Automotive Engines 

Engine Construction

• Modern engines are designed to meet the performance and fuel efficiency demands of today's car buyers. 

• Each of these engine design has its own distinct personality, based on construction materials, casting configurations and design. 

• The cylinder block is a large casting of metal that is drilled with holes to allow for passage lubricants and coolants through the block and provide spaces for movement of mechanical parts. 

• The cylinder head sits on top of the cylinder block and closes off and seals off the top of the cylinder. 

 Engine Classifications

• Today's automotive engines can be classified in several ways depending on the following design features:

• • Operating cycles.

  • • Most people will generally come in contact with only four-stroke engines.
       

  • Number of cylinders.

  • • Current engine designs include 4, 5, 6, 8, 10, and 12 cylinders.
       

  • Cylinder arrangement.

  • • An engine can be flat (opposed), inline or V-type.
       

  • Valve train type.

  • • Overhead camshaft (OHC), overhead Valve (OHV) and dual overhead camshaft (DOHC)
       

  • Ignition type.

  • • There are two types of ignition systems; spark and compression. 
       

  • Cooling systems.

  • • There are both air-cooled and liquid-cooled. 
      

  • Fuel type.

  • • Several types of fuel currently used in automobile engines, including gasoline, natural gas, methanol, diesel and propane. 

Four-Stroke Gasoline Engines

•  Passenger cars and truck:

• • The engine provides the rotating power to drive the wheels through the transmission and driving axles. 
    

• Vehicle engines:
   

• • Both gasoline and diesel, are classified as internal combustion because the combustion or burning takes place inside the engine.
     
  • These systems require an air/fuel mixture that arrives in the combustion chamber at the correct time and an engine constructed to withstand the temperatures and pressures created by the burning of thousands of fuel droplets. 
  
  
  
• • The combustion chamber is the space above the top of the piston and the cylinder head. It is an enclosed area in which the fuel and air mixture is burned. 

Four stroke cycle:

• Cycle

• • A series of events that happen over and over.
     

• Stroke 

• • The movement of the piston in the cylinder from top to bottom of the cylinder or from bottom to top of the cylinder. 
• • Four strokes are required to complete one cycle. 

• Intake Stroke:

  
   
• • As the piston moves away from top dead center (TDC), the intake valve opens. 
     
  • • The downward movement of the piston increases the volume of the cylinder above it, reducing the pressure in the cylinder. 
    • This reduced pressure, commonly referred to as engine vacuum, causes the atmospheric pressure to push a mixture of air and fuel through the open intake valve. 
    • As the piston reaches the bottom of its stroke, the reduction in pressure stops, causing the intake air/fuel mixture to slow down. 
    • It does not stop because of the weight and movement of the air/fuel mixture. 
    • • it continues to enter the cylinder until the intake valve closes. 
    • The intake valve closes after the piston reaches bottom dead center (BDC). 

• Compression Stroke:
   

• • The compression stroke begins as the piston starts to move from BDC. 
  • • The intake valve closes, trapping the air/fuel mixture in the cylinder. 
    • The upward movement of the piston compresses the air/fuel mixture, thus heating it up. 
    • At TDC, the piston and cylinder walls form a combustion chamber in which the fuel will be burned. 
    • • The volume of the cylinder with the piston at BDC compared to the volume of the cylinder with the piston at TDC determines the compression ratio of the engine. (8:1, 9:1, 10:1 etc.) 

• Power Stroke:

• 
  
• • The power stroke begins as the compressed fuel mixture is ignited. 
  • • With the valves still closed, an electrical spark across the electrodes of a spark plug ignites the air/fuel mixture. 
    • The burning fuel rapidly expands, creating a very high pressure against the top of the piston. 
    • This drives the piston down toward BDC. 
    • The downward movement of the piston is transmitted through the connecting rod to the crankshaft. 

• Exhaust Stroke:
   

• • The exhaust valve opens just before the piston reaches BDC and on the power stroke. 
  • • Pressure within the cylinder causes the exhaust gas to rush the open valve and into the exhaust system. 
    • The movement of the piston from BDC pushes most of the remaining exhaust gas from the cylinder. 
    • As the piston nears TDC, the exhaust valve begins to close and the intake valve starts to open. 
    • The exhaust stroke competes the four-stroke cycle. 
    • • The opening of the intake valve starts the cycle over. 

• Cylinder Design:

• • Depending on the vehicle, each of the following cylinder design can be used: 
  • • In-Line 
    • V-Type 

   

  • In-line engines 
    
    
  • • In the in-line engine design the cylinders are all placed in a row. 
    • • There is one crankshaft and one cylinder head for all the cylinders. 
      • The block is cast so all cylinders are located in an up-right position. 

• The V-type engine design has two rows of cylinders located 60 to 90 degrees away from each other. 
  

•  Valves and Camshafts:
   

• • There two basic valve and camshaft placement configurations of the four-stroke gasoline engines used in vehicles. 
  • • Overhead Valve (OHV) 
    • Overhead Cam (OHC) 

• Overhead Valve (OHV):

• • As the name implies, the intake and exhaust valves on an overhead valve engine are mounted in the cylinder head and are operated by a camshaft located in the cylinder block. 
  • In these engine, with the camshaft in the block, the valves are operated by valve lifters and push rods that are actuated by the camshaft. 

•  Overhead Cam

   (OHC): 
   
• • An overhead cam engine also has the intake and exhaust valves located in the cylinder head. But, as the name implies, the cam is also located in the cylinder head. 
  • • Some engines have separate camshafts for the intake and the exhaust valves. 
    • These are called dual overhead camshaft (DOHC) engines. 

 Valve and Camshaft Operation
 

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