Flat floor, diffuser and hot air

[Tomek]

HI,

I am a huge fan of F1 and I have watched almost every race since 1989. I think there is less than 10 that I have missed since then 

I am reading this forum for two years at lease and it was very helpful. I am currently building truck day car, mid-front engine with rear wheel drive, around 950kg with driver and 250hp engine and I need some help in terms of flat floor, exhausts and cooling. Car is equipped with flat floor and rear diffuser.

Can you explain me if there is any advantage/disadvantage of directing hot air from radiator under the floor. Radiator is about 50 cm after the front bumper and I can choose to direct hot air through the bonnet or under the floor.
The same question is for exhaust gasses. Should I put them under the floor or at the rear of the car above the diffuser?

Thanks for your help and understanding (I am not an engineer)

Regards

[bazanaius]

I believe there are a few threads discussing exhausts exiting into the diffuser area. I know Macca has done it a few times on their cars.

In general, I believe that the argument against this is that the exhaust gases are not a constant stream, but rather a series of pulses as the exhaust valves open/close. This could lead to an inconsistent level of downforce generated by the diffuser/floor and produce a rather unpredictable driver.

With the radiator, I'm not sure how fast the air would be going out of this area, and whether all you'd effectively be doing is unsealing your floor.

[newbie]

I believe there are a few threads discussing exhausts exiting into the diffuser area. I know Macca has done it a few times on their cars.

In general, I believe that the argument against this is that the exhaust gases are not a constant stream, but rather a series of pulses as the exhaust valves open/close. This could lead to an inconsistent level of downforce generated by the diffuser/floor and produce a rather unpredictable driver.

With the radiator, I'm not sure how fast the air would be going out of this area, and whether all you'd effectively be doing is unsealing your floor.

exactly what bazanaius said. i believe that the effect of the underbody exhaust was that, in order to get the increased downforce around the curve, you had to boot the throttle while entering the corner. Very unpredictable and also counter-intuitive from the drivers point of view. Do you have a front splitter? or a front 'duct wing' through the engine cover?

[Ciro Pabón]

Wouldn't less dense air (that is, hot air) produce less downforce? Besides, the speed of the exhaust is not as high as the speed of the car.

[Belatti]

... Besides, the speed of the exhaust is not as high as the speed of the car.

How can you be so sure?

[ringo]

I think ducting the radiator exhaust under the car is a bad idea. There is going to be a pressure increase as soon as the air under the floor sees that break to the exhaust opening. Now if you had a fan at the exhaust driving the air out underneath it could make a difference, but the fan has a velocity pressure, so it could also lead to unwanted results especially if the airflow underneath is moving at a lower velocity than the fan.
A good idea is to have your radiator at the back of the car and exhaust the air right above the diffuser.

The engine exhaust is better left to exit above the diffuser not in it's throat. It will accelerate the flow but Imagine letting off the pedal and having a massive drop in downforce!

[Ciro Pabón]

... Besides, the speed of the exhaust is not as high as the speed of the car.

How can you be so sure?

Good question. The simple answer: I'm not.

However, why should we have a simple thread, a thread that goes directly to the point, based on experience, when we can complicate our life and be nagged by some anonymous genius?

So, this is, in a crude fashion, the numbers I got when I thought about that for a minute (without a calculator, so...) :

- "Static" volume of exhaust (yes, I know there is a thread on that specific subject, but I said crude) :

2.4 liters x 18.000 rpm / 4 = 10.800 lt/min

I'll assume an square exhaust with a 10 cm side (Toro Rosso probably has one of these) which gives me an area of 1 square dm.

I'll simplify that even more, assuming the gas is at 1 atm and 20 degrees. If you want to take in account stupid things that might (or might not) influence the volume, like temperature and pressure, be my guest or check the thread on gas volume, where I'm sure Belatti posted like 132 times, when he was young and hadn't been involved with half the pitbabes in TC.

So, it's like 10.000 dm3/1 dm2 = 10.000 dm/min = 1.000 m/min = 60 kph

Some could argue that's more or less the speed of a Toro Rosso, btw.

QED

Well, I know, it's "demonstrandum" only if Belatti invents an engine with 1 atm and 20 degrees in the exhaust, which I think implies a thermal efficiency of 99% or so, but I said "crude" and I have full faith in Belatti's ability to invent anything.

I would love for somebody to post the true speed taking in account that the gas is at 3 atm and 300 degrees or whatever, but, now that you're into it, please, take in account the Doppler effect on the exhaust speed. Ahhhh.. so, you haven't thought about that? So, in the end I get a negative speed of 240 kph... right?

Besides, the gas of a typical exhaust can move my hair, but it won't cause ripples in my face if I place myself in front of it, while my face feels different when I'm in free fall at 200 kph or so (jumping from an airplane... oh, the memories).

[Belatti]

wich one?

this one is interesting...
http://www.f1technical.net/forum/viewtopic.php?f=6&t=6435

and the result is not 60kph...

But I would say a little less that what our friend Kilcoo says.

Check out to some of the Scarbs comments:

http://www.scarbsf1.com/diffuser.html

Exhausts
Teams use the exhaust pipes exiting into the diffuser to “energise” the flow, there speed of the airflow through the diffuser can be sped up by the high velocity gasses exiting the exhaust pipes when the engine is revving. This apparently free increase in diffuser performance was very popular in the 90s and was encouraged by bodywork restrictions preventing the exhaust pipes routing over the gearbox. As the performance of the diffuser grew and it became a larger component of the cars rear downforce, it became a problem that the effect of the exhausts blowing was reduced when the driver lifted off the throttle and the speed of the flow reduced, teams found this made the cars handling very sensitive to engine speed and slowly the practice was dropped first by routing the exhausts into less effectual positions in the diffuser and then later through the top of the sidepods using the Ferrari inspired “periscope” exhausts.
Three teams still blow the exhausts into the diffuser, Williams and Minardi blow the exhausts over the inner side channel, in fact Minardi have shaped the ends of the exhaust pipes to form the top of the diffuser channel, Williams flattened the bottom of the pipes to merge the flow of the exhaust and out the top of the diffuser channel. McLaren blow their exhausts above the shadow plate in the centre of the diffuser, there may be a few reasons for this practice, one of the main reasons is to energise the flow coming from the transition area of step plane, McLaren use the exhausts blowing against a knife edge formed between the pipe and the vertical step, this speeds up the flow, improving the scavenging effect ton this area, it also adds t the downforce generated by the central diffuser tunnel.

[hecti]

What kind of car its is exactly?
i have never heard of radiator exhaust exiting under a car. i dont think that it would be wise to have such an influx or air into a flat bottom car. also on the exhaust, i've heard and belive that exhaust exits like those of a dodge viper or mclaren slr help air flow around a car, ie: the dodge viper is the fastest street car to lap the nurburgring so i guess its a good indication and the slr is not that far off i believe.
you sould also think about a front spliter too, it should help reduce under steer through corners and decrease excess tire ware

[F1_eng]

Averaging volumetric inlet air flow and assuming 100% vol eff @ 18,000 rpm, the average speed in a 60mm dia exhaust would be around 140mph

[speedsense]

A airplane must use a higher top speed to become airborne on a hot day, as the heat causes less lift. However if you can increase the heat on the high velocity side of a wing and have a lesser temp on the low pressure side, you can actually increase lift.
When it comes to a diffuser, you would want the higher temp to be above the diffuser and not in it. IMHO

[speedsense]

Averaging volumetric inlet air flow and assuming 100% vol eff @ 18,000 rpm, the average speed in a 60mm dia exhaust would be around 140mph

Try this study on a rotary engine... the air flow through the intake and the exhaust is supersonic....

[BreezyRacer]

HI,

I am a huge fan of F1 and I have watched almost every race since 1989. I think there is less than 10 that I have missed since then 

I am reading this forum for two years at lease and it was very helpful. I am currently building truck day car, mid-front engine with rear wheel drive, around 950kg with driver and 250hp engine and I need some help in terms of flat floor, exhausts and cooling. Car is equipped with flat floor and rear diffuser.

Can you explain me if there is any advantage/disadvantage of directing hot air from radiator under the floor. Radiator is about 50 cm after the front bumper and I can choose to direct hot air through the bonnet or under the floor.
The same question is for exhaust gasses. Should I put them under the floor or at the rear of the car above the diffuser?

Thanks for your help and understanding (I am not an engineer)

Regards

I feel this is a very bad idea UNLESS .. there was a Japanese Touring car team a few years ago that did this but with the intent of driving air into or maybe out of the diffuser with a cooling fan. Thus they needed the radiator to be incorporated into the undertray/diffuser so they could use the "cooling fan" to produce additional DF with the diffuser. Without a fan like this setup was, it's truly a bad idea. And to pull something like this off you need tons of CFD time, radiator and duct tuning, etc. I wish I had pics of the Japanese solution (which was subsequently banned) but I never found any that surfaced. Sure would have been interesting. I was apparently a "dominant" idea.

[F1_eng]

Averaging volumetric inlet air flow and assuming 100% vol eff @ 18,000 rpm, the average speed in a 60mm dia exhaust would be around 140mph

Try this study on a rotary engine... the air flow through the intake and the exhaust is supersonic....

Can you elaborate on your point please?
As far as my knowledge goes, this type of flow condition wouldn't yield a possible supersonic flow situation but you may have a special case in mind?

[rjsa]

A airplane must use a higher top speed to become airborne on a hot day, as the heat causes less lift. However if you can increase the heat on the high velocity side of a wing and have a lesser temp on the low pressure side, you can actually increase lift.
When it comes to a diffuser, you would want the higher temp to be above the diffuser and not in it. IMHO

This is kind of messed up. On a wing, the upper side is the high speed, low pressure one. So by this theory hot air on top of the wing would help.

The difuser is not exactly a wing. It's more of a vacuum pump. It pulls air from the underbody, reducing the pressure there. When air hits the difuser the growing cross section will reduce the speed and increase the pressure. If the hot air on top of the wing helps*, then hot air from the radiator towards the underbody would do also.


* (which I don't know, because hot air will be less dense, but I can't figure out the effect on pressure)