Introduction
Significant growth of all sectors of the economy requires the movement of large quantities of cargo and passengers. High maneuverability, mobility and adaptability to work in different conditions makes the car one of the primary means of transportation of goods and passengers. The share of road transport accounts for over 80% of goods transported by all modes of transport combined, and more than 70% of passenger traffic. In recent years, the factories of the automobile industry mastered many samples of upgraded and new automotive vehicles, including those for agriculture, construction, trade, oil, gas and forest industries. Currently, there are many devices that use the thermal expansion of gases. These devices refer to gasoline engines, diesel engines, turbojet engines, etc.
1. The engines with external combustion.
2. The internal combustion engines.
The internal combustion engine is by far the most common type of engine. The list of vehicles in which it is set is huge. Internal combustion engines can be found in cars, helicopters, tanks, tractors, boats, etc.
The internal combustion engine is a heat engine, which converts the chemical energy of burning fuel into mechanical energy. They can be divided into different categories: by working cycles - 2-and 4-stroke; according to the method of preparation of the combustible mixture from an external (in particular gasoline) and internal (e.g. diesel engines) carburetion. By the form of the energy converter of the internal combustion, engines are divided into piston, turbine, jet and combined.
Advantages of piston internal combustion engine, ensured by its widespread use, are: autonomy, universality, low cost, compactness, low weight, fast start, variety of compatible fuel.
However, internal combustion engines have a number of disadvantages, which include: high level of noise, a large frequency of rotation of the crankshaft, the exhaust emissions, low resource, low efficiency.
First four-stroke engine was introduced by Nikolaus Otto in 1876, and it therefore also bears the name of the engine with the Otto cycle. A more proper name of such a cycle - four-stroke cycle. Currently this is the most common type of engine for cars.
The working cycles of engines
In the first cycle, the fuel is atomized or vaporized and is sucked into the working chamber together with air. The mixture of fuel and air is compressed and then ignited by an external source (often an electric-spark discharge), which is the beginning of power generation due to the heat of the burning mixture. In the second cycle, working fluid, i.e. air, is compressed independently, and the fuel is injected in liquid form at the end of the compression period. The ignition is carried out after the fuel is transferred from the hot compressed air. Requirements for fuel depend on the type of engine. The gasoline engine operating on the Otto cycle, should work with the fuel, causing detonation to compress the fuel-air mixture. It is necessary that it burns evenly, without premature ignition and has no unburned residue (Grc.nasa.gov).
Thermodynamic cycles of engines
Different requirements for internal combustion engines, depending on their purpose, led to the creation of various types of these engines. However, from the thermodynamic point of view they can be classified according to the nature of the processes. The cycles used in engines, can be divided into the following three types:
1) cycle with heat supply at constant volume;
2) cycle with heat supply at constant pressure;
3) a mixed cycle in which heat is supplied at constant volume and at constant pressure.
Cycle with heat supply at constant volume.
A feature of engines operating on this cycle is the external preparation of the working mixture, which is then fed into the cylinder where it is compressed and ignited by electric sparks, the combustion occurs very quickly and the process can be viewed as occurring at constant volume. Since the external mixture formation is performed at low temperature, the engine can run on light fuels, which are well mixed with air. This engine was first built in 1876 by Otto and it worked with the gas mixture (Encyclopedia Britannica).
Theoretical cycle with heat supply at υ = const consists of two adiabatics and two isochors (Fig. 2). In the processes 1-2, adiabatic compression of working mixture takes place. At the point 2, it is ignited by an electric spark and burns in the process 2-3 at constant volume. In process 3-4, adiabatic expansion of the combustion products occurs, the piston moves and work is done for the. At point 4 the exhaust valve opens and the pressure in the cylinder drops to atmospheric.
Thus, portion of the exhaust products of combustion leaves the chamber of the cylinder. As a result, the reciprocating motion of the piston pushes the remains of products of combustion and absorbs the next portion of the working mixture. On the theoretical diagram (Fig. 2) these processes coincide with Isobaric Pa, but conventionally, they combine with the isochoric process 4-1, which gives the amount of heat q2, actually carried out together with the removed gases.
Real cycles of internal combustion engines considerably differ from the theoretical, therefore the theoretical analysis also introduces different assumptions. As a working body in the study of the cycles of internal combustion engines was adopted by an ideal gas, the number and properties of which are constant (in reality they change as a result of combustion of the atomized fuel).
The processes of compression and expansion are not adiabatic, because there are friction and the heat transfer between the cylinder walls and the gas in a real engine. Process 2-3 in reality also differs from the isochoric due to the movement of the piston during fuel combustion. Due to the development of all processes in time, certain points of transition from one process to another (point 1, 2, 3, and 4) are absent in real cycles and the processes succeed each other gradually (Fig. 1).
In engines operating on this cycle, instead of working mixture, they compress the, whose temperature at the end of the compression process (point 2 in Fig. 3) exceeds the ignition temperature of the fuel and amounts to 600-800°C. Because of this, atomized liquid fuel, supplied to the cylinder, mixing with air, ignites spontaneously and burns, and the fuel feed is regulated so that the combustion is at constant pressure (Isobar 2-3). The atomization of fuel, supplied to the cylinder is performed by compressed air (pressure of 5-9 MPa) supplied from a special compressor (such engines are often called compressor engines). In process 3-4 occurs adiabatic expansion of the combustion products, and process 4-1 similar to the same in the cycle with combustion at υ=const. This cycle was first proposed and constructed by Diesel.
The disadvantage of these engines is the presence of a high pressure compressor, which reduces the reliability and complicates the design and consumes some engine power. So they are now superseded by natural pressure engines, in which the atomization of fuel is the fuel pump.
Mixed cycle
The engines on the mixed cycle, are more perfect in comparison with engines with Isobaric combustion, as they have no compressor. The first patent for a high pressure engine was given in 1906 by Russian engineer G. Trinkler (Trinkler). However, these engines became widespread much later when engineers managed to make fine atomization of fuel with fuel pump and injectors of a special construction. Currently, transport engines that use heavy fuel mainly work on the mixed cycle.
In the combined cycle, as in the cycle with Isobaric combustion, compression is exposed to the air. The fuel is fed into the cylinder by a pump at the end of compression (point 2 in Fig. 4) at a pressure of 30-150 MPa and due to the high temperature of air, it ignites spontaneously. The supply of fuel under high pressure creates favorable conditions for good cutting and mixing thereof with air, which provides sufficiently complete combustion of the fuel and increases the efficiency of the engine. The combustion process is at constant volume (Isochore 2-3), and then at constant pressure.
Works Cited
Encyclopedia Britannica,. "Nikolaus August Otto | German Engineer". N.p., 2016. Web. 15 Feb. 2016.
Grc.nasa.gov,. "Four Stroke Internal Combustion Engine". N.p., 2016. Web. 15 Feb. 2016.
TRINKLER, Gustav. "INTERNAL GOMBUSTION ENGINE FOR LIQUID COMBUSTIBLE;". 1906: n. pag. Print.
