VALID ICE CYCLES
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, a 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.
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 displayed as dependences of pressure changes ( R) from changes in volume ( V) (collapsed indicator diagram) or pressure changes with the angle of rotation crankshaft(φ), which is called an 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 beginning 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 (release 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.
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.
In a four-stroke engine, the work processes are as follows:
- 1. Intake stroke. When the piston moves from TDC to BDC due to the resulting vacuum from the air cleaner, atmospheric air enters the cylinder cavity through the open intake valve. The air pressure in the cylinder is 0.08 - 0.095 MPa, and the temperature is 40 - 60 C.
- 2. Compression cycle. The piston moves from BDC to TDC; the inlet and outlet valves are closed, as a result of which the upwardly moving piston compresses the incoming air. To ignite the fuel, the temperature of the compressed air must be higher than the autoignition temperature of the fuel. During the piston stroke to TDC, the cylinder is injected through the nozzle diesel fuel supplied by the fuel pump.
- 3. Expansion stroke, or working stroke. The fuel injected at the end of the compression stroke, mixing with the heated air, ignites, and the combustion process begins, characterized by a rapid increase in temperature and pressure. In this case, the maximum gas pressure reaches 6-9 MPa, and the temperature is 1800-2000 C. Under the action of the gas pressure, piston 2 moves from TDC to BDC - a working stroke occurs. Around BDC, the pressure drops to 0.3-0.5 MPa, and the temperature drops to 700-900 C.
- 4. Cycle of release. The piston moves from BDC to TDC and through the open exhaust valve 6 the exhaust gases are pushed out of the cylinder. The gas pressure drops to 0.11-0.12 MPa, and the temperature drops to 500-700 C. After the end of the exhaust stroke, with further rotation of the crankshaft, the operating cycle is repeated in the same sequence.
An indicator diagram taken with an indicator device is called an indicator diagram (Fig. 1).
Rice. 1
Consider the diagram:
- 0-1 - filling the cylinder with air (at internal mixture formation) or a working mixture (at external mixture formation) at a pressure slightly below atmospheric due to the hydrodynamic resistance of the intake valves and the suction pipe,
- 1-2 - compression of air or working mixture,
- 2-3 "-3 - the period of combustion of the working mixture,
- 3-4 - the working stroke of the piston (expansion of combustion products), mechanical work is performed,
- 4-5 - exhaust gas exhaust, pressure drop to atmospheric occurs at almost constant volume,
- 5-0 - freeing the cylinder from combustion products.
In real heat engines, the conversion of heat into work is associated with complex irreversible processes (there is friction, chemical reactions in the working fluid, final piston speeds, heat exchange, etc.). Thermodynamic analysis of such a cycle is impossible VM Gelman, MV Moskvin. Agricultural tractors and cars. - M .: Agropromizdat, 1987, part I and P ..
SCHEME OF 4-STROKE DIESEL OPERATION.
ICE MARKING.
Domestic diesel engines are labeled 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
P - reversible
C - with a reversible clutch
P - s gear transmission
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; numbers after letters - 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 working stroke (cycle) is carried out in four piston strokes, or two revolutions of the crankshaft. The strokes are: inlet (filling), compression, working stroke (expansion), outlet (exhaust).
I cycle - 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 is increasing all the time. The valve closes behind the 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 cycle - COMPRESSION... The piston moves in the opposite direction and compresses the fresh air charge. The volume in the cylinder decreases. Pressure and temperature rise to 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 at a high pressure of 20-150 MPa (200-1200 kg / cm 2), which spontaneously ignites under the influence of high temperature and quickly burns out. Thus, during the second stroke, the air is compressed, the fuel is prepared for combustion, the working mixture is formed and the beginning of its combustion. 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 cycle - EXPANSION... Under the action of the forces arising from the pressure of the fuel combustion products, the piston moves to the BDC. The thermal energy of the gases is converted into mechanical work by moving the piston. At the end of the expansion stroke, the gas parameters decrease 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 cycle - ISSUE... At the end of the expansion stroke (up to BDC), the exhaust valve opens and gases that have energy and pressure higher than atmospheric pressure rush into the exhaust manifold, moreover, when the piston moves to TDC, 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). At TDC, the outlet valve closes. The work cycle is over.
Depending on the position of the piston, the change in pressure in the cylinder of the engine can be plotted in the PV (pressure - volume) axes of a closed curve, which is called an indicator diagram. In the diagram, each line corresponds to a certain process (clock):
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 - line of atmospheric pressure.
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 in 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. - full 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 ordinate (on a selected scale) represents the pressure in the cylinder.
The indicator diagram under consideration shows the theoretical (settlement) cycle, where the assumptions are made, i.e. strokes start and end at dead centers, the piston is at TDC, the combustion chamber is filled with the remains of exhaust gases.
V real engines the moments of opening and closing the valves begin and end not at the dead points of the piston position, but with a certain displacement, which can be clearly seen on the circular timing diagram. The moments of opening and closing the valves, expressed in degrees of crankshaft rotation (r.p.), are called valve timing. The optimal angles of opening and closing the valves, as well as the beginning of the fuel supply, are determined experimentally when testing a prototype at the manufacturer's stand. All angles (phases) are indicated in the engine log.
When 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 suction valve opens before TDC and closes after the BDC piston moves to an angle equal to 20-40 ° SCC, which is designated as the lead and lag angle of the intake valve. Usually the angle of the f.c.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 for the fuel to self-ignite, it takes time for it to heat up and evaporate. This period of time is called the autoignition lag period. Therefore, fuel injection is performed with some advance until the piston arrives at TDC at an angle of 10-35 ° sc.c.v.
FUEL ADVANCE ANGLE
The angle between the direction of the crank and the cylinder axis 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 grade and engine speed. Diesel engines have a PSA from 15 to 32 ° and are of great importance for ICE operation... 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 SPS is essential for proper engine operation and economy. With proper regulation, fuel combustion should begin before the piston arrives at TDC by 3-6 ° sc.c. Highest pressure Pz, equal to the calculated one, is reached when the piston moves to TDC at an angle of 2-3 ° sc.c.v. (see "Combustion phases").
With an increase in the SOPT, the autoignition delay period ( 1st phase) increases and the bulk of the fuel burns out at the moment the piston moves 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 KShM.
A decrease in the SOPP leads to the fact that the main part of the fuel enters the cylinder when the piston moves to 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 the exhaust gases by the piston, the exhaust valve is opened with an advance before the piston arrives at BDC by an angle equal to 18-45 ° c.c., which is called the advance angle of the exhaust valve opening. Point (). For better cleaning of cylinders from combustion products, the exhaust valve is closed after the piston TDC moves to a retard angle equal to 12-20 ° sc.c., corresponding to the point () on the circular diagram.
However, it can be seen from the diagram that the suction and discharge valves are simultaneously open for some time. This opening of the valves is called the valve phase overlap angle, which adds up to 25-55 ° c.c.
The indicator diagram - the dependence of the pressure of the working fluid on the volume of the cylinder (Fig. 2) - is the most informative source that allows you to analyze the processes occurring in the cylinder of an internal combustion engine. The engine strokes, carried out in four piston strokes from TDC to BDC, are shown on the indicator diagram in coordinates p - V the following curve segments:
r 0 – a 0 - intake stroke;
a 0 – c - compression stroke;
c – z - b 0 – stroke of the working stroke (expansion);
b 0 – r 0 – cycle of release.
The following characteristic points are marked on the diagram:
b, r - the moments of opening and closing of the exhaust valve, respectively;
u, a - the moments of opening and closing of the intake valve, respectively;
Rice. 2. Typical indicator diagram of a four-stroke
internal combustion engine
The area of the diagram, which determines the work per cycle, consists of the area corresponding to the positive indicator work obtained during the compression and stroke strokes, and the area corresponding to the negative work spent on cleaning and filling the cylinder in the intake and exhaust strokes. The negative work of the cycle is usually attributed to mechanical losses in the engine.
Thus, the total energy imparted to the shaft piston engine in one cycle L, can be determined by the algebraic addition of the work of the steps L = L vp + L sr + L px + L no. The power transmitted to the shaft is determined by the product of this sum by the number of strokes of the working stroke per unit of time ( n/ 2) and the number of engine cylinders i:
The engine power determined in this way is called the average indicated power.
The indicator diagram allows you to divide the cycle of a four-stroke engine into the following processes:
u –r 0 - r - a 0 - a - inlet;
a - θ - c "- compression;
θ – c "- c - z - f - mixture formation and combustion;
z - f - b - extension;
b –b 0 - u - r 0 - r - release.
The above typical indicator chart is also valid for diesel engine... In this case, the point θ will correspond to the moment of fuel supply to the cylinder.
The diagram indicates:
V c – the volume of the combustion chamber (the volume of the cylinder above the piston at TDC);
V a - the total volume of the cylinder (the volume of the cylinder above the piston at the beginning of the compression stroke);
V n – working volume of the cylinder, V n = V a - V c.
Compression ratio.
The indicator diagram describes the duty cycle of the engine, and its limited area – indicator work of the cycle. Really, [ p ∙ ∆V] = (N / m 2) ∙ m 3 = N ∙ m = J.
If we assume that a certain conditional constant pressure acts on the piston p i, performing work during one piston stroke, equal to work gases per cycle L, then
L = p i ∙ V h ()
where V h is the working volume of the cylinder.
This is the conditional pressure p i it is customary to call the average indicator pressure.
The average indicated pressure is numerically equal to the height of a rectangle with a base equal to the working volume of the cylinder V h with an area equal to the area corresponding to the work L.
Since the useful indicator work is proportional to the average indicator pressure p i, the perfection of the working process in the engine can be assessed by the value of this pressure. The more the pressure p i, the more work L, and therefore the cylinder displacement is used better.
Knowing the average indicator pressure p i, cylinder displacement V h, number of cylinders i and crankshaft speed n(rpm), you can determine the average indicated power of the four-stroke engine N i
Work i ∙ V h is the engine displacement.
The transfer of the indicated power to the engine shaft is accompanied by mechanical losses due to friction of the pistons and piston rings on the cylinder walls, friction in the bearings of the crank mechanism. In addition, part of the indicated power is spent on overcoming aerodynamic losses arising from the rotation and vibration of parts, on actuating the gas distribution mechanism, fuel, oil and water pumps and other auxiliary engine mechanisms. Part of the indicator power is spent on removing combustion products and filling the cylinder with a fresh charge. The power corresponding to all these losses is called the power of mechanical losses. N m.
In contrast to the indicated power, the net power that can be obtained on the motor shaft is called the effective power. N e. The effective power is less than the indicator power by the amount of mechanical losses, i.e.
N e = N i - N m. ()
Power N m corresponding to mechanical losses and effective engine power N e is determined empirically during bench tests using special loading devices.
One of the main indicators of the quality of a piston engine, which characterizes the use of the indicated power for performing useful work, is the mechanical efficiency, defined as the ratio of the effective power to the indicator:
η m = N e / N i. ()
The total energy imparted to the shaft of a piston engine can be determined by algebraic addition of the work of strokes and multiplying the sum by the number of working strokes per unit of time ( n/ 2) and the number of engine cylinders. The power determined in this way can be obtained by integrating the dependence of pressure as a function of volume shown in the indicator diagram (Figure 4.2, b), and is called the average indicated power N... This power is often associated with the concept of the indicated average effective pressure. R i calculated as follows:
Effective power N e is the product of the indicated power N on the mechanical efficiency of the engine. The mechanical efficiency of a motor decreases with increasing engine speed due to friction and drive losses.
To build the characteristics of an aircraft piston engine, it is tested on a balancing machine using a variable-pitch propeller. The balancer provides measurement of torque, crankshaft revolutions and fuel consumption. According to the value of the measured torque M cr and the number of revolutions n the measured effective motor power is determined
If the engine is equipped with a gearbox that reduces the propeller speed, then the formula for the measured effective power is:
where i R - ratio reducer.
Taking into account the dependence of the effective engine power on atmospheric conditions, the measured power for comparison of test results is brought to standard atmospheric conditions according to the formula
where N e - effective engine power, normalized to standard atmospheric conditions;
t measure - outdoor air temperature during testing, ºС;
B- outside air pressure, mm Hg,
∆R- absolute air humidity, mm Hg
Effective specific fuel consumption g e is determined by the formula:
where G T and - fuel consumption and effective engine power, measured during tests.
Construction of indicator diagrams
Indicator charts are plotted 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 the plot, on the abscissa axis, a segment AB is laid, corresponding to the working volume of the cylinder, and equal in magnitude to the stroke of the piston on a scale that, depending on the magnitude of the stroke of the piston of the engine being designed, can be taken as 1: 1, 1.5: 1 or 2: 1.
A segment OA corresponding to the volume of the combustion chamber,
is determined from the ratio:
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)
pressures = 0.02; 0.025; 0.04; 0.05; 0.07; 0.10 MPa in mm so that
get the height of the diagram equal to 1.2 ... 1.7 of its base.
Then, according to the thermal calculation data, the diagram is laid in
the selected scale of the pressure at the characteristic points a, c, z ", z,
b, r. Point z for gasoline engine corresponds to pzT.
Indicator diagram of a four-stroke diesel engine
According to the most common graphic 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 (it is recommended to press = 15… 20 °). Further, from the origin of coordinates, beams OD and OE are drawn at certain angles and to the ordinate axis. These angles are determined from the relations
0.46 = 25 °, (3.13)
The compression polytrope is constructed using the OK and OD rays. From point C, a horizontal line is drawn to the intersection with the ordinate; from the point of intersection - a line at an angle of 45 ° to the vertical to the intersection with the ray OD, and from this point - a second horizontal line parallel to the abscissa axis.
Then, from point C, vertical line before crossing the OK beam. From this point of intersection at an angle of 45 ° to the vertical, draw a line to the intersection with the abscissa axis, and from this point, the second vertical line, parallel to the ordinate, to the intersection with the second horizontal line. The point of intersection 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 constructed using the rays OK and OE, starting from the point Z ", similarly to the construction of the compression polytrope.
The criterion for the correctness of the 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 polytropic curve of expansion should start from the point z, not z ..
After the construction of the polytrope of compression and expansion,
rounding of the indicator diagram taking into account the anticipation of the opening of the exhaust valve, the ignition timing and the rate of pressure rise, as well as the intake and exhaust lines. For this purpose, under the abscissa axis, a semicircle with a radius of R = S / 2 is drawn along the length of the piston stroke S as on the diameter. From the geometric center Оґ towards the NMT. postponed segment
where L- the length of the connecting rod, 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 ( Qо= 20 ... 30 ° to VMT), and the point WITH 1 demolished on
polytropic compression, obtaining point c1.
To build lines for cleaning and filling the cylinder, a beam is laid O 1?V 1 at an angle g= 66 °. This angle corresponds to the opening advance angle of the outlet valve or outlet ports. Then a vertical line is drawn up to the intersection with the expansion polytropic (point b 1?).
From point b 1.draw a line defining the law of change
pressure in the area 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. can also
find by the value of the lost share 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.
