Influence of the equivalence ratio on the knock and performance of a hydrogen direct injection internal combustion engine under different compression ratios

Hydrogen has become an ideal alternative fuel for internal combustion engines. However, with an increase in the equivalence ratio and compression ratio, knock combustion is more likely to occur, which limits its engineering application. In this study, the effects of the equivalence ratio on the knock under different compression ratios were studied through numerical simulation. The signal energy ratio (SER) were used to evaluate the knock onset (KO). The knock intensity (KI) and engine performance were compared and analyzed under different equivalence ratios and compression ratios. The results revealed that a high compression ratio can significantly amplify the effect of the equivalence ratio on combustion and knock. Under the compression ratio of 17.5, KI increases more quickly, with the constant equivalence ratio rise and is more sensitive to ignition timing with equivalence ratio increasing. For the compression ratio of 11, the ignition timing limited by knock is about 4°CA earlier than that of compression ratio of 17.5, and the engine performance is more stable in the low-knock zone. However, when KI exceeds 1 MPa, the power and ITE decreases 20.6% and 20.9% respectively.     

Effect of supercharging on improving thermal efficiency and modifying combustion characteristics in lean-burn direct-injection near-zero-emission hydrogen engines

The authors have proposed a new combustion process called the Plume Ignition Combustion Concept (PCC), in which with an optimal combination of hydrogen injection timing and controlled jet geometry, the plume of the hydrogen jet is spark-ignited to accomplish combustion of a rich mixture. This combustion process markedly improves thermal efficiency by reducing cooling loss, which is essential for increasing thermal efficiency in a hydrogen engine while maintaining high power. In order to improve thermal efficiency and reduce NOx formation further, PCC was applied to a lean-burn regime to burn a leaner mixture globally. In this study, the effect of supercharging which was applied to recover the reduced output power due to the leaner mixture on improving thermal efficiency was confirmed along with clarifying the cause.     

Numerical investigation on combustion regulation for a stoichiometric heavy-duty natural gas engine with hydrogen addition considering knock limitation

Aiming to further improve the thermal efficiency and reduce NO_x emissions in the stoichiometric hydrogen-enriched natural gas (NG) engine, a detailed 3-D simulation model of stoichiometric operation heavy-duty NG engine is built based on the actual boundary conditions from high load bench test. The superimposed methods for knock regulation, combustion and emission control, including Miller valve timing, hydrogen volume fraction and EGR rate were proposed and investigated comprehensively. It reveals that the typically bimodal characteristic of heat release rate (HRR) curve is caused by knock, which seriously restricts the performance improvement of stoichiometric NG engine under high load condition. To predict and control the occurrence of the second peak of HHR accurately, a new parameter BI is defined. Moreover, the Miller timing with 20°CA of the intake valve late closing shows better combustion performance within the knock limit, accompanied by a slight increase in NO_x emissions. Additionally, the 5% hydrogen blend, coupled with the Miller cycle, can further enhance the indicated thermal efficiency (ITE) of the NG engine due to the stronger effects on acceleration of laminar flame propagation velocity than the promotion of end-gas auto-ignition. Besides, the great potential of EGR rate for balancing NO_x and ITE is also confirmed in the heavy-duty hydrogen-enriched NG engine adopting Miller cycle. Compared to the original indexes, combing with the regulation strategies of intake valve late closing (20°CA), hydrogen addition (5%) and EGR (17%) are proved to increase the indicated thermal efficiency by 1.89% and reduce NO_x emissions by 11.47% within the knock limit.     

Investigation of the gas injection rate shape on combustion,knock and emissions behavior of a rotary engine with hydrogen direct-injection enrichment

The application of hydrogen direct-injection enrichment improves the performance of gasoline Wankel rotary engine, and the hydrogen injection strategy has a significant impact on combustion, knock, and emissions. The Z160F Wankel rotary engine was used as the investigated compact engine, and the simulation model was developed using CONVERGE software. The combustion, knock and emissions characteristics of the engine were studied with the different mass flow of hydrogen injection, i.e., the trapezoid, wedge, slope, triangle and rectangle type of gas injection rate shape. In the numerical simulations, the in-cylinder pressure oscillations were monitored using monitoring points, and the knock index (KI) was used as an evaluation indicator. The study revealed that the gas injection rate shape significantly affected the mixture of hydrogen and air, thus impacting combustion, knock and emissions. When the injection rate shape was rectangle, the flame speed was faster, the peak pressure in the cylinder was higher, and the corresponding crank angle was earlier, which led to higher pressure oscillations in the cylinder and larger KI. Based on the rectangle injection rate shape, the KI decreased by 75.81%, 33.47%, 26.46% and 76.58% for trapezoid, wedge, slope, and triangle, respectively, and the indicated mean effective pressure increased by 15.68%, 5.07%, 0.56% and 14.98%, respectively. Due to the small difference in maximum temperature, which resulted in very little variation in nitrogen oxides for each injection rate shape, the total hydrocarbon emissions of the trapezoid and triangle injection rate shape was high due to the delayed combustion phase. This paper provides a solution for direct hydrogen injection to improve the combustion, knock and emissions behavior of the rotary engine.     

Comparisons of hydrogen and gasoline combustion knock in a spark ignition engine

Combustion knock is one of the primary constraints limiting the performance of spark-ignition hydrogen fuelled internal combustion engines (H2-ICE) as it limits the torque output and efficiency, particularly as the equivalence ratio nears stoichiometric operation. Understanding the characteristic of combustion knock in a H2-ICE will provide better techniques for its detection, prevention and control while enabling operation at conditions of improved efficiency.

Engine studies examining combustion knock characteristics were conducted with hydrogen and gasoline fuels in a port-injected, spark-ignited, single cylinder cooperative fuel research (CFR) engine. Characterization of the signals at varying levels of combustion knock from cylinder pressure and a block mounted piezoelectric accelerometer were conducted including frequency, signal intensity, and statistical attributes. Further, through the comparisons with gasoline combustion knock, it was found that knock detection techniques used for gasoline engines, can be applied to a H2-ICE with appropriate modifications. This work provides insight for further development in real time knock detection. This would help in improving reliability of hydrogen engines while allowing the engine to be operated closer to combustion knock limits to increase engine performance and reducing possibility of engine damage due to knock.     

Cycle variations in a hydrogen internal combustion engine

The cycle variation characteristics of a port fuel injection hydrogen internal combustion engine (PFI-HICE) have been extensively investigated. The covariance of indicated mean effective pressure (COV_imep) is the best parameter for evaluating the cycle variations in the PFI-HICE. COV_imep decreases as fuel–air ratio increases from 1000 to 5500 rpm, and engine speed minimally affects COV_imep. The effect of ignition advance angle on COV_imep is determined by fuel–air ratio. The ignition advance angles that correspond to the minimum COV_imep of the PFI-HICE decrease as fuel–air ratio increases. The effect of ignition advance angle on COV_imep diminishes as fuel–air ratio increases. The COV_imep of the PFI-HICE rapidly decreases as throttle increases when the throttle is less than 20%. Injection timing only slightly affects COV_imep under high-speed conditions, and COV_imep increases when hydrogen is injected in intake periods under low-speed conditions. These results indicate that studying COV_imep improves the stability of PFI-HICEs.     

Diagnosis and control of abnormal combustion of hydrogen internal combustion engine based on the hydrogen injection parameters

In order to improve the limitation of evaluating the abnormal combustion problem of hydrogen internal combustion engine by single index, the abnormal combustion risk coefficient is proposed and defined based on AHP(Analytic Hierarchy Process)-entropy method. The abnormal combustion risk of PFI hydrogen internal combustion engine is comprehensively evaluated from multiple indexes such as the uniformity coefficient of the mixture, the temperature of the hot area, the maximum temperature rise rate, the residual amount of hydrogen in the intake port and the cylinder temperature at the end of the exhaust. The influence of hydrogen injection parameters on abnormal combustion was explored. The results show that the temperature and the maximum temperature rise rate in the hot area decrease first and then increase with the increase of hydrogen injection angle and hydrogen injection flow rate. Although large hydrogen injection angle and hydrogen injection flow rate can reduce the cylinder temperature at the end of exhaust, they will increase the residual hydrogen amount in the intake port. Appropriate hydrogen injection angle and hydrogen injection flow scheme can ensure that all parameters are at a better level, so that the risk coefficient of abnormal combustion decreases by 2.1%–5.5%, and the possibility of abnormal combustion is reduced.     

Statistically discussing impacts of knock type on the heat release process in hydrogen-fueled Wankel rotary engine

Hydrogen-fueled internal combustion engine is proposed to resist the threat of global warming. Wankel rotary engine (WRE) has been proven to be an excellent hydrogen-fueled power device, which can overcome the shortcomings of hydrogen, such as poor power, serious backfire and large storage volume, to some extent when it is used as fuel in the internal combustion engine. However, due to its unique structure, WRE suffers from severe knock. Therefore, the goal of this work is to investigate the impacts of knock type on the heat release process in hydrogen-fueled WRE. This present work is conducted at 2000 r/min and wide-open throttle. The main results are as follows: In hydrogen-fueled WRE, the peak knock pressure of knock caused by rapid and unstable combustion of hydrogen is usually earlier than CA50 and that of knock caused by spontaneous combustion of end gas is usually later than CA50. The sequence between the crank angle of peak knock pressure and CA50 combined with knock intensity can be used to determine the knock type in hydrogen-fueled WRE. Besides, a means for knock detection is proposed according to the distribution of crank angle corresponding to peak knock pressure. In addition, the distribution of CA0-10, CA50 and CA10-90 of 1000 consecutive cycles under two kinds of knock in hydrogen-fueled WRE are discussed in detail, and regular conclusions are drawn. In particular, the limitation of CA50 as a metric for evaluating knock level is also demonstrated.     

Experimental investigation of combustion characteristics and NOx emission of a turbocharged hydrogen internal combustion engine

In order to alleviate the contradictions of increasingly prominent environmental pollution, greenhouse gas emissions and oil resource security issues, the search for renewable and clean alternative energy sources is getting more and more attention. Hydrogen energy is known as a future energy source because of its safety, reliability, wide range of resources and non-polluting products. Hydrogen internal combustion engine combines the technical advantages of traditional internal combustion engines and has comprehensive comparative advantages in terms of manufacturing cost, fuel adaptability and reliability. It is one of the practical ways to realize hydrogen energy utilization. In this paper, the combustion characteristics and NOx emission of a turbocharged hydrogen engine were investigated using the test data. The results showed the combustion duration (the crank angle of 10%–90% fuel burned) at 1500 rpm and 2000 rpm was equal and the combustion duration is much bigger than the other loads when the BMEP is 0.27 MPa. The reason is the effect of the turbocharger on the gas exchange process, which will influence the combustion process. The cylinder pressure and pressure rise rate were also investigated and the peak pressure rise rate was lower than 0.25 MPa/°CA at all working conditions. Moreover, the NOx emission changed from 300 ppm to 1200 ppm with engine speed increasing and the maximum value can reach to 7000 ppm when the equivalence ratio is 0.88 at 2500 rpm, maximum brake torque. The NOx emission shows different changing tendencies with different working conditions. Finally, these conclusions can be used to develop controlling strategies to solve the contradictions among power, brake thermal efficiency and NOx emission for the turbocharged hydrogen internal combustion engines.     

The distinctive characteristics of combustion duration in hydrogen internal combustion engine

As a practical solution to reduce the emission pollution and energy crisis, the research and development of HICE has been processed in several decades. The focus of this paper is trying to explore the new features of the combustion duration in HICE not only by engine experiment, but also by analysis of the physical properties of hydrogen, especially the obvious difference from that of gasoline. Firstly, the laminar flame speed difference between hydrogen and gasoline was studied and discussed. Secondly, a distinctive rule of combustion duration in HICE was discovered by analyzing the experiment data. Finally, as a key reference point to the HICE operation, a new characteristic of the location of 50% mixture combust up was proposed and analyzed, this will be helpful for the calibration of optimum ignition timing.     

Effects study of injection strategies on hydrogen-air formation and performance of hydrogen direct injection internal combustion engine

Based on the dual challenges of the global energy crisis and environmental pollution, hydrogen has been recognized as an ideal alternative internal combustion engine (ICE) fuel. To improve the combustion efficiency of hydrogen direct injection ICE, we numerically analyzed the effects of different injection parameters, including injection timing, injection pressure, and dual injection, on the formation of a hydrogen-air mixture using the CONVERGE software from the perspective of mass transfer and flow state. It was determined that it is enough to set the injection timing to −88° after top dead center (ATDC) for both uniform mixture and desirable indicated thermal efficiency (ITE). However, when the injection timing is set to −43° ATDC, an acceptable ITE and effective combustion can be achieved by employing the “jet-room coordination” effect of the ω chamber. Injection pressure has a minimal effect on mixture formation and combustion. In contrast, the timing and mass fraction of secondary injection have a significant influence on tumble strength, which is a key factor for the mixture improvement.     

Simulation of the combustion process for a CI hydrogen engine in an argon-oxygen atmosphere

Hydrogen combustion in a noble gas atmosphere increases the combustion chamber temperature, and the high specific heat ratio of the gas increases the thermal efficiency. In this study, nitrogen was replaced by argon as the intake air along with pure oxygen to supply the engine. The objectives of this study are to determine the effects of different engine parameters on combustion and to analyse the emissions from hydrogen combustion in an argon-oxygen atmosphere. This research was conducted through simulations using CONVERGE 2.2.0 software, and the YANMAR engine NF19SK model was used to determine the basic parameters. Changing the injector location affects the pressure and temperature in the combustion chamber. With increasing compression ratio, the pressure increases more rapidly than the temperature. However, combustion at high compression ratios decreases the maximum heat release rate and increases the combustion duration. Hydrogen combustion at ambient temperatures below 1200 K follows the Arrhenius equation.     

Effects of hydrogen addition on engine performance in a spark ignition engine with a high compression ratio under lean burn conditions

In this study, the effects of hydrogen addition on the engine performance were investigated using spark ignition engine fueled gasoline with a compression ratio of 15 at an air excess ratio (λ) of 1.8 and above. At λ = 1.8, the indicated thermal efficiency at the spark timing of the knock limit reached the maximum level under the conditions in which the hydrogen fraction was set to 4% of the heating value of the total fuel. Based on a heat balance analysis, the best hydrogen fraction was found as a balance between the improvement in the burning efficiency and the increase in heat loss. The lean limit was extended when the hydrogen fraction was increased from λ = 1.80 to λ = 2.28. The hydrogen addition achieved the maximum indicated thermal efficiency at spark timing of the knock limit was obtained at λ = 2.04, where the hydrogen fraction was 10%.     

Energy and exergy analysis of a turbocharged hydrogen internal combustion engine

This paper has analyzed the energy and exergy distribution of a 2.3 L turbocharged hydrogen engine by mapping characteristics experiment. The energy loss during fuel energy conversion mainly includes: exhaust energy (23.5–34.7%), cooling medium (coolant and oil) energy (21.3–34.8%), intercooler energy (0.5–3.6%) and uncounted energy (5.8–14.1%), while the proportion of effective work ranges from 25.7% to 35.1%. Results show that all kinds of energies increase with engine speeds and they are not sensitive to the loads. However, the proportions of different kind of energy exhibit different characteristics. Moreover, the turbocharger can increase the brake thermal efficiency and the maximum can be increased by 4.8%. Exergy analysis shows exergy efficiency of the coolant energy does not exceed 5%, while the exergy efficiency of the exhaust energy can reach up to 23%. And the total hydrogen fuel thermal efficiency limit is theoretically above 59%.     

Research on emission characteristics of hydrogen fuel internal combustion engine based on more detailed mechanism

A CFD simulation model with simplified chemical reaction mechanism was built based on CONVERGE software to study the in-cylinder combustion progress and NO generation mechanism of hydrogen fueled internal combustion engine (HICE). Simulation results show that the in-cylinder combustion progress experiences the ellipsoidal flame stable propagation stage and the rapid turbulent combustion stage. At the end of rapid turbulent combustion the OH concentration decreases quickly, the peak temperature and maximum NO mass appear at that time, and then the in-cylinder temperature and NO mass decrease step by step. The final emission depends on the peak temperature and NO decomposition time of high-temperature regions. The higher the maximum temperature, the greater the NO peak mass; and the faster the temperature drop, the less the NO decomposes. Adoption of EGR can reduce the in-cylinder maximum temperature, and NO decomposes sufficiently at low speed, which in turn leads to lower NO emission of HICE.     

Numerical study on knock characteristics and mechanism of a heavy duty natural gas/diesel RCCI engine

In this paper, the knock phenomenon of reactivity controlled compression ignition (RCCI) engine fueled with natural gas/diesel was numerically studied. The knock mechanism of the RCCI engine is explained and the strategy of suppressing knock is put forward. The knock characteristics were studied by setting monitoring points in different spaces positions of the cylinder. The results show that the pressure oscillation amplitude at the center and edge of the cylinder is large under the high load condition of RCCI engine. In addition, the knock mechanism was studied by using pressure difference method, maximum amplitude of pressure oscillation, important components, temperature isosurface, pressure fluctuation and heat release rate. The results show that the knock of RCCI engine is mainly caused by the end-gas auto-ignition. The pressure difference results show that the characteristic frequency is consistent with the natural resonance mode (0,1) of the cylindrical combustion chamber. On this basis, the effects of pilot oil injection timing and compression ratio on engine knock are further studied. It is confirmed that diesel knock and end-gas knock may exist simultaneously in the same cycle when RCCI engine knock occurs. And diesel knock occurs before top dead center, and end-gas knock occurs after top dead center. Proper adjustment of pilot oil injection timing and reduction of compression ratio can effectively suppress engine knock.     

Near-zero emissions with high thermal efficiency realized by optimizing jet plume location relative to combustion chamber wall,jet geometry and injection timing in a direct-injection hydrogen engine

Thermal efficiency was substantially improved and NOx emissions were reduced to a level at a single-digit ppm with PCC combustion by optimizing such characteristics as the direction, number and diameter of the injected jet and controlling the injection timing and also by combining with combustion of lean mixture. Output power declined by lean mixture was recovered by supercharging in keeping NOx emissions remained at the same level, while thermal efficiency was improved furthermore by slightly re-optimizing jet conditions. As a result, hydrogen engine which does not emit any CO2 and particulate matter in principle is worth to be called near-zero emission engines in both name and reality.