Operating cycle - a set of thermal, chemical and gas-dynamic processes, successively, periodically repeating in the engine cylinder in order to convert the thermal energy of the fuel into mechanical energy. The cycle includes five processes: intake, compression, combustion (combustion), expansion, release.
Diesel and carburetor four-stroke engines are installed on tractors and vehicles used in the timber industry and forestry. Forestry vehicles are mainly equipped with four-stroke diesel engines,
During the intake process, the engine cylinder is filled with fresh charge, which is purified air at diesel engine or a combustible mixture of purified air with fuel (gas) for a carburetor engine and a gas diesel engine. A combustible mixture of air with finely dispersed fuel, its vapors or combustible gases must ensure the spread of the flame front in the entire occupied space.
The compression process in the cylinder compresses a working mixture consisting of fresh charge and residual gases (carburetor and gas engines) or fresh charge, atomized fuel and residual gases (diesels, multi-fuel and gasoline injection engines and gas diesel engines).
Residual gases are called combustion products remaining after the completion of the previous cycle and participating in the next cycle.
In engines with external mixture formation, the operating cycle proceeds in four cycles: intake, compression, expansion and exhaust. Intake stroke (Fig. 4.2a). Piston 1, under the influence of rotation of the crankshaft 9 and connecting rod 5, moving to BDC, creates a vacuum in cylinder 2, resulting in a fresh charge combustible mixture flows through pipeline 3 through inlet valve 4 to cylinder 2.
The compression stroke (Fig. 4.2b). After filling the cylinder with a fresh charge, the intake valve closes, and the piston, moving to TDC, compresses the working mixture. This increases the temperature and pressure in the cylinder. At the end of the cycle, the working mixture is ignited by a spark that occurs between the electrodes of spark plug 5, and the combustion process begins.
Extension stroke or power stroke (fig. 4.2e). As a result of the combustion of the working mixture, gases (combustion products) are formed, the temperature and pressure of which increase sharply by the time the piston reaches TDC. Under the influence of high gas pressure, the piston moves to the BDC, while doing useful work transmitted to the rotating crankshaft.
Release stroke (see Fig. 4.2d). In this stroke, the cylinder is cleaned of combustion products. The piston, moving to TDC, through the open exhaust valve 6 and pipeline 7 pushes the combustion products into the atmosphere. At the end of the stroke, the pressure in the cylinder slightly exceeds atmospheric pressure, so some of the combustion products remain in the cylinder, which mix with the combustible mixture that fills the cylinder during the intake stroke of the next working cycle.
The fundamental difference between the operating cycle of an engine with internal mixture formation (diesel, gas-diesel, multi-fuel) is that on the compression stroke, the fuel supply equipment of the engine power system injects finely atomized liquid motor fuel, which is mixed with air (or a mixture of air with gas) and ignites. The high compression ratio of a compression ignition engine allows the mixture in the cylinder to be heated above the autoignition temperature of the liquid fuel.
The working cycle of a two-stroke carburetor engine (Fig. 4.3) used to start a skidder diesel is completed in two piston strokes or in one revolution of the crankshaft. In this case, one cycle is working, and the second is auxiliary. In a two-stroke carburetor engine, there are no intake and exhaust valves, their function is performed by intake, exhaust and purge windows, which open and close with the piston as it moves. Through these windows, the working cavity of the cylinder communicates with the inlet and outlet pipelines, as well as with the sealed crankcase of the engine.
Indicator diagram. Operating or actual engine cycle internal combustion differs from the theoretical one studied in thermodynamics by the properties of the working fluid, which is real gases of variable chemical composition, the rate of heat supply and removal, the nature of the heat exchange between the working fluid and the parts surrounding it, and other factors.
Actual engine cycles are graphically depicted in the coordinates: pressure - volume (p, V) or in coordinates: pressure - crankshaft rotation angle (p, φ). Such graphical dependences on the specified parameters are called indicator diagrams.
The most reliable indicator diagrams are obtained experimentally, instrumental methods, directly on the engines. The indicator diagrams obtained by calculation on the basis of thermal calculation data differ from the actual cycles due to the imperfection of the calculation methods and the assumptions used.
On fig. 4.4 shows indicator diagrams for four-stroke carburetor and diesel engines.
The circuit r, a, c, z, b, r is a diagram of the operating cycle of a four-stroke engine. It reflects five alternating and partially overlapping processes: intake, compression, combustion, expansion and exhaust. The intake process (r, a) starts before the piston arrives at BMT (near point r) and ends after HMT (at point k). The compression process ends at point c, at the moment of ignition of the working mixture in a carburetor engine or at the moment fuel injection begins in a diesel engine. At point c, the combustion process begins, which ends after point r. The expansion process or work stroke (r, b) ends at point b. The release process begins at point b, i.e. at the moment the exhaust valve opens, and ends after point r.
The area r, a, c, b, r is built in p-V coordinates, therefore, on a certain scale it characterizes the work developed by the gases in the cylinder. The indicator diagram of a four-stroke engine consists of positive and negative areas. The positive area is limited by the lines of compression and expansion k, c, z, b, k and characterizes the useful work of gases; the negative one is limited by the intake and exhaust lines and characterizes the work of gases expended to overcome the resistance during intake and exhaust. The negative area of the diagram is insignificant, its value can be neglected, and the calculation is made only along the contour of the diagram. The area of this contour is equivalent to the indicator work, it is planned to determine the average indicator pressure.
The indicator work of the cycle is called the work in one cycle, determined by the indicator diagram.
The average indicator pressure is such a conditional constant pressure in the engine cylinder at which the work of the gas in one piston stroke is equal to the indicator work of the cycle.
The average indicator pressure p is determined from the indicator diagram:
Lecture 4
ACTUAL 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 gas distribution phases on gas exchange processes
2.2. Parameters of the gas exchange process
2.3. Factors affecting gas exchange processes
2.4. Exhaust gas toxicity and ways to prevent environmental pollution
3. Compression process
3.1. Compression Process Options
4. Combustion process
4.1. combustion rate
4.2. Chemical reactions during combustion
4.3. The combustion process in a carburetor engine
4.4. Factors affecting the combustion process in a carburetor engine
4.5. Detonation
4.6. combustion process fuel mixture in diesel
4.7. Diesel hard work
5. Expansion process
5.1. The purpose and course of the expansion process
5.2. Extension Process Options
The difference between the actual cycles of four-stroke engines from the theoretical ones
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:
the immutability of the working fluid;
· the absence of any heat and gas-dynamic losses, except for the obligatory removal of heat by the refrigerator.
In real reciprocating internal combustion engines, mechanical work is obtained as a result of the flow of real cycles.
The actual cycle of the engine 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:
The 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, a combustion process takes place, which proceeds at finite rates;
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 specific volume ( υ ), then the actual cycles are depicted as dependences of the pressure change ( R) from volume change ( V) (collapsed indicator chart) or change in pressure with crank angle (φ), which is called an expanded indicator chart.
On fig. 1 and 2 show collapsed and expanded indicator diagrams for 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 less accurate.
Rice. 1. Collapsed indicator diagram of a four-stroke engine
forced ignition
Rice. 2. Expanded indicator diagram of a four-stroke diesel
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 Q1- the amount of heat supplied in the actual cycle;
Q2- thermal losses of the actual cycle.
In the actual cycle Q1 depends on the mass and heat of combustion of the fuel introduced into the engine per cycle.
The degree of use 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 Q1:
, (3.2)
Taking into account formula (1), formula (2) of the indicator efficiency can be written as follows:
, (3.3)
Therefore, heat use in the actual cycle depends on the amount of heat loss. In modern internal combustion engines, these losses are 55–70%.
The main components of heat loss Q2:
Loss of heat with exhaust gases to the environment;
Heat loss through the cylinder walls;
Incomplete combustion of fuel due to local lack of oxygen in the combustion zones;
Leakage of the working fluid from the working cavity of the cylinder due to the leakage of adjacent parts;
Premature release of exhaust gases.
To compare the degree of heat utilization in real and thermodynamic cycles, relative efficiency is used
AT automotive enginesη o from 0.65 to 0.8.
The actual cycle of a four-stroke engine is completed in two revolutions of the crankshaft and consists of the following processes:
Gas exchange - fresh charge inlet (see Fig. 1, curve fraction) and exhaust gases (curve b"b"rd);
Compression (curve aks"s");
combustion (curve c"c"zz");
Extensions (curve z z"b"b").
When a fresh charge is admitted, the piston moves, releasing a volume above it, which is filled with a mixture of air and fuel in carbureted engines and clean air in diesel engines.
The start of intake is determined by the opening of the intake valve (point f), the end of the inlet - by its closing (point k). The beginning and end of the release correspond to the opening and closing of the exhaust valve, respectively, at the points b" and d.
Not shaded area b"bb" on the indicator diagram corresponds to the loss of indicator work due to pressure drop as a result of the opening of the exhaust valve before the piston arrives at BDC (pre-exhaust).
Compression is actually carried out from the moment the intake valve closes (curve k-s"). Before closing the intake valve (curve a-k) the pressure in the cylinder remains below atmospheric ( p0).
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 ignition of a fresh charge does not occur at TDC, and combustion proceeds with continued movement of the piston, the calculated 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.
Ignition of a fresh charge in gasoline and gas engines is carried out from an electric discharge between the electrodes of a spark plug.
In diesel engines, fuel is ignited by the heat of air heated by 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 power stroke is performed. In this case, the energy of thermal expansion of the gas is converted into mechanical work.
Study of the work of the real piston engine it is advisable to carry out according to the diagram, which gives the change in pressure in the cylinder 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.
On fig. Figure 7.6.1 shows the indicator diagram of an engine operating with fast-burning fuel at constant volume. As a fuel for these engines, light fuel gasoline, lighting or generator gas, alcohols, etc. are used.
When the piston moves from the left dead position to the extreme right, a combustible mixture is sucked in through the suction valve, consisting of vapors and small particles of fuel and air. This process is depicted in a 0-1 curve diagram, which is called the suction line. Obviously, the 0-1 line is not a thermodynamic process, since the main parameters do not change in it, but only the mass and volume of the mixture in the cylinder change. When the piston moves back, the suction valve closes, and 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, the combustible mixture is ignited by an electric spark. Combustion of the combustible mixture occurs almost instantly, i.e., almost 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. On the indicator diagram, the expansion process is depicted by a 3-4 curve, called the expansion line. At point 4, the exhaust valve opens and the pressure in the cylinder drops to almost 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 higher than atmospheric pressure. This process is depicted in the 4-0 curve diagram and is called the exhaust line.
Effective power N e is called the power received on crankshaft engine. She is smaller indicator power N i on the amount of power expended on friction in the engine (friction of pistons against cylinder walls, crankshaft journals against bearings, etc.) and actuation of auxiliary mechanisms (gas distribution mechanism, fan, water, oil and fuel pumps, generator, etc. ).
To determine the value of the effective power of the engine, 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 amount of mechanical losses in the engine)
indicator power N i is the power developed by the gases inside the engine cylinder. The power units are horsepower(hp) or kilowatts (kW); 1 l. with. = 0.7355 kW.
To determine the indicated power of the engine, it is necessary to know the average indicated pressure p i i.e. such a conditional pressure constant in magnitude, which, acting on the piston for only one cycle of combustion-expansion, could do work equal to the work of gases in the cylinder for the entire cycle.
Thermal balance represents the distribution of heat that 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 loss. There are such basic heat losses:
- caused by overcoming friction;
- arising from heat radiation from heated external surfaces of the engine;
- losses on the drive of some auxiliary mechanisms.
The normal level of thermal balance of the engine may vary depending on the mode of operation. It is determined by the results of tests in conditions of a steady thermal regime. The thermal balance helps to determine the degree of compliance with the design of the engine and the economy of its operation, and then take measures to adjust certain processes in order to achieve better operation.
SCHEME OF OPERATION OF A 4-STROKE DIESEL.
ICE MARKING.
Domestic diesel engines are marked in accordance with GOST 4393-74. Each type of engine has a conventional letter and number designation:
H - four-stroke
D - two-stroke
DD - two-stroke double action
R - reversible
C - with reverse clutch
P - s reduction gear
K - crosshead
H - supercharged
G - for operation on gas fuel
GZh - for operation on gas-liquid fuel
The numbers in front of the letters indicate the number of cylinders; the numbers after the letters are the cylinder bore/stroke in centimeters. For example: 8DKRN 74/160, 6ChSP 18/22, 6Ch 12/14
Marking of foreign diesel companies:
Engines of the SKL plant in Germany (former GDR)
Four-stroke internal combustion engines are called engines in which one stroke (stroke) is carried out in four piston strokes, or two revolutions of the crankshaft. The strokes are: intake (filling), compression, stroke (expansion), exhaust (exhaust).
I measure - FILLING. The piston moves from TDC to BDC, as a result of which a vacuum is created in the over-piston cavity of the cylinder, and air from the atmosphere enters the cylinder through the open intake (suction) valve. The volume in the cylinder is constantly increasing. The valve closes at BDC. At the end of the filling process, the air in the cylinder has the following parameters: pressure Pa=0.85-0.95 kg/cm 2 (86-96 kPa); temperature Ta=37-57°C (310-330 K).
II measure - COMPRESSION. The piston moves in the opposite direction and compresses a fresh charge of air. The volume in the cylinder decreases. Pressure and temperature rise to the following values: Pc=30-45kg/cm 2 (3-4 MPa); Tc = 600-700°C (800-900 K). These parameters must be such that self-ignition of the fuel occurs.
At the end of the compression process, finely atomized fuel is injected into the engine cylinder from a nozzle under a high pressure of 20-150 MPa (200-1200 kg / cm 2), which spontaneously ignites under the action of high temperature and burns out quickly. Thus, during the second cycle, air is compressed, fuel is prepared for combustion, the working mixture is formed and its combustion begins. As a result of the combustion process, the gas parameters increase to the following values: Pz=55-80kg/cm 2 (6-8.1 MPa); Tz=1500-2000°C (1700-2200 K).
III beat - EXPANSION. Under the action of forces arising from the pressure of the products of combustion of the fuel, the piston moves to the BDC. The thermal energy of gases is converted into mechanical work of moving the piston. At the end of the expansion stroke, the gas parameters are reduced to the following values: Pb=3.0-5.0 kg/cm 2 (0.35-0.5 MPa); Tb=750-900°C (850-1100 K).
IV measure - RELEASE. At the end of the expansion stroke (up to BDC), the exhaust valve opens and gases with energy and pressure greater than atmospheric rush into exhaust manifold, moreover, when the piston moves to TDC, the exhaust gases are forced to be removed by the piston. At the end of the exhaust cycle, the parameters in the cylinder will be as follows: pressure P 1 =1.1-1.2 kg/cm 2 (110-120 kPa); temperature T 1 =700-800°C (800-1000 K). After TDC, the exhaust valve closes. The work cycle is over.
Depending on the position of the piston, the change in pressure in the engine cylinder can be depicted graphically in the coordinate axes PV (pressure - volume) of a closed curve, which is called an indicator diagram. In the diagram, each line corresponds to a specific process (cycle):
1-a - filling process;
a-c - compression process;
c-z" - combustion process at constant volume (V=const);
z"-z - combustion process at constant pressure (P=const);
z-b - expansion process (work stroke);
b-1 - release process;
Po - atmospheric pressure line.
Note: if the diagram is located above the Po line, then the engine is equipped with a pressurization system and has a large power.
The extreme positions of the piston (TDC and BDC) are shown by dotted lines.
The volumes occupied by the working fluid, in any position of the piston and enclosed between its bottom and the cylinder cover, are plotted on the abscissa axis of the diagram, which have the following designations:
Vc is the volume of the compression chamber; Vs is the working volume of the cylinder;
Va. is the total volume of the cylinder; Vx is the volume above the piston at any moment of its movement. Knowing the position of the piston, you can always determine the volume of the cylinder above it.
On the y-axis (in the selected scale) lay the pressure in the cylinder.
The considered indicator diagram shows the theoretical (calculated) cycle, where assumptions are made, i.e. strokes begin and end at dead points, the piston is at TDC, the combustion chamber is filled with residual exhaust gases.
AT real engines the moments of opening and closing of the valves do not begin and end at the dead points of the piston position, but with a certain offset, which is clearly seen in the circular valve timing diagram. The moments of opening and closing of valves, expressed in degrees of rotation of the crankshaft (p.k.v.) are called valve timing. The optimal opening and closing angles of the valves, as well as the start of the fuel supply, are determined experimentally when testing a prototype at the manufacturer's stand. All angles (phases) are indicated in the motor log.
By the time the air charge enters the engine cylinder, the suction valve opens. Point 1 corresponds to the position of the crank when the valve opens. For better filling of the cylinder with air, the intake valve opens up to TDC and closes after the BDC piston passes through an angle equal to 20-40 ° c.c.v., which is designated as the lead and lag angle of the intake valve. Usually the angle p.k.v. corresponds to an intake process of 220-240°. When the valve closes, the filling of the cylinder ends and the crank takes the position corresponding to point (2).
After the compression process, self-ignition of the fuel takes time for it to heat up and evaporate. This period of time is called the ignition delay period. Therefore, fuel injection is carried out with some advance until the piston reaches TDC at an angle of 10-35 ° c.c.v.
FUEL ADVANCE ANGLE
The angle between the direction of the crank and the axis of the cylinder at the time of the start of fuel injection is called the fuel advance angle. UOPT is counted from the start of supply to TDC and depends on the supply system, fuel type and engine speed. UOPT in diesel engines is from 15 to 32 ° and is of great importance for the operation of the internal combustion engine. It is very important to determine the optimal feed advance angle, which must correspond to the manufacturer's value specified in the engine passport.
Optimal SPTA is of great importance for normal operation engine and its economy. With proper regulation, fuel combustion should begin before the piston reaches TDC by 3-6 ° p.c.v. Maximum pressure Pz, equal to the calculated one, is achieved when the piston passes the TDC at an angle of 2-3 ° c.c.v. (see "Combustion phases").
With an increase in UOPT, the self-ignition delay period (I-th phase) increases and the bulk of the fuel burns out at the moment the piston goes to TDC. This leads to a hard operation of the diesel engine, as well as to increased wear of the parts of the CPG and the crankshaft.
A decrease in the UOPT leads to the fact that the main part of the fuel enters the cylinder when the piston passes the TDC and burns in a larger volume of the combustion chamber. This reduces the cylinder power of the engine.
After the expansion process, in order to reduce the cost of expelling exhaust gases by the piston, the exhaust valve is opened ahead of time until the piston arrives at BDC by an angle equal to 18-45 ° p.c.v., which is called the exhaust valve opening advance angle. Dot (). For better cleaning of the cylinders from combustion products, the exhaust valve closes after the TDC piston passes to a retard angle equal to 12-20 ° c.c.v., corresponding to the point () on the pie chart.
However, it can be seen from the diagram that the suction and exhaust valves are simultaneously in the open position for some time. This opening of the valves is called the valve phase overlap angle, which amounts to a total of 25-55 ° c.c.v.
Construction of indicator charts
Indicator diagrams are built in coordinates p-V.
The construction of an indicator diagram of an internal combustion engine is based on a thermal calculation.
At the beginning of construction, on the abscissa axis, a segment AB is plotted, corresponding to the working volume of the cylinder, and in magnitude equal to the piston stroke on a scale, which, depending on the piston stroke of the designed engine, can be taken as 1:1, 1.5:1 or 2:1.
Segment OA, corresponding to the volume of the combustion chamber,
is determined from the ratio:
Segment z "z for diesel engines (Fig. 3.4) is determined by the equation
Z,Z=OA(p-1)=8(1.66-1)=5.28mm, (3.11)
pressures = 0.02; 0.025; 0.04; 0.05; 0.07; 0.10 MPa in mm so that
get the height of the chart equal to 1.2 ... 1.7 of its base.
Then, according to the thermal calculation data on the diagram, they are laid in
the chosen scale of pressure values at the characteristic points a, c, z", z,
b, r. z point for gasoline engine corresponds pzT.
Four-stroke diesel engine indicator diagram
According to the most common Brouwer graphical method, compression and expansion polytropes are constructed as follows.
Draw a ray from the origin OK at an arbitrary angle to the abscissa axis (it is recommended to take = 15 ... 20 °). Further, from the origin, rays OD and OE are drawn at certain angles and to the y-axis. These angles are determined from the relations
0.46 = 25°, (3.13)
The compression polytrope is built using the rays OK and OD. From point C, a horizontal line is drawn until it intersects with the y-axis; from the intersection point - a line at an angle of 45 ° to the vertical until it intersects with the OD beam, and from this point - a second horizontal line parallel to the abscissa axis.
Then a vertical line is drawn from point C until it intersects with the OK beam. From this point of intersection at an angle of 45 ° to the vertical, we draw a line until it intersects with the abscissa axis, and from this point?? the second vertical line parallel to the y-axis, until it intersects with the second horizontal line. The intersection point of these lines will be the intermediate point 1 of the compression polytrope. Point 2 is found similarly, taking point 1 as the beginning of the construction.
The expansion polytrope is built using the rays OK and OE, starting from the point Z", similar to the construction of the compression polytrope.
The criterion for the correct construction of the extension polytrope is its arrival at the previously plotted point b.
It should be borne in mind that the construction of the expansion polytropic curve should be started from the point z , and not z ..
After constructing the contraction and expansion polytropes, they produce
rounding the indicator diagram taking into account the pre-opening of the exhaust valve, ignition timing and the rate of pressure rise, and also apply the intake and exhaust lines. For this purpose, under the abscissa axis, a semicircle with radius R=S/2 is drawn on the piston stroke length S as on the diameter. From the geometric center Оґ in the direction of n.m.t. a segment is postponed
where L- the length of the connecting rod, is selected from the table. 7 or prototype.
Ray O 1.With 1 is carried out at an angle Q o = 30° corresponding to the angle
ignition timing ( Qo= 20…30° to w.m.t.), and the point With 1 demolished on
contraction polytrope, obtaining the point c1.
To build lines for cleaning and filling the cylinder, a beam is laid O 1?AT 1 at an angle g=66°. This angle corresponds to the pre-opening angle of the exhaust valve or exhaust ports. Then a vertical line is drawn until it intersects with the expansion polytrope (point b 1?).
From a point b 1. draw a line that defines the law of change
pressure in the section of the indicator diagram (line b 1.s). Line as,
characterizing the continuation of cleaning and filling the cylinder, can
be held straight. It should be noted that the points s. b 1. you can also
find by the value of the lost fraction of the piston stroke y.
as=y.S. (3.16)
Indicator diagram two-stroke engines just like supercharged engines, it always lies above the atmospheric pressure line.
In a supercharged engine indicator chart, the intake line may be higher than the exhaust line.