Ram Airboxes in F1 ?

[Tommy Cookers]

A vehicle with suitable air intake can potentially access induction air at about eg 5% above ambient pressure at 200mph, MotoGP machines use such ram airboxes.

Such airboxes (sealed?) were conspicuous in F1 in the 70s.
Later they were required to be lower and not sealed ? (ie to the induction system on the delivery side).

What pressure is available to the induction system of current F1 cars driven in undisturbed air, and why/how ?

[xpensive]

The "ram-effect" of an airbox can hardly be any more than the dynamic pressure; Rho * speed squared /2, which at 200 mph (89.33 m/s) would be 4.79 kPa or about 4.8% of the atmosphere at an air-density of 1.2 kg/m^3.

However, when the above relative speed will be car-speed minus air-inlet speed, the effective ram-effect will be considerably less and the gain in horsepower also partly balanced by increased air-resistance, there is obviously an optimum airbox cross-section area for a certain car-speed and engine air-intake volume per time-unit.

[xpensive]

Guess what I found, a little something yours truly did on the subject way back when;

viewtopic.php?f=6&t=6190&p=88302&hilit=Airbox#p88302

A simplified calculation of course, but I reckon I wasn't that way off?

[riff_raff]

What pressure is available to the induction system of current F1 cars driven in undisturbed air, and why/how ?

Tommy Cookers-

The absolute air pressure at the airbox inlet duct is whatever the ambient atmospheric air pressure is. The dynamic air pressure at the inlet end of the engine intake manifold runner (trumpet) would vary depending upon the speed of the car. A well designed airbox minimizes drag and also produces a uniform dynamic pressure distribution at each inlet runner. If you want to get technical, we could also consider the local air pressure fluctuations produced right at the trumpet mouth due to acoustic wave oscillations within the inlet runners.

Regardless, the small increase in air pressure within the airbox is a cheap way to increase engine performance with little drag penalty.

[matt21]

IMO the most important factor of using an airbox is to get cool air into the engine withot too mouching bends in the ducting.
If you think about an air inlet like that on the Benetton B188 you probably face the the problem of getting the air heated up by the radiators and you´re maybe loosing pressure due to the routing.

[strad]

The airboxs function is to provide stable, calmer, undisturbed air for the engine to draw in. A reservoir of sorts..IF it can gain a bit of positive pressure, so much the better.

[Tommy Cookers]

Thanks, guys !

xpensive's link was very useful (leading to other earlier threads)

(I had tried searching before, honest !)

[xpensive]

My point TC, is that while the more cross-section area of the airbox inlet, the higher the speed-differential and the higher pressure, but at the same time, that very inlet pressure will act as a very effective air-brake on the same inlet.

The idea is to find a balance there, if you look at some of those airboxes of the 70s, they were huge and probably did more harm than good, with the aerodynamiscists of today the designers know this why airboxes all have the same size.

http://www.best-used-car.org/lola/lola-t370/

[strad]

We found that the airbox should be able to contain twice the volume of the engines capacity.

[olefud]

My point TC, is that while the more cross-section area of the airbox inlet, the higher the speed-differential and the higher pressure, but at the same time, that very inlet pressure will act as a very effective air-brake on the same inlet.

The idea is to find a balance there, if you look at some of those airboxes of the 70s, they were huge and probably did more harm than good, with the aerodynamiscists of today the designers know this why airboxes all have the same size.

http://www.best-used-car.org/lola/lola-t370/

I may not begetting the cross-sedction area point, but as I understand it, once the inlet flow volume reaches the outlet flow, pressure within the plenum will not increase with increased inlet area. The inlet will have a velocity dependant kinetic pressure and that's the upper pressure limit throughout the fluid.

[xpensive]

The "ram-effect" is in reality just the stagnation-pressure at the airbox inlet, which is created by the difference between
car-speed and airbox inlet-speed, as Rho * (Speed-diff^2)/2, with speeds being the same, nothing happens.

I humbly believe there's a useful calculation xample in that link above?

[PhillipM]

We found that the airbox should be able to contain twice the volume of the engines capacity.

Really? Tend to find with much higher RPM engines the airboxes are far larger than that?

[strad]

Good point,,,that was only up to 10,000RPM

[riff_raff]

I may not begetting the cross-sedction area point, but as I understand it, once the inlet flow volume reaches the outlet flow, pressure within the plenum will not increase with increased inlet area. The inlet will have a velocity dependant kinetic pressure and that's the upper pressure limit throughout the fluid.

The duct cross section is important because the airbox functions similar to a diffuser. It slows flow velocity and increases pressure. The volume of the airbox is important because each of the intake runners produces large pressure pulses, and a large airbox volume mitigates their effect.