One of the most significant aero modifications of the current WRC car generation unveiled in 2017 was the inclusion of a rear diffuser, as we already reviewed here. The advantage of the rear diffuser is to facilitate removing the air flowing below the car. The more the amount of air removed, the faster the airflow under the car and, by Bernouilli, we know that this means lower pressure under the car, and, by extension, higher pressure difference, that is, higher downforce. Also, diffuser contributes to a fast, smooth exit of air, reducing the turbulence in the wake of the car, which contributes to a lower drag.
E.Lappi/J.Ferm, Toyota Yaris WRC, Rally Monte Carlo 2018, 4th – picture by Toyota Motor Corporation
But the efficiency of a rear diffuser can be improved, as F1 designers have proven since the 80s, by using the gases from the exhaust. As these gases are expelled at high velocity (especially when the engine is at high RPMs), if the exhaust is located at an adequate point, they can contribute to faster removal of the air coming from under the car, by dragging this air out of the car. A faster removal and faster airflow result in lower pressure, so an additional gain in downforce. Also, the car’s wake interaction with the flow coming from the rear wing is smoother, and drag is reduced.
K.Meeke/P.Nagle, Citroën C3 WRC, Tour de Corse 2018, 9th – picture by Honza Fronĕk – eWRC.cz
To get this contribution, the most adequate location of the exhaust would be inside the diffuser, in between strakes, following the idea of F1 Renault’s Jean-Claude Migeot designer in 1983. But, as it happened in F1, such practice is forbidden, so designers had to locate the exhaust on the outer top of the diffuser.
E.Evans/D.Barritt, Ford Fiesta WRC, Rally Montecarlo 2018, 6th – picture by M-Sport
The most logical location then is on top of the diffuser, in the centerline, the closest to the underbody airflow path. At the center, the wake is more homogeneous and interaction with wheels is minimal, allowing an optimal contribution from exhaust gases. And this is what Citroën, Ford and Toyota did when they designed their 2017 WRC cars.
Toyota Yaris WRC (top), Citroën C3 WRC (center) and Ford Fiesta WRC (bottom) – pictures by Michelin, Honza Fronĕk and Bogdan Barabas – eWRC.cz
Notice that the angle is also important: the exhaust is fixed at a small positive angle, in order to allow gases to flow tangentially with the airflow coming from below the car path, to better drag this airflow without interfering into the flow path, so the dragging effect is more effective.
On the other side, Hyundai kept the exhaust on one side, probably because relocating the exhaust to the center required too complex modifications in the engine and exhaust system. The main problem of such a location is that exhaust gases are released at one side of the car’s wake, where interaction with rear wheels’ wake is higher. Airflow in this area is less homogeneous, due to the presence of rear wheels, so the dragging effect of the exhaust is less effective. Also, in the case of the Hyundai the exhaust seems to be located horizontally, so gases interact directly into diffuser airflow inlet, instead of flowing tangentially, as for the other cars.
T.Neuville/N.Gilsoul, Hyundai i20 Coupé WRC, Rally of Sweden 2018, 1st – picture by Michelin
Exhaust blown diffusers perform very well while the driver keeps full throttle; the problem could come when the driver lifts-off and the engine revs fall down. However, due to the use of anti-lag or exhaust gas recirculation systems, to keep turbo rotating at high speed, a continuous flow of exhaust gases is ensured and the blown effect is maintained.
We have not found any published evidence of the advantages of using exhaust blown diffusers in a rally car, but for an F1 at full throttle, some authors report a gain of around 1% in downforce and a reduction of 1% in drag. It might seem a small contribution, but due to the limitations of current regulations, any gain is welcome.
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