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A combustion chamber is the area in an engine where fuel is burned. In an internal combustion engine, fuel is burned in the space directly on top of its pistons. Internal combustion engines are widely used in small, mobile vehicles. In the external combustion engine, fuel is burned in a space physically separated from the walls of its cylinders. External combustion engines were extensively used in the 19th century to power large steamships and trains.
The combustion chamber of an internal combustion engine is bounded in part by the surface of a piston. When fuel is ignited in this combustion chamber, hot gasses can directly act upon the piston to push it down. Since the pressure of the gas is high, it rapidly expands to follow the piston and continue pushing on it. By the time the gas pressure is low again, the gas has pushed the piston to its bottom position. A crankshaft connected to the pistons will turn as the pistons oscillate.
One widespread application of the internal combustion engine is power for mobile vehicles, such as automobiles, boats and propeller airplanes. The heat in an internal combustion chamber is created inside the engine itself, so it can rapidly act on engine pistons. This relative efficiency allows internal combustion engines to be smaller and lighter than their external-combustion counterparts. For this reason, internal combustion engines are widely used for mobile applications with relatively low power needs.
The steam engine, on the other hand, is an external combustion engine. This means that the combustion chamber is located outside of the cylinders. External combustion engines require heat transfer through the solid walls of the engine. Heat transfer through a stationary solid object is called conduction. Conduction can be a slow process when temperature differences are small, so steam engines typically use large combustion chambers that produce a lot of heat.
In the steam engine, coal is burned in a combustion chamber known as a firebox to produce heat. The heat flows through the walls of another container full of water known as a boiler. When the water boils, high-pressure steam is temporarily directed into one or more cylinders to act on a piston. The linear movement of the piston is converted into the circular movement of a crankshaft to drive the engine.
Both internal and external combustion engines are forms of the reciprocating engine. These engines ultimately convert chemical energy into mechanical work, with heat being the intermediate form of energy. Even under ideal conditions, they cannot convert 100% of the stored chemical energy into mechanical work because some heat will always escape the system.
This is the type of basic information anyone needs to know to understand engines and how they work. Engines have gotten more powerful and efficient over the years, but all the technological advances haven't altered the basic physics behind how engines work.