777Driver clearly shows the obvious part of mpg loss. The OP's mpg difference was about 36% and 777Driver accounted for about 35% loss. Below is a detailed write up on the many forces at play. I know this may be beyond some readers. But at least you can see the complexity and identify some of the factors. Most of these factors are outside of our control once we buy a particular model. The main factors we can control is the vehicle speed, tire inflation and smoothness of the exterior surface. Oh and always drive going downhill!
Vehicle aerodynamics – effects of side winds
1. Vehicle Aerodynamics – Effects of side winds, Vehicle exposed to cross wind
2. Vehicle Body Aerodynamics? Is a branch of physics and is concerned when air flows around a body. Deals with a solid body moving through atmosphere and the interaction which takes place. Depends on varying wind speeds and wind direction. The most important factor is reducing fuel economy in aerodynamics
3. Importance of aerodynamic study?
Drag force is reduced. So maximum speed and acceleration are obtained for the same power output.
Fuel consumption of the vehicle can be reduced to the maximum (about 35% of fuel cost).
Gives better appearance and styling. By reducing the various forces and moments, good stability and safety can be achieved.
Helps to provide proper ventilation system.
Aerodynamic noise could be reduced which results in quiet running of the vehicle.
4. Features incorporated to aerodynamics?
Large corner radii on the front section
Low level front skirts
Fairings above and behind the driver’s cab
Aerodynamically shaped mirrors
5. Aerodynamic Drag Aerodynamic drag is usually insignificant at low vehicle speed but the magnitude of air resistance becomes considerable with rising speed.
6. Aerodynamic Drag Drag force depends on the following factors:
The size and shape of the vehicle (area of nose end, vehicle super structures)
Wind direction and strength
7. FL = ½ Cd Aρv2 FL : Drag Force Cd : Vehicle’s drag co-efficient A : Area of vehicle’s nose end ρ : Density of air v : Velocity of the vehicle PL = FL v = ½ Cd Aρv3 PL = Power to overcome drag
8. Types of Drag
Pressure drag (or) Form drag (or) Profile drag - 57%
Induced drag - 8%
Friction drag - 10%
Inference drag - 15%
Cooling and ventilation system drag – 10%
9. Pressure Drag
10. Typical static pressure coefficient distribution
11. Induced Drag (or) Lift Drag
This lift force depends on the contour of the body
Normal Speed - Not a serious problem
High Speed – Serious problem
Lift affects stability and braking performance
To reduce the accelerated flow upper side
To reduce the deceleration flow under side will reduce the aerodynamics lift
12. Friction Drag (or) surface drag (or) Skin friction
This is caused by friction force between the boundary layer and the body surface
Shear stress generated in the boundary layer
Laminar boundary layer should be maintained
Well polished surface is not only attractive but also makes the vehicle more economical
Body smoothness is of the order of 0.5 to 1.0 microns.
13. Interference drag
The flow over many exterior components interact with the flow over basic body shape and this leads to drag
Exterior components includes door handles, mirrors, aerials and badges which project out from normal surface
Mechanical components Engine parts, suspension system, exhaust system, frame rail
Exterior ornaments must be placed where the velocity is minimum
Door closer must be placed in a close proximity and longitudinally in line with each other
14. Internal Drag (or) Cooling and ventilation system Drag
Arising from cooling of the engine
Brakes, cabin ventilation flows
Contributes 10% of the overall drag
15. Drag Co-efficient The aerodynamic drag coefficient (Cd) is a measure of the effectiveness of a streamline aerodynamic body shape in reducing the air resistance to the forward motion of the vehicle.
16. Aerodynamic forces
Force of air drag in the direction of motion with wind angle along longitudinal axis (Px)
Cross wind force (Py)
Aerodynamic lift (Pz)
17. Longitudinal air drag (Px). The longitudinal component of the resultant of pressure distribution. Magnitude is represented by Px = (Cx p A V2 ) / 2
18. Cross wind air drag (Py). It's formed by asymmetric flow of air around the vehicle body when the wind angle is not equal to zero. Magnitude is represented by Py= (Cy p A V2 ) / 2
19. Aerodynamic lift (Pz). It’s the vertical component of the resultant of the pressure distribution over the vehicle body due to flow of air around it. Magnitude is represented by Pz = (Cz p A V2 ) / 2
20. Effect of cross wind
21. Aerodynamic moments
Rolling moment - Mx
Pitching moment – My
Yawing moment - Mz
22. Rolling moment - Mx. This moment caused by the cross wind force Py about the longitudinal axis. Magnitude is given by Mx = Py a = Cmx p A L V2 / 2. This effect is dangerous for tall van, where side force acts much above the C.G. The only solution is increase the wheel track
23. Pitching moment - My. This moment caused about y-axis by cross wind force Py or the longitudinal force Px. Magnitude is given by My = Pz b = Cmy p A L V2 / 2
24. Yawing moment - Mz. This moment caused about z axis by cross wind force Py. Magnitude is given by Mz = Py c = Cmz p A L V2 / 2