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Car Engines

 

Overview

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.

Activity

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. Materials
  • 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. Questions 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?
  • Resources

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

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

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

      Web sites
      Society of Automotive Engineers
      www.sae.org
      University of Michigan Automotive
      Research Center
      arc.engin.umich.edu
      Woman Motorist Magazine On-Line
      www.womanmotorist.com