Lesson3/Learning Event 1
This power is then carried by high-voltage wires to a step-down transformer near your home. This
reduced voltage is transferred through wires to the fuse box in your home. From the fuse box, the
electrical power is carried by wires to the switch you flipped and eventually to the light fixture that
When you step into your car and start it, you again perform what appears to be a simple act. You
merely turn the ignition switch to the start position until the engine is running and then release the
switch. Now, let's see what was actually involved in this act.
From previous studies, you know that a spark produced at the instant the fuel-air mixture of a
cylinder is compressed to the proper pressure will cause a combustion that will drive the piston
We also know that when that piston returns to the same position again, another spark will ignite
the mixture again.
Just think how fast these sparks must occur at just the right instant in an eight-cylinder engine
running at 4,000 RPM.
The ignition system is one of the most interesting (and troublesome) systems found on a gasoline
engine. It is interesting because it must build up the vehicle's battery voltage from about 24 volts
or less to as much as 25,000 or 30,000 volts, and it must do this many times per second. It is
troublesome because so many things can and do go wrong in the system.
To give you some idea about how fast the ignition system builds up the battery voltage to as much
as 30,000 volts at the spark plug, let us take a six-cylinder engine turning at 4,000 RPM and see
what the ignition system is doing. As you know, in a fourstroke cycle engine, one-half of the
cylinders fire during each revolution of the crankshaft. This means that three cylinders of a six-
cylinder engine will fire during each revolution. By multiplying the number of RPM by the number
of cylinders firing each revolution, we find that the ignition system in our example must deliver 3 x
4,000 or 12,000 high-voltage surges or sparks per minute. This is equal to 200 sparks per second.