HCCI Combustion Characteristics and influencing fa

2022-09-19
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Abstract: This paper briefly introduces the concept and characteristics of homogeneous charge compression ignition (HCCI) combustion, and further analyzes the characteristics of HCCI Combustion in diesel engines, as well as some important factors affecting HCCI Combustion in diesel engines, such as mixture formation mode, intake air temperature, load, exhaust gas recirculation (EGR), valve timing and compression ratio

key words: diesel engine, homogeneous charge compression ignition, influencing factors

HCCI (homogeneous charge compression ignition) combustion is a new combustion mode. It can effectively reduce fuel consumption and emissions of traditional internal combustion engines, especially reduce NOx and particulate matter (PM) emissions of diesel engines at the same time, and further increase thermal efficiency. Therefore, it is considered as a major progress in engine combustion technology

with the increasingly stringent emission regulations and the progress of engine technology, the potential of HCCI Combustion Technology in saving energy and reducing emissions has attracted great attention all over the world. Some research institutions and enterprises in the United States, Europe and Japan are vigorously carrying out research in this field, and have become a research and development hotspot in the engine field

I. overview of HCCI combustion mode

hcci is a mode in which the uniform combustible mixture is compressed in the cylinder until it ignites by itself. With the progress of the compression process, the temperature and pressure in the cylinder continue to rise, and the combustible mixture that has been evenly mixed or basically evenly mixed reaches the conditions of spontaneous combustion at multiple points at the same time, so that the combustion occurs at multiple points at the same time, and there is no obvious flame front. The combustion reaction is rapid, and the combustion temperature is low and evenly distributed. Therefore, only a small amount of NOx and particles (PM) are generated, which has high thermal efficiency at low load. HCCI engine mainly has the following characteristics

1. Ultra low NOx and PM emissions. Under some working conditions, the NOx emission of HCCI engine can be reduced by 95% - 98% compared with that of diesel direct injection engine (DI)

2. High combustion thermal efficiency. The thermal efficiency of HCCI engine is even higher than that of direct injection diesel engine

3.hcci combustion process is mainly controlled by combustion chemical kinetics

4. The operating range of HCCI engine is narrow

hcci engine combustion is limited by misfire (too lean mixture) and deflagration (too rich mixture), which narrows the operating range of the engine. For high cetane fuel, HCCI engine burns very fast, and it is easy to knock under high load conditions (high mixture concentration); For fuels with high octane number, because HCCI combustion is lean combustion, the engine is prone to misfire under low load conditions

5. HC and CO emissions of HCCI engine are high. This is mainly because HCCI combustion usually uses thinner mixture and stronger EGR, which is caused by the low temperature in the cylinder

second, the characteristics of HCCI Combustion in diesel engines

there are two difficulties in realizing HCCI Combustion in diesel engines: first, diesel fuel has high viscosity and poor volatility, which makes it difficult to form a homogeneous mixture; Second, as a fuel with high cetane number, diesel is prone to low-temperature spontaneous combustion, and the combustion speed of homogeneous mixture is difficult to control, which is easy to cause rough combustion

diesel HCCI combustion occurs at multiple points at the same time, with no obvious flame front, rapid combustion reaction, low combustion temperature and uniform distribution, only a small amount of NOx and PM are generated, and it has high thermal efficiency at low load. The traditional diesel engine uses high-pressure injection to form non-uniform mixture diffusion combustion. The mixture and temperature distribution are extremely uneven. NOx is generated in the high-temperature area of the diffusion flame shell, and PM is generated in the internal high-temperature hypoxia

the combustion heat release of diesel HCCI shows two special stages. The first stage (the smaller peak on the exothermic curve) is related to low-temperature chemical kinetics (cold flame or blue flame); The second stage (the larger peak on the exothermic curve) is the main combustion period; The first stage is the second stage of pre flame reaction. The heat released by the pre flame reaction heats the remaining charge, while the remaining charge continues to be compressed. After a short delay, the remaining charge reaches the ignition conditions, and fires almost at the same time, which makes the heat release rate rise rapidly, and now there is a large peak on the heat release curve

therefore, HCCI burns faster, and the combustion start point and heat release rate are very sensitive to the temperature and pressure of the charge during the compression process, so it is difficult to control. If HCCI combustion is well controlled, efficient and stable combustion can be carried out within the wide range of large air-fuel ratio, with low cycle fluctuation pressure and soft operation

the ignition time of HCCI combustion is mainly affected by chemical reaction dynamics, and is less affected by load and speed, so it cannot be controlled by conventional load, speed and other feedback signals. Generally, a larger air-fuel ratio or a higher EGR rate is used to slow down the combustion speed to prevent detonation, but at the same time, it is difficult for the average indicated pressure in the engine cylinder to reach a higher level, which makes HCCI Engine vulnerable to misfire, detonation, power and other restrictions, and the range of operation is not wide. The current technology can not realize the HCCI combustion process of diesel engine under all working conditions. From the perspective of practicality, it is inevitable to adopt the dual-mode operation scheme, that is, low and medium load, using HCCI combustion process; At high load, the traditional combustion mode can still be used to obtain the best economic performance of the engine

III. influencing factors of HCCI Combustion in diesel engines

(I) three kinds of mixture formation methods that affect HCCI Combustion in diesel engines

the formation of homogeneous mixture is the first step to realize HCCI combustion control. The diesel homogeneous premixed gas methods adopted internationally include: inlet port out of cylinder premixed gas, in cylinder early injection and late injection

1. Out of cylinder premixed hcci

that is, diesel is injected into the intake pipe during the intake stroke and mixed with air to form premixed gas. It is a relatively simple method to improve the uniformity of the mixture by using inlet injection and intake swirl to strengthen the formation of the mixture. However, higher intake air temperature is required to promote the evaporation of diesel, which requires the installation of heating devices and intake fuel injection system, and is not conducive to the cold start of diesel engine. In the early research, this method is often used to prepare the mixture. The Southwest Research Institute of the United States, which first conducted the research, used this method to introduce the mixture. After the fuel is ejected from the inlet, it mixes with the air to form a uniform mixture. When the inlet valve is opened, the mixture enters the cylinder for compression and ignition. Due to the poor volatility and wall impact of diesel, this method will lead to higher HC and CO emissions and increased fuel consumption

2. Early HCCI injection in cylinder

this method is the widely used method of diesel HCCI pre mixture formation at present. That is, in the early stage of the compression stroke, diesel oil is injected into the cylinder and gradually mixed with air as the piston goes up until spontaneous combustion occurs. Due to the high density of diesel, the low density of air in the cylinder at the early stage of the compression stroke, and the large penetration of high-density diesel injected into the low-density environment, the phenomenon of fuel hitting the wall is serious, which will reduce the atomization and mixing degree of fuel, resulting in increased emissions, increased fuel consumption and other problems

in order to improve the atomization and mixing of fuel, the HCCI injection advance angle of diesel engine is much larger than that of traditional diesel engine, so that the diesel and air can be fully mixed before ignition. In addition, the atomization and mixing of diesel oil can be improved to a certain extent by improving the design of fuel injector, changing the shape of combustion chamber, and properly organizing the air flow in the cylinder. The typical representative of the successful application of in cylinder diesel early injection in products is the unibus combustion system of Toyota in Japan. Compared with pre mixed diesel HCCI outside the cylinder, early injection diesel HCCI has the following two advantages

the temperature and pressure in the cylinder during the compression stroke are higher than those in the intake pipe when the intake valve is opened, which is conducive to the atomization and mixing of diesel. Injection at the early stage of compression stroke reduces the requirement of intake air temperature and the tendency of mixture deflagration

with the early injection scheme of compression stroke, only one set of oil supply system is required to meet HCCI and traditional direct injection diesel

3. Late HCCI injection in the cylinder

after approaching or at the top dead center, inject diesel into the cylinder, and adopt a large number of pre cooled EGR, strengthen eddy current and reduce compression ratio to achieve ignition delay, so that diesel ignition happens just after the end of injection. Although the uniformity of oil and gas formed by late injection in the cylinder is not as uniform as that of inlet injection and early injection in the cylinder, NOx and PM emissions are still lower than those of traditional diesel engines. The typical representative of HCCI combustion with late injection in diesel engine cylinder is the MK system of Nissan company in Japan. By delaying the fuel injection, the MK system extends the ignition delay period by increasing the EGR rate (reducing the oxygen concentration to 15% - 16%), so that the fuel injection is completely completed within the ignition delay period. In order to improve the mixing rate, the swirl ratio of MK engine is increased by 12, and the combustion chamber design is optimized to speed up the oil-gas mixing. Within the load range of MK combustion, NOx can be reduced by more than 90%, and the smoke intensity is less than 1 Bosch unit

(II) influence of intake air temperature

the ignition time of HCCI combustion is very sensitive to intake air temperature. With the increase of intake air temperature, the phenomenon of ignition advance will occur. Therefore, controlling the temperature in the cylinder will be a key factor in controlling the ignition time of HCCI combustion. Generally, HCCI Combustion and ignition start point are controlled by adjusting intake air temperature

adding an intake heating device to the intake pipe and introducing exhaust gas recirculation (EGR) can improve the intake temperature. The HCCI research of najt, foster and later Thring on four stroke diesel engine is to heat the mixture through the electric heating device after exhaust gas recirculation. The effects of air flow rate and inlet air temperature on the engine are experimentally studied. With the increase of inlet air temperature or air flow rate, the combustion efficiency increases, and HC and CO emissions decrease; In the low temperature and low flow rate area, the pressure of each cylinder changes relatively little. Under a certain temperature condition, the ignition time of each cylinder varies greatly. When the temperature is very low, even one cylinder catches fire and the other cylinder stalls; Considering the HC and CO emissions under idle conditions, measures of low flow rate and high inlet temperature can be adopted to improve the combustion efficiency; For high load conditions, the average effective pressure increases with the decrease of intake air temperature. In order to improve efficiency and increase power output, feedback control must be carried out on each cylinder to optimize the combustion phase of each cylinder; Spraying water into the intake manifold can reduce the temperature of the mixture, thus reducing the temperature in the compression stroke cylinder, delaying the ignition time, reducing the combustion speed and peak pressure, and increasing the operating range of HCCI under heavy load; The voltage required by the air inlet heating device is high, and the device itself is very large, which is only suitable for basic research in the laboratory; Water spray will lead to the increase of HC and CO emissions, but has little control effect on NOx, which hinders the application of this method

(III) influence of load

1. Under low load condition

hcci diesel engine operates under low load condition, the circulating oil supply is small, the mixture concentration is thin, and the reactant concentration is an important factor affecting the combustion reaction. In addition, the temperature in the cylinder is low at this time, which significantly delays the ignition time of HCCI combustion, and even causes misfire. In 2009, after the occurrence of the misfire cycle, the explosion pressure of the subsequent cycle often rises suddenly. This is because the electromechanical immediately stops rotating at this time of the misfire cycle, and part of the residual fuel is in the cylinder, resulting in the increase of the fuel volume of the next cycle, causing the explosion pressure to rise suddenly

2. High load condition

hcci diesel engine runs at high load, and

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