A diffuser, in an automotive context, is a shaped section of the

car A car, or an automobile, is a motor vehicle with wheels. Most definitions of cars state that they run primarily on roads, seat one to eight people, have four wheels, and mainly transport people rather than cargo. There are around one billio ...

rear which improves the car's

aerodynamic Aerodynamics () is the study of the motion of atmosphere of Earth, air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dynamics and its subfield of gas dynamics, and is an ...

properties by enhancing the transition between the high-velocity

airflow Airflow, or air flow, is the movement of air. Air behaves in a fluid manner, meaning particles naturally flow from areas of higher pressure to those where the pressure is lower. Atmospheric air pressure is directly related to altitude, temperat ...

underneath the car and the much slower freestream airflow of the ambient

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

. It works by providing a space for the underbody airflow to decelerate and expand (in volume, as density is assumed to be constant at the speeds that cars travel) so that it does not cause excessive

flow separation In fluid dynamics, flow separation or boundary layer separation is the detachment of a boundary layer from a surface into a wake. A boundary layer exists whenever there is relative movement between a fluid and a solid surface with viscous fo ...

and drag, by providing a degree of " wake infill" or more accurately, pressure recovery. The diffuser itself accelerates the flow in front of it, which helps generate downforce. This is achieved by creating a change in velocity of the air flowing under the diffuser by giving it a rake angle which in turn generates a change in pressure and hence increases downforce. __TOC__

Overview

When a diffuser is used, the air flows into the underbody from the front of the car, accelerates and reduces

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...

. There is a suction peak at the transition of the flat bottom and diffuser. This transition is where the lowest pressure generally is located and is called the throat of the diffuser. The diffuser then eases this high velocity air back to normal velocity and also helps fill in the area behind the car making the whole underbody a more efficient downforce producing device by reducing drag on the car. The diffuser also imparts upward momentum to the air which further increases downforce. The trailing or leading edge of a diffuser may receive a nolder—a precise small lip, protuberance or wing to enhance its performance.

Operation (rear diffuser)

The aft part of a car underbody is where a rear diffuser is usually located. It works by accelerating the velocity of the airflow underneath the car. At the diffuser exit station the air flow is at the same pressure and speed of the ambient. Since its geometry is expanding the rear region, the exit area is much bigger than the inlet, so for the

conservation of mass In physics and chemistry, the law of conservation of mass or principle of mass conservation states that for any system closed to all transfers of matter the mass of the system must remain constant over time. The law implies that mass can neith ...

principle the air flow will have a much bigger speed at the diffuser inlet and as a consequence under the whole car underbody. The consequence of the increased flow speed is a reduction in pressure according to the

Bernoulli's principle Bernoulli's principle is a key concept in fluid dynamics that relates pressure, speed and height. For example, for a fluid flowing horizontally Bernoulli's principle states that an increase in the speed occurs simultaneously with a decrease i ...

. Since the pressure below the car is lower than on the side and above the car, downforce is produced if implemented correctly. Front diffusers also exist (especially on

Le Mans Prototypes A Le Mans Prototype (LMP) is a type of sports prototype race car used in various races and championships, including the 24 Hours of Le Mans, FIA World Endurance Championship, IMSA SportsCar Championship, European Le Mans Series, and Asian ...

or similar cars); however, they generate downforce purely from momentum exchange with the air, as there is nothing ahead of them to drive. A poorly designed front diffuser can create a low pressure region toward the front of the car which slows the air behind it down and reduces the effectiveness of the rest of the underbody. Front diffusers usually route air away from the car so that it doesn't affect the rest of the underbody. The air can be vented through a channel or expelled near the front wheels. Injecting the

exhaust Exhaust, exhaustive, or exhaustion may refer to: Law * Exhaustion of intellectual property rights, limits to intellectual property rights in patent and copyright law ** Exhaustion doctrine, in patent law ** Exhaustion doctrine under U.S. law, i ...

into the rear diffuser can also help extract the air from below the car. The exhaust gasses effectively energize the boundary layer, helping to raise the pressure of the low-pressure, fast-moving airstream back to the ambient atmospheric pressure at the exit of the diffuser. This fast-moving air helps evacuate the diffuser more quickly, which helps drop the pressure at the underbody. However, this makes the diffuser rather sensitive to engine speed. When the driver lifts off the

throttle A throttle is a mechanism by which fluid flow is managed by construction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases (by the use of a throttle), but usually decreased. The term ''throttle'' ha ...

, the exhaust flow is greatly reduced, which makes the diffuser less effective, robbing the vehicle of downforce. Thus, handling is negatively affected. Rear splitter and diffuser

The car's bodywork also interacts with the flow through the diffuser. In addition to creating downforce, the front wing and nose try to keep "clean air" flowing around, and more importantly under, the car. Clean air under the car prevents flow separation from occurring in the diffuser, which would severely rob its performance. The rear wing also affects the diffuser. When the wing is mounted low and close to the diffuser, the low pressure under the wing helps suck air through the diffuser. Cars, such as the Toyota Eagle MkIII and the Jaguar XJR-14 employed two-tier wings to enhance this effect. One profile was mounted high, in order to hit relatively clean air. The other profile was mounted almost flush with the bodywork behind the chassis. This wing's profile is used to drive the diffuser, creating that low pressure area to help move air from the underbody. According to Hiro Fujimori, aerodynamicist for the Toyota Eagle MkIII project, this bi-plane wing produced 18% more downforce for the same drag than a normal wing. Conversely, equal downforce levels could be attained for significantly reduced drag with this "Red Baron" wing.

Multi-deck diffusers

In 2009, the Formula 1 grid was embroiled in controversy. The culprit was the so-called ''double-decker diffuser'' introduced at first by

Brawn GP Brawn GP was a Formula One constructor which competed in the 2009 Formula One World Championship, with drivers Jenson Button and Rubens Barrichello. The team was formed in 2009 by a management buyout led by Ross Brawn of the Honda Racing F1 T ...

WilliamsF1 Williams Racing, legally known as Williams Grand Prix Engineering Limited and competing as Atlassian Williams Racing, is a British Formula One team and constructor. It was founded by Frank Williams (1942–2021) and Patrick Head. The team w ...

, and Toyota Racing, but later put into use by every team. These three teams had exploited a loophole in the rules that allowed for more volume in the diffuser. The rules stated that the diffuser must start at a point aligned with the centerline of the rear wheels. The loophole allowed for holes in the underbody, perpendicular to the reference plane (not visible as a hole when viewed from directly above), that fed a diffuser channel that was above the main diffuser. This greatly increased the available downforce, and was worth about half a second per lap, according to Mike Gascoyne. The teams decided to allow the double-decker diffusers again for 2010. However, for 2011, the Formula 1 Technical Working Group decided to ban multi-deck diffusers.

Splitters

As the front of the car slows down the air without a diffuser, this is the ideal place for an inlet. A splitter is commonly used here, serving to increase the amount of downforce at the front of the car. The airstream is brought to stagnation above the splitter by an air dam, causing an area of high pressure. Below the splitter, the air is redirected away from the stagnation zone and is accelerated, causing the pressure to drop. This, combined with the high pressure over the splitter, creates downforce. The larger the area of the splitter, the more downforce is generated. In most closed-wheel race cars, the underside of the splitter smoothly integrates with the undertray, creating one large flat plane that is driven by the rear diffuser. Some race cars, such as the

Toyota GT-One The Toyota GT-One (model code TS020) is a Auto racing, racing car initially developed for Group GT1 rules, but later adapted into an IMSA GT Championship, LMGTP car. It raced in the 1998 24 Hours of Le Mans, 1998 and 1999 24 Hours of Le Mans. H ...

, use an additional diffuser immediately behind the splitter to help create more downforce. The air extracted by this diffuser is exhausted through vents in the sidepods or above the car around the cockpit.

Examples of diffusers and splitters

File:Corvette C6.R Atlanta esses.jpg, Rear diffuser protruding from below bumper on a Chevrolet Corvette C6.R. Note the strakes for keeping out tyre wake. File:TF109 rear detail.jpg, Double Decker diffuser of a

Formula One car A Formula One car or F1 car is a single-seat, open-cockpit, open-wheel racing car, open-wheel formula racing car used to compete in Formula One racing events. It has substantial front and rear wings, large wheels, and a turbocharged engine mid ...

( Toyota TF109). Note channels above the main diffuser on either side of the light. File:2013 IAA DSC02024 DxO (9879145825) (2).jpg, Rear diffuser visible below the license plate on a Porsche 918 Spyder. File:Neckarsulm-AudiForum-Audi-R10-TDI.jpg, Large front splitter below nose on an Audi R10 TDI File:Gulf DBR9.jpg, Front splitter visible below the nose on an Aston Martin DBR9 File:Lotus Exige S front.jpg, Front splitter integrated into bumper of a Lotus Exige S. File:Sub im WRX.jpg, Small front splitter below the bumper of Subaru Impreza WRX. File:2006FOS 1990NissanR90.jpg, Two-tier rear wing, with lower profile mounted to enhance diffuser airflow. File:SplitterNASCARKaseyKahneFront.jpg, Front view of a splitter on a

NASCAR The National Association for Stock Car Auto Racing, LLC (NASCAR) is an American auto racing sanctioning and operating company that is best known for stock car racing. It is considered to be one of the top ranked motorsports organizations in ...

car (white) File:SplitterNASCARKaseyKahneSide.jpg, Side view of a splitter on a

car (white)

Overview

Operation (rear diffuser)

Multi-deck diffusers

Splitters

Examples of diffusers and splitters

See also

References

External links