You've Got Permission to Be a Backseat Driver!
AbstractDo you like to tell adults where to go and how to get there? Well, here's your chance to do it in the name of science! In this science fair project, you will see what happens to your car's fuel efficiency when it takes you downtown to see a movie, up a hill for a great view, or out for a cruise on a flat country road.
Kenneth L. Hess, Science Buddies
Kristin Strong, Science Buddies
Edited by Peter Boretsky, Lockheed Martin
To determine your car's fuel efficiency under different driving conditions.
Imagine filling an empty milk jug with gasoline (gas). What could that gallon of gas do? Well, it won't sing or walk your dog for you, but if you could convert its energy content into electricity, it could run your refrigerator for ten days or wash your laundry for more than four months! Not bad, huh? Gas is a very concentrated source of energy that powers internal combustion engines like those found in most lawn mowers, leaf blowers, and cars. People don't worry too much about how many lawns they can mow (or leaves they can blow) on a gallon of gas, but they do think a lot about how many miles they can drive on a gallon of gas.
The number of miles a car can get out of one gallon of gas is known as fuel efficiency, and is measured (in the United States) in miles per gallon (MPG, for short). There are two types of MPG that people measure—instantaneous and average. Instantaneous means what is happening right now, at this instant, while average means what is happening over a period of time.
For example, if you took a snapshot every second of your instantaneous MPG for 5 seconds, you might get instantaneous readings of 24, 19, 23, 21, and 18, but your average over those 5 seconds would be 21 (the sum of those 5 numbers, divided by 5). The instantaneous MPG number is always changing, while the average MPG is more stable. In this science fair project you'll investigate the instantaneous MPG numbers and see how they change depending on what your car is doing—taking you downtown to a movie or up a hill for a great view.
Now what determines how many miles per gallon you can get out of your car? That depends on a lot of different factors, like:
- How good your car engine is at changing the energy in gas into rolling (mechanical) energy. This is known as your engine's efficiency.
- How you drive.
- Where you drive.
- Your car's activity; for example, running an air conditioner, moving at a constant velocity, or changing velocity.
- The forces on your car.
The forces (the pushes and pulls) on your car are very important factors. On a flat road, the main forces on your car in the horizontal direction are shown in Figure 1. Friction makes it difficult for the car to move across the road. It is the force that opposes motion at the points where the wheels contact the road. Air drag makes it difficult for the car to move through the air. It is the force that opposes or resists motion through the air. The wind can produce a force in any direction, or not at all. The car produces a force that is applied to the wheels when gas is burned inside the engine.
Diagram showing forces on a moving car. Forward force is applied by the wheels from burning gas, while friction acts on the car in the opposite direction. Also, the force of air drag opposes the forward motion of the car.
Figure 1. This drawing shows the horizontal forces on a car on a flat road.
If the sum of the forces pushing on the car from the right equals the sum of the forces pushing on the car from the left, then the forces are balanced. Newton's First Law of Motion says that if the forces are balanced, then the car will keep on doing what it is doing. If, for example, it is stopped, it will remain stopped. If, however, it is rolling at a constant velocity, it will stay at that same velocity. In these cases, the car's acceleration, which means a change in its velocity, will be zero.
If, however, the forces become unbalanced, there will be an acceleration that is not zero. The velocity of the car will change. This is Newton's Second Law of Motion.
According to Newton's laws, in an imaginary world (or in outer space) without any drag, friction, or wind, all your car would need to do is produce a single force and with one big push to the wheels, it would roll along on a flat surface forever! Your MPG number would be almost infinite! You get a sense of this if you have ever gone ice skating before, where the friction force is very low. All you need to do is push off with one foot and you can glide along for quite some distance before having to push off again. Bend over to reduce your air drag, and you can go even farther before you have to push off again!
In the real word though, the friction, drag, and wind forces are all very big and very real, so your car must burn gas to make a rotational force, which turns the wheels to keep your car moving at a constant velocity. Your car must burn even more gas to increase its velocity and accelerate.
So what's going on under your car's hood? How does your car burn gas and create this rotational force? Well, a small drop of gasoline and air are squeezed into a small space and then ignited with a spark. The resulting small explosion creates an expanding gas, which pushes a piston that is attached to a crankshaft, and that rotates your car's wheels. Don't miss this exciting HowStuffWorks animation of a piston and crankshaft in action. It's amazing to think there are thousands of small, controlled explosions going on every day inside cars on the road!
You've learned about the forces on a car when it's out cruising on a flat stretch of road, but what happens when it comes to a hill? As shown in Figure 2, when a car drives to the top of a hill, not only is it overcoming friction, drag, and wind, but it is doing the additional work of raising the weight of the car to the height of the hill. This additional work requires that the car burn more gas than it would out on a flat road. What do you think happens to the instantaneous MPG numbers your car can achieve when it is going up a hill? Do the numbers go up or down?
Figure 2. This drawing shows the additional work the car must do when climbing a hill.
When the car does this hard work of climbing the hill, it burns more gas to gain potential energy. Potential energy is a stored form of energy that has the potential or ability to do work. Potential energy can be changed into moving or kinetic energy when the car comes back down the hill. You have experienced this gain in potential energy and the change back into kinetic energy if you have ever huffed and puffed your way up a hill on a bike, and then coasted back down.
What do you think will happen to the instantaneous MPG measurements on your way back down a hill? If you don't know, there is one good way to find out! So buckle up, because here's your chance to play backseat driver and tell an adult where and how to drive!
Terms and Concepts
- Internal combustion engine
- Fuel efficiency
- Miles per gallon (MPG)
- Air drag
- Newton's First Law of Motion
- Newton's Second Law of Motion
- Potential energy
- Kinetic energy
- What does miles per gallon mean?
- What is the difference between instantaneous miles per gallon and average miles per gallon?
- What factors determine how many miles per gallon you can get out of your car?
- What do Newton's First and Second Laws of Motion say?
- What horizontal forces does a car experience on a flat road?
- How does an engine create a rotational force on the wheels?
- What happens to a car as it climbs or goes down a hill?
This source shows an animation of a piston:
- Brain, M. (2008). How Car Engines Work. HowStuffWorks, Inc. Retrieved June 25, 2008.
This source shows an animation of a crankshaft:
- Wikipedia Contributors. (2008, June 10). Crankshaft. Wikipedia: The Free Encyclopedia. Retrieved June 25, 2008.
Materials and Equipment
- Two adult volunteers, one who can drive
- Watch with a second hand
- Car with an instantaneous MPG meter
- Access to a flat road
- Access to a flat highway
- Access to a road or highway with a hill
- Access to a parking lot or road that you can safely stop on
Familiarizing Yourself with the Data Table
- Make a data table in your lab notebook, similar to the one shown. You and the adults who are going to help you should go through the data table and talk about where and how you are going to test each car activity. If you don't feel you can do a car activity safely, then you should skip that activity.
Getting the Passengers and the Car Ready
- Your first command as a backseat driver is to tell the two adults to get in the front seats of your car and buckle up. Have one adult sit in the driver's seat. He or she will drive and pay attention to safety and the road. Have the other adult sit in the front passenger seat. He or she will read the instantaneous MPG meter and call out numbers for you, when you are ready to start a trial.
- You, the student, should buckle up in the back seat with a stopwatch, a pen, and your lab notebook.
- Have an adult set the MPG meter for instantaneous mode rather than average mode. Consult your car's Owner's Manual for instructions on how to do this, if necessary.
Reading Your Car's Instantaneous MPG Meter While Driving Under Different Conditions
- Tell the driver what car activity you want him or her to perform using the data table. For example, for the first car activity, you might say, "Please go at a constant, low velocity of 25 miles per hour on a flat road."
- When the car is performing the desired activity, or ready to perform the desired activity, the driver should tell you that the car is now ready. You then tell the adults that you are going to start recording readings now. You say "Go," and start watching the second hand on your watch, and have the adult in the passenger seat call out instantaneous MPG readings so you can write them down in the data table. When 5 seconds have passed, you say "Done," and stop recording instantaneous MPG reading for this trial. For example, in 5 seconds you might write down 22, 19, and 20. The instantaneous MPG reading changes often because it is a "snapshot" of your car's fuel efficiency. The instantaneous MPG meter tells you what is happening to your car's fuel efficiency in each instant.
- Repeat steps 1-2 of this section for the car activity you are currently working on two more times so that you have a total of three trials for the activity.
- Repeat steps 1-2 of this section until you have completed all the car activities in the data table three times.
|Car Activity||Trial 1:
5 Seconds of Instantaneous MPG Readings
5 Seconds of Instantaneous MPG Readings
5 Seconds of Instantaneous MPG Readings
|Average of the Fuel Efficiencies from the Three Trials (MPG)|
|The car is going at a constant low, velocity of __________ miles per hour on a flat road. Choose and record a velocity that is appropriate for the speed limits on your road. Cruise control, if available and appropriate for your driving conditions, will help maintain a constant velocity.|
|The car is going at a constant, high velocity of ___________ miles per hour on a flat highway. Choose and record a velocity that is appropriate for the speed limits on your road. Cruise control, if available and appropriate for your driving conditions, will help maintain a constant velocity.|
|The car is speeding up from a stop to a constant velocity of ________ miles per hour. Choose and record a velocity that is appropriate for the speed limits on your road.|
|The car is coasting on a flat road with the driver's foot off the accelerator.|
|The car is braking.|
|The car is going up a hill at a velocity of __________ miles per hour. Choose and record a velocity that is appropriate for the speed limits on your hill.|
|The car is going down a hill with the driver's foot off the accelerator (and braking, if necessary).|
Analyzing Your Data Table
- For each car activity, average the instantaneous MPG readings you obtained in each trial. To get the average, add all the numbers together and divide by the number of readings.
- For each car activity, add up the averages for each trial that you calculated in step 1 and divide by 3 to get an average fuel efficiency, and record this average in the last column of your data table.
- Look at the last column and compare it to the car activities. Which activity gave you the best fuel efficiency? Which gave you the worst? Was it more efficient to drive at a slow, constant speed or at a faster, constant speed? Make a list of the car activities and rank them from most efficient to least efficient.
Ask an Expert
- Compare the fuel efficiency of different cars and trucks performing the same activity.
- Compare the fuel efficiency when your car is going up different grades or slopes of roads at the same velocity.
- Compare your car's fuel efficiency with the air conditioning turned on and off while the car is performing the same activity.
- Compare the fuel efficiency when the engine is cold and when the engine is hot. For each cold engine trial, you will need to cool down the engine before taking your MPG readings.
- Compare fuel efficiency on a highway with the windows open and with the windows closed. What force accounts for the difference?
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