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  1. #1
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    2 out of 2 members liked this post. Yes Reputation No

    Cobb keeps your head clean?

    So I was discussing some things with Rob from Cobb the other day and one thing I asked about was EGR. My old car had an egr valve and piped it right into the intake manifold.

    For those that don't know, EGR is exhaust gas recirculation and it is an emissions trick. If you pipe exhaust gas into a motor you it won't burn. The engine needs to open it's throttle more to compensate. An open throttle results in less pumping loss and a more efficient engine with less fuel consumption under cruise/light load.

    The N54 does not have an EGR valve. It accomplishes the EPA required EGR by overlap of the intake and exhaust cams using double vanos. By overlapping the cams the exhaust valves and intake valves can be open in such a way as to recirculate exhaust gases back into the combustion chamber. EGR is no bueno for DI motors as there is no fuel to wash the valves clean. As a result we get carbon build up from it and the PCV system. If you look at BMW DI patents they even mention that there is no advantage whatsoever from EGR for DI motor.

    Anyway, Rob said that Cobb had modified the cam phasing to significantly reduce this overalap on their tunes to improve response to throttle imputs. I would love to see the exact tables and am really hopeful they did this cam phasing at low and mid loads as it should have the additional benefit of reducing carbon buildup on the head. Of course this would be very difficult to verify but I'll take anything I can get when it comes to carbon buildup mitigation.

    In short, I'm speculating that a quality catch can + the Cobb accessport are the best ways to reduce carbon buildup. Add meth and that's probably the best it will ever be.

    Thoughts?

  2. #2
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    That's very interesting..how would they change cam phasing to accomplish that?
    Click here to enlarge

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    Click here to enlarge Originally Posted by dzenno Click here to enlarge
    That's very interesting..how would they change cam phasing to accomplish that?
    that is not viable

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    Retard intake and advance exhaust. Very easy to diagnose if someone would log map0 and staged vanos setpoints. Ive been asking this for awhile... Lets do it. The channel is lobe centerpoint in crank degrees.

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    Click here to enlarge Originally Posted by boom Click here to enlarge
    For those that don't know, EGR is exhaust gas recirculation and it is an emissions trick. If you pipe exhaust gas into a motor you it won't burn. The engine needs to open it's throttle more to compensate. An open throttle results in less pumping loss and a more efficient engine with less fuel consumption under cruise/light load.

    The N54 does not have an EGR valve. It accomplishes the EPA required EGR by overlap of the intake and exhaust cams using double vanos. By overlapping the cams the exhaust valves and intake valves can be open in such a way as to recirculate exhaust gases back into the combustion chamber. EGR is no bueno for DI motors as there is no fuel to wash the valves clean. As a result we get carbon build up from it and the PCV system. If you look at BMW DI patents they even mention that there is no advantage whatsoever from EGR for DI motor.

    Thoughts?
    Your description of the purpose and effect of EGR is not exactly correct. Emission of oxides of nitrogen (NOx) occur more at higher combustion temperatures. Running a lean AFR results in higher combustion temperatures. The use of EGR allows the engine to run leaner in light load situations, such as highway cruising, while lowering the combustion temperature to levels where NOx is not created as much. I haven't read BMW's DI patents, but I can't see how they could accomplish this effect without EGR. Perhaps they can vary injector timing, although according to the Continental engineer in the fuel injector thread, BMW does not use the stratified charge capability on the N54.

    That being said, it does make sense that eliminating the EGR effect would reduce carbon buildup on the intake valves. Definitely worth looking into.

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    I think you're misunderstanding me. BMW does recirculate the exhaust gas by overlapping the intake and exhaust cams. Cobb eliminates this overlap to improve response but the side effect is that egr is reduced and likely so is carbon buildup. You're correct about the NOx emissions. It has that effect as well but the main reason behind EGR implementation, I thought, was to improve fuel efficiency by reducing pumping losses due to a closed throttle. In any case, BMW has EGR and Cobb reduces/eliminates it. Again, i do believe this would be difficult to conclusively verify but it does make sense to me.

    Click here to enlarge Originally Posted by andrew20195 Click here to enlarge
    Your description of the purpose and effect of EGR is not exactly correct. Emission of oxides of nitrogen (NOx) occur more at higher combustion temperatures. Running a lean AFR results in higher combustion temperatures. The use of EGR allows the engine to run leaner in light load situations, such as highway cruising, while lowering the combustion temperature to levels where NOx is not created as much. I haven't read BMW's DI patents, but I can't see how they could accomplish this effect without EGR. Perhaps they can vary injector timing, although according to the Continental engineer in the fuel injector thread, BMW does not use the stratified charge capability on the N54.

    That being said, it does make sense that eliminating the EGR effect would reduce carbon buildup on the intake valves. Definitely worth looking into.

  7. #7
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    Click here to enlarge Originally Posted by boom Click here to enlarge
    So I was discussing some things with Rob from Cobb the other day and one thing I asked about was EGR. My old car had an egr valve and piped it right into the intake manifold.

    For those that don't know, EGR is exhaust gas recirculation and it is an emissions trick. If you pipe exhaust gas into a motor you it won't burn. The engine needs to open it's throttle more to compensate. An open throttle results in less pumping loss and a more efficient engine with less fuel consumption under cruise/light load.

    The N54 does not have an EGR valve. It accomplishes the EPA required EGR by overlap of the intake and exhaust cams using double vanos. By overlapping the cams the exhaust valves and intake valves can be open in such a way as to recirculate exhaust gases back into the combustion chamber. EGR is no bueno for DI motors as there is no fuel to wash the valves clean. As a result we get carbon build up from it and the PCV system. If you look at BMW DI patents they even mention that there is no advantage whatsoever from EGR for DI motor.

    Anyway, Rob said that Cobb had modified the cam phasing to significantly reduce this overalap on their tunes to improve response to throttle imputs. I would love to see the exact tables and am really hopeful they did this cam phasing at low and mid loads as it should have the additional benefit of reducing carbon buildup on the head. Of course this would be very difficult to verify but I'll take anything I can get when it comes to carbon buildup mitigation.

    In short, I'm speculating that a quality catch can + the Cobb accessport are the best ways to reduce carbon buildup. Add meth and that's probably the best it will ever be.

    Thoughts?
    Interesting topic to explore and I hope Cobb comments on this.

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    Click here to enlarge Originally Posted by boom Click here to enlarge
    I think you're misunderstanding me. BMW does recirculate the exhaust gas by overlapping the intake and exhaust cams. Cobb eliminates this overlap to improve response but the side effect is that egr is reduced and likely so is carbon buildup.
    I understand BMW uses cam overlap to create an EGR effect on the N54. Honda does the same in their newer DOHC engines. You mentioned BMW says in their patent that EGR is unnecessary for DI engines. I am no expert on the capabilities of DI, and was curious as to how BMW can control NOx emissions without using an EGR effect. Click here to enlarge

  9. #9
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    NOx emissions result from high temps in the combustion chamber. Diluting with EGR reduces temps. I would speculate that DI mitigates the high temps b/c of the cooling effect of the atomization at high pressure. Not sure though.

    I do know that the cars are capable of running in lean burn mode (50-60:1 afr) under light loads. This mode is disallowed in the US however, because of high sulfur content in the fuels. Lean burn definitely makes NOx but there are catalysts that can deal with it. They get poisoned though with too much sulfur and US gas has too much sulfur.

    Click here to enlarge Originally Posted by andrew20195 Click here to enlarge
    I understand BMW uses cam overlap to create an EGR effect on the N54. Honda does the same in their newer DOHC engines. You mentioned BMW says in their patent that EGR is unnecessary for DI engines. I am no expert on the capabilities of DI, and was curious as to how BMW can control NOx emissions without using an EGR effect. Click here to enlarge

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