15 replies to this topic

[Aubwi]

A few questions about airboxes today. In F1, how much additional power does the ram effect produce? I read an article on grandprix.com that said the vents that were mandated in late 1994 and early 1995 reduced power about 14%. Safe to say then, that the total ram effect is significantly more than this? Seems surprisingly large! But then I've heard claims of 30% plus for road cars. I take it with a grain of salt of course, but still...

Also I saw an old thread on the nostalgia forum where everyone seemed to think the purpose of the giant 70's airboxes was just to "slow the air down", and they vociferously denied the importance of any ram-effect. I think they must be confused. Otherwise, why point the airbox inlet directly into the airstream? I can see that the airbox would slow down the air, with the inlet being smaller than the area of the engine trumpets, but the primary purpose was to pressurize the air, correct?

Third question: Today's engines are said to produce about 800 horsepower. Is this with or without ram-effect?

Fourth question: Does a turbo or supercharged engine gain even more advantage from a ram effect?

[murpia]

Airboxes do slow the air down. That converts the kinetic energy of the air into static pressure. A good airbox is shaped like a diffuser 'bent' round a corner. A bad one isn't... If you assume incompressibility so Bernoulli's equation is valid, you can easily calculate the effect.

Regards, Ian

[NRoshier]

hmmm...I've spoken much to an engineer familiar to Greg who works in engine tuning. He has experimented with different systems on cars and feels a 1-1.5kpa increase is about as good as you will normally see. Having said that it was not at the speeds that an F1 car would see.
I would be interested to learn more.

[Aubwi]

Ian,

Refreshing myself on my old college physics text, I can now understand that Bernoulli's equation tells us that the pressure increases because the air slows down. So the nostalgiaphiles are half wrong as I suspected.

But I dont' see how you can calculate the pressure increase without knowing the air velocity decrease if that's what you're getting at.

[exFSAE]

Don't believe turbo cars gain much or anything from a ram duct. Take a look at any turbo open wheel car, you'll find the ram deal non-existant.

[Greg Locock]

Air temperature at the throttle body is vastly more important than ram effect, for road cars.

The speed of the air in the intake is easy enough to work out, but you'll get almost the same delta p whetehr you include that or not (at a guess - check that)

delta p=1/2 rho v^2, v~100 rho=1.2, so you'll get 6 kpa, or about 1 psi at 360 km/h. So you'd get a 6% increase in VE - worth having, for sure, but only equivalent to a 20 deg C temperature rise in the intake, and you'd easily get that in a road car - not that that would be doing 360 very often.

[Aubwi]

Greg, I see, you're assuming v sub2 = 0. I guess that would give a theoretical maximum for delta P.

Another thought, when the car is stationary, does the airbox inlet actually act like a restrictor plate, and reduce power? It's easy to imagine shrinking the inlet and getting less and less power, just like a restrictor plate. But I'm not sure how Bernoulli's law applies at zero velocity. The engine is just sucking the air through the inlet and... it's making my head hurt. I think I should go eat something.

[exFSAE]

Restrictor plates and venturis restrict power because you can only push air, at best, through an orifice at Mach 1. I figure a 20mm venturi is at best, good for about 85hp (rough number).

Move that up to say a 5" diameter inlet (not sure actual size on a grand prix car these days) and your restriction is now 3500hp No where NEAR what an F1 powerplant would make. So no, the inlet isn't really a restriction in any way.

With regard to inlet flow, you don't want to funnel it down too quickly. Same goes for radiator inlets. Funneling down, velocity goes up, pressure goes down, and you get all sorts of nasty flow reversion where the air wants to go back OUT your inlet rather than where you want it.

"Ram" air might be kind of a misleading phrase since it gives the impression you're trying to take that high speed air and dump it down into the engine, which isn't entirely the case. As Ian points out, the whole thing is a diffuser and the box is designed to spread available airflow equally to the intake trumpets, in the process bumping the pressure up so you have a nice P differential across your butterflies and it'll go through instead of trying to go back out.

How much power increase? You can figure out how much the velocity is going to change if you assume the airbox is fairly well designed and the flow isn't going to separate, and is incompressible.

Let's say you're truckin along at 200mph (89.4 m/s). That is v_1. v_2 is NOT 0 m/s since the air isn't going to stop and fully stagnate against the intake trumpets. Let's assume the airbox cross section increases by a factor of 3 after the inlet. v_2 = 29.8 m/s, delta_v = 59.6 m/s. Call rho 1.2 kg/m^3.. increase in pressure is about 2kPa (0.3 psi). I'll take a 2% VE increase. Not anywhere near claims of 30%, but you take every percent you can. Wins are determined by hundreths of a percent different in lap time.

Or if you have the airbox designed to increase cross section by a factor of 5 (increasing this factor makes it more difficult to prevent flow separation).. v1 = 89.4 m/s, v2 = 17.9 m/s, deltav = 71.5 m/s. 0.5 * rho * v^2 = about 3 kPa (almost 1/2 psi).

Make sense?

[RDV]

Below measured pressures vs. calc airbox ram.
Coludn't find my F1 ones, but basically behaved the same way...the ram pressure will be contingent to airspeed, and from memory the best I've seen was at Monza @ 33Mbar...quite helpful one those hot afternoons, you would take all you could get.
Increasing opening size over the minimum required (then it would be a restrictor....;) ) would be not very useful, as making the scoop about the size of a football field would give only @ 2 Mbar more...



ENG CC= 2000 VOL EFF 0.88 INLTCM2=153 CWP= 0.263157895 DIA(in)= 25.1 xxxxxx 93dtheor=148
RATIO1st= 14/40 17/39 20/35 22/33 23/33 23/26 ambprs 956 637.54 DIA MM
ENG AIR AIRSCOOP PRESS RAMspeed ambtemp 12 1 FUJI Mbar overpressure
RPM= m3/sec RAM M/S KM/H 2ND 3RD 4TH 5TH 6TH 2ND 3RD 4TH 5TH 6TH
4500 .0660 4.31 15.53 62.0 81.3 94.9 99.2 125.9 1.50 2.67 3.70 4.06 6.66
4750 .0697 4.55 16.39 65.5 85.8 100.1 104.7 132.9 1.67 2.98 4.12 4.52 7.42
5000 .0733 4.79 17.25 68.9 90.3 105.4 110.2 139.8 1.85 3.30 4.56 5.01 8.23
5250 .0770 5.03 18.12 72.4 94.9 110.7 115.7 146.8 2.04 3.64 5.03 5.52 9.07
5500 .0807 5.27 18.98 75.8 99.4 115.9 121.2 153.8 2.24 3.99 5.52 6.06 9.95
5750 .0843 5.51 19.84 79.2 103.9 121.2 126.7 160.8 2.45 4.36 6.04 6.62 10.88
6000 .0880 5.75 20.71 82.7 108.4 126.5 132.2 167.8 2.67 4.75 6.57 7.21 11.84
6250 .0917 5.99 21.57 86.1 112.9 131.7 137.7 174.8 2.90 5.16 7.13 7.83 12.85
6500 .0953 6.23 22.43 89.6 117.4 137.0 143.2 181.8 3.13 5.58 7.71 8.46 13.90
6750 .0990 6.47 23.29 93.0 122.0 142.3 148.7 188.8 3.38 6.01 8.32 9.13 14.99
7000 .1027 6.71 24.16 96.5 126.5 147.6 154.3 195.8 3.63 6.47 8.94 9.82 16.12
7250 .1063 6.95 25.02 99.9 131.0 152.8 159.8 202.8 3.90 6.94 9.60 10.53 17.29
7500 .1100 7.19 25.88 103.4 135.5 158.1 165.3 209.8 4.17 7.42 10.27 11.27 18.51
7750 .1137 7.43 26.75 106.8 140.0 163.4 170.8 216.8 4.45 7.93 10.96 12.03 19.76
8000 .1173 7.67 27.61 110.3 144.5 168.6 176.3 223.8 4.75 8.45 11.68 12.82 21.06
8250 .1210 7.91 28.47 113.7 149.1 173.9 181.8 230.7 5.05 8.98 12.42 13.64 22.39
8500 .1247 8.15 29.33 117.1 153.6 179.2 187.3 237.7 5.36 9.54 13.19 14.48 23.77
8300 .1217 7.96 28.64 114.4 150.0 175.0 182.9 232.1 5.11 9.09 12.58 13.80 22.67
MAX JX MMHG= 16
EQUIV Mbar= 21.33048927
MEASURED TOP= 21.33 mBAR
DYN RECOVERY 94.11 %

[murpia]

http://en.wikipedia....nation_pressure

That would dictate an upper limit on airbox ram effect. So for 300kph = 83.3m/s and a density of 1.2kg/m^3 you get 4163Pa or 41.63mBar.

Of course a real airbox has an outflow though the engine so RDVs 33mBar looks good to me...

Regards, Ian

[NTSOS]

I think a ram air inlet/box arrangement would work very well, especially at speed, if it was
not for the unfortunate fact that a very large and hungry air pump is trying to get at each and
every unsuspecting air molecule that inadvertently bumbles it's way into the beasts limited in
volume inlet trap/torture chamber.

As they desperately attempt to avoid capture, their efforts are futile as the angry/hungry air
pump is relentless as it methodically tries to stalk, scour and consume every available air molecule
and their valiant replacements.........the attack so savage and vicious that the very vessel
that imprisons them is on the verge of implosion....saved only by its strength, inlet size and
volume. It is a scene of unimaginable horror, as the mass slaughter of innocent.........oh,
sorry.....lost my head!

I think the primary function of a "ram air" device is that of a cooler air supply and
distribution center.

John

[cheapracer]

Originally posted by RDV

would be not very useful, as making the scoop about the size of a football field would give only @ 2 Mbar more...

%

So it's not true that Coultards greatest advantage is his chin scooping all that air up into the airbox?

[RDV]

....on the other hand windshileds and driver head positioning can detract from a good working airscoop/ram intake....

[McGuire]

Looking over the figures, it is clear that the cars are not going fast enough. At > 400 mph the ram air effect is quite significant. Please install higher gear ratios and instruct all drivers to go faster, effective immediately. - the management

[DOHC]

Originally posted by RDV
....on the other hand windshields and driver head positioning can detract from a good working airscoop/ram intake....

Cant' see that it would make much more than a 1.36 mbar difference @ equivalent airspeed of 147.66 knots and 288.15 Kelvin though... Maybe loss of one third horsepower for Ferrari, two thirds bullpower for Red Bull.

[zac510]

Originally posted by exFSAE
Don't believe turbo cars gain much or anything from a ram duct. Take a look at any turbo open wheel car, you'll find the ram deal non-existant.

I'm thinking of a few turbo racing cars from the last 20 yrs at the moment, open wheelers and prototypes, and i can only think of snorkels facing the front of the car leading to turbo inlets, especially where restrictors are present. Are you sure you're not thinking of open wheelers where the engine is a control engine?