Investigating exhaust aided diffusers

[xpensive]

http://en.wikipedia.org/wiki/Mythomania

[autogyro]

Just explaining how an exhaust driven diffusser could logicaly work, I though the aero nerds would approve.
I had no idea that Ferraris inability to make one work would cause such a problem.

[marcush.]

to my understanding Difussers are flow enhancers and used to work the underfloor harder .
I do appreciate that the upsweep of the difusser topsurface in itself is a downforce generating surface as it is a wedge.
but when speaking only of difusser induced DF this will materialise near the forward edge of the underfloor ,right?

[ringo]

Mods!

Any chance you could auto-remove autogyro's continued and inappropriate injection of his "wonder-box" in to every other thread on this forum?

It's getting extremely tiresome seeing it crop up again and again and again and again and....

Guess what mate? You can do it yourself!!!

Just a fan, meet the "foe" list. "Foe" list, meet Just a fan.

Back on topic...

It could be a possibility, but i think we need to look at the cars themselves and find out why it is an exhaust blown diffuser is a net gain on a 2010 car, than say a car from the early 90s or late eighties.
What i find stands out is the engine speeds. Cars today rev at 18000~20,000 depending on which year and engines you are looking at.

Well, I believe McLaren abandoned the blown diffuser with the MP4/16 of 2001. The 2000 car still had it, and that engine reved between 17,000 and 18,000 rpms. So, about the same as they are now...and yet they dropped the concept...

Hmmm...what does that mean?

Yes now we are getting somewhere, would you care to post pics. What i really wanted to see is more numbers as it relates to finding the exhaust velocity.
The Mp4 16





pics from : http://scarbsf1.com/mac_detail/index.htm

where does the exhaust go, is it in the diffuser throat, or simply exits under the crash structure?
I really want to get into the details of this, because i have no idea why the method was dropped for so many years, and it suddenly re emerged on the RB6, which happens to be the fastest car on the track. This is why i use the word stigma; it seems all teams overlooked it, and even amongst the users here the method is met with an explanation about DF senstitivity to throttle etc.

[ringo]

I've changed the topic to investigating and removed stigma. This topic is to focus on the different exhaust blown diffusers over the years and putting some numbers to them.
It would be helpful if the more experienced members could list out these cars.

[xpensive]

Just explaining how an exhaust driven diffusser could logicaly work, I though the aero nerds would approve.
I had no idea that Ferraris inability to make one work would cause such a problem.

Not being the most modest man on the face of the earth myself, I honestly don't think that you are either Auto?

Xplain to me the groudbreaking inventions taken place over the past 30 years, John Barnard was far from an idiot in my book and neither was Gordon Murray, Neil Oatley or Adrian Newey. Colin Chapman could make giant strides from one season to the other, but I don't think you will ever see that, 30 years later?

Technical developments seem to move in small incremrents in these optimized days.

Besides, the diffusers can be outlawed woith a penstroke, just like that, back to square one.

[marcush.]

Neweys CG 901 had an exhaust driven difusser ,I bet his cars before did as well...and the williams of Newey did...


http://www.gurneyflap.com/Resources/LH-Engine-Bay.jpg CG901
http://www.gurneyflap.com/Resources/MARCH-911b-451.jpg
http://www.gurneyflap.com/Resources/MARCH-911b-473.jpg
http://www.gurneyflap.com/Resources/MARCH-911b-444.jpg
http://www.gurneyflap.com/Resources/DSC_0255.jpg FW16
http://www.gurneyflap.com/Resources/DSC_0259.jpg FW16
http://www.gurneyflap.com/Resources/P1020453.jpg FW14b

[ringo]

Nice shots you got there. Looks as though the exhaust is either in the centre; mp4 16, or 1 pipe per side; FW14b. Looks like the same old tricks are being used today, except that the RB6 doesn't have the pipes sealed in the diffuser. There is an orifice in the diffuser which allows outside air to pass through as well. RB6 concept is much different.
It could be this hole that reduces the sensitivity of the diffuser to the throttle.



I am planning to do a cfd, but when i get some free time. And try to look at the effect of engine speed.

[marcush.]

placing the exhaust exit upstream and have a zone where it can mix with the car flow
will for sure take out the peaks you would sustain with a direct port into this area?
secondy I´d say there is a difference introducing that flow at the transition from flat floor to difusser (wich does add massflow into the diffusser)whereas positioning the exaustpipe above the diffusser creates a quickflowing hot air area that might as well work as a difusser extension? so does he use both effects?

[DaveW]

placing the exhaust exit upstream and have a zone where it can mix with the car flow....

I like that thought. RBR's pull rod layout moves weight (& volume) forward & down (all good but, perhaps, comparatively insignificant). It ensures that the rod is clear of the exhaust flow (probably more for thermal than aero reasons). It would also, presumably, provide more flexibility to position & direct the exhaust flow to make best use of the energy. Closeups of the rear of the vehicle would be useful to suggest what (if anything) might interact with the flow & whether some of it might be diverted (a diffuser energizer, perhaps).

A rear down force generator under the driver's right foot would be something of a challenge during braking & corner entry, but the enhanced exit traction would probably be seductive (slow in, fast out again)....

[ringo]

I think that is probably the same reason the RB5 was no good in the wet in china. Horner said he knew exactly why the performance was not there in the car but he did not say what it was.
It could may well be the diffuser opening. The exhaust stream gets pretty noisy in the wet, on top of that, water will take take away some of the sensible heat from the exhaust, reducing the calculated effectiveness of the diffuser.

[vonk]

Sorry, late to the party.

IMHO, the exhaust mass flow is a function of the power produced by the engine regardless of its RPM. So it’s throttle dependent. You could use the exhaust flow to somehow increase down force or you could use the diffuser to “suck” on the exhaust for better engine breathing. This would reduce the diffuser down force where the exhaust comes out, so it is segmented off from the rest of the diffuser bottom (arrow). Like most tricks, it’s a trade-off, but helpful down the back straight.

[marcush.]

I think that is probably the same reason the RB5 was no good in the wet in china. Horner said he knew exactly why the performance was not there in the car but he did not say what it was.
It could may well be the diffuser opening. The exhaust stream gets pretty noisy in the wet, on top of that, water will take take away some of the sensible heat from the exhaust, reducing the calculated effectiveness of the diffuser.

quite an intriguing thought .
as well it surely is of no help funneling water from top of the car through the slot into the difusser ....

[engineguru00]

IMHO, the exhaust mass flow is a function of the power produced by the engine regardless of its RPM. So it’s throttle dependent. You could use the exhaust flow to somehow increase down force or you could use the diffuser to “suck” on the exhaust for better engine breathing. This would reduce the diffuser down force where the exhaust comes out, so it is segmented off from the rest of the diffuser bottom (arrow). Like most tricks, it’s a trade-off, but helpful down the back straight.
]

This idea has been floating around in my head for a while. While I am sure it helps with downforce generation as well (by either increased mass flow rate or energizing the boundary layer), I was thinking more along the effect on the engine of this system. Back pressure in the exhaust system is detrimental to engine performance, with every 7.6kPa taking away roughly 1% top end power. By putting the exits in a high velocity, you lower the total pressure that you are fighting against (Ptotal=Pstatic+.5*density*velocity^2). In the case of exhaust exiting parallel to the outside high velocity air, the dynamic pressure actually takes away from the static pressure. This lowers your back pressure and, as a result, your pumping losses.

My bet is that some of the performance loss in the rain is due to the loss in heat from the exhaust. If they are using the high temperature of the exhaust to energize the boundary layer as been speculated, the presence of water (or even humidity in the air) would lower this due to water's high relative specific heat.

[riff_raff]

vonk,

IMHO, the exhaust mass flow is a function of the power produced by the engine regardless of its RPM

The N/A piston engine is a positive displacement device. So at constant ambient conditions, simple air mass flow through the engine is mostly a function of engine speed (the injected fuel mass contributes a little bit).

On the other hand, the amount of energy or momentum within the exhaust gas mass flow is a function of both engine power and piston speed.

When the exhaust flows are directed into the underbody diffuser zone, the term for such an arrangement is commonly called an "ejector". Ejectors are commonly used on turbine engines to improve bypass flows. To be effective an ejector should produce exhaust flow velocities that match or exceed the ambient flow velocity. This is not easy to do with a recip engine exhaust flow that varies widely at any given instant, or at any given throttle position.

As for estimating the mass and velocity of an engine's exhaust flow at the header collector's exit point at any given instant, that is very difficult. Since the exhaust manifold's flow dynamic pressure, temperature and velocity can vary widely during the course of an engine cycle. The instantaneous local flow conditions within the pipes are affected by acoustics, cylinder blowdown pressures, valve overlap, and engine load.

The issue with using the exhaust flows to gain an ejector effect at the diffuser is that going into a corner the driver will slam the throttle closed right at the same time he is applying the brakes hard. The loss of downforce from the exhaust ejector effect changes the center of pressure within the underwing and thus changes the front/rear grip of the chassis precisely while braking and turning into a corner. The exhaust flows cannot be maintained during this time without keeping the throttle wide open, which would require using the clutch. And using the clutch at each corner would destroy the clutch within a lap or two.

Regards,
riff_raff