Figure 28 is an image the inside of a cylinder of an internal combustion engine probably created by a 3D computer model. Part of the cylinder wall and the cylinder head has been removed so that the viewer can see what else is going on in the cylinder. The focus is on the top of the cylinder. In the cylinder there is a piston which is close to the top of the cylinder. Above the cylinder and piston there are two valves, a fuel injector and a spark plug. The valves are shaped like upside-down mushrooms except that they have long slender stems and very wide flat heads. The fuel injector looks like a nozzle on the end of a pipe. The spark plug is cylindrical and its central and side electrodes are visible. The 3D modeller has attempted to show the injector spraying fuel into the cylinder head cavity above the piston and that fuel subsequent burning. That burning was started by the spark plug but since a spark is not being shown it is logical to presume that the spark has already occurred. Since the fuel is now burning and the gases in the cylinder head cavity are now expanding, it follows that the piston is moving down the cylinder doing what is known as the power stroke.