AUTOMOTIVE
Ignition System 

Purpose of the Ignition System

• For each cylinder in an engine, the ignition system has three main jobs.
  
• • First
  • • It must generate an electrical spark that has enough heat to ignite the air/fuel mixture in the combustion chamber.
      
  • Second
  • • It must maintain that spark long enough to allow for the combustion of all the air and fuel in the cylinders.
  • Last
  • • It must deliver the spark to each cylinder so combustion can begin at the right time during the compression stroke of each cylinder.

• A very high pressure is exerted against the top of the piston when the combustion process is completed. This pressure pushes the piston down on its power stroke and is the force that gives the engine power.

• For the engine to produce the maximum amount of power it can, the maximum pressure from combustion should do present when the piston is at 10 to 23 degrees after top dead center. Therefore, the delivery of the spark must be timed to arrive at some point before the piston reaches top dead center.

• How much before TDC the spark should begin gets complicated because of the fact that as the speed of the piston moving from its compression stroke to its power stroke increases, the time needed for combustion stays about the same. This means the spark should be delivered earlier as the engine's speed increases.
  
• • However, as the engine has to provide more power to do more work, the load on the crankshaft tends to slow down the acceleration of the piston and the spark should be somewhat delayed.
    
  • Figuring out when the spark should begin gets more complicated due the fact that the rate of combustion varies according to certain factors.
    
  • • Higher compression pressures tend to speed up combustion.
    • Higher octane gasoline ignite less easily and require more burning time.
    • Increased vaporization and turbulence tend to decrease combustion times.
    • Intake air temperature, humidity, and barometric pressure also affect combustion.
      
  • Because of all of all these complication, delivering the spark at the right time is a difficult task.

• There are two types of ignition systems found on today's car.
   
• • Distributor ignition (DI)

• Electronic ignition (EI)
  
• • Called distributor less or direct ignition (DIS)

Ignition Timing

• Ignition timing refers to the precise time spark occurs and is specified by referring to the position of the number 1 piston in relation to crankshaft rotation.

• Ignition timing reference marks are located on engine parts and on a pulley or flywheel to indicate the position of the number one piston. Vehicle manufacturers specify initial or base ignition timing.

• When the marks are aligned at top dead center (TDC), or 0, the piston in cylinder number 1 is at TDC of its compression stroke.

• Additional numbers on a scale indicate the number of degrees of crankshaft rotation before TDC (BTDC) of after TDC (ATDC).

• In a majority of engines, the initial timing is specified at a point between TDC and 20 degrees BTDC.

• If optimum engine performance is to be maintained, the ignition timing of the engine must change as the operating conditions of the engine change.

• These conditions affect the speed of the engine and the load on the engine.

• All ignition timing changes are made in response to these primary factors.

Engine RPM

• For best performance, two factor must be considered.

1. At higher rpm's, the crankshaft turns thought more degrees in a given period of time. If combustion is to be completed by 10 degrees ATDC, ignition timing must occur sooner or be advanced.
2. However, air/fuel mixture turbulence (swirling) increases with rpm. This causes the mixture inside the cylinder to turn faster. Increased turbulence requires that ignition must occur slightly faster or be slightly retarded.

• These two factors must be balanced for best engine performance.

• Therefore, while the ignition timing must be advanced as engine speed increases, the amount of advance must be decreased some to compensate for the increased turbulence.

 

Engine Load

• The load on an engine is related to the work it must do.

• Driving up hills of pulling extra weight increases engine load.

• Under load there is resistance on the crankshaft, therefore the pistons have a harder time moving through their strokes.

• Under light loads
• • With the throttle plate partially opened the amount of air/fuel mixture drawn into the manifold and cylinders is small.
  • On compression, this thin mixture produces less combustion pressure and combustion time is slow.
  • To complete combustion by 10 degrees ATDC, ignition timing must be advanced.
• Under heavy loads
• • When the throttle is opened fully, a larger mass of air/fuel mixture can be drawn in.
  • High combustion pressure and raid burning results.
  • The ignition timing must be retarded to prevent complete burning from occurring before 10 degrees ATDC.

 

Firing Order

• Each cylinder of an engine must produce power once in every 720 degrees of crankshaft rotation. Each cylinder must have a power stroke at its own appropriate time during the rotation.

• To make this possible, the pistons, and rods are arranged in a precise fashion called the engines firing order.

 

 

 

 

• The firing order is arranged to reduce rocking and imbalance problems.

 

• Because the potential for this rocking is determined by the design and construction of the engine, the firing order varies from engine to engine.

 

 

 

 

 

• Vehicle manufacturers simplify cylinder identification by numbering each cylinder.

 

 

 

 

 

<< Previous    [1]  2    Next >>