I have noticed this as well with my rs/6. Mentioned it to the shop that did my apr chipset and he said it is consisten with audis. He mentioned that some people had a stop nob so to speak installed under the accelerator.
I have noticed this as well with my rs/6. Mentioned it to the shop that did my apr chipset and he said it is consisten with audis. He mentioned that some people had a stop nob so to speak installed under the accelerator.
i have the same on my A3 1.8L. is that on all cars (BMW, Mercedes.....) or just on Audi???
please help ! ! !
anyoneOriginally Posted by tvrfan
This could get REALLY interesting....this is long so skip to the last five lines if you're impatient.
I have a pretty good background in both Physics and Fluid Dynamics (BS biomedical engineering) and certainly appreciate that many complex devices function in a way that we THINK we understand but really don't, be that an Engine or a Human Being.
What I was saying is that I don't believe the actual linear velocity of the intake air at the throttle body has any effect on the Torque developed by the engine.
The Throttle Body diameter and geometry and position of the Throttle Plate will effect two things for sure. Total amount of air passing per unit time and the amount of turbulence in that air. What the engine needs is a certain MASS of air, i.e. number of moles of a given gas (oxygen for combustion), and does not really depend on how fast the molecules are rushing into the cylinder.
I agree that the same MASS of air passing through two different sized openings will move at different linear velocities if indeed the same MASS of air moves in the same amount of time. But for a given fixed pressure and temperature of that moving air,you may end up with some faster moving molecules through that gate, but less of them per unit time, thus less MASS of air moving.
Fluid dynamics is a complex science. While a gas such as common Air is considered a fluid, just as much as gasoline or water are fluids, the fluid dynamics of a gas is different from that of a liquid. One particular difference is the compressibility of a gas vs a liquid. Gasoline is not that compressible compaired to air. So when the pressure produced by the fuel pump behind the fuel injector is allowed to produce flow of fuel through the injector of a given geometry and open for a specific pulse width, a very predictable amount of fuel is delivered.
Now the issue of suction vs pushing is interesting. The movement of air into a normally aspirated internal combusion engine is accomplished by a pressure gradient producted by the movement of the piston during the intake phase of the 4 strokes.
This pressure gradiant results in the movement of air from a higher pressure area to a lower pressure area which is the definition of SUCTION. Air is also PUSHED into an engine by a Turbo or Supercharger or by the "Ram Air" effect.
The linear velocity of that air now needs to be considered. What possible effect could the linear velocity of the air have on the development of torque as a function of that linear velocity alone???
The Throttle body is connected to the MAF then to the intake manifold where the fuel injectors are found. The fuel injectors inject gasoline right into the intake valve. (newer direct injections engines have either an injector in the intake manifold AND an injector in the cylinder head (Lexus) or just in the cylinder head (Audi RS4, S6, S8).)
The MAF sensor determines what MASS of Air is moving into the engine. The ECU needs to know the mass (ie. the number of molecules of AIR) to match the proper amount of fuel to mix. The resultant messurement at the Oxygen sensors helps fine tune the mixture further.
Torque is created by the combined internal explosions in each cylinder per unit time through a mechanical link of each piston to the crank staft, etc, and the explosive force of each internal combustion depends on the volume of the cylinder, compression ratio, starting pressure, temperature, position of the spark (better progression of ignition), completeness of air/fuel mixture, etc,etc. The linear velocity of the intake air not mentioned here.
Putting more air/fuel into a given sized cylinder at a given compression ratio etc, producted MORE torque. Thus add a turbo, add a supercharger, add both, etc. The other way to get more torque is to add more cylinders or use larger cylinders, but these both add weight to the engine and more mechanical complexity.
The throttle plate position is going to determine how much air can flow into the intake manifold in a given time at a given pressure/temperature/air density. Those variables are less of a concern to the ECU which relies on the MAF sensor to tell it the mass of air entering the intake manifold, then combines that information with other sensors (intake manifold pressure, oxygen sensor, temperature sensors etc) to determine how much fuel to mix with the intake air to optimize combustion. Again there is no Air Linear Velocity here.
Turbulence created by the partially closed throttle plate is not an issue as the MIXING of the air and fuel occurs at the intake valve, thus any prior turbulence in the intake manifold is not an issue by the time the air gets to that tiny intake valve opening.
How about resistence to air flow at the partially closed throttle body which causes increased intake air linear velocity???
If you consider the fluid (air) as an ideal fluid instead of a gas, and if the total potential across the system (pressure gradiant from outside air to inside of the "sucking" cylinder) is unchanged and then the resistance to flow is increased (the throttle body is partially closed) that the velocity of flow would actually be decreased (i.e. V=IR V= potential difference ie voltage, I = flow, ie current, R= resistance).
But since the Air is not an ideal fluid but a gas (not even an IDEAL gas) it is a compressible fluid, and the relationship of total mass flow to pressure to temperature to linear velocity of the air is MUCH MUCH more complex. And the simple Ohms law becomes much more complex for anything more complex than a DC circuit, and I'll argue that air flow into an engine is not a like a simple DC circuit, but rather complex with added consideration of the true pulsitile flow of air into and out of the engine, and the impedance, inductance and capacitance of each tube/air box/MAF/ etc. in the flow path.
Now the nuts and bolts - for the RS6...
Drive - By - Wire.
Period..
The actual position of the throttle plate is controlled by the ECU...and even though at many times the position of the
throttle place IS directly related to the position of the gas pedal, the ECU will change it to match the needs of the given
LOAD and many other variables at a given moment.
So where your FOOT is on the gas pedal vs where the Throttle body plate is position may not be what you think.
and....I don't think linear velocity of air has anything to do with torque. I may be wrong...but I doubt it..
Mike
@ Aronis,
i have the same on my A3 1.8L on half throttle it accelerates faster.
SO is that just on audi models or on other cars too ( BMW, Mercedes........)???
Add me to the list that I find this true...if i fully gun it from a stop the car bogs down...one (and primarily in my opinion) from the traction control kicking in (from starting to break away the tires) and two the engine as noted. My best launches are to ease into it (roll off) then go to 3/4 in the next seconds then all in. If i dive all the way in I find I have to let off to 3/4 then go all in again. When already moving such as an interstate acceleration I find I get the random all out occasionally that someone mentioned at half or more throttle...the car does have a mind of its own on what it thinks i want to do with the drive by wire (I sense it especially as it reads your recent behavior).
2012 R8 GT #275 Suzuka, 2014 RS7 Suzuka/Lunar Silver, 2013 RS5 Suzuka/Lunar Silver
2000 Audi TT Neiman Marcus Edition #47, 1960 Cadillac Eldorado Biarritz Convertible
Past...03 RS6, 10 S5 Cabriolet, 00 NMTT #25, 94 Audi Cabriolet
Is it Automatic or Manual?
Traction control?
AWD or Front?
Mike
Nothing like two geeks debating . . . .
[quote=Aronis;115865]This could get REALLY interesting....this is long so skip to the last five lines if you're impatient.
I have a pretty good background in both Physics and Fluid Dynamics (BS biomedical engineering) and certainly appreciate that many complex devices function in a way that we THINK we understand but really don't, be that an Engine or a Human Being. I ACKNOWLEDGE YOUR PRESENCE
What I was saying is that I don't believe the actual linear velocity of the intake air at the throttle body has any effect on the Torque developed by the engine. I DISAGREE
The Throttle Body diameter and geometry and position of the Throttle Plate will effect two things for sure. Total amount of air passing per unit time and the amount of turbulence in that air. KEY POINT What the engine needs is a certain MASS of air, i.e. number of moles of a given gas (oxygen for combustion), and does not really depend on how fast the molecules are rushing into the cylinder. DISAGREE AGAIN, see notes on camshaft profile effects.
I agree that the same MASS of air passing through two different sized openings will move at different linear velocities if indeed the same MASS of air moves in the same amount of time. But for a given fixed pressure and temperature of that moving air,you may end up with some faster moving molecules through that gate, but less of them per unit time, thus less MASS of air moving. AGREED
Fluid dynamics is a complex science. While a gas such as common Air is considered a fluid, just as much as gasoline or water are fluids, the fluid dynamics of a gas is different from that of a liquid. One particular difference is the compressibility of a gas vs a liquid. Gasoline is not that compressible compaired to air. So when the pressure produced by the fuel pump behind the fuel injector is allowed to produce flow of fuel through the injector of a given geometry and open for a specific pulse width, a very predictable amount of fuel is delivered. AGREED
Now the issue of suction vs pushing is interesting. The movement of air into a normally aspirated internal combusion engine is accomplished by a pressure gradient producted by the movement of the piston during the intake phase of the 4 strokes.
This pressure gradiant results in the movement of air from a higher pressure area to a lower pressure area which is the definition of SUCTION. Air is also PUSHED into an engine by a Turbo or Supercharger or by the "Ram Air" effect. SEMANTICS, you're making a distinction based on how the higher pressure is created. I understand the definition of suction, was simply pointing out the more technically correct definition (e.g., strictly speaking - there is no such thing as cold, only the absence of heat). So it doesn't matter why the higher pressure exists on one side of the throttle, it's just a question of magnitude. The turbo/supercharger is creating roughly 12psi of pressure on the upstream side, the engine is still sucking, to use your term, creating a loss of pressure (maybe 8-10psi) on the downstream side for a total difference of roughly 20psi across the plate. With a normally aspirated version, there is only the 8-10psi differential.
The linear velocity of that air now needs to be considered. What possible effect could the linear velocity of the air have on the development of torque as a function of that linear velocity alone???
The Throttle body is connected to the MAF then to the intake manifold where the fuel injectors are found. The fuel injectors inject gasoline right into the intake valve. (newer direct injections engines have either an injector in the intake manifold AND an injector in the cylinder head (Lexus) or just in the cylinder head (Audi RS4, S6, S8).) AGREED
The MAF sensor determines what MASS of Air is moving into the engine. The ECU needs to know the mass (ie. the number of molecules of AIR) to match the proper amount of fuel to mix. The resultant messurement at the Oxygen sensors helps fine tune the mixture further. AGREED
Torque is created by the combined internal explosions in each cylinder per unit time through a mechanical link of each piston to the crank staft, etc, and the explosive force (KEY PHRASE, remember this one) of each internal combustion depends on the volume of the cylinder, compression ratio, starting pressure, temperature, position of the spark (better progression of ignition)BINGO, completeness of air/fuel mixture, etc,etc. The linear velocity of the intake air not mentioned here. Not directly, but it's effect is there. The more turbulent air HAS an effect on combustion. You're idealizing a bit much here. The speed of the flame front has a big impact on torque. The faster it happens, the more pressure is built up in a relatively fixed volume cylinder. That's why twin plugs have an effect. If it travels more slowly, the piston has already started it's downward path, lessening the effect of the expanding gasses.
Putting more air/fuel into a given sized cylinder at a given compression ratio etc, producted MORE torque. Thus add a turbo, add a supercharger, add both, etc. The other way to get more torque is to add more cylinders or use larger cylinders, but these both add weight to the engine and more mechanical complexity. These are big picture generalities, irrelevant to our fun little debate.
The throttle plate position is going to determine how much air can flow into the intake manifold in a given time at a given pressure/temperature/air density. Those variables are less of a concern to the ECU which relies on the MAF sensor to tell it the mass of air entering the intake manifold, then combines that information with other sensors (intake manifold pressure, oxygen sensor, temperature sensors etc) to determine how much fuel to mix with the intake air to optimize combustion. Again there is no Air Linear Velocity here. Again you're oversimplifying. MAF's are not perfect and are taking an average airflow reading. If my understanding is correct, they measure the ability of a wire to retain heat. The more airflow, the less heat retained. In your ideal model, there is perfectly laminar flow with perfectly even pressure distribution. I think we have both acknolwedged that there is turbulence in the airflow. While I am not positive of this, it is certainly not wild to theorize that turbulence could "fool" the MAF briefly into an incorrect assumption.
Turbulence created by the partially closed throttle plate is not an issue as the MIXING of the air and fuel occurs at the intake valve, thus any prior turbulence in the intake manifold is not an issue by the time the air gets to that tiny intake valve opening.
How about resistence to air flow at the partially closed throttle body which causes increased intake air linear velocity?????
If you consider the fluid (air) as an ideal fluid instead of a gas, and if the total potential across the system (pressure gradiant from outside air to inside of the "sucking" cylinder) is unchanged and then the resistance to flow is increased (the throttle body is partially closed) that the velocity of flow would actually be decreased (i.e. V=IR V= potential difference ie voltage, I = flow, ie current, R= resistance). I think you really meant to reference Bernoulli's equation, not Ohm's law. But anyway, ON AVERAGE, yes. But not at the throttle plate and possibly not at the MAF
But since the Air is not an ideal fluid but a gas (not even an IDEAL gas) it is a compressible fluid, and the relationship of total mass flow to pressure to temperature to linear velocity of the air is MUCH MUCH more complex. And the simple Ohms law becomes much more complex for anything more complex than a DC circuit, and I'll argue that air flow into an engine is not a like a simple DC circuit, but rather complex with added consideration of the true pulsitile flow of air into and out of the engine, and the impedance, inductance and capacitance of each tube/air box/MAF/ etc. in the flow path. (I'm appreciative of the many analogies of electricity to fluid flow, but I think your biomedical roots are showing here, we really are talking about fluids, not electricity)
Now the nuts and bolts - for the RS6...
Drive - By - Wire.
Period..
The actual position of the throttle plate is controlled by the ECU...and even though at many times the position of the
throttle place IS directly related to the position of the gas pedal, the ECU will change it to match the needs of the given
LOAD and many other variables at a given moment.
So where your FOOT is on the gas pedal vs where the Throttle body plate is position may not be what you think. Interesting twist indeed, forgot about that little detail. However, it seems highly illogical (ESPECIALLY given Audi's past experience with the mythical unintended acceleration issue) that they would program the actual throttle to provide more power at a decreased level of pedal depression, don't you think?
and....I don't think linear velocity of air has anything to do with torque. I may be wrong...but I doubt it.. While you've certainly shown your engineering mettle, and I'll grant you that electronic controls may have a lot more to do with it than either of us know, to deny that the velocity of intake air has an impact on the performance of an engine flies directly in the face of the real world. Refer to my examples of camshaft profile, intake runner cross sections, carburetor sizing, etc. They DO impact performance directly. If you take all of the points you made, especially one of the later paragraphs about looking at the total system, if you were correct, these components would have zero effect on an engine's performance, but in fact they most definitely do. I'll also concede that I have explained this effect to the best of my ability, and any better explanation and/or proof will have to be left up to someone brighter and more educated than myself.
It's been fun.
Last edited by RS6-4dr911; December 5th, 2007 at 22:14.
The linear velocity AT the throttle body is not necessarily the same at the Valve opening at the head.
This is Pulsitile flow, and the flow of the air actually STOPS until the valve opens again, so what ever velocity the molecules had at the THROTTLE BODY location is not NOW present.
I'll go through your entire post later, I have to run....
I think its GREAT discussing things we basically know little about....LOL....better than politics,
Mike
If you think I'm gonna let you have time to research some actual facts to continue this debate you're nuts. I did, however. My RS6 training manual indicates that the MAF's are well upstream of the throttle body. So, any comments I previously made about the throttle plate affecting the MAF's must, sadly, be retracted.
I'll grant you that the activity at the throttle body may begin to diverge from that at the valve seat, although it's position just ahead of the manifold means it's impact is greater than when I thought it was farther upstream of the valve seats. So now we're arguing over whether the linear velocity of airflow through the "system" has any impact. On small bore throttle bodies or carburetors, the higher velocity provides a stronger signal to the fuel metering circuits improving throttle response. Regardless of the size of the given intake runners, a change in throttle body bore size impacts performance, on a carburetor. Can't say I am convinced it would do so on an EFI throttle body.
You're right on the pulsitile flow (I admit, I learned a new word today) however, think of it this way - since momentum is m x v, and given we have the same m for a given rpm, the higher the velocity, the higher the momentum. If we have higher momentum, then when the valve slams shut "stopping" the flow (this is where our ideal gas becomes non-ideal, and the flow doesn't really stop, it just re-directs momentarily) the momentum starts to create a packing effect of the intake airstream, compressing it. When the valve opens up, a dozen milliseconds later, the high-pressure packed airstream wants very badly to enter the combustion chamber. Here's the ram air effect you doubted existed. I also wouldn't be so sure that the "pulsitile" flow doesn't find it's way back to the throttle body. I remember seeing a dyno test of a high performance 911 engine and at certain steady rpm's you could see a standing wave of fuel vapor just above the air horns (several inches UPSTREAM of the throttle plates) resulting from this very effect (of the valves slamming shut).
Summary, I'm ready to concede that I don't know, for SURE, why there is better performance at part throttle, but I'll fight to the rhetorical death over airstream velocity in the combustion chamber making a difference.
Your turn.(it's still fun, lol. And if we keep it at it in this way, we may very well solve this mystery!!)
Well if there is one thing I've learned here it's that I really need to go back to school. GREAT debate! I think you two are impressive to say the least. Thank you for making me feel like a complete idiot! :trash:
I have noticed the improvement in acceleration from a digg at 1/2-3/4 throttle. I raced a BMW today from a stand still and she really bogged for a couple of seconds. Every now and then I'll unintentionally rocket away from a light at what seems like 1/4 throttle. Always leaves me scratching my head....kinda like this debate.
Are all the cars you guys are talking about automatics? I dont notice this in any of the manual transmission Audis Ive driven. They also dont have that extra bit of effort required to press the gas pedal in the last bit, the action is smooth for the entire travel of it.
MrBucket, my B5 S4 6-speed stick had this issue, and quite a few other B5 S4 owners (mostly 6-speed) on audiworld's forum reported the same (back in ~2002ish).
@ aronis
its manual and front. no traction control
few questions tough:
1::: is that problem on all cars (bmw mercedes.......) or just on audi ???
2::: that means the car acceleration faster with half throttle from 100-200km/h then with full throttle???
1) This behavior COULD show itself on any car with a DBW (drive by wire) throttle, which means most cars sold today.
2) Not exactly, but it depends:
With DBW throttles, unless you can see the programming curve of the actual throttle butterfly, there's no way to predict how the engine will respond. I remember a thread on AW where someone actually looked at the programming of the "Sport" button on the RS4. In that case, the throttle mapping was MUCH different when "Sport" was engaged. Bascially, the first 1/4 of the pedal travel delivered about 70 % of *actual* full throttle, and the last 3/4 only the remaining 30%. Some RS4 drivers like that feeling, some don't.
Personally, I can also add my family's 3 total 2.7Ts to the list of cars that feel quicker at 3/4 throttle than at full throttle, in some instances.
tvrfan -
1. LOL, I don't think there are many folks here who have Mercs or BMWs as you've asked that question quite a number of times. Though, I guess I can ask my friend what it's like in his E55.
2. I think this is talking about acceleration, at any point. Though we're primarily addressing off the line.
its confusing me, a car should accelerate faster on full throttle as not 3/4 or 1/2 throttle!
LOL, that's why you gotta read the physicist/mechanical engineers go at it up there for information!
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