Investigating exhaust aided diffusers

[autogyro]

+1 Although I have an answer to the clutch problem and keeping rpm up (I will say nothing more on that).
However, I do not believe it is a sensible thing to use the engine at high revs just to supply a high velocity gas flow.
There are far to many down sides to fuel efficiency and car control for this to be viable.
I think the Rb uses the exhaust flow simply to help smooth flow and keep the boundary layer attached.

[DaveW]

In Turkey, RBR appeared to be:

1. quick through the qualifying speed trap (though still 6 kph slower than the McLarens)
2. relatively slow through slow corners (Webber c.f. Hamilton), despite what appeared to be a better mechanical set-up.
3. outstandingly quick through turn 8 (best sector 2 qualifying time by 0.4 secs)

My conclusion is that they ran with a lowish drag configuration, lacked down force in slow speed corners, but still had down force to spare through "flat" corners.

Over to you....

[vonk]

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).

This would be true for an incompressible fluid and no throttle. But, as you know, an engine can be operated at a given speed with full, partial or no throttle opening. Each case will result in a different exhaust mass flow rate at the same RPM, which is simply the air and fuel required for the power produced at each throttle position.

[donskar]

@ riff raff

Thanks for the fine post -- the sort of thing that keeps me coming back to F1technical, despite the screaming, bickering children.

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.

IIRC, the fluctuaton of exhaust effect on downforce, especially the reduction when it would be needed most -- in cornering -- was a reason that exhaust aided diffusers were tried and dismissed in the recent past. I hope other posters will comment on this apparent contradiction (keeping in mind Newey's aero prowess.)

Keeping the throttle wide open through the corner would also reduce engine braking AND use more fuel.

[vonk]

@ riff raff

Thanks for the fine post -- the sort of thing that keeps me coming back to F1technical, despite the screaming, bickering children.

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.

IIRC, the fluctuaton of exhaust effect on downforce, especially the reduction when it would be needed most -- in cornering -- was a reason that exhaust aided diffusers were tried and dismissed in the recent past. I hope other posters will comment on this apparent contradiction (keeping in mind Newey's aero prowess.)

Keeping the throttle wide open through the corner would also reduce engine braking AND use more fuel.

Keeping the throttle wide open in a turn without load on the engine would immediately send it into the rev limiter.

[wesley123]

well it is hard to say, i think it is in what you inject it.

For example the nissan P35 used in generating udnerbody downforce the use of the exhaust alot, it had rounded tunnels with Vgs in it, it probably has to do with injecting this into the Vortex airflow. For example the Allard J2X-C had its exhaust placed as far at the end of the tunnels as possible, to reduce the negative emntioned effect, the Allard didnt had those strakes in the diffuser area, so i thing the exhaust airflow has to do something with vortex generation

[marcush.]

in braking area the car will have a steep rake and accordingly see a big increase in downforce anyways.
with higher speeds the rear will go down reducing rake .Maybe the system is really meant to help only at high speeds?

[f1plank]

Seem to remember Shumacher keeping the engine on the limiter during downshifts in the early 90,s with the manual shift Benneton B192.
Will see if i can trawl up some in car footage from the net.
Can't be bothered going up the loft for the vcr and the vhs tapes of G.P's

[wesley123]

@marcush; could be possible yes, but you will still have an balance shift between these things, wich made me think, could it be that vettel went off the throttle causing his car sway to the right? i think it would certainly be plausible.

But note the RBR has way softer suspension then the Mclaren, so it can go beyond the optimum rake simply reducing downforce. Although there were rumours about the RBR having diagonal damping, thus increasing wheelbase when pushed down, by this the car can run lower

[ringo]

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.

I believe this is more accurately being used on the redbull than the other cars of the past.
Having the exhaust blowing from a distance and through an orifice allows entrainment of the free stream air to take place. So we have high velocity exhaust coupled with free steam velocity. In the event of off throttle, the exhaust flow will drop, but there will still be freestream air introduced into the diffuser through the orifice. Virtually making it impossible for the flow through the orifice to drop bellow the limit.

On the other hand the other examples of ejectors on f1 cars aren't really accurate to the aeronautical application, i think the exhaust is not encircled by the free stream air since the pipes are sealed into the diffuser walls. Air is only coming from underneath the diffuser, not as an annulus around the exhaust flow. During off throttle, the exhaust flow conditions go bellow free stream, i would suspect, and reduce suction under the floor. The exhaust flow is not replaced with free stream air like in the case of the RB6.

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

What do you suppose will happen if an Modern F1 car only goes from say 18000 to 16000 rpm during a gear change, will this be so detrimental? At worst during a braking maneuver going from 18000 to 14000?
This was really one of the main things i was interested in. What if in a modern f1 car, the engines speeds are so high, that even in an off throttle moment, during a downshift or period of braking, the engine speeds do not go bellow the critical level where the exhaust flow would markedly affect the loss in down force.
And keeping with what vonk said, i think the mass flow would decrease during off throttle positions, on the basis the throttle bodies are closed. The velocity flow would correspond to the engine speed though, only that the exhaust will have less density, internal energy and momentum.
I believe the velocity is just as important as the mass flow to aid the diffuser in the case of a car because thrust is not a concern as in an aircraft. With that the blown diffusers on the RB6 may be working very consistently.

I don't necessarily want a direct answer but I am just putting some thoughts out there.

[PlatinumZealot]

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

My theory on the arrangement is that it helps the diffuser only at high speeds. At high speeds the air should emerge from the diffuser at a narrow angle relative to the ground, and having a steep angled diffuser can cause turbulence because the narrow angle air stream separates from the steep angled roof of the diffuser. At low speed the diffuser can be steep because the air is moving slower it can better attach to the roof of the diffuser. In other words you can't maximise the benefit of a large upper diffuser opening at higher speeds.

The exhaust stream might give only a minimal improvement at low speeds in a high angle diffuser since the air is already attached and flowing well. So when the drivers brake down to a slow speed the downforce level doesn't change that much.

I think the hot stream enables a steep angle, or large upper diffuser opening to be maximised at high speeds because of energising and the entrainment or ejector effect you mentioned. THe normally low angle dettached high speed air can now follow the steep angle of the diffuser roof. So at high speeds there is good scaling of downforce, but at low speeds the difference is small because the diffuser was already maximised. I don't know if I explained it clear enough though.

Come to think of it, I think that is why Mclaren and Renault have the largest upper diffusers but yet they cannot maximise them in the ultra high speed corners.

[riff_raff]

"At worst during a braking maneuver going from 18000 to 14000?"

ringo,

The flow rate through the engine while braking is very much reduced, in spite of the relatively small (4000 rpm?) rev reduction. The purpose of downshifting into a corner is to get a braking effect from the engine. In order to maximize this the intake is fully throttled, so intake flows are severely choked off.

Just to make my point clear, it's my understanding that the actual amount of downforce lost during braking with an exhaust blown diffuser is not so much a problem. The real problem is the rapid, unpredictable change in aero balance front-to-rear that occurs. A car that is being braked or turned right at the limits of traction would go into under/oversteer or brake lock with a small change in aero balance quickly, and the driver would not be quick enough to catch it.

Regards,
riff_raff

[DaveW]

Back in the days of venturi tunnels, down force was assumed to be proportional to the product of a (dimensional) "pressure coefficient" (Cp) and free stream dynamic pressure. Cp was increased dramatically by sealing the sides of the tunnels with skirts (I recall building a "pressure coefficient meter" to warn a driver when one of his skirts wasn't sealing properly). Cp was also increased by increasing the expansion ratio of the venturi (ratio of exit to throat areas). This happened as speed increased, increasing down force, compressing the suspension & lowering the average ride height of the aero platform. All was well until the expansion ratio became too large, causing the venturis to stall & Cp to decrease (again dramatically). This caused the aero platform to leap into the air, allowing the duct flow to re-attach, which re-started the cycle. The result was a violent aeroelastic instability, commonly called "porpoising". Without question, the onset of porpoising could have been delayed (& the critical airspeed increased) by "re-energizing" the duct boundary layer.

I would guess that a current flat bottom/diffuser behaves in much the same way. Certainly occasional mild porpoising can still be observed.

RBR's clear performance advantage (assuming it has an aero source) implies that they have found a way of lowering the average ride height, thus increasing the effective expansion ratio of the diffuser, without stalling the diffuser, &/or have found a way of improving duct "sealing", &/or, perhaps, improving extraction from the undertray directly. The unique position & orientation of the exhaust would suggest it is implicated. Enhancing the photograph posted by n anirudh clearly shows the exhaust heading for the trailing edge of the undertray. What is not clear (not for me anyway) is whether some of the exhaust flow is diverted to re-energize the diffuser boundary layer.

It would appear that the "cost" of the solution (if it is the solution) is increased drag (from increased down force at the same L/D or, perhaps, from a reduced L/D) as indicated by the low Barcelona speed trap figures. It would also appear that RBR forfeited some of their advantage at Turkey by reducing "conventional" down force to help top speeds at the expense of low speed (non-augmented) cornering ability. The augmented traction available at corner exit would, presumably, compensate, as would the clear advantage gained through high speed (power on) corners.

What of aero oversteer under braking & corner entry? This is a feature of most aero race vehicles anyway, & several solutions have been devised to mitigate the problem. One, made public at the time of the "spygate" enquiry, is to delay the application of the rear brakes so that the rear tyres initially act as a "keel", presumably at some cost in terms of increased front disc/pad wear.

The above is mostly a re-hash of thoughts & ideas posted by others. My thanks to them & apologies for being too idle to acknowledge them individually.

[marcush.]

Back in the days of venturi tunnels, down force was assumed to be proportional to the product of a (dimensional) "pressure coefficient" (Cp) and free stream dynamic pressure. Cp was increased dramatically by sealing the sides of the tunnels with skirts (I recall building a "pressure coefficient meter" to warn a driver when one of his skirts wasn't sealing properly). Cp was also increased by increasing the expansion ratio of the venturi (ratio of exit to throat areas). This happened as speed increased, increasing down force, compressing the suspension & lowering the average ride height of the aero platform. All was well until the expansion ratio became too large, causing the venturis to stall & Cp to decrease (again dramatically). This caused the aero platform to leap into the air, allowing the duct flow to re-attach, which re-started the cycle. The result was a violent aeroelastic instability, commonly called "porpoising". Without question, the onset of porpoising could have been delayed (& the critical airspeed increased) by "re-energizing" the duct boundary layer.

I would guess that a current flat bottom/diffuser behaves in much the same way. Certainly occasional mild porpoising can still be observed.

RBR's clear performance advantage (assuming it has an aero source) implies that they have found a way of lowering the average ride height, thus increasing the effective expansion ratio of the diffuser, without stalling the diffuser, &/or have found a way of improving duct "sealing". The unique position & orientation of the exhaust would suggest it is implicated. Enhancing the photograph posted by n anirudh clearly shows the exhaust heading for the trailing edge of the undertray. What is not clear (not for me anyway) is whether some of the exhaust flow is diverted to re-energize the diffuser boundary layer.

It would appear that the "cost" of the solution (if it is the solution) is increased drag (from increased down force at the same L/D or, perhaps, from a reduced L/D) as indicated by the low Barcelona speed trap figures. It would also appear that RBR forfeited some of their advantage at Turkey by reducing "conventional" down force to help top speeds at the expense of low speed (non-augmented) cornering ability. The augmented traction available at corner exit would, presumably, compensate, as would the clear advantage gained through high speed (power on) corners.

What of aero oversteer under braking & corner entry? This is a feature of most aero race vehicles anyway, & several solutions have been devised to mitigate the problem. One, made public at the time of the "spygate" enquiry, is to delay the application of the rear brakes so that the rear tyres initially act as a "keel", presumably at some cost in terms of increased front disc/pad wear.

The above is mostly a re-hash of thoughts & ideas posted by others. My thanks to them & apologies for being too idle to acknowledge them individually.

+1

btw
the delayed but not reduced brake pressure at the rear does work very well .did that on my fWD race cars with great sucess .
It was commercially available at that time in the states by Stewart components..

[vonk]

Speaking for myself, I have not seen a clear case made for a significant contribution to diffuser down force by exhaust ducting. Exhaust flow is a highly variable parameter that can introduce much noise into diffuser performance. While engine breathing can be aided by exhausting into a low pressure region, in the case of the diffuser, it would quickly “fill” that low pressure region, thereby defeating the purpose of the diffuser. Therefore, if the region receiving the exhaust flow is separated from the rest of the diffuser volume, benefit for the exhaust can be had at little or no expense of down force, depending on the power produced by the engine. In fact, at full throttle, the high energy (heat and velocity) exhaust flow could even produce a little thrust, while under braking the full diffuser would be effective.






The arrow points to what I think is the exhaust segment. If these separators are just flow straightners, why not space them equally across the diffuser?