Hydrogen enrichment effects on the second law analysis of a lean burn natural gas engine

This paper presents a computational work aimed at investigating the effects of hydrogen addition on the exergy (or availability) balance in a lean burn natural gas spark ignition (SI) engine. A thermodynamic engine cycle simulation was extended to perform the exergy analysis. A zero dimensional, two-zone computational model of the engine operation was used for the closed part of the cycle. The results of the model were compared with experimental data to demonstrate the validation of the model. Exergetic terms, such as exergy transfer with heat, exergy transfer with work, irreversibilities, fuel chemical exergy, and total exergy, were computed based on principles of the second law. The exergetic (the second law) efficiency was also calculated. The results of exergy analysis show that increasing hydrogen content and lean burn have considerably affected the exergy transfers, irreversibilities and second law efficiency. With increasing hydrogen content, the irreversibility produced during combustion decreases, and the second-law efficiency sharply increases at near the lean limit.     

The lean burn direct injection jet ignition gas engine

This paper presents a new in-cylinder mixture preparation and ignition system for various fuels including hydrogen, methane and propane. The system comprises a centrally located direct injection (DI) injector and a jet ignition (JI) device for combustion of the main chamber (MC) mixture. The fuel is injected in the MC with a new generation, fast actuating, high pressure, high flow rate DI injector capable of injection shaping and multiple events. This injector produces a bulk, lean stratified mixture. The JI system uses a second DI injector to inject a small amount of fuel in a small pre-chamber (PC). In the spark ignition (SI) version, a spark plug then ignites a slightly rich mixture. In the auto ignition version, a DI injector injects a small amount of higher pressure fuel in the small PC having a hot glow plug (GP) surface, and the fuel auto ignites in the hot air or when in contact with the hot surface. Either way the MC mixture is then bulk ignited through multiple jets of hot reacting gases. Bulk ignition of the lean, jet controlled, stratified MC mixture resulting from coupling DI with JI makes it possible to burn MC mixtures with fuel to air equivalence ratios reducing almost to zero for a throttle-less control of load diesel-like and high efficiencies over almost the full range of loads.     

Experimental study on thermal efficiency and emission characteristics of a lean burn hydrogen enriched natural gas engine

In order to analyze the effect of hydrogen addition on natural gas (NG) engine's thermal efficiency and emission, an experimental research was conducted on a spark ignition NG engine using variable composition hydrogen/CNG mixtures (HCNG). The results showed that hydrogen enrichment could significantly extend the lean operation limit, improve the engine's lean burn ability, and decrease burn duration. However, nitrogen oxides (NO_x)$({\text{NO}}_x)$ were found to increase with hydrogen addition if spark timing was not optimized according to hydrogen's high burn speed. Also found when spark timing was set at constant was that hydrogen addition actually increases heat transfer out of the cylinder due to smaller quenching distance and higher combustion temperature, thus is not good to improve thermal efficiency if combined with the effect of non-ideal spark timing. But if spark timing was retarded to MBT, taking advantage of hydrogen's high burn speed, _NOx${\mathrm{NO}}_x$ emissions exhibited no obvious increase after hydrogen addition and engine thermal efficiency increased with the increase of hydrogen fraction. Unburned hydrocarbon always decreased with the increase of hydrogen fraction.     

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%.     

Study on combustion and emission characteristics of a n-butanol engine with hydrogen direct injection under lean burn conditions

The n-butanol fuel, as a renewable and clean biofuel, could ease the energy crisis and decrease the harmful emissions. As another clean and renewable energy, hydrogen properly offset the high HC emissions and the insufficient of dynamic property of pure n-butanol fuel in SI engines, because of the high diffusion coefficient, high adiabatic flame velocity and low heat value. Hydrogen direct injection not only avoids backfire and lower intake efficiency but also promotes to form in-cylinder stratified mixture, which is helpful to enhance combustion and reduce emissions. This experimental study focused on the combustion and emissions characteristics of a hydrogen direct injection stratified n-butanol engine. Three different hydrogen addition fractions (0%, 2.5%, 5%) were used under five different spark timing (10° ,15° ,20° ,25° ,30° CA BTDC). Engine speed and excess air ratio stabled at 1500 rpm and 1.2 respectively. The direct injection timing of the hydrogen was optimized to form a beter stratified mixture. The obtained results demonstrated that brake power and brake thermal efficiency are increased by addition hydrogen directly injected. The BSFC is decreased with the addition of hydrogen. The peak cylinder pressure and the instantaneous heat release rate raises with the increase of the hydrogen addition fraction. In addition, the HC and CO emissions drop while the NOx emissions sharply rise with the addition of hydrogen. As a whole, with hydrogen direct injection, the power and fuel economy performance of n-butanol engine are markedly improved, harmful emissions are partly decreased.     

Study on combustion behaviors and cycle-by-cycle variations in a turbocharged lean burn natural gas S.I. engine with hydrogen enrichment

Lean burn is widely accepted as an effective approach to simultaneously improve spark-ignition engine's thermal efficiency and decrease exhaust emissions. But although lean burn has a lot of advantages it is also associated with several difficulties including slower flame propagation speed and increased cycle-by-cycle variations. Hydrogen addition is thought to be an ideal approach to tackle these problems. This paper presents an experimental work aimed at investigating the effects of hydrogen addition on the combustion behaviors and cycle-by-cycle variations in a turbocharged lean burn natural gas SI engine. The experiments were conducted over a wide range of hydrogen enhancement levels, equivalence ratios, spark timings, manifold absolute pressures and engine speeds.     

A comparative study of lean burn and exhaust gas recirculation in an HCNG-fueled heavy-duty engine

Two dilution strategies, exhaust gas recirculation (EGR) with a stoichiometric mixture and excess air with a lean mixture, were investigated for an 11 L, 6-cylinder H₂-blended compressed natural gas (HCNG) engine. The engine was operated at 1260 rpm and 50% of maximum engine load (575 Nm) at maximum brake torque for each strategy. To evaluate the EGR approach, the stoichiometric combustion mode was varied, and to evaluate the lean combustion mode, the excess air ratio was varied. The maximum EGR rate and lean flammability limit were constrained by the combustion stability. The dilution rate was employed to compare the dilution effect on engine performance and emission levels under identical levels of the dilution for both combustion modes. The thermal efficiencies under stoichiometric combustion with EGR were lower than those under lean combustion, owing to a higher pumping loss and a lower combustion speed. The total hydrocarbon emissions under the lean combustion mode were lower than those under the stoichiometric combustion mode only when the combustion speed was relatively slow, due to the higher mixing rate caused by the active combustion. As the dilution rate was increased in the lean combustion mode, the rate of decrease in NO_x emissions slowed compared to the stoichiometric combustion mode. The lowest level of engine-out NO_x emissions was observed under lean combustion.     

The effects of hydrogen enriched natural gas under different engine loads in a diesel engine

Natural gas, which is among the alternative fuels, has become widespread in the transportation as it is both economical and environmentally friendly. While the use of natural gas is at a significant level in spark ignition engines, it has not yet been implemented in compression ignition engines (CI) as it worsens combustion due to ignition delay. In CI engines, however, the combustion properties of natural gas (NG) can be improved by adding hydrogen (H₂) to NG. This is one of the methods applied to use natural gas in CI engines. In this experimental study, two different volumetric rates of NG and NG/H₂ mixtures were added to the combustion air in a CI engine, and engine performance and emissions were examined under different engine loads. The experiments were performed at two different engine speeds, four different engine loads and no-load condition. An engine cylinder pressure of 59.16 bar, which is the closest value to the 59.39 bar obtained in the use of diesel fuel, was obtained at 1500 rpm for “Diesel + NG(500 g/h)” and 59.9 bar (highest values) was obtained for “Diesel + (500 g/h) [80%NG+20%H₂]" at 1750 rpm. For “Diesel + NG(250 g/h)” (Mix1) and “Diesel + NG(500 g/h)” (Mix2), as the engine speed increases, at the point where the maximum in-cylinder pressure is obtained occurs further to the right from top dead center (TDC). With the addition of 500 g/h NG, an increase of 4.5% was achieved in the cylinder pressure at full load, while an increase of 6.5% was achieved in the case of using “Diesel + (500 g/h) [80%NG+20%H₂]". Although the effect of the NG and NG/H₂ mixtures on in-cylinder pressure was small, the fuel consumption and thermal efficiency improved. Substantial improvements in hydrocarbon (HC) emissions were observed with the use of “Diesel + (250 g/h)[80%NG+20%H₂]”. Carbon dioxide (CO₂) emissions decreased with speed increase, but no significant differences in terms of CO₂ emissions were observed between the mixtures. There was a maximum difference of 15% between the diesel and the mixtures in CO₂ emissions. Although there was a decrease in nitrogen oxide (NO_x) levels with the increase in engine speed, the lowest NO_x emissions of 447.6 ppmvol was observed in “Diesel + NG(250 g/h)” (Mix1) at 1750 rpm at maximum load.     

Study on the extension of lean operation limit through hydrogen enrichment in a natural gas spark-ignition engine

An experimental study aimed at investigating the extension of lean operation limit through hydrogen addition in a SI engine was conducted on a six-cylinder throttle body injection natural gas engine. Four levels of hydrogen enhancement were used for comparison purposes: 0%, 10%, 30% and 50% by volume. The effects of various engine operating conditions on engine's lean burn capability were also examined. Test results were then analyzed from a combustion point of view. The results show that engine's lean operation limit could be extended through adding hydrogen and increasing load level (intake manifold pressure). Effect of engine speed on lean operation limit is smaller. At low load level increase in engine speed is beneficial to extending lean operation limit but this is not true at high load level. The effects of engine speed are even weaker when the engine is switched to hydrogen enriched fuelling. Spark timing also influences on lean operation limit and both over-retarded and over-advanced spark timing are not advisable. It is also observed there existed a limiting value imposed on spark-90% MFB burn duration if lean operation limit is not to be exceeded and interestingly, this limiting value was independent on hydrogen enhancement level and engine operating conditions examined in this study.     

Effect of hydrogen direct injection on natural gas/hydrogen engine performance under high compression-ratio conditions

In traffic transportation, the use of low-carbon fuels is the key to being carbon-neutral. Hydrogen-enhanced natural gas gets more and more attention, but practical engines fueled with it often suffer from low engine power output. In this study, the inner mechanism of hydrogen direct injection on methane combustion was optically studied based on a dual-fuel supply system. Simultaneous pressure acquisition and high-speed direct photography were used to analyze engine performance and flame characteristics. The results show that lean combustion can improve methane engine's thermal efficiency, but is limited by cyclic variations under high excess air coefficient conditions. Hydrogen addition mainly acts as an ignition promoter for methane lean combustion, as a result, the lean combustion limit and thermal efficiency can be improved. As for hydrogen injection timing, late injection can increase the in-cylinder turbulence intensity but also the inhomogeneity, so a suitable injection timing is needed for improving the engine's performance. Besides, late hydrogen injection is more effective under lean conditions because of the reduced mixture inhomogeneity. The current study shall give some insights into the controlling strategies for natural gas/hydrogen engines.     

Experimental study of stratified lean burn characteristics on a dual injection gasoline engine

Due to increasingly stringent fuel consumption and emission regulation, improving thermal efficiency and reducing particulate matter emissions are two main issues for next generation gasoline engine. Lean burn mode could greatly reduce pumping loss and decrease the fuel consumption of gasoline engines, although the burning rate is decreased by higher diluted intake air. In this study, dual injection stratified combustion mode is used to accelerate the burning rate of lean burn by increasing the fuel concentration near the spark plug. The effects of engine control parameters such as the excess air coefficient (Lambda), direct injection (DI) ratio, spark interval with DI, and DI timing on combustion, fuel consumption, gaseous emissions, and particulate emissions of a dual injection gasoline engine are studied. It is shown that the lean burn limit can be extended to Lambda= 1.8 with a low compression ratio of 10, while the fuel consumption can be obviously improved at Lambda= 1.4. There exists a spark window for dual injection stratified lean burn mode, in which the spark timing has a weak effect on combustion. With optimization of the control parameters, the brake specific fuel consumption (BSFC) decreases 9.05% more than that of original stoichiometric combustion with DI as 2 bar brake mean effective pressure (BMEP) at a 2000 r/min engine speed. The NO_x emissions before three-way catalyst (TWC) are 71.31% lower than that of the original engine while the particle number (PN) is 81.45% lower than the original engine. The dual injection stratified lean burn has a wide range of applications which can effectively reduce fuel consumption and particulate emissions. The BSFC reduction rate is higher than 5% and the PN reduction rate is more than 50% with the speed lower than 2400 r/min and the load lower than 5 bar.     

Performance and emission characteristics of a turbocharged spark-ignition hydrogen-enriched compressed natural gas engine under wide open throttle operating conditions

This paper investigates the effect of various hydrogen ratios in HCNG (hydrogen-enriched compressed natural gas) fuels on performance and emission characteristics at wide open throttle operating conditions using a turbocharged spark-ignition natural gas engine. The experimental data was taken at hydrogen fractions of 0%, 30% and 55% by volume and was conducted under different excess air ratio (λ) at MBT operating conditions. It is found that under various λ, the addition of hydrogen can significantly reduce CO, CH₄ emissions and the NO_x emission remain at an acceptable level when ignition timing is optimized. Using the same excess air ratio, as more hydrogen is added the power, exhaust temperatures and max cylinder pressure decrease slowly until the mixture’s lower heating value remains unchanged with the hydrogen enrichment, then they rise gradually. In addition, the early flame development period and the flame propagation duration are both shorter, and the indicated thermal efficiency and maximum heat release rate both increase with more hydrogen addition.     

Cyclic variations on LPG and gasoline-fuelled lean burn SI engine

Lean operation is an attractive operational condition; it is known as one of the methods to increase thermal efficiency, and to decrease exhaust emissions and fuel consumption. However, as the mixture leans, cyclic variations increase. Cyclic variations are usually attributed to the result of random fluctuations, excess air ratio and flow field due to the turbulent nature of the flow in the cylinder that limits the range of operating conditions of the spark ignition engine. Gaseous fuels as clean, economical and abundant fuels can improve the lean operating limits and decrease the cyclic variations. Therefore, the purpose of this research is to investigate the use of liquefied petroleum gas (LPG) as a fuel for spark ignition engine in terms of lean operation, and focuses on the cyclic variations and exhaust emissions. The results of this study showed that use of LPG decreased the coefficient of variation in the indicated mean effective pressure, and emission.     

Part-load performance characteristics of a lean burn catalytic combustion gas turbine system

The purpose of this study is to compare the part-load performance of a lean burn catalytic combustion gas turbine (LBCCGT) system in three different control modes: varying fuel, bleeding off the fuel mixture flow after the compressor and varying rotational speed. The conversions of methane species for chemical process are considered. A 1D heterogeneous plug flow model was utilized to analyze the system performance. The actual turbomachinery components were designed and predicted performance maps were applied to system performance research. The part-load characteristics under three control strategies were numerically investigated. The main results show that: the combustor inlet temperature is a significant factor that can significantly affect the part-load characteristics of the LBCCGT system; the rotational speed control mode can provide the best performance characteristics for part-load operations; the operation range of the bleed off mode is narrower than that of the speed control mode and wider than that of the fuel only mode; with reduced power, methane does not achieve full conversion over the reactor at the fuel only control mode, which will not warrant stable operation of the turbine system; the thermal efficiency of the LBCCGT system at fuel only control strategy is higher than that at bleed off control strategy within the operation range.     

汽油发动机稀薄燃烧NOx后处理系统试验研究

在一台1.5 L涡轮增压缸内直喷汽油发动机上,使用不同的三效催化反应器(three-way catalytic,TWC)、稀燃NOx捕集器(lean NOx trap,LNT)、被动选择性催化还原器(passive selective catalytic reduction,PSCR)等排气后处理组合,研究了汽油发动机...     

Comparison of the effects of EGR and lean burn on an SI engine fueled by hydrogen-enriched low calorific gas

A naturally aspirated spark ignition (SI) engine fueled by hydrogen-blended low calorific gas (LCG) was tested in both exhaust gas recirculation (EGR) and lean burn modes. The “dilution ratio” was introduced to compare their effects on engine performance and emissions under identical levels of dilution. LCG composed of 40% natural gas and 60% nitrogen was used as a main fuel, and hydrogen was blended with the LCG in volumes ranging from 0 to 20%. The engine test results demonstrated that EGR operations at stoichiometry showed a narrower dilution range, inferior combustion characteristics, lower brake thermal efficiency, faster nitrogen oxides (NO_x) suppression, and higher total hydrocarbon (THC) emissions for all hydrogen blending rates compared to lean burn. These trends were mainly due to the increased oxygen deficiency as a result of using EGR in LCG/air mixtures. Hydrogen enrichment of the LCG improved combustion stability and reduced THC emissions while increasing NO_x. In terms of efficiency, hydrogen addition induced a competition between combustion enhancement and increases in the cooling loss, so that the peak thermal efficiency occurred at 10% H₂ with excess air ratio of 1.5. The engine test results also indicated that a close-to-linear NO_x-efficiency relationship occurred for all hydrogen blending rates in both operations as long as stable combustion was achieved. NO_x versus combustion duration analysis showed that adding H₂ reduced combustion duration while maintaining the same level of NO_x. The methane fraction contained in the THC emissions decreased slightly with an increase in hydrogen enrichment at low EGR or excess air dilution ratios, but this tendency was diminished at higher dilution ratios because of the combined dilution effects from the inert gas in the LCG and the diluents (EGR or excess air).     

Degradation analysis of a heavy-duty gas turbine engine under full and part load conditions

This paper describes the degradation analysis and the performance diagnostics of the gas-turbine (GT) cycle of a combined cycle power plant (CCPP). Three different operating loads, which are 100%, 75%, and 50%, were tested at different ambient conditions, namely, temperature, pressure, and humidity. First, a degradation model to simulate the GT performance with these various operating loads and conditions has been developed. It is then demonstrated how this degradation affects the GT and its components. The degradation analysis has been performed on 2500 readings obtained during 2 years of operation. After applying the load determining criteria, 60 readings were obtained to represent the full load operation and 40 reading points for each part-load operation. The degradation analysis has been carried out on the basis of actual data obtained from a CCPP; this differentiates this study from the others in the same area. Based on the commissioning test performance of the GT cycle, the model has been validated. The results show that the rate of degradation increases dramatically as the load increases. Moreover, the degradation rate also increases with an increase in the ambient air temperature. However, the degradation rate for the various studied parameters, namely, polytropic efficiencies, GT exhaust mass flow rate, and the overall GT efficiency, has been found to decrease with time. The maximum degradation percentage has been estimated to be −1.71% at full load conditions in comparison with −1.33% and −1.16% at 75% and 50% load, respectively.     

Dynamics of cycle-to-cycle variations in a natural gas direct-injection spark-ignition engine

The dynamics of cycle-to-cycle variations (CCV) was investigated in a natural gas direct-injection spark-ignition engine. The method of continuous wavelet transform was used to analyze the time series of the indicated mean effective pressure (IMEP) and other combustion variables. The dominant oscillatory modes in the CCV were identified, and the engine cycles over which these modes may persist were delineated. Results were obtained for four compression ratios: CR = 8, 10, 12 and 14, at two engine speeds of 1200 and 1800 rpm. The results reveal that the CCV exhibit multiscale dynamics with fluctuations occurring at different timescales. At the engine speed of 1200 rpm, the spectral power of CCV for CR = 12 was found to be significantly reduced at the different timescales compared to the CCV at other values of CR. At the higher engine speed of 1800 rpm, this reduction was less pronounced. In addition, cross wavelet transform was used to explore the relationships between the CCV of IMEP and those of flame development duration, main combustion duration and total combustion duration. Strong interdependence was found to exist between the IMEP and main combustion duration as well as total combustion duration, over a wide range of frequencies and engine cycles.