1990 5.7 TBI EGR delete pit falls

Discussion in 'Engine Performance + Maintenance' started by kenh, Sep 26, 2020.

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  1. PlayingWithTBI

    PlayingWithTBI Desert Old Guy

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    Yeah, they originally developed the EGR valve for gas engines because they couldn't meet NOx requirements.

    FWIU it varies depending on how much Oxygen is detected by the O2 sensor. The ECM will pulse the EGR module to maintain its AFR demand. Even if you don't have an SES light, the rolling dyno will fail your vehicle for high NOx.

    https://en.wikipedia.org/wiki/Exhaust_gas_recirculation
     
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  2. kenh

    kenh I'm Awesome

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    I "think" all I'm going to do is clean up what the MFG missed under the valves, smooth the "lip" out of the valve pocket area and round off the lumps where the spark plugs protrude. Like I said just "clean them up" a bit.

    Ken
     
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  3. Pinger

    Pinger I'm Awesome

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    This is where it gets a bit confusing.
    With AFR there is a lean flammability above which the mixture will not ignite with a spark. The excess of air implies an excess of oxygen.
    EGR contains no oxygen so shouldn't have any effect on AFR from the oxygen content perspective.

    From the Wiki page:
    ''In a typical automotive spark-ignited (SI) engine, 5% to 15% of the exhaust gas is routed back to the intake as EGR. The maximum quantity is limited by the need of the mixture to sustain a continuous flame front during the combustion event; excessive EGR in poorly set up applications can cause misfires and partial burns''
    And that kind of puts the kybosh on leaner mixtures - or you're adding to your initial flammability problem.
    (With EGR it obviously isn't an excess of oxygen leading to a 'lean flammability' limiting condition but EGR does inhibit initial combustion and therefor only the additional dilution can be the cause - preventing the oxygen from the intimate contact with fuel that it requires (fuel attached to EGR not oxygen requires more fuel to find the oxygen)).

    Stoich for gasoline is an AFR of 14.7. You are at TPS sub 50% running AFR of 16.7. 16.7/14.7 = 1.14. You are providing 14% excess air to reduce throttling losses which compares very well with the quoted '5% to 15% ' of EGR in the Wiki article.

    My conclusion is that EGR is used to mitigate throttling losses not because it is necessarily more effective but because (due to running AFRs closer to stoich) the fuelling is easier (and possibly the combustion products the cat has to deal with).
     
  4. Erik the Awful

    Erik the Awful I'm Awesome

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    The sole purpose of EGR on a gas motor is to reduce NOx emissions. If NOx wasn't a concern, you could lean mixtures out and get both great fuel economy and minimal hydrocarbon emissions. In the '60s and '70s NOx pollution in the northeastern US was so bad that we were getting acid rain. If you went out in the rain, the next time you washed your clothes they developed holes and fell apart.
     
  5. DerekTheGreat

    DerekTheGreat I'm Awesome

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    Yes, that's the primary one. Lower combustion chamber temps and a slight boon to MPG are another result of recirculated exhaust gas displacing fuel/air mix in the combustion chamber.
     

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