Basal Skull

why do you have to keep volume the same - it just changes with the temp. It's probably a combination of very small amount of pressure dropping and volume expanding - that's why the temp decreases.

TB993tt

So back to my earlier question, if the air is at 1.4bar as it enters the combustion chamber AND the air is cooler (than a "normal" 1.4bar charge) 'cos it has expanded then with a CR of 9:1 surely an ideal combustion should liberate more like 650hp not a measily 530 ????

It makes more sense to me that the turbo is working to produce 1.4bar to make the whole expansion thing "do its thing" with the resultant air escaping from the narrow intake tubes into the combustion chamber being actually quite a bit less if one measured the average pressure of the volume of air allowed into the chamber during a stroke ?
The density/volume stuff surely just changes with working temperature

Can someone direct some of the clever guys over here: Steve W, Porsche PhD, Red Rooster, Geoffrey etc ??

eclou

Keeping the volume the same would even be a stretch. A cooling gas will tend to contract, not expand in volume. That is why there must be a pressure drop. These gas laws have not been circumvented by Porsche engineers.

pole position

Out of curiosity, at what manufacturers R&D are you employed at ? You are making some definite statements and I was wondering if you could back them up with facts, not net fiction. I am also puzzled by a man of your abilities that you use generic GIAC software................are you the brain behind Lin and his mediocre offerings with 6000 revisions until he gets it right ?

eclou

I think you are correct that the 1.4 bar is seen only prior to the expansion chamber, which then creates another pressure drop to 1.2 bar or so. I just cannot imagine that the benefit is worthwhile. Empirically since the RUF 550(using the stock 997 VTG and 1.2 bar) car is actually faster than a Gt2 , it would hold that the "modified VTG" and "expansion intake" is not a step forward.

eclou

Nice post PP. You are adding little value to this discussion. I have stated the facts for my assertions:

1)factory published dyno plots
2)real world performance figures
3)basic gas laws

I am going to PM you my background for your satisfaction

TB993tt

I am happy that this explains what the Porsche marketing people are trying to describe above.

Does it follow that to make a VTG (or any other blower) produce 1.4bar in a "normally" sized intake pipe (linear 997tt) would take more energy than producing 1.4bar in a narrower pipe (GT2) so the VTG is probably not actually working THAT much harder (quantifying how much is obviously difficult) than on the 997tt.

I suspect Porsche marketing did not want to take the discussion(training for sales people) any further than mentioning 1.4bar -which lets face it MOST people asscociate with the size of their manhood and it will be well recieved by customers

I don't agree that this is not a breakthrough and using marketing torque graphs is too simplistic IMO. Porsche will have spent many many hours R&Ding this using VERY sophisticated test stand. Whilst the difference may not be as massive as the "blurb" would have us think, Porsche would not use it if it wasn't significant.
The modified 997tt cannot be compared unless the 997GT2 is tuned to the same limits instead of the (almost certainly) conservative factory limits.

What of tuning the 997GT2, will the relative small tube mean that the VTGs can wind up to give even more boost before the expansion phase ?? All good stuff which will no doubt come out in the months to come - I can see a bigger market for the Secan Intercoolers

eclou

Theorhetically if the intake plumbing was of smaller caliber, the GT2 should be able to produce more boost and subsequent torque at lower rpms than if using a larger caliber intake manifold. I just don't think this is the case based upon the figures Porsche has released to date.

The other issue I see is that it means the GT2 VTG's may have little to no headroom for additional power output. The expansion intake is a mandatory pressure drop/restriction point and certainly pushing the VTG's to any higher boost should be of great detrimental consequence.

Basal Skull

ahh, this is where you are incorrect sir. You are right in that when a certain volume of air is cooled, it will decrease in volume, but, if you have compressed air/gas then you allow it to expand, it gets colder - absorbs heat as it expands.

eclou

You are correct, but your question was what happened if you kept pressure constant. In the above situation, the pressure will also drop if you allow the volume to expand.

Basal Skull

But the pressure drop is very small, not anything near the 1.4 to 1.2 bar that's been talked about in the above posts. It's the change in pressure related to just increasing the length of the 'distributor tube' and decreasing the length othe the 'intake port' and the flow created by such change in the intake. Pulsatile flow comes into the equation as well.

Porsche is talking about small differences in efficiency that they likely have proven to make a difference. I doubt it's just marketing, it must make a difference for them to use it. (I think)

I just pm'ed a 'automotive air induction engineer' who I've seen post on the 997 forum - hopefully he can also comment.

Al Pettee

You got it Basal Skull (I'm just another humble MD, as well).

The expansion intake was engineered to decrease temps. at the impeller since those on early VTG's on test mules were melting.

It wasn't merely a marketing exercise.

Al Pettee

I forgot to add it also was a nod to the CO2 ****'s blitzkrieging Europe to demonstrate fuel consumption counter-measures.

eclou

Back to the ideal gas law- c'mon guys, I know that chem was part of my pre-med req's - the volume could be held as a constant, since the volume of each cylinder is .6L, say intake temp is 320K, pressure is 140kPa (1.4bar)

If pv= nRT, then (140kPa)(0.6L)=n(8.314)(320k)
n=0.03 moles of "gas" in each cylinder

if you want to generate even a modest drop in the intake temp, say 20 degrees then we can figure out how much pressure drop would need to occur

p2=nRT2/V

p2=(0.03)(8.314)(300)/0.6L

p2=124.7 kPa, or 1.25 bar

This is just for a 20 degree drop. Good intercoolers can generate much more drop.

Now, a 20 degree drop in intake temps would probably not drop EGT significantly, since EGT is more dependent on the pressure increases in the actual turbine of the turbo. Elimination of exhaust backpressure is far more important a factor in dropping EGT's

8haggis

If I can just jump in here with a few thoughts,
This post is an interesting read and I have to say that when I saw the posting of the GT2 pamphlet on-line, I was surprised to read about this manifold design concept as it does appear to depart from the norms of intake manifold design.
I hope I can clarify some of the discussion here by talking a little about manifold resonance design (and apologize in advance if this runs a bit long):
Manifold resonance design is the management of pressure transfer from left to right bank manifold for what is effectively natural supercharging. When the engine is running, there are pressure waves which travel back up from the intake valve through the intake manifold. When designing your intake manifold, you want to use these pressure waves to "push" more air into the next firing cylinder to increase the charge entering the cylinder and increase its power. Except for the manifold in discussion, Porsche intake manifolds are designed with this in mind. To increase the engine operating range in which this happens at an ideal time (the pressure waves travel at a fixed speed and therefore the distance that they have to travel will determine the RPM value at which all things line up perfectly), Porsche added a communication tube between left and right bank manfolds. When this tube is shut, the path is longer. When open, shorter and therefore a different RPM for natural supercharging.
Now, what the press release is saying about the GT2 manifold is that it is designed for the opposite effect. It is a bit of a simplification, but the diagrams shown in the topic starter basically show this by pointing to the valley vs the peak of the sinusoidal pressure wave. This means that when a cylinder is trying to draw in air (intake valve open and cylinder traveling downward), the pressure wave is traveling away from that cylinder. (By the way, the ideal gas law that has been discussed does not include the effects of a fluid in motion. Fluid Mechanics is a field all its own, and a complex one at that and I can get into that more if needed, but I'll skip it for now.) So, since you are pulling the air into the combusion chamber at the same time that it is trying to pull away from that chamber, it expands. When it expands, the temperature will drop. This has been indicated by the gas law discussions, but there is another greek letter which is influenced by this situation. Rho, or the density of the fluid. When this air is expanded, the density will drop as well as the temperature. In the area of combusion, this is a bad thing. Lower temperature is good as this gets you farther away from the cusp of pre-ignition, however the lower density doesn't give you as much oxygen to burn. Less oxygen means less HC (gasoline) can be combusted with it, and therefore less energy and less work. So, how do you get around this problem? Increase the boost pressure.

I think this might answer the intial question which was asked.

However, the most interesting question (for me) is "Why did Porsche take this approach on this vehicle/powertrain?" This is a VERY hard question to answer from an armchair and really would require some "inside" information from the development team on why this tact was taken. There are many, many factors which could be players in this final decision on building this car with an "expansion manifold."
I do not believe that this is a marketing invention to sell the car. I also believe that the basic reasoning is sound, if a bit unconventional. Is it the best choice forward? Well, time will be the judge of that. If this is a one-off, then the expected theoretical benefits were likely not realized (or were realized, but at a unacceptable price). If we see more expansion manifolds in the future Porsche line-up, then maybe they figured out something we all missed. It might be worth a check on www.sae.org in April to see if Porsche release a paper explaining their work on this manifold design. (Kinda like a Journal of Medicine for Automotive Engineers)

I'm not sure if this helps or hurts.