Lesson 1/Learning Event 1
FIGURE 13. POWER STROKE.
Just as the piston finishes the compression stroke, the ignition system causes an electrical spark to jump across
the spark plug gap. (The spark occurs inside the combustion chamber.) This really happens just before the
piston reaches TDC, but for our purposes here, we will think of it as being at TDC.
The compressed fuel-air mixture is ignited or set on fire by an electric spark. The mixture burns so fast that it
explodes. This causes high pressure in the cylinder that drives down on the piston head with great force.