Upper exhaust exit - aero aid?

[Sparky]

As an addition to the aero package this year, many teams are using upper bodywork exhaust exits. Is this as beneficial as they would have us believe?

During the 'Ground Effects' era, the exhaust was used to fill the voids under the car, as the heat made the air less dense, of a lower pressure and therefore created more downforce over the car.

This is an extreme version of the Bernoulli effect.

(Air over a wing section has further to travel than the air under it, so has a higher velocity and thus less pressure, creating higher downforce over the wing.)

Why would the engineers elect to expel the thinner air over the surface? Surely this would have the reverse effect to the wing example above.

Or does the air travel under the rear wing and put the effect I've described to good use?

What say you?

[Franklin]

F1 cars are STILL in the ground effects era.

The exhaust air is not thinner air but HIGHER ENERGY air.

Injecting it into the diffusers at the back of an F1 car increases downforce by re-accelerating the air moving through the diffusers, but can lead to inconsistent downforce since downforce then becomes influenced by engine RPM.

[THR]

I dont really agree with franklin.

speed affects downforce not rpm.
downforce squares with speed. which would have a far greater effect than rpm

i dont know why they have all gone for top exhausts, perhaps the power gain from the cleaner flow from the engine outwieghs the downforce effect?
they reckon that F1 cars understeer more than over steer, so the downforce is an acceptable loss?
more power, more speed,= more downforce, so perhaps power is the main thinking behind it?

[Sparky]

The point I was trying to make, is that nobody has given a good reason for their placement.
Prost used them last year on the AP02.
Prost tested this configuration i the wind tunnel, and suggest it might be worth about 3-4% extra downforce.
The air flow in front of the rear wheels speeds up with the extra heat coming from the exhaust gases. This cleans the air flow through the rear wing, allowing greater downforce.

Apparently!

[KC]

Could the whole idea of cowling exhaust be just a "red herring" to throw everyone off of the scent? And now some of the other teams are copying it because it fashionable to do so. The Ferrari design looks like the exhaust comes out near the front of the gearbox, while their chief competitor McLaren's exits atthe outer edge of the side pod. Who can say what gain is being made. Personally I think its more about trapped exhause heat than anything else. With today's rumored 19,000rpm cielings, the exhaust manifold temeprature must be exceeding what can be managed with a rear exhaust outlet.

[Sparky]

I've done a little more detective work.

Ferrari's engine dyno test results showed that short exhausts might be worth an extra 10 - 15 hp.

So, rather than route the exhaust into the diffuser and cause aerodynamic difficulties, or exit under the engine cover, resulting in rear suspension arms and driveshafts failure, the answer was to exit the exhaust pipes level with the engine cover surface, at the desired length and use the heat to speed the air and reduce the turbulence in front of the rear wheels.

Can anyone think of any other benefits?

[Sparky]

Here's a pic that shows the way in which the exhaust is aimed at the rear wing...

[Red]

Sparky, I also think that periscopes were used because they did not want to blow heated gas into suspension arms. Also wanted to avoid the throttle-sensitive downforce.

The picture you posted is a Ferrari F399 (Eddie's car). At F1-2000, the exhausts are pointed even more toward the central pilon to eliminate the negative effect they might have on the rear wing aerodynamics. This design could not be used before (this is my opinion and of course it could be complete wrong!) because the central pilon was overheated and could crack. (I will look for a pic).

But there is something I don't understand, maybe some of you guys will help:
At MP4/15, Newey & Co decided to expell the exhaust straight into the central diffuser. As far as my knowledge this was tested 10-15 years ago and deemed worthless (inconsistent downforce).

[Red]

Here:
F399

F1 2000

[Dino IV]

Well actually THR, Franklin's point is correct.

The removal of the exhaust from the diffusor is not because of an aero-advantage the periscopes might give (they don't actually) but because of the disadvantage they provide when exiting inside the diffusor.

The differences in downforce between full throttle and no throttle are quite large.
McLaren has found a way to use the advantage of diffusor exhaust and minimise the disturbance in car stability the alternating revs provide.

The principle of diffusor exhausts is quite simple:
When on the gas you'd like your downforce pressure-point to move backwards for better traction. This is possible by entering hot gasses form the exhaust in the diffusor. Why? Because an airstream can be forced to curve (that's what's happening in the diffusor) but at a certain point the air's energy is too low and it is not possible to make the curve anymore. By adding the high energy exhaust gasses it becomes possible.

The principle may seem simple, the McLaren-solution is immensely difficult:
The exhausts of both cylinderrows come together in one hole in a place usually occupied by the gearbox. Their gerabox therefore must be extremely compact and placed high. Let alone the immense heat the exhausts provide. It's very difficult to keep nearby systems in good shape.

Technically this was - until now - impossible. Williams for instance tried the same two years ago, but they burnt their diffusors the first race in Melbourne. They had to move the exhaust more and more to the sides and had lots of difficulties with all kinds of overheating failures. At McLaren it seems to be working fine and I am really impressed, I must say. Those are the kinds of engineering achievements that sets F1 apart from any other series.

Regards,

Dino IV.

[yelwoci]

I'd just like to thank you guys for providing such a well argued, informed and presented discussion here.
This is exactly the type of stuff that makes this forum so worthwhile.
Thank you again
IanC

[Sparky]

Check out the different routing for the McLaren manifolds. You can't actually see the exit tip, but it must arrive at the bodywork in a near-vertical position. Is this the reason for the Mac's 'Stuka' shriek?




[This message has been edited by Sparky (edited 05 June 2000).]

[KC]

I think Sparky has hit opon the different noise made by the McLaren. In NASCAR some of the teams use whats called a 180 degree header. The exhaust headers turn forward before being routed to the rear of the car to generate a certain level of exhaust back pressure. The exhaust sound has a much higher pitch to it even on the large displacement V8s used in WC racing.

[Dino IV]

(WOW lotsa new UBB goodies around )

Quite difficult to find a good picture of the McLarens rear end on the web. Best I came across is this one taken at the Nurburgring: (Sorry for posting the big one but otherwise things still remain unvisible)



The 2-1 exhaust tip can be found exactly between the two little white metal glances at the bottom of the central part of the diffuser. Directly underneath the gearbox!
Remarkable stuff.

About the 'Stukka' shriek I think KC might be right. Compare the headers of the Mac and the Ferrari shown above and 180 degree headers could be the answer.

Regards,

Dino IV.

[Sparky]

Of course, it could just be that the McLaren's manifolds are like that to equalize the primary lengths, or to tune the length of the entire system.
No. 1 port is almost a straight run to the collector, while No. 5 port has to go 'round the houses' to be at the same length.

By the way Dino, I thought the Mac's were using periscopes this year as well?!

Also, schumi's exhaust/suspension failure: Could it have just been high ambient / circuit temps, a cracked manifold due to fatigue (constant acceleration / deceleration) or too many elements in the rear wing? Did the 'Armco kiss' add to the problem, or cause it outright?



[Edited by Sparky on 6th June 2000 at 03:29]

[IAM]

Well it would appear that Ferrari must have moved the exhaust again or just had a problem with it, as I understand that the exhaust caused the suspension on Shuey's red car to break!

Ian

[Mackmot]

I was thinking along the lines of exhausts last week and I was wondering if this idea would work. If it doesnt dont call me stupid cos I'm only 16 and havent yet been to Uni where I will learn all I need to know.

If you look at the picture below that I made, well just pretend that that is the cross section of the diffuser showing the way the air spreads out causing downforce, right, well if you fed the exhaust into this area would the low pressure basically suck out the gases and give an increase in HP or is the low pressure in this area not significant enough to do this.

[Dino IV]

I guess you're right too, Sparky about just sticking to primary lenghts.

No, indeed, no periscopes at McLaren. Ah, now I understand the 'nearly vertical' part in your posting.
Like I wanted to point out in my first posting the Mac-exhausts trail back at bottom level (you can see one side at your engine picture above) and come together in one piece underneath the gearbox and exit into the middle part of the diffuser. Imagine the exhaust tip between the 2 little white metal parts inside the middle diffuser at my rear end picture above.
Hardly comprehensible how they managed to accomplish this without 'killing' all the other heat sensitive equipment around it.

[desmo]

2000 McLaren Diffuser

Sorry if I am dredging up an lold topic, but this is central to the drama at the moment.

It appears to me that airflow is being diverted into the central section of the
diffuser from the adjacent sections via a NACA-type duct on either side. My guess
is Adrian is trying to tune the velocity in the central channel to the median speed
of the exhaust flow being fed into the central section to use the energy of the
exhaust while minimising the sensitivity to changes in the velocity of the exhaust
flow. The siamesed outlets seem to direct the exhaust flow right on the top of the
airfoil in the central section of the diffuser. This could actally create local
downforce or could be just to more cleanly re-introduce the flow under the car to
the wake of the car.

When the Macs engines blow you can see the airflow exiting the diffuser describing
two counter-rotating vortices originating from the outboard portion of the diffuser
and then being pulled up into the low-pressure zone behind the bottom of the rear
wing. Looks kind of like the vortices that are visible off the endplates of the front
wing when the drivers lock a front under braking.

I wonder if there aren't useful lessons to be learned from intentionally introducing
smoke into the airflow around the car to help visualise how the car interacts with
the air in real-life track conditions as opposed to the idealised conditions of the
wind tunnel. Unless you know how the air behaves around the car on the track,
you cannot be certain how accurate wind tunnel simulations are.

[Dino IV]

That's an interesting drawing, desmo, thanks for posting it.
I couldn't find any text on that hungarian site which shed some light on it so I'll give it my own thoughts.

I don't think they use both exhaust exits as shown here.
What I heard from winter testing was that they had wrapped the exhausts exist in the top layer of the diffuser, as shown here.
What they are actually racing with right now is the system down in the middle of the diffuser.
So I guess it's both systems in one drawing.

Your proposal of aero-testing with smoke on the track could be interesting, but it would prove difficult to measure in an appropriate fashion. Firstly one would have to fit all measuring equipment on the car and secondly that would make way too many disturbing materials on it so it should data aquiring should be made in steps and in turn require lots of similar testing circumstances, which is exactly what's so hard to achieve in real life over lab-like circumstances. The extremely advanced windtunnel facilities of some teams offer all they could virtually need. 1:1 scale, rolling floors, suspension loads, bump simulation, slip angles etc etc. I guess they way they're simulating reality right now gives data, almost as accurate as real track data would give minus the error for changing circumstances while measuring.

Cheers,
Dino

[desmo]

I should have mentioned that the other arrow in the drawing pointed to the old exhaust outlet location.

As someone pointed out earlier in the thread, it was quite a feat to get the outlets that close to the centerline of the diffuser. Note what appears to be an airfoil just behind and beneath the siamesed exhaust outlet. This in concert with the central outlet location and the apparent tuning of the airflow velocity in the central channel seem to somehow be the means Adrian used to address the "diffuser stall" phenomenon.

If you saved tapes of the early-season races when the Ilmors in the Macs were blowing up, watch the smoke from the Macs diffusers. It is a perfect illustration of the airflow exiting the diffuser and rejoining the wake behind the car.