• Parts of the semi engine.engine image by Tomasz Pawlowski from Fotolia.com
  
  Engines on a semi truck are very similar to those on a regular car or truck. The primary differences are that engines on a semi truck are far larger and run on diesel instead of gasoline. The reason for the larger size is that they need to produce greater torque in order to get the heavy loads they carry to move. There are four main parts of any engine: pistons, crankshaft, valvetrain and spark plugs. The parts of the engine are essentially the same as a regular car, with only a difference in size and at times the number of certain parts.
  
  

Pistons

• Most engines have a number of pistons encased in cylinders which slide back and forth, creating the power in the engine. On top of the piston is the combustion chamber, where fuel and oxygen ignite, creating downward force that drives the engine. While every part of an engine is important, the pistons are especially important in that if there is a problem the whole engine often has to be rebuilt, as such a large and fundamental part is damaged. In a car, there are generally between four and eight pistons, whereas a semi truck many times has 12 to 16 pistons.
  
  

Crankshaft

• Connected to the pistons is the crankshaft, which converts the up and down motion of the pistons into turning power. Essentially, the pistons turn the crankshaft, which builds the necessary force to propel the vehicle. It is similar to the way that legs moving up and down on bicycle pedals turn the wheels.
  
  

Valvetrain

• There are a number of parts in the valvetrain: valves, rocker arms, pushrods, lifters, and the camshaft. The function of the valvetrain is to let fuel and air in and out of the engine at the correct time. If air or fuel is not let in at the right moment, then combustion cannot take place, and if fuel is let in at the wrong time, the resulting combustion can cause serious damage to the engine. The camshaft opens and closes the valves, which let in the air and fuel.
  
  

Spark Plugs

• Spark plugs are located toward the top of the cylinder, and function by igniting the fuel and oxygen combination once the proper amount of pressure is created. As the pistons lower, fuel is injected into the chamber of the cylinder, and as the piston moves back up, the fuel is compressed, created pressure which will make the resulting combustion more powerful. Once the piston nears the top of the cylinder, the spark plug ignites the fuel and the resulting combustion drives the piston downward, which repeats the process.