velocity of exaust compared to airflow

[gambler]

Has anyone done the math on the velocity of the exaust exiting
the header at 16000 to 18000 rpm. Then compared it to airflow
at 100mph 150mph 200mph ect.
Wondering if the airflow helps pull exaust out , and when.

[riff_raff]

I haven't done the math, but in general having an exhaust velocity at or above the local airflow velocity would only be beneficial if you were trying to generate thrust from the exhaust flow. One of the main objectives for the exhaust outlet location (besides regulations) is to locate it in an area of relatively low dynamic air pressure to assist exhaust scavenging. I also believe the aero guys like it to be in an area where the varying flow does not adversely affect downforce, but still also helps energize the trailing wake flows.

[marcush.]

I haven't done the math, but in general having an exhaust velocity at or above the local airflow velocity would only be beneficial if you were trying to generate thrust from the exhaust flow. One of the main objectives for the exhaust outlet location (besides regulations) is to locate it in an area of relatively low dynamic air pressure to assist exhaust scavenging. I also believe the aero guys like it to be in an area where the varying flow does not adversely affect downforce, but still also helps energize the trailing wake flows.

I remember one Indy car exploiting exhaust gas flow on the upperside of the undertray ,wich was the famous Galmer wich was also winning in Indy with AlUnser jr..
Alan Mertens stated something like :exhaust flow can be used in a ways of the effect of a gurney ,to energise the flow coming out of the diffuser ,or in a way that the diffusser is acting as if it were longer...due to the hot quick exhaust stream actually extending the upper deck of the difusser...on the galmer the exhaust was actually a slit at the rearmost edge of the diffusser almost over the whole width of the car..

http://images.google.de/imgres?imgurl=h ... sAbW4NWSDg

[gambler]

The gills on the F10 are there for a reason, I cant help but wonder
Are they there to soften the scavenging, or are they increasing flow
by pulling side pod air out to increase it.

[HungryHebbo]

I remembered reading a while back about exhausts and how it's beneficial to have a resonance situation. Did a quick google search, and came up with this. It's a bit outdated (2000), but the same principles apply.

The gist I got from it was that the exhaust velocity will be faster than the airflow around the car, hence the blowing of diffusers and such which Red Bull are kind of doing this year. This set-up though is less than optimum with exhaust length for power, so it's a trade off.

NB: On re-read of this, it doesn't make appear to make much sense! Ah well, PhD lit reviews on a saturday tend to do that

[gambler]

I had always thought the exaust would surpass the airflow, but
havent seen any proof.
The varibles are so complex,with the size of the exaust and camshaft profiles,
along with the expansion.
I would figure the most true way would be to inject a smoke additive to fuel
and run the engine in the wind tunnel to see where its going.

[volarchico]

I would figure the most true way would be to inject a smoke additive to fuel
and run the engine in the wind tunnel to see where its going.

But they don't run full sized vehicles in the tunnel. Or at least not ones with functioning engines. Fully functional wind tunnel models would be crazy.

[Belatti]

Check out this thread:

viewtopic.php?f=6&t=7501&hilit=exhaust

Ciro attemps:

- "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.

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

And this thread:

viewtopic.php?f=6&t=6435

Kilkoo attemps:

The volume flow in will be approx dependant on engine displacement and RPM.

0.0024*18000/60 ~ 0.72 m^3 per second

The massflow can then be obtained from this by multiplying by atmospheric air density.

0.72*1.225 ~ 0.882 kg/s

The volume flow out will depend on local density, which is dependant on the air temperature at the exhaust exit point.

(anyone any idea of the temp?)

Vol(out) = 0.882/density

where:

density ~ 101325/(287*temp)

(I'm assuming the exhaust exit is subsonic)

The exhaust exit velocity can then be quickly be calculated from the exhaust exit area.

Vol/Area = Velocity

and then:

*Assuming* a subsonic exit - that gives an air density of

P/RT = rho

101325/(287*1273)= 0.277 kg/m^3

So using the massflow rate from earlier, 0.220 kg/s - which gives a volume flow rate of ~ 0.97 m^3/second...

That is ~ 0.48m^3/second per exhaust

Now... how big is an exhaust exit area? Say its 5cm diameter circle, thats then 0.0020m^2

0.48/0.0020 = 244 m/s

Or 880 kph.

What do you think, 60 or 880 kph

[riff_raff]

Not an easy thing to calculate. Too many variables.

Couple of things to consider though:

-Volumetric discharge flow of a 2.4L 4-stroke F1 engine is only 1.2L/rev.

-Air mass flow through the engine is much greater than simply displaced volume times ambient air density, due to intake manifold acoustics, fuel latent heat, and intake charge inertia effects.

-The mass of the fuel combusted should be included in the mass flow rate of the exhaust gas products.

-The flow velocity at the exhaust valve is likely at or near sonic for a brief period when the exhaust valve first opens. Flow conditions in the rest of the exhaust system are definitely sub-sonic.

-The local gas velocity, pressure, density, temperature, reynolds number, etc. at any given point along the exhaust manifold, at any given instant, can vary widely. Definitely not steady state flow conditions.

Nice discussion though.
riff_raff

[hollus]

A factor of 4 of the difference between 60 and 880Km/h comes just from the exhaust diameter, a factor of five really after Kilkoo rounded down more than necessary. Then, Ciro has exhaust at 20C and Kilkoo at 1000C. Another factor of 4. I am willing to bet that the real exhaust gas temparature is somewhere in between.
But this raises an interesting point, that by changing the exhaust final diameter one can control the gas speed to make it suit what is meeded by the aerodynamics. Of course within reason, one does not want to sacrifice too much horsepower or engine life for that.

[riff_raff]

After the brief initial blow down phase immediately after the exhaust valve opens, the exhaust flow velocity past the exhaust valve is essentially proportional to the piston velocity x D^2/d^2 (where "D" is the bore diameter and "d" is the equivalent effective exhaust port orifice diameter). Velocity will be highest past the valve, and will decrease as the flow moves into the larger volumes of the exhaust primary tubes and collector.

You can assume that the flow past the valves will be at an average velocity below something like 100 ft/sec (ie. 6000 ft/min mean piston speed). And the average flow velocity in the primaries and collector will be even less. 100 ft/sec works out to 110 kph. Reducing the collector pipe size to force the exhaust flow exit velocity up by 100% or more would produce unacceptable back pressure in the system. So I would think that this idea is probably not going to pan out.

Regardless, it still presented a good topic for discussion.

riff_raff

[F1_eng]

What a meal you guys are making of this. This is as simple as things get.

Whatever goes in, has to come out, unless there is somewhere in the cylinder than stores the air....quite unlikely.

Just under 60m/s as a general case.

[mep]

What a meal you guys are making of this. This is as simple as things get.

You make such a derogative comment on the topic and still get it wrong.

Whatever goes in, has to come out, unless there is somewhere in the cylinder than stores the air....quite unlikely.

It's not a air-pump it's a motor. [-X
You add some fuel to the air and burn it. This changes some things mainly rising gas temperature. Otherwise the engine would not run at all.
You are also changing gas composition this might be insignificant but at least we should check for it.

Nevertheless you have higher air temperature and higher pressure when your exhaust valve opens. In the end a bigger volumetric flow passes through your exhaust pipe than through your intake.

The only problem we have is we don't really know the temperature but hey we could do a thermodynamic calculation to get it.
http://de.wikipedia.org/w/index.php?tit ... 0302122730

We need temperature on point 4. Then we don't continue to go back to 1 because that would mean we are doing a closed process. We just continue with a constant temperature until we have 1bar pressure and calculate the Volume.
Anyone interested in?
From earlier calculations I think the temperature could be around 1000°K.

In the end it might be correct that we end up with a already posted speed but then we know it for sure.

The attempt from Ciro is to simple because it doesn't take into account the Volume rise. Furthermore I think the exhaust area is to big.
I would take 2 50mm pipes, makes about 39cm^2 compared to 100cm^2.

[F1_eng]

Very sorry, I didn't try to mislead anyone, I tried to remember the number as I was quite busy and didn't have time to do the calc.

Have done it now and it 144m/s

It's not possible for you to do the cyclic calculation to work out the temperture at exhaust valve open because you don't know the point of exhaust valve open, the burn rate, 50%MFB angle or the polytropic index of compression or expansion for the engine. Sure you could take a rough guess at 4 or 5 variable but it's probably better to guesstimate a temperature from a typical gasoline engine, there's nothing particularly special happening in the combustion process of an F1 engine.