Aerodynamics and Air Routing
Aerodynamics and Air Routing
One objective in developing the new vehicle was to further expand the competitive lead of the Boxster/S (987) in the area of aerodynamics and that was to focus on reducing the aerodynamic lifting forces to enhance performance. Another aim in this connection was to achieve a further reduction in the drag coefficient, despite the high levels of cooling air required for the brakes and engine.
The main challenges to be faced in enhancing the aerodynamics of the new Boxster series were:
- The larger frontal area, caused by flared wheel housings and in particular the wider tires
- The lower lift coefficients and therefore the increased wheel load
- The higher cooling air requirement of the brakes due to the increase in performance
- A more efficient cooling system required on account of the increased engine power
The development objectives have been attained in particular by:
- Aerodynamic optimization of the styling
- Optimization of the cooling-air guide
- New underbody paneling
- Optimization of add-on parts
The result of the efforts to optimize aerodynamic is a marked improvement in aerodynamic coefficients in comparison with the previous Boxster/S (986) models. The drag coefficient has been reduced to cw = 0.29 on the new Boxster (987) (previous Boxster 0.31) and to cw = 0.30 on the Boxster S (987) (previous Boxster S 0.32), while the lift coefficients at the front and rear axles of the Boxster and Boxster S have been reduced to cAV = 0.09 and cAH = 0.07 respectively. These are unique, exceptional levels which are unrivalled by any competitors in the international arena.
Optimization of the Outer Skin
In redesigning the exterior, the goal was on reconciling stylistic and aerodynamic requirements. The main starting points when it came to optimizing the aerodynamic outer skin, apart from the front and rear fenders the vehicle, were the fender and rear quarter panel sections. Particular attention had to be paid to aerodynamic integration of the flared wheel housings.
The sweep and the radii of the front apron have been optimized to produce an aerodynamically efficient air flow at the front end and significantly better prerequisites for cooling air flow without affecting the styling of the outline. The lateral openings on the front apron required to supply cool air to the radiators have also been carefully optimized and integrated in the front end concept.
The lateral front apron radii and wheel housings are designed to shield the front wheels from the air flow in an optimum manner and to provide much greater wheel housing ventilation. These measures reduce drag and, by providing greater wheel housing ventilation, also reduce lift at the front axle.
The shape of the A-pillar has also been optimized in terms of flow characteristics and is now more vaulted. This reduces resistance and cuts down on wind noises at high speeds for noticeably more comfort on long trips.
The door mirror has been redesigned and is now connected to the mirror triangle via a new double arm. The mirror housing and the air duct at the mirror and the side windows have been optimized for minimum resistance and maximum protection against drops of water landing on the mirror glass and the side windows. In addition, a new hydrophobic surface coating which virtually eliminates soiling of the side windows is being used for the first time on the side windows.
In the rear area, the flared wheel housings, the retractable spoiler and rear end have all been optimized. Like the front, the radii of the rear side sections have been designed to produce an aerodynamically efficient air flow at the rear wheels and the rear end, which improves resistance and reduces lift at the rear axle.
The increased extension height of the retractable spoiler ensures that the spoiler functions at the optimum aerodynamic operating point and that the desired rear axle lift is attained with the optimum resistance advantage.
The redesigned air inlet in the rear quarter panel takes up the graceful geometric lines of the Carrera GT. The positioning and the opening cross section of the air inlet have been carefully optimized to ensure the maximum possible increase in air flow rate combined with the minimum possible effects on lift and resistance. This way, it has been possible to improve the flow of air through the engine compartment, to lower the temperature in the engine compartment and to optimize the intake of combustion air.
Optimization of the Cooling Air Guide
The main objective when designing the cooling air guide is to ensure the necessary cooling air requirement for the engine and brakes in all operating states of the engine. The general idea behind both cooling concepts is to achieve a flow with as little resistance as possible to minimize the effect of the cooling air flow on the resistance coefficient of the vehicle as a whole. The effect of the flow on the lifting forces must also be reduced or used as cleverly as possible to achieve the objectives for lifting forces and lift balance.
The higher output of the 3.2 liter engine of the Boxster S (987) results in more engine waste heat, increasing the cooling air requirement by approx. 15 %. In addition to using larger and more efficient radiators, the optimized cooling air guide is a significant factor in the increased cooling performance. As on the previous Boxster (986) model, the new cooling air guide is completely closed. This prevents leakages, and at the same time, effective guidance of the cooling air ensures an optimum air flow for the radiators. When designing the cooling air guidance, particular attention was paid to keeping the air ducts short and the deflection as low as possible. The exhaust air flow from the radiators is therefore now expelled laterally into the wheel housing liners instead of just vertically downwards in front of the front wheels.
This reduces flow loss for the cooling air guidance system while lateral expulsion of the air reduces the lift at the front axle. Another benefit of lateral expulsion is that it results in much less dust being raised when the radiator fan is running on dusty surfaces. Ventilation flaps have been inserted into the corners of the square fan frames to further increase air throughput while underway. These ventilation flaps open above speeds of approx. 43 mph (70 km/h) to facilitate an additional flow of cool air. Despite the increased cooling air throughput, the new resistance optimized air routing concept has enabled the air flow resistance to be limited to approx. 2.5 % of the total resistance - an extremely low level in comparison with the competition. Requirement driven supply of cool air has enabled significantly smaller air inlet openings in the front end in comparison with the competition, which fits in well with stylistic requirements.
The improved driving performance has also resulted in increased requirements for brake cooling. An optimized brake air deflector has been developed for the Boxster/S. This ensures more efficient deflection of the air routed through the brake air ducts on the underbody to the brake discs and significantly improves brake cooling as a result of the higher air throughput.