What Is a Combustion Chamber?

The combustion chamber is a device in which fuel or propellant is burned to generate high-temperature gas, and is a combustion device made of a high-temperature resistant alloy material. After the piston reaches the top dead center, the space between the top of the piston and the cylinder head is where the fuel burns. It is an important component of gas turbine engines, ram engines, and rocket engines.

Diffuser, combustion chamber housing, flame tube, fuel nozzle, ignition device

parameter

There are combustion chamber efficiency, pressure loss, stability, ignition range, outlet temperature distribution and heat capacity.

However, the parameters closely related to the variable operating conditions of the gas turbine are mainly the combustion chamber efficiency and pressure loss. The former directly affects the fuel consumption of the gas turbine (affects the efficiency of the gas turbine), and also affects the gas flow through the turbine; Expansion ratio to turbine. Due to the complexity of the combustion process inside the combustion chamber, one cannot yet use theoretical calculation methods to give the relationship between the efficiency of the combustion chamber and the pressure loss as a function of the operating conditions. These relations are mainly experimentally based empirical formulas.

Total pressure loss of air flow through the combustion chamber

1) Flow loss in the diffuser. It includes frictional resistance and flow resistance that causes the airflow to escape when the expansion angle is too large;

2) Flow loss when the airflow flows through the various parts of the combustion chamber. Including the pressure loss of air flow through the head device (spoiler or combustion bowl, etc.) and the pressure loss when flowing through the wall air inlet or gap;

3) Mixed loss between the inlet jet and the main flow in the flame tube;

4) Additional pressure loss caused by air flow through various obstacles (support plates, baffles, nozzles, etc.) in the channel;

5) When the airflow is heated, the thermal resistance is lost due to the change in airflow density.

The pressure loss in the five parts of the combustion chamber can be divided into two categories:

One is loss of flow resistance; the other is loss of thermal resistance.

The total loss of the combustion chamber can be obtained by adding the above two types of resistance losses. ^[5]

Single tube combustion chamber, combined tube combustion chamber, annular combustion chamber

With the development of modern technology, the limits of global emission standards and human requirements for the environment are getting higher and higher. Energy efficiency and low pollution have become the development trend of modern automobiles. Comprehensive consideration of engine fuel economy, power and emissions and other indicators, the new combustion chamber is now more and more widely used in automobiles.

Fireball high compression ratio combustion chamber

The main part of the combustion chamber is located below the recessed exhaust valve of the cylinder head. The diameter is small, the structure is compact, and there is a certain squeezing surface, which can form a strong squeezing turbulence. At the same time, the shallow recess of the intake valve communicates with a shallow groove in the main combustion chamber. Its advantages are: it can burn very lean mixture, and the air-fuel ratio can reach 26, which reduces the fuel consumption rate. The disadvantage is that high octane gasoline must be used, which is sensitive to carbon deposition, and the compression ratio needs to be strictly controlled.

Texaco combustion system tccs combustion chamber

TCCS combustion chamber is a type of combustion chamber in the United States taxaco controlled combustion process. Belongs to gasoline injection uniform type.

In this system, only the air is sucked into the cylinder, and a strong suction vortex is organized by the air guide screen. At the end of the compression, the crankshaft angle is about 30 ° before the top dead center. A mixed gas is formed with the air flow. Because the spark plug is arranged under the air flow, there is just a relatively thick mixture that is liable to catch fire near the spark gap. After the fire, the flame and gas expand with the airflow. After the ignition, the mixture of fuel injected and the swirling airflow immediately burns when encountering the flame.

This type of combustion system does not necessarily use all the air in the cylinder. When the load is small, the area where the combustion products expand is not large. As the load increases, the duration of the injection is extended, and the area of the combustion products also expands, but only at full load. Only extended to the entire cylinder. Therefore, its total air-fuel ratio can reach 100.

Advantages: 1) Power can be used

The design of the combustion chamber of a gasoline engine has a great influence on the engine's power, economy, working stability and emission characteristics. Therefore, the design of the combustion chamber should meet the following requirements.

As compact as possible

Combustion chamber area-to-volume ratio is used to characterize the compactness of the combustion chamber. The face to face ratio is small, and the combustion chamber structure is compact, so that the flame propagation distance is short, the combustion can be completed in a short time, the detonation tendency is reduced, and the engine compression ratio can be improved. At the same time, because the surface area per unit volume is small, the relative heat dissipation area is small, the heat loss is reduced, the engine has high thermal efficiency, and the face-to-surface ratio is small, the chilling zone of the cylinder wall is reduced, and HC emissions are reduced. The combustion chamber surface volume ratio depends on the cylinder diameter and the shape of the combustion chamber. In the case of using a small combustion chamber, in order to reduce the surface area per unit volume, a hemispherical combustion chamber is often used.

Spark plugs in place

Depending on the position of the spark plug, the rate of change of the flame propagation distance and the combustion speed are different, which affects the working performance of the gasoline engine. Therefore, when determining the position of the spark plug, the following aspects should be considered: 1) The flame propagation distance should be short, such as the arrangement of the spark plug In the center of the combustion chamber. 2) Reduce the heating of the end gas, such as the spark plug is arranged near the exhaust valve.

3) Reduce combustion variation between cycles to ensure good warm-up and low-speed stability. For example, the spark plug is arranged between the intake and exhaust valves, which is convenient to use fresh mixed gas to remove residual exhaust gas around the spark plug, so that the mixed gas is easy to ignite. At the same time, the intensity of the airflow should be controlled to avoid blowing sparks.

4) Ensure that the engine runs smoothly. The position of the spark plug should be able to gradually expand the flame surface spread from the spark plug.

Reasonable combustion chamber shape

The volume distribution of the combustion chamber reflects the distribution of the mixed gas. Coordination with the position of the spark plug determines the heat release rule of combustion and the distance from the flame to the flammable mixture at the edge, which affects the explosion resistance of the engine, rough work, performance, economy, pressure rise speed, and work stability. For example, when the circular combustion chamber is ignited at its bottom, the combustion rate is fast first and then slow. Wedge-shaped combustion chambers are similar. When igniting at the top of the circular combustion chamber, the combustion rate is slow first and then fast. In between, the bathtub-shaped combustion chamber is similar to this

In short, the volume distribution of the combustion chamber should be considered in conjunction with the position of the spark plug. The most favorable distribution is to make the pressure increase rate in the initial stage of the combustion process small, the engine to work softly, and the medium-term heat release to maximize the cycle work. The post-combustion hinge is small and has high thermal efficiency.

With high inflation efficiency

The arrangement of the air inlet and the air inlet minimizes the resistance of the air intake and increases the air charge. The shape of the combustion chamber should be considered to allow a larger diameter of the intake valve. If the wedge-shaped combustion chamber can be arranged with a larger diameter intake valve, the mixed air flow should be as smooth as possible with fewer turns. The figure below shows a hemispherical and oblique bathtub shape. Comparison of combustion chamber charge factors.

The intake duct of the hemispherical combustion chamber has fewer curves, and the combustion chamber has a slightly higher bow height (large slope area), which is conducive to the layout of a large area of the intake and exhaust valves, so the performance is good and the charging efficiency is high.

Create proper turbulent motion

Forming a gas flow of appropriate intensity in the combustion chamber can accelerate the flame propagation; increase the cooling of the end mixture; reduce the combustion fluctuations between cycles, and expand the ignition limit of the mixture, which is conducive to the combustion of leaner mixtures; reduce HC emissions, but the turbulence is too strong, Heat loss to the cylinder wall increases, and it is possible to blow out the nucleus and misfire, which will increase Hc emissions. The following figure shows the comparison of the combustion pressure change when the turbulence is appropriate and the turbulence is too strong.

It can be seen that when the turbulence is too strong, even if the ignition advance angle is reduced, the pressure increase rate is still high, making the work rough and the thermal efficiency reduced. Practice has proved that when the turbulence intensity makes the pressure increase rate of 196-245 (kPa / degree), the thermal efficiency of the engine is the highest.

In recent years, a low-pollution, low-fuel-consumption, high-swirl combustion chamberfire spherical combustion chamber has been developed.

The swirl formed by the intake process is pressed into the small diameter chamber under the exhaust valve during the compression process. The high-speed swirl combined with the compact combustion chamber allows the use of high compression ratios without causing surface ignition or deflagration.

Proper cooling of the end mixture

Proper cooling of the end mixture can avoid local hot spots in the combustion chamber, reduce the final combustion temperature, and reduce the tendency to deflagration. At the same time, pay attention to the cooling intensity should not be too large, otherwise it will increase HC emissions. ^[9]