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.     

An analytic study of applying Miller cycle to reduce NOx emission from petrol engine

An analytic investigation of applying Miller cycle to reduce nitrogen oxides (NO_x) emissions from a petrol engine is carried out. The Miller cycle used in the investigation is a late intake valve closing version. A detailed thermodynamic analysis of the cycle is presented. A comparison of the characters of Miller cycle with Otto cycle is presented. From the results of thermodynamic analyses, it can be seen that the application of Miller cycle is able to reduce the compression pressure and temperature in the cylinder at the end of compression stroke. Therefore, it lowers down the combustion temperature and NO_x formation in engine cylinder. These results in a lower exhaust temperature and less NO_x emissions compared with that of Otto cycle. The analytic results also show that Miller cycle ratio is a main factor to influence the combustion temperature, and then the NO_x emissions and the exhaust temperature. The results from the analytic study are used to analyse and to compare with the previous experimental results. An empirical formula from the previous experimental results that showed the relation of NO_x emissions with the exhaust temperature at different engine speed is presented. The results from the study showed that the application of Miller cycle may reduce NO_x emissions from petrol engine.     

A novel gas turbine cycle with hydrogen-fueled chemical-looping combustion

In this paper we have proposed a novel gas turbine cycle with hydrogen-fueled chemical-looping combustion, and the system study on two hydrogen-fueled power plants, the new gas turbine cycle and an advanced gas turbine cycle with H₂/O₂ combustion, has been investigated with the aid of exergy principle (EUD methodology). The hydrogen fueled chemical-looping combustion in the new gas turbine cycle consists of two successive reactions: hydrogen fuel is reacted with metal oxide (reduction of metal oxide), and then instead of air or pure oxygen, the reduced metal is successively oxidized by the saturated air. As a result, the new hydrogen-fueled gas turbine cycle has a breakthrough performance, with at least about 12 percentage-point higher efficiency compared to the gas turbine cycle with H₂/O₂ combustion, and will be environmentally superior due to complete elimination of NO_x formation. The promising results obtained here indicated that this novel gas turbine cycle with hydrogen-fueled chemical looping combustion could make a breakthrough in efficient use of hydrogen energy in power plants.     

Application of the Miller cycle to reduce NOx emissions from petrol engines

A conceptual analysis of the mechanism of the Miller cycle for reducing NO_x emissions is presented. Two versions of selected Miller cycle (1 and 2) were designed and realized on a Rover “K” series 16-valve twin-camshaft petrol engine. The test results showed that the application of the Miller cycle could reduce the NO_x emissions from the petrol engine. For Miller cycle 1, the least reduction rate of NO_x emission was 8% with an engine-power-loss of 1% at the engine’s full-load, compared with that of standard Otto cycle. For Miller cycle 2, the least reduction rate of NO_x emission was 46% with an engine-power-loss of 13% at the engine’s full-load, compared with that of standard Otto cycle.     

Optimizing thermal performances and NOx emission in a premixed ammonia-hydrogen blended meso-scale combustor for thermophotovoltaic applications

As a carbon-free energy carrier, ammonia has attracted significant interest in the combustion field as a potential substitute for fossil fuels. However, the focus has been given to the application at meso-scale conditions, particularly with regard to thermal performance and NO_x emissions. Therefore, the present study numerically investigates a 3-dimensional time-domain premixed ammonia/oxygen meso-scale combustor to optimize its' thermal performance and NO_x emission for power generation applications. The numerical model is firstly validated by using experimental data available in the literature. Then, the effects of 1) the inlet pressure (P_in), 2) the equivalence ratio, and 3) the hydrogen blended ratio on the temperature uniformity, the combustor outer wall mean temperature (OWMT), NO emission, and exergy efficiency are examined. The results indicate that increasing P_in intensifies the mixing process of the mixture gases, thus reducing the residence time for the high-temperature flame in the combustion chamber. The optimized OWMT and NO emissions are up to 26% and 40.3% respectively, with only 9% compensation of the standard deviation achieved, when the inlet velocity is set to 0.5 m/s and P_in is 3.0 bar. Furthermore, varying the equivalence ratio in the range of 0.95–1.1 has a minor influence on improving thermal performances, but a significant impact on mitigating the NO_x emission performance. Additionally, blending less than 15% hydrogen has a significant reduction in the maximum NO_x emission (up to 53%); however, the influence on the OWMT can be neglected. Further exergy analysis reveals that elevating P_in results in a decrease in the exergy efficiency due to the increased inlet exergy. In general, this work provides a preliminary method for improving the thermal performance and NO_x emission of an ammonia/hydrogen-oxygen-fueled meso-scale combustor for power generation purpose.     

A modified diesel engine for natural gas operation: Performance and emission tests

A diesel engine was modified for natural gas operation to optimize performance using gaseous fuel. A variation of combustion ratios (CR) including 9.0:1, 9.5:1, 10.0:1 and 10.5:1 was utilized to evaluate engine performance and emissions from the same engine over the engine speeds between 1000 and 4000 rpm. Tested engine performance parameters include brake torque, brake power, specific fuel consumption (SFC) and brake thermal efficiency. Emissions tests recorded total hydrocarbon (THC), nitrogen oxides (NO_x) and carbon monoxide (CO). The results showed that a CR of 9.5:1 had the highest thermal efficiency and the lowest SFC while a CR of 10:1 showed a high torque at low speed. THC emissions were directly proportional to the CR. NO_x emissions increased with increasing CR and then declined after a CR of 10:1.     

Emission and performance estimation in hydrogen injection strategies on diesel engines

Recently, the increasing demand for energy requires the use of alternative fuels, especially in fossil fueled power systems. As a promising alternative fuel for next-generation diesel engines that utilize fossil fuel, hydrogen fuel is one step ahead due to its positive properties. In this study, the effects of hydrogen on the performance of a diesel engine have been numerically investigated with respect to different injection ratios and timings. The numerical results of the study for 25% load conditions on a single-cylinder, four-stroke diesel engine have been validated against experimental data taken from literature and good agreement has been observed for pressure results. Emission parameters such as NO_x, CO and performance parameters such as cylinder temperature, pressure, power, thermal efficiency and IMEP are presented comparatively.The results of numerical analyses show that the maximum pressure, temperature and heat release rate are observed with injection ratio of H15 and early injection timing (20° CA BTDC). Besides that, engine power, thermal efficiency and IMEP are greatly improved with increasing injection ratio and early injection timing. Although combustion chamber performance parameters improve with rising the hydrogen injection ratio, higher NO_x emissions have also been detected as a negative side effect. Furthermore, while early injection timing increases diesel engine performance, it also causes an increase in NO_x emissions. Therefore, precise determination of injection timing together with the optimum amount of hydrogen has revealed that it brings crucial improvement in engine performance and emissions.     

Efficiency improvement and NOx emission reduction potentials of two‐stage turbocharged Miller cycle for stationary natural gas engines

As a part of an engine research and development project in cooperation with industry, a V20 engine is designed with two‐stage high‐pressure turbocharging, and the potential of the Miller cycle is examined through calculations using computational models based on experimental as well as computational studies. The stationary gas engines must produce less than 250 mg Nm^?3 in 5% excess of O₂, additionally the amount of NO_x emissions and their dependence on engine operational and design conditions are investigated by using a zero‐dimensional reaction kinetic model. The results show an increase in efficiency, also the amount of NO_x emissions is kept under the constraint value of 250 mg Nm^?3 for stationary engines. The results obtained here promise a very high improvement potential for future emission regulations. Copyright © 2004 John Wiley & Sons, Ltd.     

Remarkable improvement of NOx–PM trade-off in a diesel engine by means of bioethanol and EGR

In order to realize a premixed compression ignition (PCI) engine, the effects of bioethanol–gas oil blends and exhaust gas recirculation (EGR) on PM–NO_x trade-off have been investigated focusing on ignition delay, premixed combustion, diffusion combustion, smoke, NO_x and thermal efficiency. The present experiment was done by increasing the ethanol blend ratio and ethanol and by increasing the EGR ratio in a single cylinder direct injection diesel engine. It is found that a remarkable improvement in PM–NO_x trade-off can be achieved by promoting the premixing based on the ethanol blend fuel having low evaporation temperature, large latent heat and low cetane number as well, in addition, based on a marked elongation of ignition delay due to the low cetane number fuel and the low oxygen intake charge. As a result, very low levels of NO_x and PM, which satisfies the 2009 emission standards imposed on heavy duty diesel engines in Japan, were achieved without deterioration of brake thermal efficiency in the PCI engine fuelled with the 50% ethanol blend diesel fuel and the high EGR ratio. It is noticed that smoke can be reduced even by increasing the EGR ratio under the highly premixed condition.     

Analysis of exhaust waste heat recovery from a dual fuel low temperature combustion engine using an Organic Rankine Cycle

This paper examines the exhaust waste heat recovery potential of a high-efficiency, low-emissions dual fuel low temperature combustion engine using an Organic Rankine Cycle (ORC). Potential improvements in fuel conversion efficiency (FCE) and specific emissions (NO_x and CO₂) with hot exhaust gas recirculation (EGR) and ORC turbocompounding were quantified over a range of injection timings and engine loads. With hot EGR and ORC turbocompounding, FCE improved by an average of 7 percentage points for all injection timings and loads while NO_x and CO₂ emissions recorded an 18 percent (average) decrease. From pinch-point analysis of the ORC evaporator, ORC heat exchanger effectiveness (?), percent EGR, and exhaust manifold pressure were identified as important design parameters. Higher pinch point temperature differences (PPTD) uniformly yielded greater exergy destruction in the ORC evaporator, irrespective of engine operating conditions. Increasing percent EGR yielded higher FCEs and stable engine operation but also increased exergy destruction in the ORC evaporator. It was observed that hot EGR can prevent water condensation in the ORC evaporator, thereby reducing corrosion potential in the exhaust piping. Higher ? values yielded lower PPTD and higher exergy efficiencies while lower ? values decreased post-evaporator exhaust temperatures below water condensation temperatures and reduced exergy efficiencies.     

Theoretical investigation of the combustion performance of ammonia/hydrogen mixtures on a marine diesel engine

Ammonia is a good hydrogen carrier and can be well combined with hydrogen for combustion. The combustion performance of the mixtures of ammonia and hydrogen in a medium-speed marine diesel engine was investigated theoretically. The HCCI combustion mode was selected for reducing thermal-NO_x production. The start fire characteristic of the NH₃–H₂ mixtures was studied under different equivalence ratio, hydrogen-doped ratio, and intake air temperature and pressure. Then, the combustion performance of the NH₃–H₂ mixtures (doping 30% hydrogen) was analyzed at a typical operation condition of engine. The addition hydrogen improved the laminar flame velocity of ammonia, and affected the NO_x emission. For the medium-speed marine engine fueled with NH₃–H₂, reducing combustion temperature, introducing EGR and combining with post-treatment technology would be a feasible scheme to reduce NO_x emission.     

The influences of hydrogen on the performance and emission characteristics of a heavy duty natural gas engine

Because blending hydrogen with natural gas can allow the mixture to burn leaner, reducing the emission of nitrogen oxide (NO_x), hydrogen blended with natural gas (HCNG) is a viable alternative to pure fossil fuels because of the effective reduction in total pollutant emissions and the increased engine efficiency.In this research, the performance and emission characteristics of an 11-L heavy duty lean burn engine using HCNG were examined, and an optimization strategy for the control of excess air ratio and of spark advance timing was assessed, in consideration of combustion stability. The thermal efficiency increased with the hydrogen addition, allowing stable combustion under leaner operating conditions. The efficiency of NO_x reduction is closely related to the excess air ratio of the mixture and to the spark advance timing. With the optimization of excess air ratio and spark advance timing, HCNG can effectively reduce NO_x as much as 80%.     

Generating efficiency and emissions of a spark-ignition gas engine generator fuelled with biogas–hydrogen blends

We investigated the generating efficiency and pollutant emissions of a four-stroke spark-ignition gas engine generator operating on biogas–hydrogen blends of varying excess air ratios and hydrogen concentrations. Experiments were carried out at a constant engine speed of 1200 rpm and a constant electric power output of 10 kW. The experimental results showed that the peak values of generating efficiency, maximum cylinder pressure, and NO_x emissions were elevated at an excess air ratio of around 1.2 as the hydrogen concentration was increased. CO₂ emissions decreased as the excess air ratio and hydrogen concentration increased, due to lean-burn conditions and hydrogen combustion. An efficiency per NO_x emissions ratio (EPN) was defined to consider the relationship between the generating efficiency and NO_x emissions. A maximum EPN value of 0.7502 was obtained with a hydrogen concentration of 15%, for an excess air ratio of 2.0. At this EPN value, the NO_x and CO₂ emissions were 39 ppm and 1678.32 g/kWh, respectively, and the generating efficiency was 29.26%. These results demonstrated that the addition of hydrogen to biogas enabled the effective generation of electricity using a gas engine generator through lean-burn combustion.     

The modification of the fuel injection rate in heavy-duty diesel engines. Part 1: Effects on engine performance and emissions

An experimental study has been performed on the effects of injection rate shaping on the combustion process and exhaust emissions of a direct-injection diesel engine. Boot-type injections were generated by means of a modified pump-line-nozzle system, which is able to modulate the instantaneous fuel injection rate. The interest of the study reported here was the evaluation of the effective changes produced in the injection rate at different engine operating conditions, when the engine rotating speed and the total fuel injected were changed. In addition, the influence of these new injection rates was quantified on the global engine performance and pollutant emissions. In particular, the focus was placed on producing “boot-like” injection rate shapes, with the main objective of reducing NO_x emissions.Results show how this system is capable of achieving boot-type injections at different boot pressures and boot durations. Also, even though the general trend of the system is to reduce NO_x and to increase soot and fuel consumption, emissions and performance trade-offs can be improved for some specific boot shapes. On the contrary, the modulation of the injection rate showed to be ineffective at medium engine load, since the increase in soot was greater than the relative decrease in NO_x.The analysis of the modifications produced by these strategies on the combustion process, and on the rate of heat release are the base of a second paper.     

The effects of injection timing on NOx emissions of a low heat rejection indirect diesel injection engine

Higher NO_x is one of the major problems to be overcomed in a low heat rejection (LHR) diesel engine as insulation leads to an increase in combustion temperature about 200–250 °C compared to an identical standard (STD) diesel engine. High combustion temperatures alter optimum injection timing of a LHR engine. With the proper adjustment of the injection timing, it is possible to partially offset the adverse effect of insulation on heat release rate and hence to obtain improved performance and lower NO_x. However, the injection timing and brake specific fuel consumption (BSFC) trade-off must be considered together in performance and NO_x emission point of view. In this study, optimum injection timing was found with 4 crank angle (34° CA) retarded before top dead centre (BTDC) in LHR diesel engine in comparison to that of STD diesel engine (38° CA BTDC). When the LHR engine was operated with the injection timing of the 38 crank angle, which is the optimum value of the STD engine, it was shown that NO_x emission increased about 15%. However, when the injection timing was retarded to 34° CA in the LHR case, it was observed a decrease on the NO_x emissions with about 40% and the brake specific fuel consumption (BSFC) with about 6% compared to that of the STD case. Thus, by retarding the injection timing, an additional 1.5% saving in fuel consumption was obtained.     

Experimental investigation of control of NOx emissions in biodiesel-fueled compression ignition engine

Biodiesel is an alternative fuel consisting of the alkyl esters of fatty acids from vegetable oils or animal fats. Vegetable oils are produced from numerous oil seed crops (edible and non-edible), e.g., rapeseed oil, linseed oil, rice bran oil, soybean oil, etc. Research has shown that biodiesel-fueled engines produce less carbon monoxide (CO), unburned hydrocarbon (HC), and particulate emissions compared to mineral diesel fuel but higher NO_x emissions. Exhaust gas recirculation (EGR) is effective to reduce NO_x from diesel engines because it lowers the flame temperature and the oxygen concentration in the combustion chamber. However, EGR results in higher particulate matter (PM) emissions. Thus, the drawback of higher NO_x emissions while using biodiesel may be overcome by employing EGR. The objective of current research work is to investigate the usage of biodiesel and EGR simultaneously in order to reduce the emissions of all regulated pollutants from diesel engines. A two-cylinder, air-cooled, constant speed direct injection diesel engine was used for experiments. HCs, NO_x, CO, and opacity of the exhaust gas were measured to estimate the emissions. Various engine performance parameters such as thermal efficiency, brake specific fuel consumption (BSFC), and brake specific energy consumption (BSEC), etc. were calculated from the acquired data. Application of EGR with biodiesel blends resulted in reductions in NO_x emissions without any significant penalty in PM emissions or BSEC.     

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).     

Staged combustion properties for pulverized coals at high temperature

Staged combustion properties for pulverized coals have been investigated by using a new-concept drop-tube furnace. Two high-temperature electric furnaces were connected in series. Coal was burnt under fuel-rich conditions in the first furnace, then, staged air was supplied at the connection between the two furnaces. Reaction temperature (1800–2100 K) and time (1–2 s) were similar to those used in actual boilers. When coal was burnt at the same stoichiometric ratio as in actual boilers, similar combustion performance values as for actual boilers were obtained regarding NO_x emission and carbon in ash. The most important factor for low NO_x combustion was to raise the combustion temperature above the present range (1800–2100 K) in the fuel-rich zone. The NO_x emission was significantly increased with decrease of burning temperature in the fuel-rich zone when the temperature was lower than 1800 K. But, NO_x emission was cut to around 100–150 ppm, for sub-bituminous coal and hv-bituminous coal, in the latest commercial plants by forming this high-temperature fuel-rich region in the boilers. If the temperature and stoichiometric ratio could be set to the most suitable conditions, and, burning gas and air were mixed well, it would be possible to lower NO_x emission to 30–60 ppm (6% O₂). The most important NO_x reduction reaction in the fuel-rich zone was the NO_x reduction by hydrocarbons. The hydrocarbon formation rate in the flame was varied with coal properties and combustion conditions. The NO_x was easily reduced when coals which easily formed hydrocarbons were used, or, when burning conditions which easily formed hydrocarbons were chosen. Effects of burning temperature and stoichiometric ratio on NO_x emission were reproduced by the previously proposed reaction model. When solid fuel was used, plant performance values varied with fuel properties. The proposed drop-tube furnace system was also found to be a useful analysis technique to evaluate the difference in combustion performance due to the fuel properties.     

Experimental study of hydrogen enriched compressed natural gas (HCNG) engine and application of support vector machine (SVM) on prediction of engine performance at specific condition

The effect of excess air ratio (λ) and ignition advance angle (θ_ig) on the combustion and emission characteristics of hydrogen enriched compressed natural gas (HCNG) on a 6-cylinder compressed natural gas (CNG) engine has been experimental studied in an engine test bench, aiming at enriching the sophisticated calibration of HCNG fueled engine and increasing the prediction accuracy of the SVM method on automobile engines. Three different fuel blends were selected for the experiment: 0%, 20% and 40% volumetric hydrogen blend ratios. It is noted that combustion intensity varies with the excess air ratio and the ignition advance angle, so are the emissions. The optimal value of λ or θ_ig has been explored in the specific engine condition. Results show that blending hydrogen can enhance and advance the combustion and stability of CNG engine, and it also has some benefic influence on the emissions such as reducing the CO and CH₄. Meanwhile, a simulation research on forecasting the engine performance by using the support vector machine (SVM) method was conducted in detail. The torque, brake specific fuel consumption and NO_x emission have been selected to characterize the power, economic and emissions of the engine with various HCNG fuels, respectively. It can be seen that the optimal model built by the SVM method can highly describe the relationship of the engine properties and condition parameters, since the value of the complex correlation coefficient is larger than 0.97. Secondly, the prediction performance of the optimal model for torque or BSFC is much better than the case of NO_x. Besides, the optimal model built by the PSO optimization method has the best prediction accuracy, and the accuracy of the model obtained based on the training group with 20% hydrogen blend ratio is the best compared with those of others. The upshots in this article provide experimental support and theoretical basis for the adoption of HCNG fuel on internal combustion engines as well as the application of intelligent algorithmic in the engine calibration technology field.     

Performance and emission comparison of a supercharged dual-fuel engine fueled by producer gases with varying hydrogen content

This study investigated the effect of hydrogen content in producer gas on the performance and exhaust emissions of a supercharged producer gas–diesel dual-fuel engine. Two types of producer gases were used in this study, one with low hydrogen content (H₂ = 13.7%) and the other with high hydrogen content (H₂ = 20%). The engine was tested for use as a co-generation engine, so power output while maintaining a reasonable thermal efficiency was important. Experiments were carried out at a constant injection pressure and injection quantity for different fuel–air equivalence ratios and at various injection timings. The experimental strategy was to optimize the injection timing to maximize engine power at different fuel–air equivalence ratios without knocking and within the limit of the maximum cylinder pressure. Two-stage combustion was obtained; this is an indicator of maximum power output conditions and a precursor of knocking combustion. Better combustion, engine performance, and exhaust emissions (except NO_x) were obtained with the high H₂-content producer gas than with the low H₂-content producer gas, especially under leaner conditions. Moreover, a broader window of fuel–air equivalence ratio was found with highest thermal efficiencies for the high H₂-content producer gas.