Indeed I mentioned the inertia / momentum argument due to a greater pressure difference, however; how many milliseconds do you really think that would last?
I still really doubt that such an effect would even be measurable, however without some serious math I don't think we're going to get anywhere with respect to being able to prove or disprove it. If I get the time (and some serious motivation) perhaps I could look into trying to figure out A) what the vacuum would be in a certain engine condition B) how quickly the two pressures (pre/post throttle) would equalize, C) how much inerita the mass of air rushing in would create and D) how long it would take for the engine to re-equalize that rebound effect. TBH it seems pretty involved; but I think could be done.
That being said - a more plausible reason for the said phenomenon would be the slack in the driveline being taken up in a big hurry. If the engine is in softish mounts this would be even more pronounced. However, if all of the momentum of the car is pushing back on the engine, all of the driveline's components are engaged in "opposite" relation compared to when the engine is doing the pushing on the driveline. When you hammer the throttle, the cumulative play / lash between all the driveline components can cause a pretty significant jolt - which would far better explain the phenonmenon described.
Yeah that would play a big role, but it's probably one of those things you'd notice at very high RPM when driving in anger? Like he says with the longi mounted engine having a stabilising effect in corners. Maybe it's acting like a Gyro? Maybe if you're that used to the car you start to notice things like these. Who knows. He did sort of make sense though.
doesn't the weight of the flywheel also have a significant influence on engine braking? ie, heavier flywheel of a typical road car giving less engine braking, and the lighter flywheel of a race car giving more?
According to my engine simulation transient effects like this don't last long at all. The pressure traces in the cylinders, intake and exhaust manifolds reach a stable point in two or three engine cycles (four to six revolutions) usually at the most, and this is slamming the engine from idle/closed throttle @ 9000 rpm to full throttle @ 1-2000 rpm in an instant, so it's a much more severe case than simply opening the throttle.
4 revolutions at 2000 rpm = 0.12 seconds
4 revolutions at 6000 rpm = 0.04 seconds
This is an instantaneous full closed to full open throttle change though (vice versa is about the same). In reality though, even "booting it" quickly will take some fraction of a second. I'd be surprised to see any increase in torque at all during this period. Depending on the precise instant you changed the throttle, some cylinders will probably get a slight increase in volumetric efficiency (torque), while others get a slight decrease. Here you're probably talking more about instantaneous torque changes over the course of a cycle rather than the average torque you feel in your butt or measure on the dyno over several cycles.
My guess is that it would be immeasurable on a dyno and not something you'd feel in your behind. The momentum effects in any single intake track might increase, but they are lost the moment the intake valve closes which causes the intake gases to come to a screeching halt at the valve. You'll have a little different reflection dynamic the next time the intake valve opens and closes, but again, this would be different from one cylinder to the next. Some will most likely lose while others gain, and all a very small amount I suspect...
Nothing like the straight goods from Todd to put a myth to rest
Been thinking about this a bit more and I don't think that's totally true, but don't hesitate to correct me should I need it.
While in principle, many of the flow characterisitcs have fundamental resemblence, I think that there is some important distinctions that have to be made. Firstly, when I talk about scavenging, I mean specifically removal of exhaust from the cylinders. The same phenomenon by extention also serves to provide some aid towards induction, provided approprite intake valve timing.
This being said, on a turbocharged engine, there is tremendous backpressure between the exhaust valve and the turbo's manifold, since they are generally specifically designed to create very high heat/pressure on the inlet side of the turbine. This creates a better pressure differential across the turbine and allows cooling to take place inside the turbine housing; which is most of what drives it. It is therefore impossible to create proper pulse tuning (on the exhaust side) on a turbocharged engine, since the manifold is always under pressure (when on the throttle at least). The extra inlet pressure however provides some scavenging effect, otherwise perhaps valve overlap would be counter productive. So don't think the dyanmics are really the same at all. Perhaps on a supercharged engine?
Yep - a heavier flywheel has more momentum so it reduces the engine braking effect.
Compression affects engine braking as does the size of the cylinders themselves. Gear ratios obviously and the efficiency of the transmissions also.
I read someone saying that in the US cars have lower compression ratios than in Europe and this is because there is no point running a high compression engine on a low RON fuel e.g. 89 RON.
With a higher RON fuel you are able to run higher compression ratios before you have detonation/pinking/pre-ignition problems. A higher RON fuel burns slower. So its not that there's no point, it's more that the engine will not be able to have as much compression or ignition advance on these low RON fuels.
I'd say that US engines have lower compression ratios because they are still using 1920 design (hemi is still a buzzword), and make up for cylinder efficiency by having engines of at least 23121 litres in hatchbacks (producing 0.2hp/litre).
I don't think even US fuel is as low as 89RON - I think you'll find it's on par with EU fuel at 95 - 98RON, and correspondingly 85 - 89MON. The 'Octane Rating' might be lower, but only because most people (incl. oil companies on forecourts) rarely state MON/RON.
Well, again, every pump I have seen in Canada states the octane as (R+M)/2, and they deem that "pump octane number" because it's a better indication of the fuels performance than either a Research or Motor octane number alone. (Since both by themselves can be misleading to the performance of the fuel because they are measured under different air temperature & load conditions) AFAIK, this is the same in the US.
Compression is most of what affects engine braking, whereas more rotating / reciprocating mass will hamper the effect.
I said the same things apply to forced induction, not just scavenging on a turbo-charged engine.
Turbocharging is only one method of forced induction as you've got supercharging, nitrous injection and even ram-air.
When Turbocharging; exhaust scavenging doesn't play as big a role that I'm aware of but good manifold design still plays a massive role!
But any inlet modification will be just as beneficial to F/I as with N/A. Pulse tuning/runner lengths/porting/smoothing/shaping etc.
Alot of people wrongly assume that because they've now got a way of forcing higher pressure air in, all these things don't count, when in actual fact they are now even more important than before.
Yes, which I why I brought up superchargers on my last sentence. I realize that some people classify nitrous oxide use as "forced induction", and I suppose in a literal sense it kind of is... I just don't lump it in with mechanically, forcibly charging the engine with ambient air. I've never met a ram-air system I liked either, just my opinon
I'm not sure I agree on that point, especially the "more important than before" portion. The biggest point being that the ridiculous deltas in charge temperature and pressure would make manifold tuning futile. The speed of the reflecting pressure waves is proportional to the square root of the absolute temperature of the gas, so charge temperature plays a big part.
So then, if the density of the charge is constantly fluctuating tuning an intake for one particular set of circumstances seems like waste of time to me. This was written in haste, and could be explained in more detail but I think the points comes across.
The greatest volumetric efficiency that I've ever read about due to intake runner tuning is 115%, which is nothing compared to 1 bar of boost so whatever effects that would (but probably don't really very much) exist would be silly compared to the 1 bar of boost.
As to your first answer, really no problem. Criticism is always welcome, as i'm no exper in this stuff really and may be as well wrong.
Some tech terms i may use wrongly cause of the langage bareers.
My car is nothing sophisitcated really. A tuned BMW E21 323i (inline 6) http://img408.imageshack.us/img408/1372/bmwe21gz4.jpg
Cours as every good old car it has a carburator which is the main reaosn actually why i love the car so much (know that it sounds odd but.. :} ). Were it not for the fact that i have driven also computer controlled cars that react properly in the matter we talk about (my definition of properly is speed up when you push the gas pedal and slow down when you take it off) I'd say no computer controlled car can behave well. Cours i'd be wrong as there are cars that "know how to behave". Also usuall cars. For example all M power later products of BMW behave really good while their noon M-power versions suck on that a lot. With my lil BMW it's not advised to take your foot of the gas pedal too abruptly already when the road is wet. :} As far as i know in very light sports cars with big engines and RWD you can spin just by too sharply stepping off the pedal to the right.
The ratio is a good point. My car has a only 4 forward gears
The aerodynamics of the car... What aerodynamics ? Noone seemed to care yet at that time lol.
But don't be mistaken by that. 140km/h is not yet a speed of sound and believe me the wind can be blowing 80km/h in the back of the car but it'll also slow down the same fast. In common people having to do with older cars will get my point right a way. With computer steering comes whole set of nasty automatation modules that often stay like really whole seconds behind what we want the car to do by pressing the gas pedal, these systems (in usuall cars) hardly tell a difference if you want to speed up or slow down the first second since your decision not to say about reacting to some little things like uderpressure... However sport computer handled systems will. The main thing is that first of all throttles in the intake system are used there ( they, as someone before me nicely explained, cutt off almost whole air intake when off gas pedal). No system of air flow support then works, (in usuall cars executed by bloody small electric "step engines"[however you guys call that] that let the air flow inside the engine freely every time we step off the pedal just tu save the kinematic energy of our vehicle and thus save fuel, enviroment and not to make more noise etc. etc. BULLSHIT.)
"but what you're describing is either REALLY severe spark retardation (in which case you would be firing buring fuel/air out of the exhaust valve)"
Ever seen a WRC rally car in action? What happens before it enters a corner? Cours you see flames from the exhaust! That's exactly the sign of the ignition retardation i'm talking about (meaningn the spark is generated earlier than normally, when the exhaust valves are not yet fully closed). My car and not only my also does that (you can actually only hear that cause flames are suppressed by the quite usuall exhaust) There is nothing worng about that. Actually it's one of the signs that the engine is beeing steered properly!
The fuel is also an issue. Same engine runnig at a lower octane gas (for example using 95 unstead of 98) wont brake so much and will in all be weaker ( my car engine compression ratio is 11.7:1 if i'm not wrong btw).
The outcomes of your test indicate that your car engine braking must be stongly reduced and that is probably done on purpouse. Ever tried getting rid of the step engine and stuffing a rag in the suction pipe for the time of a test (justa as much as not to make the engine suffocate and turn off at lowest rpm)? Well i can only tell you that it does make the engine brake more properly. Not that i tried it no no no, cours not.
I will re-read and talk more about your post tonight; I am having a bit of trouble understanding some of your ideas (language barrier indeed) at this point. The only thing I can see that's obvious right now is that when you mentioned the WRC car; you are referring to their anti-lag systems to alleviate turbo.... lag. They burn that extra fuel simply to keep the turbocharger's RPMs as high as possible. Sometimes they don't actually even burn "more", but rather the "same" fuel much later (not earlier) than normal (around 50 degrees after TDC I think), so that is late ignition not early ignition I think only some systems burn the fuel actually right in the manifold itself, and only at extrememly low RPM. I'll have to re-read your post a bit to respond properly to everything
I am glad this interesting conversation arised as well!
On a G40 scroll charger with a big FMIC and Aquamist on my Polo the temperature measured at the throttle plate was around 30degC when moving, which may aswell be a warm day.
What I was trying to get across was cranking up the boost isn't always the answer for more power. If you're limited to a certain boost level, or want to stick with a smaller charger, there's lots of power to be had by optimising flow with porting/shaping/valve throat work etc just as there is with N/A tuning.
If you want to get your "moneys worth" from a certain charger CFM so to speak. I've met lots of people who think the only thing you can do with F/I engines is put a bigger charger/intercooler on, when ideally you want to make sure the rest of the engine is using the extra pressure efficiently first.
Btw, this is ecu what I had, I'm quite sure I will not get OBD II car and if I get some other car I will put such system in it, so much better than any stock ecu
I think that could be mistaken in this discussion.
what I found is that by advancing, not by retardation, of the ignition the engine bahaves in such way that work done by burning the fuel (d*mn, I dont even know how you call each stroke ) would be lower than energy needed to exhaust the fumes, thus it is called negative work (or other correct term in English ). And I suppose this is the phenomena standing behind your nose hit by the wheel, DaDriver
Well, the point is that doing that would be extremely detrimental to the engine and I just cannot accept that any engine would seek to use preignition in said fashion. If someone comes up with some kind of concrete proof, then so be it. But I've never heard of that, and in fact most sane people try and avoid those very circumstances like the plague.
I was also addressing a different topic in your quote; since he noted WRC cars having flames shooting from them. That is not for the reason that you or him are talking about, so I explained the reason (ALS).
Eeekie I have more comments for you but don't have the energy right now
OT:I would like to ride in your car by the sounds of it!
Ignition map This was not quite tuned yet, bit too agressive at WOT, but you get idea, top line (100) is naturally WOT and bottom line you can get only when in heavy engine braking (NA engine).
Normal cruising range is adjusted quite close to edge of pinging/detonation, so it gives good power and low fuel consumption, but retarded a little as you always like to have bit safe marginal for hot day and city traffic etc. Fueling in cruising was set to 14.7:1 as stupid engine won't accept anything leaner
In WRC they use valve and spark timing so that you get bang that keeps turbo spinning, spark is shot when exhaust valve is open so it causes shockwave that spools up turbo, so spark is retarded a lot.
Yes you described the idea i have in mind all the time. Negative work name fits right in.
And yes you'r right. I called the process the wrong way it's indeed advancing, not retarding, my bad!
JTbo
Whoa that ignition map looks quite heavy ! 6-54 deg difference X_X wow. would like to take a ride in sth like that! In my car it works somewhere in between 30-42deg as far as i know.
What you said about the safe margin gained by retardation in case of some harder outside contitions i find very true. Usuall car manufacturers do that to make sure their cars are less unreliable and never have problems with engine swithching off by itself whem on idle, in a very wide range of conditions. For that unfortunatelly we pay a price of car behaving like a gay, heavy, full of inersion piece of metal on wheels (having my own oppion :tilt.
JTbo/Ball Bearing Turbo
Thats interesting what you say about WRC and turbos. Didn't know about it (basically not i know noything about turbo)
Good point i guess.
Still i know cars without turbo which do that...
Ball Bearing Turbo
"But I've never heard of that, and in fact most sane people try and avoid those very circumstances like the plague. "
Yes ideed the process of a the negative engine work sound quite insane. Same are motor sorts . The whole idea is old as the idea of a car is.
PS, (bit off topic)
Sanity you talk about is something we (at least a lot of people, i feel excluded tho) got used to over years. That sanity has different goals that a, lets say, real motorization fan has. They go more or less in this order: income, ecology and economy, design, reliability and handyness only then goes safety. And that last one is mostly passive (like tenths of airbags) since after a collision you need a new car. Clear deal. Maybe that's an answer why new Peugeot 307 behaves so gay within every move of the steering wheel and gas pedal prooving also that thenths of years of human ingeenering expirience mans nothing to say compared to business demands. That's sad and more and more engulfing in every aspect of our lifes..ooops sounds like i start changing topic... ehh... eghm Yeah i probably sound extreem to you people... well
FWD was invention of those boring fellows that had swallowed steel bar and worked at financial department, today even looks seem to be made by those, every damn soap box looks copy of each other only difference is minor details.
My car's engine is designed at 70's so it can't use too lean mixture or even too much advance at low rpm/full throttle because combustion camber is what it is. Stock map for comparison (here 0 is WOT and 8 is engine braking).
I wasn't saying it should be implimented. Just that it makes sense.
