NOx emission characteristics in rich–lean combustion of hydrogen

Characteristics of NO_x formation in a gas turbine fuelled with hydrogen were analyzed with both an experimental and a numerical approach. This research experimentally investigated NO_x reduction effect of rich–lean combustion in a coaxial burner. Hydrogen emits no Prompt NO even in rich mixture conditions, and can be more effective to reduce NO_x in the rich–lean combustion system than hydrocarbons. The results show that the rich–lean combustion of hydrogen successfully reduces NO_x emission compared with diffusive combustion. In the rich–lean combustion, hydrogen combustion has lower NO_x emission compared to methane combustion, especially with larger equivalence ratio of richer side mixture. Calculations of NO_x formation in the rich–lean combustion were also done employing the extended Zel’dovich NO formation mechanism.     

Emission analysis of a compressed natural gas directinjection engine with a homogenous mixture

In an era in which environmental pollution and depletion of world oil reserves are of major concern, emissions produced by automotive vehicles need to be controlled and reduced. An ideal solution is to switch to a cleaner fuel such as natural gas, which generates cleaner emissions. In addition, control over the in-cylinder air-fuel mixture can be best achieved through a direct-injection mechanism, which can further improve combustion efficiency. This need for cleaner automobiles provides the motivation for this paper’s examination of the use of computational fluid dynamic (CFD) simulations to analyze the concentrations of the exhaust gases produced by a compressed natural gas engine with a direct-fuel-injection system. In this work, a compressed natural gas direct-injection engine has been designed and developed through a numerical simulation using computational fluid dynamics (CFD) to provide an insight into complex in-cylinder behavior. The emissions analyzed in this study were carbon monoxide (CO), nitric oxide (NO) and carbon dioxide (CO₂), i.e. the main pollutants produced by natural gas combustion. Based on a stoichiometric mixture, the concentrations of CO and NO were computed using the dissociation of carbon dioxide and the extended Zeldovich mechanism. CO₂ was calculated using a mass balance of the species involved in the combustion process. The simulation results were then compared with the experimental data generated by a single-cylinder research engine test rig. A good agreement was obtained with the experimental data for the engine speeds considered for all emissions concentrations.     

Effect of UWS injection at low exhaust gas temperature on NO<Subscript>x</Subscript> removal efficiency of diesel engine

Urea-SCR systems have been widely used in diesel vehicles according to the strengthened NO_x (Nitrogen Oxides) emission standard. The NO_x removal efficiencies of the latest well optimized urea-SCR system are above 90 % at moderate exhaust gas temperature of 250 ~ 450 °C. However, a large amount of NO_x is emitted from diesel vehicles at cold start or urban driving conditions, when the exhaust gas temperature is not high enough for SCR catalyst activation. Although many researchs have been stuied to improve NO_x conversion efficiency at these low temperature conditions, it is still one of important technical issues. In this study, the effect of UWS injection at low exhaust gas temperature conditions is studied. This study uses a 3.4 L diesel engine equipped with a commertial urea SCR system. As a result, it is found that about 5 % of NO_x removal efficiency is improved in the NRTC test when UWS injection starts at the SCR inlet temperature of 150 °C compared to 200 °C. It is also found that urea deposits can be formed on the wall of exhaust pipe, when the local wall temperature is lower than temperature of urea decomposition.     

Numerical analysis of the effect of inhomogeneous pre-mixture on the pressure rise rate in an HCCI engine using multi-zone chemical kinetics

The HCCI (Homogeneous Charge Compression Ignition) engine is an internal combustion engine under development, which is capable of providing both high diesel-like efficiency and very low NO_x and particulate emissions. However, several technical issues must be resolved before the HCCI engine is ready for widespread application. One issue is that its operating range is limited by an excessive pressure rise rate which is caused by the excessive heat release from its selfaccelerated combustion reaction and the resulting engine knock in high-load conditions. The purpose of this study was to evaluate the potential of thermal and fuel stratification for reducing the pressure rise rate in HCCI engines. The NO_x and CO concentrations in the exhaust gas were also evaluated to confirm combustion completeness and NO_x emissions. The computational work was conducted using a multi-zone code with detailed chemical kinetics, including the effects of thermal and fuel stratification on the onset of ignition and the rate of combustion. The engine was fueled with dimethyl ether (DME) which has a unique two-stage heat release, and methane which has a one-stage heat release.     

Effect of operating parameters on diesel/propane dual fuel premixed compression ignition in a diesel engine

In this research, the effects of three operating parameters (Diesel injection timing, propane ratio, and exhaust gas recirculation (EGR) rates) in a diesel-propane dual fuel combustion were investigated. The characteristics of dual-fuel combustion were analyzed by engine parameters, such as emission levels (Nitrogen oxides (NO_x) and particulate matter (PM)), gross indicated thermal efficiency (GIE) and gross IMEP Coefficient of Variance (CoV). Based on the results, improving operating strategies of the four main operating points were conducted for dual-fuel PCCI combustion with restrictions on the emissions and the maximum pressure rise rate. The NOx emission was restricted to below 0.21 g/kWh in terms of the indicated specific NO_x (ISNO_x), PM was restricted to under 0.2 FSN, and the maximum pressure rise rate (MPRR) was restricted to 10 bar/deg. Dual-fuel PCI combustion can be available with low NO_x, PM emission and the maximum pressure rise rate in relatively low load condition. However, exceeding of PM and MPRR regulation was occurred in high load condition, therefore, design of optimal piston shape for early diesel injection and modification of hardware optimizing for dual-fuel combustion should be taken into consideration.     

Exhaust emissions and its control methods in compression ignition engines: A review

Extensive usage of automobiles has certain disadvantages and one of them is its negative effect on environment. Carbon dioxide (CO₂), carbon monoxide (CO), hydrocarbons (HC), oxides of nitrogen (NO_x), sulphur dioxide (SO₂) and particulate matter (PM) come out as harmful products during incomplete combustion from internal combustion (IC) engines. As these substances affect human health, regulatory bodies impose increasingly stringent restrictions on the level of emissions coming out from IC engines. This trend suggests the urgent need for the investigation of all aspects relevant to emissions. It is required to modify existing engine technologies and to develop a better after-treatment system to achieve the upcoming emission norms. Diesel engines are generally preferred over gasoline engines due to their undisputed benefit of fuel economy and higher torque output. However, diesel engines produce higher emissions, particularly NO_x and PM. Aftertreatment systems are costly and occupy more space, hence, in-cylinder solutions are preferred in reducing emissions. Exhaust gas recirculation (EGR) technology has been utilized previously to reduce NO_x. Though it is quite successful for small engines, problem persists with large bore engines and with high rate of EGR. EGR helps in reducing NO_x, but increases particulate emissions and fuel consumption. Many in-cylinder solutions such as lower compression ratios, modified injection characteristics, improved air intake system etc. are required along with EGR to accomplish the future emission norms. Modern combustion techniques such as low temperature combustion (LTC), homogeneous charge compression ignition (HCCI), premixed charge compression ignition (PCCI) etc. would be helpful for reducing the exhaust emissions and improving the engine performance. However, controlling of autoignition timing and achieving wider operating range are the major challenges with these techniques. A comprehensive review of diesel engine performance and emission characteristics is given in this paper.     

Reduction of emissions with propane addition to a diesel engine

Recent studies on dual-fuel combustion in compression-ignition (CI) engines, also known as diesel engines, fall into two categories. In the first category are studies focused on the addition of small amounts of gaseous fuel to CI engines. In these studies, gaseous fuel is regarded as a secondary fuel and diesel fuel is regarded as the main fuel for combustion. The objectives of these studies typically involve reducing particulate matter (PM) emissions by using gaseous fuel as a partial substitution for diesel fuel. However, the addition of gaseous fuel raises the combustion temperature, which increases emissions of nitrogen oxides (NO_x). In the second category are studies focused on reactivity-controlled compression-ignition (RCCI) combustion. RCCI combustion can be implemented by early diesel injection with a large amount of low-reactivity fuel such as gasoline or gaseous fuel. Although RCCI combustion promises lower NO_x and PM emissions and higher thermal efficiency than conventional diesel combustion, it requires a higher intake pressure (usually more than 1.7 bars) to maintain a lean fuel mixture. Therefore, in this study, practical applications of dual-fuel combustion with a low air-fuel ratio (AFR), which implies a low intake pressure, were systemically evaluated using propane in a diesel engine. The characteristics of dualfuel combustion for high and low AFRs were first evaluated. The proportion of propane used for four different operating conditions was then increased to decrease emissions and to identify the optimal condition for dual-fuel combustion. Although the four operating conditions differ, the AFR was maintained at 20 (? approximately equal to 0.72) and the 50% mass fraction burned (MFB 50) was also fixed. The results show that dual-fuel combustion can reduce NO_x and PM emissions in comparison to conventional diesel combustion.     

Evaluation and visualization of stratified ultra-lean combustion characteristics in a spray-guided type gasoline direct-injection engine

To comply with reinforced emission regulations for harmful exhaust gases, including carbon dioxide (CO₂) emitted as a greenhouse gas, improved technologies for reducing CO₂ and fuel consumption are being developed. Stable lean combustion, which has the advantage of improved fuel economy and reduced emission levels, can be achieved using a sprayguided-type direct-injection (DI) combustion system. The system comprises a centrally mounted injector and closely positioned spark plugs, which ensure the combustion reliability of a stratified mixture under ultra-lean conditions. The aim of this study is to investigate the combustion and emission characteristics of a lean-burn gasoline DI engine. At an excess air ratio of 4.0, approximately 23% improvement in fuel economy was achieved through optimal event timing, which was delayed for injection and advanced for ignition, compared to that under stoichiometric conditions, while NO_x and HC emissions increased. The combustion characteristics of a stratified mixture in a spray-guided-type DI system were similar to those in DI diesel engines, resulting in smoke generation and difficulty in three-way catalystutilization. Although a different operating strategy might decrease fuel consumption, it will not be helpful in reducing NO_x and smoke emissions; therefore, alternatives should be pursued to achieve compliance with emission regulations.     

Measurement of soot oxidation with No<Subscript>2</Subscript>-O<Subscript>2</Subscript>-H<Subscript>2</Subscript>O in a flow reactor simulating diesel engine DPF

Understanding the mechanism of carbon oxidation is important for the successful modeling of diesel particulate filter regeneration. Characteristics of soot oxidation were investigated with carbon black (Printex-U). A flow reactor system that could simulate the condition of a diesel particulate filter and diesel exhaust gas was designed. Kinetic constants were derived and the reaction mechanisms were proposed using the experimental results and a simple reaction scheme, which approximated the overall oxidation process in TPO as well as CTO. From the experiments, the apparent activation energy for carbon oxidation with NO₂-O₂-H₂O was determined to be 40±2 kJ/mol, with the first order of carbon in the range of 10∼90% oxidation and a temperature range of 250∼500°C. This value was exceedingly lower than the activation energy of NO₂-O₂ oxidation, which was 60±3 kJ/mol. When NO₂ exists with O₂ and H₂O, the reaction rate increases in proportion to NO₂. It increases nonlinearly with O₂ or H₂O concentration when the other two oxidants are fixed.     

Comparison of HC species from diesel combustion modes and characterization of a heat-up doc formulation

This study summarizes engine speed and load effects on HC species emissions from premixed charge compression ignition (PCI) and conventional diesel combustion, and it evaluates diesel oxidation catalyst (DOC) formulations on a gas flow reactor for the purpose of diesel particulate filter regeneration or lean NO_x trap desulfation. HC emissions are sampled simultaneously by a Tedlar bag for light HC species and by a Tenax TA™ adsorption trap for semi-volatile HC species, and they are analyzed by gas chromatography with a flame ionization detector. The bulk temperature and residence time during combustion are key parameters that are important for understanding the effects of speed and load on engine-out HC emissions. The degree of post-flame oxidation is higher in PCI than in conventional combustion, and it is increased for PCI with a higher speed and load, as indicated by a lower fuel alkanes/THC ratio, a higher alkenes/fuel alkanes ratio, and a higher methane/THC ratio. Ethene and n-undecane are two representative HC species, and they are used as a surrogate mixture in the gas flow reactor to simulate PCI and conventional combustion with in-cylinder post fuel injection. Among the three DOC formulations tested, the catalyst with constituent precious metals of platinum and palladium (PtPd) showed the best light-off performance, followed by PtPd with an addition of cerium dioxide (PtPd+CeO₂), and platinum (Pt), regardless of exhaust compositions. Conventional combustion exhaust composition shows a lower light-off temperature than that of PCI, regardless of catalyst formulation.     

Effect of direct in-cylinder CO2 injection on HCCI combustion and emission characteristics

Fuel injection during negative valve overlap period was used to realize diesel homogeneous charge compression ignition (HCCI) combustion. In order to control the combustion, CO₂ in-cylinder injection was used to simulate external EGR. Effects of CO₂ injection parameters (injection timing, quantity, pressure) on HCCI combustion and emission characteristics were investigated. Experimental results revealed that CO₂ in-cylinder injection can control the start of combustion and effectively reduce NO_x emission. Either advancing CO₂ injection timing or increasing CO₂ injection quantity can reduce peak cylinder pressure and mean gas temperature, delay the starts of low temperature reaction (LTR) and high temperature reaction (HTR), and lower pressure rise rate; NO_x emission was reduced, while smoke, HC, and CO emissions increased. Since the combustion phase was improved, the indicated thermal efficiency was also improved. Injection pressure determines the amount of disturbance introduced into the cylinder. Generally, with the same injection quantity, higher injection pressure results in higher momentum flux and total momentum. Larger momentum flux and momentum has a stronger disturbance to air-fuel mixture, resulting in a more homogeneous mixture; therefore, larger injection pressure leads to lower NO_x and smoke emissions.     

Study of the effects on exhaust emissions in direct injection diesel engines: Effects of fuel injection system,distillation properties and cetane number

The effect of injection nozzle, diesel fuel density (volatility) and cetane number on diesel exhaust emissions were investigated. Decreasing injection nozzle hole diameter decreases PM emission. However, a small nozzle hole increases NO_x emission and decreases the effect of fuel on PM emission. Decreasing fuel density is effective for reduction of NO_x emission. But the effect is smaller than that of nozzle hole diameter and injection pressure. Furthermore injection timing retardation decreases the effect of fuel density on NO_x emission.     

NOx reduction characteristics of Pt-ZSM-5 catalyst with diesel engine exhaust

The platinum ion-exchanged ZSM-5 zeolite catalyst (Pt-5), which reduces nitrogen oxides (NO_x) in the presence of oxygen and hydrocarbons, was applied to actual diesel engine exhaust. Compared to the Cooper ion-exchanged ZSM-5 zeolite catalyst, the Pt-Z had higher NO_x reduction efficiency, η_NOx = 33%, and lower activation temperature, 250°C, in normal engine operation. It was found that water in the exhaust gas did not apparently affect the NO_x reduction, while the reduction efficiency was significantly affected by the aspect ratio of the catalyst reactor and by the shape of the catalyst, i.e. pellet or honeycomb.     

Performance of the NOx sensor based on mixed potential for automobiles in exhaust gases

The NO_x sensor based on mixed potential was made by laminating YSZ green sheets, on which electrodes including an NO_x sensing electrode, an NO_x conversion electrode, a Pt heater and thermocouple were printed and sintered. The output signal of this sensor was fairly independent of gas temperatures and the velocity of gas flows in the test gases. The engine test for the exhaust gases at around λ=1 showed that the sensor outputs changed corresponding to NO_x concentrations from the gas analyzer. It is expected that this sensor based on mixed potential can be utilized for automobiles.     

Heat exchanger/catalytic system for reducing the exhaust emissions from diesel engines

A system has been researched over the past 3 years for reducing the exhaust pollutants from diesel engines for light commercial vehicles. The system researched achieves Euro 6 standards for reduction of polluting gases (CO, HC, PM, NO). It consists of 4 main sections: 1. A heater and heat exchanger (HE); 2. A CO/HC oxidising catalyst (D°C); 3. Pt catalyst on a diesel particulate filter (DPF); 4. A NO reducing reaction (SCR) within the DPF. The system operates as follows. The exhaust gas contains oxidising gases, namely both O₂ and NO₂. The levels of CO and HC are oxidised by O₂ to CO₂ for temperatures above 200°C. Carbon (PM) is oxidised to CO₂ by NO₂ but requires a temperature above 250°C. The operating exhaust temperature of 300°C is ideal for the removal of NO by using the Pt catalyst and the CO generated within the DPF. The heater is required to be able to raise the exhaust temperature at any time to 300°C in order to optimise the performance of the system, since diesel engine exhaust temperatures vary between 160°C (slow speeds) to 350°C (high speeds). Considerable heat is required (??3 kW) to maintain the exhaust gas for a 2l engine at 300°C for engine idle conditions. Therefore a heat exchanger is required to re-circulate the input heat and thereby reduce the maximum power consumption to a maximum of 500W over the engine full operating test cycle. This energy is supplied by the engine battery and alternator. Experimental results have been obtained for the exhaust from a Kubota diesel engine and the reductions in exhaust emissions of 83% (CO/ HC), 58% (NOx) and 99% (PM) were obtained. The PM was continuously cleaned so that there was no build up of back pressure.     

Combustion characteristics of LPG and biodiesel mixed fuel in two blending ratios under compression ignition in a constant volume chamber

This study aims to investigate the combustion characteristics of mixed fuel of liquefied propane gas (LPG) and biodiesel under compression ignition (CI) in an effort to develop highly efficient and environmentally friendly mixed fuelbased CI engines. Although LPG fuel is known to be eco-friendly due to its low CO₂ emission, LPG has not yet been widely applied for highly efficient CI engines because of its low cetane number and is usually mixed with other types of CI-friendly fuels. In this study, a number of experiments were prepared with a constant volume chamber (CVC) setup to understand the fundamental combustion characteristics of mixed fuel with LPG and biodiesel in two weight-based ratios and exhaust gas recirculation (EGR) conditions. The results from the current investigations verify the applicability of mixed fuel of LPG and biodiesel in CI engines with a carefully designed combustion control strategy that maximizes the benefits of the mixed fuel. Based on the results of this study, ignition is improved by increasing the cetane value by using higher blending ratios of biodiesel. As the blending ratios of biodiesel increased, CO and HC decreased and CO₂ and NO_x increases.     

Majorization of working parameters for non-thermal plasma reactor and impact on no oxidation of diesel engine

The main target of this work is to realize the function of pre-oxidizing NO from diesel engine’s exhaust by using self-designed double-dielectric Non-thermal Plasma (NTP) reactor. The majorized discharge frequency and discharge peak to peak voltage (V_p-p) range for NTP reactor were obtained through air discharge test. The diesel engine test bench was established to observe the effect of NTP on the volume fraction of NO. The results showed that there were more active substances and fewer by-products in NTP reactor when discharge frequency was 9 kHz and V_p-p was between 9 kV and 23 kV; Exhaust flows had insignificant effect on the performance of NTP pre-oxidizing NO; The ability of NTP to pre-oxidize NO gradually weakened with the increase of engine load, and when the engine load were 0 % and 25 %, the ratio of NO/NO₂ could reach 1. In such working conditions, SCR system could improve the conversion rate of the NO_x at low-temperature zone through quick reaction combined with NTP.     

Reduction of engine emissions via a real-time engine combustion control with an egr rate estimation model

Vehicle emissions regulations are becoming increasingly severe and remain a principal issue for vehicle manufacturers. Since, WLTP (Worldwide harmonized Light vehicles Test Procedures) and RDE (real driving emission) regulations have been recently introduced, the engine operating conditions have been rapidly changed during the emission tests. Significantly more emissions are emitted during transient operation conditions compared to those at steady state operation conditions. For a diesel engine, combustion control is one of the most effective approaches to reduce engine exhaust emissions, particularly during the transient operation. The concern of this paper is about reducing emissions using a closed loop combustion control system which includes a EGR rate estimation model. The combustion control system calculates the angular position where 50 % of the injected fuel mass is burned (MFB50) using in-cylinder pressure for every cycle. In addition, the fuel injection timing is changed to make current MFB50 follow the target values. The EGR rate can be estimated by using trapped air mass and in-cylinder pressure when the intake valves are closed. When the EGR rate is different from the normal steady conditions, the target of MFB50 and the fuel injection timing are changed. The accuracy of the model is verified through engine tests, as well as the effect of combustion control. The peaks in NO level was decreased during transient conditions after adoption of the EGR model-based closed loop combustion control system.     

Experimental study on the characteristics of nano-particle emissions from a heavy-duty diesel engine using a urea-SCR system

Particulate matter in diesel engine exhaust, particularly nano-particles, can cause serious human health problems including diseases such as lung cancer. Because diesel nano-particle issues are of global concern, regulations on particulate matter emissions specify that not only the weight of particulate matter emitted but also the concentration of nanoparticles must be controlled. This study aimed to determine the effects on nano-particle and PM emissions from a diesel engine when applying a urea-SCR system for NO_x reduction. We found that PM weight increases by approximately 90% when urea is injected in ND-13 mode over the emission without urea injection. Additionally, PM weight increases as the NH₃/NO_x mole ratio is increased at 250 °C. In SEM scans of the collected PM, spherical particles were observed during urea injection, with sizes of approximately 200 nm to 1 μm. This study was designed to determine the conditions under which nano-particles and PM are formed in a urea-SCR system and to relate these conditions to particle size and shape via a quantitative analysis in ND-13 mode.     

车用直喷式柴油机排气净化的途径

在改善车用柴油机燃油经济性的同时，需进一步降低氮氧化物和微粒排放，关键是进一步优化燃烧过程，减少有害排放物的生成，也要改善燃料品质，甚至进一步采用排气后处理技术，本文阐述了喷油系统和进气系统的改进，燃烧室设计的优化，增压中冷，废气再循环等技术措施的潜力，以及燃油改质，排气后处理等措施的效果。