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"Innovation for a brighter future"
Launched August 2020

OGAB® Sustainable Driving

Through our patented Active Flow Control technology, Ogab® has developed a breakthrough combination system that reduces aerodynamic drag and cools brakes by utilising energy that would otherwise be lost.

Our patented technology has achieved a considerable reduction in drag force on an Audi A4 test vehicle. This results in a 24.13% reduction in fuel consumption and gains an extra 174 miles on the same tank of fuel.

Flow trajectories of temperature from the brake disc cooling nozzles at t=30s for carbon-carbon material.

1. Introduction

Environmental concerns, resource scarcity and excessive cost of fossil fuels have obligated governments and industries to reduce the Greenhouse Gas (GHG) emissions and deploy effective fuel-saving strategies in various industrial sectors. The transportation sector is one of the most energy-intensive industries which is responsible for a substantial portion of CO2 emission footprint, as a significant amount of fuel is consumed in this industry. The transportation sector accounts for 28% of the total USA energy consumption. Therefore, immediate actions are required to reduce fuel consumption and associated GHG emissions to achieve a sustainable future.

Vehicle manufacturers seek novel techniques and solutions to reduce the road resistance of the motor vehicles and, therefore, the fuel consumption and GHG emissions. This resistance consists of a mechanical part as well as an aerodynamic facet. 

For example, in an urban environment the power dissipated through acceleration and braking of the vehicle is the dominant loss, whereas on the highway aerodynamic losses are dominant in which the aerodynamic pressure drag is predominant. Therefore, researchers have explored various active and passive flow control strategies to reduce the aerodynamic pressure drag, which can be expressed in non-dimensionalized form as drag coefficient.

 Aerodynamic drag is the force that resists the movement of a body through a fluid medium. The majority of the studies claim drag reduction improvements in the range of 1% to 10% for passive and active flow control strategies. Though, often the gain in active methods is offset by the power requirements for running the system, while the passive improvements are minimal and not economical to be deployed.

All the vehicles in the transportation industry are equipped with brakes. Their proper functioning is relied upon for safe operation of vehicles on the road. The brakes slow down the vehicle and stop it in a reasonable amount of time. On most vehicles, each of the main wheels is equipped with a brake unit. Performance and reliability of brake systems are crucial for safety of drivers and pedestrians.

Due to the weight of vehicle and short stopping distances, the design of brake system is very demanding. Almost all vehicles use disc brakes which operate based on an induced friction between rotating and stationary discs inside the assembly. Disc brakes consist of two major components – the rotor and stator. The rotating disc (rotor) is keyed to the wheel rim and rotates with the tyre while the stator (brake lining) is fixed and attached to the calliper and then to the chassis. 

When driver activates the brakes, brake linings squeeze the rotor and create friction which reduces speed. Brakes are often made of grey cast iron or steel due to its desirable mechanical properties, however, on sport vehicles carbon-carbon composite brakes are employed. The carbon fibre discs are noticeably thicker than grey cast iron rotors but are extremely light. They are able to withstand temperatures fifty percent higher than cast iron brakes. Carbon rotors also dissipate heat faster than cast iron counterparts. A carbon rotor maintains its strength and dimensions at high temperatures. Moreover, carbon brakes last 20% to 50% longer than cast iron brakes, which results in reduced maintenance. 

The only drawback of carbon brakes is the high cost of manufacturing. Since the primary function of brakes is to convert the kinetic energy of the vehicle into heat/thermal energy by creating friction, a significant amount of heat is generated during the braking procedure. Hence, brake components need to withstand a very harsh environment. As the brake linings (stators or brake pads) are wearable, activating brakes causes considerable material consumption and environmentally harmful debris to be released into the atmosphere by each brake activation (which also increases the wear of the brakes and the subsequent maintenance cost). 

This study aims at proposing a system that can be used for both reducing the drag coefficient (hence the fuel consumption) as well as reducing the brake disc temperature (hence extending the brake linings lifetime and controlling the adverse environmental effects). 

This novel system employs an Active Flow Control strategy which consists of a turbocharger, a vortex tube and a set of specially designed nozzles that are subtly deployed to achieve the aforementioned goals. During the cruising speed of vehicle,, the proposed system switches to the drag reduction mode while during the onset of braking, the system switches to the brake disc cooling mode. In other words, such system has a dual-function capability in which when one system is activated the other is de-activated and vice versa. 

The proposed system recovers the otherwise wasted compressed air from the turbocharger (which is not used for boosting engine performance). For the aerodynamic drag reduction mode and at high cruising speeds, the proposed system improves the flow field around the vehicle. The proposed technology will create a flow field which is in the favour of parameters that reduce the drag coefficient. For the brake disc cooling mode and on the onset of braking action, the proposed system changes the functionality and the recovered air from the turbocharger will be used to decrease the brake disc temperature.

2. Methodology

For this study Computational Fluid Dynamics (CFD) tool was used to demonstrate the effectiveness of the proposed system described in Section 1. CFD is a virtual laboratory that solves Navier-Stokes equations and allows for fast, yet accurate investigations of various design changes in which the precise physics that are involved in any specific problem can be taken into account. CFD analyses were performed for two cases as a) Aerodynamic drag reduction mode and b) Brake disc cooling mode. Each part was also divided into 2 sections in which in one section the performance was obtained without the proposed system and in the second section the parameters of interest were achieved with inclusion of the innovative technology so that comparison can be made. The simulation scenario for conducting the analyses for the above cases was defined as following:

A sedan car is moving at the cruising speed of 112 km/h (70 mile/h) and the drag coefficient of the vehicle is calculated for two configurations as 1) without nozzles 2) with nozzles both under steady-state conditions. With these simulations, the effectiveness of the first part of current study (i.e. aerodynamic drag reduction) will be demonstrated. For the second part of the scenario, it was assumed that the driver pushes the brake pedal and stopped the vehicle from the cruising speed of 112km/h (70 mile/h) to complete halt in a few seconds and the brake disc temperature is evaluated for two configurations 1) without nozzles 2) with nozzles both under transient conditions.

2.1 Aerodynamic Drag Reduction

To investigate the effect of injecting hot air in the front region of the vehicle on the drag coefficient, it was necessary to specify a vehicle. In this study, the selected vehicle is a commercial sedan Audi A4 where its 3D CAD geometry and key dimensions are depicted in Figure 1.

Figure 1 CAD model of the selected vehicle (top) key dimensions in millimetre (bottom)

The aim of these CFD simulations was to obtain the drag coefficient, which was calculated by equation 1.

Where, Fd is drag force (N) and is the outcome of the CFD simulations, ρ is average free stream density (kg/m3) calculated over the computational domain (or the wind tunnel), V is the wind or car velocity (m/s) and A is the vehicle frontal area (m2). The frontal area is the projection of the car body onto the plane, normal to the direction of wind speed. By conducting the CFD analyses, the solver calculates the drag force (Fd) exerted on all vehicle surfaces normal to the wind speed direction. 

This parameter is the summation of pressure and friction forces. At that the pressure force is determined with replacing the calculated static pressure P by the difference P – Pref, where Pref is the specified reference pressure on the wall side lying outside of the computational domain. In other words, pressure or normal force is the fluid force normal component acting on the selected vehicle surfaces. Friction force also accounts for the fluid’s friction on surfaces, which includes the shear stress term to consider the boundary layer and viscous effects. Since these forces exist in all directions, only that component of force is extracted, which is in the direction of wind speed (i.e. in Z direction – refer to Figure 2). Eventually, with the known vehicle velocity, vehicle frontal area and the calculated free stream density (averaged over the entire computational domain), the drag coefficient was calculated via equation 1.

Figure 2 Selected vehicle surfaces for calculation of drag force

To ensure that the results obtained are independent of the grid quality, a mesh independence study was conducted for the case without injecting nozzles using the advance solution adaptive meshing technique. This approach allows addition of mesh cells in the areas with high gradients in the parameters of the interest. The mesh independence study results for the case without nozzle are shown in Table 1.

Table 1 Mesh independence study results for the case without injecting nozzles

Number of mesh cellsDrag coefficient (-)Variation (%)
2,532,1490.2
3,849,4540.19084.6
4,547,5930.18532.88

This study shows the sensitivity of drag coefficient to the quality of the generated grid. As evident from this table, by increasing the number of mesh cells from about 2,532,000 to 3,849,000 the variation in drag coefficient is about 4.6% while by increasing the number of cells to about 4,547,000 this variation reduces to 2.88%. This variation was considered an acceptable trade-off between the quality of the results and the computational time. Therefore, the mesh quality in the second row was selected for the rest of the analyses. The mesh quality at two planes in the wind tunnel as well as on the vehicle’s body is shown in Figure 3.

Figure 3 Generated mesh quality

2.2 Brake Disc Cooling

The brake disc assembly consists of a single rotating disc and two brake pads that are clipped to the calliper. Figure 4 shows the CAD model of the brake system assembly, together with its key dimensions that was used in the current study. 

Figure 4 CAD geometry of the Audi A4 brake system (top) key dimensions in millimetre (bottom)

As mentioned in the introduction, the function of brake is to convert the kinetic energy to thermal energy or heat. Therefore, it was necessary to calculate the amount of heat that the vehicle released during the braking procedure. Table 2 outlines the key characteristics required for the calculation.

Table 2 Key characteristics for calculation of heat generation during brake

ParameterValue
Weight of Audi A4 [kg]1850
Speed [m/s]31.1
Number of wheels [-]4
Number of brake assemblies [-]4
Standard braking distance at 112 km/h [m]67
Deceleration [m/s2]7.22
Braking duration [s]4.31

Generated heat is calculated from the velocity of 112 km/h (or 31.1 m/s) to a complete halt (0 km/h). The kinetic energy of vehicle is calculated via equation 2.

Considering that this amount of kinetic energy is dissipated within 4.31 seconds (which is the braking period) then the heating power from all brake disc assemblies can be obtained as below:

Then the heating power per brake disc assembly is achieved as following:

Therefore, such value was used as heating power that is dissipated by each brake’s disc and pads during the braking period. 

2.2.3 CFD Simulation Setup

The selected materials for the brake pads and disc were grey cast iron and carbon-carbon composite with properties shown in Tables 3 and 4, respectively. Therefore, for this part of the study the simulations were conducted for 2 materials at 2 different conditions (i.e. without brake disc cooling and with brake disc cooling).

Table 3 Grey cast iron thermo-physical properties

PropertyValueUnit
Density7200kg/m^3
Thermal Conductivity45W/(m-C)
Specific Heat510J/(kg-C)
Melting temperature1200C

Table 4 Carbon-carbon composite thermo-physical properties

PropertyValueUnit
Mass Density1980kg/m^3
Thermal Conductivity120.32W/(m-C)
Specific Heat750 @ T=25C / 1970 @ T=1300C (Varying linearly)J/(kg-C)
Melting temperature1300C

Due to the nature of the physics involved in this project, transient analysis was employed to accurately resolve the development of temperature gradients and accurately model the real physics of the phenomena. Following the information provided in the methodology, it was assumed that the driver initiates the braking procedure at the speed of 112 km/h and stops the car completely in 4.31 seconds. However, a total time of 30 seconds (i.e. additional 25.69 seconds) was simulated to investigate the effect of cooling technology even after the braking procedure. The time step used for this study was set to 0.05s.

The calculated heating power of 51.96kW obtained in section was assigned to the two sides of the brake disc (as a surface heat source) and to the two sides of the brake pads (again as a surface heat source), which are in contact with the rotating disc (divided equally between 4 surfaces) as shown in Figure 5. Such value was assigned to those wheels that are included in the computational domain (i.e. only two wheels were included). This heating power was continuously active during the first 4.31 seconds only (which was the duration of the braking). 

Figure 5 Surface heat source applied to the 2 sides of rotating disc and 2 sides of brake pads for each wheel

Figure 6 shows the mesh resolution on the brake assembly which highlights the high quality mesh created for this analysis.

Figure 6 Mesh resolution on the brake assembly

3. CFD Simulation Results:

In this section, CFD results are presented for both parts of the study. In the first section, we present the aerodynamic drag reduction results for two conditions as without nozzle and with nozzle. In the second part, CFD results related to the brake disc cooling will be presented for two conditions as without cooling system and with cooling system. 

3.1 Aerodynamic Drag Reduction Results

Figure 7 compares the key quantitative results for both cases as without Advanced TRS and with Advanced TRS. The obtained drag coefficient of 0.206 for the case without Advanced TRS and for the selected vehicle is in a reasonable proximity of the reported values of drag coefficient for the similar vehicle type in the literature (in the range of 0.25 to 0.3). It should be noted that slight discrepancies between the current CFD result and experimental data can be related to several factors such as test conditions, surface roughness values, wind velocity, vehicle speed, yaw angle and many other parameters that are controlled during experimental testing. It can be concluded that the results presented here are of high accuracy, quality and reliability, and the developed CFD model can be used as a benchmarking model.  

Figure 7 Comparison of the key results for the cases without and with nozzle

3.2 Brake Disc Cooling Results:

Disc and pad solid temperature contours for carbon-carbon material (top) without cooling nozzles (bottom) with cooling nozzles at t=30s.

For the case with proposed brake cooling technology, the system was activated onset of the braking function (brakes engaged), then it remained active for the braking period (i.e. for 4.31 seconds) and then it was remained operative for an additional 25.69 seconds (i.e. till t=30s). During this 30 seconds of transient simulation, the brake linings and the rotating disc were in contact only for the first 4.31 seconds (i.e. from t=0s to t=4.31s) and all the heat that was generated as the result of friction was released during this time. 

At t=4.31s till t=30s the brakes were released but the cooling system remained operative. The qualitative and quantitative results of the brake system without cooling and with cooling are presented in this section for 2 brake disc material as carbon-carbon and grey cast iron.

Figure 8 presents the quantitative results of the brake disc average volume temperature for the front and rear wheels’ brake disc systems for the case without cooling and the case with cooling for the carbon-carbon material at the end of the transient simulation (i.e. t=30s). As can be seen from this figure, deploying the cooling technology proved to be effective as it reduces the rotating disc average volume temperature by about 12.45%.

Figure 9 demonstrates the quantitative results of the brake disc average volume temperature for the front and rear wheel’s brake disc systems for the case without cooling and the case with cooling for the grey cast iron material at the end of the transient simulation (i.e. t=30s). As can be seen from this figure, deploying the cooling technology proved to be effective as it reduces the rotating disc average volume temperature by about 6.07%.

Figure 8 Comparison of temperature between the case without cooling and the case with cooling for carbon-carbon material at t=30s

Figure 9 Comparison of temperature between the case without cooling and the case with cooling for grey cast iron material at t=30s

Figures 10 and 11 present the evolution of disc average volume temperature for carbon-carbon material and grey cast iron material, respectively. 

Figure 10 Evolution of disc average volume temperature during simulation - Carbon-carbon material

Figure 11 Evolution of disc average volume temperature during simulation – Grey cast iron material

It can be seen that by deploying the cooling system the escalation rate of the temperature during the braking period is reduced, leading to lower temperature at the end of the braking period, as well as the end of the simulation for both materials. Overall, the proposed cooling system showed better performance with the carbon-carbon composite material compared to the grey cast iron. 

Comparison of turbulence intensity between the case without injecting nozzles and the case with injecting nozzles for the carbon-carbon material.

To ensure that the proposed brake cooling technology does not affect the flow field behaviour around the vehicle and especially in the vicinity of rotating wheels, the drag force was compared for the case without cooling and the case with cooling system. As evident from Figure 12, inclusion of the brake cooling system didn’t have considerable effects on the drag force and hence on the drag coefficient. 

Figure 12 Comparison of drag force for the case without cooling and the case with cooling system for carbon-carbon material

4. Conclusions

Following the results presented in Figure 7, the inclusion of Advanced TRS® proves to be very effective in reducing the drag coefficient. The implementation of Advanced TRS® led to a substantial reduction in drag force with the relative improvement of 34.81% and hence resulted in drag coefficient reduction of 34.47% compared to the case without Advanced TRS®. It is known that a 20% reduction in drag will contribute to about 10% reduction in fuel consumption at 80 km/h. These fuel savings would rise as speed increases to a value of approximately 15% at 120 km/h (https://www.tc.gc.ca/en/programs-policies/programs/documents/AERODYNAMICS_REPORT-MAY_2012.pdf). 

Therefore, at 112 km/h, a 20% reduction in drag will lead to a 14% reduction in fuel consumption (using an interpolation). With the Advanced TRS®, it was possible to reduce the drag coefficient by 34.47%. Therefore, such a system can reduce fuel consumption by 24.13% at 112 km/h for the investigated vehicle.

For the selected vehicle (i.e. Audi A4, 50 TDI, V6, 286 Hp, quattro Tiptronic) the combined fuel economy (i.e. urban and extra-urban) is 6 litres per 100 kilometres. Deploying the Advanced TRS results in a 24.13% reduction in fuel consumption, which leads to the fuel consumption of 4.55 litres per 100 kilometres.

This means that about 1.45 litres of fuel are saved per  100 kilometres. With the fuel tank capacity of 63 litres and combined fuel economy of 17.26 km/litre, the vehicle’s range is 1087.4km. With deploying the proposed drag reduction technology, such range can be extended by 272.14km leading to the total range of 1359.5km for the investigated vehicle.

On the other hand, it was shown that the proposed cooling technology was also effective in reducing the rotating disc average temperature. It was possible to reduce the overall disc average volume temperature by 12.45% for the carbon-carbon material and by 6.07% for the grey cast iron material. This technology can significantly extend the lifespan of the brake linings while reducing the maintenance cost, as well as stopping the release of environmentally harmful debris from brake assembly consumable materials.

Consequently, the aforementioned improvements in the drag reduction and brake disc cooling were solely achieved by recovering the wasted air that is captured by the vehicle’s turbocharger system. In other words, the excess air from the turbocharger that is not used in the engine cycle will be recovered for re-use either in drag reduction system or in the brake disc cooling system (i.e. the recovered air switches between the two functionality). Since this technology uses the otherwise wasted energy without consuming any fuel, it outperforms other technologies in the market as the majority of them require an excessive power supply and typically, their gain is offset by fuel or power consumption. 

It should be mentioned that the proposed brake cooling technology has the potential to be integrated within a more advanced system. Such system encloses the brake disc and pads as well as the cooling technology within a sealed compartment in which the particulate matter and environmentally harmful debris that are released as a result of the braking function can be effectively captured. Afterwards, in a proper location (for example, in a dealer workshop) such debris can be discharged safely from the confined space in the vehicle into a specially designed vessel for treatment to alleviate their adverse environmental effects. With such a method, the harmful impact of braking can be minimised by controlling the release of particulate matter.