The reciprocating engine uses the reciprocating motion of one or more pistons to convert pressure in a fluid into work. Generally this work is in the form of rotational movement, that is, we obtain mechanical energy. In the opposite direction, we find ourselves with rotating machines in which the movement of the machine parts is already rotating, such as turbines or the Wankel engine.
Reciprocating Engine Types
Reciprocating engines can be classified in many different ways. The main forms of classification are the following:
- According to the arrangement of the cylinders in the engine
- According to the displacement
- According to the compression ratio
- According to the ratio of diameter and stroke
- According to the number of movements of each cycle
Combustion engines are further classified in two ways:
- Spark ignition engines. It is also known as the gasoline engine, Otto engine, or internal combustion engine. In this case, the spark plug initiates fuel combustion.
- The diesel engine. It is also known as a compression ignition engine. In the diesel engine the air inside the cylinder is compressed, when it is compressed it heats up allowing the fuel to ignite.
Despite the importance of endothermic engines, there are other types of reciprocating engines:
Types of Positive Ignition Engines
The main types of positive ignition engines are as follows:
- Gasoline engine: It is the most common version since it is installed in an important part of cars. You have two fuel delivery options: an injector and a carburetor.
- Carburetor engine. One feature is the production of a mixture of fuel and gas in a special mixer, carburetor. Previously, such gasoline engines prevailed; Now, with the development of microprocessors, its field of application is rapidly diminishing.
- Injection engine. One characteristic is the reception of the fuel mixture in the manifold or in the engine cylinders feeding the fuel injection system.
- Gas engines. An engine that burns gaseous hydrocarbons as fuel under normal conditions: liquefied gas mixtures, compressed natural gases, gas obtained by converting solid fuel into gaseous fuel. As solid fuel are used: coal, peat, wood.
These engines are widely used, for example, in small and medium-power power plants, using natural gas as fuel (in the high-power area, gas turbine power units reign supreme).
Types of Compression Ignition Engines
The diesel engine is characterized by the ignition of fuel without the use of a spark plug. A portion of fuel is injected into the heated air into the cylinder from adiabatic compression (at a temperature that exceeds the ignition temperature of the fuel) through the nozzle.
During the injection of the fuel mixture, it is sprayed, and then around the individual droplets of the fuel mixture there are sites of combustion, as the fuel is injected, the fuel mixture burns in the form of a torch. Since diesel engines are not subject to detonation, the use of higher compression ratios is allowed.
Raising it more than 15 practically does not increase efficiency, since in this case the maximum pressure is limited by a longer combustion and a decrease in the injection advance angle. However, small size high speed vortex chamber diesel engines can have a compression ratio of up to 26, for reliable ignition under high heat removal conditions and for less stiffness (stiffness is caused by lag on ignition, characterized by an increase in pressure during combustion, measured in MPa / degree of crankshaft rotation).
Large supercharged marine diesel engines have a compression ratio of approximately 11..14 and an efficiency of more than 50%.
Diesel engines tend to be less fast and with the same power as gasoline, they are characterized by high shaft torque. Also, some large diesel engines are adapted to run on heavy fuels, such as fuel oils.
The launch of large diesel engines is carried out, as a rule, due to the pneumatic circuit with supply of compressed air or, in the case of diesel generator sets, from a connected electric generator, which, when launched, acts as an engine Boot.
Contrary to popular belief, modern engines, traditionally called diesel engines, do not operate according to the Diesel cycle, but rather according to the Trinkler - Sabate cycle with a mixed heat supply. The disadvantages of diesel engines are due to the peculiarities of the duty cycle: increased mechanical stress, which requires greater structural strength and, as a consequence, an increase in their dimensions, weight and increased costs due to complicated design and the use of more expensive materials.
In addition, diesel engines due to heterogeneous combustion are characterized by unavoidable soot emissions and a higher content of nitrogen oxides in the exhaust gases.
The main portion of the fuel is primed, as in one of the varieties of gas engines, but it is ignited not by an electric candle, but by a pilot portion of diesel fuel injected into the cylinder similar to a diesel engine.
Usually there is the possibility of working on a purely diesel cycle. Application: heavy trucks. Gas-diesel engines, like gas engines, produce less harmful emissions, and natural gas is cheaper.
Such an engine is often obtained by retrofitting one in series, while the diesel fuel saving (degree of gas substitution) is about 60%. Foreign companies are also actively developing such designs.
History of the Alternative Engine
A first example in the history of the known rotary to reciprocating motion is the crank mechanism. The oldest manual cranks appeared in China during the Han Dynasty (202 BC-220 AD). Several sawmills in Roman Asia and Byzantine Syria during the 3rd-6th centuries AD had a connecting rod mechanism that converted the rotary motion of a water wheel into the linear motion of saw blades. In 1206, the Arab engineer Al-Jazari invented a crankshaft.
The reciprocating engine was developed in Europe during the 18th century, first as a naturally aspirated engine and later as the steam engine. These were followed by the Stirling engine and the internal combustion engine in the 19th century. Today, the most common form of reciprocating engine is the internal combustion engine that runs on the combustion of gasoline, diesel, liquefied petroleum gas (LPG), or compressed natural gas (CNG) and is used to power motor vehicles and motor plants.
Reciprocating Engine Terminology
Not all reciprocating engines correspond to the scheme described, but the essential parts and their operation are similar.
The following is a terminology used universally in the field of internal combustion engines or endothermic engines. This terminology is used to indicate some fundamental dimensions and values of this type of engine.
- Top dead center (TDC). Position of the piston closest to the cylinder head. Lower Dead Center (PMI). Position of the piston furthest from the cylinder head.
- Lower Dead Center (PMI). Position of the piston furthest from the cylinder head.
- Diameter (in English: Bore). Inner diameter of the cylinder. Generally expressed in millimeters (mm).
- Carrera (in English: Stroke). It includes the distance between the TDC and the PMI, it is equal, with rare exceptions, to twice the radius of the crank shaft of the crankshaft. It is generally expressed in mm.
- Total volume of the cylinder (V 1 ). It is the space between the cylinder head and the piston when it is in the PMI It is generally expressed in cm 3
- Combustion chamber volume (V 2 ). It is between the cylinder head and the piston when the latter is at TDC It is usually expressed in cm 3
- Volume dislodged by the piston or displacement (V 1 - V 2 ). It is the generator by the piston in its reciprocating movement from TDC to PMI: It is usually expressed in cm 3 .
- Volumetric compression ratio ( ). It is understood by such that there is between the total volume of the cylinder V1 and the volume of the combustion chamber V2. In general, for short, it is simply called a compression ratio: