Frontier development and prospect trend of HCCI engine

Nowadays, with the rapid development of information technology, the level of science and technology is constantly improving. All fields are developing rapidly. In recent years, the development of the automotive engine industry has been particularly rapid. To cope with increasingly strict environmental requirements and the impact of electric motors on the market, a new engine model is gradually being valued. Homogeneous Charge Compression Ignition (HCCI) internal combustion engine is a new type of engine. It has the technical advantages of traditional diesel and gasoline engines. This combustion mode greatly improves the emission of pollutants and the engine’s overall efficiency. However, there are still certain challenges to this technology at present. But with the continuous improvement and optimization of related technologies. HCCI engines are bound to have broad development prospects. This article elaborates on the current status and development trends of HCCI engines from four perspectives: basic principles, combustion characteristics, emission characteristics, and the latest developments.


Introduction
The degradation of the environment is getting worse as oil supplies become exhausted more quickly.Traditional internal combustion engines cannot meet the needs of the public and do not conform to the concept of environmental protection.Nowadays, it is urgent to find a more reasonable combustion mode.In this regard, the focus of combustion technology research in internal combustion engines is gradually developing toward improving fuel utilization and reducing harmful pollutant emissions.Traditional internal combustion engines are divided into two forms: spark ignition (SI), represented by gasoline engines and compression ignition (CI), represented by diesel engines.However, whether it is sparking or compression ignition, combustion efficiency and pollutants are inevitable issues.Homogeneous Charge Compression Ignition (HCCI) engine can solve these two problems well.The HCCI model was proposed at the end of the 1970s, which combines the characteristics of spark ignition and compression ignition combustion and has two major features: multi-point simultaneous combustion and low-temperature combustion [1].
To some extent, it has broken through the combustion efficiency of traditional internal combustion engines and greatly reduced particulate matter (PM) and nitrogen oxides (NOX) emissions.The mechanism roughly combines diluted and premixed fuel and air mixtures with multiple ignition points throughout the entire combustion chamber, eliminating high combustion temperature zones and preventing the generation of smoke and dust particles, resulting in ultra-low particulate matter (PM) and nitrogen oxides (NOX) emissions [2].The working principle of the HCCI engine can be understood as using the piston to compress and work on the combustible mixture in the cylinder, raising the temperature to the ignition point of the combustible mixture, and enabling it to burn evenly in the combustion chamber.Compared to traditional gasoline engines, HCCI engines do not require an ignition system.It is possible to use a much higher compression ratio than traditional gasoline engines, allowing it to vary over a wide range.Compared with diesel engines, HCCI engines can solve the problem of simultaneous mixing, diffusion, and combustion of the combustible mixture of diesel and air after being ignited by uniformly mixing the combustible mixture in advance, achieving the effect of reducing pollutant emissions.

Technical characteristics of HCCI engines
In HCCI engines, the mixture's temperature, pressure, and composition can be changed through piston movement.The working process of the HCCI engine is shown in Figure 1.When the piston approaches the top dead center of compression, the mixture in the cylinder is simultaneously compressed and ignited at several points, achieving the effect of homogeneous charge compression ignition.Due to the HCCI engine being ignited through compression ignition.It is not a direct combustion triggering measure.This indirectly leads to difficulty in controlling HCCI engines.Therefore, HCCI internal combustion engines can adopt a dual-mode combustion system.Switch to spark plug ignition mode or compression ignition mode when homogeneous charge compression ignition cannot operate normally (usually in high operating conditions).When the operating conditions meet the homogeneous charge compression ignition mode (usually idle, low to medium operating conditions), it automatically switches to achieve high thermal efficiency [3].This solves the control problem of HCCI engines and meets the requirements of improving economic performance and reducing emissions.

Homogeneous low-temperature combustion process.
The combustion characteristics of the HCCI engine are homogeneous, compression ignition and low-temperature flame combustion.Homogeneity means that the combustible mixture filled into the cylinder is mixed evenly in advance, and there will be no locally rich areas or locally lean areas inside the cylinder.This can effectively avoid the problem of oxygen deficiency when the engine is running at high temperatures.This reduces the generation of carbon smoke and the accumulation of carbon in the cylinder.A low-temperature chemical reaction will rapidly increase the mixture's temperature, reaching the temperature required for ignition [4].Because high-temperature and oxygen-rich circumstances are efficiently avoided by low-temperature flame combustion, NOx production is significantly reduced.

Fuel economy.
The working cycle of HCCI engine combustion mode is usually close to the ideal constant volume heating cycle, and higher compression ratios can be used.A higher compression ratio can increase the temperature difference between the cycle's high and low heat sources, while a lower combustion temperature can reduce heat transfer losses.Both of these are beneficial for improving thermal efficiency.However, the mixture concentration used in HCCI engine combustion is relatively low, making it suitable for operation under medium to low operating conditions, where the fuel consumption rate is low.Under high operating conditions, the combustion mode of HCCI engines will greatly increase fuel supply, resulting in a sharp increase in fuel consumption [5].As mentioned earlier, adopting a dual-mode combustion system also improves fuel economy.

High efficiency.
The HCCI engine employs lean mixture combustion, enabling it to immediately modify the fuel injection rate following degradation regulation to adjust torque.This method reduces the pump air loss caused by throttle intake and improves the engine's cycle efficiency.

Combustion characteristics of HCCI engines
HCCI combustion mode combines the advantages of spark ignition and pressure ignition combustion modes.The combustion mode of traditional internal combustion engines can be influenced by the composition of the mixture and the ignition method.These two aspects can be summarized into four conditions: homogeneous mixture, heterogeneous mixture, spark ignition, and compression ignition.Combine these four conditions in pairs to obtain four possible combustion modes for internal combustion engines: traditional gasoline engines, traditional diesel engines, direct injection gasoline engines, and HCCI engines.HCCI combustion is a simultaneous compression ignition process with multiple points and large areas, completing heat release quickly.HCCI engines use a thin and homogeneous mixture with a high EGR rate, which can reduce the maximum combustion temperature of gasoline engines and eliminate the conditions for hot NO generation [6].

Gasoline HCCI engine
In gasoline engines, due to the high volatility of gasoline fuel, it is very easy to form a homogeneous mixture [7].The research focuses on combustion rate control and combustion mode switching under high operating conditions.Enable the engine to achieve the most suitable mode under current conditions through the sensing and electronic control systems.The collaborative work of flexible realtime control of fuel injection and valve throttle, optimizing the stability during work mode switching, is a common feature of current HCCI combustion application technology in gasoline engines.

Diesel HCCI engine
Because diesel has a low ignition point and is difficult to evaporate, heating procedures must be implemented when utilized as a fuel in HCCI internal combustion engines.Furthermore, the compression ratio is poor, resulting in low thermal efficiency and excessive pollutant emissions.Injecting diesel fuel under delayed injection and high swirl circumstances also aids fuel evaporation and mixing [8].

Other new models and composite fuels HCCI engine
HCCI engines have good fuel flexibility.The octane number of fuel used in HCCI engines allows for a wide range of variations.In addition to gasoline and diesel, fuels such as methanol, ethanol, natural gas, dimethyl ether, and hydrogen have been experimentally confirmed as usable fuels for HCCI engines [9].Pure hydrogen fuel provides inherent benefits over conventional fuels for HCCI engines that run on hydrogen.Since hydrogen does not contain carbon compounds, burning won't produce carbon-containing pollutants like PM, HC, CO, etc. [10].

Emission characteristics of HCCI engines
Take the hydrogen-fueled HCCI engine as an example.Nitrogen oxides are the main harmful emission of hydrogen-fueled internal combustion engines.The nitrogen oxides emitted by hydrogen-fueled internal combustion engines are mainly NO and N2O.The hydrogen fuel internal combustion engine used for modeling is an inline 6-cylinder four-stroke diesel engine with a displacement of 1.583 L per cylinder.The main parameters of the internal combustion engine are shown in Table 1.
Table 1.Main parameters of hydrogen fuel internal combustion engines.single-zone and multi-zone combustion models for hydrogen-fueled HCCI engines.The single-zone combustion model is similar to the ideal homogeneous charge compression ignition, while the multi-zone combustion model is closer to the actual combustion mode.Set two options, as shown in Table 2 [11].5 500 608 Options 1 and 2 simulate the single-region and multi-region models.According to SALVADOR M. ACEVES's findings, the non-uniformity of the equivalence ratio has little influence on the simulation outcomes [12].Therefore, the equivalence ratio of both schemes is taken as 0.6, assuming that the combustible mixture gas is uniformly distributed.The reduction in nitrogen oxide emissions is related to the temperature after combustion.The gas temperature in the cylinder accurately affects the formation of nitrogen oxides.Due to the increase in temperature, the formation rate of nitrogen oxides increases exponentially.The emission of nitrogen oxides significantly increases at temperatures above 1800 K [13].Based on the previous experiment, this internal combustion engine ignites when the initial temperature is 365 K, the rotating speed is 1800 r/min, a standard atmospheric pressure, and the equivalence ratio is 0.6.Based on the above conditions, N2O and NO emission values were calculated using a simulation model, as presented in Figures 2 and 3    Figure 2 shows the effects of single-zone and multi-zone models on N2O emissions.Figure 3 shows the effects of single-zone and multi-zone models on NO emissions.Nitrogen oxide levels are extremely low in the HCCI combustion phase.This is mostly due to burning a thin and homogenous mixture, which results in a low peak temperature in the combustion chamber.

Conclusion
This article combines the principles and performance advantages of HCCI engines.The HCCI combustion mode has great potential in fuel economy and emission characteristics.To deal with a series of environmental problems, such as global warming and resource depletion of oil, HCCI technology will undoubtedly be an important direction for developing internal combustion engines.As a high-efficiency and low-pollution engine, the HCCI engine provides new ideas and options for pollution and combustion efficiency issues.HCCI engines have obvious technical advantages.And it combines the technical advantages of traditional gasoline and diesel engines.Compared with traditional gasoline and direct injection diesel engines, HCCI engines operate smoothly and excellently.HCCI engines are less dependent on rare precious metals, and the cost of emission control systems may also be reduced.
However, there is still room for improvement in the HCCI engine.Research has pointed out that to commercialize HCCI technology, some challenging issues need to be addressed.For example, injection timing, maximum levels of Noise, UHC and CO emissions, cold start capability, operation range, homogeneous mixture preparation, and intake charge temperature.Nowadays is an era of continuous progress in science and technology.As long as the above issues are resolved, HCCI engine technology will continue improving due to its strong technical support and application value.HCCI engines will gradually become the mainstream of the internal combustion engine industry, with promising prospects.

Figure 2 .
Figure 2. Emission of N2O from the single zone and multi zone.

Figure 3 .
Figure 3. Emission of NO from the single zone and multi zone.Figure2shows the effects of single-zone and multi-zone models on N2O emissions.Figure3shows the effects of single-zone and multi-zone models on NO emissions.Nitrogen oxide levels are extremely low in the HCCI combustion phase.This is mostly due to burning a thin and homogenous mixture, which results in a low peak temperature in the combustion chamber.

Table 2 .
The setting of initial conditions.