Utilizing the Effect of Air Speed to Improve Automobile Moving Performance

A senior design project to practically implement the effect of wind speed on a race car is conducted. This project exposed student with actual system rather than some simulation kits. The student experienced the difference between the theoretical and actual result and learned how to design a signal conditioning circuit to convert the measured values into a realistic and meaningful data. The effect of air pressure on moving cars was investigated to find the relationship between air speed and lift forces. The measured values are utilized to adjust spoilers and wings attack angles. The race cars, by the use of spoilers and wings, capture the airflow, manipulate it and turn it into down force to increase traction at higher speeds. This seems counterintuitive since race cars need to be light. However, it is the mass that is important and is limiting. The mass doesn’t change, regardless of the gravity and the atmosphere. Weight, on the other hand, as it relates to cars is always changing with bumps on the road, speed and direction. Measuring these effects on cars has been achieved through the use of wind tunnels in the past. The attack angles for the wings and spoilers play a major role in suppressing the lifting problem. This paper investigates the lifting problem for a sport car on a race track and measures the lifting forces exerted on each corner of the car. The data collected from each sensor is used to adjust the spoiler attack angle at different speeds under variety of turning conditions. The proposed system has been installed on a sport car and its performance has been monitored and the relevant data has been collected. The adjustment of the spoiler led to improved performance of the car. This paper describes: 1. The theory of airflow and its lift effect on moving cars 2. Measurement mechanism, electronic sensors, data manipulation, data management, and spoiler adjustment 3. Track testing and analysis of results 4. Senior design project as an experiential learning tool INTRODUCTION As a car travels forward, it is attacked by moving air particles. These air particles go over, under and around the sides of a car. The air particles on top of a car experience lower pressure and consequently with less air density while the air particles under the car are compressed and result in higher air density. The lower air pressure on the top and the higher pressure under the car cause the car to lift. The lifting forces are not distributed uniformly for every corner of a car, especially when a car makes a turn [1-3]. These uneven lifting forces limit the capability of a car to make turns at higher speed. This phenomenon is more pronounced in racing cars than average passenger cars. For example a 2700 pounds car can produce 742 lbs of lift at 124 mph which is nearly 1/4 of its weight at that speed [4-5]. This lifting force reduces a car’s capability of gripping the road and may cause stability problems [6-7]. In this project, it is intended to measure the aerodynamic effect on a car with respect to speed changes, as it relates to weight. Several small film pressure transducers are installed between the coilover spring and spring perch of a Mazda P ge 26694.2 Miata sports car. At rest, the pressure transducers show the weight of the car: minus the wheels, tires, control arms, brakes, shock, springs, and steering knuckles. This group of parts in the automotive world is known as unsprung weight. Transducer signals are conditioned to isolate the effect of bumps on the road, and are combined separately into two groups, one for the front and another for the rear. This set-up provides two sets of readings, for the front and the rear weight. Graphs are plotted for weight versus speed changes indicating the increase of weight caused by down force or decrease of weight due to the lift force (Figure 9). The car is modified with the installation of diffusers, splitters, wings, and canards to measure lift, or down force at different speeds. Finally the car is driven on a race track; a road course featuring both left and right turns. At the race track, the car is driven a series of laps with and without aerodynamic pieces and comparisons were made between: lap times, minimum cornering speed, and maximum straight away speed. The graph of the down force vs. speed shows any gain or loss in the car performance. The purpose of this experiment is to duplicate some of the measurements that large engineering firms or automotive companies can produce in wind tunnels. There is little research or evidence of anyone trying to make these measurements on the fly in the past. The purpose of this individual measurement is to help a race team tune their car to produce the right amount of down force regardless of equal distribution of weight on front and rear, or a type of race. THEORY OF OPERATION The lift force can be calculated using the Bernoulli’s or its derivation Euler’s equations. In the calculation the effect of viscosity is assumed to be negligible, the air to be incompressible and frictionless. Furthermore, there can be no energy sources or sinks along the streamline. The Bernoulli’s equation is applied along a streamline, taking the form [6-7]: (P1/p) + (1/2)v1 2 + gz1 = (P2/p) + (1/2)v2 2 + gz2 = a constant (1) Where: P: the pressure of the fluid (Pa, PSI) ρ: the density of fluid (Kg/m 3 , lbm/ft 3 ) v: the velocity of the fluid relative to the airfoil (m/s, ft/s) g: the magnitude of acceleration for body (m/s 2 , ft/s 2 ) z: the height at that point (m, ft) The subscripts 1 and 2 represent different points along the same streamline of fluid flow. When a car turns, a force must accelerate the car towards the center of the turn. AERODYNAMIC OVERVIEW There are many different aerodynamic effects taking place on a car at different locations. For some locations the car is producing lift while others the car is experiencing down-force. Figure 1, shows a Mazda Miata (the test car) and all the different forces taking place due to air speed. P ge 26694.3 Figure-1 Test car with all different forces due to air speed The arrows represent areas of high pressure pushing towards areas of low pressure. For example the arrow on the roof pointing up is due to the pressure under the car is higher than the pressure on the roof. The arrow pushing down on the windshield wipers is a high pressure area on top pushes the car towards the relative low pressure area under the car. The reason for modifying a car shape is to produce more downforce to increase the high-pressure on the top of the car and to increase the low pressure under the car. Race and sports cars add both front air dam as well as a splitter to achieve such a goal. An air dam (a vertical section from the further point forward on a car extending down to the road) forces most of the air hitting the car to flow over the hood as opposed to under the car. This helps creating a low pressure zone under the whole car. The splitter (A horizontal board extending forward from the bottom of the bumper or air dam) is the divider between high pressure and low pressure. The high pressure on top of the splitter pushes the splitter towards the low pressure below it. A wing which is no more than an inverted airplane wing can be installed to improve the car stability. Its shape produces high pressure on top and low pressure on bottom, which results in down force. A spoiler on the other hand, like those used in NASCAR, produces the same effects of a wing except it slows the flow of air over the trunk making a high pressure zone that pushes down towards the lower pressure under the car. Another aerodynamic element is the rear diffuser. The diffuser creates additional low pressure under the rear of the car while it helps makes a smoother transition of air flow from the body of the car to the near vacuum directly behind the car. Finally the last element added to the car is simply adding smooth underbody panels under the car to keep the air flowing fast and generating low pressure [8-9]. SYSTEM DESCRIPTION The design includes 24 force sensors, 6 at each corner of the car. The front and rear 12 sensors are combined to give the user 2 readings, front and rear weight. An analog circuit is developed to process the signals, dampen noise, and average their magnitudes. The two voltage signals are recorded and graphed vs. speed. SENSOR PLACEMENT The sensors are placed where the sprung mass and unsprung mass meet on the car. This location is at the bottom of the Coil-over (shock and spring assembly). This configuration is shown in Figures 2 and 3. P ge 26694.4 Figure-2 A cut-away view of a Miata with an arrow pointing the location of the sensor placement. (Forum.miata.net) Figure-3 Top-hat where the sensor are mounted This prelimenary design, made from sheet metal, didn’t offer a flat enough mating surface. The individual output voltages between sensors were very far apart indicating uneven and non-flat rings. A set of specially made rings were machined from aluminum to meet this stringent requirement. These rings were almost perfectly flat. The rings serve as the mounting platform to insure that all the weight is transferred from the spring to the spring perch through the sensors. In Figure 4 on the left hand, the arrow points to the sensor mounting location where on the right the machined aluminum mating surfaces is shown.