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.
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.
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 cells||Drag coefficient (-)||Variation (%)|
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
|Weight of Audi A4 [kg]||1850|
|Number of wheels [-]||4|
|Number of brake assemblies [-]||4|
|Standard braking distance at 112 km/h [m]||67|
|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
Table 4 Carbon-carbon composite thermo-physical properties
|Specific Heat||750 @ T=25C / 1970 @ T=1300C (Varying linearly)||J/(kg-C)|