in the engine is modified in that the exhaust gas temperature rises to the soot burn off temperature. One method of achieving this is to retard the diesel fuel injection point, which will raise the exhaust gas temperature. the additive in the fuel is retained as 'ash' after regeneration. This ash as well as ash from the engine oil is what gradually clogs the DPF/CDPF filters, therefore raising the exhaust gas back pressure. it is worth noting that the exhaust gas back pressure is lower in the sintered metal filter than the ceramic filter, with the same ash content.
Catalyzed Diesel Particulate Filter (CDPF)
Soot particle burn off can also be improved by coating the filter with noble metals such as platinum, however the desired effect is less than using an additive system. The CDPF system also requires further measures for regeneration to raise the exhaust gas temperature, similar to the measures taken with the additive system. By comparison with the additive system, the catalyzed coating has the advantage that no additive ash occurs in the filter.
CRT system
This system operates close to the maximum torque more frequently than everyday cars and light vans in use, which causes higher NOx emissions. In the commercial truck industry because these trucks are running hot continuously it is therefore possible to perform continuously the regeneration process of the particulate filter based on the CRT principle system.
I think up to this point I have explained enough about DPF's and CDPF's and their different types of vehicles in use and why problems occur.
Now what to do when problems occur?
This is a typical guide that I have written and used to diagnose and repair DPF/CDPF systems.
The technician should;
- Always refer to the manufacturer’s recommendations and available technical data.
- Familiarise himself with the layout of the system and component locations.
- Carry out a thorough visual inspection of all the components and wiring.
- Carry out a battery test, including a starter cranking test.
- Read out any fault codes and follow the manufacturer recommended procedure.
- The differential pressure sensor is normally located in the engine compartment adjacent to the chassis. The sensor will have an electrical connector fitted incorporating three wires.
- ECM supply voltage normally 5 V
- A negative ground return which should read 0 V, and
- A signal reference voltage which should be 0.5 V with the ignition on.
- The active sensor operating range varies from 0.5 V to 4.5 V.
- The DPF sensor is linear in operation.
- The working pressure in the system can be predicted or calculated from the voltage range.
Testing the differential pressure sensor circuit
With reference to the manufacturer’s data and using an oscilloscope connect the scope to the output terminal of the DPF pressure sensor. This is normally the 0.5 V signal with the ignition on. Back probe each of the three terminals to establish which are the ECM voltage supply, the signal and the ground return.
Once the terminal wire has been located for the ground, using the scopes probe connect to the ground terminal of the DPF sensor, which is referred to as a floating ground.
Set the oscilloscope sample rate to read 4 MS. 1 ms, 100 ms, 5 s/div will show a 50 ms screen free run, no filtering should be used and no triggering should be set.
Scope setup
- 0 – 5 V
- 5 seconds time base
- Plausibility voltage should read about 0.5 V
NOTE: Differential pressure in the exhaust stream should not increase at idle
With the KOEO engine not running and the vehicle standing, depress the throttle pedal to WOT and release immediately. The voltage should not increase more than 1.0 to 1.3 V; however reference to the vehicle manufacturer’s data for exact values should be sought. Ideally the voltage should have a maximum reading of 1 V.
Start the engine and allow the engine to idle, and then rev the engine to WOT to gain maximum revs and then return back to idle. Shut off the engine and stop the scope waveform.
First examination of the differential pressure waveform
- How much vertical deflection is present!
- Looking at pressure in the system (positive direction).
- Examine the scope trace and look at the signature where the line thickens as the engine is revved, which shows the real time measurement in the differential pressure in the exhaust system.
- Reading the scope trace signature the plausibility voltage should be 0.5 V as before.
- Referring to the left hand side of the scope trace, look for the differential pressure during the cranking phase. The trace will increase at the left hand side at that position.
- Look at the voltage at that point and measure it.
- Look for an average voltage change from a static engine to running the engine at idle.
- Moving to the right hand side of the scope trace as the engine is revved, look for an increase in differential pressure.
- Use the zoom function of the scope and look in detail in different sections of the trace, look at and measure the maximum positive voltage.
- Typically 50 mill-bar pressure is about 0.8 V
- Look for maximum voltage reading displayed at idle when the pressure is between 100 – 120 mbar.
- A road test should see a maximum differential pressure reading of 220 mbar.
The diesel particulate sensor (DPF) measures pressure before and after the DPF filter, the residual pressure, and atmospheric pressure.
A blocked DPF will have a positive pressure in front of the DPF filter.
A vacuum pump can be used on the rear section of the DPF differential pressure sensor (output of the DPF filter). The pump is used to measure the vacuum pressure and linear increase in the DPF sensor voltage, which should increase to 4.5 V approximately.
At this point I'd of liked to have included a line drawing of the results of testing the differential pressure sensor but the forum software seems to create text boxes for some reason?
The vacuum readings should be taken from the gauge
NOTE: Always refer to the vehicle manufacturer recommendations for specific test data. The following is for guidance purposes only.
- 27 inch Hg is 914 mbar, 30 inch Hg is 1015 mbar approximately 1 bar, and 1000 mbar is approximately 1.0 atmosphere, so looking at about 0.8 V. This is negative pressure equivalent of about 0.8 atmospheres.
- This test above proves the sensor can report back to the ECM the pressure readings. Any reading recorded above 300 mbar is a problem.
- Apply vacuum to the atmospheric port of the differential pressure sensor. This is normally the DPF exhaust outlet side. Record the reading.
- Using the scope or a digital voltmeter read off the plausibility voltage measurement from the differential pressure sensor.
800/1013 = 0.8
Hence 800 mbar is 0.8 bar of pressure, and is equal to standard atmospheric pressure at 0.8 atmospheres.
How does a DPF vaporizer work?
Rather than being injected directly into the combustion chamber, fuel is directed to the Fuel Vaporizer. The Vaporizer then uses an integrated, electrically heated glow plug to evaporate the fuel before injecting it into the exhaust gas stream ahead of the catalytic converter/ diesel oxidation catalyst
How do you diagnose a DPF?
You will know whether active regeneration is taking place by the following symptoms:
- Engine note change.
- Cooling fans running.
- A slight increase in fuel consumption.
- Increased idle speed.
- Deactivation of automatic Stop/Start.
- A hot, acrid smell from the exhaust.
Testing the fuel vaporizer
First read out any stored fault codes. Typical codes can be;
- P244C – Exhaust temperature too low for particulate filter regeneration, or;
- P26A1 – Exhaust after treatment glow plug circuit open.
- As a first check, look at and test the fuses that protect the fuel vaporizer circuit.
- Physically check all the electrical wiring and harness connectors for damage and or corrosion.
- If a fuse or wiring is observed damage that is not caused by corrosion, then the vaporizer operation must be tested for the correct function and operation.
- The vaporizer may have an internal short circuit caused by the glow plug over heating or the pump failing.
- Gain access to the vaporizer and connect a vacuum pump. Ideally, it would be beneficial to test the supply hose pipe also.
- Apply vacuum to the vaporizer and observe if a vacuum can be generated.
- If the vaporizer can generate and hold a vacuum then the unit is blocked. Some of these vaporizers can be unblocked and become serviceable without having to renew them.
- If the fuse of the fuel vaporizer glow plug or pump has blown, then test the glow plug resistance.
- Glow plug nominal resistance should be 1.1 ohms. If the resistance is less than 0.8 ohms, or greater than 1.3 ohms, then the vaporizer glow plug is faulty.
- If the resistance value is within specifications, then disconnect the fuel line from the pump and place the end of the fuel line in a clean container and run the bleed/prime routine.
- If the pump is noisy or does not run, then check the wiring to the pump, and that the fuel lines are not blocked.
- If the pump is found to be faulty, replace it and prime the system as necessary.
