aero: rear diffusers

Rear Bumper Aerodynamics, Concerning the Implementation of a Rear Diffuser,
Lancer Evolution 8

Identification of Problem
On the factory Lancer Evolution 8 rear bumper, there exists a problem. As air travels under the car it increases above atmospheric pressure. This increase in atmospheric pressure coupled with the decrease below atmospheric pressure of the airflow over the car, leads to a lift force produced on the car. Additionally there is a pressure differential created at the rear of the car from two different air pressure flows coming together. This pressure differential creates pressure drag on the car as a whole. The general shape of the car lends to a large bubble of low-pressure air that is constantly behind the rear of the car when moving. This low pressure behind the car “sucks” the car back while driving similar to how a vacuum uses low pressure to suck items off the floor. This is also another form of pressure drag, which inhibits foreword movement to an extent. The most commonly identified source of drag on a car is the “parachute effect” caused by the rear bumper. The rear bumper sticks down into the natural underbody airflow of the car and due to its shape, that very much like a parachute, it naturally catches air from the air stream, and “parachutes” thereby creating drag.

Identification of a Solution
The solution to these drag problems can be solved or reduced, however. The way to reduce the parachuting effect of the rear bumper would be to place a flat plate under the rear bumper that covers the lip of the rear bumper and through the smooth transition of the plate from behind the axle, to block air from going over the top of the diffuser, to the rear bumper, will create less possibility for the air to get “trapped” on some underbody pieces and create drag as a result.
The low-pressure bubble at the rear of the car cannot be completely eliminated without the use of some radical mechanical devices, but it however can be reduced. The way to eliminate this is to move air into the space, as the air from any other part of the car is at a higher pressure than in the low-pressure bubble. To “blow” air into this area can be accomplished through two different approaches. One approach would be to put an airfoil hanging at the rear bumper that has a steep angle of attack to move air into this area, where the other would be to add fins to the rear diffuser main plate and use the vortices created to move some air into this low-pressure area. The airfoil would move much more air into the low-pressure area, and reduce much more drag than the fins would; however the airfoil produces some ill side effects. The airfoil, since at a very steep angle of attack, would create drag, as drag is a by-product of lift. Additionally since the airfoil would need to be located under the farthest rear part of the car, it would succumb to constant dragging on the ground as the car is driven on inclines, unless the car is sufficiently raised (which would in turn allow more air under the car, which would create more lift and more drag on the car as a whole). Therefore fins will be used because they create very little drag, help to straighten underbody airflow, and can be used to decrease the low-pressure area behind the car.
The pressure differential that is created by airflow over the car and under the car cannot ever be completely eliminated, as air will always be able to get under the car. However you can reduce the amount of air that gets under the car through various other ways, namely lowering the car and the body’s proximity to the ground. The pressure differential that comes back together at the rear of the car creates drag. However the rear diffuser can help that. If the diffuser is placed at a negative angle of attack, so that is still does not catch air from the underbody air stream, one can effectively increase the area under the rear bumper per unit length from the beginning of the diffuser. Through the continuity equation we find that an increase in area leads to a decrease in air stream velocity, and Bernoulli states that an increase in velocity leads to a decrease in underbody airflow pressure. Since the underbody airflow pressure is the highest air pressure of the airstreams around the car, lowering it will decrease the pressure differential, as the two pressures will be closer to one another when they meet at the rear of the car.

Rear Diffuser, and its Implementation of the Solution
The rear diffuser is based off a flat plate with minor modifications to its shape and to accommodate the features of the Lancer Evolution 8, such as the axle and suspension, and the exhaust system. The diffuser uses this flat plate to reduce the drag created by the rear bumper’s “parachute”, and to gradually decrease the air pressure through increasing the area, so as to reduce drag created by the pressure differential at the rear of the car. The implementation of fins on the design will reduce the low-pressure area behind the car while maintaining a very low contribution to total drag on its own. The fins also help to straighten the airflow as it comes across the diffuser, as the air becomes very turbulent as is passes over and under all the irregularities under the car. The “box” design around the exhaust system both serves to straighten air flow with the use of its sides, but also uses the bottom plate of the box as an inverted airfoil to create down force mechanically rather than through pressure like the rest of the diffuser does. The box design was created in the size that it is to accommodate the stock and the different aftermarket exhaust systems available for the Lancer Evolution 8.

Data and Graphs
All runs done in the east and west direction, to account for wind and elevation change, with all windows up

Without Diffuser
Speed During Test [MPH] Drag Force [lbf]
45-40 92.13
65-60 132.22
75-70 204.46

With Diffuser
Speed During Test [MPH] Drag Force [lbf]
45-40 106.85
65-60 141.02
75-70 214.61



With Modified Diffuser (box reduced vertically by 2”, box angle of attack reduced to 6°)
Speed During Test [MPH] Drag Force [lbf]
45-40 95.69
65-60 140.86
75-70 158.88


Data Analysis
Although the diffuser contains all the aforementioned drag-reducing elements, the data shows that the diffuser increased drag. This is due to the fact that the diffuser is creating down force. As stated earlier, drag is a component of down force, since the diffuser is reducing drag from the parts on the factory version of the car, yet showing a net increase in drag, one is lead to believe that down force is causing the calculated drag created by the diffuser, since the drag creating elements on the car have been severely reduced through the implementation of the rear diffuser.

Error Analysis
In the manner that the tests for drag calculation were ran, this involved using the car’s speedometer for a speed change reference and a stopwatch. The error would be involved with the human error associated with starting and stopping a stopwatch and ability to react to a condition. It was calculated that the human’s ability to react was less than one quarter of one second. Since the time values vary, the error accounts for between 0.5% and 7.5%, on average about 4% or between 4 and 8 pounds-force of drag. Additionally, the tests were carried out on different days, therefore one much account for atmospheric changes. It was however calculated that temperature and density changes would account for a difference of less than 1 pound-force of drag between the calculated values and expected values, so those calculations were omitted from the drag calculations for simplicity.