• The fuel/air mixture inside the cylinder must be compressed for the engine to work.engine image by peter Hires Images from Fotolia.com
  
  A car or truck engine draws a fuel/air mixture into the combustion chamber and compresses it before firing; that is, the piston pushes the fuel/air mixture into the smallest space that can efficiently support ignition. The purpose of compression is to pack the fuel molecules as close together as possible so ignition will exert the most pressure against the top of the piston.
  
  

Compression Ratio

• Designers of engines use the phrase "compression ratio" to measure the compression used by a particular engine. They measure the volume of the combustion chamber when the piston is at its lowest point and the volume when the piston is at its highest point and compare the two numbers. For example, in a 427-cubic-inch V-8 engine, the volume of each combustion chamber might measure 55 cubic inches at it's maximum size and five cubic inches when the piston is all the way to the top. The compression ratio would be 10-to-1.
  
  

Measuring Compression

• The compression inside a combustion chamber is usually around 200 pounds per square inch, depending on the engine design. A mechanic removes a spark plug from the cylinder and screws a fitting attached to the hose of a compression test gauge into the spark plug hole. Another mechanic turns the engine over without starting it. The compression test gauge shows the psi of the cylinder. The poorest performing cylinder should be within 20 per cent of the best and all the cylinders should be within ten percent of the of the manufacturers specifications.
  
  

Examples

• Gasoline engines use compression ratios between 7.5-to-1 and 13-to-1 depending on the grade of gasoline used. Diesel engines depend on compression to set off the fuel. As air is compressed, it heats up. A diesel engine will have a compression ratio of 14-to-1 to 23-to-1. The piston compresses the air inside the combustion chamber before the fuel is sprayed inside. The temperature of the air is high enough to ignite the fuel immediately.
  
  

Complexities

• Pistons and cylinder heads are rarely flat. Engine designers carve out special hollows in the pistons and change the shape of the top of the cylinder to make combustion more complete. The designers want the fuel/air mixture to swirl around in a circle or move quickly from one side of the combustion chamber to the other. Two combustion chambers with the same compression ratio will not necessarily produce the same horsepower.