The Ram Air effect while related to cars/bikes intakes is a misnomer.
The real ram effect is what happens in a ram jet engine, a particular jet engine that works without compressor and turbines, it has just an air intake, the combustors and the nozzle. All the pressure increment needed for the first part of the thermodynamic cycle is achieved via the air intake that slows down the air from freestream very high velocity to the lot lower speed required in combustors and consequently raises the pressure enough for the cycle to work.
The problem is that this is achievable only at very high flight speed, you need supersonic speed in the order of Mach 1.6-1.8 more or less to obtain the required pressure jump (and in fact you’ll see that the intake of a ram jet is a convergent-divergent design, first the convergent slows down from supersonic to sonic, and then the divergent slows down from sonic to low subsonic). At lower speed the ram jet simply can’t work and that’s why you have the “normal” jet engines with, after the air intake that can increases pressures just a bit, several stages of axial compressors to achieve the pressure jump required and downstream the combustors some stages of turbine to drive the compressors.
A car/bike works at very low speed in term of fluid dynamics so will never go fast enough to exploit that effect that is related with air compressibility.
So what you have is not the real ram air effect, but simply the conversion of the dynamic pressure in static pressure. The best you can do is to convert all the dynamic in static and get a minimal increment. How much ? Well dynamic pressure is 0.5 rho v^2 so do the math and you see that at 360 km/h = 100 m/s the maximum you get is in the order of 5-6k Pa, while the typical atmospheric pressure is in the order of 100k Pa or thereabout, so in the ideal case at 360 km/h we get a 5-6% pressure increment.
Obviously it’s proportional to speed squared hence at half the speed, 180 km/h, you’ll get 1/4 of that, less than 2%.
So if we, for simplicity, assume that the % increment of pressure gives an equal % increment of power (not strictly true, you get a bit less), you can easily calculate the gain at any speed and see that it’s minimal, at least for normal cars and bikes at usual speed. (for a F1 with 700+ hp and running at 300+ km/h even a few % gain is certainly worth the time to optimise the design)
All the above is the maximum ideal increment with an ideal design at any speed.
In real word though comparing engine power at, say, 100 km/h with that at 200 km/h, you could actually find a gain lot larger than the negligible % of the theory.
Obviously that’s not because physics laws are wrong, it’s simply because you are using the same intake of fixed geometry at both speeds, and the result means that at 100 km/h that intake wasn’t very efficient and was actually reducing static pressure compared with freestream, while at 200 km/h it works well and achieves a pressure recovery.
Consequently when you measure big gains at high speed, it’s not because of the so called “ram effect” doing marvels, but because the intake wasn’t particularly efficient at low speed.
That happens, to make it simple, because the volume of air actually entering in the intake doesn’t depend by intake size but by engine request.
Assume for example you have a F1 engine, 2.4 litres revving at 19000k rpm constant, and assume volumetric efficiency = 1 for simplicity (it’s lot higher than that in reality, up to 1.2). This means that the engine asks for 0.0024 m3 * 19000 rpm/120 = 0.38 m3/s of air. Now assume you have an air intake of diameter 10 cm => area = 0.0078 m2. At car’s speed 100 km/h = 28 m/s thru that intake could pass theoretically 0.22 m3/s, that is lot less than the engine needs. Consequently to respect engine request the air upstream the intake will accelerate so in the intake it will still enter the air needed by the engine, but at higher velocity than freestream hence with a loss of static pressure.
At 200 km/h on the contrary the intake could take 0.44 m3/s, which is more than the engine needs, consequently the air upstream the intake will slow down so that only the air needed enters in the intake and it does at increased static pressure and obtaining a slight power increment.
Now if you look at any F1 car, do the math with air needed by the engine at any speed and compare with air intake dimensions you’ll see that the intake is bigger than theoretically needed also at low speeds, exactly to force the flow to slow down before entering the intake and achieve a pressure increment.
Obviously that’s a simplification of a complex phenomenon, you have to keep in mind that this acceleration/deceleration of the flow upstream is also affected by intake design (particularly the radius of the borders) and of elements, like the drivers helmet, upstream the intake, everything is important in achieving the optimal pressure recovery.
G-rock wrote:
F1 cars use the ram air system and it does work. I think it was 93 or 94 when, to slow the cars down, the FIA imposed holes of a certain diameter in all of the cars intakes (behind or beside the roll hoop) This according to the FIA was supposed to reduce up to 30 hp and slowed the cars down by 10 km/hr down the straights.
It was a measure adopted in 94 and 95, one of the several reactions after Senna’s death.
But it had more to do with the internal airbox functioning and its “games of waves” than with the air intake per se.
