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Thread: DPF theory

  1. #1
    Regular Member Teapot's Avatar
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    Default DPF theory

    Paulcotes message regarding DPFs got me thinking about something;

    I've observed a manual regeneration process. Initially, the engine speed is raised to approximately 3500rpm. The exhaust temperature sensor shows the temperature gradually rising.

    As soon as the temperature reaches 200 deg C, the additional post-injection process is started and the temperature rapidly rises to approximately 600 deg C (downstream of the pre-catalyst).

    Now comes the bit I'm not sure about;

    The main injection timing is retarded to approximatey 15-30 deg ATDC. However this alone can't cause the temperature to rise as high as 600 deg C when stationary (no load). Plus the vapourisation of the post-injected fuel would lower the temperature.

    I assume therefore that the post-injected fuel is oxidised in the pre-cat, and it is this that actually gives the huge rise in exhaust gas temperature.

    If my theory is correct, then a faulty pre-cat would not oxidise the post-injected fuel and hence the temperature would never get high enough to oxidise the soot particles in the downstream DPF. This would result in a strong smell of unburnt diesel fuel to be emitted from the exhaust.

    So, given that the ECU won't start regen until EGT is > 200degC, we can assume that this is the required EGT for proper function of the pre-cat.

    Therefore, if there is a smell of unburnt fuel from the exhaust, then one of the following must be true:
    1. The EGT sensor is giving an inaccurate reading (higher than reality) and so the ECU is allowing regen to start before the required EGT is condition is met in reality.
    2. The pre-cat is faulty and not oxidising the post-injected fuel despite the required EGT condition being met.

    I'm hoping someone with a better understanding of DPF theory can confirm or debunk my theory as I can't find such detailed info elsewhere on the net.

    Cheers,
    Paul.
    Last edited by paulhb; 29th December 2012 at 23:27.

  2. #2
    Regular Member Teapot's Avatar
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    Default

    I should also have said - the first EGT sensor is mounted AFTER the pre-cat, so unfortunately we cannot see the 'pre pre-cat' EGT, else I would have been able to answer my own question :-)

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    nobody got any thoughts on this theory?

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    You could run the Tech 2 or Vaux-Com DPF re-gen on a DPF vehicle and read off the parameters.

    Also faulty pressure sensors can cause problems with unburnt fuel when a dpf cycle is running.

    VauxhallDieselParticulateFilter.jpg

    Personally I have done nearly all my customers DPF removals and turned them off with a remap, the difference in mpg and power is noticeable enough for them to comment a few days later, something people never do is complement you on a good job only phone me when something bad happens
    Last edited by C-Owner; 3rd January 2013 at 17:22.

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    Quote Originally Posted by littleteapot View Post
    Paulcotes message regarding DPFs got me thinking about something;

    I've observed a manual regeneration process. Initially, the engine speed is raised to approximately 3500rpm. The exhaust temperature sensor shows the temperature gradually rising.

    As soon as the temperature reaches 200 deg C, the additional post-injection process is started and the temperature rapidly rises to approximately 600 deg C (downstream of the pre-catalyst).

    Now comes the bit I'm not sure about;

    The main injection timing is retarded to approximatey 15-30 deg ATDC. However this alone can't cause the temperature to rise as high as 600 deg C when stationary (no load). Plus the vapourisation of the post-injected fuel would lower the temperature.

    I assume therefore that the post-injected fuel is oxidised in the pre-cat, and it is this that actually gives the huge rise in exhaust gas temperature.

    If my theory is correct, then a faulty pre-cat would not oxidise the post-injected fuel and hence the temperature would never get high enough to oxidise the soot particles in the downstream DPF. This would result in a strong smell of unburnt diesel fuel to be emitted from the exhaust.

    So, given that the ECU won't start regen until EGT is > 200degC, we can assume that this is the required EGT for proper function of the pre-cat.

    Therefore, if there is a smell of unburnt fuel from the exhaust, then one of the following must be true:
    1. The EGT sensor is giving an inaccurate reading (higher than reality) and so the ECU is allowing regen to start before the required EGT is condition is met in reality.
    2. The pre-cat is faulty and not oxidising the post-injected fuel despite the required EGT condition being met.

    I'm hoping someone with a better understanding of DPF theory can confirm or debunk my theory as I can't find such detailed info elsewhere on the net.

    Cheers,
    Paul.

    Might throw some light on your theory.



    From WIS

    Cars with Z19DT and Z19DTH engine alternatives can be equipped with particle trap (market dependent / option) to trap soot, which is burned once the trap is full. This is called regeneration.

    Construction
    The particle trap comprises a porous body, similar to a catalytic converter with many small passages with porous walls.

    Every other passage is blocked at the back (inlet passage) and every other at the front (outlet passage).

    The exhaust gas is forced to flow into a passage that is blocked at the rear (inlet passage).

    This passage is surrounded by outlet passages. The exhaust gas in the inlet passage will pass through the porous walls while the particles of soot are too large to pass through and will stick inside the particle trap.

    Soot calculation
    ECM calculates the amount of soot in the particle trap. Note that the differential pressure sensor (744) is not used to measure whether the trap is full but for diagnosis purposes and to prevent manual regeneration using the diagnostic tool if the particle trap is too full. The calculated amount of soot is based mainly on engine load and speed.

    Automatic regeneration
    ECM monitors driving style and selects a suitable time to employ regeneration. Cars driven a lot at idling speed and low load will attempt to regenerate earlier than cars driven more with high loads and In order for regeneration to take place, a certain temperature must be attained in the particle trap so that the soot can be burned. A temperature above +550°C (1022 F) is required in the particle trap if the soot is to be burned. A particle trap exhaust temperature sensor (602R) is used to measure the temperature. If the temperature cannot be attained, ECM assumes regeneration is not possible and will make a new attempt at the next opportunity.

    To increase the temperature in the particle trap so that regeneration will take place, an extra injection is performed during the exhaust stroke. At around 160°C (320 F) a small amount of fuel is injected into the cylinder and as this is so late (the piston is almost at BDC with the exhaust valve open) the extra fuel will not contribute to the engine torque, nor will the exhaust temperature increase appreciably.Quite simply, the exhaust gas is enriched with HC (hydrocarbon), which initiates a reaction in the front catalytic converter. The gas temperature increases, which is monitored by the front exhaust temperature sensor (602F). The heated exhaust passes into the particle trap where it reaches the rear catalytic converter first and is further heated to at least +550°C (1022 F), the temperature required for regeneration, or the soot will not be burned.

    Regeneration continues until ECM calculates that all the soot has been burned. This calculation is based on the value from the exhaust temperature sensor, the engine load and the engine speed. Regeneration can take up to 15 minutes but the driver will not notice any difference in the engine or performance.

    In the event of the car being driven in such a way that regeneration is not possible, a diagnostic trouble code will eventually be registered and the "Check Engine" indicator come on. Regeneration must now be done manually using the diagnostic tool.

    Manual regeneration
    Regeneration is not possible in certain driving conditions, e.g. driving under extremely light loads and extensive idling. This will cause a DTC to be generated and the "Check Engine" indicator to come on. Regeneration must now take place manually using the diagnostic tool.

    Regeneration must be done manually using the diagnostic tool whenever the ECM is changed, see Manual regeneration.
    Last edited by C-Owner; 3rd January 2013 at 17:39.

  6. #6
    Regular Member WeDesign's Avatar
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    Will the car perform a re-gen with the EGR unplugged or mapped out of the ECU?

  7. #7
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    EGR has nothing to do with the DPF

  8. #8
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    Thanks C-owner - the bit in bold clarifies what I was unsure about!

  9. #9
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    Quote Originally Posted by C-Owner View Post
    EGR has nothing to do with the DPF
    ^^ What he said ^^

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    Quote Originally Posted by littleteapot View Post
    Thanks C-owner - the bit in bold clarifies what I was unsure about!
    Cool

    HNY


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