Situated on the inside as opposed to the surface or exterior.

An engine that burns fuel inside closed metal cylinders.

External combustion engines; such as, steam engines, produce power less efficiently by burning fuel in an external chamber to heat a liquid or gas, which then moves a piston or a turbine.

Each piston in an internal combustion engine makes four "strokes"

The four-stroke cycles include explosions inside the engine's cylinders, on top of the pistons, and the blast force pushes the pistons down; then, the crankshaft swings around and pushes the pistons back up for the next stage in the cycle. The cycles for one piston are described in the following sequences:

1. The piston moves down, sucking in air through the inlet valve while a tiny squirt of gasoline is injected into the air.
2. The inlet valve at the top closes, trapping everything inside and then the piston moves up, squeezing the air and gasoline tightly together.
3. When the piston reaches the top, a carefully timed spark sets fire to the gasoline; as the gas burns explosively, forcing the piston back down.
4. Finally, the piston moves back up and pushes the burned gases out of the outlet valve which leave the car through the exhaust.

The trapping of light by refraction and reflection at critical angles inside a semiconductor device so that it cannot escape the device and must be eventually absorbed by the semiconductor.