Rear cooling in RX Supercars

One of the most significative differences between WRC and RX cars (apart from the engine power) is the different location of the radiator. While in all WRC cars it is located at the car front, most of the RX supercars have moved it to the car rear, while the intercooler is kept in the engine bay. This is our review of the different engine cooling solutions implemented by the WRX teams taking part in the 2019 season of the World Rally Cross Championship.

G.Chicherit, Renault Megane RX, Loheac 2019

The main reason for placing the radiator at the rear of the car is to improve the weight distribution, by relocating some of the weight in the car rear. But there are some other reasons behind:

  • removal of radiator allows better airflow to intercooler, which translates into better turbo cooling and higher power available,

  • the radiator has less risk of being damaged at the car front than at the car rear, due to frequent collisions between rallycross cars, and

  • the radiator is less exposed to dust and mud at the rear of the car, so its efficiency remains stable all over the race time.

However, there are also some disadvantages:

  • the flow of air reaching the car rear can be smaller (although fresher) than at the car front: to overcome it, fans are included downstream the radiator, to ensure enough airflow, and

  • depending on its more or less advanced position in the car, the radiator can represent an extra overhang that will surely have an impact on car behavior.

There is still an additional impact to consider: the airflow entering by side inlets, crossing through the radiator and exiting through the rear door vent has obviously an impact on the car aerodynamics: whether this is positive or negative, we will review it at the end of the post.

Peugeot 208, Ford Fiesta, Mini and Hyundai i20 RX Supercars at Barcelona event, 2019

Seven out of the nine cars regularly participating in the Championship have their radiator at the car rear. Only the ALL-INKL.COM Munnich Motorsport’s Seat Ibiza and the ESMotorsport Skoda Fabia keep the radiator at the car front. Even so, both cars have been able to access the final several times this year, for what their solution proves not to be fully penalizing them.

Renault Clio, Renault Megane and Citroën DS3 RX Supercars at Barcelona event, 2019

The easiest way to identify which car implements a rear cooler is by the side air intakes, located at the height of the rear door. The size and shape of the side air intake are different for each car. Seven of the current cars include the hole on the rear door.

A. Bakkerud, Audi S1, Barcelona 2019

The Audi S1 is the only car to include four air intakes, the usual two on the car sides and two additional on the car roof, allowing a bigger airflow to reach the cooler. Smaller in size, the Audi intakes follow the shape of a NACA duct, which is the best solution for a minimal pressure loss, which at the end means better cooling as air pressure remains high.

The goal of the air intakes is to send the maximum amount of airflow, with the minimal pressure loss and homogeneously distributed across the pipe area, so the rear cooler performance is optimal. To minimize pressure loss, ducts bring the air from the inlet to the radiator, as shown in the picture below.

J.C.Dubourg, Peugeot 208 RX, Loheac 2019

The location and orientation of the radiator, as well as the number of fans, are different for each car. The Audi S1 (with just one fan) and the Peugeot 208 (with 2 fans) have both a radiator inclined backward (angle higher than 90º). The difference is that the radiator is located at a more retarded position in the case of the Audi.

A. Bakkerud, Audi S1, Barcelona 2019

A radiator in an angle higher than 90º (as in the case of the Audi or the Peugeot) sends the air exiting the radiator towards the car base, thus generating some additional downforce, but losing some energy that could be used in the car wake.

T. Hansen, Peugeot 208, Barcelona 2019

In the case of the Hyundai i20, the Renault Clio and the Renault Megane, the radiator has also 2 fans, but it is inclined upwards (angle smaller than 90º). Note that the Peugeot 208 and the Renault Megane are the only cars with no air conduct downstream of the radiator.

T.Timerzyanov, Hyundai i20 RX, Loheac 2019

If the angle of inclination of the radiator is smaller than 90º, air exits the car at a higher speed, which has advantages in terms of a reduced wake.

G.Chicherit, Renault Megane RX, Loheac 2019

The Mini is a special case, with 3 fans and a vertical radiator.

O. Bennett, Mini RX, Loheac 2019

A radiator in a vertical position ensures a homogeneous airflow across all the radiator surface, thus better cooling capacity. In the other cars, the optimal configuration is to send air perpendicular to the radiator, for what the design of the air ducts needs to ensure such orientation for optimal cooling.

In all cars, the air exit is done through the car rear door vent, and the air is discharged into the area below the rear wing and above the rear diffuser, at the center of the car wake. In a normal car, in this area, there is some air recirculation, which contributes to increasing the wake size.

In a RX Supercar with rear radiator, the airflow leaving the car through the rear exit brings some additional energy into the car wake, allowing to remove faster the flow coming through the diffuser (similar to the exhaust blown diffuser concept). At the same time, it contributes to remove or to reduce the recirculation region. The final result is a shorter wake. As the wake is responsible for a significant proportion of the aerodynamic drag, the shorter the wake the lower the generated drag. So, the use of a rear radiator has the additional advantage of significantly reducing the car drag.


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