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External combustion is a process in which a device, such as a motor or engine, is powered by fuel burned outside of the device. It is an alternative to traditional combustion engines, where fuel is burned within the engine itself. The steam engine is the classic example of external combustion. While steam technology is rarely used today, it has served as inspiration for other modern technologies that rely on external combustion to power a device.
During external combustion, some type of fuel is burned within an independent combustion chamber. Lines filled with fluid transfer this heat energy into the motor, where the sudden increase in heat is used to drive one or more pistons. These pistons transfer force or motion, creating a functional engine that can be used to power many types of machines. Once the heat has been removed from the fluid, the cooled fluid passes back into the combustion chamber for reheating. This transfer fluid may consist of gaseous materials, or simple deionized water.
While steam was once used to power trains and ships, modern external combustion systems are primarily used in electrical production. They are also popular with the military, which benefits from the flexible fuel sources used by this technology. External combustion engines may be used in manufacturing or industrial equipment, as well as equipment used in agriculture or construction. Some scientists even expect these engines to be used in standard passenger cars in the future.
The primary advantage of these engines is that they can be powered by any type of fuel. This includes traditional fossil fuels like coal and gas, as well as alternate fuel sources like biofuel or ethanol. Some external combustion units may also run off of burning wood or even trash. This makes it easy and convenient for people in remote areas to run equipment and machinery. It also helps to reduce dependence on limited fossil fuel supplies, and may cut pollution emissions.
These engines may also pose several drawbacks, depending on how and where they are used. In much of the world, the internal combustion engine has long been the preferred source of power for machinery and equipment. Despite the pollution and other problems associated with this technology, consumers are slow to adopt any new technologies. External combustion units are also relatively new compared to more widely used methods of power, and have not been tested to their full capabilities.
I just don't understand how the detonation occurs in a four stroke engine.