I. Abstract

INTRODUCTION: Air resistance (Drag) affects vehicle acceleration and its ability to handle and achieve good fuel efficiency. A car designed with better airflow has less difficulty accelerating and requires less engine power to push the car through the air. This results in better fuel consumption. There are several ways to improve the quality of the vehicle shape to reduce drag. Rounded designs and shapes on the exterior of the vehicle as well as components on the underside of the body channel the air to flow with the least possible resistance. Our purpose during this experiment was to evaluate and compare the effect of vehicle shape on the fuel efficiency.

METHOD: This experiment was performed using three different car models of variable shape and size that are present in today’s industry; an SUV, a sedan and a sports car were used. Each vehicle was placed individually inside the wind tunnel and the drag force was measured at 5 different wind speeds for each respective vehicle. Measurements were also performed in increasing and decreasing order for each vehicle for a total of 10 measurements per vehicle.

RESULTS: The SUV was observed to have the largest drag force over the variable wind speeds, while the Sports car was observed to have the smallest drag force compared to the other two vehicles. A linear relationship between the drag force and the wind speed is observed for the 3 different vehicles.

CONCLUSION: Our data indicates that larger vehicles with sharp angles and less channeling of air produce the highest drag force, and result in more fuel consumption as more power is needed to overcome the drag.

II. Introduction

Air resistance (Drag) affects vehicle acceleration and its ability to handle and achieve good fuel efficiency. Drag is a force that opposes the direction of motion of an object in a fluid. It is dependent on the vehicle’s velocity relative to the wind, coefficient of drag, and cross sectional area. The drag coefficient (Cd) is a quantifiable representation of the aerodynamics of the vehicle. Car manufacturers and engineers want to reduce the drag coefficient as much as possible to allow the vehicle to move easier through the wall of air pushing against it as it travels along the road. The Drag force is calculated using the following equation: Fd = ½ ρv2CdA (Equation 1).

At low speeds and when it’s not very windy, air resistance is very hard to notice as it acts upon our vehicles. Once wind speeds increase or, it’s a very windy day, air resistance (Drag) plays an important role on the way a car accelerates and achieves fuel mileage. The vehicle’s engine has to apply more power to overcome the air resistance opposing its motion. Higher air resistance increases the difficulty for the vehicle to accelerate and causes the engine to work harder to produce more power to overcome the drag, which results in higher fuel consumption.

The power required to overcome drag can be mathematically represented by Pd = Fd×v (Equation 2). Combining Equations 1 and 2 results in Pd = ½ ρv3CdA (Equation 3). The drag coefficient is directly proportional to the power required; a lower drag coefficient results in less power required and consequently less fuel consumption.

Engineers have developed several different ways to improve vehicular airflow and minimize air resistance. More rounded designs and shapes on the exterior of the vehicle channel the air around the car to minimize air resistance. Components on the underside of the body of the vehicle also promote air to move more smoothly to further minimize air resistance. Another modification is the addition of spoilers, which prevent air from lifting the vehicle’s wheels and making it unstable at high speeds.

Vehicles come in different shapes and sizes, resulting in different drag coefficients. In this experiment, we measured the drag forces acting on three differently shaped cars, allowing us to draw conclusions about the relationship between vehicle shape and fuel consumption.

III. Experimental Plan

The wind tunnel was chosen to show the effects of vehicle shape on fuel efficiency, because it provides an accurate simulation of a vehicle on the road. The wind tunnel’s airflow is created by a large fan, which creates a suction force that pulls air through the tunnel. The fan also has straightening vanes, which smooth the airflow to allow for more consistent forces.

[pic]

The air then passes over the vehicle located in the test section of the wind tunnel. The moving air simulates the effects of air passing over a driving vehicle. The flow of air creates the resultant drag force that is experienced by the vehicle, which is then measured by a load cell. This is the force we are interested in as the majority of fuel burned by a vehicle is used in order to overcome the negative drag force created by the air. Five different wind speeds were used in the experiment, which were tested in increasing and decreasing order to eliminate the effect of hysteresis. Three different vehicles with varying shapes were used to show the effect of vehicle shape on fuel efficiency. These vehicles were chosen as they represented three of popular vehicle types: sports cars, sedans, and SUVs.

EXPERIMENTAL PROCEDURE:

1) The first step in completing this lab was to secure the model vehicle to the support plate. This was done by tightly taping the car following its contours to the plate. 2) The next step was to place the support plate onto the load cell. This was done by aligning the holes in the support plate with the pins on the load cell. 3) The load cell was then zeroed out by adjusting the scale on the measurement monitor. 4) The door of the wind tunnel was then close and secured shut. The wind tunnel motor was then turned on after ensuring the break was applied. 5) The break was then deactivated and the wind tunnel motor was adjusted until the desired speed was reached. 6) The break was then reapplied and the horizontal load cell measurement was recorded. This process was then repeated for all the desired speeds. This was done in both ascending and descending order. 7) The wind tunnel motor was then turned off and the wind tunnel door was opened. The support plate was then removed from the load cell and the vehicle was detached. This entire process was completed for each of the other two cars.

MATERIALS: - 1 model SUV [pic]

- 1 model sedan [pic]

- 1 model sports car [pic]

- 1 load cell [pic]

- 1 static pitot tube - 2 data acquisition monitors [pic]
IV. Results & Discussion

Measurements for the drag force acting on the three car shapes were taken at even increments of wind speed. The recorded voltages from our experiments were converted to their proper units using appropriate calibration curves. Data from these experiments can be seen below. (Tables 1, 2, and 3)

|Drag (N) |Wind Speed (m/s) |
|0.063583342 |5.246097692 |
|0.123570425 |6.986832678 |
|0.193530638 |8.91486544 |
|0.293261944 |11.1746865 |
|0.34368372 |13.18321026 |

Table 1. Drag and Wind speed for Sports

|Drag (N) |Wind Speed (m/s) |
|0.080637766 |5.203184463 |
|0.147372469 |6.911019485 |
|0.219668397 |8.836349441 |
|0.321624193 |11.0479907 |
|0.434331691 |13.17723724 |

Table 2. Drag and Wind Speed for Sedan

|Drag (N) |Wind Speed (m/s) |
|0.081008514 |5.246097692 |
|0.154416687 |6.986832678 |
|0.252479626 |8.91486544 |
|0.367040866 |11.1746865 |
|0.519789185 |13.18321026 |

Table 3. Drag and Wind Speed for SUV

Comparing these results in a plot of Drag Force vs. Wind Speed, we are able to see a relationship between the speed of the car and the drag force exerted on the vehicle. We are also able to compare the drag force exerted on all three shapes of cars at a common wind speed. This plot can be seen below. (Graph 1)

[pic]
Graph 1. Plot of Drag vs. Wind Speed for three cars

DISCUSSION:

The results obtained from this experiment can be used to show two main relationships: the relationship between wind speed and resultant drag force, and more importantly, the relationship between drag force and vehicle shape.
The data shows that there exists a linear relationship between wind speed and resultant drag force. As wind speed increases, we also see a positive and proportional increase in drag force. Using the data, we can derive equations that represent the proportionality between wind speed and drag force. These equations can be used to predict drag force (y), given a certain wind speed (x). Using our graph, we are able to visually notice a relationship between vehicle shape and drag force. At the lowest of our tested wind speeds, we see that there is not much difference between the drag forces on each of the three cars. As we increase the wind speed, the drag force increases most rapidly with the SUV, and least rapidly with the sports car.

From this experiment, we can draw the conclusion that large cars with sharper edges, like the SUV, produce the highest amount of drag when driving. The least amount of drag was seen with the sleek sports car that was low to the ground and had no edges. The sedan, being of medium height and having smooth but noticeable edges, found itself between the other two cars in terms of drag force. The difference in drag force between the three cars was less noticeable at low speeds, but became more noticeable as the speeds increased.

As experimental data, our results are subject to error. Error may have been introduced to our data from improper taping of vehicles, improper reading of voltages, conversion of data with calibration curves and rounding. The load cell and pitot-tube introduced systematic errors, having accuracies of 1% and 4% respectively. In an attempt to negate the effects of hysteresis, the readings were taken in both ascending and descending order.

V. Conclusions

The purpose of this experiment was to verify the relationship between the shape of a vehicle and its average fuel consumption. Without taking 3 differently shaped cars and driving them around until their fuel ran out, the experimental procedure was conducted in a manor that would be ideal for observing the drag force felt by a vehicle. Drag force is an important factor when comparing vehicle shape to fuel efficiency, because drag force values portray how much work the engine must overcome in order to accelerate the car forward. This means that the vehicle shape that results in the least drag force will be the most fuel-efficient.

The three types of vehicles tested were scaled models of a sedan, SUV, and sports car. Tests were done at scaled wind speeds to replicate drag forces commonly felt by every day vehicles. By conducting the experiment with common vehicle types, and at typical vehicle speeds, the results achieved would be fairly coincident to what these different vehicle types experience on most Canadian roads.

Observing the drag force equation (Fd = ½Cd ρv2A), it can be seen that the drag force value has a linear relationship with the reference area of an object. This indicates that as the reference area of an object goes up, the drag force felt by the object will also increase linearly. By keeping the value of air density (ρ) constant, and testing the vehicles at the same air velocities, these variables can be removed from the equation. Therefore, by this logic, it was hypothesizes that the sports car would feel the least drag, with the sedan and SUV coming after, respectively. The results of the experiment portray this perfectly, with linear relationships between drag force and wind speed. The results also allude to the fact that the equation and hypotheses are valid: a larger reference area does result in a larger drag force, and the sports car’s shape is the most fuel efficient.

Though there are various factors that contribute to the fuel-efficiency of a vehicle, vehicle shape is one of the most important. By reducing the drag force felt by the wind, the engine of the vehicle has to work less, consuming less fuel. With the demand for more fuel-efficient cars, engineers all over the world are designing vehicles that can “cut” through the air. By understanding the effects of vehicle size and shape, and utilizing things such as spoilers, and ducts, engineers have the tools to create more fuel-efficient vehicles.

VI. References

"Car Aerodynamics." Vehicle. Web. 12 Apr. 2013.

"Drag (physics)." Wikipedia. Wikimedia Foundation, 04 Oct. 2013. Web. 12 Apr. 2013.

"Fuel Economy in Automobiles." Wikipedia. Wikimedia Foundation, 04 Nov. 2013. Web. 12 Apr. 2013.

Hagens, Nate. "The Oil Drum | Fuel Economy Factors - Part 1: The Role of Aerodynamic "Drag." Web. 12 Apr. 2013.

"How Aerodynamics Work." How Stuff Works. Web. 12 Apr. 2013.

GRAPHICS

VII. Contribution of Each Member of the Group

Spencer – Experimental Plan, Editing
Sean – Results & Discussion, Editing
Yordano – Abstract, Introduction, Editing
Jason – Conclusion, References, Organize, Editing