6 replies to this topic

[jennerabc]

Hey all,



This is my first post, so you know, be kind!

I have been thinking about different types of exhausts of late, and wondering how to help 'pull' the exhaust out, much like a properexhaust manifold should, by basically creating a low pressure region, which then draws out the gasses.

So this brought me to think about putting a Gurney Flap around the exhaust opening. If you are familiar with front and rear wings, a Gurney helps to create a low pressure region to the upper side of the respective wing, therefore drawing the air flow from the underside out at an increased velocity, therefore lower pressure which creates more downforce.

In my exhaust Gurney you would be doing the same thing, although with my example the exhaust would have to be in some flow of air. So something like Ferrari's would be useless having them, but maybe on the BAR's for example and many others where the exhaust exit is flush with the body work it may work.


As I see it, it would work by sucking the air from the eninge, and creating my BHP, an obvious benefit, but at the expense of some drag and loss of downforce no doubt. But if the extra BHP outweighed the losses then this I feel could be an interesting tweak.



Would be interested in your feedback.

Jenner

[eldougo]

Well thats an interesting idea firsty i would like to see your thoughts on paper, the shape of the gurney would be the main thing .2nd
the heat around the bodywould could be a problem just my initial thoughts.

[kodandaram]

I think its Feasible but the flap would have to be manufactured as a part of the exhaust outlets I think.

The problem with either bolting them or welding them over the exhaust outlet is that the mechanical vibration and heat will result in breakage at the point of contact b/w the flap and exhaust. Also the material used to manufacture this Flap will have to be able to withstand high temperatures and vibration - which means that we can probably use the same material as the exhaust itself. The shape will be quite tough to perfect as well sa the dimensions . If the dimensions are incorrect then it will lead to wrong amount of airflow over the flap and result in adverse consequences due to the difference b/w pressure over and below the flap.Another difficulty may arise due to the amount of material needed to make the flap structurally strong.

If the same material is used then the designing and manufacturing process will get simplified . But If the flap has a large surface area - it will increase the drag ....so it wil have to be something like what the Ferrari car has right now ....does anyone know what those protrusions ver the exhaust outlets are for ? It may well be for the same reason.

[kodandaram]

Now wait a minute ......It suddenly strikes me that this won;t work . Why ? because I think we got our ideas wrong here . The FLap on the front wing - does it really suck the air from the lower plane of the wing ? I don't think so ....it is there only to reduce the velocity of aire above the wing. Below the wing there is no resistance for air - so it moves faster below the wing than above the wing.

The air above the wing travels slowly and exerts more Pressure . The air below goes thru faster and exerts very little pressure. SO it results in downforce . In case of the flap - it will only increase grag and not suck the air from the outlets .......

[Jezztor]

the idea is sound, but it won't work because the void has to have the 'capacity' to draw the air faster than the exhaust is expelling air. I don't have any accurate figures but a source has told me that the gas flows substantially faster than the speed of the car. The low pressure could help in 'free-moving' gas particles but exhaust design is VERY complex and variable outlet pressure like this could disturb things.

This idea is used to extract air from radiator exhausts, but the air in the engine bay is not travelling as fast as the car. A small bernoulli lip is often seen on the lips of the opening of the radiator exhaust. It creates complex swirls which create a low pressure void behind the lip and the low pressure air 'extracts' the hot air because it is 'easier' to flow through low pressure than high pressure.

Jezz

[DOHC]

Originally posted by jezztor
the gas flows substantially faster than the speed of the car.

Right, the average speed of an F1 car is about 60 m/s, and a typical exhaust gas speed is 100 m/s. Maybe not a huge gap, but gasses do flow faster than the car travels.

[DOHC]

Originally posted by kodandaram
it is there only to reduce the velocity of aire above the wing.

Correct, and that's what the Gurney flaps do too. But sometimes you'd see it expressed differently, like that the flap is there to change the trailing edge boundary condition, or to change the overall curvature and circulation and the like. That's just different languages for the same thing, depending on what you would like to stress for the moment.