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MultiAir and Throttle Plate
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Thread: MultiAir and Throttle Plate

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    Senior Member HalfPint's Avatar
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    MultiAir and Throttle Plate

    I know the MultiAir engine does not use the throttle plate most of the time, but it does have have one. I assume it is used at idle, but when else? Is it closed on overrun/compression braking?

    It is just open or closed, or does is the position controlled between those points?
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    Senior Member HalfPint's Avatar
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    Are you serious? Did you even read my post?

    For reference, I've been playing with carburetors and fuel systems for nearly 45 years, and I'm passing familiar with what a throttle is.
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    Quote Originally Posted by HalfPint View Post
    Are you serious? Did you even read my post?

    For reference, I've been playing with carburetors and fuel systems for nearly 45 years, and I'm passing familiar with what a throttle is.
    That wasn’t meant to insult you. I read you post, you should know the answer, if if you been playing with carburetors, and fuel injection.

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    Moderator map's Avatar
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    I have never seen a mention of the throttle body positioning... just how the intake valves are controlled. I'd imagine, if you have a Scangauge (or one of the multiple diagnosis tools) you could track the TBPS numbers.

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    Quote Originally Posted by map View Post
    I have never seen a mention of the throttle body positioning... just how the intake valves are controlled. I'd imagine, if you have a Scangauge (or one of the multiple diagnosis tools) you could track the TBPS numbers.
    The Throttle Plate Position is controlled by the Throttle Pedal Position Sensor. (Throttle Pedal)
    Closed 0% - 80% WOT Open Max.

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    I was told by the engineer in charge of adapting the MultiAir Turbo to the NA 500 that it is mainly used to boost intake manifold vacuum during certain operating conditions. Exactly when, I don't know. Driving around with one of those scanners would be interesting to see when. I could imagine that adjusting the throttle angle and valve opening you could change the velocity of the air going into the engine, so that would be interesting to see how they tuned that.

    The engine can also do "internal EGR" which eliminates an external EGR valve. That is one of the enhancements of "MultiAir II" but our engines may also do that to some measure. I believe I read that in some of the engineering literature I have but don't have access to it right now. This is accomplished through the intake valve* and valve opening overlap. All pretty interesting.

    "The updated MultiAir 2 system is an enhanced version of the original that brings about further improvements in terms of efficiency and driving pleasure. Specifically, because MultiAir 2 can decouple and open the intake valves independently from the intake cam lobe, the intake valve opening has been optimized to precisely modulate the amount of exhaust gas recirculated in the combustion chamber via valve overlap, giving the engine internal EGR. "

    http://www.fiat500usa.com/2014/03/ne...neva-show.html
    Last edited by Fiat500USA; 01-16-2021 at 07:06 PM. Reason: I had to correct myself. I forgot what I wrote in 2014! ;)
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    Senior Member HalfPint's Avatar
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    Quote Originally Posted by Fiat500USA View Post
    I was told by the engineer in charge of adapting the MultiAir Turbo to the NA 500 that it is mainly used to boost intake manifold vacuum during certain operating conditions. Exactly when, I don't know. Driving around with one of those scanners would be interesting to see when. I could imagine that adjusting the throttle angle and valve opening you could change the velocity of the air going into the engine, so that would be interesting to see how they tuned that.

    The engine can also do "internal EGR" which eliminates an external EGR valve. That is one of the enhancements of "MultiAir II" but our engines may also do that to some measure. I believe I read that in some of the engineering literature I have but don't have access to it right now. This is accomplished through the intake valve* and valve opening overlap. All pretty interesting.

    "The updated MultiAir 2 system is an enhanced version of the original that brings about further improvements in terms of efficiency and driving pleasure. Specifically, because MultiAir 2 can decouple and open the intake valves independently from the intake cam lobe, the intake valve opening has been optimized to precisely modulate the amount of exhaust gas recirculated in the combustion chamber via valve overlap, giving the engine internal EGR. "

    http://www.fiat500usa.com/2014/03/ne...neva-show.html
    Interesting. Without a throttle plate there would be no intake manifold vacuum at all, as the intake would be at atmospheric pressure - but I'm not sure how well it would idle, as they'd have to cut back the lift & duration very far. That might be beyond the dynamic range of the MultiAir system, hence the need for at throttle.

    What got me thinking about this was the oil consumption issue on the 2.4 Tigershark, which is supposedly caused by "vacuum being created in the cylinders during deceleration".

    Of course that's not a turbo engine, so it would not be under positive cylinder pressure as much. Anyway, without the throttle closed the valve guides and stem seals are never under vacuum, so would never be a way to pull oil in. The vacuum would always be inside the cylinders, and would probably be strongly pulsating. Those pulsations might pull oil past the rings more easily.
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    Moderator map's Avatar
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    The way I'm seeing it, there would be more vacuum in the cylinder with the intake valve is clamped shut. The exhaust valve opens and most of the air is expelled. Then you have 2 downward strokes where crankcase pressure would exceed pressure in the cylinders for much of the stroke. Then the exhaust opens and expels air/fumes/oil out the exhaust, that leaked past the rings and valve guides.

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    Senior Member HalfPint's Avatar
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    Quote Originally Posted by map View Post
    The way I'm seeing it, there would be more vacuum in the cylinder with the intake valve is clamped shut. The exhaust valve opens and most of the air is expelled. Then you have 2 downward strokes where crankcase pressure would exceed pressure in the cylinders for much of the stroke. Then the exhaust opens and expels air/fumes/oil out the exhaust, that leaked past the rings and valve guides.
    All the intake vacuum in every engine comes from the pumping of the cylinder, that is the source. With the throttle plate closed upstream of the intake valves, a low pressure is created in the intake manifold, everywhere downstream of the throttle plate.

    So thinking about it further, they must close the throttle plate on overrun or there would be no compression braking, just like on a diesel which has no throttle.

    Taking that further, if you were to try to reduce oil consumption with a firmware change as they have done, the only strategy would be to change the cam timing to reduce pumping efficiency on overrun, thus reducing the intake vacuum in general. Since they only have control over the intake valves, you'd just try to keep the intake valve open longer, along with the throttle plate, while having the fuel shut off. That way there would be no vacuum in the intake, and little in the cylinder. There would also be little compression braking, but drivers with automatics are used to that.

    It still doesn't tell if the oil is coming past the rings or valve guides.
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