ACTUAL ICE CYCLES

The difference between actual four-stroke engine cycles and theoretical ones

The highest efficiency can theoretically be obtained only by using a thermodynamic cycle, options for which were discussed in the previous chapter.

The most important conditions for the occurrence of thermodynamic cycles:

· constancy of the working fluid;

· absence of any thermal and gas-dynamic losses, except for the mandatory 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.

Valid cycles have the following fundamental differences from thermodynamic cycles:

Real 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 there is a combustion process that occurs at finite rates;

The chemical composition of the working fluid changes;

The heat capacity of the working fluid, which is real gases of changing chemical composition, in real cycles is constantly changing;

There is a constant heat exchange between the working fluid and the parts surrounding it.

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 changes in absolute pressure ( r) from changes in specific volume ( υ ), then the actual cycles are depicted as pressure changes ( r) from changes in volume ( V) (collapsed indicator diagram) or changes in pressure from the crankshaft angle (φ), which is called an expanded indicator diagram.

In Fig. 1 and 2 show collapsed and expanded indicator diagrams of four-stroke engines.

An expanded indicator diagram can be obtained experimentally using special device- 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. Collapsed indicator diagram of a four-stroke engine
with positive 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 folded indicator diagram, limited by the lines of compression, combustion and expansion, corresponds to the useful or indicator work Li of the actual cycle. The magnitude of the indicator work characterizes the beneficial 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 real cycle Q 1 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 assessed by 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 formula (1), formula (2) of indicator efficiency can be written as follows:

, (3.3)

Consequently, heat use in an actual cycle depends on the amount of heat loss. IN modern internal combustion engines these losses amount to 55–70%.

Main components of heat losses Q 2:

Heat loss from exhaust gases in environment;

Heat loss through the cylinder walls;

Incomplete combustion of fuel due to local lack of oxygen in combustion zones;

Leakage of the working fluid from the working cavity of the cylinder due to 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

.

IN car 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 frak) and exhaust gas release (curve b"b"rd);

Compression (curve акс"с");

Combustion (curve c"c"zz");

Extensions (curve z z"b"b").

When a fresh charge is introduced, the piston moves, freeing up a volume above it, which is filled with a mixture of air and fuel in carburetor engines and clean air in diesel engines.

The start of intake is determined by the opening intake valve(dot f), the end of the inlet - by closing it (point k). The beginning and end of the exhaust correspond to the opening and closing of the exhaust valve, respectively, at the points b" And d.

Unshaded area b"bb" on the indicator diagram corresponds to the loss of indicator work due to a drop in pressure as a result of the opening of the exhaust valve before the piston reaches BDC (pre-exhaust).

Compression actually occurs from the moment the intake valve closes (curve k-s"). Before the intake valve closes (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 ignition of the fresh charge does not occur at TDC, and combustion occurs with 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 therefore the useful work of the cycle, is less than the thermodynamic or calculated one.

Ignition of the fresh charge in gasoline and gas engines is carried out by an electrical discharge between the electrodes of the 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 occurs. In this case, the energy of thermal expansion of the gas is converted into mechanical work.

The operating cycle is a set of thermal, chemical and gas-dynamic processes that are sequentially and periodically repeated 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 (burning), expansion, exhaust.
Tractors and cars used in the timber industry and forestry are equipped with diesel and carburetor four-stroke engines. Forest transport vehicles are mainly equipped with four-stroke diesel engines,
During the intake process, the engine cylinder is filled with a fresh charge, which is purified air from diesel engine or a combustible mixture of purified air with fuel (gas) for a carburetor engine and gas diesel engine. A flammable mixture of air with finely atomized fuel, its vapors or flammable gases must ensure the spread of the flame front throughout the entire occupied space.
During the compression process, a working mixture consisting of a fresh charge and residual gases (carburetor and gas engines) or a fresh charge, atomized fuel and residual gases (diesels, multi-fuel and gasoline-injected engines and gas diesel engines) is compressed in the cylinder.
Residual gases are 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 occurs in four strokes: 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, as a result of which a fresh charge of the combustible mixture enters through pipeline 3 through the inlet valve 4 into cylinder 2.

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. At the same time, the temperature and pressure in the cylinder increase. At the end of the stroke, the working mixture is ignited by a spark that occurs between the electrodes of spark plug 5, and the combustion process begins.
Expansion stroke or power stroke (Fig. 4.2e). As a result of combustion of the working mixture, gases (combustion products) are formed, the temperature and pressure of which increase sharply when the piston reaches TDC. Under the influence of high gas pressure, the piston moves to BDC, and useful work is performed and transferred to the rotating crankshaft.
Release stroke (see Fig. 4.2d). During this stroke, the cylinder is cleaned of combustion products. The piston, moving to TDC, through the open exhaust valve 6 and pipeline 7, pushes 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 are mixed with the combustible mixture that fills the cylinder during the intake stroke of the next working cycle.
The fundamental difference between the engine operating cycle and internal mixture formation(diesel, gas-diesel, multi-fuel) is that during the compression stroke the fuel supply equipment of the engine power system injects finely atomized liquid motor fuel, which mixes with air (or a mixture of air and gas) and ignites. The high compression ratio of a compression ignition engine allows the working mixture in the cylinder to be heated above the auto-ignition temperature of the liquid fuel.
The operating cycle of a two-stroke carburetor engine (Fig. 4.3) used to start a diesel skidder is completed in two strokes of the piston or 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 the intake, exhaust and purge ports, which are opened and closed by the piston as it moves. Through these windows, the working cavity of the cylinder communicates with the intake and exhaust pipelines, as well as with the sealed engine crankcase.

Building indicator charts

Indicator charts are plotted in coordinates p-V.

The construction of an indicator diagram of an internal combustion engine is carried out on the basis of thermal calculations.

At the beginning of the construction, a segment AB is laid out on the abscissa axis, corresponding to the working volume of the cylinder, and in magnitude equal to the piston stroke on a scale that, depending on the piston stroke of the designed engine, can be taken as 1:1, 1.5:1 or 2:1.

Section OA corresponding to the volume of the combustion chamber,

is determined from the relation:

The 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)

pressure = 0.02; 0.025; 0.04; 0.05; 0.07; 0.10 MPa in mm so that

obtain a diagram height equal to 1.2...1.7 of its base.

Then, according to the thermal calculation data, the diagram is plotted in

selected scale of pressure values ​​at characteristic points a, c, z", z,

b, r. Point z for gasoline engine corresponds pzT.

Indicator diagram of a four-stroke diesel engine

According to the most common graphical method of Brouwer, polytropes of compression and expansion are constructed as follows.

A ray is drawn from the origin OK at an arbitrary angle to the abscissa axis (recommended = 15...20°). Next, rays OD and OE are drawn from the origin of coordinates at certain angles to the ordinate axis. These angles are determined from the relations

0.46 = 25°, (3.13)

The compression polytrope is constructed using rays OK and OD. From point C, draw a horizontal line until it intersects with the ordinate axis; from the point of intersection - a line at an angle of 45° to the vertical until it intersects with the ray OD, and from this point - a second horizontal line parallel to the x-axis.

Then a vertical line is drawn from point C until it intersects with the ray OK. From this intersection point at an angle of 45° to the vertical, we draw a line until it intersects with the abscissa axis, and from this point we draw a second vertical line, parallel to the ordinate axis, until it intersects with the second horizontal line. The intersection point of these lines will be intermediate point 1 of the compression polytrope. Point 2 is found in the same way, taking point 1 as the beginning of the construction.

The expansion polytrope is constructed using rays OK and OE, starting from point Z", similar to the construction of the compression polytrope.

The criterion for the correct construction of an expansion 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 start from point z, not z ..

After constructing the compression and expansion polytropes, they produce

rounding of the indicator diagram taking into account the advance of the opening of the exhaust valve, ignition timing and the rate of pressure rise, and also drawing the intake and exhaust lines. For this purpose, under the abscissa axis, a semicircle with radius R=S/2 is drawn along the piston stroke length S as on the diameter. From the geometric center Oґ towards the b.m.t. segment is postponed

Where L- length of the connecting rod, selected from the table. 7 or according to the prototype.

Beam ABOUT 1.WITH 1 is carried out at an angle Q o = 30° corresponding angle

ignition timing ( Qо= 20...30° to w.m.t.), and the point WITH 1 demolished on

compression polytrope, obtaining point c1.

To build lines for cleaning and filling the cylinder, a beam is laid down ABOUT 1?IN 1 at an angle g=66°. This angle corresponds to the pre-opening angle of the exhaust valve or exhaust ports. Then draw a vertical line until it intersects with the expansion polytrope (point b 1?).

From the point b 1. draw a line defining 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, may

be carried out 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 the indicator diagram of a supercharged engine, the intake line may be higher than the exhaust line.

The indicator diagram of the internal combustion engine (Fig. 1) is constructed using data from the calculation of engine operating cycle processes. When constructing a diagram, it is necessary to choose a scale in such a way as to obtain a height equal to 1.2... 1.7 of its base.

Fig.1 Diesel engine indicator diagram

Rice. 1 Diesel engine indicator diagram

At the beginning of construction, the segment S a = S c + S is plotted on the abscissa axis (the base of the diagram),

where S is the piston stroke (from TDC to BDC).

The segment S c corresponding to the volume of the compression chamber (V c) is determined by the expression S c = S / - 1.

The segment S corresponds to the working volume V h of the cylinder, and is equal in magnitude to the piston stroke. Mark the points corresponding to the position of the piston at TDC, points A, B, BDC.

The ordinate axis (diagram height) displays pressure on a scale of 0.1 MPa per millimeter.

Pressure points p g, p c, p z are plotted on the TDC line.

Pressure points p a, p b are marked on the BDC line.

For a diesel engine, it is also necessary to plot the coordinates of the point corresponding to the end of the calculated combustion process. The ordinate of this point will be equal to p z, and the abscissa is determined by the expression

S z = S s   , mm. (2.28)

The construction of a line of compression and expansion of gases can be carried out in the following sequence. At least 3 volumes or piston stroke segments V x1, V x2, V x3 (or S x1, S x2, S x3) are selected arbitrarily between TDC and BDC.

And the gas pressure is calculated

On the compression line

On the expansion line

All constructed points are smoothly connected to each other.

Then the transitions are rounded (with each change in pressure at the junctions of the design cycles), which is taken into account in the calculations by the coefficient of completeness of the diagram.

For carburetor engines rounding at the end of combustion (point Z) is carried out along the ordinate р z = 0.85 Р z max.

2.7 Determination of average indicator pressure from the indicator diagram

The average theoretical indicator pressure p" i is the height of the rectangle, equal to the area pressure scale indicator chart

MPa (2.31)

where F i is the area of ​​the theoretical indicator diagram, mm 2, limited by the lines of TDC, BDC, compression and expansion, can be determined using a planimeter, the integration method, or another method; S - length of the indicator diagram (piston stroke), mm (distance between the lines TDC, BDC);

 p - pressure scale selected when constructing the indicator diagram, MPa / mm.

Actual indicator pressure

р i = р i ΄ ∙ φ p, MPa, (2.32)

where  p is the coefficient of incompleteness of the area of ​​the indicator diagram; takes into account the deviation of the actual process from the theoretical one (rounding with a sharp change in pressure, for carburetor engines  p = 0.94.. .0.97; for diesel engines  p = 0.92.. .0.95);

р = р r - р а - average pressure of pumping losses during intake and exhaust for naturally aspirated engines.

After determining p i from the indicator diagram, compare it with the previously calculated one (formula 1.4) and determine the difference as a percentage.

The average effective pressure p is equal to

r e = r i – r mp,

where pmp is determined by formula 1.6.

Then calculate the power according to the dependence
and compare with the given one. The discrepancy should be no more than 10...15%, if more processes need to be recalculated.

OPERATING DIAGRAM OF A 4-STROKE DIESEL.

ICE MARKING.

Marking of domestic diesel engines is carried out in accordance with GOST 4393-74. Each engine type 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 gear transmission

K - crosshead

N - 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 diameter/piston stroke in centimeters. For example: 8DKRN 74/160, 6ChSP 18/22, 6Ch 12/14

Marking of foreign diesel manufacturing companies:

Engines from the SKL plant in Germany (former GDR)

Four-stroke internal combustion engines are engines in which one power stroke (stroke) is carried out in four piston strokes, or two revolutions of the crankshaft. The strokes are: intake (filling), compression, power stroke (expansion), exhaust (exhaust).

I bar - FILLING. The piston moves from TDC to BDC, as a result of which a vacuum is created in the above-piston cavity of the cylinder, and through the open inlet (suction) valve, air from the atmosphere enters the cylinder. The volume in the cylinder increases all the time. At BDC the valve closes. At the end of the filling process, the air in the cylinder has following parameters: pressure Pa=0.85-0.95 kg/cm 2 (86-96 kPa); temperature Ta=37-57°C (310-330 K).

2nd bar - COMPRESSION. The piston moves in the opposite direction and compresses a fresh charge of air. The volume in the cylinder decreases. Pressure and temperature increase to the following values: Pc=30-45kg/cm2, (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 high pressure of 20-150 MPa (200-1200 kg/cm2), which spontaneously ignites under the influence of high temperature and burns out quickly. Thus, during the second stroke, air is compressed, fuel is prepared for combustion, the working mixture is formed and its combustion begins. As a result of the combustion process, gas parameters increase to the following values: Pz = 55-80 kg/cm 2 (6-8.1 MPa); Tz=1500-2000°C (1700-2200 K).

III bar - EXPANSION. Under the influence of forces arising from the pressure of fuel combustion products, the piston moves to 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 bar - RELEASE. At the end of the expansion stroke (before BDC), the exhaust valve opens and gases, having energy and pressure greater than atmospheric, rush into the exhaust manifold, and when the piston moves to TDC, a forced removal occurs exhaust gases piston At the end of the exhaust stroke, 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). The exhaust valve closes at TDC. The work cycle is completed.

Depending on the position of the piston, the change in pressure in the engine cylinder can be depicted graphically in coordinate axes PV (pressure - volume) 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 (working 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 supercharging system and has more power.

The extreme positions of the piston (TDC and BDC) are shown with 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 – cylinder working volume;

Va. – 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.

The pressure in the cylinder is plotted on the ordinate axis (on a selected scale).

The indicator diagram under consideration shows the theoretical (calculation) cycle, where assumptions are made, i.e. strokes begin and end at dead centers, the piston is at TDC, the combustion chamber is filled with exhaust gas residues.

IN real engines the moments of opening and closing the valves begin and end not at the dead centers of the piston position, but with a certain offset, which is clearly visible in the pie chart of the gas distribution. The opening and closing moments of valves, expressed in degrees of crankshaft rotation (c.c.c.), are called valve timing. The optimal angles for opening and closing valves, as well as the start of fuel supply, are determined experimentally when testing a prototype at the manufacturer’s stand. All angles (phases) are indicated in the engine form.

By the time the air charge enters the engine cylinder, the suction valve opens. Point 1 corresponds to the position of the crank at the moment the valve opens. To better fill the cylinder with air, the suction valve opens before TDC and closes after the piston moves to BDC at an angle equal to 20-40° p.k.v., which is designated as the angle of advance and retardation of the intake valve. Usually the angle of p.k.v. corresponds to an intake process equal to 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 requires time for it to heat up and evaporate. This period of time is called the auto-ignition delay period. Therefore, fuel injection is carried out with some advance until the piston reaches TDC at an angle of 10-35° p.k.v.

FUEL SUPPLY ADVANCE ANGLE

The angle between the direction of the crank and the cylinder axis at the start of fuel injection is called the fuel advance angle. OOPT is counted from the beginning of feed to TDC and depends on the feed system, fuel type and engine shaft speed. OOPT for diesel engines varies from 15 to 32° and has great value to operate 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 data sheet.

Optimal OOPT is of great importance for normal operation engine and its efficiency. With proper regulation, fuel combustion should begin before the piston reaches TDC at 3-6° p.k.v. The highest pressure Pz, equal to the calculated one, is achieved when the piston moves to TDC at an angle of 2-3° p.k.v. (see "Combustion phases").

With increasing OOPT, the auto-ignition delay period ( Phase I) increases and the bulk of the fuel burns at the moment the piston passes TDC. This leads to harsh operation of the diesel engine, as well as increased wear of parts of the CPG and the crankshaft.

A decrease in CVD leads to the fact that the main part of the fuel enters the cylinder when the piston passes 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 pushing out exhaust gases by the piston, the exhaust valve is opened with advance until the piston reaches BDC at an angle equal to 18-45° p.k.v., which is called the advance angle of the exhaust valve opening. Dot (). To better clean the cylinders from combustion products, the exhaust valve is closed after the piston TDC moves to a retardation angle equal to 12-20° p.k.v., corresponding to point () on the pie chart.

However, the diagram shows 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° p.k.v.