Lecture 4
VALID ICE CYCLES
1. The difference between the actual cycles of four-stroke engines from theoretical
1.1. Indicator diagram
2. Gas exchange processes
2.1. Influence of valve timing on gas exchange processes
2.2. Gas exchange process parameters
2.3. Factors affecting gas exchange processes
2.4. Exhaust Gas Toxicity and Pollution Prevention Ways
3. Compression process
3.1. Compression process parameters
4. Combustion process
4.1. Combustion rate
4.2. Combustion chemical reactions
4.3. Combustion process in a carburetor engine
4.4. Factors affecting the combustion process in a carburetor engine
4.5. Detonation
4.6. The combustion process of a fuel mixture in a diesel engine
4.7. Diesel hard work
5. Expansion process
5.1. Purpose and course of the expansion process
5.2. Expansion process parameters
Difference of actual cycles of four-stroke engines from theoretical
The highest efficiency can theoretically be obtained only as a result of using the thermodynamic cycle, the variants of which were considered in the previous chapter.
The most important conditions for the flow of thermodynamic cycles:
· Invariability of the working fluid;
· The absence of any heat and gas-dynamic losses, except for the obligatory heat removal by the refrigerator.
In real piston internal combustion engines mechanical work is obtained as a result of actual cycles.
The actual engine cycle is a set of periodically repeating thermal, chemical and gas-dynamic processes, as a result of which the thermochemical energy of the fuel is converted into mechanical work.
Real cycles have the following fundamental differences from thermodynamic cycles:
Actual cycles are open, and each of them is carried out using its own portion of the working fluid;
Instead of supplying heat in actual cycles, the combustion process takes place, which proceeds at finite speeds;
The chemical composition of the working fluid changes;
The heat capacity of the working fluid, which is real gases of varying chemical composition, is constantly changing in actual cycles;
There is a constant heat exchange between the working fluid and the surrounding parts.
All this leads to additional heat losses, which in turn leads to a decrease in the efficiency of actual cycles.
Indicator diagram
If thermodynamic cycles depict the dependence of the change in absolute pressure ( R) from the change in the specific volume ( υ ), then actual cycles are shown as dependences of pressure changes ( R) from changes in volume ( V) (collapsed indicator chart) or pressure changes from the crankshaft angle (φ), which is called the expanded indicator chart.
In fig. 1 and 2 show a collapsed and expanded indicator diagrams of four-stroke engines.
A detailed indicator diagram can be obtained experimentally using a special device - a pressure indicator. Indicator diagrams can also be obtained by calculation based on the thermal calculation of the engine, but they are less accurate.
Rice. 1. Rolled up indicator diagram of a four-stroke engine
forced ignition
Rice. 2. Expanded indicator diagram of a four-stroke diesel engine
Indicator diagrams are used to study and analyze the processes occurring in the engine cylinder. So, for example, the area of the collapsed indicator diagram, limited by the lines of compression, combustion and expansion, corresponds to the useful or indicator work L i of the actual cycle. The value of the indicator work characterizes the useful effect of the actual cycle:
, (3.1)
where Q 1- the amount of heat supplied in the actual cycle;
Q 2- heat losses of the actual cycle.
In a valid loop Q 1 depends on the mass and heat of combustion of the fuel introduced into the engine per cycle.
The degree of utilization of the supplied heat (or the efficiency of the actual cycle) is estimated by the indicator efficiency η i, which is the ratio of heat converted to useful work L i, to the heat of the fuel supplied to the engine Q 1:
, (3.2)
Taking into account the formula (1), the formula (2) of the indicator efficiency can be written as follows:
, (3.3)
Consequently, the heat use in the actual cycle depends on the magnitude of the heat loss. In modern internal combustion engines, these losses are 55–70%.
The main components of heat loss Q 2:
Losses of heat with exhaust gases to the environment;
Heat loss through the cylinder walls;
Incomplete combustion of fuel due to a local lack of oxygen in the combustion zones;
Leakage of the working fluid from the working cavity of the cylinder due to leaks of adjacent parts;
Premature release of exhaust gases.
To compare the degree of heat utilization in real and thermodynamic cycles, the relative efficiency is used
V car enginesη o from 0.65 to 0.8.
The actual cycle of a four-stroke engine takes two revolutions of the crankshaft and consists of the following processes:
Gas exchange - fresh charge inlet (see Fig. 1, curve frak) and exhaust gas release (curve b "b" rd);
Compression (curve akc "c");
Combustion (curve c "c" zz ");
Extensions (curve z z "b" b ").
When a fresh charge is injected, the piston moves, releasing a volume above itself, which is filled with a mixture of air and fuel in carburetor engines and clean air in diesel engines.
The start of the intake is determined by the opening of the intake valve (point f), the end of the inlet - by closing it (point k). The beginning and end of the release correspond to the opening and closing of the outlet valve, respectively, at points b " and d.
Not shaded area b "bb" on the indicator diagram corresponds to the loss of indicator work due to a pressure drop as a result of opening the exhaust valve before the piston reaches BDC (pre-release).
Compression is actually carried out from the moment the intake valve is closed (curve k-c "). Before closing the intake valve (curve a-k) the pressure in the cylinder remains below atmospheric ( p 0).
At the end of the compression process, the fuel ignites (point with") and quickly burns out with a sharp increase in pressure (point z).
Since the ignition of a fresh charge does not occur at TDC, and combustion proceeds with the continued movement of the piston, the design points with and z do not correspond to the actual processes of compression and combustion. As a result, the area of the indicator diagram (shaded area), and hence the useful work of the cycle, is less than the thermodynamic or calculated one.
The ignition of a fresh charge in gasoline and gas engines is carried out from an electrical discharge between the electrodes of the spark plug.
In diesel engines, fuel is ignited by the heat of the air heated from compression.
The gaseous products formed as a result of fuel combustion create pressure on the piston, as a result of which an expansion stroke or a working stroke is performed. In this case, the energy of thermal expansion of the gas is converted into mechanical work.
Engine indication. Determination of power
Indicator diagrams, taken in compliance with the necessary conditions, allow you to determine the indicated power and its distribution over the engine cylinders, to study the gas distribution, the operation of injectors, fuel pumps, and also to determine the maximum cycle pressure p z , compression pressure p with, etc.
Removal of indicator diagrams is carried out after warming up the engine with a steady thermal regime. After removing each diagram, the indicator should be disconnected from the cylinder by a 3-way indicator valve and an indicator valve on the engine. The indicator drums are stopped by disconnecting the cord from the drive. The indicator piston and its rod should be lightly lubricated from time to time after taking a few diagrams. The engine should not be indicated when the sea state exceeds 5 points. When removing indicator diagrams, the indicator drive must be in good working order, indicator cocks are fully open. It is recommended to take the diagrams simultaneously from all cylinders; if the latter is not possible, then their sequential removal must be carried out as soon as possible at a constant engine speed.
Before displaying, it is necessary to check the serviceability of the indicator and its drive. The piston and indicator bushing must be completely seated; when the spring is removed from the upper position, the lubricated piston must descend in the cylinder slowly and evenly under its own weight. The piston and indicator sleeve are lubricated only by the cylinder or engine oil, but not instrumental, which is included in the indicator kit and is designed to lubricate the joints of the writing mechanism and the upper part of the piston rod. The spring and the nut (cap) holding the spring must be screwed in completely. The lifting height of the indicator writing pin must be proportional to the gas pressure in the indicated cylinder, and the drum rotation angle must be proportional to the piston stroke. The clearances in the pivot joints of the transmission mechanism should be small, which is checked by slight swinging of the lever with the piston stationary, and there should also be no backlash. When the indicator communicates with the working cavity of the cylinder with a stationary drum, the writing pin of the indicator should draw a vertical straight line.
The indicator is connected to the drive either with a special indicator cord or with a special steel tape measuring 8 x 0.05 mm. Drive cord - linen, braided; before installation, a new cord is pulled out during the day, suspending a weight of 2 - 3 kg to it. In case of an unsatisfactory condition of the cord, significant distortions of the indicator diagram are obtained. Steel tape is used for engines with a speed of 500 rpm and above, as well as if the speed is less than 500 rpm, but the connection between the indicator and the drive looks like a broken line 2 - 3 m long. compression diagrams with fuel off. If the compression line matches the expansion line, then the cord is serviceable. The length of the indicator cord must be adjusted so that in the extreme positions the drum does not reach the stop. With a short cord, it breaks, with a long one - the diagram has a shortened form ("cut off"), since at the end of the piston stroke the drum will be stationary. During the indication, the cord must be constantly taut.
When drawing the atmospheric line, make sure that it is located at a distance of 12 mm from the bottom edge of the paper for indicators of model 50 and 9 mm - model 30. In this case, the writing mechanism will operate in the most optimal measuring range and maintain a correct record of the suction line under line of atmospheric pressure. The length of the diagram should be no more than 90% of the maximum drum stroke.
The indicator cord must lie in the swinging plane of the indicator drive arm. In the middle position of the lever, the cord should be perpendicular to its axis. The indicator should be installed so that the cord does not touch pipelines, machine grids and other parts. If it touches, and this is not eliminated by changing the position of the indicator, then a transition roller is installed. In this case, it is necessary to maintain the perpendicularity of the cord from the roller to the axis of the lever of the drive indicator at the middle position of the latter. The pressure of the pencil (writing pin) should be adjusted so that it does not tear the paper, but leaves a thin, clearly visible mark. The copper pin must always be well sharpened. Strong pencil pressure will increase the area of the diagrams. The paper should fit snugly against the indicator drum.
Before installing the indicator, thoroughly purge the indicator valve of the engine to avoid clogging of the channels and piston. Before reading the diagram, repeat the purge through the 3-way indicator valve. Before indicating the engine, the indicator should be well warmed up. Failure to comply with this requirement leads to distortion of indicator diagrams. When installing and removing the indicator, do not use an impact tool when tightening and kickbacking the union nut. This is done using a special key included in the indicator kit.
Indicators and indicator springs must be checked by the supervisory authorities at least once every two years and have a certificate of validity. The state of the indicator drive is checked with the engine running by taking compression diagrams with the fuel supply turned off. With a properly adjusted indicator drive, the compression and expansion lines should match. If defects are detected in the gas distribution mechanism during the analysis of the indicator diagrams, it is necessary to take measures to eliminate them. After correcting the defects, re-index and process (analyze) the indicator diagrams.
Conventional indicator charts for analyzing the change in work flow of motors operating with variable load. They are filmed in series on a continuous tape, followed one after the other at a set interval.
The taken indicator diagrams are analyzed before processing, since due to insufficient motor regulation or due to a malfunction of the indicator, its drive or violation of the indication rules, the indicator diagrams may have various distortions.
Planimetry.
Indicator diagrams are processed in the following sequence: adjust the planimeter and planimetry all diagrams; determine their areas; measure the lengths of all diagrams and the values of the ordinates p c and p z, count p i , for each cylinder. The planimeter is adjusted according to the area of the circle outlined by the bar attached to the planimeter. In the absence of a special bar, the planimeter readings are checked squarely on graph paper. Planimetry is performed on a smooth board covered with a sheet of paper. When installing the planimeter, its levers in relation to the diagram are positioned at an angle of 90 °. When tracing the diagram, the angle between the planimeter levers should be 60 - 120 °.
The length of the indicator diagram is measured along the atmospheric line. The actuator travel should be selected so that the length of the diagram is 70 and 90 - 120 mm for indicators of models 30 and 50, respectively.
In the absence of a planimeter, the average indicated pressure p i is found with sufficient accuracy by the trapezoid method. To do this, the diagram is split vertical lines into 10 equal parts.Average indicatorpressure is determined by the formula
pi = Σ h/ (10m),
where Σ h- sum of heights h1, h2 h10,
mm; T -
scale of indicator spring, mm / MPa. Method of measuring ordinatesh, p
z
and R
with
shown in Fig. 4.6. When taking indicator diagrams in each individual case, for a comparative assessment of the distribution of the load on the cylinders, it is necessary to take into account the temperature of the exhaust gases.
Each site is divided in half and its height is measured in the middle. When registering the results of the indication on the form of the taken diesel diagram, it is necessary to indicate the name of the vessel, the date of the indication, the brand of the diesel engine, the cylinder number, the scale of the spring, the length and area of the diagram, the obtained parameters p z, p s, p, -, N e, n... The processed indicator diagrams of each engine are pasted into the "Indexing log" with a corresponding analysis of the indexing results. The explanatory text should indicate the identified deficiencies in the engine adjustment and the measures taken to eliminate them. At the end of the voyage, the "indexing log" and the set of processed diagrams must be submitted to the MSS of the fleet together with the voyage machine report. When processing diagrams taken from high-speed diesel engines, it is necessary to make a correction for the error of the indicator writing mechanism, which in some cases can reach 0.02-0.04 MPa (added to the main value).
Analysis of the combustion process using diagrams and oscillograms
An indicator diagram is a graphical representation of the relationship between cylinder pressure and piston stroke.
Methods for obtaining (removing) indicator diagrams
To obtain indicator charts, mechanical indicators are used either electronic systems measuring the pressure of gases in the cylinder and fuel during the injection process (MIPCalculator, pressureanalyzer) (NK-5 Autronica and CyldetABB). To obtain complete indicator diagrams using a mechanical indicator, the engine should be used. equipped with an indicator drive.
Types of indicator charts
With the help of mechanical indicators, the following types of indicator diagrams can be obtained: normal, displaced, comb diagrams, compression, gas exchange and expanded.
Normal indicator charts serve to determine the average indicator pressure and general analysis of the nature of the indicator process.
Rice. 1 Types of indicator diagrams
Displaced diagrams are used to analyze the combustion process, identify shortcomings in the operation of the fuel equipment, assess the correctness of setting the fuel feed advance angle, and also to determine the maximum combustion pressurep z and the pressure of the beginning of visible combustionR" with which is usually equated to the compression pressure pwith... The offset diagram is removed by attaching an indicator cord to the adjacent cylinder, if its crank is jammed at 90 or 120 °, or by using a rotary head drive, or by quickly turning the indicator drum by the cord by hand.
Comb charts serve to determine the pressure at the end of compressionR with and maximum combustion pressureR G on engines without indicatordrives.In this case, the indicator drum is turned by hand using a cord. To determine pwiththe diagram is taken with the fuel supply to the cylinder turned off.
Compression diagrams
as indicated, are used to test the indicator drive. They can also determine the pressure pwithand evaluate the tightness piston rings by the size of the area between the compression line 1
and extension line2.
Gas exchange diagrams are filmingin the usual way, but weak springs with a scale of 1 kgf / cm are used2 = 5 mm (or more) and normal ("steam") piston. These diagrams are used to analyze the processes of release, purging and filling of the cylinder. The upper part of the diagram is limited by a horizontal line, since the indicator piston, under the influence of a weak spring, reaches the uppermost position and remains in it until the pressure in the cylinder drops to 5 kgf / cm2 .
Expanded charts
serve to analyze the combustion process in the TDC area, as well as to determine p, in engines that do not have an indicator drive. Expanded diagrams are taken with an electric or mechanical indicator with a drive independent of the motor shaft (for example, from a clockwork).
To remove all of the above diagrams, with the exception of the comb, an indicator drive is required
Distortions of indicator charts occur most often when the indicator piston gets stuck (fig. 2,a), installation of a weak (Fig. 2, b) or stiff spring (Fig. 2,v), loosening the nut fastening the indicator spring, drawing out the indicator cord (Fig. 2,G) or its great length (Fig.2, e).
Rice.2. Distortionindicatorcharts
Processing indicator charts is carried out in order to determine the values of the average indicator pressure on themR i , maximum combustion pressurep z and pressure at the end of compressionR with ... The parameters are determined most simplyp z and pwithby comb charts and offset charts. To do this, use a scale bar to remove ordinates from the atmospheric line to the corresponding points (see Fig. 1,b, c) or, if not available, a simple ruler. In the latter case, the valuesR z and pwithwill be equal:
whereT is the scale of the spring.
The maximum combustion pressure can also be determined from the normal indicator diagram, and the pressure at the end of compression can be determined from the compression diagram.
The average indicator pressure is determined from normal or expanded indicator charts. By expanded diagramsp i are found in a graphic-analytical way, by rebuilding an expanded diagram into a normal one or using a special nomogram.
According to a normal indicator chart, the valueR i determined by the formula
(130)
whereF i - area of the indicator diagram, mm2 ;
T - scale of the indicator spring, mm / (kgf / cm2 );
l - length of the diagram, mm.
The length of each indicator diagram is measured between the tangents to the extreme points of the diagram contour, which are drawn perpendicular to the atmospheric line. The area of the diagram is measured with a planimeter.
It should be noted that when determining the average indicator pressureR i according to the indicator diagram, the measurement error can reach 10-15% or more. At the same time, in marine low-speed diesel engines at normal technical condition fuel supply and boost systems pressure ratioR i R τ , p z , fuel pump index and cyclic fuel deliveryg c usually remain fairly stable for a long time. Therefore, any of the above parameters can be selected to estimate the cylinder load.
In this regard, some diesel plants consider the installation of indicator drives to be impractical., and the diagnostic system developed for these engines uses the valueR z .
Therefore, the most common types of indicator charts taken by a mechanical indicator are combs and unfolded "by hand".
The comb diagram allows you to determine the pressure at the end of compression (R with ) and maximum cycle pressure (p z ), and to removeR with it is necessary to turn off the fuel supply to this cylinder. Disabling the cylinder will lead to a decrease in the power and speed of the engine, the gas turbine and the boost pressure, which in turn will affect the magnitude of the compression pressure. For measuring compression pressure, a freehand swept chart is preferred. This diagram, with a certain skill, resembles a detailed diagram taken using an indicator drive, but there is no connection between pressure and piston stroke.
Received valuesp with andp z needs to be analyzed. To obtain more accurate conclusions, while taking the diagram, it is necessary to record the following data: gas temperatures behind the cylinders, before and after the turbine, charge air pressure and temperature, engine and turbine speed, engine load indicator. It is advisable to know the fuel consumption at the time of taking the diagram.
The best way analysis of the engine condition is to compare the measured values with the values obtained during the factory or running tests of the engine at the same load.
In the absence of test data, it is necessary to compare the obtained values with the average.
For exampleTable 1
date
Dv-l
GNT
Additional values
Time
Turnovers
R n
Steam / No.c
Wed zn
p z bar
165
156
167
156
175
164
163,8
Δp z
0,71%
-4,78%
1,93%
-4,78%
6,82%
0,10%
3,5%*
p c bar
124
120
125
128
127
122
124,3
Δp c
0,27%
3,49%
0,54%
2,95%
2,14%
1,88%
2,5%*
T G ° C
370
390
380
390
372
350
375,3
ΔT G
-1,42%
3,91%
1,24%
3,91%
0,89%
-6,75%
5,0%*
Fuel pump index
Action
Rings,
valve
TP ↓
ϕ↓
TR
* RD 31.21.30-97 Rules technical operation STS and K p. 99
p z bar
T G ° C
Action
TR
ϕ↓
TR ↓
Rice. 3. Diagnostic complex of Autronica» NK-5
Complex NK-5 by Autronica ... With the help of the complex (Fig. 3), it is possible to obtain the most complete information about the course of the working process in all cylinders of the engine and to recognize the disturbances arising in it, including in the operation of the fuel injection equipment. For this purpose, a sensor is provided6 high pressure, installed on the high pressure fuel line at the injector, as well as sensors:4 - boost pressure; 5 - TDC and shaft rotation angle; 7 - gas pressure(3 - intermediate amplifiers of sensor signals). Measurement results in the form of pressure curves and digital values of the measured parameters are displayed on a color display 1 and a printing device2 . The microprocessor built into the system allows the measurement data to be stored in the memory and later to compare the new data with
old or reference.
As an example, the curves of gas pressures in the cylinder and in the fuel line at the injector (Fig. 4) illustrate typical disturbances in the course of processes. Reference curve 1 reflects the nature of the pressure change at the considered operating mode of the engine in a technically sound condition, curve2 characterizes the actual process with various distortions caused by malfunctions.
Leakage of the nozzle needle (fig. 4,a) due to the deterioration of fuel atomization leads to a slight increase in the angleφ z , decrease in pressureR z and significant afterburning of fuel on the expansion line. The expansion curve is flatter and higher than the reference. Exhaust gas temperature risest G and pressureR exp on the extension line at 36 ° after TDC.
With a delay in fuel injection (Fig. 4, b), the beginning of visible combustion and the entire process of fuel combustion are shifted to the right. Pressure decreases at the same timeR z temperature is risingt G and pressureR exp . A similar picture is observed when the plunger pair of the fuel pump is worn and the density of its intake valve is lost. In the latter case, the cyclic fuel supply decreases and, accordingly, the pressure decreases slightly.p i
Due to the early fuel supply (Fig. 4,v) the entire combustion process shifts to the left in the direction of advance, the angle φ decreases Gand the pressure is growingR z . As the process becomes more economical, thep i . Early feeding is also confirmed by the fuel pressure curve at the injector (Fig. 4, d).
Changes in the fuel pressure curve due to increased cycle flow (fig. 4,e) accompanied by an increase in the valuesR f T a NS and the duration of the supply φ f.
The drop in the rate of rise of fuel pressure Δр f/ Δφ in the section from the beginning of its rise to the moment of opening the needle, as well as the total injection pressure drop (Fig. 4,e) causes a decrease in the feed advance angle φ npand maximum pressureR f max . The reason lies in an increase in fuel leakage through the plunger pair, the needle-guide pair of the injectors due to their wear or in the loss of tightness of the pump valves, fuel pipe fittings. Coking in the nozzle holes or excessive increase in fuel viscosity (fig. 4,g) leads to an increase in injection pressure due to an increase in the resistance of the fuel outflow from the holes.
220
-15 40 -5 VMT 5 10 15 f, 9 №8
Fig. 4. Pressure of gases in the cylinder and fuel in the high pressure line
Rice. 6.4. Pressure of gases in the cylinder and fuel in the fuel line at the injector220
-15 40 -5 VMT 5 10 15 f, 9 №8
As well as the diagram of the thermodynamic cycle, it is possible to depict in the coordinates p-V and the actual cycle of the engine internal combustion... The resulting diagram is called an indicator diagram.
Diagram of a four-stroke diesel engine. First, consider the operating cycle of a four-stroke, uncharged diesel engine.
The first bar is filling. When the diesel piston moves from left to right, intake valve 3 (Fig. 19) opens and air from the atmosphere enters the cylinder. In naturally aspirated engines, the process of filling the cylinder occurs due to vacuum
Rice. 19. Diagram of the operating cycle of a four-stroke diesel engine and a diagram of its device:
1 - piston; 2 - cylinder; 3 - inlet valve; 4 - nozzle; 5 - the exhaust valve in it, and the air pressure in the cylinder reaches 0.085-0.09 MPa, therefore the cylinder filling line is located below atmospheric (0.1 MPa). In reality, the filling line is not straight, as it is influenced by the unevenness of the piston speed, the opening and closing phases of the valves, the design of the inlet pipe and other factors. For a more complete charge of the cylinder with air, measures are taken to reduce the resistance to the passage of air into the cylinder. The quality of cylinder charging is assessed by the filling ratio c „, which is usually 0.8-0.88. This means that the diesel cylinder is filled with air only 80-88% compared to the amount of air that would fit in the working volume of the cylinder under normal ambient conditions. The filling factor depends mainly on the temperature and air pressure at point a (see Fig. 19). The higher the pressure and the lower the air temperature at point a, the greater the filling factor (Fig. 20).
The second measure is compression. The piston moves from right to left, the intake valve closes, and the air in the cylinder is compressed. In this case, its temperature at point c rises to 500-750 ° C, and the pressure can increase to 5-7 MPa. The compression process is shown in the diagram by line ac (see Fig. 19). When the piston has not yet reached the top dead center (TDC) at 18-30 ° of the crankshaft rotation angle, liquid fuel is injected into the cylinder through injector 4, which ignites at point c and starts to burn. The fuel supply stops after the piston has already passed the TDM. 10-15 ° and again starts moving from left to right. The fuel entering the cylinder is mixed with air and starts to burn. In the diagram, the combustion process is depicted by a broken line cr "r.
The third stroke is gas expansion. At the beginning of the third stroke of the piston, fuel combustion occurs, which theoretically ends at point d. The pressure at point r increases to 8-13 MPa, and the temperature up to 1750-2100 K. After point r, gas expansion occurs, which continues until the outlet valve opens. The latter opens at point e "by 40-55 ° to the lower position of the piston, when the pressure in the cylinder reaches 0.5-0.8 MPa, and the temperature is 1000-1100 K. and, consequently, better cleaning of the cylinder from spent
Rice. 20.Changing the filling ratio of the cylinders d), depending on the pressure and temperature of the air in the cylinder at the beginning of compression
Rice. 21. Indicator diagram of a four-stroke gas turbine diesel engine:
rya - pressure during filling; pr cylinder pressure during release; рк - air pressure in the charge manifold; V, the volume of the compression chamber: the volume described by the piston, V * is the total volume of the gas cylinder. The expansion stroke is a useful working stroke, since during this period gases with high pressure act on the diesel piston in the direction of its movement and perform useful work, giving it to the load unit.
The fourth stroke is the release of gases. The piston moves from right to left, you
Rice. 22. Diagram of the operating cycle of a two-stroke diesel engine and a diagram of its device:
A - purge window; B - outlet window. 1 - cylinder; ) - piston; , 3 - injector, starting valve 5 is open and gases are pushed out of the cylinder. The process of gas release in the diagram is shown by the line e "er. Gas removal occurs at a pressure of 0.11-0.12 MPa, therefore the gas discharge line is located above the atmospheric line. The gas temperature behind the exhaust valve is 700-900 K-
For more perfect purging and charging of the cylinder with air, the intake and exhaust valves are open simultaneously for 50-100 ° of rotation of the crankshaft crank. This so-called "overlap" of valves ensures good cleaning of the cylinders from fuel combustion products and more complete filling of the working volume with air, as well as cooling the piston crown and exhaust valves with a stream of cold air. The quality of cleaning the cylinder from exhaust gases is assessed by the coefficient of residual gases y, which is the ratio of the amount of gases remaining in the cylinder from the previous cycle to the amount of fresh air charge that entered the cylinder. Usually y - = 0.024-0.1.
Features of the working cycle of a four-stroke diesel engine with gas turbine supercharging. In supercharged diesel engines, the cylinder charging process is different than in naturally aspirated engines. The turbocharger sucks in air from the atmosphere at a pressure p0 (Fig. 21) and compresses it to a pressure pk. Before entering the cylinder, the air compressed in the turbocharger passes through the cooler, intake manifold and exhaust valves; on the way from the turbocharger to the cylinder, its pressure decreases from pk to p „. Therefore, the inlet pressure line is located below the pk line and above the atmospheric line (Po).
After filling the cylinder with air, the piston, moving from point a to the left, compresses the air. The compression process is depicted by the ac curve. At the end of compression, fuel is injected into the cylinder, which ignites at point c. The combustion process is shown by the lines cz "and g" r. The expansion of gases occurs along the curve r. At point e, the exhaust valves open and the exhaust gases are pushed into gas turbine(at a pressure of RT), and then released into the atmosphere. Thus, the gas outlet line from the cylinder is located above atmospheric and below the filling line. In four-stroke engines, the energy of the exhaust gases is quite sufficient for the supercharger to compress the air to a pressure pk, which is higher than pg. As a result of the boost, the area of the indicator diagram, and, consequently, the power of the diesel engine, increases significantly.
It should be noted that in reality the combustion process occurs not along straight lines with r "and r" r, but along a dashed line (see Fig. 21).
Diagram of a two-stroke diesel engine. Compression of air in the cylinder when the piston moves from right to left begins at point a and continues to point c (Fig. 22). For 16-25 ° of the angle of rotation of the crankshaft to the extreme left position of the piston through injector 3 into the cylinder at high pressure liquid fuel (in a finely atomized form) is supplied, which, in contact with compressed air heated to a high temperature, ignites. The resulting gases, trying to expand, move the piston to the right. A moving piston through a connecting rod rotates the crankshaft. Before reaching the extreme right position, the piston 2 with its edge opens the outlet window B, allowing the exhaust gases to escape through the muffler to the outside. Moving further to the right, the piston opens the blow-out window L, through which fresh air, which has an increased pressure, rushes into the cylinder. Air displaces the exhaust gases and fills the cylinder. When the piston changes direction and begins to move from right to left, it will first close the purge port A and then the outlet B, after which the air remaining in the cylinder will begin to compress. Thus, a full working process (cycle) in a two-stroke diesel engine takes two piston (stroke) codes, while the crankshaft makes one revolution.
In two-stroke diesel engines, purge air is supplied to the cylinders by a supercharger driven by the diesel shaft or by a turbocharger. Power and efficiency depend on the quality of cylinder blowing. diesel engine. To ensure good air purging of the cylinders and reduce the thermal stress of diesel parts in contact with hot gases, much more air is supplied to the cylinders than is required for fuel combustion; during blowing off, some of the air escapes through the exhaust ports. Considering this, the supply of the purge air blower must be 30-40% more than necessary to ensure complete combustion of the fuel. When designing two-stroke engines, designers strive to ensure that with the least loss of compressed air, the best possible blowing and charging of the cylinders is obtained. In two-stroke diesel engines, the energy of the exhaust gases is usually insufficient to compress the charge air to the required pressure, since this pressure must be higher than the pressure in the exhaust pipe for high-quality cleaning of the cylinders, and the energy of the exhaust gases (all other things being equal) is lower than in four-stroke engines , due to the dilution of gases with cold purge air. Therefore, in two-stroke diesel engines, combined supercharging is used, in which part of the energy required to compress the charge air is taken from the engine crankshaft (see above).
Blowdown circuits for two-stroke diesel engines. The simplest, but at the same time the most imperfect scheme is the so-called cross-slot blowdown, in which 15-20% of the exhaust gases can remain in the cylinder (Fig. 23, a). Such blowdown is used in low-power diesel engines, for which simplicity of design, rather than economy, is of decisive importance. The blowdown circuit shown in fig. 23.6 is more perfect. Thanks to the non-return valve 3, this design provides some cylinder pressurization. This scavenging scheme is used on low-speed marine engines.
Direct-flow valve-slit blowing is more perfect (Fig. 23, c). Compressed air from the supercharger enters the cylinder through the lower windows, and the exhaust gases are removed through exhaust valves 3 located in the cylinder head. With such a purge on the diesel engine, set camshaft... Valve-slot blowing is used in 11D45 and 14D40 diesel locomotives.
The most perfect is direct-flow slot blowing (Fig. 23, d), which can be carried out in engines with counter-moving pistons. Compressed air from the supercharger enters through the upper ports (purge), and the exhaust gases are removed from the cylinder through the lower (exhaust) ports. In order to be able to fully charge the cylinder, the lower piston overlapping the exhaust ports is slightly ahead (by 10-12 ° of the crankshaft angle) the upper piston overlapping the intake ports.
With this method of purging, almost no exhaust gases remain in the cylinder. Direct-flow slot blowing is used in 2D100 and 1 OD 100 diesel locomotives.
Indicator diagram graphical representation of the change in the pressure of gas or steam in the cylinder of a piston machine, depending on the position of the piston. The I.D. is usually plotted with a pressure indicator (See Pressure Indicator). The abscissa shows the volume occupied by gases in the cylinder, and the ordinate shows the pressure. Each point on the I. d. ( rice.
) shows the pressure in the engine cylinder at a given volume, i.e. at a given position of the piston (point r corresponds to the beginning of the intake; point a- the beginning of compression; point with- the end of compression; point z -
the beginning of the expansion; point b- the end of the extension). I. d. Gives an idea of the value of the work produced by an internal combustion engine or pump, and about their power. Working body performs useful work only during the working stroke. Therefore, to determine the useful work, it is necessary from the area limited by the expansion curve zb, subtract the area bounded by the compression curve ac. Distinguish between theoretical and real I. The theoretical is constructed according to the data of thermal calculation and characterizes the theoretical cycle; the real I. d. is removed from the running machine using the indicator and characterizes the actual cycle (see. rice.
). For the convenience of calculating and comparing with each other different engines the pressure variables along the piston stroke are replaced by a conventional constant pressure, at which work is obtained in one piston stroke equal to the work of gases per cycle with variable pressure. This constant pressure is called the average indicator pressure and represents the work of gases in relation to the working volume of the piston engine. B. A. Kurov.
Great Soviet Encyclopedia. - M .: Soviet encyclopedia. 1969-1978 .
See what "Indicator Chart" is in other dictionaries:
Indicator diagram for various piston mechanisms is a graphical dependence of the pressure in the cylinder on the piston stroke (or depending on the volume occupied by the gas or liquid in the cylinder). Indicator charts are built when researching ... Wikipedia
indicator diagram- Diagram of the dependence of the pressure in the cylinder of a piston machine on its variable volume. [GOST 28567 90] Topics compressor EN pressure volume diagram DE Indikatordiagramm ... Technical translator's guide
Graphical representation of the dependence of the pressure of the working fluid (steam, gas) in the cylinder of a piston machine (engine, pump) on the movement of the piston. It is a closed curve, the area inside which is proportional to the work done by the worker ... Big Encyclopedic Dictionary
Graphical representation of the dependence of the pressure of the working fluid (steam, gas) in the cylinder of a piston machine (engine, pump) on the movement of the piston. It is a closed curve, the area inside which is proportional to the work done by the worker ... encyclopedic Dictionary
Graphic. image of the change in the pressure of steam or gas in the cylinder of a piston machine, depending on the movement of the piston or the angle of rotation of the crankshaft (see Fig.). The area of I. d. Is proportional to the work done. working fluid inside the cylinder for ... ... Big Encyclopedic Polytechnic Dictionary
Graphic. image of the dependence of the pressure of the working fluid (steam, gas) in the cylinder of a piston machine (engine, pump) on the movement of the piston. It is a closed curve, the area inside the swarm is proportional to the work done by the working fluid ... Natural science. encyclopedic Dictionary
Indicator diagram- 97. Indicator diagram D. Indikalorcliagramm E. Pressure volume diagram Diagram of the dependence of the pressure in the cylinder of a piston machine on its variable volume
Researching the work of real piston engine it is advisable to carry out according to the diagram in which the change in pressure in the cylinder is given depending on the position of the piston for the entire
cycle. Such a diagram, taken using a special indicator device, is called an indicator diagram. The area of the closed figure of the indicator diagram depicts, on a certain scale, the indicator work of the gas in one cycle.
In fig. 7.6.1 depicts an indicator diagram of an engine operating with fast fuel combustion at a constant volume. As fuel for these engines, light fuel gasoline, lighting or generator gas, alcohols, etc. are used.
During the stroke of the piston from the left dead position to the extreme right through the suction valve, a combustible mixture is sucked in, consisting of vapors and small particles of fuel and air. This process is depicted in a 0-1 curve diagram called the suction line. Obviously, line 0-1 is not a thermodynamic process, since the main parameters in it do not change, but only the mass and volume of the mixture in the cylinder change. With the reverse movement of the piston, the suction valve closes, the combustible mixture is compressed. The compression process in the diagram is depicted by a curve 1-2, which is called the compression line. At point 2, when the piston has not yet reached the left dead position a little, the combustible mixture is ignited from an electric spark. Combustion of the combustible mixture occurs almost instantaneously, that is, practically at a constant volume. This process is depicted in the diagram by curve 2-3. As a result of fuel combustion, the gas temperature rises sharply and the pressure increases (point 3). Then the combustion products expand. The piston moves to the right dead position and the gases do useful work. In the indicator diagram, the expansion process is depicted on a 3-4 curve, called the expansion line. At point 4, the exhaust valve opens and the pressure in the cylinder drops to almost the outside pressure. With further movement of the piston from right to left, combustion products are removed from the cylinder through the exhaust valve at a pressure slightly exceeding atmospheric pressure. This process is depicted in the 4-0 curve diagram and is called the exhaust line.
Effective power N e is the power received on crankshaft engine. It is smaller indicator power N i by the amount of power spent on friction in the engine (friction of pistons against cylinder walls, crankshaft journals against bearings, etc.) and actuating auxiliary mechanisms (gas distribution mechanism, fan, water, oil and fuel pumps, generator, etc.) ).
To determine the value of the effective engine power, you can use the above formula for the indicated power, replacing the average indicated pressure p i in it with the average effective pressure p e (p e is less than p i by the value mechanical losses in the engine)
Indicator power N i is called the power developed by gases inside the engine cylinder. Power units are horsepower(hp) or kilowatts (kW); 1 l. with. = 0.7355 kW.
To determine the indicated engine power, it is necessary to know the average indicated pressure p i, i.e., such a conditional constant pressure, which, acting on the piston during only one combustion-expansion stroke, could perform work, equal to work gases in the cylinder for the entire cycle.
Heat balance is the distribution of heat, which appears in the engine during the combustion of fuel, into useful heat for the full functioning of the car and heat, which can be qualified as heat losses. There are such basic heat losses:
- caused by overcoming friction;
- arising from the radiation of heat from the heated outer surfaces of the engine;
- losses on the drive of some auxiliary mechanisms.
The normal level of thermal balance of the engine may differ depending on the operating mode. Determined by test results under steady-state thermal conditions. Thermal balance helps to determine the degree to which the engine design matches the efficiency of its operation, and in the future to take measures to regulate certain processes in order to achieve better performance.
