Written by Frank M. Lin May 9th, 2019 7:47 am – While working up close and personal with a 2017 Honda Honda Civic Type-R I was able to see all the little details it has for air flow management. While thinking about how I’m going to build a exhaust system for it I was looking at the layout from the bottom. Then an idea hit me… exhaust assisted blown diffuser/venturi setup. I had read/heard about such a setup for Formula 1 but have never examined it closely before. But I just did, and it blows my mind. First check out the awesome sounds of these cars, read about how off throttle overrun works, and then go below to understand the evolution of ground effects; the under body work and the how the diffuser works with the entire package.
Would you just listen to how awesome the sound is?! But then something doesn’t seem right… why does it sound like that? A little further look someone explained:
Well technically, what you’re hearing is the off-throttle overrun.
If you just divert your normal exhaust gasses to the rear diffuser you will only get the increased downforce while exhaust gasses are present… i.e. when the throttle is pressed.
But drivers lift OFF the throttle while cornering, which is when they need downforce the most.
So F1 engineers developed engine mapping systems that allowed air to be drawn into the engine when the throttle was off, this air then exited via the exhaust ensuring that the diffuser was still being blown even when the driver lifted off the throttle.
OMG WTF BBQ right?! Leave it to Formula 1 engineers to come up with such cool tricks.
Engine Off – Throttle Overrun
Red Bull’s Adrian Newey initially pioneered the new generation of blown diffusers and he positioned exhaust outlet low above the floor of the Red Bull RB6. This was extremely beneficial to energize airflow going through and over the diffuser. This kind of diffuser was known as Blown diffuser. Basically, teams have been blowing exhaust gases over the rear floor of their cars even when the driver is off the accelerator going into a corner.
There are two way to blow a diffuser – hot and cold.
In Thursday’s Press Conference at Monaco 2011, Adrian Newey apparently tried to defend the legality of exhaust-blown diffusers on the over-run, by claiming that the primary function of an open throttle on the over-run is to cool the exhaust valves:
“In the case of Renault, (RedBull Racing engine supplier) when they open the throttle to full open on the over-run for exhaust valve cooling, and that’s part of the reliability of the engine…Obviously if other people are going further and perhaps firing the engine on the over-run then clearly exhaust valve cooling is not part of that and that would be something that presumably they would need to explain to keep Charlie (Whiting, Fia technical delegate) happy.”
Much more about blown diffuser you can read here, but just short recap:
What is a blown diffuser?
On road cars, the engine exhaust exits are normally located at the rear of the car. On a Formula One car they are deliberately located in front of the rear wheels so that the hot, fast flowing exhaust gases can be channeled towards the car’s diffuser. This increases airflow through the diffuser and in turn increases the amount of downforcethe diffuser produces. This is perfectly legal under current F1 regulations.
What is „cold blowing“?
Normally the engine will only produce exhaust gases requested for good diffuser blowing when the driver is on the throttle. This means when the driver lifts off, the blown diffuser is suddenly deprived of the additional airflow. To get around this, some teams have modified their engine mapping so that when the driver lifts off, although fuel supply and ignition are cut, airflow through the exhaust to the diffuser continues. This technique has become known as ‘cold blowing’ – the exhaust is still ‘blowing’ into the diffuser, but that airflow is now ‘cold’ since no fuel or ignition is involved. Renault is using this technique.
What is „hot blowing’?
Some teams (read Mclaren and Ferrari) have taken things a step further. To make the off-throttle ‘blowing’ as similar ( hot and fast flowing gasses) to the on-throttle ‘blowing’ as possible, they cut the ignition (supply for sparkplugs) when the driver lifts off the throttle, but continue to inject some fuel through the engine’s valves into the exhaust. This fuel ignites on the hot exhaust, increasing the amount, speed and temperature of the airflow exiting towards the diffuser.
Leading engineers say hot-blowing can give an advantage of as much as second a lap over no blowing at all, while cold-blowing is worth about 0.3-0.4 second over no blowing at all. The row started when the FIA decided to introduce a limit of 10% of throttle when the driver was not pressing the accelerator.
The brief history of it is that Charlie Whiting, head of the FIA technical department, feels that the use of exhaust gas to create downforce is illegal. This comes from the regulation change that bans the F-Duct, which says “no driver movement shall deliberately affect the aerodynamics of the car”. Clearly this involved driver movement (a foot lifting off the accelerator pedal).
Therefore any engine mapping with the prime purpose of using the engine as a thrust-producer is illegal. Charlie issued a series of technical directives to that intent culminating in a technical directive, which came to teams roughly two weeks ago, a bit before Valencia GP 2011. It said hot-blowing – ie fuelling and igniting on over-run – is banned and for cold-blowing – ie without injection and ignition – the maximum throttle opening would be 20 per cent at 18,000 rpm tapering to 10 per cent at 12,000 rpm.
It seems that Mercedes in particular lobbied that they should be able to retain over-run (with zero pedal) fired by 4 cylinders (half the cylinders in an F1 engine). They persuaded Charlie they were doing that in 2009 for the prime purpose of brake balance modification and therefore should be allowed to continue doing so for reliability reasons. Charlie’s philosophy is that things teams did prior to using the exhaust to create downforce should still be legal. So Charlie agreed to what Mercedes requested.
Renault engines had been operating from 2009 with a strategy of running throttles 50 per cent open (so called cold-blowing) on over-run for reasons of throttle response and engine braking balance. As this was also for non-downforce generating reasons, Renault believed they should be allowed to continue doing it, for the same reason Mercedes were allowed to continue hot-blowing. So Charlie agreed to allow 50% of throttle opening for Renault.
But Renault’s rivals object because the French engine company has now been allowed to have a 50% throttle opening when the driver is lifting the throttle.
This is what Whitmarsh calls “a very substantial performance benefit”. To which Horner responds: “Why is it any more of a performance benefit than fired overrun?”
Leading Formula One engineers have spoken out about the influence of cold and hot blown diffusers, and how their teams have had to adopt their cars for the upcoming rule change. As I can see, a lot of fans on forums think that this is a quite simple thing of matter to change these configurations of engine mappings. No problem, change a few parameters and things is done.
But it’s not that simple, and it affects how you operate the engine.
We need to understand what is going on within the engine when a driver lifts off the throttle and the subsequent effect that has on other aspects of the car. Unlike in road cars the driver in an F1 car does not leisurely lift off the throttle and delay the braking phase. Instead the driver may be at near maximum revs, when he will simultaneously lift off the throttle pedal completely and hit the brake pedal hard for the initial downforce aided braking. During the braking, the lower gears will be sequentially selected, further peaking revs all the time as the car slows down. This sudden closing of the throttles blocks the inlet to the combustion chamber, but the pistons in the cylinders will continue to pump up and down at a great speed. This creates huge stresses inside the combustion chamber and the vacuum created will suck air past the piston rings (so called blow-by). This will rapidly slow the engine, creating too much engine braking effect, which in turns creates stresses in the drive train and over-brakes the engine. The excessive engine braking effect will make the car nervous on throttle lift off, regardless of any subsequent aerodynamic effect. So engine manufacturers find different solutions to ease the stresses and braking effect of the driver lifting off the throttle.
In the past there were several different engine strategies in place and the driver was able to change overrun setting to tunes the cars handling, and driver switching between teams found the change in overrun settings needed some adjustment to both their driving style and sometimes with the engines settings.
Renault engine for example runs open throttles on the overrun (but no fuel injected or spark), this both eases the blow-by, reducing the vacuum effect inside the pistons and stress issues, it also useful for cooling the exhaust valves, as a great alternative to using excess fuel to cool the back of the valve. Renault sport is believed to be running as much as 90% open throttle on the overrun. This is what’s best known as cold-blown mapping. During this season and through out free practice, the three Renault engined teams, had a distinctive loud overrun note, which continues briefly as the drivers picked up the throttle out of slow turns. As the throttles are open more than other teams, the induction noise is far greater.
Mercedes High Performance Engine manufacturer have their solution, this is the so called fired or hot overrun. When the driver lifts off, fuel continues to be injected into the engine and spark fired within the combustion chamber but ignition was delayed as much as 45%. This offsets the engine braking effect created by the engine, giving a smoother transition from on throttle to the overrun when off it and again reducing the vacuum effect inside the pistons and stress issues. As a result this means there is less engine braking effect. This gives Mercedes the freedom to define braking bias and KERS charging, without having to account for engine braking. Effectively decoupling the engine braking effect from the actual action of the braking system. As with Renault’s mapping in past times, Mercedes solution is analogous to the hot blowing mapping. At pre Silverstone free practices and races, Mercedes engined teams had a particularly clean overrun sound. Where as Ferrari had far more cracks and pops as the engine slowed.
Cosworth engines are using same system as Renault to ease stresses on their engines and transmissions.
With all engine manufacturers having long established overrun strategies that have critical impacts on the basic engine design or the braking system, it was hard to rapidly switch to a very strict overrun mapping as demanded by the blown diffuser rule after Valencia GP race 2011. Renault and Mercedes lobbied the FIA to be allowed to retain elements of these old overrun strategies, while still emasculating their current strategies. The FIA have been able to see the mappings used in 2009 through to the current day, as the ECU code is held by the FIA since the advent of the single ECU. They’ve been able to see the engines have had these long established mappings, but also how they have become more aggressive since the “Blown Diffuser”has been developed.
So the FIA relented. This sounds like a climb down by the FIA and unfair to different engine manufacturers. But the unreported events at Silverstone are fairer than the picture being painted by the teams and the media. Its true that Renault were given their greater throttle opening, but also Mercedes were given their fired-overrun, but these dispensations have been given to every engine manufacturer, so Ferrari could have more throttle opening or Cosworth could develop a fired overrun.
A couple of weeks ago I find an excellent article on Gordon McCabe blog explaining one phenomenon regarding Formula 1 airbox and Airbox air spillage. Link on this article you can find here. Picture is my small add.
Monday, October 31, 2011
Just to make it clear for normal people. Under full throttle (lover half of the image), all air approaching airbox is sucked inside, trough engine and out trough exhaust. Same happened with hot or cold blowing. Engine pumps all air in. So, no air spillage around airbox.
With off throttle situation (upper half of the image) or without engine hot or cold blowing (engine pumping effect), some of air approaching airbox is not sucked in and as air spill (turbulent air) will continue his traveling toward rear wing.
A couple of weeks ago, the FIA issued a Technical Directive to the Formula One teams, announcing that off-throttle blowing of the exhausts will be severely curtailed in 2012 by engine mapping restrictions.
In combination with stringent requirements on the position and angle of the exhaust exits, this is intended to minimise the exploitation of exhaust flow for aerodynamic purposes. It will, however, have a secondary consequence. As Gary Anderson recently explained, off-throttle exhaust flow also serves to reduce spillage from the airbox:
“In the past when the driver closed the throttle to slow for a corner, the airbox spillage became a lot worse. If the airflow attachment on the sides of the engine cover was not good, the performance of the rear wing would be compromised – not something the driver wants under braking or on corner entry.
“Step forward the blown diffuser. Hot or cold blowing allows the engine to work like an air pump, moving this airflow through (Airbox) and out of the exhausts. This reduces the potential turbulent airflow creating negative performance on the rear wing.
If off-throttle blowing of the exhausts is genuinely to be prohibited next year by means of engine mapping restrictions, this will presumably re-create the problem of airflow spilling out of the airbox when the driver lifts off the throttle on turn-in to a corner.
So here’s an idea: Why not introduce a fluidic switch which, under certain circumstances, re-routes the airbox airflow through the chassis to the lower leading edge of the sidepods? This could have the joint benefit of boosting the velocity of the underbody flow, and improving airflow to the rear wing, just at the time when the driver most needs it, when the car is in pitch under braking and turn-in.
Once the potential of using aerodynamic downforce to win races was realized, designers began experimenting with methods other than simply attaching inverted wings.
The diffuser is an area of bodywork at the rear of the car, although the term “Diffuser” is technically incorrect, it is the most popular term applied to this part of the car.
The air flowing below the car, exits through the diffuser on the rear of the car. The diffuser is usually find on each side of the central engine and gearbox fairings and is, by the rules, located behind the rear axle line. The diffuser creates nearly 50% of the cars downforce. Being so powerful the rules have progressively been tightened, making the diffuser smaller and smaller to cap cornering speeds.
Although wings and diffusers work similarly, they are based under different concepts. A diffuser serves to eject air out from the underside of the car. This pulling action increases the velocity of the air below the car, so that the more slowly moving air above the car will push the car into the ground. The suction effect is a result of Bernoulli’s equation, which states that where speed of the fluid is higher, pressure must be lower. Therefore the pressure below the race car must be lower than the pressure at the outlet since the speed of the air below the racecar will be higher than the speed of the air at the outlet.
The diffuser in itself doesn’t produce a reduction in pressure. The role of the diffuser is to expand the flow from underneath the car to the rear, decrease the flow’s velocity from inlet of the diffuser to outlet (so that at the outlet the flow velocity is similar to the free stream velocity), inturn produce a pressure potential, which will accelerate the flow underneath the car resulting in reduced pressure and as such, a desired increased downforce generation. This pressure difference is a function of the ratio of the areas at the inlet and the outlet of the diffuser, where this area ratio is set by the diffuser angle and the vehicle ride height.
Diffuser can be considered to have “pumped-down” the underbody, inducing a component of downward force on the vehicle.
It was found during testing that the diffuser actually acted as a pump to generate downforce over the underbody flow path. This was not deemed to be the only identifiable fluidmechanical mechanism affecting the flow path around the diffuser. The three main aspects were; “ground effect‟, “underbody upsweep‟ and “diffuser pumping‟.
– “Ground Effect” plays a role when an object is used in the vicinity of a moving ground. Flow asymmetry is developed from the flow accelerating as it travels underneath the body due to ground constraint as a result the static pressure underneath the body is reduced which provides the resulting downforce.
– “Underbody Upsweep” refers to the upsweep of the diffuser at the rear. This is typically cambered and up-curved in shape, similar to the upper surface of an airfoil. Due to the direction of this curving, a resulting downward directed lift force will result during flow interaction.
– “Diffuser Pumping” refers to the increased cross-section area over the diffuser length, which can be used to increase the flow rate through diffuser because of pressure potential. As the ratio of the inlet to outlet area becomes increasingly
greater, this generates greater pressure recovery that, due to the base pressure remaining constant will increasingly depress the base pressure at the inlet. The diffuser acts to reduce the underbody pressure due to the expansion resulting in increased flow rate under the body. This increase results in further decrease in underbody pressure, which produces the “pumping down‟. This scavenging both produces a lower pressure area under the car and also acts to reduce the boundary layer. This effect is reduced with the higher underbody heights regulated since the 90’s.
Diffuser design before 2009 rule change
Its design includes vertical fences, some of which are curved, some stepped, and some angled, but all are developed through constant tweaking and evolution in the wind tunnel. The basic job of these fences is to keep apart the many different types of air flows found at the rear end of an racing car – areas of low pressure air due to the rear wheels, and the rear wing, and the air coming under the floor. All these different air flows have different energy levels and different speeds, and their separation makes them easier to deal with.
Showing precious little of the secretive diffuser, F1 rules prohibit under-car shaping or venturis, and mandate a minimum ride height enforced by a relatively low-tech wear plank or skidblock attached underneath the car. However, there is still scope to shape the area directly under and behind the rear axle line.
Diffuser on Mercedes GPW01 formula 1 car
Front underbody diffuser
Here we can see a typical diffuser on prototype LMP car. They have one more “diffuser” area below the nose of the car. We can see building up of the negative pressure in these two areas
LMP cars have a flat bottom, but the front of the car can have ducts to feed the engine and such. These ducts are used also to gain a certain downforce. As you can see in the following image of a Toyota GT-One undertray, the front part of the undertray of the car is highly contoured, so that air coming under the car splits up, some of it going through to the rear of the car, but another part goes to the exits behind the front wheels. This flow, goes upward into a diffuser like compartment, where it’s pressure is reduced, thus generating downforce just ahead of the front axle. The effect is basically the same as an air splitter, but here the flow is also directed towards the sides and the splitter that marks the beginning of the flat floor.
Looking at the diagram shown right, it can be seen that the car’s front is very similar to a wing in ground effect, even though looking at the vehicle there is no wing present. This design is not as effective as a “real” wing due to the redirection of flow. However, the fact the sides are relatively sealed increases the effect of the shape.
We can see there are the different ways of handling the airflow in the front of the car. Here the designers are trying to achieve the same thing as in the rear diffuser: more front end downforce by giving the air a place to expand and thus reduce pressure.
Diffusers, when good designed and working properly, can be extraordinarily important to the aerodynamics of a car. Diffuser, combined with the airfoil in the lower portion of the rear wing is employed to produce significant downforce, approximately 40 – 50% of the total car downforce. When not working properly, diffuser can mess even with the most experienced drivers.
A common problem for some badly designed diffusers is known as “diffuser stall.” Sometime the problem with the diffuser stalling is linked to the bad designe of the front wing.
Every team’s diffuser will stall to some degree when the rear of the car, because of high speed and higher downforce is closer to the ground, which reduces the drag and allows higher top speeds. But with that it reduces the downforce, and it is vitally important that this airflow reattaches the moment the rear of the car starts to rise. Otherwise, the braking area becomes a bit of a nightmare for the driver.
If a driver believes he has a certain grip level while taking a corner, he will take it at the highest speed possible assuming this level of grip. While braking, their car pitchedforward, as you might notice when you see a car on the road braking moderately. This forward pitch lifts the rear of the car. If not designed properly, a diffuser will lose a very large percentage of its effectiveness if lifted a small amount (with the stiffness of F1 suspensions, this “small amount” is 5-10 mm). When this happens a large amount of downforce is lost, and in turn, a large amount of grip is lost. If most of the 40 – 50 percent of a driver’s aerodynamic grip is lost mid-corner, it is very difficult to keep the car from twitching and you can see on onboard TV that driver start “hunting” the car with steering wheel. To sometimes add to this problem, diffuser does not reattach air flow effectively when it stalls.
During the past there were a few ideas how to improve diffuser efficiency. Well known, but not the first try, is McLaren’s try to speed up air exiting diffuser by blowing high speed exhaust gases inside diffuser area to “energise” the flow. There, speed of the airflow through the diffuser can be speed up by the high velocity gasses exiting the exhaust pipes when the engine is revving. This apparently free increase in diffuser performance was very popular in the 90s and was encouraged by bodywork restrictions preventing the exhaust pipes routing over the gearbox. The high velocity gases entrained air, energizing the thick boundary layer, and effectively powering up diffuser as it drew air under the car.
This idea was excellent but not very successful. Problem was that exhaust gases don’t have constant speed. More driver press the throttle pedal, higher the speed of the exhaust gasses and better efficiency of diffuser. Less throttle, lower the speed. With lower speed of gasses, diffuser efficiency drop and driving the car with lower downforce on the back of the car is not so pleasant thing to do. The worst part of the story is that during the cornering, when you need more downforce, drivers usually need to lift a foot a bit, losing downforce in that critical point.
Slowly the practice was dropped first by routing the exhausts into less effectual positions in the diffuser and then later through the top of the sidepods using the Ferrari inspired “periscope” exhausts.
The teams need to leave a hole in the center of the diffuser to insert the outboard starter mechanism, this usually results in small cut outs in the lower ramp of the central part of the diffuser and in the Vee formed above the shadow plate. The effect of this on diffuser efficiency is minimal.
Not withstanding the 2009 downforce reduction rules, the diffuser continues to be the dominant factor in aero design. Making the most of creating low pressure under the rear of the cars bodywork is as important as ever. Year 2009 we saw teams exploit rule loopholes to create additional underbody inlets feeding larger exit areas, known as the double diffuser (read about that later in the article). Year 2010 teams have further exploited these rules for ever larger inlets and outlets. However it has again fallen to Red Bulls Adrian Newey to look at the history book and re-invent a concept that has since fallen out of favour. Last year he did this with the pull rod rear suspension and this year it has been the exhaust driven diffuser. By mounting the exhaust outlets in line with the floor, they blow through the diffuser driving greater airflow and hence creating more downforce. Already with mid season upgrades, many teams followed Red Bulls lead. Read here how Blown diffuser work.
Though Bernoulli’s principle is a major source of lift or downforce in an aircraft or racing car wing, Coanda effect plays an even larger role in producing lift. To know more about interaction of Bernoulli principle and Coanda effect check my article here.
Ferrari Enzo front and rear diffuser
Rear diffuser on Ferrari F430
Rear diffuser on Audi R8 ready for DTM racing
Diffuser on Hyundai Genesis Coupe D1 Drift car
Rear diffuser on Shevrolet Corvette Can-Am
As part of the package of aero changes designed to reduce downforce for 2009, the diffuser has undergone quite complex modifications. Moves to create a more standardised shape have removed the difference in height between the central and two outer sections and all three channels are now taller – 175mm rather than 125mm. In addition, the diffuser has been moved rearwards, with its trailing edge now 350 mm behind the rear axle – previously, it was level with the rear axle.
Diffusers on Ferrari and McLaren 2009 designed by the letter of the new regulations
McLaren, Ferrari, Renault and BMW Sauber have all made very literal interpretations of the revised 2009 rules regarding the rear diffuser. All of the channels are the same height and length, with no difference in height between the main central section and the side channels.
This contrasts with the designs of Toyota, Brown GP and Williams (see subsequent illustrations), which interpret the new regulations slightly differently.
Toyota’s diffuser makes a very interesting interpretation of the revised 2009 rules (and one that has prompted speculation regarding its legality). By exploiting regulations that allow extra bodywork within a 150mm zone in the centre of the car, the team appear to have cleverly shaped the TF109’s rear crash structure (upper red arrow) so that it effectively lengthens and heightens the diffuser’s central section, which also features a very low splitter at its base (lower red arrows). Because of this upper section of diffuser (extra bodywork), this type of construction was named “double decker”, or “double deck diffuser”.
Like, in this time Williams engine supplier Toyota’s, Williams’ interpretation of the revised diffuser regulations is highly innovative. Much of the diffuser’s central section is actually lower than the outer sections. However, clever shaping of the rear crash structure immediately below the rear light effectively creates a second central section (see upper arrow). In combination, the result is a central section that exceeds the 175mm height allowance that applies to the diffuser alone.
BrownGP version of diffuser
The diffusers of Brawn GP, Toyota and Williams caused a major stir during the opening two races of the season in Australia and Malesia, and Ferrari, Renault, Red Bull and BMW decided to challenge the trio at ICA (International Court of Appeal). BrownGP win first two races with Janson Buton as a winner and Rubens Barrichello as second.
The FIA International Court of Appeal (ICA) has rejected protests against the diffusers used by the Brawn, Toyota and Williams teams, after concluding that their ‘double decker’ designs comply with the 2009 regulations.
BMW Sauber, Ferrari, Red Bull and Renault had all questioned the legality of the diffusers, but following Tuesday’s (14.04.2009) hearing in Paris, the ICA decided that race stewards in Australia and Malaysia had made the right call in declaring them legal.
Full statement from the FIA:
The FIA International Court of Appeal has decided to deny the appeals submitted against decisions numbered 16 to 24 taken by the Panel of the Stewards on 26 March at the 2009 Grand Prix of Australia and counting towards the 2009 FIA Formula One World Championship.
Based on the arguments heard and evidence before it, the Court has concluded that the Stewards were correct to find that the cars in question comply with the applicable regulations.
After a year of development, 2010 diffusers turned out as real monsters with huge downforce value. Red Bull Racing (2010 constructor champions) with their 2010 racer turned out with double diffuser and with exhaust gas blowing inside and over it. Trough the year 2010 they were almost unbeatable.
Diffuser on Ferrari f10 during 2010
Diffuser on Mercedes GPW01 – 2010
Diffuser on Red Bull Racing RB6 during 2010