Does the Prius C have an EGR circuit or not?

Discussion in 'Prius c Main Forum' started by farmecologist, Jan 14, 2020.

  1. sam spade 2

    sam spade 2 Senior Member

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    Exhaust gasses are HOT.
    EGR routes a tiny bit of that back to the intake to mix with the incoming air/fuel charge.
    Nobody has been able to explain to me how the ABSENCE of that bit of hot exhaust in the intake will make it hotter somehow.
     
  2. ChapmanF

    ChapmanF Senior Member

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    Well ... they don't burn, is the thing.

    As for how hot they are ... they were burning hot compressed gases not long ago, but they expanded as the piston moved down, and that cooled them (Boyle's law). Then an exhaust valve opened and they expanded further into the manifold, and that cooled them further.

    So the question, like so many things in engineering, is "compared to what?". Yes, they are hot, compared to a cup of coffee. They are also cool, compared to what goes on in a cylinder on the power stroke.

    But it really comes back to they don't burn. They're being used to dilute an incoming fuel-air mixture, which does burn. So not only is there less stuff in the cylinder to burn, the molecules that burn are farther apart. It takes them longer to meet each other. The combustion goes slower and with a flatter temperature curve, instead of quicker and with a higher temperature peak. (Hey! Social distancing for hydrocarbons!)

    The whole reason the EGR is done is to keep the temperature from peaking into the regime where NOx emissions are formed. So we know the functioning EGR keeps the combustion cooler than that. If it didn't, they wouldn't be putting this stuff in cars.

    The main problem the "so therefore head gasket" proponents still avoid confronting is once again the old engineering "compared to what?". Yes, it's agreed, no question, combustion without EGR gets hot enough to make NOx, and EGR keeps it cooler than that. But how does "temperature to form NOx" compare to "temperature to damage head gasket?" Are they at all in the same ballpark? Engines were built without EGR for a long time, happily making NOx emissions, and those engines had head gaskets.
     
  3. PriusII&C

    PriusII&C Member

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    @ChapmanF You explained it in a scientific and logic way! :)

    Wonder if you could help me understand two more questions? 1) While the exhaust gas from EGR reduces combustion temperature by diluting air/fuel mixture, would this also lead to a reduction in the thermal efficiency of the engine? 2) Could the reduction in temperature be achieved similarly with a smaller throttle opening without EGR?
     
  4. ChapmanF

    ChapmanF Senior Member

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    They are good questions, and I know the answer to one (it will surprise you!). I don't strictly know the answer to (1), but I'm sure it is in plenty of technical articles about EGR findable in libraries.

    What will surprise you about (2) is that you really don't want a smaller throttle opening! A certain amount of work spark-ignited (SI) gasoline engines do is just to suck on air across a small throttle opening. That 'pumping loss' is reduced if you can find some other way to modulate the power while letting the throttle opening be larger.

    One of the ways Prius gets its efficiency is by being programmed so when it needs X amount of power from the engine, it tends to pick a point on the power curve where X power is available at fairly low RPM and fairly open throttle, so the pumping loss is minimized.

    Some other Toyota engines (but not ours :() use their Valvematic system, where both intake and exhaust valve timing are changeable and they can use that to modulate power with less need for throttling.

    For that same reason, EGR in a SI engine has a bonus benefit: besides reducing NOx emission, which is why it has to be there, it also reduces the manifold vacuum and pumping loss, and that gains you MPG. (The manifold vacuum decreases both because of the EGR being piped right into the manifold, and also because you may end up using a slightly wider throttle opening to get the target power.)

    By contrast, diesel engines don't have pumping losses over a throttle, because their output isn't controlled by a throttle. They also have to have EGR to meet emission limits, but they lose MPG rather than gaining. Hence the enthusiasm for "EGR delete" among diesel owners. If you see somebody wanting to EGR-delete their SI gasoline engine, it's either because they haven't thought about it much, or because they're imitating their best buddy who drives a diesel.

    It turns out that vacuum-pumping across the closed throttle is also the source of most of the resistance when the gasoline engine is used for braking. It's often mistakenly ascribed to the work of compression, but that is much more like a spring: energy is used pushing the piston up but immediately returned to the piston going down, and the air leaves the engine at about the same pressure it had coming in.

    Diesels, again, can't get their engine braking from sucking across the throttle they don't have, so they have to rely on compression, and they need a way to kill the 'spring' effect so the energy isn't just immediately returned to the crankshaft. Hence the Jacobs brake, which shifts the diesel's exhaust valve timing to 'pop' the air right out of the cylinder at the point of maximum compression, before it has a chance to return its energy on the downstroke. That's why Jake-braking trucks are so loud.
     
    #24 ChapmanF, Feb 3, 2021
    Last edited: Feb 3, 2021
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  5. PriusII&C

    PriusII&C Member

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    @ChapmanF Obviously you are an expert in ICE. I learned a lot. Thank you!:)
     
  6. ChapmanF

    ChapmanF Senior Member

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    You happened to pose a question (one out of two) that I happened to have learned the answer to myself. :) Put me next to an actual expert in ICE and the difference would be unmistakable.
     
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  7. farmecologist

    farmecologist Senior Member

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    Yeah..the reason I'm wondering is because I'm thinking of getting a Prius c at some point. And it is *well* known that Gen3's can have clogged EGR's...which may or may not lead to a head gasket issue. I'm just trying to inquire here if any Prius c owners have ran into the EGR and/or head gasket problem. It is really encouraging that the answer so far is 'no'.

    However, I'm not sure I'm buying the "it just doesn't happen to the Prius c" defense. Remember...there are *far* fewer Prius c's on the road than Gen3's....and I'd wager a lot of the c's have fewer miles on the clock than the Gen3's that are running into the EGR and/or head gasket issue. But definitely encouraging nonetheless! (y)
     
  8. ChapmanF

    ChapmanF Senior Member

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    Seems curious to me too. It doesn't appear there's anything notably different about the design.
     
  9. farmecologist

    farmecologist Senior Member

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    I know...if anything..it looks like the 'EGR pipe(s)' are *longer* than the Gen3...Not that it matters.

    For reference :


    EXHAUST GAS RECIRCULATION SYSTEM. Toyota Prius c | Toyota


    Maybe I need to get a c and do my own investigation...haha. (y)
     
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  10. Raytheeagle

    Raytheeagle Senior Member

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    You buy the 'c' and I'll get data on the Prime:).

    Your cooler cleaning will be MUCH easier than mine :cool:.

    But I'll give it a shot when the time comes(y).
     
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  11. sam spade 2

    sam spade 2 Senior Member

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    It is also WELL known that forums like this tend to MAGNIFY problems that really are no more prevalent than they are in other makes and models.
    The fact that owners who tend toward economy and longevity will run into more problems like that are is overlooked too.
     
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  12. Mendel Leisk

    Mendel Leisk Sunday driver DIY’r

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    Maybe you’re the best man for the job? ;)
     
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  13. ChapmanF

    ChapmanF Senior Member

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    The right response hasn't changed much since your earlier use of that cop-out last November.

    But it may as well be expanded a little, because there's a further important part of how that all works. Suppose you do have all the time in the world and you do jump down the rabbit hole and make your best effort to find support for Fred's claim, and at the end you tell Fred that you didn't.

    Fred will say that's no wonder, because you weren't invested enough in the claim to begin with, of course you didn't try hard enough, look in the right places, find the right stuff.

    That's why it's always expected that the person advancing a claim be the one who has done the basic homework to check its plausibility. It's not only the matter of disrespect for other people's time in trying to offload one's own homework onto them, it's that the person advancing the claim is the person most motivated to find the support, and if somebody else doesn't, there's always "well, they didn't try hard enough" and nothing has been accomplished.

    Out of whatever number of hours I fritter away on PriusChat stuff, I spend a good bit of it doing the background homework on stuff I'm about to say in some post, to make reasonably sure before I post it that what I'm about to say is supportable. That's a natural, expected use of my time. Doing the same homework for your claims isn't, and wouldn't be necessary if you did it yourself, making the natural, expected use of your time.

    You've made some number of posts now, surely in the dozens, possibly hundreds, claiming a line of causality from "EGR lowers combustion temperature" through "EGR clogging raises combustion temperature" to "the raised combustion temperature destroys the head gasket".

    It's been brought to your attention many of those times, probably not hundreds yet, does feel like dozens, that your claim has a plausibility question built in that involves some numbers you have never provided. Namely: what are those combustion temperatures? By how much might EGR clogging change them? What temperature does it take to destroy a head gasket?

    I would not be surprised if every one of those numbers could be found in a library, without putting you to any of the effort of an experiment. I have not gone off to find them myself; this is not the kind of trick post where I'm looking at the answers right here but refusing to post them. I have my guesses but it's completely possible what you find could surprise me.

    But it's increasingly surprising that, as motivated as you are to keep repeating the claim dozens or hundreds of times, you don't seem to have more of an interest in finding out what those numbers are so you can supply them alongside your repetitions of the claim.
     
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