Exhaust Driven Diffusers

Red Bulls Exhaust exits low and blows through the diffuser

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. Last year we saw teams exploit rule loopholes to create additional underbody inlets feeding larger exit areas, known as the double diffuser. This year 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. It seems for the team’s midseason upgrades, many will follow Red Bulls lead.

A diffuser is a simple device; a diverging duct creates low pressure under the car, creating negative lift, i.e. downforce. The FIA has acted several times since the mid eighties to cap the potential of the diffuser by reducing its length, height, ride height and position relative to the rear axle. Moving through the diffuser is the key to it producing downforce, or Mass flow as the aerodynamicists call it. This can be achieved with the size of the diffuser itself, effectively capped by the rules, but teams still are split on how large an exit they want to create within the current bodywork rules (McLaren\Renault large exit, Red Bull\Ferrari smaller exit). Onset airflow is another factor controlled by the front wing, bargeboards and the floor itself, but this is somewhat capped by what can be achieved with the limited devices the rules allow for. Then there is the flow over the top of the diffuser, this has been perhaps the biggest area of development in recent years. By ending the diffuser with a gurney flap, the airflow over the top of the diffuser can actually aid airflow underneath the diffuser. This is the reason sidepods have become slimmer\undercut and the diffuser appears more exposed amongst the coke bottle bodywork. Effectively the harder the air flows over the diffuser, the more powerful the gurney can be in puling airflow from inside the diffuser; this makes the diffuser act as though the exit is larger and makes more downforce. As long as a car needs bulky sidepods (even bulkier with this years fuel tanks) then the potential power of the airflow over the diffuser is limited. However we have a secondary source of powerful airflow at the rear of the car and that’s the exhaust pipes. Using the flow from the exhaust pipes can actually drive airflow through the diffuser, either by blowing inside the diffuser or over the top and driving the gurney flap. This isn’t a new solution, in fact Renault exploited this as early as 1983, when diffusers first appears in place of the banned full-length ground effect tunnels. Renault split the pipes exiting the turbocharger into three and directed them exactly at the point where the flat floor kicks up the form the diffuser. Soon most teams followed this format and for twenty or so years teams experimented with different exhaust outlet positions within the diffuser. As F1 switched from turbocharged engines to normally aspirated, the flow out of the exhausts was no longer ‘smoothed’ by the action of the turbo, the flow became much more abruptly on or off. along with the increasing dominance of the downforce created by the diffuser, this made the amount of downforce produced vary depending on throttle position, i.e. more downforce at full throttle where the flow was aided by the engine, then less downforce as the driver lifted off reducing the through flow. To negate the effect teams moved the exhaust outlets from the diffusers kick line to a less sensitive position, normally further up the diffuser roof. Eventually teams sought to avoid any sensitivity and move the exhausts clear of the diffuser and blew them over the top of the exit. Until Ferrari shifted their exhausts to exit periscope style in 1998. Most teams followed this approach aside from a few teams, which wanted to keep the blown effect, notably this was Both McLaren and Minardi. Eventually both teams had to divert from blown diffusers in order to package the much shorter exhaust pipe lengths demanded by the engine suppliers. It was Adrian Newey at McLaren that raced the last heavily blown diffuser, the MP4-16 exited its pipes low down in the middle of the diffuser. In 2002 the MP4-17 went to periscope exits due the demands of the Mercedes engine. At the cars 2002 launch he told me “Requests from the engine supplier, from Ilmor, was different exhaust system requirements which meant we could no longer continue with putting the exhausts exits out through the floor so we had to go for top exits”. I asked if this was an engine related requirement not aero, Newey said “yes”. I further prompted him if this was for shorter pipe lengths? He replied “I’d rather not go into details; we couldn’t accommodate what was wanted”. Underlining his commitment to the blown diffuser philosophy, I asked he’d tried try top exits on the old car (mp4-16)? Newey said “No never”.

McLarens MP-15 blew its exhausts into the diffuser

But Newey reverted to a blown diffuser for the highly experimental MP4-18 in 2003. The exhausts exited relatively high in the side channels to blow into the taller middle tunnel. However the routing of the exhaust past the all new carbon fibre (double clutch) gearbox lead to problems and along with other technical issues the car never raced. Replaced by the MP4-17D and MP4-19 both with the by now conventional periscope exhausts.

McLarens still born MP4-18 blew its exhausts towards the middle of the diffuser

This year Newey designed the RB6 to have a blown diffuser, although it was first tested with the conventional RB5 exhausts, it was only at the last test the team unveiled the secret exhaust development. Even replacing the old exhausts with look-a-like stickers to fool the unwary. This development was posted here back in early march, as was the opening into the diffuser. Strangely many fans back then denied the systems appearance and the fact it blew through the diffuser.
The RB5 that preceded this year’s car, already had high placed rear wishbones, and this allowed the subsequent car to run exhausts mounted low down and exit well below the wishbone, avoiding any overheating issues of the carbon fibre components. Teams have run exhausts in very close proximity to the wishbones now for many years, the differing strategies teams employ reduce the thermal load on the carbon fibre wishbones. Either gold foil film, extra carbon fibre heat shield or these are often coated with ceramic finishes to reflect heat. This latter finish being made obvious by the matt silver finish tot he parts. Sauber have run these on their top rear wishbone for many years. The heat shield even having a small air inlet to feed cooling air in-between the heat shield and wishbone beneath. Teething troubles may be expected as the teams start to run the new exhaust positions, but the heat protection will be a solution relatively easy to overcome. 

the inlets for the diffuser (yellow) are visible behind the exhaust outlets

The inlets for the exhaust flow are visible within the diffuser (yellow)

Contrary to the popular belief the low exhaust position is not related to the Red Bulls Pull rod suspension, in some respects having the exhaust in close proximity to the pull rod\rocker linkage is undesirable. But the exhaust positioning is probably more sensitive to wishbone position, such that teams aiming for low wishbones may have problems packaging the exhaust under the suspension. McLaren and Virgin have notably low wishbones.
In the RB6’s case Newey made an opening in the diffuser to allow the diffuser to be blown both under and over by the exhaust. This probably helps the airflow going up the outside shoulder of the upper diffuser deck, which probably has little energy and struggles to keep attached. Other teams this weekend may be expected to run a diffuser blown over the top, which perhaps offers less potential then a through blown diffuser, but at least will be legal next year when double diffuser are by banned by new rules preventing openings in the diffuser.
Another misconception of the low exhaust is the effect on tyre temperature. It’s possible the exhaust does affect the inner shoulder of the rear tyres, but this may well be an effect teams want to discourage. Any tyre heating will certainly be secondary benefit of the system and the sole reason for going with low exhausts. Its interesting to note Red Bull have run a fence on the floor between the exhaust and rear tyre. This probably helps keep unwanted heat from the tyres. But in Canada, where tyre temperatures were, this fence was removed. It could be that the tyre heating effect could be a tuneable parameter, by varying the heat shielding around the coke bottle area.

So far we have seen Ferrari, Renault and Mercedes have followed Red Bulls ‘back to the future’ exhaust\diffuser solution. McLaren and Williams are expected to follow suit for the enxt race At Silverstone.

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29 thoughts on “Exhaust Driven Diffusers

  1. Fascinating, a superbly comprehensive explanation. I have one question: if blown diffusers were abandoned because of the sensitivity of downforce to throttle position, has something changed to make that less of a problem on the current cars?

    • I think the teams are better able to map the exhaust gas flow around a lap and then simulate the effect. Teams are placing their exhaust still someway from the diffuser, this reduces the amount the diffuser gains from the effect, hence also reduces the sensitivity to power on\power off.

      • The red bulls and other teams this year are not actually running blown diffusers though. These latest designs use the exhaust flow to isolate the turbulant air flow generated by the tyre from the flow through the diffuser/rear wing. Like using the exhaust gases as a shield. The idea is not actually to blow the exhaust gases through the diffuser. As mentioned in your article, this makes the car unstable when the drive comes on and off the throttle.

      • Nice article, too bad it’s not correct. Mike Gascoyne talked about it on the BBC coverage of the race. Matt here also explaines why your article is not correct.

      • Re: Dominic and Matt

        The majority of the gas does not flow into the diffuser tunnel, no, but some does on the RB – through the slot. It enters in very near to the exit, and therefore reduces the on/off throttle sensitivity. The real problem came when the exhaust gas was entering entirely into the diffuser tunnel, right at the very beginning. The further towards the exit you go, the better.

        Additionally, flowing the gas over the diffuser helps to activate the flow within.

        RB definitely runs a blow-through, though, not a blow over.

  2. Cracking article Craig. I’ve always been interested in the MP4-18’s mystery, as though it was a title winner that came a few years too early.

  3. Very good explanation. I wonder if Red Bull is telling Vettel and Webber to keep their engines revving high through corners to get the full benefit of the diffuser. You could see through turn 8 in Turkey that they both preferred going flat out, 18,000 rpms in 6th gear.

  4. I was surprised to hear that Ferrari were working on it since pre-season, it seems like quite a simple thing to implement to at least gain a tenth, and then tune it later on.

    Do you know if these new exhausts have any effect on the engine?

  5. So the advantage of exhaust flow is that it has a faster velocity than the normal flow through the diffuser? I don’t understand how the gurney flap helps the diffuser, does it create lower pressure behind that sucks the air? Then, how is the exhaust flow similar to the gurney?

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  11. Interesting – an article on diffusers just days before Mark Webber gives the world a spectacular view of the underside of his car 🙂

    Do you think there were any surprises revealed by his flip in Valencia?

    • Was definitely interesting to see how far forward the “plank” extended, for me. Also, it was sort of an “upskirt” shot at the the diffuser channels – quite interesting.

  12. I don’t necessarily totally agree with either of these posts. I managed to find Gascoyne’s BBC slot on you tube…

    MG: “What the teams are doing now, are bringing the exhausts down into this area in front of the tyre, because how you manage airflow around the tyre and around the double diffuser is critical. They are using the exhaust gasses to control that flow, they are not directly blowing into the diffuser, which is what they did in the late eighties and nineties. That creates a very sensitive car on and off the throttle. They are using to control airflow around the tyre and improve downforce.”

    Looking at what he’s saying he does indeed put a lot of emphasis on the flow around the tyre, but other tech directors are calling these blown diffusers and looking at the teams implementations of the set up, they are aiming to fence off the flow from the exhaust from going anywhere near the rear tyre. instead they are sending the flow right up the intersection of the upper diffuser deck and the lower deck. Ferrari in particular has added extra gurneys the the trailing edges of the diffuser in this area. while I’m sure the whole flow structure between the chassis and wheels is affected, My point is the effect is primarily aimed at improving the flow coming out from under the diffuser for downfroce, rather than flow seperation and rear wheel wake.

    However I will send some emails to MG and other Tech directors to get their clarifications.

    • I believe that the effect is primarily aimed at improving the flow coming out from under the diffuser, but achieved by ensuring the turbulence around the rear tyre does not impinge upon the flow area that the diffuser feeds into. Having seen several of the RBR’s aero traces around the rear wheels, it certainly appeared they were concentrating upon influencing the turbulent impact of the rear wheels upon the total rear aero?

      The RBR works upon some aero and down force functions no other cars employ. You only have to look at the RB5 to see that it could produce near comparative down force without any DDD. This year they have added to that with a restricted DD of their own. This goes a long way to explaining why the RBR is so good in qualifying, because not being totally dependent upon the DDD, ride height has far less effect upon down force than other cars.

      The RBR maximises down force from managing the pocket of low pressure air running behind the car. The capability to move that closer to the exit point from air travelling underneath the car will have significant importance on the volume of air passing through. Addressing the turbulence from the rear wheels would then be a simple, small refinement of that basic principle.

      I have no inside knowledge from RBR, with the exception of having seen some of their aero work around the rear wheels in testing. When you look at all the extremes that RBR have gone to with with the design of the RB5 and it’s continuation in the RB6, I can come to no other conclusion. The reverse suspension is technically inferior and it’s only favour being it allows a narrow rear end. Mirrored by the narrow ended gearbox, a function that does not enhance reliability. Then shrink rap the engine covers to heat the unit to maximum and you wonder what great gain Newey is looking at in compromising reliability to such a degree?

      • Reading and researching a little more, it seems that Red Bull are following a practice used on turbo cars (i.e. the old F1 turbos and WRC cars) to keep the turbo spooled up. If Red Bull can keep the flow out of the exhaust pipe relatively constant, even when the throttle is closed going into a turn, then the diffuser will see a more constant air flow and maintain downfroce. Relieving it of the on\off throttle sensitvity often used as a critisism of EBD systems. In effect an antilag system is trying to do the same as the Red Bull EBD mapping, maintaining a constant exhaust gas pressure, on or off the throttle.
        Retarding the ignition on the overrun means the fuel ignites after the exhaust valve opens and keeps burning & expanding in the exhaust, this keep exhaust flow going. Effectively its like keepign the throttle open without the car acellerating. This sort of system will overheat the engine, in particular the exhaust valve and exhaust pipe, if used too much. Plus it burns more fuel, making its use in the fuel critical race less attractive. Although I’m sure theres a setting for using it in races too. A little like a push-to-pass engine mapping, that also speeds you up in the corners. This mapping would be controlled via the SECU vie the driver seelcting a steering wheel control, using quite normal tuning parameters and not some clever workaround. Of course this is all quite legal.
        If the overheating issues can be contained, this would be a relatively simple mapping to introduce for another EBD team. As mentioned Renault Sport, Red Bulls engine supplier would have to know about this. Copying the concept, but not the actual SECU code would be quite easy.

        Info on antilag systems http://bit.ly/d4pQN1

      • Either way with a blown diffuser or the alternative, this would give a similar effect. Bearing in mind that the RBR diffuser is compromised by the design of the rear end, I would expect the opposition to gain more than RBR from this.

      • Hmmm, just checked the Bahrain GP laptimes – out of nowhere Alonso went a second faster than anyone else ontrack, another couple of pretty quick laps, and he has halved the gap with Vettel to a second or so; Vettel at the same time responds with his own fastest laps of the race, and then his sparkplug problem.

        Any chance that feeling the breath on his neck, Seb engaged this mode during the race (for too long?), and this contibuted to the failure.

        I guess what I am asking, as well as exhaust ports, does this F1 version of anti-lag stress the plugs too?

  13. So would the perfect scenario for a blown exhaust (ignoring regulations) be… A high speed corner with no throttle blip on entry, for exhaust gasses to be wholly funnelled into the diffuser prior to its upper lip and for the lip to be flared/flapped/tweaked to further enhance the pressure differential?

    I guess then to flatten out the response to throttle changes and make things more stable one tweaks the exhaust exit to blow the gasses over the diffuser with a large whole allowing fast flow into in, classic feedback loop engineering?

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