MarkCup said:
How does a diffuser work then?
I thought it was because it smooths and speeds up the airflow exit under the car at the back, which in turn reduces lift?
So it reduces "drag" and "lift"?
Yes, right, if you want to be technically correct, what a diffuser produces is "negative" lift. Ie, downforce. But there isn't any point in "positive" lift in a car, for the same reason there isn't any point in producing "negative" lift in a plane, so everyone just calls it "lift" and "l/d ratios" (lift to drag ratios) and leaves the sign of the number to everyone's common sense when they're talking about aerodynamic devices. Except when they are doing the arithmetic. The sign certainly matters then.
The roof of a Clio would naturally produce positive lift. The rear of the car would press down less hard on the rear wheels the higher the speed. The negative lift that a diffuser would produce would reduce that rear end and reduce "nervouseness" and increase grip on the road. Only Renault knows whether the diffuser reduces the lift from a nett positive lift figure to a nett negative figure.
Contrary to what another contributor said a diffuser does NOT work courtesy of the bernoulli principle of higher air speed generating lower pressure. That's the way a wing on a plane works. The lower surface is flat, and the upper surface is curved, so the air has to accelerate over the upper surface, producing a lower pressure on the top surface of the wing that the lower, generating a nett lift.
Wings on cars usually take little advantage of the bernoulli principle. They simply force the air striking them to change direction upwards, which produces an equal downward force on the wing. That way of doing it has a much poorer lift to drag ratio - ie the amount of drag produced to get a given amount of lift - but it allows much more lift (yes, negative lift) to be produced with a smaller size and mass of wing. You simply can't fit as big a wing on a car as a plane.
Most of the (negative) lift a car wing produces is on the inner surface. The "front" surface. The one the air strikes straight on. But a small amount also gets generated as air flows under the wing, and as the wing curves upwards the air tries to follow it upwards too, and that pulls the wing down.
A diffuser under the rear of a car is like that lower surface of a wing. Air that has flowed under the car reaches the diffuser, which is an up-curving surface, and gets pulled upwards, and that pulls down the diffuser and the back of the car. It only works if the airflow under the car is clean, not turbulent, which is very hard to achieve without a flat underbody. And the trick with diffusers is making the air follow the curved surface of the diffuser upwards. If you don't have much of an upward curve you don't get much (negative) lift. If you have too much of a curve the air just separates from the surface rather than following it, and you get no lift and lots of turbulence and drag. One of the biggest aerodynamic secrets of F1 is how to make diffusers generate downforce at low speeds for the corners without getting airflow separation and drag-producing turbulence at the higher speeds down the straights.
