By on December 2, 2015

1.6 TDI Motor ( EA 189 ):  Strömungsgleichrichter (Einbau: Bild 1 von 6)

Volkswagen announced a fix for some of its diesel engines in Europe last week that included fitting a “flow straightener” device to their 1.6-liter engines. The device provides something tangible for Volkswagen to trot out, but has been criticized for being a low-tech distraction from the actual fix.

The “flow transformer” serves a purpose to provide a more accurate reading by the mass air flow sensor, but its initial exclusion may point to how far the cheating reached. To understand the implications of installing such a device, we need to take a look at the role of the mass air flow sensor on fuel and emissions mapping.

In a gasoline engine, the mass air flow sensor plays an integral role in calculating the fuel mixture for an engine and many of these sensors come with some sort of screen or grid in order to smooth out the air flow. The smoothed air flow allows for a more exact calculation of fuel, which can improve performance and fuel economy. It works so well that many even add them to large aftermarket intakes in order to achieve a better tune for theirs cars.

Diesel engines that deploy a mass air flow sensor do not use them for fueling calculations and instead they deploy the sensor as part of the EGR control strategy. The primary purpose of an EGR control strategy is to reduce nitrogen oxide emissions and the mass air flow sensor plays a role in calculating the EGR flow rate.

The telling part of excluding the “flow transformer” in the initial design is that multiple design teams would have been affected and would have to work around the limitation. The engineers that worked on the EGR system would have to calculate EGR flow based on their testing equipment and set it to a minimal level for production since real-time data would not be reliable enough to calculate it on the fly.

Without a reliable reading, the engineers may not have had enough data to create a functional EGR flow map and would need to let the car leave production with a dirty map. The preset EGR flow rate may work to clean up some of the emissions but it may not be optimal to meet all of the government standards hence the need for a “defeat device” map which could temporarily increase flow in order to pass the test.

While the “flow straightener” has been criticized by some as just a gimmick, it will actually serve a purpose, but the main driving force behind the fix is the programming. The device will assist programming the ECM with a proper EGR flow map that will reduce emissions and should meet the government standards in Europe.

Since the fix seems so simple now, many will ask why it was not installed and programmed on the cars when it was originally built and the answer lies in engine reliability. Increased EGR flow can cause increases in soot build up along with the need for increased maintenance on an engine. It can also cause premature failures on some engine components which could lead bean-counters to push for modifications in order to decrease the amount of warranty claims.

MAFsensorsThe 2-liter TDI engine is excluded from the “flow straightener” upgrade and for good reason. Taking a look at the sensor used on the engines, we can see an example of the 1.6-liter sensor made by Continental exposed the sensor element directly to the air stream, while the 2-liter sensor made by Bosch has a “flow straightener” screen built directly into the sensor thus negating the need to add one. All of the U.S. market diesels appear to use the Bosch sensor so the “flow straightener” is unlikely to make it to our shores as part of the upcoming recall fix.

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22 Comments on “How The Volkswagen Dirty Diesel ‘Flow Straightener’ Really Works...”

  • avatar

    And I thought a Flo Straightener was that Snapshot device. My bad.

  • avatar

    I don’t argue the explanation about what a flow straightener does to a MAF and how that affects a diesel engine tune. That is all correct.

    But that piece of plastic is a minor red herring at best, in terms of what is needed to bring the affected TDIs into compliance.

    “Since the fix seems so simple now …”

    There are two assumptions behind this simple statement that are unwarranted:

    1- that the fix will reduce emissions below limits — how about we wait for some real-world emissions results first, and also see how performance and longevity of parts are affected?

    2- that the fix is largely a simple device like the flow straightener. The fix for the emissions starts and ends with changing the tune to increase duty cycle of EGR and a whole lot more. If in the process, they need to add a flow straightener to that 1.6 engine to get more reliable air mass readings, that is a significantly minor extra on all they have to do for a fix. Seriously, why are we focusing on that piece of plastic as somehow significant?

    You want more proof of why the flow straightener is largely insignificant? Why hasn’t VW announced a fix for the 2.0 yet?

    • 0 avatar

      It may not have come across as such but I agree with your statement and believe the change to the tune is the main driving force in fixing the emissions. Although VW wants to make it look like a bigger piece of the puzzle, the flow straightener is really just a tool to assist in remapping the EGR control strategy.

      • 0 avatar

        A diesel engine is unthrottled. Does anyone know at what pressure the EGR outlow to the intake manifold is and how it is integrated with the pressurized air flow from the turbo? Because it seems to me that with a fixed set of intake hardware and cylinder displacement and rpm, calculating/calibrating mass air flow is relatively simple.

        Sure the flow straightener may well allow a tighter calibration map, i/e. more linear, that is about all it does, and we’re not talking of a need for a 10 or 20% tighter calculation in a diesel to reduce NOx by 10 or 20%.

        No, we need anywhere from 500% to 4000% less NOx to meet Euro5 depending on engine load. The load requirement itself is reacted to merely by adding more fuel because airflow is 100% all the time in a diesel. Greater power is made simply by adding more fuel at a given rpm, and if even more is needed then more rpm will be required as well..

        In any case, the German DUH environmental group is suing the German Transport Board who approved this “fix”, demanding the technical details of whether it actually works and what it does. They’re also after Renault and Opel. The German Vehicle Manufacturers Assn have already been hard at it lobbying the bureaucrats in Brussels for Euro6 to be relaxed and phased in over 5 years, with full German government backing.

        Meanwhile the EPA NOx limit for 2016 is HALF of Euro6 as it presently stands. I am waiting in some anticipation to see what excuse of a solution VW is going to come up with that’ll satisfy that standard for EPA and CARB. It may be impossible without towing a clean-up cart behind each vehicle.

        A royal mess.

        • 0 avatar

          Calculating the flow is not that hard but it takes proper modeling and testing. However the flow rate is just a small part of the problem they need to know the number of O2 molecules IE the actual density of that flow. That is why everyone switched from the old VAF (Vane Air Flow) system that just reported volume, to the MAF which indicates the mass of air flowing.

          They certainly could have taken the time an created an accurate MAF transfer curve w/o the flow straightener. The idea behind this device is provide a more uniform density of the flow through the MAF across all operating parameters making developing an accurate transfer curve easier and more importantly quicker.

          • 0 avatar


            Which is also the reason you can’t just use – on a gas engine – a throttle position sensor and infer from that “what the flow must be” from the known displacement and RPM.

            Air density varies.

          • 0 avatar

            Actually you can use a Speed Density system on a gas engine buy determining the barometric pressure, manifold pressure and temp of the air. It just is a much more messy algorithm and requires more programming and testing work than directly measuring the mass of air entering the system. Honda used speed density long after other mfgs had turned to the MAF system. A couple of Hondas use SD for the fueling calculations yet still have a MAF which is used for…. you guessed it, setting proper EGR flow.

            The throttle position sensor’s purpose is to determine acceleration enrichment, deceleration fuel cutoff and as a rationality check and it is that way on both SD, VAF, and MAF systems. The only type of system that uses the TPS in the basic fueling calculation is the Alpha-N systems which just aren’t used in production vehicles.

            The problem is that speed density doesn’t work so well on an engine without a throttle, and with they way diesel works you don’t need to know how much air is entering the engine, at least until you start worrying about emissions.

        • 0 avatar

          “Because it seems to me that with a fixed set of intake hardware and cylinder displacement and rpm, calculating/calibrating mass air flow is relatively simple.”

          Not certain, but doesn’t the turbocharger complicated this?

    • 0 avatar

      Screw a fix for the 2.0 – I want a buy back

      • 0 avatar

        As a 2.0 TDi flogger, I agree. I like the car, but the fact remains that if I imported a 959, or a JDM GT-R, and didn’t do my DOT and EPA properly (if even possible, and notoriously not for the 959), things would happen.

        1. EPA comes to my location and seizes the noncompliant vehicle, saving our air.

        2. If it turns out I filed false paperwork about a bogus test and relied on that to get an EPA OK, then unsmiling folks come to my house, and I get a very expensive trip through the criminal justice system….personally, not a “corporate person”.

        I can’t see how VW is any different, just in scale.

        Constant readers have seen this, but VW needs a “Recall Edition”. Gas motor, TDi equal interior bits, auto or stick. Start building now, swap to original owners even up. Public will be impressed you did the right thing. Govt will not be able to beat you up too bad. What does it cost to build a car ? Probably cheaper per unit than the fines, etc, especially if you ARE forced to buy back.

    • 0 avatar

      The fix has always “seemed simple” in that it requires programming to run the EGR at sufficient flow rates all the time.

      This device just means that it will be easier for them to properly program the system w/o having to go back and properly develop the transfer curve for the readings from the MAF under all possible conditions.

      • 0 avatar

        You are probably right. The excessive EGR needed would explain why BKW 335 series cars in the US have such clogged intake issues, and why VW didnt have the same problem….VW didn’t EGR as much.

        In an alternate reality I bought a 335d and am now bankrupt. I would still have a smile from the first 60k, though…great car.

    • 0 avatar

      One could guess that unless VW has a total death wish, this fix (along with the corresponding S/W changes) is sufficient to meet European emissions requirements (which are much looser for diesels than the US.)

      The article explained why the 2.0 fix won’t include a flow straightener. Not because it’s an insignificant part, but rather because it already has one.

      • 0 avatar

        Just from the picture it looks like the difference with the 2.0 MAF is not so much that it has a flow straightener but that it is a grid type MAF that covers the entire air tube. The 1.6 MAF only measures airflow in the center of the tube, so without the flow straightener a disproportionate amount of air can bypass the measuring elements on the MAF. With the flow straightener the amount of air entering the measurement area is directly proportional to the airflow that is not measured on a per square inch basis.

      • 0 avatar
        SCE to AUX


        Agreed. VW would be stupid to produce a fix that won’t work, since everybody will be checking their test results.

        • 0 avatar

          Yeah that already learned that lesson when CARB initially called them out. The new programming did little more than increase the time needed before the engine would go out of cheat mode. When CARB then tested a “fixed” vehicle for long enough they found that it still reverted to a dirty mode and that is what prompted the stop sale action.

          One would think that they aren’t stupid enough to try the same thing again, at least with CARB and the EPA. Other countries may be a different story.

    • 0 avatar

      “The device will assist programming the ECM with a proper EGR flow map that will reduce emissions and should meet the government standards in Europe.”

      I mean, he SAYS right there that it also needs reprogramming.

      The focus on the piece of plastic is to explain WHY it actually has an effect and is necessary on cars that don’t ship with one to make the programming fix actually work?

  • avatar

    I know people who modify their MAF/MAS sensors and remove these screens. DSMs, GMs, Kompressors, BMWs, for example. Claims of increased throttle response and horsepower gains are always reported with ill affects noticed only at idle. Interesting.

    Makes sense, turbulent air is being forced to straighten its direction, there must be some loss?

    Anyway, how long before said screens destroy turbocharger compressor wheels? :-)

    • 0 avatar

      Yes this will reduce the maximum air flow potential. So assuming that the fueling had be set to consume all the O2 the engine can ingest then this will cut peak torque and HP.

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