One of the
requirement for an efficient engine is the correct amount of heat shock,
delivered at the right time. This requirement is the responsibility of the
ignition system. This ignition system supplies properly timed, high-voltage
surges to the spark plugs. These voltage surges cause cause combustion inside
the cylinder. 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. Secondly, it must maintain that
spark long enough to allow for the combustion of all the air and fuel in the
cylinders. Lastly, it must deliver the spark to each cylinder so combustion can
begin at the right time during the compression stroke of each cylinder.
combustion process is completed, a very high pressure is exerted against the top
of the piston. This pressure pushes the piston down on its power stroke. This
pressure is the force that fives the engine power. For an engine to produce the
maximum amount of power it can, the maximum pressure from combustion should be
present when the piston is at 10 to 23 degrees after top dead center (ATDC).
Because combustion of the air/fuel mixture within a cylinder takes a short
period of time, usually measured in thousandths of a second (milliseconds), the
combustion process must begin before the piston is on its power stroke.
Therefore, the delivery of the spark must be timed to arrive at some point
before the piston reaches top dead center.
Determining how much before TDC the spark should begin gets complicated by the
fact that as the speed of the piston as it moves 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 tend to slow down the acceleration of the piston and the
spark should be somewhat delayed.
out when the spark should begin gets more complicated with 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. Other factors, including intake air temperature,
humidity, and barometric pressure, also affect combustion. Because of all of
these complications, delivering the spark at the right time is a difficult task.