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Heat engines can generally be considered energy conversion machines. They convert chemical energy in a fuel, commonly a fossil fuel, into work by combusting the fuel in air to produce heat. This heat is used to raise the temperature and pressure of a working fluid that is then used to perform useful work. Heat engines can be classified as:
They can also be classified as either reciprocating or rotary. In reciprocating engines, the working fluid is used to move a piston in a linear fashion. The linear motion is then commonly converted to rotary motion through a connecting rod/crankshaft mechanism. In a rotary engine, the working fluid spins a rotor that is directly connected to the output shaft.
In external combustion engines, the working fluid is entirely separated from the fuel-air mixture. Heat from the combustion products is transferred to the working fluid through the walls of a heat exchanger. An example of a reciprocating external combustion engine is the Stirling engine where heat is added to the working fluid at high temperature and rejected at low temperature. Heat added to the working fluid can be generated from practically any heat source, such as burning fossil fuels, wood, or any other organic material. The steam engine is another example of a reciprocating external combustion engine. Heat added from an external source elevates water temperature until it is converted into steam that provides pressure and eventually net work. Steam engines powered cars in the USA between 1900 and 1916, however, they all but disappeared by 1924. Reasons for their demise included the size and number of the major components required for their operation such as furnace, boiler, turbine, valving, as well as their complicated controls . The steam turbine—still in operation—is an example of a rotary external combustion engine.
In internal combustion engines, the working fluid consists of the products of combustion of the fuel-air mixture itself. Reciprocating piston engines are perhaps the most common for of internal combustion engine known. They power cars, trucks, trains and most marine vessels. They are also used in many small utility applications. They can be fuelled with liquid fuels such as gasoline and diesel fuel or gaseous fuels such as natural gas and LPG. Two common sub categories of reciprocating piston engines are the two-stroke and the four-stroke engine. Examples of rotary internal combustion engines include the Wankel rotary engine and the gas turbine.
Common goals in the design and development of all heat engines include: maximizing work (power output), minimizing energy consumption and reducing pollutants that may be formed in the process of converting chemical energy to work. Figure 1 shows the main components of a reciprocating internal combustion engine. Intake and exhaust valves are omitted for simplicity, however it is worth noting that in some two-stroke engine designs inlet and exhaust ports are used rather than valves which are commonly used in four-stroke engines.
Figure 1. Basic Components of a Reciprocating Engine
Both two- and four-stroke reciprocating internal combustion engine may be equipped with either a spark-ignited (SI) combustion system or a compression-ignited (CI) system. Spark-ignited engines use a combustion cycle known as the Otto cycle, while compression-ignited engines use the Diesel cycle. Spark-ignited systems are characterized by a homogeneous or mostly well mixed charge of fuel and air. In this medium combustion is initiated by a spark and the flame propagates along a front from the spark location to the opposite side of the combustion chamber.
In diesel engines, air is compressed to a high pressure and temperature into which fuel is injected. The charge of air and fuel in these engines is described as heterogeneous meaning that a large spatial variation in the fuel-air mixture proportions exists. Some regions of the combustion chamber will be entirely air and others entirely fuel. Between these extremes, a mixture of fuel and air will exist in varying proportions. The temperature of the air rises when it is compressed in the cylinder. Upon injection, the fuel evaporates in this high temperature environment and mixes with the hot surrounding air in the combustion chamber. The temperature of the evaporated fuel reaches its auto-ignition temperature and ignites to start the combustion process. The auto-ignition temperature of fuel depends on its chemistry. Unlike the SI system, combustion in compression-ignited engines occurs at many points where the air-fuel ratio and temperature can sustain this process.