So what is a diffuser and what does it do for an F1 car?

Since the 2009 F1 season started in Australia, there’s been a new order on the grid.  This year the sports technical regulations were changed, particularly those dealing with aerodynamics.  A few teams have got the march on the others by interpreting the rules in a different way, and have developed faster, better cars.  The old guard of McLaren and Ferrari are on their back foot, whilst teams like Brawn find themselves leading the championship.

If you believe the press this change of form has been down to the cars diffusers.  But what are they?  Most reports simply say that diffusers generate downforce, but strictly speaking that’s not really what they’re for.

Having had a few conversations about this recently I thought I’d attempt to write an explanation of what a diffuser does.  I’m not exactly an aerodynamicist, but hopefully this will make sense!  Right… so a few basics… sorry if they’re obvious! 

Ultimately a cars performance is dictated by it’s tyres.  The more friction or grip they have with the road, the faster a car can corner, accelerate and brake.  There are many weird and wonderful ways to get the tyres themselves to grip better, from compounds of rubber to their internal construction.  One simple way to increase grip though is just to push them onto the road more.  Sounds obvious eh?  Of course you could do this by adding weight, but that would compromise other aspects of the car’s performance.  The more mass a car has the more energy that is needed to accelerate it and keep it on the road, so really you want a way to push the car down, but without adding mass.  That’s where downforce comes in, using the airflow passing over – and under – the car to push it down into the road.

An important idea to understand when thinking about downforce is the Bernouilli Principle for fluid dynamics.  In short all this says is that the faster a fluid flows the lower its pressure.  In our case, the fast that air is flowing over the surface of a car, the lower it’s pressure.  (This is a vast simplification, but in general true.)

An example of this in practice is an aeroplane wing

Airflow over a wing
Airflow over a wing

A wing is shaped so that the air flowing over the top of it is faster than the air flowing under it.  The faster moving air has a lower pressure than the slower air, which generates an upward force that makes a plane fly.

In many forms of motorsport wings are used to generate a downward force to push the car onto the road.  In doing this huge amounts of downforce can be created.  But wings aren’t the  only way of achieving this. 

How about using the other surfaces on the car?  If you can make the air under the car flow faster than the air over it, the car itself would generate downforce.   Of course the airflow around a car should be as smooth as possible to avoid drag that would slow the car down, so the best route to achieving our goal is to accelerate the air under the rather than slow down the air over the top. 

The next principles to consider are the Conservation of Mass and Venturi Effect.  These principles are quite hard to explain (for me at least!) but can be demonstrated using the venturi tube below.

VenturiTube

As the air passes through the tube it meets a narrow throat through which it must pass.  The amount of air that enters through the inlet must be the same as the amount that exits at the end, so to pass through the throat the airflow must accelerate, and by increasing in velocity it reduces in pressure.  Once through the throat the diffuser increases the diameter of the tube back to that of the inlet, and therefore slows the airflow back to its original velocity.

A practical example of a venturi tube in use is a carburettor choke, where the low pressure is used to suck fuel into the airflow and then into the engine cylinders.

So what has this got to do with downforce?  Well imagine that half of the tube was flat (as below).

VenturiTubeHalf 

The same still applies, the air in the throat accelerates and slows.  So if the flat surface was a road and the throat the underside of a car you would get low pressure under the car – just what we want.

For this to work the car has to have a flat underside, any interference in the airflow will slow it down. 

The closer the car is to the road the faster the air will have to travel, hence the desire to run cars as low as possible.  One thing to consider here though is that whilst the airflow will go faster the lower the car, air still has to be able to get under it.  At some point the car will be so low that not enough air will pass under the front lip and the effect will stall, leading to a drop in downforce.  This is a particular problem under braking when the front of the car will dip towards the road.  If it drops too far the floor will stall and the driver will experience a sudden drop in grip just when he needs it.  This was what the active suspension of a few years ago was intended to avoid, it actively managed the suspension to keep the ride height at an optimum level.

Another effect of having lower than ambient pressure under the car is that air will leak in from the sides reducing it’s effect.  This is a real problem and one that on the old Lotus 78 and 79 race cars was solved by running skirts along the side of the car to stop air getting in.  Skirts are now banned, but other techniques have been adopted over the years such as creating vortex’s along the sides – but that’s all a bit off topic.

So if the flat floor generates downforce why have the diffuser at the rear of the car?  After all the more flat area you have the more downforce you’ll get. 

The diffusers purpose is to control the way the airflow at the back of the car decelerates back to its normal velocity.  So it less about creating downforce, it’s actually an essential part of the venturi whose purpose is to slow the airflow by increasing the space between the road and the underside of the car.  This allows the pressure to rise back up to ambient so that the airflow leaves the underside of the car as smoothly as possible decreasing drag that would slow the car down.  Having said that, the pressure in the diffuser will on average be lower than ambient, so it will create some downforce, but as a side effect.

A common misconception, and one that I made before I read a book about aerodynamics, is that the diffuser expands the air to produce low pressure.  This isn’t the case as it would require the airs density to change, and in an open system like the underside of a car a diffuser wouldn’t be able to do.

So how are Brawn and the other teams getting such an advantage?  Well by interpreting the aero rules differently they have been able to build diffusers that are much more efficient than their rivals.  By being able to start their diffusers earlier they are able to run them up closer to the upper bodywork of the car, and importantly closer to the rear wing elements.  The low pressures of the underside of the rear wing and diffuser interact and increase the effectiveness of each other.  There’s some really good information on the specific differences between the Brawn/Toyota/Williams cars and the others in the FIA’s explanation of their appeal ruling here.

A good book on this subject is Competition Car Aerodynamics: A Practical Handbook by Simon McBeath.  

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