Minimising the Cooling System Drag for the New Porsche 911 Carrera Thomas Wolf Dr. Ing. h.c. F. Porsche AG, Stuttgart, Germany

ABSTRACT

The new Porsche 911 Carrera went into production in Summer 2004. Both the drag coefficient and the lift coefficient of the new 911 have been reduced significantly as compared with the predecessor car. One of the main reasons of the successful reduction of the drag coefficient by CD=0.02 is the low cooling-air drag by which the 911 is distinguished. The present paper describes the layout and development of the optimised cooling-system concept of the new 911, which is essentially based on the cooling concept of the highly successful predecessor model. Following the brief description of the aerodynamic features and cooling requirements of the new 911, the current know-how regarding the layout of low-drag cooling air concepts is briefly addressed. Then, the layout philosophy, development and optimisation of the cooling-air concept of the new 911 are described in detail. Finally, a summary of the results obtained as well as the outcome of a benchmarking campaign are presented.

1 THE NEW PORSCHE 911 CARRERA

Among the major engineering goals of the new Porsche Carrera 911 number outstanding driving performances and excellent driving dynamics (Figures 1 & 2). The significantly improved aerodynamic performances (Table 1) make an important contribution to achieving these goals.

Figures 1 & 2: The new Porsche 911 Carrera (CD=0.28)

911 Carrera Pe [hp] CD CLf CLr Af [m2] CD x Af [m2] MY 1997 300 0.30 0.08 0.05 1.95 0.59 MY 2002 320 0.30 0.06 0.03 1.95 0.59

911 325 0.28 0.05 0.02 2.00 0.56 MY 2005 911 S 355 0.29 0.05 0.02 2.00 0.57

Table 1: Aerodynamics of the water-cooled 911 models

Fig. 3: Drag & lift coefficients of the new 911 Carrera and its competitors (all data determined in the full-scale Porsche wind tunnel under static conditions)

Figure 3 compares the aerodynamic coefficients of the new 911 Carrera and some of its competitors. As can be seen, the aerodynamic evolution of the 911 series has

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been successfully continued: With a drag coefficient of CD=0.28 for the basic version the new 911 has reconquererd the top position in its market segment. The improvement of the drag coefficient by CD=0.02 compared to its predecessor is the result of thorough aerodynamic optimisations which were mainly focussed on the outer shape, the almost 100% enlargement of the aerodynamic undershield and the extremely low cooling-air losses compared with those of the cars competitors.

2 DEVELOPMENT TARGETS FOR THE 911 COOLING-AIR SYSTEM

2.1 Cooling drag situation of the 911 The cooling and cooling-air systems of the new 911 are based on the proven concept of the highly successful predecessor model which was first launched in 1996. The two water coolers of the rear-engine-powered 911 are placed beneath the front fenders at an angle of 45 to the longitudinal axis of the car. This configuration allows relatively large radiators to be used (Fig. 4). In the predecessor model, the cooling-air outlets are in the underfloor located laterally ahead of the front wheels. The outlet air from the radiators is guided vertically downwards towards the road surface (Figure 5).

Fig. 4: Radiator arrangement and Fig. 5: Sketch of the cooling-air system radiator size of the 911 (MY 1997) of the first water-cooled 911 (MY 1997) With a cooling drag of CDc=0.002-0.003 the first water-cooled 911 (MY 1997) has almost zero cooling-system drag. This low drag level mainly results from the vertical downward deflection of the outlet air ahead of the front wheels, which acts like a virtual wheel spoiler and improves the flow conditions about the wheels [1]. The resulting positive intereference effect almost completely compensates for the overall-drag contribution resulting from the internal drag. The predecessor 911 MY 2002 has a cooling drag of about CDc=0.006 which is slightly higher than that of the first water-cooled 911 MY 1997. This results from the roughly 15% higher cooling-air mass flow rate which is necessary to cope with the higher cooling demand of the 320 hp engine. The cooling-air system of the 911 MY 2002 is basically the same as that of the 911 MY 1997. The increase of mass flow rate was achieved solely by optimising the front air-inlet area and inlet ducts. As the rest of the cooling-air ducts are the same for both

models the interference situation, too, is supposed to be unchanged. Thus it can be assumed that the cooling drag increase in the 911 MY 2002 is primarily due to the higher internal pressure losses caused by the higher mass flow rates through the radiators. Figure 6 compares the cooling drags and mass flow rates of the 911 MY 1997 and MY 2002. As can be seen, there is a clear trend: Quite obviously, the cooling drag increase is proportional to the increase of the cooling-air mass flow rate.

Fig. 6: Extrapolation of the cooling drag for the new 911 based on the predecessors At the beginning of the project, the trend illustrated in Figure 6 was used to roughly estimate the cooling drag to be expected in the new 911. Based on the anticipate

engine-power increase by about 11% and the experience gained with the predecessor models the required increase of the cooling-air mass flow rate was estimated at about 21% (Table 2).

Table 2: Estimation of the cooling air demand of the new 911 Figure 6 shows that a further increase of the cooling-air mass flow by 21% relative to the predecessor with unchanged cooling-air concept would have led to a cooling drag of about 10 to 11 drag counts, corresponding to an increase by 4 to 5 drag counts. This was definitely not acceptable with regard to the ambitious aerodynamic goals defined for the new 911.

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Therefore a limit of CDc=0.005 was defined for the cooling drag of the new 911 and stipulated in the Specifications. This meant that the cooling drag of the 911 had to be reduced even below the drag level of the predecessor model. The situation illustrated in Fig. 6 had made it clear that it would not be possible to adopt the cooling-air concept of the predecessor unchanged and that considerable modifications would have to be carried out. The primary engineering target, however, was to guarantee a sufficiently high cooling capacity regardless of the driving condition. For sports cars, circuit racing and high-speed driving are the most relevant operating modes with the highest loads on the cooling system occurring during blower-assisted operation on racing tracks. During aerodynamically relevant high-speed driving, the demand for cooling is considerably lower. The different cooling-capacity and cooling-air demands under those operating conditions had to be taken into due account. And in order to realize a most efficient and demand-adapted cooling-air concept it was important not to overdimension the cooling-air requirements either. 2.2 Optimising the cooling system for low drag Regarding the 911, the question was how to increase the cooling-air mass flow while reducing the cooling drag. Let us first have a look at the theoretical background:

The cooling drag and cooling-air flow rate are dependent on a lot of parameters which are shown in Figure 7 taking the example of a closed cooling-air ducting. Besides the air flow about the vehicle the most important of these parameters for a given inlet area Ai are the following: the radiator core area Ac, internal losses (), driving differential pressure (CpO) and the cross-section of the cooling-air outlet AO.

Fig. 7: Parameters relevant for cooling drag and cooling-air flow rate When considering the cooling drag CDc it must be kept in mind that it is basically composed of two elements: the drag due to internal flow resistance and the interference drag: , , = + Dc Dc ITD Dc IFD

Internal drag Interference drag

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Since the interference drag can be both positive and negative - with the latter being welcome - there are two main approaches to optimise the cooling drag:

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Minimising the internal drag Making use of the interference effects, i.e. trying to realize the highest possible

negative interference drag

Low internal pressure losses and specific influencing of the interference drag can only be obtained with a strictly ducted cooling-air system with air inlet and outlet ducts as illustrated in Figure 7. Interferences of the cooling air flow with the air flows about the car can occur anywhere - i.e. in the front-end region as shown here but also in the wake of the car. In the past, little consideration was given to the fact that the interference drag can have a considerable influence - and be negative at that. According to the results of pertinent investigations at Porsche, the interference drag can reach almost the same order of magnitude as the internal drag. Negative interference effects occur in nearly all Porsche sports cars and make a significant contribution to the low overall cooling drag levels of the 911 model series (see Fig. 21). It is very difficult, however, to specifically influence or predict such interferences. While the interference contribution eludes even the most simple kind of formulation, it is possible to formulate rather uncomplicated theoretical approaches at least for the internal drag and the cooling-air volume flow rate, which clearly illustrate the influence of the relevant parameters (see also Appendix A):

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As can be seen, the internal drag is influenced by the volume flow raised to the power of three, the radiator area raised to the power of two and the pressure loss coefficient. Thus formula (2) clearly underlines the priorities to be observed during optimisation1:

Lowest possible cooling-air volume flow rate ( cV ) Large radiator core area ( cA ) Low internal pressure losses ( )

1 According to equation (2) the internal drag increases by 1.21 = 1.772, i.e. by 77%, if the volume flow rate is raised by 21% with otherwise unchanged parameters. Based on the 911 MY 2002 value of CDc=0.006 this means a cooling drag increase by about 0.005 to CDc = 0.011. Thus the experimental trend shown in Figure 6 is clearly confirmed by theory.

So, the internal drag is influenced - with increasing priority - by internal losses, the radiator core area and mainly by the cooling-air flow rate. The cooling-air volume flow itself, on the other hand, depends on the internal losses, the radiator front surface and - additionally - on the driving differential pressure (1-CpO ) and the outlet cross-section AO. Therefore, the relationships of equations (2) and (3) for the internal drag and cooling-air volume flow rate cannot be treated separately. The question is, whether there are combinations of individual parameters which, according to equations (2) and (3), result in a higher cooling-air volume flow rate while reducing the internal drag at the same time. Based on the data of the MY 2002 model, corresponding parameter variations were performed which are given in Appendix A. The results obtained confirm the afore-mentioned theory. Fig. 8 shows some of the most efficient parameter combinations.

Fig. 8: Parameter variation based on the 911 MY 2002 model As the results in Appendix A show, the greatest benefit for the internal drag with smallest possible penalty for the cooling-air volume flow is obtained by reducing the outlet cross-section2. According to Fig. 8, for example, the volume air flow can be increased by 5 to 15% by reducing the air outlet area AO by 50%, the pressure loss coefficient by 50% and the pressure coefficient at the outlet to CpO= -0.25 or CpO= -0.50 respectively. At the same time, the internal drag drops by 47% to 26%. Any increase of the radiator core area - as small as it may be - will further improve the situation because the cooling capacity increase is directly proportional to the enlargement of the radiator core area and the heat transfer is more efficient with low internal flow velocities [3]. The main finding, however, has been that the size of the outlet cross-section AO is of decisive importance for the attainment of the targets. 2 Similar conclusions were derived in Ref. [2] based on the momentum approach.

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2.3 Development targets The further goal was to decrease the cooling drag of the 911 to the target level by consistently applying the layout rules worked out above. Thus the following targets were specified for the cooling air system in a very early phase of the development project:

Cooling drag CDc 0.005 Maintaining the favourable interference effects Minimising the increase of the cooling-air mass flow rate (max. 5-10%) Significantly reducing the outlet cross-section of the cooling air duct and

making use of the post-acceleration effect [4] in order to increase the momentum recovery

Minimisation of the internal pressure losses (installation drags, turning losses and leakages)

Increase of the radiator core area by approximately 4% Use of an improved radiator whose efficiency is at least 5% higher Increase of the fan drive power by about 25% and installation of an electronic

speed control system allowing the cooling-air mass flow to be precisely adapted to the requirements mainly during operation on race circuits.

Maintaining the size of the air inlets in the car front for styling reasons Possibility of adapting the cooling-air mass flow to the requirements by varying

the outlet area The purpose of the thermodynamically more efficient radiator was to additionally minimise the increase of the cooling-air mass flow. The main target, however, was to control the cooling drag and the cooling-air mass flow via the outlet cross-sections of the cooling-air ductings. It is obvious that a drag-relevant throttling effect at the cooling-air outlet requires a cooling-air duct with minimal leakages. 3 EXPERIMENTAL DEVELOPMENT AND OPTIMISATION 3.1 Preliminary concept layout As investigations of the predecessor model had shown, it was not possible to increase the air flow rate while maintaining the vertically downward exhaust without significantly increasing the cooling drag [5]. This is due to the fact that for space reasons the 90 downward flow deflector in the predecessor model is located immediately downstream of the radiator. The pressure increase caused by the flow deflection extends even in the radiator area. This means that the throttling area which restricts the air-flow rate in the predecessor model is not located in the cooling-air outlet at the underfloor but in the deflecting scoop directly downstream of the radiator. Of course, it would be possible to considerably increase the air-flow rate by unthrottling this area - e.g. by cutting or completely omitting the air scoop. However, such measures would impair the positive interference between the outlet air and the air flow about the wheels, as the cooling-air speed at the underfloor outlet which is required to maintain the interference effect would drop considerably as a result.

As far as the new 911 was concerned, it was clear that an alternative air outlet in the wheel house area would have to be provided for, allowing to unthrottle the air flow downstream of the radiator and to control the air flow rate and internal drag via the outlet area. However, earlier investigations with air outlet openings in the wheel houses carried out within the scope of the predecessor development used to result in strongly increased cooling drag and overall drag coefficients. From an aerodynamic point of view, the question was which outlet concept would allow similar positive interference effects to be reached as with the predecessor, i.e. at least how to avoid negative interferences with a cooling-air outlet into the wheel house. To answer this question, three different outlet variants were tested, which are illustrated in Figure 9 below.

Variant 1 (Basis) Variant 2 Variant 3

Fig. 9: Concept variants examined in the early project phase The first variant is based on the predecessor concept and was used as a starting point. The second (so-called combined) variant was provided with an additional air outlet into the wheel house. With the third variant, the lower outlet was closed leaving only the air outlet into the wheel house. 3.2 Testing and Optimisation Preliminary basic investigations with alternative cooling-air outlets were carried out in a very early phase of the project using a 1:4-scale model with a radiator simulator. The results showed similar lift and drag effects as in the original full-scale 911. Subsequent tests with various front ends and air outlet variants furnished valuable information about the cooling drags and interferences obtained with the styling and outlet variants examined. The first full-scale investigations with the three basic concepts as shown in Figure 9 were carried in the early concept phase using a predecessor model [5]. The results showed that the mass flow target could be met, but none of the variants reached the target for the cooling drag. However, with variant 3, it was possible - as expected - to efficiently control the cooling-air mass flow by varying the cross-section of the air outlet into the wheel house. In the following, the outlet-air concepts were therefore examined in more detail. These investigations were carried out in close cooperation with the thermodynamics experts in order to obtain direct information about the cooling potential achievable

with the variants under examination. Wind tunnel measurements were performed in order to determine and evaluate the cooling drag and air flow rates. With regard to the thermodynamics, driving tests were carried out with the aim of evaluating the alternative air outlet configurations. The test vehicle used was a predecessor model fitted with a tuned-up engine and new sample radiators. The air outlet variants were examined under both circuit-racing and top-speed conditions. The first evaluation criterion checked was the cooling-water temperature. The temperature changes and measured cooling-air flow rates were used as input data for mathematical analysis based on a thermal model. Figure 10 sums up the most important results obtained with the basic system and the two main variants.

Fig. 10: Wind-tunnel and driving-test results obtained with the air-outlet variants All variants shown in Figure 10 were fitted with the tuned-up engine and new sample radiators. Variant V1 which featured the cooling-air concept of the predecessor clearly reflected the potential of the new radiators: Despite the tuned-up engine and the lower cooling-air flow rate the coolant temperatures were practically identical with those of the predecessor. As can be seen in Figure 10, the greatest potential with regard to cooling-air flow rates and cooling capacities was offered by the variant V2 with the largest possible downward air outlet and the largest outlet into the wheel house. On the other hand, this variant had the largest penalty for cooling drag. However, from the results yielded by the other variants it was concluded, that the cooling drag targets can be reached by reducing the cooling-air mass flow increase relative to the predecessor to about 10% even though the coolant-temperature target is still slightly exceeded.

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Altogether, the results obtained were rather promising and confirmed that the cooling-air outlet into the wheel house would allow a sufficiently high cooling capacity to be realized. In addition, the measurements delivered a simple correlation between the change of cooling-air mass flow and the change in cooling-water temperature. This correlation was used to assess the subsequent wind tunnel tests. For the aerodynamic tests performend after the concept phase, a full-scale through-flow concept vehicle whose outside panelling was continuously updated to the respective styling status was used (Fig. 11). The concept vehicle consisted of the basic structure of a stripped predecessor model equipped with a GFRP outside panelling corresponding to the respective styling status. The through-flow vehicle is a full vehicle equipped with radiators and air ducts for engine cooling, engine-compartment cooling, brake cooling etc. Along with the 1:4-scale and 1:3-scale models it is Porsches main aerodynamic test device which has yielded excellent results so far. The cooling-air mass flows are measured by means of radiators fitted with cylindrical pressure sensors previously calibrated on a flow test bench. The procedure [6] uses the static pressure difference across the radiator as a basis (Fig. 12) and allows the volume flow rate to be measured with a relative precision of about 3% [7].

Fig. 11: 911 through-flow concept Fig. 12: Porsches method for measuring vehicle the mass-flow through radiators The subsequent investigations were aimed at further optimising the cooling-air mass flow and drag of the air-outlet variants favoured in the preliminary phase with the help of the concept vehicle and to adapt them to the respective styling status. It had to be made sure in particular, that the front-end air inlet openings designed by the styling department were sufficiently large. During the fine-optimising phase, more than 30 variants of the lower and wheel-house air outlets with different dimensions and configurations were submitted to wind tunnel tests. The definite configuration chosen was the combined variant with downward air outlet and the outlet into the wheel house as it offered the greatest scope for subsequent optimisation. Another beneficial measure was to use pressure flaps in the fan shroud

which unthrottle the air flow downstream of the radiator and produce homogeneous flow conditions about the radiator. As far as thermodynamics were concerned, the air outlet variants were tested in one of the first 911 prototypes. As of that stage, the primary evaluation criterion was the absolute coolant temperature which was not allowed to exceed given limits. Experience has shown that by the end of a project the coolant temperatures frequently increase unexpectedly as a result of final modifications to the engine management system. It was therefore decided at the start of the project to keep a safe 5 degree distance from the thresholds. Following the Styling-Freeze of the outer shape the fine tuning in the wind tunnel could be tackled: The outlets leading the air downwards into the wheel house were closed one after the other. The minimum air flow rate required to remain within the given temperature limits was known from the correlation between the air flow rate in the wind tunnel and the coolant temperature in the driving test. Thus, the individual measures taken could be precisely evaluated. Figure 13 summarizes the influence of these measures on the air-flow rate and the cooling drag.

Fig. 13: Fine tuning of the combined air-outlet variant in the wind tunnel As illustrated in Figure 13, the cooling-air drag drops proportionally to the cooling-air volume flow rate. For the initial status with maximum downward air outlet and maximum outlets into the wheel house the cooling drag is CDc=0.009. Closing the downward air outlets reduces the cooling drag by CDc=-0.002. By gradually closing the remaining air outlets leading into the wheel house (from the vehicle center line to the outside) leaving about 50% of the initital outlet area it is even possible to lower

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the cooling drag to less than the specified limit of CDc=0.005 with the cooling-air flow rate target derived from the correlation being only just reached. The 50% outlet-variant with closed downward outlet which was developed in the wind tunnel was subsequently submitted to corresponding driving test which fully confirmed the afore-mentioned results at all operating points.

4 FINAL COOLING AIR CONFIGURATION

4.1 Optimised cooling air concept

Figures 14 and 15 show the final cooling-air ducting as it has been implemented in the production car. It consists of the front end with its air inlet openings, air duct, condenser, radiator, fan shroud with pressure flaps and fan, air outlet duct into the wheel house and the wheel-house liner with integrated outlet opening into the wheel house. The former downward outlet still exists but is closed from below by the wheel-house liner. The cooling-air system is an extremely compact unit. The air flow overcomes the short and strongly enlarging section between the air inlet in the front end and the radiator (Ac/Ai 4) almost without any flow separations because the radiator and condenser drags favour the expansion of the air flow, i.e. the inlet duct and the radiator/condenser unit act as a wide-angle diffuser with back end resistance.

Fig. 14: Cooling air duct exploded view Fig. 15: 911 MY2005 radiator arrangement Fig. 16: Wheel-house outlet and undershield Fig. 17: Wheelhouse outlet (backside)

Figure 17 shows an enlarged view of the 50% air outlet into the wheelhouse. As can be seen, the outlet air is horizontally deflected to the exterior at an angle of approximately 45 by corresponding deflector fins. This configuration improves the lift conditions at the front axle and has been used to comply with the targets in terms of lift and lift balance. To minimise the thermodynamic engineering risks before SOP, the achievable maximum wheelhouse outlet opening was maintained. The aerodynamically favourable 50%-outlet is realized by closing half of the openings (gaps) by means of a thin plastic foil. The closed gaps are relatively easy to reopen. This allows maximum scope for trouble shooting and offers a sufficiently great cooling-air reserve for further engine tuning during subsequent model updating.

Fig. 18: Evolution of the cooling drag from the concept phase to SOP Figure 18 illustrates the evolution of the cooling drag versus the engineering period and up to SOP. At the beginning of the project, there is a large scatter to be seen which results from the many concept variants examined. As the project progresses the cooling drag took shape which was forecast by means of the through-flow concept vehicle. Fig. 19: Scatter of 911 cooling drag Fig. 20: Cooling drag of an individual car

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Figure 19 shows the cooling drags measured with a total of 30 production cars. The average of these measurements is CDc=0.004-0.005 with a scatter of 0.001 to 0.007. This scatter results from the assembly tolerances and the measuring accuracy in the wind tunnel. It must be kept in mind, however, that the variations of the 911 cooling drag are in the millesimal CD range. In Figure 20 the cooling drag for a particular car has been plotted versus the wind velocity. To obtain this plot, a velocity polare was carried out with the air inlets opened and closed, respectively.

4.2 Interference effects

When dealing with the cooling drag, distinction must be made between two different interference effects: The most important one is the interference between the cooling air flow and the air flow about the car which results in an additional - either positive of negative -interference- related drag contribution. As already mentioned, interferences can contribute considerably to the overall cooling drag level and be used to the advantage of the system. The low cooling-air losses of the Porsche production cars in particular are largely due to negative interference drags. Figure 21 shows the individual drag contributions in the new 911 and its predecessors.

Fig. 21: Drag contributions in the new 911 Fig. 22: Determination of the internal and its predecessors losses by means of a hydraulic pressure test The interference contributions in the various cars were determined according to equation (1) by subtracting the internal drag portion from the overall cooling drag measured in the wind tunnel. The internal drag was determined with the help of equation (2). The velocity ratio Vc/V and the pressure loss coefficient were measured in corresponding wind tunnel tests and on a flow test bench by pressure-testing the complete front-end (Figure 22). In order to calculate the drag coefficients, both the cooling-air mass flow and the overall total pressure difference between the cooling-air inlets and outlets of each individual car were measured. The second interference effect depends on the way in which the cooling-air losses are measured in the wind tunnel. Usually, the air inlets in the front-end are closed. The cooling drag then corresponds to the difference between the drag coefficients obtained with the inlets in open and closed condition. Recently, the question has

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arisen, whether this method is really suited to determine the cooling drag correctly. In the past already, it has been pointed out that the surprisingly low cooling-air losses of quite a number of cars might be due to front-end interferences [4]. Such an interference can be caused by a flow separation which occurs at the front end - e.g. above the hood - if the air inlet openings are closed and which does not appear if the the inlets are open. This means that the CD-value measured with the inlets closed is too high and that the cooling drag determined on that basis is too low, i.e. it seems to be low but in fact it is not. To date, it has not been easy to prove that such effects occur in wind tunnel testing. The existence of such front-end interferences was detected quite by chance some years ago when examining a competitors car in Porsches wind tunnel. And it was also found out that these interferences can be of considerable magnitude. The car in question was tested using the standard method when all of a sudden a negative cooling-air loss occurred, i.e. the drag coefficient increased when the inlets were closed. The reason of this low cooling drag was supposed to be the above-mentioned front end interference. The question was, how to measure and confirm this phenomenon. To this end, a simple method was developed based on the following assumptions: If the front-end interference really exists it must be caused as explained above. In that case, the drag level measured with the inlets closed includes and is unduly increased by the drag portion resulting from flow separation. In order to determine the actual cooling drag increase without this separation-dependent share, it must be made sure that the separation occurs also when the inlets are open. To this end, the front inlets of the car in question were fitted with circumferential, sharp 50-mm-high separation edges3 as shown for the 911 in Figures 23 and 24.

Fig. 23 & 24: Measurement of cooling drag with separation edges (911 MY 05). 3 Due to their very typical look, these separation edges have been called fish-mouth by Porsches aerodynamics experts. Accordingly, the aerodynamic test is called fish-mouth test.

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Fig. 25: Influence of separation edges (fish-mouth) on the cooling drag of various

911 models and a competitor car. Figure 25 illustrates the impacts of this measure on the competitor car in question and on some 911 models - including the 911 MY 2005 - examined in the same way. As can be seen, the effects measured with the competitor car are of a large magnitude, whereas the influence on the cooling drag of the 911 models is insignificant. With no separation edges, a negative cooling drag of CDc= -0.001 is obtained. With separation edges, by contrast, there is a positive cooling drag of CDc=0.014. This is precisely the magnitude one would expect from the theoretical approach in equation (2). So, this result confirms that the car in question actually shows a front-end interference. Of course, the separation-edge or fish-mouth test described above is no absolutely perfect method as the separation edges might also have an effect on the pressure conditions and air flow about the cars. However, it indicates whether there is a distinct front-end interference or not. In addition, it allows the true cooling-air drag to be at least approximately measured.

5 COMPARISON WITH PREDECESSORS AND COMPETITORS

Figure 26 compares the 911 production status with the predecessor models. In comparison with the direct predecessor MY 2002 the cooling air mass flow rate could be increased by about 10 to 15%, while the cooling drag was reduced to CDc = 0.004 for the 911 basic model and to CDc=0.005 for the 911 S. This increase of the cooling air mass flow rate was achieved inspite of the significant 60% reduction of the cooling-air outlet area when compared with the predecessor MY 2002, see Figure 27.

Fig. 26: Cooling drags and cooling air mass flows of the new 911 and its predecessors

Fig. 27: Cooling-air duct areas of the new 911 and its predecessor Figure 28 compares the cooling-air efficiencies of various models of the 911 family. The efficiency criterion used is the cooling drag related to the velocity ratio =VC/V . With the cooling-air velocity VC resulting from the cooling-air volume flow and radiator area, the latter is implicitly taken into account. Of two cars, having identical cooling drags and cooling-air volume flows, the car which does with the smaller radiator core area is the most efficient one.

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5

6

7

8

9

10

Coo

ling

drag

CD

c *1

03

Mass flow rateCooling drag

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Streamline coordinarte counted from inlet position [m]

Cro

ss s

ectio

nal a

rea

A A

911 MY 2002911 MY 2005

Cooler

OutletInlet

010

20

30

40

50

Boxster MY 1996

Boxster MY 2001

911 CarreraMY 1996

911 Turbo MY 2000

911 CarreraMY 2002

911 CarreraMY 2005

Competitor400hp

Rel

ativ

e in

let a

rea

Ai /

Ac

[%] Usual design rule: Ai /Ac = 40 %

Fig. 28: Efficiencies of the cooling concepts of the 911 model family versus a competitors car

From Figure 28 it can be seen that the efficiency criterion allows the various cooling- concepts to be more easily compared: As can be seen, the new 911 yields approximately the same cooling air efficiency as the first water-cooled 911 MY 1997, whose cooling drag level of about 0.002 is still setting the standard today against which all the other Porsche cars and their competitors have to match themselves.

Fig. 29: Relative inlet cross-sections of various Porsche cars and a competitor

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

911 MY 97 911 MY 02 911 TurboMY 02

911 MY 05 911S MY05

Competitor400hp

Coo

ling

drag

rela

ted

to v

eloc

ity ra

tio C

Dc /

= Vc / V

Another criterion - though mostly a stylistically relevant one - is the size of the inlet openings in the front end. Big inlets which are not necessarily negative from an aerodynamic point of view are often rejected for styling reasons. Other automotive manufacturers apply the 40% rule - i.e. the target size of the air inlet areas is 40% of the radiator core area. By contrast, clearly smaller inlet areas can be realized by providing for efficient cooling-air concepts, see Figure 29.

Fig. 30: Cooling drags of the new 911 and its competitors In Figure 30, the cooling drag of the new 911 is compared with those of various competitors. All cars were tested under static conditions in Porsches full-scale slotted-wall wind tunnel. As can be seen, the new 911 shows by far the lowest cooling drag.

6 CONCLUSIONS

One of the major engineering challenges in the aerodynamic development of the new 911 was to realize the required increase of the cooling-air volume flow rate while preventing the cooling drag from growing. To solve this problem, the air ducting concept of the predecessor with vertically downward cooling-air outlets ahead of the front wheels had to be given up. Under strict observance of the given theoretical layout principles, an alternative low-drag air-ducting concept with air outlets opening into the wheel houses was developed. The aerodynamic part of the production-car development was performed in the wind tunnel with the help of conventional methods. The test car used was Porsches service-proven full-scale through-flow concept vehicle. The entire development

0.00

0.01

0.02

0.03

0.04

0.05

100 200 300 400 500

Engine power Pe [hp]

Coo

ling

drag

C

Dc

911 MY 05 911S MY 05

process drew benefit from the close interactions between the aerodynamic concept layout in the wind tunnel and the thermodynamic driving tests. The optimised cooling-air concept of the new 911 covers all of the highly differing demands of circuit racing and high-speed driving. Inspite of the roughly 15% increase of the cooling-air flow through the radiators, it has even been possible to reduce the cooling drag level of the new 911 to below that of the predecessor model. The low cooling-air losses of the 911 are primarily due to the integrally designed cooling-air ducting from the inlet to the outlet openings, the large radiator areas, low internal pressure losses and mainly to the control of the cooling-air mass flow rate and cooling drag through the clearly reduced outlet cross-sections. Similar to all water-cooled variants of the 911 family, negative interference drags, too, are contributing to the low cooling drag of the new 911. With a share of about 1.4% in the overall drag, the cooling drag of the new 911 is one of the lowest among its competitors.

7 TERMINOLOGY

A area Ac radiatior core area Af projected frontal area Ai duct inlet area AO duct outlet area CD vehicle drag coefficient, CD=D / (V2/2 Af) CDc drag coefficient contribution due to the cooling system CDc measured cooling drag, CDc = CD,inlets open - CD,inlets closed CDc,IL cooling drag due to internal pressure losses CDc,IF interference drag CpO static pressure coefficient at outlet CL total lift coefficient CLf lift coefficient front axle CLr lift coefficient rear axle D aerodynamic drag force DC aerodynamic drag due to the cooling system GFRP glass-fibre reinforced plastic MY model year p static pressure pt total pressure P power Pc power to overcome the cooling losses Pe engine power Tc coolant temperature

cV cooling air volume flow rate Vc radiator normalizing or core speed VO outlet air speed V free stream air speed (synonymous with car speed on road) exit-flow inclination angle to vertical

air density pressure loss coefficient, =pt / (Vc2/2 ) velocity ratio, =Vc / V

8 REFERENCES

[1] PORSCHE AG: Kraftfahrzeug mit einer aufbauseitig angeordneten Luftfhrungsvorrichtung. (Road vehicle equipped with cooling air ducting). German Patent Office, DE 3530494 A1, 1987. [2] BARNARD, R.H.: Theoretical and experimental investigation of the

aerodynamic drag due to automotive cooling systems. Proc. Instn. Mech. Engrs. Vol. 214, Part D, ImechE, 2000.

[3] BOSNJAKOVIC, F.: Technische Thermodynamik (Technical Thermodynamics)

1. Teil, 5. Auflage, Seite 474-475, Verlag Theodor Steinkopff, Dresden, 1967. [4] POTTHOFF, J.: Luftwiderstand und Auftrieb moderner Kraftfahrzeuge. (Drag

and lift of modern road vehicles). Proc. First Symposium on Road Vehicle Aerodynamics, Edited by A. J. Scibor-Rylski, The City University, 6 & 7 November 1969 (MIRA Translation No. 40/71).

[5] DEL GAIZO, C.: Entwicklung einer widerstandsarmen Khlluftfhrung durch Optimierung der Khler-Abluftseite. (Development of a low-drag cooling air

ducting by optimising the cooler exit flow). Diploma thesis, Stuttgart University / Porsche AG, 2001.

[6] PORSCHE AG: Verfahren und Vorrichtung zur Bestimmung von

Massenstrmen gasfrmiger Medien durch Wrmetauscher. (Method and device to determine the mass-flow rate of gaseous media flowing through heat exchangers), German Patent Office, DE 39 16 529 A1, 1990.

[7] BRAUN, T.: Vergleich und Bewertung verschiedener Messverfahren zur

Ermittlung des Luftdurchsatzes bei Khlluftstrmungen. (Comparison and validation of different measuring techniques to determinate the air-flow-rate in cooling air flows). Diploma thesis, FH Coburg / Porsche AG, 1999.

[8] KCHEMANN, D. und WEBER, J.: Aerodynamics of Propulsion, McGraw-Hill

Book Company, Inc., New York, 1953, p. 281. [9] SCHMITT, H.: Leistungsbedarf zur Khlung des Fahrzeugmotors und seine Verminderung. (Power demand required for engine cooling and its reduction).

Deutsche Kraftfahrtforschung, Heft 45, VDI-Verlag, 1940.

-70

-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

70

Cha

nge

of d

rag

or fl

ow ra

te [%

]

Volume flow rate

Cooling drag

Parameter +50% CpO, AO, Ac,

Parameter -50% CpO, AO, Ac,

CpOCpO AcAO Ac AO

9 APPENDIX A Parameter variation for low cooling drag The basic relationships between the cooling drag and volume flow rate are represented by the following simple equations: Cooling drag: , ,= +Dc Dc ITD Dc IFDC C C (A1)

Internal drag (App. B): 3

,

= c c

Dc ITDf

V ACV A

(A2)

Volume flow rate: = c c cV V A (A3)

Velocity ratio [8]: 21-

= +

pOc

c

O

CVV A

A

(A4)

When considering the internal drag only, the question arises whether there is a particularly suited combination of parameters, which reduces the internal drag inspite of the volume flow increase. In order to get a proper idea of the basic relationships, the parameters were varied. Using the data of the 911 MY 2002 each parameter was varied by +/- 50%, Figure A5:

Fig. A5: Influence of the parameters in equations (2/3) on the cooling drag and cooling air flow rate

As can be seen, the greatest possible internal drag reduction with lowest possible negative impacts on the volumetric flow is achieved by reducing the air outlet areas.

On the other hand, the cooling-air volume flow can be increased with almost no rise in drag by lowering either the pressure loss coefficient or enlarging the radiator area. Accordingly, it should be possible to reach the desired target by cleverly combining these parameters as shown in Fig. 8. B Cooling drag formulation based on power requirement Power required to overcome the aerodynamic drag:

With cooling air flow: 32

= = D fP D V C V A (B1) Without internal drag: ' ' ' 3

2 = = =D fP D V C V A

( )3 , - 2 D Dc ITD fV C C A = (B2) Power required to overcome the internal losses:

32

= = c c c c cP D V V A (B3) Overall power requirement: ' = + cP P P (B4) By inserting equations (B1)-(B3) in equation (B4) we obtain the following equation for the internal drag coefficient:

3

,

= c c

Dc ITDf

V ACV A

(B5)

Equation (B5) was already derived by H. Schmitt in 1940 [9]. It can be considered as equivalent to the momentum approach. Momentum changes or the the influence of the outlet angle of the cooling air exit flow (see Fig. 7) which is usually taken into consideration in the momentum approach are implicitly accounted for in the pressure loss coefficient . However, there is an interesting difference: According to equation (B5) the drag due to internal flow resistance cannot be negative as the total pressure loss coefficient is not negative if there is no energy input from outside. Thus according to equation (A1), the overall cooling drag of a car can only be negative if there is a negative interference drag which must be higher than the internal drag. The main advantage of equation (B5) over the ususal momentum approach is in the simplicity of the represenation and, above all, in the fact that the parameters used are easy to measure and allow the internal drag to be determined in a simple manner. Using equation (A1) the overall cooling drag can then be split up into the internal and interference drags as shown in Figure 21 for the new 911 and its predecessors.