Diesel Homogeneous Charge Compression Ignition Research Articles

Synthesized evaluation of various injection regimens on hydrogen propelled homogeneous charge compression ignition and dual fuel modes for an automotive application

The present work aimed to perform a comparative study between the hydrogen diesel homogeneous charge compression ignition (HDHCCI) and hydrogen diesel dual fuel (HDDF) modes with multiple injection regimens. The results showed that the maximum feasible hydrogen energy shares (HES) were 73.99% and 27.46% for the HDDF and HDHCCI modes in a single pulse injection. The level of efficiency increased while increasing HES in the HDHCCI mode for any injection regimen. Conversely, the level of efficiency decreased on increasing HES for the HDDF mode, however it yields higher efficiency than the HDHCCI mode at a steady HES operation. The extremely low NO emission was attained using single pulse and twin pulse HDHCCI modes as compared with HDDF mode, and vice versa for smoke and HC emissions, since the existence of cool flame due to too early injection timings drastically altered the combustion characteristics of HDHCCI mode.

Experimental optimization of homogeneous charge compression ignition through fuel modifications and a relative comparison with reactivity controlled compression ignition

Reactivity controlled compression ignition (RCCI) and homogeneous charge compression ignition (HCCI) are suitable alternatives to improve the performance and emission metrics of conventional diesel combustion. Previous studies have shown that the gasoline/diesel RCCI outperforms diesel HCCI as it has a wider load range, higher brake thermal efficiency, and lower smoke and oxides of nitrogen emissions. The present work addressed the shortcomings of diesel HCCI by replacing diesel with low reactivity, high volatile methanol–gasoline blends. 2-ethylhexyl nitrate, an ignition quality improver, was added in small amounts (6 vol.%) to the blends to avoid misfiring, particularly at lower loads. There were two approaches to improve engine performance metrics of methanol–gasoline HCCI: exhaust gas recirculation and fuel composition variation. The best approach was chosen, and HCCI performed using the selected approach was compared with baseline RCCI. The methanol-gasoline HCCI was compared to baseline-optimized gasoline/diesel RCCI to demonstrate that the challenges associated with diesel HCCI can be overcome through fuel modifications. It was revealed that the strategy of varying methanol concentration in fuel blends was more effective than the strategy of varying EGR rate in terms of efficiency improvement and reduction of soot, carbon monoxide (CO) and unburned hydrocarbon (UHC) emissions. The parametric investigation showed that HCCI outperformed RCCI at mid to higher loads. For instance: at 60% load (3.19 BMEP), indicated thermal efficiency decreased by 7%, specific CO, UHC, nitrogen oxides, and smoke emissions elevated by 1.3, 1.2, 2.5, and 20.5 times for baseline-optimized RCCI than 40% methanol/54% gasoline/6% 2-EHN HCCI. Also, increased methanol in fuel during HCCI improved indicated thermal efficiency at a particular load. For instance, at 60% load, the indicated thermal efficiency improved by 30% during HCCI of fuel containing 40% methanol than 20% methanol. Overall, the present work discusses a practical method to efficiently use renewable methanol fuel in HCCI engines.

Experimental study of homogeneous charge compression ignition combustion in a light-duty diesel engine fueled with isopropanol–gasoline blends

Homogeneous charge compression ignition (HCCI) is a high-efficiency, ultra-low emission generating combustion technique that can potentially phase out conventional diesel combustion (CDC). Due to wide misfire and knocking limits, a limited load range impedes its commercial success. The present work addresses HCCI limitations using isopropanol–gasoline blends in a port fuel injected light-duty diesel engine. A cetane booster additive, 2-ethylhexyl nitrate (2-EHN), at 5vol.% was mixed with the test fuels to broaden the HCCI lower load limits. The tested fuels include biofuel isopropanol that replaced non-renewable gasoline in 20% increments to achieve higher loads during HCCI. The results show a substantial increment in HCCI operating load range (24%–86%) with test fuels over diesel HCCI (20%–38%). The characteristics of HCCI combustion in the modified test engine were benchmarked with CDC in the unmodified production engine. The significant benefits of HCCI over CDC were the concomitant reduction of indicated specific energy consumption (ISEC), oxides of nitrogen (NOx), and soot emissions, especially at higher loads. For instance, at 80% load, the ISEC, soot, and NOx were decreased by 5%, 99%, and 77%, respectively, for 60% isopropanol/35% gasoline/5% 2-EHN fueled HCCI than CDC. The exergy analysis was conducted to gain insight into irreversibility sources in thermodynamic processes during HCCI and CDC. The second law efficiency of HCCI improved with increased isopropanol content in the test fuels at a specific load. Overall, the present work shows that isopropanol–gasoline blends are viable alternatives to diesel to operate the HCCI engine that meets the desired emission and performance targets.

Investigations on load range extension of a homogeneous charge compression ignited light-duty diesel engine operated with diisopropyl ether and gasoline blends

A major obstacle for commercial implementation of homogeneous charge compression ignition (HCCI) combustion mode is a narrow operating load range. Several thermal and fuel management strategies have been investigated so far to address the shortcomings of HCCI, among which fuel management strategies are generally more effective. The present study intends to utilize high volatile, optimum reactivity fuel to extend the operating load range of a light-duty diesel engine operated in HCCI mode. Experiments are conducted at the rated engine speed, and varying load conditions. The tested fuels include blends of gasoline and diisopropyl ether and the additive 2-ethyl hexyl nitrate (2-EHN). Diisopropyl ether is a low reactivity, and high volatile renewable fuel that has the potential to replace crude oil derived gasoline. In the fuel blends, the proportion of diisopropyl ether is increased from 10% to 60% by volume with a corresponding decrease in gasoline proportion. The results obtained are compared with diesel HCCI considered as the reference. The engine can operate only up to 38% of rated load (2.02 bar brake mean effective pressure) in diesel HCCI mode, which is increased up to 58% of rated load (3.08 bar brake mean effective pressure) with diisopropyl ether-gasoline-2-EHN fuel blends. An improvement in indicated thermal efficiency is observed upon increasing the diisopropyl ether content in the tested fuel blends. The oxides of nitrogen and smoke emissions are low in HCCI with all the fuel blends, while the unburned hydrocarbon and carbon monoxide emissions increase compared to diesel HCCI. Overall, the present research shows that diisopropyl ether is a potential fuel to achieve desired performance and emission targets in an HCCI engine.

Parametric investigation on hydrogen driven homogeneous charge compression ignition mode with advanced diesel injection using chemical kinetics based numerical model

In the present study, a numerical model based on chemical kinetics was developed to investigate the effect of different parameters on characteristics of hydrogen diesel homogeneous charge compression ignition (HDHCCI) mode. The results showed that the combustion process was retarded with diminished cool flame as increasing hydrogen energy share (HES) due to reduction of H2O2, H2O and OH radicals with hydrogen addition. Thus, it increased the indicated thermal efficiency (ITE) and decreased the indicated specific energy consumption (ISEC). The levels of NOx and soot emissions were reduced by 22% and 63% at maximum HES. A spray angle of 10° decreased the total fuel impingement compared with a spray angle of 74° because of improvement in air-fuel mixing phenomena. Incomplete combustion was recovered at HES of 25% when increasing the charge temperature or adapting the twin-pulse in HDHCCI mode, consequently the ITE was improved with a decrease in ISEC.

Comparison of the hydrogen powered homogeneous charge compression ignition mode with multiple injection schedules and the dual fuel mode using a twin-cylinder engine

The present work discusses the influence of injection schedules in hydrogen diesel homogeneous charge compression ignition (HDHCCI) mode, the experiments were carried out in three distinct diesel injection schedules like single pulse, double pulse and multi-pulse at low load operation. The maximum reachable hydrogen energy shares (HES) are 27.46%, 26.97% and 39.96% for a single pulse, double pulse and multi-pulse respectively. For almost all HES levels, the single pulse and double pulse HDHCCI mode provided ultra-low NO emission when compared with hydrogen diesel dual fuel (HDDF) mode. The multi-pulse injection increased the brake thermal efficiency (BTE) compared with single pulse and double pulse injection, since the timing of injection pulse 3 (IP3) in multi-pulse significantly altered the combustion characteristics of HDHCCI mode. Locating the IP3 in the window of 30–40 °BTDC (before cool flame) enhanced the BTE and reduced the brake specific energy consumption (BSEC) of the engine.

Performance studies on homogeneous charge compression ignition (HCCI) engine powered with alternative fuels

Abstract Experiments were carried out on homogeneous charge compression ignition (HCCI) engine to study its performance at different hydrogen fuel energy ratio (HFER) and loads. The engine was powered with two fuels: Hydrogen (H2) as inducted fuel along with air and diesel/Biodiesel fuel (BDF) as injected fuel. Diesel and BDF were injected into the cylinder using common rail direct injection (CRDI) facility developed. Electronic control unit (ECU) was developed to inject the fuel at any required crank angle (CA) for any magnitude of duration. At 80% load, engine operation was associated with severe knocking without exhaust gas recirculation (EGR), however engine operated well only for EGR percentage about 50% and 54% with diesel and BDFs respectively. Beyond this EGR level, HCCI engine showed misfiring and unstable combustion. At this condition, maximum HFER possible was less than 8% in both diesel and biodiesel HCCI operations and misfiring occurred beyond HFER of 8%. The maximum brake thermal efficiency (BTE) of 31.5%, 30.6% and 30.2% were reported with diesel, honge biodiesel (BHO) and cotton seed biodiesel (BCO) respectively in diesel hydrogen HCCI (DHHCCI) and biodiesel hydrogen HCCI (BHHCCI) modes. At 80% load, HCCI engine powered with BDFs showed 65–67% lower smoke, and 98–99% lower NOx emissions with HFER of 7% and EGR of 54% as compared to conventional compression ignition (CI) mode. However this engine operation resulted in nearly 11 times higher HC emissions with HFER of 7% and EGR of 54%. Hence HCCI mode of engine operation could be a feasible technological option to minimize the pollution levels besides improving the fuel economy.

Comparative analysis of performance of dual fuel (DF) and homogeneous charge compression ignition (HCCI) engines fuelled with honne oil methyl ester (HOME) and compressed natural gas (CNG)

Experiments were conducted at different compressed natural gas energy ratio (CNGER), brake mean effective pressure (BMEP) and charge temperatures to perform the comparative analysis of homogeneous charge compression ignition (HCCI) and dual fuel (DF) engines. Compressed natural gas (CNG) was inducted along with air and diesel/honne oil methyl ester (HOME) was injected into the intake air to control the start of combustion. The engine could not run at BMEP below 2.5bar in the HCCI mode as the intake charge temperature could not be raised beyond 145°C due to system limitations and beyond a BMEP of 4bar, combustion rate became very high and very careful control over the charge temperature and CNG flow rate was needed. In DF mode the liquid fuel was injected into the cylinder and CNG was inducted along the intake air. Brake thermal efficiency (BTE) of compressed natural gas diesel homogeneous charge compression ignition (CNGDHCCI) and compressed natural gas biodiesel homogeneous charge compression ignition (CNGBHCCI) as well as the compressed natural gas diesel dual fuel (CNGDDF) and compressed natural gas biodiesel dual fuel (CNGBDF) modes are lower than that of neat diesel powered compression ignition (CI) mode of engine operation. However, the BTE of CNGDHCCI and CNGBHCCI is always better than that of CNGDDF and CNGBDF modes. Nitrogen oxide (NOx) and smoke emission drastically reduced in HCCI mode of engine operation but carbon monoxide (CO) and hydrocarbon (HC) emissions are higher.

Fuel adjustment to achieve a smooth net indicated mean effective pressure during mode switching from homogeneous charge compression ignition to conventional diesel compression ignition in a transient hydrocarbon emissions study

For diesel homogeneous charge compression ignition engines, combustion is unstable during the mode-switching process because of an unfavourable evaporation environment, especially at low speeds. Unusual fluctuations in the net indicated mean effective pressure and a spike in the transient emissions occur in the transitional cycles between modes. In this study, first, experiments were performed to characterize the hydrocarbon emissions during the combustion mode-switching process between homogeneous charge compression ignition and conventional diesel compression ignition. The results showed that the hydrocarbon emissions decreased as the speed increased at a constant load. When the load increased at a constant speed, the hydrocarbon emissions decreased. A spike in the hydrocarbon emissions during the transitional cycles was observed at 1000 r/min when mode switching from homogeneous charge compression ignition to compression ignition. Second, based on the spike phenomenon, a method of fuel adjustment was proposed to eliminate the fluctuations in the net indicated mean effective pressure and the spike in the hydrocarbon emissions in the five transitional cycles. Finally, by regulating the fuel adjustment coefficient in the five transitional cycles, the spike phenomenon of the transient hydrocarbon emissions was eliminated, and the fluctuations in the net indicated mean effective pressure decreased. The coefficient of variation in the net indicated mean effective pressure decreased to 2%.

Diesoline, Diesohol, and Diesosene Fuelled HCCI Engine Development

Compression ignition (CI) engines are facing strong restrictive emission norms globally, which demand extremely low oxides of nitrogen (NOx) and particulate matter (PM) emissions. Homogeneous charge compression ignition (HCCI) engine is a very attractive solution to meet these stringent emission challenges due to its capability to simultaneously reduce NOx and PM. In this study, HCCI combustion was investigated using different test fuels such as diesoline (15% v/v gasoline with diesel), diesohol (15% v/v ethanol with diesel), and diesosene (15% v/v kerosene with diesel) vis-a-vis baseline mineral diesel. A dedicated fuel vaporizer was used for homogeneous fuel–air mixture preparation. The experiments were performed at constant intake charge temperature (180 °C), fixed exhaust gas recirculation (EGR) (15%) at different engine loads. Stable combustion characteristics were determined for diesosene at lower engine loads, however, diesoline and diesohol yielded improved emissions compared to baseline diesel HCCI combustion. At higher loads, diesoline and diesosene showed higher knocking tendency compared to baseline diesel and diesohol. Diesohol showed lower NOx and smoke opacity, however, diesoline and diesosene showed slightly lower hydrocarbon (HC) and carbon monoxide (CO) emissions compared to baseline diesel HCCI combustion. Performance results of diesohol and diesosene were slightly inferior compared to diesel and diesoline HCCI combustion. Physical characterization of exhaust particulates was done for these test fuels using engine exhaust particle sizer (EEPS). Particle number-size distribution showed that most particles emitted from diesoline and diesohol were in ultrafine size range and baseline diesel and diesosene emitted relatively larger particles. Reduction in total particle number concentration with addition of volatile fuel components in mineral diesel was another important observation of this study.

Influence of multiple fuel injection strategies on performance and combustion characteristics of a diesel fuelled HCCI engine – An experimental investigation

Homogeneous charge compression ignition (HCCI) engines have the potential to operate with low nitric oxide (NOx) and soot emissions. Diesel HCCI engines pose challenges related to too early combustion, high combustion rates and wall wetting of the fuel. Injecting diesel in pulses has been shown to improve combustion in these engines. In this work, diesel injected in five timed pulses has been investigated in a compression ignition (CI) engine operating in HCCI mode. The influence of varying the first, middle and last injection pulse durations alone, in sequence, while maintaining all the other injection pulse durations equal has been studied. In addition, the influence of the injection timing of the last injection pulse was also studied. A comparison of the results has been made with HCCI operation using diesel injection in a single pulse at an injection timing of 100°CA before top dead centre (TDC) and also with the conventional compression ignition mode of operation under similar outputs. The results show that, multiple pulse (MP) injection is better than injecting the fuel in a single pulse (SP) as it leads to lower emissions and higher thermal efficiency. This is because of better combustion phasing and higher heat release rates.

Investigation of butanol-fuelled HCCI combustion on a high efficiency diesel engine

Highly-diluted diesel homogeneous charge compression ignition (HCCI) combustion can achieve ultra-low NOx and soot emissions but its implementation is impeded by the lack of control on the ignition timing and excessively early combustion phasing (before TDC) that limit the achievable engine load and result in a reduced energy efficiency. The low volatility and high reactivity of common diesel fuels make it non-conducive for HCCI combustion; hence in this work, n-butanol that has a low reactivity and high volatility is studied for HCCI combustion on a single-cylinder high compression ratio (18.2:1) diesel engine without any modifications to the air-path system. The results indicate that n-butanol-fuelled HCCI combustion offers the benefit of ultra-low NOx and smoke emissions with minimal requirements for intake dilution through exhaust gas recirculation (EGR). The low reactivity helps in realizing an optimal combustion phasing, and thermal efficiency levels comparable to that of conventional diesel combustion are consistently achieved. At low-to-mid engine loads (4–7bar IMEP), the emissions are largely insensitive to the boost pressure, and the boost selection is primarily governed by the trade-off between the combustion instability and the thermal efficiency. At higher engine loads, both boost and EGR are required to limit the high pressure rise rates and to modulate the combustion phasing for high thermal efficiency. The load range is extended up to 10bar IMEP in n-butanol HCCI mode while maintaining ultra-low NOx and soot emissions with improved performance characteristics compared to diesel HCCI.

Computational research on load extension route of diesel homogeneous charge compression ignition engine

A quasi-dimension combustion that considers fuel film evaporation is built to simulate rate of heat release of diesel HCCI combustion. The calculation results are in good qualitative and quantitative agreement with the experiment results. Load extension route of HCCI engine is studied based on the validated simulation model. In the process of load extension, the maximum combustion temperature and pressure rise ratio are the single limitation condition in low and high intake pressure respectively. The transformation boundary of single limitation condition is at intake pressure about 1.4 bar. The matching relation between intake pressure and excessive air ratio indicates that excessive air ratio can be taken as feedback signal to control load extension route. Along the ultimate load extension route, load of HCCI engine (IMEP) can be extended to 0.889 MPa by single supercharging mean and be further extended from 0.889 MPa to 1.029 MPa by combined supercharging and LIVC means.

Computational and statistical analysis of diesel-homogenous charge compression ignition combustion

This paper explores the auto-ignition chemistry of n-heptane and toluene mixture to represent diesel-homogenous charge compression ignition (HCCI) combustion by means of a reduced chemical kinetic mechanism using CHEMKIN-PRO. In particular, pressure and temperature histories of n-heptane–toluene mixture are demonstrated at the stoichiometric condition and at the lean operating condition with 0%, 10%, and 20% exhaust gas recirculation (EGR) rates, respectively. Since pressure and temperature profiles play a vital role in a reaction path at a certain operating condition, an endeavour has been made here to present a comprehensive reaction path analysis on the formation/destruction of chemical species at a peak operating condition. Also, validation of the proposed model was done by comparing the results with the experimental analysis carried out by prior researchers. Besides, multiple linear regression analysis was performed to compute the pressure and temperature histories of various lean operating conditions (Φ = 0.5–0.9) with and without EGR through the fresh intake stream.

Fuel injection and EGR control strategy on smooth switching of CI/HCCI mode in a diesel engine

Homogeneous charge compression ignition(HCCI) combustion has been demonstrated abilities to lower engine emissions due to its nature of even mixture and low temperature combustion. This combustion mode is only applicable in part load and low speed area so far. This range limitation results in frequently combustion mode switching from CI mode to HCCI mode(CI–HCCI) and HCCI mode to CI mode(HCCI–CI) in real applications. Large variation of IMEP and combustion phase occurs during mode switching process. This is unfavorable to the performance of engine. In the research, diesel HCCI realized by early injection with negative valve overlap(NVO) is investigated.Fuel injection control strategy is studied to improve the smoothness of IMEP during mode switching firstly. Based on the concluded fuel injection control strategy, EGR strategy is studied to improve the smoothness of CA50 during mode switching. In the analysis, fuel compensation coefficient(η) and dimensionless cycle number(σ) are defined. Results of this study show fuel compensation of transition cycle is necessary in CI–HCCI mode switching, but direct mode switching is adequate in HCCI–CI mode switching. The optimum fuel compensation coefficient for CI–HCCI mode switching is at about 1.06, and the COVIMEP is about 5.6. Too late and early switching of EGR are both unfavorable to the smoothness of CA50 during mode switching. The optimum dimensionless cycle number of CI–HCCI and HCCI–CI mode switching are 4.8 and 0.6 respectively.

Experimental investigations of combustion, performance and emission characterization of biodiesel fuelled HCCI engine using external mixture formation technique

In the present research, effect of biodiesel content on homogeneous charge compression ignition (HCCI) engine combustion, performance and emission characteristics has been investigated experimentally. Experiments were performed in a modified two cylinder engine, in which, one cylinder was operated in HCCI mode while other was operated in conventional CI mode. HCCI engine can be operated with a wide variety of fuels starting from mineral diesel to various blends of biodiesel (B20 and B40). The basic requirement of the HCCI engines is homogeneous mixing of fuel and air, which is done by using port fuel injection strategy in this study. An external device was used for fuel vaporization and mixture formation. For controlling HCCI combustion, different EGR conditions (0%, 15% and 30%) were also applied. In the experiments, combustion results were found to be more stable for biodiesel HCCI compared to diesel HCCI due to lower rate of heat release (RoHR) for biodiesel. A reduction in power output and an increase in ISFC were observed upon increasing the biodiesel content in the test fuel. A small increase in CO, HC and smoke emissions was observed with increasing biodiesel content due to slower evaporation rate of biodiesel. A significant reduction in NOx emissions was also observed with for biodiesel blends.

A review of hydrogen and natural gas addition in diesel HCCI engines

Homogeneous charge compression ignition (HCCI) engine uses a relatively new mode of combustion technology. In principle, there is no spark plug or injector to assist the combustion, and the combustion auto-ignites in multiple spots once the mixture has reached its chemical activation energy. The challenges in developing HCCI engines are the difficulties in: controlling the auto-ignition of the mixture and the heat release rate at high load operations, achieving a cold start, meeting emission standards and controlling knock. At low engine speeds, early auto-ignition can occur, possibly leading to knocking, while late auto-ignition at high engine speeds will make HCCI susceptible to misfire. Hydrogen greatly reduces emissions levels but with reduced power. However, when hydrogen is combined with diesel in dual-fuel mode, low NOx, CO and particulate matter (PM) emissions levels can be achieved, and engine efficiency can be increased by 13–16%. Numerical methods are commonly used to predict HCCI engines' performance (i.e. emissions levels, brake thermal efficiency and combustion phasing), which is cost-effective compared to solely relying on experimentation. The multi-zone method promises better simulation results compared to the single-zone model by combining detailed chemical kinetics with simplified 3D modeling so that turbulence and inhomogeneity in the mixture are considered; good agreement between simulations and experiments have been achieved. Specific strategies used in the experimental method (e.g. fuel additives, inlet air heating, inlet air pressurizing, exhaust gas recirculation (EGR) and injection methods), and numerical method (e.g. single-zone and multi-zone models, mixing model, turbulence model and multi-dimensional model), and other issues associated with HCCI engines are discussed in this paper.

English

Homogeneous charge compression ignition (HCCI) engine technology is relatively new and has not matured sufficiently to be commercialized compared with conventional engines. It can use spark ignition (SI) or compression ignition (CI) engine configurations, capitalizing on the advantages of both: high engine efficiency with low emissions levels. However, the combustion behavior in an HCCI engine is difficult to predict because it has no spark plug or injector. The chemical kinetics mechanism influences the combustion with some heat losses to the cylinder wall. The effect of different heat loss models in a diesel HCCI engine has to be investigated further. A single-zone thermodynamics model was used in this study along with three different heat loss models: Woshcni, modified Woschni, and Hohenberg correlations. The results will be validated against experiments, and a good heat loss model will be used for further investigation of combustion behavior.

Experimental study on compound HCCI (homogenous charge compression ignition) combustion fueled with gasoline and diesel blends

Because the energy crisis and environmental pollution are significant concern, a next-generation combustion mode for internal combustion engines that can simultaneously reduce exhaust emissions and substantially improve thermal efficiency has attracted increasing attention. In the last two decades, diesel-fueled HCCI (homogenous charge compression ignition) combustion has been widely researched. It has been determined that diesel HCCI combustion has the potential to improve NOx and soot emissions at low-to-medium loads; however, it suffers from higher HC (hydrocarbon)/CO (carbon monoxide) emissions, narrow operating ranges, and uncontrollable ignition timing and combustion rates due to low volatility and high ignitability. For this reason, 30%–50% (v/v, briefly G30, G40, and G50) gasoline/diesel fuel blends are used in a novel technology of a compound HCCI combustion mode, in which port fuel injection and in-cylinder direct injection are combined. The combustion and emission characteristics of the compound HCCI combustion using blend fuels are investigated on a single-cylinder engine. The effect of the gasoline volume in the blends, the premixed ratio, and the overall fuel supply rate on compound HCCI combustion are initially evaluated, and the effects of the intake air boost on G30 compound HCCI combustion is also investigated.The experimental results indicate that the maximum heat release rate, maximum in-cylinder pressure, and NOx emissions of G40 and G50 compound HCCI combustion significantly increase when compared to that of G30 compound HCCI combustion. Moreover, it is determined that the intake air boost has great potential to reduce the NOx and soot emissions of compound HCCI combustion simultaneously. CO and HC emissions of optimized G30 compound HCCI combustion with/without boost are relatively higher compared to that of traditional DICI (direct injection compression ignition) combustion. The NOx and soot emissions of optimized G30 compound HCCI combustion with intake air boost are far lower than those of the DICI diesel engine. Specifically, the NOx emissions can be maintained within 100 ppm, and the soot emissions are below 10% at the full load ranges.

Effects of fuel compensation in transitional cycles on the smoothness of combustion mode switching in a diesel engine

Because indicated mean effective pressure (IMEP) fluctuation often occurs in diesel homogeneous charge compression ignition (HCCI) engines during the conventional compression-ignition (CI) combustion mode and HCCI combustion mode switching process, in this paper, the effects of a fuel injection compensation strategy on the smoothness in the CI-HCCI combustion mode switching process at equal load were studied. The fuel compensation in one transitional cycle, two transitional cycles and three transitional cycles were discussed, and the return maps of the IMEP variation were analyzed. The results showed that the fuel increase of the first transitional cycle can enhance the combustion of this cycle and prevent the IMEP from decreasing. More transitional cycles with decreased fuel compensation ratio can improve the smoothness of the IMEP. However, the magnitude of the decreasing trend of the IMEP fluctuation ratio decreases and the fuel control strategy becomes complicated as the number of transitional cycles increases. The fuel injection compensation strategy for the diesel engine is optimized when two transitional cycles and fuel ratios of 1.07 for the first cycle and 1.03 for the second cycle are used, and the fluctuation of the IMEP decreased by 68% compared to the direct combustion mode switching process.