CAR ENGINES How do car engines work?  SuChin looks under the hood to examine the workings of an engine.
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Getting Started

Begin the lesson by having small groups of students do the following activity: Place two teaspoons of baking soda in a gallon-size, zipper-type storage bag. Put a cup containing three ounces of vinegar inside the bag, being careful not to spill any vinegar from the cup. Zip the bag closed. Hold the bag out away from you over a sink or garbage container. Pour the vinegar onto the baking soda.

Ask the students the following questions: What happens to the bag? What happened inside the bag to cause this reaction? How is this type of reaction, where chemical energy is transformed into mechanical energy, similar to what happens inside a car engine? How are they different?


It's hard to imagine living in a world where there are no cars, buses, or trucks. When it comes to important inventions, the internal combustion engine has to be near the top of the list. Unlike most steam engines that preceded them, internal combustion engines are small enough to fit in personal vehicles, such as cars. Unlike electric motors, these little powerhouses can travel a long distance on a compact fuel source.

As with most inventions, the internal combustion engine is really the product of many individuals working over a long period of time. Early experiments with engines that burned liquid fuel started back in 1838, but it wasn't until 1876 that a German engineer named Nikolaus Otto created one that actually worked.

Little has changed in the Otto-cycle engine over the last 120 years. The key element behind its power involves igniting a small amount of gasoline inside a confined space called the cylinder. As the fuel explodes, it produces a great deal of hot gas, which presses against the face of a piston, pushing it down in the cylinder. The other end of the piston is connected to a piston rod that turns a rotating crankshaft, which in turn is linked to the car wheels. The up-and-down motion of the piston makes the crankshaft turn, just as the up-and-down pedaling turns the crank of a bike. It is this rotary motion of the crankshaft that runs the car's engine.

For all this motion to take place smoothly, four distinct actions or strokes occur in the engine. During the intake stroke, the piston moves down in the cylinder and a mixture of air and fuel enters the cylinder through a valve in the top. In the compression stroke, the valve closes and the piston begins to move back up the cylinder, compressing the mixture.

Once the piston reaches the top of the cylinder, the spark plug ignites the fuel, which drives the piston back down. This is called the power stroke because it's where the power comes from. In the final exhaust stroke, the piston moves back up again and a second valve opens to allow the spent gas to escape. Then the cycle starts all over again.

Typical car engines have either four, six, or eight cylinders. It is important that all of the piston movements are timed to move in an orderly way. Otherwise, the engine won't run smoothly.


1. How did the development of the internal combustion engine change the settlement pattern of people over the last 100 years? 2. What are some of the disadvantages associated with the internal combustion engine and how might they be corrected?

Student Activity
Build a piston system to convert linear to rotary motion.


For a car to move, the wheels have to turn in a rotary motion. Internal combustion engines are powered by pistons which move in a back-and-forth or linear motion. By building and testing a model piston/crankshaft system, you'll discover how this energy transfer takes place and learn why there is an upper limit to how fast an engine can run.


  • two 11" x 17" pieces of cardboard or chipboard
  • strong pair of scissors and glue
  • three 2" brass paper fasteners
  • 2" roofing nail
  • metric ruler and stopwatch
  • 1. Create a piston by copying and enlarging the illustration below. Cut the parts (piston head, piston rod, crankshaft, and cylinder sides) out of one of the pieces of cardboard or chipboard. You will mount the parts on the other piece.

    2. Using the point of the nail, carefully punch out four holes in the crankshaft, one hole at each end of the piston rod, and one hole midcenter at the bottom of the piston head. Assemble the piston head, piston rod, and crankshaft using the brass fasteners. Glue the cylinder sides to your base board, allowing enough room between them for the piston head to move up and down without getting stuck. Finally, punch a hole in the base board so that you can attach the free end of the crankshaft to it. Make sure you've allowed enough room so that the piston rod can turn the crankshaft completely when the piston head moves up and down.

    3. Gently push the piston up and down in the cylinder.

    4. Using the ruler, measure the length of the stroke that the piston makes during one complete turn of the crankshaft. The stroke distance (D) is the difference between the highest and lowest point in the piston's head during one rotation of the crankshaft.

    5. Determine the maximum number of complete cycles the piston can make in 15 seconds. To do this, carefully slide the piston up and down as quickly as you can without bending the apparatus.

    6. Now connect the piston rod to the next hole toward the center of the crankshaft and repeat steps 4 and 5, recording your results. Repeat this for all the holes in the crankshaft.


    1. What happened when you attached the piston rod closer to the center of the crankshaft?

    2. Was it easier or harder to make the crankshaft turn quickly?

    3. Did the crankshaft get stuck in any part of its motion? In a real engine, would this happen, too? Why or why not?


    Brian Show Number: 1501


    Books and articles

    Heywood, J. (1988)
    Internal combustion engine fundamentals.
    New York: McGraw-Hill.

    Macaulay, D. (1988)
    The way things work.
    Boston: Houghton Mifflin Company.

    Computer software

    Microsoft Home Essentials:
    Microsoft Encarta 97 Encyclopedia.
    (800) 454-9497


    Society of Automotive Engineers
    400 Commonwealth Drive
    Warrendale, PA 15096-0001
    (412) 776-4841

    Web sites

    Society of Automotive Engineers

    University of Michigan Automotive Research Center

    Woman Motorist Magazine On-Line

    Try This:

    For an engine to run efficiently, the motor has to keep cool. Otherwise, the metal parts will begin to expand and eventually lock up or Òseize.Ó See if you can design a Òradical radiatorÓ that dissipates the heat from a cup of hot water. Try using flexible plastic tubing for the core and aluminum foil fins.

    Try This:

    Car transmissions use gears to speed up the rate at which the wheels turn. You can see how they work by checking out the gear cluster on a 10-speed bike. Turn the bike upside down. As you turn the pedals, switch gears on the rear wheel. See how the speed of the tire changes relative to the speed of the crank and size of the gear. Come up with a mathematical relationship to predict these speed changes?

    Try This:

    What role does oil play in an engine andhow do different types of oil behave? Check out samples of a 30-weight, 50-weight, and multiweight oil. How do their properties compare.

    NEWTON'S APPLE video cassettes and educational materials provide further information about this and other topics. Call 1-800-588-NEWTON.

    Copyright 1997,
    Twin Cities Public Television

    We encourage duplication for educational non-commercial use.Educational materials developed with the National Science Teachers Association.NEWTON'S Apple is a production of KTCA Saint Paul/Minneapolis.Made possible by a grant from 3M.

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