Combustion engine
An internal combustion engine (in the broadest sense) is a machine that, by burning fuel, converts its chemical energy into mechanical work. Combustion of fuel or fuel mixture can occur in the engine, but also outside the engine. In a narrower sense, the term internal combustion engine usually means a piston internal combustion engine [1]. The article deals with the combustion engine in a broader sense.
The principle of work
Some substances, generally called fuels, are capable of a chemical combustion reaction that releases heat. If the released heat heats up the working gas, according to the equation of state, its pressure or volume will increase, depending on whether it is in a closed or open space. We can use this state of gas to perform mechanical work, either by applying pressure to the moving part of the engine, or by using the law of action and reaction when the working substance flows from the engine. Bringing out the obtained mechanical work in a usable form is only a matter of a suitable design solution. The whole process takes place in accordance with the second law of thermodynamics with only a certain efficiency.
Some variant of the mentioned general principle is present in the implementation of every internal combustion engine. In doing so, the following applies:
fuels can be solid, liquid or gaseous,
the environment in which they burn is most often air, but it can also be another substance,
combustion can take place inside or outside the engine,
the working gas can be separate, or it can be the flue gas directly,
the working gas is continuously exchanged or is enclosed in the engine,
the moving part of the engine that is affected by the working gas can move in different ways,
one, the other, or both methods can be used to obtain mechanical work.
Considering the above, it is not surprising that the group of internal combustion engines includes many types and different designs, so it is useful to use the right criteria for their division.
Division
Combustion engines are divided according to several aspects. Individual points of view are mutually independent, so a simple tree structure cannot be created in this case
Basic division
Combustion engines are divided into:
depending on where the combustion takes place (inside or outside the engine) on
internal combustion engines (most current combustion engines)
engines with external combustion, where it is possible to distinguish whether the energy reaches the working space
supply of working gases (for example, a gas turbine)
by heat transfer through a heater of the working substance (for example, a Stirling engine)
according to the work cycle on
engines with a continuous duty cycle (for example, a combustion turbine)
engines with an interrupted duty cycle
two-stroke engine
four-stroke engine
According to the basic working principle on:
piston combustion engines
reciprocating piston engines (most current internal combustion engines)
reciprocating piston engines (Wankel engine)
vane engines (turbines)
jet engines (jet or rocket)
Reciprocating combustion engines are divided into:
spark ignition - combustion is caused by energy from an external source - the spark of a spark plug
diesel - when combustion causes heating of the compressed mixture above the ignition temperature
engines with combined ignition
glow engines
In common practice today, an internal combustion engine is understood as an internal combustion engine.
Other divisions
There are other more detailed considerations. The following list of criteria is not exhaustive, in addition, individual categories of internal combustion engines, such as turbines, have specific categories of their own.
according to the type of fuel burned, engines are divided into:
gas - burning gaseous fuels, for example natural gas
for liquid fuels - oil (gasoline, diesel) or other (alcohol)
for solid fuels - for example, powdered coal
multi-fuel - which can switch to burning another fuel with or without modifications to the engine
dual-fuel - which burn several types of fuel at the same time
according to the place of preparation of the mixture:
with the formation of a mixture outside the working area - for example, in the carburetor, combustion chamber
with the formation of a mixture in the working space - for example in the cylinder of diesel engines
according to the method of preparation of the mixture:
carburetor engines
injection engines
according to the way air is delivered to the cylinders:
engines with atmospheric filling of the cylinders (natural), when intake is provided only by the vacuum caused by the movement of the piston in the cylinder,
turbocharged engine, when the filling of the cylinder under a greater pressure than the surrounding atmosphere is provided by an additional device in the filling system. The charging devices can be mechanical blowers or even compressors, for example in the VW Corrado (G-Lader) car, or compressors used in Bentley cars (Bentley Blower) turbochargers, for example in TDI engines from VW.
according to work purpose for: stationary, mobile, industrial, power plant, ship, railway, vehicle, tractor, special and others.
Piston combustion engine
Further information in the main article: Reciprocating internal combustion engine
These engines are the most numerous representative of combustion engines in technical equipment. They are almost exclusively used to drive motor vehicles. Their basic part is a piston that makes a sliding or rotating movement and mediates the conversion of the pressure energy of the gas filling into the mechanical energy of the rotating shaft.
Petrol engine
More information in the main article: Spark combustion engine
The basic characteristic of spark-ignition engines is that the mixture in the cylinder above the piston is ignited most often by a spark between the spark plug electrodes. The mixture needs an external energy source for ignition. The gasoline engine can work as a two-stroke or a four-stroke.
Diesel engine
More information in the main article: Diesel engine
The basic characteristic of diesel engines is that the mixture, which is formed directly in the combustion chamber, is ignited from the so-called compression heat. These engines have such a high compression ratio that when the air sucked into the cylinder is compressed, the temperature in the combustion chamber at the moment of fuel injection is higher than the ignition temperature of the fuel. Even a diesel engine can work as a two-stroke or four-stroke.
Rocket engine
For more information, see the main article: Rocket engine
The rocket engine has a special position among other combustion engines:
it does not draw the working substance from the atmosphere during operation, but in addition to fuel, it must also have a sufficient supply of oxidizer
the useful output of the engine is not mechanical work but the reactive effect of the combustion gases
apart from auxiliary systems (e.g. pumps, turning nozzles), it does not contain moving parts in the main energy conversion system.
Taking into account the above fundamental differences, some parts of the text below do not apply to rocket engines.
History
The first proposals for using gunpowder as a source of mechanical energy appeared already in the 17th century. In France, Hautefeuille and Denis Papin dealt with the issue, and in the Netherlands, Christian Huygens. At the end of the 18th century, Street in Great Britain proposed to use a mixture of liquid fuel with air, and Barber designed the first combustion turbine. In the 19th century, Lebon patented a double-acting gas engine, S. Brown built a gas engine and built it into a vehicle in 1826. Étienne Lenoir constructed in 1860 a two-stroke double-acting slide engine for light gas. In 1867 Nikolaus Otto and Langen constructed an atmospheric gas engine, in 1873 Reithmann a four-stroke gas engine, in 1878 Otto again a recumbent four-stroke gas single-acting engine. In 1879, Kostovič constructed a spark-ignition engine with a power of up to 60 kW to drive airships. In 1884, Gottlieb Daimler built the first high-revving four-stroke spark-ignition engine. In 1893, Rudolf Diesel described a working cycle for the use of pulverized coal or other heavy fuels. In the 1910s, there were attempts to inject fuel with a pump rather than compressed air. Serial production of automobiles, and thus also their engines, began in 1908 by the Ford company. In 1912, the Swiss company Sulzer built the first locomotives powered by a diesel engine.
Advantages and disadvantages of internal combustion engines
Each type of combustion engine has its advantages and disadvantages and is therefore more or less suitable for driving individual devices.
The main advantages of combustion engines are:
in the piston design, they achieve high energy conversion efficiency. Combustion turbines achieve high efficiency only in high-power units
they can be put into operation quickly (with the exception of engines with large powers)
they can be designed for different fuels, purposes and sizes
especially with liquid fuels, they make it possible to achieve low fuel consumption, high action radii, low power weights, i.e. j. they are suitable for driving vehicles.
The main disadvantages of combustion engines are:
inappropriate impact on the environment
require a foreign source of energy for launch (except for rockets)
piston engines have a disadvantageous course of performance characteristics
they do not achieve a long service life
Subsystems
Electric starter
The engine needs other subsystems for its real work.
The fuel system ensures fuel storage, distribution and filtering.
The mixture preparation system ensures the preparation of the air and fuel combustion mixture in sufficient quantity, quality and suitable composition.
The cooling system ensures the removal of residual heat and engine operation in an optimal thermal mode.
The lubrication system ensures the lubrication of the moving parts of the engine.
The control system (today almost exclusively electronic) ensures the regulation of engine operation based on external conditions and the condition of the engine itself.
The electrical system ensures sufficient power for the electrical powered systems.
The ignition system (for gasoline engines) ensures reliable ignition of the mixture in the combustion chamber.
Power regulation
The performance of an internal combustion engine depends mainly on the amount of energy released by burning the fuel. We can regulate the amount of released energy in two ways.
Regulation of the amount of the mixture, otherwise also called quantitative regulation. With a smaller amount of mixture, less energy is released and thus the engine delivers less power. This regulation is used by spark-ignition engines without working with a homogeneous mixture. Most often, this regulation is implemented by a throttle valve in the intake manifold.
Regulation of the composition of the mixture, otherwise also called qualitative regulation. The same amount of mixture that contains less fuel releases less energy. This regulation is mainly used by diesel engines working with a heterogeneous mixture. Most often, this regulation is represented by changing the length of the fuel injection.
Mixture components
Fuels
Hydrocarbons are most often used as fuels: volatile (gasoline), volatile (diesel), compressed natural gas (CNG), liquefied propane-butane (LPG), alcohols (methanol, ethanol).
Their various mixtures are also used, or the engine is used as a multi-fuel engine with fuel switching while driving. Recently, biological additives such as rapeseed methyl ester (MERO) are beginning to be added to fuels.
Intensive work is being done to replace petroleum fuels with hydrogen.
The air
In addition to fuel, an essential component of the mixture for most internal combustion engines is air, as it contains the oxygen necessary for combustion.
Energy balance
The heat supplied in the fuel is distributed in the combustion engine as follows:
Qd = Qe + Qch + Qv + Qns + Qol + Qzv
Qd - heat supplied in the fuel
Qe - heat equivalent to the useful work of the engine
Qch - heat removed to the cooling system
Qv - heat removed by exhaust gases
Qns - heat unused by incomplete combustion of fuel, also called chemical losses
Qol - heat dissipated by the lubricating oil
Qzv - heat not included in the previous terms (other losses)
The proportion of the most prominent components of the equation is shown in the table, with Qd corresponding to 100%.
Motor type % Qe % (Qch + Qol) % Qv
petrol 21 to 28 12 to 27 30 to 55
diesel 29 to 42 15 to 35 25 to 45
Thermal energy that causes heating of the engine and exhaust gases (Qch, Qv, Qol) is unusable and is called wasted energy.
From the mentioned comparison, it follows that the diesel engine will use the supplied energy more efficiently, so it works with higher efficiency.
Of the heat equivalent to the useful work Qe, a part is still consumed for mechanical losses (friction and drive of auxiliary aggregates).
Emissions
Combustion engines are a significant source of emissions
More information in the main article: Vehicle emissions
Considering its mass distribution, the internal combustion engine is one of the important sources of environmental pollution. They are a natural product of the perfect combustion of hydrocarbon fuel
CO2 - carbon dioxide.
H2O - water.
In addition to them, other harmful compounds are also formed:
NOx - nitrogen oxides, mostly represented by nitrogen dioxide. They are formed by the dissociation of nitrogen at high temperatures and its subsequent oxidation.
NH3 - ammonia. They are caused by insufficient combustion of fuel.
CO - carbon monoxide. It is created by incomplete combustion without sufficient air (oxygen) access.
CHx - unburnt hydrocarbons. They are caused by insufficient combustion of fuel.
SO2 - sulfur dioxide. It is created by burning sulfur admixtures in fuel.
O3 - ozone. It is formed during the reactions of NOx and CHx.
solid particles - soluble or insoluble (soot) - especially in engines burning hard-to-evaporate fuels (diesel engines).
aldehydes - partially oxidized hydrocarbons.
The internal combustion engine is also a significant source of noise emissions.
Reducing emissions
At the current level of knowledge, it is not possible to mass replace the internal combustion engine with another alternative drive, suitable for automobile transport, which would preserve the advantages of the internal combustion engine and not have its disadvantages. Therefore, manufacturers of engines and motor vehicles work intensively to reduce the adverse effects of using internal combustion engines.
Some of the following techniques can be used to reduce emissions:
Increasing engine efficiency. This technique brings a reduction in fuel consumption, which will also reduce the volume of emissions created by the engine under otherwise identical conditions. For reducing C02 emissions, this technique is the only one possible, since carbon dioxide is a product of perfect (and therefore clean) combustion.
Use of additional equipment. The task of these devices is to increase the purity of exhaust gases before they are released into the atmosphere. The technical implementation is catalysts (three-way, or reacting only to a certain component) or flue gas filtration.
Design modifications of the engine. These are mainly changes in the compression ratio, valve timing, the size and shape of the injection holes, an increase in the number of valves, a change in the shape of the combustion chambers, and a reduction in frictional losses.
Affecting the course of combustion. It is realized by changing the time and volume courses of the injection, by using glow plugs even at low loads, by changing the flow of the filling in the cylinder.
Adjustment of fuels. Fuels with a higher proportion of hydrogen, a lower content of sulfur and polycyclic hydrocarbons, with an increased cetane and octane number are desirable.
Work cycle change. For engines with quantitative regulation, the cylinder deactivation technique CDA (Cylinder DeActivation) can be used.
By influencing the filling of the cylinders. At partial loads, it is possible to use the exhaust gas recirculation technique - EGR (Exhaust Gas Recirculation).