Internal Combustion Engine
In an internal combustion engine, the working fluid consists of a combustible fluid placed inside a cylinder. Four-stroke Diesel and petrol (gasoline) engines are internal combustion engines. In these engines, the fluid undergoes combustion inside the cylinder and expands. The expansion pushes a piston which is attached to a crankshaft and produces motion. There are small differences in the operating principles between petrol engines and Diesel engines: the petrol engine follows the so-called Otto cycle whereas the Diesel engine follows the Diesel cycle. The Diesel engine also does not use a spark to produce combustion, and it is more efficient than a petrol engine.
Gas turbine engines used in aircraft are also a type of internal combustion engine. Here, a compressor takes in air from the atmosphere and compresses it down to high pressures. Fuel is added to pressurized air and ignited. This air moves through a set of turbines, causing them to rotate. Eventually, the air leaves through an exhaust.
External Combustion Engine
In an external combustion engine, the combustion takes place outside the cylinder. Heat then needs to be transferred to the cylinder where work is done. Steam engines are an example of external combustion engines. In steam engines, the water is boiled in a container, producing steam. The steam then expands and travels through a set of tubes, eventually arriving at the piston, which is situated elsewhere.
Steam engines were used to power vehicles in the past, steam trains being an obvious example. Energy losses in steam engines are comparatively much greater. A significant amount of heat is lost on the way from the boiler to the piston. Steam engines are also quite bulky, giving them low power-to-weight ratios. Today, steam engines are commonly used at power stations.
Stirling engines are another type of external combustion engine. These have a low power-to-weight ratio as well, however, theoretically, they should be able to reach high levels of efficiency. A lot of research is being done to see if the actual efficiency of these engines could be improved. Since Stirling engines could theoretically use any power source, they may also have a great deal of potential in the future when renewable energy sources would be in wider use.
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