# How is the 2.5l, seet up to balance the movement of 5 cylinders, counterbalance shafts or?



## Quisp (Mar 18, 2012)

trying to learn more about the car and I was wondering how the 2.5 turbo compensates for the problem of balancing the movement of 5 cylinders. With even numbers it is easier to understand but with 5 i wondered how audi did it with the TT RS.
Any info would be great.


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## carl44 (Nov 23, 2012)

Quisp said:


> trying to learn more about the car and I was wondering how the 2.5 turbo compensates for the problem of balancing the movement of 5 cylinders. With even numbers it is easier to understand but with 5 i wondered how audi did it with the TT RS.
> Any info would be great.


Here you go . Carl

http://www.jlosee.com/images/TTRS/PDF/l5ttrs.pdf


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## Williamttrs (Mar 29, 2013)

carl44 said:


> Here you go . Carl
> 
> http://www.jlosee.com/images/TTRS/PDF/l5ttrs.pdf


Good read. Thanks Carl!


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## Quisp (Mar 18, 2012)

carl, thanks but I have that and the actual training manuals for the car. In the info you linked to I must be missing the part about the counter balancing. Did you see it mentioned in there, if so can you point me in the right direction. I may have read it and not realized it because it was called something else.


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## hightechrdn (Dec 9, 2011)

Quisp said:


> carl, thanks but I have that and the actual training manuals for the car. In the info you linked to I must be missing the part about the counter balancing. Did you see it mentioned in there, if so can you point me in the right direction. I may have read it and not realized it because it was called something else.


The crankshaft has counterbalance weights built into it, like 99.9% of all automotive engines. There is also a balancer assembly bolted to the front of the crank, on the outside of the engine crankcase. Again, this isn't anything special per say. The harmonic balancer bolted on the front of the crank does look to be a 'high performance' design compared to cheaper units used on a lot of engines, but it isn't some specific to the I5 engine design either.

Beyond those features, I don't believe the 2.5L TFSI engine uses any balance shafts from what I have read. Balancer shafts are typically not used with high performance engines as they tend to hurt engine responsiveness (both increasing and decreasing RPM).

In summary. the 2.5L TFSI uses a fairly conventional approach to engine balancing, but with a high quality design and parts spec.


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## carl44 (Nov 23, 2012)

The EA113 in the TTS has a counterbalance shaft , none in the RS. I'm suprised on how well it's balenced. There's a little harmonic vibration right off idle @ aprox 11-1200 otherwise very good for a odd cylinder motor. I had a 91 NSX when they came out and it was a 60degree V-6 . It had bad harmonics @ about 23-2500. Most 60 degree V-6 s offset the crank rod journals 30 degrees to even out the fireing impulsives ..carl


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## Quisp (Mar 18, 2012)

Thank you for the info. bolted to the front like where the pulleys are? What happens if the balance is thrown off somehow? I imagine it woudl be disastrous.
When you say harmonics, I think sounds, is that the same type of harmonics you are referring to?
The only firing impulsive i now is my wife....


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## carl44 (Nov 23, 2012)

Quisp said:


> Thank you for the info. bolted to the front like where the pulleys are? What happens if the balance is thrown off somehow? I imagine it woudl be disastrous.
> When you say harmonics, I think sounds, is that the same type of harmonics you are referring to?
> The only firing impulsive i now is my wife....


What happens with engines is many have a certan rpm that causes a vibration that goes away with either a increase or decrease in rpm. It's the same as what happens with wheel balence it may shimmy @ 65 and be smooth @ 70 or 60. Carl


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## Williamttrs (Mar 29, 2013)

Someone correct me if I am wrong, but I always thought harmonic balancing had to do with the fact that all materials resonate at a certain frequency. If left unchecked, this feedback will destroy just about anything. Examples of this have been tragically seen in some bridge and building designs and engineers now employ canceling techniques to avoid it. 

The 1940's collapse of the Tacoma Narrows Bridge is an example of what I am discussing.


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## Resolute (May 15, 2012)

I'm not quite sure what you're asking, but it's a five cylinder engine, so it's got more than its fair share of 'balancing' problems.

While the crank features counterweights like all modern IC crankshafts, each counterweight simply helps cancel the inertial loads imparted around the crank by each reciprocating assembly (conrod, wrist pin, piston, etc...). You might think of this as 'balanced' because if we attach the weight of each reciprocating assembly to the crank, and then spin it on a balancing machine, all this mass will rotate about the crank's longitudinal axis without any drama. Unfortunately, while the electric motor rotating the entire assembly on a balancing machine is nice and smooth, in the real world this whole assembly of rotating and reciprocating masses will be motivated by individual explosions of fuel and air. There isn't a counterweight for explosions.

It's important to realize that as 'smooth' as an engine might feel, each turn of the crankshaft is actually powered by separate explosions in the combustion chamber. The force of each explosion imparts some measure of instantaneous torque about the crankshaft which decreases through the rest of the power stroke. Of the 720 degree rotation which is a full combustion cycle, only about 200 degrees of that rotation are actually under power by a single cylinder- for the rest of the combustion cycle that cylinder's reciprocating assembly is simply 'freewheeling' along for the ride. So, unlike an electric motor which delivers smooth, linear torque at a given rpm; an internal combustion engine generates a varying amount of torque across each 720 degree combustion cycle. At some point in the cycle, the instantaneous torque is high, while at other points, the instantaneous torque can be negative (think of the 'freewheeling' components acting as drag, or creating a force acting opposite the direction of crank rotation)

How does this translate to the torque value we read on an engine dyno? In multiple cylinder engines, the torque generated by all the individual cylinders are superimposed with a phase shift dependent on the numbers of cylinders, their configuration, crankshaft design and firing sequence to produce the 'mean torque' at a given rpm. In other words, the average torque produced at a given rpm by all the cylinders through a full 720 degree combustion cycle. Changes in volumetric efficiency at each rpm alter the amount of air/fuel available for combustion, which in turn changes the mean torque generated by all the cylinders at that rpm, and the dyno plot we see is simply the mean torque values for each rpm across the engine's working range.

Regardless of rpm and how much torque is generated by all the cylinders at that rpm, it's critical to remember that it's still a mean value of individual 'surges' of torque generated by all the cylinders through the complete combustion cycle. As a result, the mean torque of all the cylinders (aka how much torque the engine is said to generate at that rpm) is ALWAYS less (often several times less) than the peak instantaneous torque generated by a single cylinder. (remember, at some points in the crank rotation torque is high, at other points it can be negative) Further, the instantaneous torque generated by each cylinder is variable across the full range of the power stroke and is also inconsistent from cylinder to cylinder. This regular frequency of variable torque application about the crankshaft combines with the cylinder configuration and their firing order to produce most engine vibrations.

Ideally, an engine should be designed so that forces generated in any particular cylinder are directly opposed by those of another cylinder. Think of a four cylinder engine, where as one piston is shot down a cylinder during a power stroke another piston is 180 degrees out of phase in the cycle so that it is moving up its cylinder at exactly the same rate and duration. This would ensure primary balance, or first-order harmonics (vibrations generated at engine speed), to be in good order. It requires even numbers of cylinders and an even firing order. 5 pot engines obviously don't have this.

Unfortunately, even with even firing cylinder arrangement, angular variations occur between the connecting rod and the cylinder axis as the piston performs each stroke. This is because the reciprocating assembly is driven by a rotating assembly within a cylinder that is not infinitely long. As a consequence, the connecting rod doesn't get to move in a perfect straight-line motion and the piston is forced to move more rapidly over the outer half of its stroke than it does over the inner half. Simply put, the piston doesn't travel at the same speed throughout the combustion cycle for the same angular movements of the crankshaft. This is why rod-to-stroke ratio is important for designing high-revving engines, because longer rod lengths and shorter strokes reduce piston side-loading in the cylinder and decrease instantaneous acceleration at TDC, which in turn improves dwell time. In any case, the resulting inequality of piston accelerations and decelerations produces corresponding differences in the inertia forces generated. This is secondary balance, and vibrations produced from this are seen as second-order harmonics (twice engine speed). Inline and boxer sixes can balance these forces, as can boxer fours (not inline fours though), because the differing inertia forces can be both matched and opposed in direction between one cylinder and another. 5 pot engines don't do this.

While cylinder numbers and arrangement can help mitigate some serious balance issues from all those individual surges of torque, we still have rocking couple to deal with. Essentially, the offset of each cylinder firing with a variable and diminishing amount of torque causes the crank to want to rock fore and aft with each cylinder's power stroke. Firing order and crankshaft length come into play here. Longer cranks have higher rocking moments, and therefore more couple. In this way, inline 5 cylinders are better than inline sixes, but obviously suck compared to the shorter cranks found in a V6 or inline 4.

Finally, all those individual surges of variable torque cause a disturbing amount of torsional excitations. Think of these as different amounts of 'twist' the crankshaft undergoes through the combustion cycle. While we think of the crank as a rigid piece of kit, both the material and design contribute to proportional limit stress, whereby it will slightly deform under a specific load and return to form once unloaded. Loading stores energy in the crank, and unloading releases it. The amount of energy stored and released through these torsional loadings varies with crank material and design, instantaneous torque from each cylinder, the number of cylinders, cylinder arrangement and firing order. More cylinders means less time between power strokes for a combustion cycle, which means the mean torque is closer to peak instantaneous torque generated by each cylinder. This reduces torsional load. Also, even firing order and piston arrangement can reduce torsional loadings. A 60 degree V6 can fire a power stroke every 120 degrees, but a 90 degree V6 must alternate between 150 and 90 degrees of rotation to fire a power stroke. The odd firing caused by the cylinder arrangement of a 90 degree V6 creates periods (frequencies) of higher loading, which produces torsional vibrations in the crankshaft. Handling such vibrations is the role of the harmonic damper at the front of the crankshaft. Contrary to internet myth, this heavy pulley on the snout of the crankshaft isn't there for 'external balancing' of the rotating assembly. It's there to dampen the energy released from torsional unloading. Most dampers are tuned elastomer types, specifically designed for the engine to cancel out the widest possible range of frequencies encumbered by the engine design.

What does all this mean for the Audi 2.5L TFSI? A 5-pot engine has to fire a power stroke every 144 degrees of crankshaft rotation. This means the crankpins have to be 72 degrees offset from each other if we don't want the engine to sound stupid and also expect it to work well. Remember how I said for a 720 degree combustion cycle, each cylinder only applies torque for about 200 degrees? Well, with a 5 cylinder engine firing every 144 degrees, we always have some amount of positive torque being applied through the crank. Hurray for us. This inherently makes it smoother than a single cylinder lawnmower. Unfortunately, it also means the uneven levels of torque divided among the five cylinders increases second order vibrations. The good news is that there is still a lower discrepancy between the mean torque and each cylinder's instantaneous torque than we would see in a comparable four cylinder engine. This can give the 'feel' of a 5 pot being smoother than an inline four, especially at mid-range rpm where the second and third order harmonics are better. But, a four cylinder is at least balanced for first-order harmonics (that whole even number of cylinders thing), and the 5-pot isn't. Damn. This could be solved by a single counter-rotating mass moving at engine speed (a single balance shaft), which would provide the engine with primary balance, but Audi didn't do this. Instead, the engine is designed to just deal with it, which means it won't be a good high-revving engine design. Like, at all. Even worse, at higher engine speeds, inline 5 cylinders have an uneven third-order vibration which occurs every 144 degrees. Makes sense why they designed it for a turbo now and not as a NA screamer, huh? But all is not terrible for this 5 cylinder's 'balance'. Audi wisely implemented a 1-2-4-5-3 firing order (instead of 1-5-4-3-2), which lowers the rocking couple and the first order harmonic while increasing the second order harmonic. To help deal with this, Audi fitted a cool viscous harmonic damper instead of an elastomer one. Several SAE papers have concluded that viscous dampers can not only dampen a broader range of frequencies derived from torsional excitations, but help alleviate some second-order harmonics.

So... to answer your question. No, Audi didn't use a balance shaft. Yes, you need one in an inline 5 engine to balance first-order harmonics, but Audi ain't care. No, I wouldn't worry about it, unless you plan to rev the thing beyond it's design limit. And while the engine inherently has crappy second and third order harmonic issues, Audi employed a viscous damper and the best firing order to minimize issues. Which is a damn good reason to NOT replace the damper with one of those stupid machined Al lightweight crank pulleys.

Will


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## Williamttrs (Mar 29, 2013)

Resolute said:


> I'm not quite sure what you're asking, but it's a five cylinder engine, so it's got more than its fair share of 'balancing' problems.
> 
> While the crank features counterweights like all modern IC crankshafts, each counterweight simply helps cancel the inertial loads imparted around the crank by each reciprocating assembly (conrod, wrist pin, piston, etc...). You might think of this as 'balanced' because if we attach the weight of each reciprocating assembly to the crank, and then spin it on a balancing machine, all this mass will rotate about the crank's longitudinal axis without any drama. Unfortunately, while the electric motor rotating the entire assembly on a balancing machine is nice and smooth, in the real world this whole assembly of rotating and reciprocating masses will be motivated by individual explosions of fuel and air. There isn't a counterweight for explosions.
> 
> ...


And I thought I wrote long posts. Thanks for sharing. I will have to read this one in multiple sittings.


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## cipsony (Mar 26, 2013)

Resolute said:


> I'm not quite sure what you're asking, but it's a five cylinder engine, so it's got more than its fair share of 'balancing' problems.
> 
> While the crank features counterweights like all modern IC crankshafts, each counterweight simply helps cancel the inertial loads imparted around the crank by each reciprocating assembly (conrod, wrist pin, piston, etc...). You might think of this as 'balanced' because if we attach the weight of each reciprocating assembly to the crank, and then spin it on a balancing machine, all this mass will rotate about the crank's longitudinal axis without any drama. Unfortunately, while the electric motor rotating the entire assembly on a balancing machine is nice and smooth, in the real world this whole assembly of rotating and reciprocating masses will be motivated by individual explosions of fuel and air. There isn't a counterweight for explosions.
> 
> ...


BIG LIKE thank you.


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## smack_ttrs (Mar 24, 2013)

yeah great info.
thanks!


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## Quisp (Mar 18, 2012)

That is very interesting reading. Wouldnt have thought about the surge each time it fires but n iow it seems like an "of course" moment. Could not have a constant equal torque on a gas engine, there will always be that surge. MIgh tnot be notioceable but it is initeresting none the less.
Anybody seen the wirte up on the new Mercedes SLS AMG Electric? 740 hp and 738 lb/ft torque from an electric.(and they play sounds for the car through the stereo like starting noise and driving...)


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## Resolute (May 15, 2012)

Williamttrs said:


> Someone correct me if I am wrong, but I always thought harmonic balancing had to do with the fact that all materials resonate at a certain frequency. If left unchecked, this feedback will destroy just about anything. Examples of this have been tragically seen in some bridge and building designs and engineers now employ canceling techniques to avoid it.
> 
> The 1940's collapse of the Tacoma Narrows Bridge is an example of what I am discussing.


This is more or less correct. A crankshaft has a natural frequency, just like the Tacoma Narrows bridge had a natural frequency, whereby exposure to forces of that frequency will naturally excite the part with increasing vigor until fatigue failure. As discussed in my previous post, the nature of the internal combustion engine means there will always be orders of vibration caused by the periodic application of variable amounts of torque through a reciprocating assembly acting about a rotating assembly. These vibrations can include a harmonic vibration which coincides with the natural frequency of the crankshaft at a critical rpm. The torsional vibrations (and to some degree, lateral oscillation, or 'rocking couple') of the engine can be minimized by a damper tuned to the crankshaft material and engine parameters. The damper absorbs the energy stored in the crankshaft from torsional loads and releases it as heat, which reduces harmonic vibrations in much the same way as a weight can be used to dampen vibrations in a tuning fork. As a result, harmonic dampers not only reduce overall NVH, but also reduce fatigue on the crankshaft and prolong the service life of associated components like engine mounts.

Will


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## Marty (Jul 14, 2000)

Great posts, Will!


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## TTRStud (Jul 18, 2013)

Reviving this post...has anyone noticed engine vibrations when revving while the car is stopped in place? Here's what I noticed: rev up and all feels nice and even, but then the rpms are dropping, the car vibrates right after the needly passes 1200 rpm's or so on its way down to idle. 

I'm a new owner so I'm still trying to get to know my car; I've never owned a 5-cyl car before, so I want to ensure this is all normal. At first I thought it had to do with my car having an aftermarket exhaust (louder, more poppy), but after reading some information, it appears that they are completely independent from each other.


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## carl44 (Nov 23, 2012)

TTRStud said:


> Reviving this post...has anyone noticed engine vibrations when revving while the car is stopped in place? Here's what I noticed: rev up and all feels nice and even, but then the rpms are dropping, the car vibrates right after the needly passes 1200 rpm's or so on its way down to idle.
> 
> I'm a new owner so I'm still trying to get to know my car; I've never owned a 5-cyl car before, so I want to ensure this is all normal. At first I thought it had to do with my car having an aftermarket exhaust (louder, more poppy), but after reading some information, it appears that they are completely independent from each other.


I think i posted awhile ago that there is a vibration at about 1100 . if you sit in natural and hold the gas @1000-1200 you will feel it, i think its just the nature of the engine. most cars ive had have some rpm range that you feel a vibration. i remember my 91 NSX shook @ 2300 they all did because it was a 90 degree V6 . carl 

From a NSX fourm:

Quote Originally Posted by ImolaOrange03
I just purchased a 2003 NSX with 14,000 miles on it. There was rough idle during warm up at 1300 rpm and calms down a bit once it went down to 900 rpm. When I step on throttle to get 1300 rpm, I got the rough idle back.

I took the NSX to a Acura dealer and the Acura Sr. Tech at Acura Headqtrs said that the rough idle is normal. Said that if I removed the Targa top, the roughness during idle will be less. Anyone else experience this?
That vibration you feel just above normal idle sped (1200 rpm or so?) is just a result of the engine's characteristic mechanical vibration and not misfire. It's a 90 degree v6 rather than the more balanced 60 degree. Personally, I like the shudder that you feel in your back when you blip the throttle through that range--it is part of the NSX's character.


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## TTRStud (Jul 18, 2013)

carl44 said:


> I think i posted awhile ago that there is a vibration at about 1100 . if you sit in natural and hold the gas @1000-1200 you will feel it, i think its just the nature of the engine. most cars ive had have some rpm range that you feel a vibration. i remember my 91 NSX shook @ 2300 they all did because it was a 90 degree V6 . carl
> 
> From a NSX fourm:
> 
> ...


Thank you! But what I'm experiencing is while the rpm's are dropping back to idle, not while revving and holding it. Have you noticed anything like this?


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