Date: Thu, 17 Feb 2005 23:44:28 +0000 From: Colin Stamp <col.dustbinnospamp.plus.com> Subject: Re: new saab motor for 9-3 series
On 17 Feb 2005 21:40:07 GMT, Dave Hinz <DaveHinznospamcop.net> wrote: > >Actually, thinking more about it, you'd want it to be most effective >somewhere below full power, so the area under the curve of reduced >vibration, across the entire operating range, is minimized. Then you'd have an engine with it's vibration minimum at some point between overrun an maximum output. Again, that would be really easy to spot. All the piston engines I've come across vibrate most at full power and least on overrun. > >> That would be really easy to spot >> since the vibration would increase when you lift off the accelerator. >> My balance-shafted straight four certainly doesn't behave like that, >> does yours? > >Nope, nor does my non-balance shafted I4. (2.0L in an '86 900). So >there's more to it than either of us are considering. I'm thinking >the engine mounts are coming into play. It seems to fit perfectly with my view that the balance shafts are only there to balance the moving masses. >> You seem to be forgetting that it isn't possible to turn off a balance >> shaft. If it's generating enough force to cancel out the combustion >> forces at full power (Not that that is very likely to be possible) > >But, have we determined how much of the overall vibration that force >is? That'll be pretty colinear with the axis of the bore, wouldn't it? >The "jumping up and down" that someone else mentioned. Linear forces >should be easier to dampen than forces in several directions. No. There's a torsional component too - the counter-torque to the torque pulses applied to the crank by the rods. > >> then it will still produce the same large forces on overrun at the >> same RPM, with no combustion forces to balance them out. The effect >> would be no improvement, Just an engine that vibrates as badly on >> overrun as a *real* engine vibrates on full power. >> >>> >>>IF, and only if, the combustion forces are that much more than >>>all that spinning mass going up and down. >> >> And at high powers they are. Stick your right foot down and feel it >> for yourself. > >That's the thing - tried it this morning in my non-balance-shaft 2.0L, >and the absence of vibration to the pedal, or the gearshift, is equally >low regardless. Was that double negative deliberate ;o) I feel the increase in mine alright. It never goes to irritating levels IMO, but the increase is unmistakable. > >>>> or even just off >>>> the cam. I dread to think what would happen at idle with a simulated >>>> minus god-knows-how-many BHPs worth of vibration going on! >>> >>>And yet, from empirical evidence, there isn't a problem. Therefore >>>your assumption must be flawed. >> >> You need to look a bit further up to your incorrect assumption about >> balance shafts in order to see why you're wrong here. > >I think you assumed what I was saying. They're obviously optimized >_at some point_, most likely where it'll do the most good. And that point is overrun. > >OK, I'll say that the 4 with balance shafts can be smoother than >the 6 without balance shafts. Throw 4 balance shafts into the 6, then >the 6 will probably be smoother, because then _all things are more equal_, >and then, at that point, the fact that 6 is bigger than 4 comes into play. Level playing fields and all that. Then again, no manufacturer that I know of seems to feel the need for a balance-shafted V6, whereas quite a few have gone for balance shafts in their straight fours. > >>>Not hardly. No symmetry at all in a 1. >> >> There's no symmetry at-all in an I2 either, though there could be if >> it was made with the pistons 180 degrees out of phase. They're not >> made like that though, because that would compromise the combustion >> force problem because of uneven firing. The moving-mass forces aren't >> as important as the combustion forces in engine vibration terms - >> particularly in engines making high specific powers. > >I don't know that a 1 or 2 anything is really in-scope, because they >present unique challenges as has been mentioned. a I-3 two stroke >would be in scope for Saabs, and there is quite a bit of tortional >vibration at idle, but that's more the nature of a 2-stroke's scavenging >than anythign else. It's not in-scope at-all other than as a demonstration that when engine designers have a choice between reducing inertial vibration or reducing combustion related vibration, they go for the one that will give them the most benefit - combustion vibration. >>> >>>> I'll take your word, at least as far as straight sixes having some >>>> advantage over V6s is concerned. I've never disputed that the >>>> moving-mass forces exist in a V6, and are awkward. All I did was point >>>> out the (IMHO much larger) combustion forces that also exist in both >>>> engines, but which are worse in a four than a six. >>> >>>At least in a 4, they're all going in the same direction. Heck of >>>a lot easier to deal with one vector than two intersecting ones. > >I think that's pretty key there. > >>>> If there was an engine mount that could remove *all* the vibration >>>> across *all* frequencies, then it would make the whole argument >>>> academic (which it pretty-much is anyway). Such a beast doesn't exist. >>> >>>Never said it did. >> >> I'm sorry. When you said "If the vibration never gets anywhere", I >> went and assumed you wanted to consider the possibility of a perfect >> engine mount. Silly me. > >If you can't feel it, does it matter if there's vibration? Nope. This discussion is pretty academic as far as the real world is concerned. >Sometimes, "good enough", is. Sometimes, yes. > >Seriously though, let's take a different look at this. What, other >than internal stresses, would account for the Saab High Milage list >over at (evilplace) having a whole lot more 4's than 6's? Market >share is part of it, but I wonder if it's disproportionately skewed >towards the I4 during the years when both were offered. I'd bet a >pint that it is. I'd have to bow out of that one. No experience. I think you'd be stretching it a bit to suggest that it was all down to a catastrophic lack of balance though. >>>> How does an engine mount know how many cylinders there are? If >>>> you're saying that the engine mounts might be tuned to the particular >>>> frequencies that a four might produce, then that can equally be done >>>> for a six, canceling out any possible effects. >>> >>>Can be. Is it? >> >> Who knows? Who cares? Engine mounts are irrelevant to whether one >> engine vibrates more than another. > >Not if you're saying one is more "harsh" than another; since that's >a human-based measurement, the only vibration that matters in that >context is what the human can observe in normal operation of the >vehicle. If you insist on bringing engine mounts into it, then you also need to add the seat foam, the sound deadening materials, the carpets, the resonance of the ashtrays, the Youngs modulus of the gear lever gaiter etc. etc. No point going at it half-cocked is there? Cheers, Colin.
 |
 |
 |
 |
 |
