CO2 Car Design Portfolio
By Noah Campbell
Step 1: Define the Problem
How fast can you make a model car travel down a 20 meter track powered by a CO2 cartridge?
Step 2: Brainstorming
Step 3:Research and Generating Designs
You may think these vehicles have nothing in common but they do. All land vehicles are systems made from power, suspension, guidance, control, structure and support subsystems. The most common power plant used in land transportation vehicles is the internal combustion engine. Internal combustion engines are used in the cars and trucks we use every day. These engines use a controlled explosion to create power. A four-stroke engine uses a piston and cylinder to draw fuel and air into the cylinder on its down or intake stroke. On the upstroke the air fuel mixture is compressed. At the top of this stroke, a spark ignites the fuel causing an explosion that forces the piston down, power stroke. When the piston comes back up the exhaust from the explosion is released. The horsepower produced depends on the size or displacement of the cylinders and the number of cylinders. A typical four-cylinder engine has a displacement of 1.8 liters and produces 140 hp. A six-cylinder engine has displacement around 3.0 liters and produces about 200 hp. Vehicles with eight cylinder engines produce around 300 hp with a displacement of 5.4 liters. Suspension connects the vehicle to the environment. For cars, it helps keep the rubber on the road. Suspension systems use control arms attached to the frame to allow wheels to move up and down. Springs support the vehicle. Shocks are used to dampen the movement of the spring so the car does not bounce. Wheels and tires connect the vehicle to the road surface. Working together, traction is optimized while keeping the ride comfortable. The guidance system provides information to the operator to help guide him/her to his/her destination. Speedometers in cars provide information about vehicle speed. Road signs help guide the driver by providing information about location and direction. Modern guidance systems are getting more sophisticated. Accelerate, turn, stop - vehicles need subsystems to control motion. Throttle, steering and braking subsystems control the motion of land transportation vehicles. The throttle subsystem controls the power plant to increase or decrease speed. The throttle system provides the driver with a mechanism to control the amount of air-fuel mixture that enters the combustion chamber. This control allows the operator to accelerate or decelerate the vehicle by increasing or decreasing air-fuel mixture. For cars and trucks, the gas peddle provides this control. There are many different types of steering system but the most common in land vehicles is rack and pinion steering, most vehicles with a steering wheel use rack and pinion steering. In this system, gears are used to convert the rotary motion of the steering wheel to linear motion needed to turn the wheels. To stop, brakes are attached to wheels. Braking systems use friction to slow a vehicle. Most systems use hydraulics to create the pressure needed to stop vehicles quickly. When a brake peddle is pressed, a hydraulic cylinder pushes fluid through lines to brake calipers. The calipers use hydraulic cylinders to push pads against a steel rotor to create friction. The friction caused by the pads clamping on the rotor slows down and stops the vehicle. The vehicle's chassis provides the structure that joins together the power, suspension, guidance and control subsystems. The chassis is designed to hold all of these subsystems while overcoming forces that could cause failure. Some of these forces include:
- Weight of the engine, suspension, people and cargo.
- Torsion or twisting forces created by the engine and drive train.
- Compression, tension, shear and torsion forces created by driving and road conditions (i.e. acceleration, deceleration, turns, bumps, potholes, etc.)
- Impact from accidents that could injure passengers.
Chassis designs vary depending on the purpose of the vehicle. There are three basic vehicle chassis designs - Ladder, Tubular Space Frame and Unibody.The support subsystem is not part of a land transportation vehicle. Rather it is the components that provide the energy and infrastructure that keep vehicles operating. Cars need roads and maintenance for continued safe driving. They also need gasoline. As cars developed and improved, the infrastructure needed to support travel had to develop and improve as well. Road systems began to connect communities. Gas stations became frequent enough that great distances could be traveled. The goal is to move your dragster down the track as quickly as possible. Sir Isaac Newton (the chap on the right) defines the problem. His second law of motion states:
The acceleration of an object of constant mass is proportional to the force acting upon it.
In mathematical form, the relationship of force, mass and acceleration is defined as:
To go fast, acceleration is the key. The faster the car accelerates the greater the speed. What does Newton's law tell us about acceleration? We can use algebra to change the force equation to solve for acceleration.Acceleration is related to velocity, but they are not the same. First, let’s takes a look at velocity.
Velocity is equal to distance traveled over time.
Miles per hour (mph) is a common measure of velocity. It is the distance traveled (miles) over time (hours). If you were driving at a constant velocity of 60 miles per hour (mph), in one hour you would have traveled 60 miles.
Acceleration is a change in velocity (v) over time (t).
A typical sedan can accelerate from 0 to 60 mph in 16 seconds. Therefore:
Mass is related to the weight of the vehicle. Mass is a measure of the amount of matter or material an object contains. Weight takes into account the gravitational pull on matter. Your car would weigh less on the moon but the mass would be the same. Since all races occur on earth, the difference between mass and weight factors out. If your car weighs less than your classmate's car, it has less mass.
So how do we go about changing the mass of a car? To answer this, let's take a closer look at what mass really is.
This equation shows that the mass of an object depends on a variable called density, and the volume of an object. All materials have a unique density, and as the following table shows, they vary a lot. The density of steel, for example, is approximately 7.84 grams per cubic centimeter. In other words, one cubic centimeter of steel would have a mass of 7.84 grams. Balsa wood has a density of 0.17 grams per cubic centimeter. To put things into perspective, if you designed a dragster made of steel, it would have to be about the size of your thumb to have the same mass as the other balsa wood cars. Since Newton's second law tells us that decreasing mass will increase acceleration, then the lightest possible materials and minimum volumes must be the right way to go, right? Well, not necessarily. Generally, lighter materials are not as strong as heavier materials. From our table, we can see that basswood is more than twice as heavy as balsa wood, but it is also more than twice as strong. So, if you want your car to make several runs, you may want to consider using basswood.
Body design is not the only thing to consider when it comes to mass. The total mass of your car includes the mass of the wheels, axles, bearings, and the CO2 cartridge. When considering axle selection, for example, can we say that aluminum is always a better choice than brass because it is lighter? Not necessarily. As we'll see later, a brass axle may produce less surface friction than aluminum. So, it's possible that a heavier axle will result in a faster car. Once again, we have an interesting trade off to consider. As you have learned, the total mass of the vehicle is important. How this mass is distributed is also important. Your car's behavior as it runs down the track depends on its center of mass (CM).
The center of mass is an object's "balance point". For example, if you could balance a CO2 car on the tip of your finger, your finger would be just below the center of mass. This literally means that half of the car's mass is on either side of your finger. You could also flip the car up on its end and balance the car this way. So by doing this simple experiment, you have located the center of mass along the length and height of the car.Calculating the center of mass can be pretty easy when it comes to simple objects like cubes and spheres. For these and other symmetrical objects made of a single material, the center of mass is the geometric center. But for more complicated objects, such as asymmetrical shapes or composite objects – objects with a combination of different components and materials – the center of mass is much more difficult to determine.
Your CO2 car will be one of these more complicated objects. More than likely, you will design a body shape that is anything but simple. Your design will also consist of many different components and different materials. When calculating the center of mass for your car, we’ll need to consider all these components and calculate a "composite" center of mass. The application will automatically calculate your car’s composite center of mass for you, but its important that you understand the contributing variables so that you can make informed design decisions. Let's start by considering a "barbell" with different masses on each end of the bar.
This equation defines the x coordinate of the composite center of mass for the barbell. This equation defines the x coordinate of the composite center of mass for the barbell. In this case there are only two objects (if we ignore the handle). As objects are added, the x and y coordinates for the composite center of mass will be defined as follows:
These formulas are much simpler than they look. What they essentially say is that each individual component in the system will affect the overall or composite center of mass by an amount consistent with its contribution to overall mass. If you increase the mass of the front axle assembly, for example, this will shift the overall center of mass forward and down.The center of mass helps us understand how an object will behave when external forces are applied. In an automobile, if the center of mass is high, the vehicle will tend to flip when going around a corner. SUVs and vans have a higher center of mass than sedans, which is why we sometimes hear about "rollover" problems with these vehicles. Race car designers try to keep the center of mass low and as close to the middle of the wheel base as possible. If the center of mass is located in the middle of the wheelbase, the car's mass will be evenly split between the front and rear axles, resulting in better cornering.
Likewise, the stability of a CO2 car depends on the location of the center of mass. Also important is the contact width of the car. A wider car is less likely to flip when raced. Again, there are trade-offs. As you will learn in the next section, the width of the car may affect forces that slow your vehicle. But, nothing will slow your car more than it flipping and sliding down the track on its side.
We can use a measure called the stability ratio to evaluate the stability of a dragster. The stability of a CO2 dragster is defined as the contact width of the car divided by the y-coordinate of the center of mass. The bigger this value, the better.
The contact width of the car is the average width of the front and rear axle assemblies, measured from the outside edges of the wheels. The following figure shows the front and rear contact width for a rail car. Remember to average these numbers to establish overall contact width. here are two categories of force that will affect your design - positive and negative. Positive forces accelerate (or push) the dragster toward the finish line. Negative forces decelerate (or pull) the dragster away from the finish line. The cartridge force is a positive force, while surface friction and drag are negative forces.
If you push your dragster forward, you are applying a positive force to the vehicle. When cartridge force acts on a dragster, it causes it to move. The force causes the car to increase velocity or accelerate. Remember Newton's law:
This equation tells us that there is a direct relationship between the variables force (F) and acceleration (a). Assuming mass is constant, if the force applied to an object increases, then the object will accelerate. Conversely, if the applied force is negative, then the object will decelerate. For all the dragsters in the challenge, a CO2 cartridge provides the force to move the vehicle forward. The cartridges contain a large volume of carbon dioxide gas under extreme pressure. The firing pin on the start gate makes a hole in the nozzle so the pressurized CO2 gas escapes producing thrust or force. Dragsters will accelerate until force stops. So we can assume from the test that a typical dragster will accelerate for about 0.45 seconds. At this point, maximum velocity is reached. How fast would the dragster be going at this point? If we assume an average force of 4.67N, we can use Newton's law to calculate maximum speed. This can be done by replacing acceleration with velocity divided by time, a = v / t.
We can use rules of algebra to solving for velocity:
Resistive forces are the forces working against or resisting the forward motion of your dragster. If there were no resistive forces, your dragster would continue to travel at its maximum velocity forever. If you take your foot off the accelerator in a car, the car slows down. Likewise, when the power from the CO2 cartridge stops, the dragster slows down. The forces that slow the vehicle include friction and drag. Drag occurs when a solid object moves through a gas or liquid. When your dragster is streaking down the track, air flow creates pressures and friction on the vehicle. Like surface friction force, drag force is a negative force working to slow your dragster.
To understand drag force you need to understand the variables involved. Drag force (FD) is caused by air friction on the body and the difference in air pressure on the front and rear of the body. The size and shape of the object determines the amount of friction and pressure force. The following equation shows the relationship of variables.
The net or total force acting on the vehicle is equal to:
- FC: Cartridge Force
- FD: Drag Force
- FF: Friction Force
The force from the cartridge (FC) is working to move the car forward. Drag (FD) and friction (FF) forces are working against this force. To maximize the net force, the equation tells us to increase the cartridge force and/or reduce the forces of drag and friction. Since we can't change the cartridge force, net force can only be increased by reducing all forms of drag and friction.
So, the key to building faster CO2 cars is to focus on the design factors that increase acceleration. Let's review Newton's Second Law of Motion.
This equation tells us that we can increase acceleration by increasing the net force acting on the car or decreasing its mass. Now that we have a better understanding of the forces acting on the car, we can expand our equation for acceleration to:
Citations/References: Dragster 2.0. (n.d.). Retrieved April 16, 2015, from http://front.whiteboxlearning.com/sessions/3fe1299d8da2525/index.html
Step 4: Criteria and Constraints
- 1/8” axel rod
- Small DC Electric motor
Each one of my designs needed to have a decrease in drag and weight, allowing it to go faster. Switching out the heavier materials for lighter materials and decreasing the size of the car also helped. Making it rounder also made it more streamlined, allowing my cars to go faster.
Step 6: Selecting an Approach
Constraint Design 1 Design 2 Design 3
Materials 3 3 3
Time 1 2 2
Weight 3 3 2
Surface Friction 3 3 2
Total 10 11 9
Step 7: Developing a Design Proposal
Step 9: Testing and Evaluating the Design
Step 10: Refining the Design
In order to make the fastest car, the car's mass must be close to the minimum required limit so it is not slowed down. This can be achieved by making a slimmer design of the car and changing the materials. To lower the drag of the car, its roundness must be increased as straight edges increase drag. Interior wheels also help with the speed of the car as well.
Step 11: Creating it or Making it
The car would have to be made with more professional material unlike straws and the sort. The body would also have to be made form higher quality wood. Same with the wheels as well.