Numerical and experimental investigation of hydrogen enrichment in a dual-fueled CI engine: A detailed combustion,performance, and emission discussion

An effort has been made to simulation a compression ignition engine using hydrogen-diesel, hydrogen-diethyl ether, hydrogen-n-butanol and base diesel fuel as alternatives. The engine measured for the simulation is a single cylinder, four stroke, direct injection, diesel engine. During the simulation the injection timing and engine speed are kept constant at 23°bTDC and 1500 rpm. Diesel-RK, a piece of commercial software employed for this project, can forecast an engine emission, performance and combustion characteristics. The examination of the anticipated outcomes reveals that adding hydrogen to diesel leads in a small increase in efficiency and fuel consumption. With the usage of hydrogen-blend fuels, the majority of dangerous pollutants in exhaust are greatly decreased. The shortest ignition delay was consistently given by 5H295DEE. The lowest CO₂ (578.61 g/kWh) was given by 5H295nB at CR 19.5. Hydrogen blends increase NOx emissions more than base diesel fuel. In the case of smoke and particulate matter emission, the reduce tendency was seen.     

Studies on the effect of hydrogen induction on performance,emission and combustion behaviour of a WCO emulsion based dual fuel engine

This paper aims at studying the effect of hydrogen induction on engine performance, emission and combustion behaviour of a diesel engine fuelled with the emulsion of used palm oil (called as WCO-waste cooking oil) as pilot fuel and hydrogen as primary fuel. A single cylinder water-cooled direct injection diesel engine was tested at 100% and 40% loads. Results were compared with neat diesel, neat WCO and WCO emulsion at both loads in single fuel operation. WCO emulsion in single fuel mode indicated improvement in performance and reduction in all emissions as compared to neat WCO. Dual fuel operation with hydrogen induction further reduced the emissions of smoke HC and CO with WCO as pilot fuel at all power outputs. However, hydrogen induction resulted in reduced thermal efficiency at 40% load. WCO emulsion showed higher ignition delay as compared to neat WCO. Dual fuel operation with hydrogen induction increased the ignition delay further. Heat release pattern showed higher premixed combustion rate with hydrogen induction mainly at high power outputs. Premixed combustion rate became very high at higher rates of hydrogen admission mainly at high power output. In general, hydrogen induction showed superior performance at high power output and inferior performance at low power output with WCO emulsion as injected fuel.     

Use of hydrogen in dual-fuel diesel engines

Hydrogen is a promising future energy carrier due to its potential for production from renewable resources. It can be used in existing compression ignition diesel engines in a dual-fuel mode with little modification. Hydrogen's unique physiochemical properties, such as higher calorific value, flame speed, and diffusivity in air, can effectively improve the performance and combustion characteristics of diesel engines. As a carbon-free fuel, hydrogen can also mitigate harmful emissions from diesel engines, including carbon monoxide, unburned hydrocarbons, particulate matter, soot, and smoke. However, hydrogen-fueled diesel engines suffer from knocking combustion and higher nitrogen oxide emissions. This paper comprehensively reviews the effects of hydrogen or hydrogen-containing gaseous fuels (i.e., syngas and hydroxy gas) on the behavior of dual-fuel diesel engines. The opportunities and limitations of using hydrogen in diesel engines are discussed thoroughly. It is not possible for hydrogen to improve all the performance indicators and exhaust emissions of diesel engines simultaneously. However, reformulating pilot fuel by additives, blending hydrogen with other gaseous fuels, adjusting engine parameters, optimizing operating conditions, modifying engine structure, using hydroxy gas, and employing exhaust gas catalysts could pave the way for realizing safe, efficient, and economical hydrogen-fueled diesel engines. Future work should focus on preventing knocking combustion and nitrogen oxide emissions in hydrogen-fueled diesel engines by adjusting the hydrogen inclusion rate in real time.     

Effects of hydrogenation of fossil fuels with hydrogen and hydroxy gas on performance and emissions of internal combustion engines

Energy security is an important consideration for development of future transport fuels. Among the all gaseous fuels hydrogen or hydroxy (HHO) gas is considered to be one of the clean alternative fuels. Hydrogen is very flammable gas and storing and transporting of hydrogen gas safely is very difficult. Today, vehicles using pure hydrogen as fuel require stations with compressed or liquefied hydrogen stocks at high pressures from hydrogen production centres established with large investments.Different electrode design and different electrolytes have been tested to find the best electrode design and electrolyte for higher amount of HHO production using same electric energy. HHO is used as an additional fuel without storage tanks in the four strokes, 4-cylinder compression ignition engine and two-stroke, one-cylinder spark ignition engine without any structural changes. Later, previously developed commercially available dry cell HHO reactor used as a fuel additive to neat diesel fuel and biodiesel fuel mixtures. HHO gas is used to hydrogenate the compressed natural gas (CNG) and different amounts of HHO-CNG fuel mixtures are used in a pilot injection CI engine. Pure diesel fuel and diesel fuel + biodiesel mixtures with different volumetric flow rates are also used as pilot injection fuel in the test engine. The effects of HHO enrichment on engine performance and emissions in compression-ignition and spark-ignition engines have been examined in detail. It is found from the experiments that plate type reactor with NaOH produced more HHO gas with the same amount of catalyst and electric energy. All experimental results from Gasoline and Diesel Engines show that performance and exhaust emission values have improved with hydroxy gas addition to the fossil fuels except NO_x exhaust emissions. The maximum average improvements in terms of performance and emissions of the gasoline and the diesel engine are both graphically and numerically expressed in results and discussions. The maximum average improvements obtained for brake power, brake torque and BSFC values of the gasoline engine were 27%, 32.4% and 16.3%, respectively. Furthermore, maximum improvements in performance data obtained with the use of HHO enriched biodiesel fuel mixture in diesel engine were 8.31% for brake power, 7.1% for brake torque and 10% for BSFC.     

Using HHO (Hydroxy) and hydrogen enriched castor oil biodiesel in compression ignition engine

Hydrogen and HHO enriched biodiesel fuels have not been investigated extensively for compression ignition engine. This study investigated the performance and emissions characteristics of a diesel engine fueled with hydrogen or HHO enriched Castor oil methyl ester (CME)-diesel blends. The production and blending of CME was carried out with a 20% volumetric ratio (CME20) using diesel fuel. In addition, the enrichment of intake air was carried out using pure HHO or hydrogen through the intake manifold with no structural changes – with the exception of the reduction of the amount of diesel fuel – for a naturally aspirated, four cylinder diesel engine with a volume of 3.6 L. Hydrogen amount was kept constant with a ratio of 10 L/min throughout the experiments. Engine performance parameters, including Brake Power, Brake Torque, Brake Specific Fuel Consumption and exhaust emissions – including NOx and CO, – were tested at engine speeds between 1200 and 2600 rpm. It is seen that HHO enriched CME has better results compared to pure hydrogen enrichment to CME. An average improvement of 4.3% with HHO enriched CME20 was found compared to diesel fuel results while pure hydrogen enriched CME20 fuel resulted with an average increase of 2.6%. Also, it was found that the addition of pure hydrogen to CME had a positive effect on exhaust gas emissions compared to that adding HHO. The effects of both enriched fuels on the engine performance were minimal compared to that of diesel fuel. However, the improvements on exhaust gas emissions were significant.     

Experimental investigations on a hydrogen diesel homogeneous charge compression ignition engine with exhaust gas recirculation

In this experimental study, hydrogen was inducted along with air and diesel was injected into the cylinder using a high pressure common rail system, in a single cylinder homogeneous charge compression ignition engine. An electronic controller was used to set the required injection timing of diesel for best thermal efficiency. The influences of hydrogen to diesel energy ratio, output of the engine and exhaust gas recirculation (EGR) on performance, emissions and combustion were studied in detail. An increase in the amount of hydrogen improved the thermal efficiency by retarding the combustion process. It also lowered the exhaust emissions. Large amounts of hydrogen and EGR were needed at high outputs for suppressing knock. The range of operation was brake mean effective pressures of 2–4 bar. The levels of HC and CO emitted were not significantly influenced by the amount of hydrogen that was used.     

Experimental investigation of the performance and exhaust emissions of a swirl chamber diesel engine using JP-8 aviation fuel

An experimental study is conducted to evaluate the use of JP-8 aviation fuel as a full substitute for diesel fuel in a Ricardo E-6 high-speed naturally-aspirated four-stroke experimental engine having a swirl combustion chamber. The study covers a wide range of engine load and speed operating conditions, comprising measurements of cylinder pressure diagrams, high-pressure fuel pipe pressures, exhaust gas temperatures, fuel consumptions, exhaust smokiness and exhaust gas emissions (nitrogen oxides, unburned hydrocarbons and carbon monoxide). Processing of the measurements provides important performance parameters such as maximum combustion pressure, dynamic injection timing, ignition delay, combustion irregularity and knocking tendency. The differences in the measured performance and exhaust emission parameters are determined for engine operation with JP-8 fuel, against baseline engine operation using diesel fuel. The study shows that the exhaust emission levels are not much different for operation with the two fuels. On the contrary, operation with JP-8 fuel increases combustion pressures, combustion intensity and irregularity. This is caused mainly by high pressure fluctuations present in the fuel injection system due to the different physical properties of JP-8 fuel (compared to diesel fuel), which totally change the injection characteristics. Retardation of the static injection timing is one means of improving this situation, while using the same fuel injection equipment. © 1997 John Wiley & Sons, Ltd.     

正丁醇/柴油发动机燃烧排放特性及喷油、燃烧室形状影响

针对某型号直喷柴油机,建立了该柴油机中单缸完整燃烧室及气道三维模型,使用三维计算流体力学(computational fluid dynamics,CFD)分析软件CONVERGE对其进行模拟计算,研究了正丁醇掺混比例对柴油机燃烧排放的影响。结果表明:随着正丁醇掺混比例的提高,峰值缸压、滞燃期和燃烧速度均呈递增趋势,碳烟及CO排放量逐渐减少,NO_x排放量小幅增加。为了进一步改善缸内燃烧情况和降低污染物排放,对正丁醇掺混时喷油策略、燃烧室几何形状的综合影响进行了研究,结果表明:掺混时多次喷油及采用合适的燃烧室模型可以有效改善掺混后缸内油气混合情况,增加缸内湍动能强度,进一步降低碳烟排放量。与纯柴油工况对比,掺混并采用多次喷油策略后碳烟排放明显下降,且通过掺混能够有效简化喷油策略,但弱化了燃烧室形状对碳烟排放量的影响。     

Analysis of ignition delay by taking Di-tertiary-butyl peroxide as an additive in a dual fuel diesel engine using hydrogen as a secondary fuel

Ignition delay (ID) is one of the important parameters that make influenced on the combustion process inside the cylinder. This ignition delay affects not only the performances but also the noise and emissions of the engine. In this regards the experiments were conducted on single cylinder 4–stroke compression ignition research diesel engine, power 3.50 kW at constant speed 1500 rpm Kirloskar model TV1 with base fuel as diesel and hydrogen as secondary fuel with and without Di-tertiary-butyl-peroxide (DTBP). Experiments were conducted to measure the ignition delay of the dual fuel diesel (DFD) engine at different load conditions and substitution of diesel by hydrogen with or without DTBP and then it was compared with predicted ID given by Hardenberg-Hase equation and modified Hardenberg-Hase equation.The experimental values of ignition delay were compared with theoretical ignition delay which was predicted on the basis of Hardenberg-Hase equation by considering mean cylinder temperature, pressure, activation energy and cetane number and variations are found in between 6.60% and 21.22%. While, the Hardenberg-Hase equation was modified (by considering variation in activation energy) for DFD engine working on diesel as primary fuel and hydrogen as secondary fuel shows variations 1.20%–11.96%. Furthermore, with DTBP it gives variation up to 18.01%. It was found that ID decreases with increase in percentage of DTBP and hydrogen in air-fuel mixture. This might be due to the cetane improver nature of DTBP, pre-ignition reaction rate and energy release rate of hydrogen fuel. The polytropic index get increased by addition of (Di-tert butyl peroxide) DTBP. Similarly, 5% Di tertiary butyl peroxide reduces Ignition delay.     

Numerical investigation and Pareto optimization on performance,emission and in-cylinder reforming characteristics for two cylinder reciprocating engine with COx free fuel

The main objective of this study is to introduce the applicability of ammonia to the downsized compression ignition diesel engine for power generation or range extender. For this research objective, the two cylinder engine, which was the result of the previous study, fueled with diesel-ammonia blends was considered and the performance and NOx emission tendency were identified using the numerical method. Ammonia was mixed with diesel via injection at a specific fuel energy fraction (0%, 5%, 10%, or 15%) to evaluate the engine performance and emission characteristics. In addition, concept of “in-cylinder reforming” was introduced adopting negative valve overlap (NVO) by advancing the exhaust valve closing time to investigate the effect of adding ammonia as a hydrogen carrier. Subsequently, the primary variables affecting the brake-specific fuel consumption and NO_X are determined via multi-objective Pareto analysis. The optimal Pareto front confirms that exhaust valve timing exerts a greater effect on the performance and emissions than injection timing. Moreover, in-cylinder reformed hydrogen was increased under negative valve overlap strategy.     

Analysis of ignition delay period of a dual fuel diesel engine with hydrogen and LPG as secondary fuels

In this study, experiments were performed on 4 cylinder turbocharged, intercooled with 62.5 kW gen-set diesel engine by using hydrogen, liquefied petroleum gas (LPG) and mixture of LPG and hydrogen as secondary fuels. The experiments were performed to measure ignition delay period at different load conditions and various diesel substitutions. The experimental results have been compared with ignition delay correlation laid down by other researchers for diesel and dual fuel diesel engine. It is found that ignition delay equation based on pressure, temperature and oxygen concentration for a dual fuel diesel engine run on diesel-biogas gives variation up to 6.56% and 14.6% from the present experimental results, while ignition delay equation for a pure diesel engine gives 7.55% and 33.3% variation at lower and higher gaseous fuel concentrations, respectively. It is observed that the ignition delay of dual fuel engine depends not only on the type of gaseous fuels and their concentrations but also on charge temperature, pressure and oxygen concentration.     

Hydrogen addition to tea seed oil biodiesel: Performance and emission characteristics

Compression ignition engines are the dominant tools of the modern human life especially in the field of transportation. But, the increasing problematic issues such as decreasing reserves and environmental effects of diesel fuels which is the energy source of compression ignition engines forcing researchers to investigate alternative fuels for substitution or decreasing the dependency on fossil fuels. The mostly known alternative fuel is biodiesel fuel and many researchers are investigating the possible raw materials for biodiesel production. Also, hydrogen fuel is an alternative fuel which can be used in compression ignition engines for decreasing fuel consumption and hazardous exhaust emissions by enriching the fuel. In this study, influences of hydrogen enrichment to diesel and diesel tea seed oil biodiesel blends (B10 and B20) were investigated on an unmodified compression ignition engine experimentally. In consequence of the experiments, lower torque and higher brake specific fuel consumption data were measured when the engine was fuelled diesel biodiesel blends (B10 and B20) instead of diesel fuel. Also, diesel biodiesel blends increased CO₂ and NO_x emissions while decreasing the CO emissions. Hydrogen enrichment (5 l/m and 10 l/m) was improved the both torque and brake specific fuel consumption for all test fuels. Furthermore, hydrogen enrichment reduced CO and CO₂ emissions due to absence of carbon atoms in the chemical structure for all test fuels. Increasing flow rate of hydrogen fuel from 5 l/m to 10 l/m further improved performance measures and emitted harmful gases except NO_x. The most significant drawback of the hydrogen enrichment was the increased NO_x emissions.     

A comprehensive study on performance,emission and combustion behavior of a compression ignition engine fuelled with WCO (waste cooking oil) emulsion as fuel

This work aims at evaluating the performance, emission and combustion of a diesel engine fuelled with WCO (waste cooking oil obtained from palm oil) and its emulsion as fuel. A single cylinder water-cooled diesel engine was used. Base data was generated with diesel and neat WCO as fuels. Subsequently, WCO oil was converted into its emulsion and tested. Neat WCO resulted in higher smoke, hydrocarbon and carbon monoxide emissions as compared to neat diesel. Significant reduction in all emission was achieved with the WCO emulsion. Cylinder peak pressure and maximum rate of pressure rise were found to be higher with WCO emulsion as compared to neat WCO mainly at high power outputs. Ignition delay was found as higher with neat WCO and its emulsion. It is concluded that WCO emulsion can be used in diesel engines without any modifications in the engine with superior performance and reduced emissions at high power outputs.     

Influence of hydrogen co-combustion with diesel fuel on performance,smoke and combustion phases in the compression ignition engine

The main objective of this study was to examine impact of hydrogen addition to the compression ignition engine fueled with either rapeseed methyl ester (RME) or 7% RME blended diesel fuel (RME7) on combustion phases and ignition delay as well as smoke and exhaust toxic emissions. Literature review shows in general, hydrogen in those cases is used in small amounts below lower flammability limits. Novelty of this work is in applying hydrogen at amounts up to 44% by energy as secondary fuel to the compression ignition engine. Results from experiments show that increase of hydrogen into the engine makes ignition delay shortened that also affects main combustion phase. In all tests the trends of exhaust HC and CO toxic emissions vs. hydrogen addition were negative. The trend of smokiness decreased steadily with increase of hydrogen. Amounts of hydrogen addition by energy share were limited to nearly 35% due to combustion knock occurring at nominal load.     

Performance and combustion characteristic of CI engine fueled with hydrogen enriched diesel

The combustion of hydrogen–diesel blend fuel was investigated under simulated direct injection (DI) diesel engine conditions. The investigation presented in this paper concerns numerical analysis of neat diesel combustion mode and hydrogen enriched diesel combustion in a compression ignition (CI) engine. The parameters varied in this simulation included: H₂/diesel blend fuel ratio, engine speed, and air/fuel ratio. The study on the simultaneous combustion of hydrogen and diesel fuel was conducted with various hydrogen doses in the range from 0.05% to 50% (by volume) for different engine speed from 1000 – 4000 rpm and air/fuel ratios (A/F) varies from 10 – 80. The results show that, applying hydrogen as an extra fuel, which can be added to diesel fuel in the (CI) engine results in improved engine performance and reduce emissions compared to the case of neat diesel operation because this measure approaches the combustion process to constant volume. Moreover, small amounts of hydrogen when added to a diesel engine shorten the diesel ignition lag and, in this way, decrease the rate of pressure rise which provides better conditions for soft run of the engine. Comparative results are given for various hydrogen/diesel ratio, engine speeds and loads for conventional Diesel and dual fuel operation, revealing the effect of dual fuel combustion on engine performance and exhaust emissions.     

Effect of advanced injection timing on the performance of rapeseed oil in diesel engines

Combustion studies on both diesel fuel and vegetable oil fuels, with the standard and advanced injection timing, were carried out using the same engine and test procedures so that comparative assessments may be made. The diesel engine principle demands self-ignition of the fuel as it is injected at some degrees before top dead centre (BTDC) into the hot compressed cylinder gas. Longer delays between injection and ignition lead to unacceptable rates of pressure rise with the result of diesel knock because too much fuel is ready to take part in premixed combustion. Alternative fuels have been noted to exhibit longer delay periods and slower burning rate especially at low load operating conditions hence resulting in late combustion in the expansion stroke. Advanced injection timing is expected to compensate these effects. The engine has standard injection timing of 30°C BTDC. The injection was first advanced by 5.5°C given injection timing of 35.5°C BTDC. The engine performance was very erratic on this timing. The injection was then advanced by 3.5°C and the effects are presented in this paper. The engine performance was smooth especially at low load levels. The ignition delay was reduced through advanced injection but tended to incur a slight increase in fuel consumption. Moderate advanced injection timing is recommended for low speed operations.     

Experimental investigation on a DI dual fuel engine with hydrogen injection

Over the past two decades considerable efforts have been undertaken to develop and introduce new alternative fuels for the conventional gasoline and diesel. Many alternative fuels, both liquid and gaseous, have been experimented and some have even been commercialized such as ethanol, natural gas, etc. Hydrogen has been considered as an excellent fuel to replace the petroleum‐based fuels due to its clean burning characteristics. In the present experimental investigation, hydrogen was injected in the intake manifold and diesel fuel was injected inside the engine cylinder in the conventional manner. Hydrogen injection parameters such as injection timing, injection duration and quantity of hydrogen injected were optimized based on the performance and emission characteristics. Exhaust gas recirculation (EGR) technique was adopted to reduce the oxides of nitrogen emission. From the results it was observed that for hydrogen diesel dual fuel (DF) engine, the optimal operating parameters for hydrogen injection were start of injection at gas exchange top dead centre with injection duration of 30° crank angle with the hydrogen flow rate of 7.5 litres per minute (lpm). With EGR the optimized condition was found to be 20% for the entire load. The brake thermal efficiency with 20% EGR increases by 16% at 75% load as compared with diesel, while at full load it reduces by 8% due to the recirculation of exhaust gases that results in a reduction of intake oxygen concentration compared with part load. NO_X emission decreases by five and half times, while other emissions increase by 1.4 times as compared with DF engine. Copyright © 2008 John Wiley & Sons, Ltd.     

Flame lift-off length and soot production of oxygenated fuels in relation with ignition delay in a DI heavy-duty diesel engine

The relation between ignition delay and flame lift-off length (based on OH* chemiluminescence) has been studied in an optically accessible heavy-duty diesel engine. Soot production has been studied as well, using both exhaust soot data and high-speed imaging of the in-cylinder natural luminosity. Since the luminosity is estimated to scale with T¹³ for our experimental conditions, the local temperature becomes a decisive factor in the interpretation of natural luminosity images. The fuels used include regular diesel and blends of various oxygenates (cyclohexanone, anisole, and dibutyl maleate) with synthetic diesel, to cover a wide range of cetane numbers and oxygen content.Based on a literature review, we hypothesize that the flame speed only determines an upper limit for the flame lift-off length (rather than its exact position). Under conditions that promote auto-ignition, the lift-off length may be shorter, and is likely governed by ignition chemistry.For the oxygenated fuels, the flame lift-off length is found to increase with ignition delay. An inverse correlation is found between the soot luminosity and the oxygen ratio at the lift-off length. The anisole-blend does not produce measurable amounts of exhaust soot as long as the injection duration is shorter than the ignition delay. Under cooled EGR-like conditions, the ignition delay is so large that anisole is considered a candidate fuel for soot-free operation of heavy-duty diesel engines up to medium load.     

柴油机燃用低比例小球藻生物柴油-柴油混合燃料的试验研究

以野生小球藻生物柴油(Chlorella Biodiesel Fuel,CBF)-柴油作为混合燃料,利用186FA柴油机进行台架试验。在CBF的掺混比例分别为0%,3%,5%(B0,B3,B5)时,对柴油机的动力性、燃料燃用的经济性和燃烧及排放特性进行了比较分析。试验分析表明:柴油机燃用混合燃料时,与燃用纯柴油相比,随着CBF掺混比例的增加,其扭矩和功率略有下降,最大降幅均为4%;柴油机的油耗率和能耗率略有上升,且在高、中负荷时更为明显;柴油机的缸内压力、放热率峰值稍有减小,而压力升高率峰值稍有增大,缸内压力峰值最大降幅为3.4%,放热率峰值最大降幅为12.8%,压力升高率峰值最大增幅为13.7%;柴油机滞燃期缩短了0.5~1.0°CA、燃烧持续期延长了1.0~2.0°CA,缸内压力、压力升高率和放热率峰值的出现时刻均提前了1.0~2.0°CA,燃烧速度加快;HC,CO和碳烟的排放均有所降低,而NOX的排放逐渐增多,全负荷时HC和碳烟排放的最大降幅分别为14.1%和31.7%,NOX排放的最大增幅为9.7%,CO排放的降幅为6%~12%。     

Numerical Investigation of the Effect of Hydrogen Enrichment on an Opposed-Piston Compression Ignition Diesel Engine

High power-to-weight and fuel efficiency are bounded with opposed-piston compression ignition(OPCI) engine, which makes it ideal in certain applications. In the present study, a dynamic three-dimensional CFD model was established to numerically investigate the combustion process and emission formation of a model OPCI engine with hydrogen enrichment. The simulation results indicated that a small amount of hydrogen was efficient to improve the indicated power owing to the increased in-cylinder pressure. Hydrogen tended to increase the ignition delay of diesel fuel due to both dilution and chemical effect. The burning rate of diesel fuel was apparently accelerated when mixing with hydrogen and premixed combustion became dominated. NO_x increased sharply while soot was sufficiently suppressed due to the increase of in-cylinder temperature. Preliminary modifications on diesel injection strategy including injection timing and injection pressure were conducted. It was notable that excessive delayed injection timing could reduce NO_x emission but deteriorate the indicated power which was mainly attributed to the evident decline of hydrogen combustion efficiency. This side effect could be mitigated by increasing the diesel injection pressure. Appropriate delay of injection coupled with high injection pressure was suggested to deal with trade-offs among NO_x, soot and engine power.