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 (McLarenRenault large exit, Red BullFerrari 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 slimmerundercut 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 rodrocker 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’ exhaustdiffuser solution. McLaren and Williams are expected to follow suit for the enxt race At Silverstone.

Follow ScarbsF1 on Twitter

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>