What is a combustion chamber for? Engine combustion chambers. Gas boiler Hephaestus

For good mixture formation, it is simultaneously necessary to correctly combine fuel atomization and air movement in the combustion chamber. This will improve the distribution of fuel in the chamber and carry out the combustion process with the least amount of air.

The shape of the combustion chamber should:

• correspond to the direction and range of the injected fuel jet;
• ensure organized movement of air flow, intensive mixing of fuel and air, complete combustion of fuel in a short period with the least amount of air;
• smooth increase in pressure in the cylinder, moderate maximum pressure during combustion and minimal heat losses;
• create conditions for easier engine starting.

By design, diesel engines are divided into two main categories: with undivided and divided combustion chambers. Undivided chambers have only one compartment in which both mixture formation and fuel combustion occur. The divided chambers are divided into two parts: the main and additional ones, connected to each other by a neck. In this case, fuel is injected into the additional chamber.

The method distinguishes between volumetric, film and combined mixture formation.

With volumetric mixture formation, the fuel is atomized in the volume of the combustion chamber and only a small part of it enters the wall layer. Volumetric mixture formation is carried out in undivided combustion chambers.

Film mixture formation is used in a number of combustion chamber designs, when almost all the fuel is directed to the near-wall zone. Approximately 5–10% of the fuel injected by the injector enters the central part of the combustion chamber. The rest of the fuel is distributed on the walls of the combustion chamber in the form of a thin film (10–15 microns). Initially, part of the fuel that gets into the central part of the combustion chamber is ignited, where there is usually no movement of the charge and the highest temperature is established. Subsequently, as it evaporates and mixes with air, combustion spreads to the main part of the fuel, directed into the near-wall layer. Film mixture formation requires a less fine atomization of the fuel. Nozzles with one nozzle hole are used. The fuel injection pressure does not exceed 17–20 MPa. Film mixing compared to volumetric mixing provides better economic performance of the engine and simplifies the design of fuel equipment. The main disadvantage is the low starting properties of the engine at low temperatures due to the small amount of fuel involved in the initial combustion. This disadvantage is eliminated by heating the air at the inlet or by increasing the amount of fuel involved in the formation of the initial source of combustion.

Combined mixture formation is obtained with smaller diameters of the combustion chamber, when part of the fuel reaches its wall and is concentrated in the wall layer. The other part of the fuel droplets is located in the internal volume of the charge. Approximately 50% of the fuel settles on the surface of the chamber. When entering the chamber, no rotational movement of the charge is created. The charge is set in motion when it is displaced from the space above the piston into the combustion chamber, and a vortex is created. The speed of the charge reaches 40–45 m/s. Distinctive feature from film mixture formation is the counter-movement of jets of fuel and charge displaced from the space above the piston, which helps to increase the amount of fuel suspended in the volume of the combustion chamber and brings the process closer to volumetric mixture formation. Nozzles are used with sprayers having 3–5 nozzle holes.

Combustion chambers with direct injection. In diesel engines with such chambers, fuel is injected directly into the combustion chamber by a nozzle with a working pressure of 15–30 MPa, which has multi-hole nozzles (5–7 holes) with a small diameter of nozzle channels (0.15–0.32 mm). Such high injection pressures are used due to the fact that in this case the atomization of fuel and its mixing with air is achieved mainly due to the kinetic energy imparted to the fuel during injection. To ensure uniform distribution of fuel in the chamber, the injectors of such engines are often made with several holes.

In Fig. Figure 6.4 shows the combustion chambers of engines with direct injection, providing volumetric mixture formation.

Rice. 6.4. Undivided combustion chambers for volumetric mixture formation:

a – hemispherical, b – toroidal

Rice. 9.3. Types of main combustion chambers

Tubular(top in Fig. 9.3) the combustion chamber consists of a flame tube 1, inside which the combustion process is organized, and the housing (casing) 2. Several of these cameras were usually installed on engines. In modern aviation gas turbine engines, tubular combustion chambers are practically not used.

IN tubular-ring In the chamber, all flame tubes are enclosed in a common housing, which has internal and external surfaces covering the engine shaft.

IN circular In the combustion chamber (bottom in Fig. 9.3), the flame tube has the cross-sectional shape of a ring, which also covers the engine shaft.

The location and type of injectors used to supply fuel to the combustion chambers may also vary. However, despite the wide variety of designs and design forms of the main combustion chambers, the combustion process in them is organized almost identically.

One of the most important features of organizing the combustion process in the main combustion chambers of a gas turbine engine is that it must proceed with relatively large coefficients excess air. At currently realized gas temperatures in front of the turbine are on the order of = 1800...1600 K and below, as already noted, the value of the excess air coefficient (average for the entire chamber) should be 2.0...3.0 or more. With such values a homogeneous fuel-air mixture, as stated above, does not ignite or burn. With a sharp decrease in the fuel supply to the engine, which can occur under operating conditions, the excess air coefficient can reach even significantly higher values ​​(up to 20...30 or more).

The second important feature of these chambers is that the speed of air flow or the fuel-air mixture in them (selected taking into account the requirements for the overall dimensions of the engine) significantly exceeds the speed of flame propagation. And, if special measures are not taken, the flame will be carried away by the flow outside the combustion chamber

Therefore, the organization of the fuel combustion process in the main chambers of the gas turbine engine is based on the following two principles, which make it possible to ensure stable fuel combustion at high values and high flow rates in them:

1. The entire air flow entering the combustion chamber is separated into two parts, of which only one part (usually the smaller one) is fed directly into combustion zone(where due to this the mixture composition necessary for stable combustion is created). And the other part is sent bypassing the combustion zone (cooling the flame tube from the outside) into the so-called mixing zone(in front of the turbine), where it mixes with combustion products, lowering their temperature to the required extent;

2. Stabilization of the flame in the combustion zone is ensured by creating in it reverse current zones, filled with hot combustion products, continuously igniting the fresh combustible mixture.

Rice. 9.4. Diagram of the main combustion chamber

The air entering the combustion chamber from the compressor is divided into two parts. One part is sent to the combustion zone, and the second part to the mixing zone. Part of the air entering the combustion zone is, in turn, divided into two more parts. The first part, the so-called primary air
(see Fig. 9.4), enters directly through the front device to the location of the fuel injector spray jet and is used to form a rich fuel mixture of such a composition that would ensure sufficiently fast and stable combustion in all modes.

Its second part (the so-called secondary air
) enters the chamber through the side holes in the flame tube to complete the combustion process (primary air is not enough for this). The total amount of air entering the combustion zones (i.e.
) provides it with an excess air coefficient of the order of = 1.6…1.8, which corresponds to stable combustion, complete combustion and a temperature of about 1800…1900 K.

If the permissible gas temperature in front of the turbine is below this value, it is necessary to reduce it tertiary (or mixing) air enters the flame tube through the rear rows of holes or slots, quickly reducing their temperature to an acceptable level. It is important to emphasize that if some part of the fuel does not have time to burn before entering the mixing zone, then its further combustion will practically not occur, since the excess air coefficient increases to values ​​exceeding the limit of stable combustion.

The number, location and shape of holes for supplying tertiary air are selected in such a way as to ensure the desired gas temperature field in front of the turbine.

The supply of primary and secondary air to the flame tube must be organized so that the desired flow structure is created in the combustion zone. This structure should ensure good mixing of fuel with air and the presence of powerful reverse currents, ensuring reliable ignition of the fresh mixture in all operating modes of the chamber.

Rice. 9.5. Reverse current zone

in the main combustion chamber

Other schemes of the main combustion chambers can be used - with several nozzles (several rows of nozzles), with other methods of creating a reverse flow zone, etc. But the general principles of organizing the work process in them remain the same.

AFTER COMBUSTION CHAMBERS AND ORGANIZATION OF THE PROCESS

BURNING IN THEM

Rice. 9.6. Diagram of the afterburner combustion chamber

In Fig. Figure 9.6 shows a typical diagram of an afterburner combustion chamber installed behind a turbojet engine turbine. There is a small diffuser at the entrance to the chamber 7 . Behind it is a front device consisting of several flame stabilizers 5 (plates or v-rings) and a large number (often several dozen) nozzles 1 , combined into several fuel manifolds(there are two of them in Fig. 9.6). A large number of nozzles ensures uniformity of the mixture composition throughout the chamber volume, and the presence of several collectors allows, by partially turning them off, to maintain at reduced modes (i.e., with a reduced total fuel consumption) the mixture composition necessary for stable combustion near those nozzles that have not yet been turned off.

Combustion chambers Modern gasoline engines with overhead valves predominantly use the following types of combustion chambers: hemispherical, polyspherical, wedge, flat-oval, pear-shaped, cylindrical. There are mixed combustion chamber options. The shape of the combustion chamber is determined by the location of the valves, the shape of the piston crown, the location of the spark plug, and sometimes two spark plugs, and the presence of displacers. When designing an engine, taking into account the fuel used and a given compression ratio, the following requirements are imposed on the combustion chambers: ensuring high speeds combustion, reduced requirements for the octane number of fuel, minimal losses with coolant, low toxicity, manufacturability. This is determined by the following conditions:

Compact combustion chamber;
-effective turbulization of the mixture during combustion;
-minimum surface area ratio

Combustion chambers to the working volume of the cylinders. As already noted, one way to increase the effective efficiency of an engine is to increase the compression ratio. The main reason for limiting the compression ratio is the risk of abnormal combustion processes (detonation, glow ignition, roar, etc.). In modern production engines with fairly high compression ratios, further increasing them will have a relatively small effect and is associated with the need to solve a number of problems. First of all, this is the occurrence of detonation. It is this that determines the requirements for the compression ratio and the shape of the combustion chamber. After the working mixture is ignited by a spark, the flame front spreads throughout the combustion chamber, the pressure and temperature in this part of the charge increase to 50...70 bar and 2000...2500 C, and pre-flame chemical reactions occur in the part of the working mixture farthest from the spark plug. At low crankshaft speeds, especially in engines with large cylinder diameters, the time for these reactions is sometimes sufficient for the residual charge to burn at high speeds (up to 2000 m/s).

Detonation combustion causes shock waves to travel through the combustion chamber at high speed, causing metallic knocking noises, sometimes incorrectly called finger knocking. The shock wave, destroying the wall layer of gases with a low temperature, helps to increase heat transfer into the walls of the cylinder, combustion chamber, valve plates, and piston crown, causing them to overheat and increasing heat losses in the engine. Working with strong detonation leads to general overheating of the engine, deterioration in power and economic performance. During prolonged driving with intense detonation, erosion of the walls of the combustion chamber begins, melting and scuffing of the piston, increased wear of the upper part of the cylinder due to the breakdown of the oil film, breakage of the bridges between the grooves of the piston rings and scuffing of the cylinder mirror, burnout of the cylinder head gasket. Among the factors influencing the requirements for the octane number of fuel is the compactness of the combustion chamber, characterized by the degree of increase in the volume of the burned part of the mixture (in% of the total volume of the combustion chamber) as the conventional flame front moves away from the spark plug. The most compact are hemispherical, tent-type combustion chambers, which have lower octane requirements. However, to increase the compression ratio to 9.5...10.5 in hemispherical or polyspherical chambers, sometimes it is necessary to make the piston bottom convex, which significantly worsens the degree of compactness and accordingly increases the requirements for the octane number, which increase by 3...5 units. In modern engines with 4 valves per cylinder, the spark plug is located in the center of the combustion chamber. This ensures the maximum degree of volume increase.

Another parameter characterizing anti-knock qualities is the degree of turbulization of the mixture during the combustion process. The intensity of turbulization depends on the speed and direction of the mixture flow at the entrance to the combustion chamber. One way to create intense turbulence is to increase the area of ​​the displacer (the volume located between the piston bottom and the plane of the cylinder head) in order to turbulize the charge to increase the combustion rate. Displacers have wedge, oval, pear-shaped combustion chambers. By replacing the flat-oval combustion chamber with a pear-shaped one, thereby increasing the area of ​​the displacer while simultaneously reducing its height on UAZ car engines, it was possible to increase the compression ratio by 0.5 without changing the requirements for fuel octane, due to which fuel consumption decreased by 5...7 %, and the power increased by 4... 5%. For UZAM 331 engines and for some engines trucks(ZIL-508.10) to create a vortex motion of the charge in front of the intake valve, the channel was made snail-shaped. However, at high mixture speeds this led to an increase in resistance and, accordingly, a decrease in power indicators. Therefore, the latest models of UZAM engines are produced with a conventional intake duct. Hemispherical, polyspherical cylindrical combustion chambers have practically no displacer, therefore their anti-knock qualities (according to the detonation index) are inferior to chambers with displacers. During mass production of engines, due to deviations in the dimensions of the crank mechanism parts and the volume of the combustion chamber, the actual compression ratio of an engine of one model may differ by a significant amount (within one unit). Therefore, a car of the same model often requires gasoline with different octane number. The actual compression ratio can be approximately determined using a compression gauge.

A - hemispherical; b - hemispherical with a displacer; c - spherical; g - tent; d - flat oval; e - wedge; h - cylindrical combustion chamber in the piston; g - semi-wedge with part of the chamber in the piston;

As is clear, combustion chambers must provide not only
not bad mixture formation, and even better performance
efficiency and starting properties of the motor. There are two constructive
groups of combustion chambers of diesel engines, separated from each other not only
design, and the principle of formation of the fuel mixture in the chamber. This
broken and undivided combustion chambers.

Broken combustion chambers

Such chambers have two interconnecting channels independent of the volume:

• prechamber;
• vortex chamber.

The vortex chamber can be placed either in the block head
cylinders and in the block itself. The cooling surface of the broken chambers is very
high. In this regard, the engine is prone to significant thermal losses,
which leads to a decrease in starting properties and a negative effect on the factor
efficiency. Typically, diesel engines with broken combustion chambers
provide a fairly high compression ratio.

The main advantage of broken combustion chambers is
production of virtually ideal fuel consistency. Thanks to the use
kinetic energy of gases due to flow between chamber cavities,
fuel combustion is greatly increased and exhaust smoke is minimized
systems.

In addition, the interaction of channels in broken cameras
assigns stability to the engine during its operation. The main
loads on such important parts as connecting rods, crankshaft, piston pins.
To somehow reduce the so-called roughness of diesel operation with
broken combustion chambers can also be due to an increase in temperature
mode of certain camera areas.

Undivided combustion chambers

Undivided combustion chambers, unlike broken ones, have
only volume and the simplest form, consistent with direction, number and
the size of the fuel flows of the injected fuel. Such cameras have very
small sizes, as follows, have a small cooling surface.
This is how thermal energy is lost in engines with undivided chambers
combustion is significantly less than in engines with broken chambers. Such
diesel has good starting and economic characteristics.

The shapes of undivided combustion chambers are distinguished by their
variety. More often they are designed in the piston heads. But it does occur
placement of chambers in the cylinder head, also partly in the piston heads
and partly in the head.

It is possible to break the undivided combustion chambers of diesel engines
engines according to their fundamental structural arrangement followed by
way:

1. Toroidal in the piston.
2. Hemispherical in the piston and head
   cylinders
3. Hemispherical in the piston.
4. Cylindrical in piston.
5. Cylindrical in piston with side placement.
6. Rounded in the piston.
7. Balls in the piston.
8. Toroidal with a neck in the piston.
9. Cylindrical, formed with the piston bottom and
   cylinder wall.
10. Vortex in the piston.
11. Trapezoidal in the piston.
12. Cylindrical in the cylinder head under
    exhaust valve.

In combustion chambers of types 1, 2, 3,
4, 5 a very high degree of fuel consistency formation properties is obtained
thanks to fuel atomization and coordination of the forms of its fuel flows with
camera shapes. In such combustion chambers, nozzles are often installed,
having multi-hole nozzles that allow you to control the forms of fuel
flows, also use satisfied highest pressure injection These cameras
have very small cooling surfaces. For diesel engines with
the listed types of combustion chambers are characterized by low degree characteristics
compression.

For combustion chambers type 6, 7, 8,
9 features wider cooling surfaces. Although this is unconventional,
but it still affects the starting performance of the engine. But in the process
displacement of air above the piston into the combustion chamber at the moment of compression
vortex-type flows are created, which promotes good air mixing
with fuel, forming a fairly high-quality fuel mixture.

Combustion chambers type 10, 11, 12
used not only in diesel engines, but also in engines with
possibility of using various types fuel. The corresponding feature of such cameras
is a serious direction of eddy currents that promotes evaporation
fuel and delivering it with a certain sequence to the required location
combustion. To improve performance in cylindrical chambers in the head
cylinder block under the exhaust valve use the highest exhaust temperatures
valve, which immediately forms the wall of the combustion chamber.

The main advantage of diesel engines is low fuel costs, since engines of this type have low specific fuel consumption in basic operating modes, and the fuel itself is noticeably cheaper than gasoline in many countries.

Among the disadvantages of diesel Compared to gasoline engines, they include: relatively low power indicators, more expensive to manufacture and maintain fuel equipment, worse starting qualities, increased emissions of some toxic components with exhaust gases, increased noise levels.

The economic and environmental performance of an automobile diesel engine primarily depends on the characteristics of the operating process and, in particular, on the type of combustion chamber and fuel injection system. The combustion chambers of a diesel engine are divided into separated(vortex chamber and prechamber), semi-separated And undivided .

Diesel engines with a single chamber are sometimes called direct injection engines.

Diesel engines with split combustion chamber Typically installed on light duty trucks and passenger cars. This is determined by the need to reduce noise levels and less harsh operation. As the piston approaches TDC, air from the main volume of the combustion chamber is forced into the additional volume, creating intense charge turbulence in it, which promotes better mixing of fuel droplets with air. The disadvantages of diesel engines with a divided combustion chamber are: a slight increase in fuel consumption due to increased losses into the cooling medium due to the increased surface of the combustion chamber, large losses due to the flow of air charge into the additional chamber and the burning mixture back into the cylinder. In addition, starting performance deteriorates.

Diesel engines with undivided combustion chamber have low fuel consumption and are easier to start. Their disadvantage is the increased rigidity of the work and, accordingly, high level noise.

For complete combustion of the fuel, the manufacturer selects the optimal ratio between the number of nozzle holes in the nozzle and the intensity of the vortex movement of the charge in the cylinder - so that the fuel jets completely cover the entire air charge. The smaller the nozzle holes, the more intense the rotational motion of the charge should be. In four-stroke diesel engines, the rotational movement of air during the intake stroke is ensured by the tangential arrangement of the intake channel, the presence of a screen at the valve, and a screw (snail-shaped) channel in front of the intake valve. During the compression process, as the piston approaches TDC, air flows from the space above the piston into the combustion chamber in the piston, increasing the intensity of the rotational movement of the fresh charge. Therefore, when repairing diesel engines, it is necessary to ensure that the gap between the piston bottom and the cylinder head corresponds to the value specified in the instructions. With a larger gap, the intensity of charge turbulence will be insufficient; with a smaller gap, at high loads, a knock of the piston from hitting the head may appear. During diesel engine assembly, this clearance is checked by installing lead plates on the piston crown and turning the crankshaft after tightening the head bolts.

Starting a diesel engine:

Diesel engines with a divided combustion chamber (swirl chamber or prechamber) have significantly worse starting qualities than diesel engines with a non-divided chamber.

To facilitate starting, split-chamber diesel engines are equipped with electric glow plugs installed in the pre-chamber or swirl chamber. Less commonly, spark plugs are installed in diesel engines with direct injection.

Candles come in open and closed types with an incandescent coil or heating element. They are produced by the same companies that make spark plugs. The spark plug casing is located in the combustion chamber of a diesel engine so that the cone of atomized fuel hits only its hot tip.

During the period when the toxicity of exhaust gases was assessed by the emission of CO and CH (hydrocarbons), the general press noted that diesel engines have the lowest toxicity of all internal combustion engines. However, later, when commercial gasoline began to be produced without ethyl liquid, and gasoline engines began to be equipped with three-component catalytic converters that reduce the content of CO, CH, NOx by 90-95%, the low toxicity of diesel engines compared to gasoline engines began to be modestly kept silent.

Increased toxicity of diesel engines is determined the following factors:

The first one is low efficiency of catalytic converters. This is due to the fact that the compression ratio, and therefore the expansion ratio, of diesel engines is much higher than that of gasoline engines. Therefore, the temperature of the exhaust gases is insufficient for the efficient operation of the converters. In this regard, it is not possible to reduce the emission of nitrogen oxides, which are several tens of times more toxic than CO.

Second factor - increased emissions in some modes, especially during heating, products of incomplete combustion with a characteristic unpleasant odor (acrolein, aldehydes, etc.), many of which are carcinogens. Third, soot particles are carriers of carcinogens. Once in the respiratory tract, they cause cancerous tumors. Due to the fact that none of the countries still have high-speed gas analyzers, there is no way to standardize their emissions. Therefore, legislators use indirect indicators - limiting emissions of hydrocarbons and particulate matter.

The main reasons for the increased toxicity and increased fuel consumption of diesel engines are as follows:

Low fuel quality,

Malfunction of the fuel supply system (too low excess air ratio, uneven fuel supply to the cylinders, displacement of injection phases, inter-cycle unevenness of fuel supply),

Increased oil consumption due to waste due to wear of parts of the cylinder-piston group,

In turbocharged engines, the boost pressure is too low.

One of the main characteristics of diesel fuel is its cetane number, which indicates its ability to self-ignite.

It is determined on a single-cylinder installation by comparison with a mixture of reference fuel, selected so that the ignition delay period is the same as that of the test fuel. The cetane number must be at least 45. It depends on chemical composition fuel and the presence of special additives in it. An increase in the cetane number is achieved by increasing the content of paraffin hydrocarbons in the fuel. At the same time, starting qualities are improved, but with a cetane number of 50...55, the completeness of combustion deteriorates.

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