Optical Testing Methods for Internal Combustion Engine Combustion

The combustion process of an internal combustion engine is the core of its working cycle, which directly affects the engine's power, economy and emission indicators. Since the combustion process of an internal combustion engine is very complex and difficult to test, it has long been highly valued by internal combustion engine researchers and production departments around the world.

The combustion process of an internal combustion engine is the most important and complex process in the actual engine working cycle, involving multiple disciplines such as chemical reaction kinetics, fluid mechanics, and heat and mass transfer. It is a very challenging research field. Therefore, the key to energy saving and emission reduction of internal combustion engines lies in a deep understanding of the combustion process system. Only by carefully studying the characteristic laws of the occurrence and development of internal combustion engines, clarifying the influence of various factors, and on the basis of a relatively thorough understanding of the overall process and local details of combustion, can we improve the parameter design of various parts of the internal combustion engine in a targeted manner, more effectively improve the efficiency of the internal combustion engine, and reduce emissions.

Therefore, using advanced experimental means and methods to carry out research on the combustion process in the cylinder of an internal combustion engine and obtain relevant information about the combustion flame in the cylinder (such as temperature field, concentration field, and velocity field) has very important academic value and broad application prospects. The optical test method for combustion in the cylinder of an internal combustion engine is one of the most effective research methods at present, and has been increasingly widely used at home and abroad. Using this method to study the combustion process of internal combustion engines can further deepen the understanding of the combustion process, provide a basis for the evaluation and improvement of the combustion system, and has important practical significance for guiding the design of internal combustion engine combustion systems and improving the overall level of the internal combustion engine industry.

1 Several optical test methods for internal combustion engine combustion research

The greatest advantage of the optical test method for internal combustion engine combustion is that it does not interfere with the combustion field and can intuitively obtain images of the combustion process. In recent years, the rapid development of optical technology and computer technology has provided new opportunities for more accurate research on the combustion process, and has therefore been highly valued by scientific research institutions and engine manufacturers in various countries. In the past decade, various internal combustion engine combustion test technologies based on optical principles have developed rapidly, and their practical applications have become increasingly extensive. Some advanced combustion test technologies have gradually entered the practical stage. Among the various optical test methods for internal combustion engine combustion, there are mainly two-color method, holographic method, absorption spectroscopy method, laser-induced fluorescence spectroscopy (LIF method), Raman scattering spectroscopy and coherent anti-Stokes Raman scattering (CARS method). The application of these optical test methods has made the research on internal combustion engine cylinder combustion develop in the direction of microscopic, quantitative and visual.

1.1 Two-color method

Two-color method is a traditional method for measuring high temperature. Thermal radiation is a ubiquitous phenomenon in nature. All objects, as long as their temperature is higher than absolute zero, will generate radiation to varying degrees. It can be seen from Planck's blackbody radiation law that when the temperature of a blackbody is constant, its spectral radiation emittance conforms to Planck's formula. The basic principle of the two-color method is to fit the blackbody radiation curve by measuring the luminous intensity of two wavelengths, so as to infer the temperature of the object.

In the diesel engine combustion flame, carbon particles exist throughout the combustion process. The spectrum of carbon particles is a continuous spectrum in the visible light range. From the ignition delay period, the fuel undergoes fission reaction to generate C and H atoms, and a small amount of carbon particles are still present in the fuel combustion products. Carbon particles can reach thermal equilibrium with the surrounding environment in a very short time (about 1 μs), and their radiation spectrum can represent the instantaneous temperature of the combustion flame. The radiation intensity of the monochromatic wavelength of carbon particles can be used as the basis for measuring the flame temperature. In actual measurement, by selecting the two monochromatic wavelength radiation intensities of carbon particles with wavelengths of λ1 and λ2 in the visible light range, the interference of the radiation wavelengths of other components is avoided to achieve the measurement of the instantaneous temperature of the flame.

Compared with other measurement methods, the two-color method has the following shortcomings:

the temperature measurement value is only a statistical average, and the spatial distribution of temperature cannot be obtained;
the test device is relatively complex, and the test results must be calibrated;
the two-color method uses the emission spectrum of the material for measurement. When the wavelength falls in the infrared and visible light bands, the measurement accuracy is affected due to the overlap with the high-temperature radiation spectrum of the flame.

1.2 Holographic method

Holography is based on the principles of physical chemistry. It uses the interference phenomenon of light waves to simultaneously record the amplitude and phase of the object light wave on the photosensitive film, and reproduce the three-dimensional image of the object through the diffraction phenomenon, or re-display the object light wave.

The holographic interference temperature measurement method uses a laser holographic system to record the comparison wave of the initial state of the temperature field on the holographic dry plate after exposure. After development and fixing, the processed film is accurately placed back to its original position, while keeping the other optical components of the whole system unchanged. At this time, the original reference wave is used to illuminate the hologram to reproduce the comparison wave. If the original object light wave is still used to illuminate the temperature field, an object light wave with superimposed temperature field information will be generated. The object light wave will generate interference fringes with the original comparison wave, so that the continuously changing temperature field can be expressed as the change of interference fringes.

By using laser holographic interference method and combining it with a high-speed camera, the change process of the temperature field in the combustion chamber can be continuously recorded to obtain a two-dimensional temperature image; however, this test device generally needs to be carried out on a vibration-damping table, which has extremely poor vibration resistance, seriously affecting its actual use.

1.3 Absorption spectroscopy

Absorption spectroscopy is a method of measuring temperature and concentration by using the absorption effect of the medium on light when light passes through the combustion medium. According to the Bouguer-Lamkert absorption law, after light with a frequency of γ passes through a medium with a length of L, the transmittance of the light intensity I is:

Tγ=Iγ(L)/Iγ(0)=exp(－∫dx.βγ.PI), ... (1)

where PI is the partial pressure of the absorbing particle; βγ is the absorption coefficient of the particle to light with a frequency of γ, which is determined by the properties of the medium itself.

β γ=Σδ jg
j(γ－γ0), ………(2)

wherein, δj is the spectral line intensity in the absorption medium, which is determined by the molecular energy level, quantum number, particle number and temperature of the absorption medium; gj(γ－γ0) is the absorption spectrum line function of the medium, which is generally a comprehensive broadening line. In order to obtain parameters such as particle concentration and temperature, the measured absorption spectrum must be fitted.

Absorption spectroscopy is also used to study various chemical reactions in the combustion process, such as the generation of NOx and CO. In order to improve spatial resolution and measurement accuracy, saturation absorption method (LISF) and optical tomography (Optical Tomography) have been developed on the basis of absorption method, which has increased the accuracy of absorption method to more than 10-9. Saturation absorption method uses two interlaced light beams of different intensities. A strong light beam excites the ground state particles, causing holes to appear on the absorption curve; when another weak laser beam of the same frequency passes, the absorption will weaken. LISF also makes the result expression simpler. Optical tomography is to set M beams and N beams of parallel light in two directions with a certain angle, so as to form M×N absorption points, and improve the spatial resolution by measuring the spectrum at these absorption points. Both methods have been improved to "point-by-point" measurement, which greatly improves the spatial resolution and measurement accuracy. [page]

Laser absorption spectroscopy is a relatively simple and mature method for analyzing the combustion process. It has the following characteristics:

the method is direct and the test equipment used is simple;
all measurement results are the average value on the optical path, so the spatial resolution is relatively low;
affected by the instability of the laser light source, the measurement accuracy is low.

1.4 Laser Induced Fluorescence (LIF)

Laser induced fluorescence is a highly sensitive method for detecting concentration and temperature. Its principle is that when the laser wavelength is tuned to two specific energy levels of a molecule, the molecule resonates and absorbs photon energy to be excited to a high energy state. In the process of returning from the high energy state to the ground state, the molecule emits fluorescence; the fluorescence is received by a photomultiplier tube, and its signal is:

Pf=hγ.(A 21/4π).Ωc.Vc.N2, ………………(3)

where hγ is the fluorescence photon energy, A 21 is the spontaneous emission coefficient of the upper energy level of the fluorescence, Ωc and Vc are the solid angle of the optical collection system and the effective fluorescence volume, respectively, and N2 is the number of particles in the upper energy level of the fluorescence.

When using the LIF method for quantitative analysis, in order to obtain the absolute value of the concentration, the fluorescence signal must be corrected, that is, the fluorescence volume Vc, the fluorescence collection solid angle Ωc, the fluorescence transmission efficiency of the optical system, and the absorption, capture, polarization and collision broadening factors of the fluorescence on the fluorescence signal are considered. Moreover, when using the LIF method to study high-temperature, high-pressure combustion processes such as internal combustion engines, the fluorescence quenching effect must also be considered. The quenching effect refers to the fact that when a molecule absorbs photon energy and transitions to an excited state, the energy reaches other energy levels through collision relaxation rather than fluorescence. Especially under high temperature and high pressure, the particle concentration is high and the mean free path is short, so this effect is more obvious. In severe cases, no fluorescence spectrum can be received.

In addition to its high sensitivity, the most attractive feature of the LIF method is that it can display the concentration distribution of the combustion field in a two-dimensional plane. Within one laser pulse, a two-dimensional transient combustion field distribution map can be obtained, realizing real-time processing. Therefore, the LIF method and the improved LIF method (PLIF method) have become one of the methods widely used internationally in recent years.

Compared with other spectral diagnostic techniques, laser induced fluorescence has the following characteristics:

high sensitivity, because the fluorescence signal is directly related to the upper and lower energy levels of the measured molecule;
high spatial and temporal resolution;
using OH fluorescence as the LIF spectrum can reduce infrared background radiation and particle scattering, and OH is an intermediate product of many chemical reactions, which is very important for improving combustion performance and reducing pollutants;
it can realize the two-dimensional distribution display of concentration field and temperature field.

1.5 Raman scattering spectroscopy

When light passes through gas molecules, part of the light will be scattered by the molecules and frequency shift will occur. The scattered light intensity is

Among them, I0 is the incident light intensity, σI(γ0)/ Ω is the differential scattering cross section of light with a frequency of γ0, δΩ, δS, η are the optical system collection solid angle, the scattering intensity determined by the optical system and the transmission coefficient of the optical system, respectively, and nI is the number of particles in the lower energy level of the molecule.

Raman frequency shift is caused by the uneven vibration energy level of the molecule and the interaction with the rotation energy level, so the Raman frequency shift of each vibration energy level is different; and because the number of particles at each energy level changes with temperature, the Raman signal also changes with temperature. By measuring and comparing the two intensity peaks of the Raman spectrum and then fitting them, the temperature can be obtained.

Raman spectroscopy has the following characteristics:

Raman signal is only a linear function of the incident light intensity and the number of particles, so the analysis of the test results is simple, and there is no need to consider the effects of time delay, quenching and collision;
Raman spectroscopy can be used to realize the display of two-dimensional combustion fields;
usually, the collision cross section is only 10-29 cm2~10-31 cm2, which is 10 orders of magnitude less than the molecular absorption cross section and 3 orders of magnitude less than Rayleigh scattering. Therefore, the Raman signal is very weak (1 Raman photon is generated for every 104 incident photons), which puts higher requirements on the sensitivity of the detection instrument, and can only measure high-concentration particles and can only be used in low-noise environments, not in oxygen-rich combustion because of the strong noise radiation background;
the accuracy of Raman spectroscopy temperature measurement is generally ±100 K.

1.6 Coherent Anti-Stokes Spectroscopy (CARS)

The principle of coherent anti-Stokes spectroscopy is that when two high-energy laser beams with frequencies of ω1 and ω2 are focused at one point and incident on a certain medium, if ω3=2ω1-ω2 happens to be a certain resonance spectrum line of the molecule and meets the phase matching condition in nonlinear optics, then the light of the ω3 frequency will be greatly enhanced. This signal can be used to identify the composition and concentration of the gas, which is the CARS method. Generally, ω1 is fixed, and ω2 can change the frequency by tuning the laser, so ω3 can always resonate with a certain molecular energy level. In addition, since the effect of temperature on the spectrum can be completely determined, the temperature of the gas can be determined by spectral line fitting analysis. The CARS method is a method that uses nonlinear optics. Its characteristics are:

it has high signal intensity and can produce signals 105 to 1010 times larger than the Raman signal. At the same time, the CARS signal is a coherent light with a frequency of ω3=2ω1-ω2 higher than the pump light frequency, so the signal-to-noise ratio is high, and it is not affected by the fluorescence of various components in the fuel and flame, and can be used for the study of oxygen-enriched combustion;

compared with the LIF method, the CARS method does not need to consider the influence of factors such as quenching and energy transfer, and does not need to be corrected;

the spatial resolution of the CARS method is relatively low, and in turbulent flames, its spatial resolution is only 2 mm to 4 mm. The CARS method cannot be used to display the two-dimensional combustion field;

the nonlinear conditions of the CARS method are difficult to achieve, so the experimental device is complex.

In summary, the two-color method, holographic method, absorption spectroscopy, laser induced fluorescence (LIF), Raman scattering spectroscopy and coherent anti-Stokes spectroscopy (CARS) have their own characteristics. In practical applications, they must be reasonably selected according to the actual situation. In recent years, the research and application of laser induced fluorescence (LIF) abroad has been particularly active, which represents a development trend. The main reason is that the laser induced fluorescence method has extremely high sensitivity and can obtain two-dimensional images with high spatial resolution. Therefore, in the future methods of internal combustion engine cylinder combustion testing, the laser induced fluorescence method will occupy an important position and has a broad future. At the same time, as a traditional method of measuring high temperature, the two-color method has also gained new vitality driven by the rapid development of new technologies. The latest development of fiber optic imaging technology and high-speed image acquisition and processing technology combined with the two-color method allows people to easily and quickly obtain a two-dimensional temperature image of the internal combustion engine combustion process [6], overcoming the difficulty that the traditional method can only obtain the temperature values ​​of a few points in space, but cannot obtain the two-dimensional temperature distribution. Therefore, the two-color method has also been widely used due to its simplicity and minimal impact on the combustion chamber itself.