High Exhaust Gas Recirculation Rates Research Articles

Closed-loop combustion control of biodiesel–diesel blends in premixed operating conditions enabled via high exhaust gas recirculation rates

Stringent emissions and fuel economy regulations motivate the investigation of advanced combustion strategies such as premixed charge compression ignition. However, controlling premixed charge compression ignition is challenging as there is no direct trigger for the combustion event. In addition, fuel-flexible applications must account for the impact of variable fuel properties on the premixed combustion proces. This paper identifies the control challenges in fuel-flexible premixed charge compression ignition and presents a control strategy for mitigating the differences in combustion phasing, the increases in nitrogen oxide emissions, and the decreases in power output typically encountered when operating with biodiesel in premixed operating conditions. When biodiesel is used in premixed operating conditions with the engine operating with the stock calibration for diesel fuel, the timing of the start of combustion and the peak heat release can shift by over 2° crank angle and 4° crank angle respectively, the nitrogen oxide emissions can increase by over 100%, and the torque output can drop by over 30% with respect to the engine performance with diesel. The control techniques presented in this paper use a biodiesel estimation method in conjunction with a physics-based accommodation strategy based on accounting for the differences between the fuel energy densities and the oxygen contents of diesel and biodiesel. This control method dictates fueling quantities on an energy basis instead of the traditional mass basis and maintains a nearly constant in-cylinder oxygen fraction for both diesel and biodiesel. At the premixed operating points considered in this study, this strategy is demonstrated to eliminate increases in the biodiesel nitrogen oxide emissions, decreases in the torque, and shifts in the combustion timing. The results demonstrate, first, that combustion timing control and nitrogen oxide emissions control can also be achieved through control of the in-cylinder oxygen fraction alone and, second, that energy-based fueling provides consistent in-cylinder oxygen fractions for diesel and biodiesel while also matching the torque outputs for the two fuels when considered at the same premixed operating point (the same speed, intake manifold temperature, charge flow, effective compression ratio, and combustion timing).

Corrosion Behavior of Aluminum Alloy for Heat Exchanger in an Exhaust Gas Recirculation System of Diesel Engine

This study examines the corrosion behavior of aluminum alloys in condensed water containing different concentrations of anions for a heat exchanger in an automotive exhaust gas recirculation (EGR) system of a diesel engine. Electrochemical measurements and bulk analyses revealed that the aluminum alloy is protected by the adsorption of increased nitrite and nitrate ions contained in condensed water, despite increased detrimental sulfate ions at high EGR rates. The mechanism of the beneficial anions, which are nitrite and nitrate on aluminum alloy in condensed water, was spontaneous physical adsorption (physisorption).

Numerical analysis on knock for a high compression ratio spark-ignition methanol engine

Methanol is an alternative, renewable, environmentally and economically attractive fuel. It is considered to be one of the most favorable fuels for internal combustion engines. In this paper, knock in a high compression ratio spark-ignition methanol engine was modeled and investigated using FIRE Code CFD (Computational Fluid Dynamics) calculations. Knocking combustion was simulated under different operating conditions. Finally, the effects of spark timing and EGR (Exhaust Gas Recirculation) techniques for suppressing knock in a high compression ratio spark-ignition methanol engine were studied based on the numerical analysis through the analysis of the thermal efficiency. The results showed that with the increase of the EGR, the thermal efficiency was greatly reduced and the knock occurrence timing was postponed. The use of a high EGR rate and early spark timing had a better thermal efficiency on the knock suppression as compared to the use of a low EGR rate and late spark timing.

Experimental study on the performance of and emissions from a low-speed light-duty diesel engine fueled with n-butanol–diesel and isobutanol–diesel blends

The effects of isobutanol- and n-butanol-enriched diesel fuel on the diesel engine performance and emissions are investigated. Neat diesel, 15% isobutanol–85% diesel, 30% isobutanol–70% diesel, 15% n-butanol–85% diesel, and 30% n-butanol–70% diesel blends are investigated in this study. The tests were carried out at light and medium loads and a fixed low engine speed, and by using various combinations of the exhaust gas recirculation rate and the injection timing to investigate the effect of the molecular structure difference on soot formation. The results show that n-butanol–diesel blends give a longer ignition delay than isobutanol–diesel blends do. Hence isobutanol has a higher peak cylinder pressure and a higher premixed heat release rate than n-butanol does. Adding butanol (isobutanol and/or n-butanol) to diesel fuel is able to decrease the soot emissions substantially, while the change in the nitrogen oxide emissions varies slightly. Soot emissions from n-butanol–diesel blends are lower than those from isobutanol–diesel blends. Introducing exhaust gas recirculation and retarding the injection timing are effective approaches to decrease the nitrogen oxide emissions. However, a high exhaust gas recirculation rate leads to a loss in the fuel efficiency. The combination of a low exhaust gas recirculation rate, later injection and butanol blends can achieve low-temperature combustion and simultaneously decrease the nitrogen oxide and soot emissions.

Effects of hydraulic nozzle flow rate and high injection pressure on mixture formation, combustion and emissions on a single-cylinder DI light-duty diesel engine

Injection pressure and hydraulic nozzle flow rate are significant parameters that influence mixture formation and combustion in the direct injection (DI) diesel combustion process. An injection pressure increase enhances mixture formation as well as combustion. Decreasing the hydraulic flow rate, by means of reducing spray hole orifice diameter, affects mixture formation primarily through improved gas entrainment processes. Both injection pressure increase and hydraulic flow rate decrease therefore benefit mixture formation and combustion, which cause less soot emissions. Thus, the application of higher exhaust gas recirculation rates to decrease nitrogen oxide emissions is possible. The objective of this paper is to determine the effects of hydraulic flow rate variation in combination with very high injection pressures. Engine tests are carried out using a single-cylinder research engine, which is based on a light-duty DI diesel engine. With a displacement of 755 cm3 per cylinder, the engines range between typical passenger car (≈ 500 cm3) and medium-sized heavy-duty (≈ 1000 cm3) engines. The engine is equipped with a prototype common rail fuel injection system that is capable of delivering system pressures larger than 250 MPa. Four different hydraulic nozzle layouts are tested. The effects are presented for one part load and one full load operating condition. For each engine operating point, exhaust gas recirculation variations with different injection pressure levels and boost conditions are conducted. Evaluation of engine test results is performed at a constant soot/nitrogen oxide emission ratio of 1:10, which is relevant in engine calibration. Hydraulic measurements and the application of an empirical spray model support the interpretation of the engine test results. It is observed that a reduced hydraulic flow rate is beneficial for part load emission performance. It was found that a lower hydraulic flow rate causes a higher mean relative air/fuel ratio within the fuel spray and thus induces less soot formation. Moreover, the premixed fraction of combustion is enhanced through both injection pressure and hydraulic flow rate increases. In full load operation, the beneficial effect of reduced flow rates by means of smaller nozzle orifices cannot be detected. Parameters such as injection duration, combustion duration and spray momentum during spray/wall interaction are identified as more important. Boost pressure variation has only a minor effect on part load, whereas full load results are greatly influenced. In part load and full load operation, a limitation of the potential to reduce emissions is not observed within the investigated injection pressure range.

Parametric study on particle size and SOF effects on EGR cooler fouling

Abstract Recent diesel engine technologies, developed for enhanced regulation of exhaust emissions, are characterized by high exhaust gas recirculation (EGR) rates and high-pressure fuel injection. The use of high EGR rates, by which high temperatures can be avoided in a cylinder, is an effective method for reduction of nitrogen oxide (NO x ) emissions. High-pressure fuel injection leads to smaller soot particles, which decreases mass-based soot emission. These technologies, however, also have effects on particulate matter (PM) characteristics that are closely related to EGR cooler fouling. High-pressure fuel injection, which makes smaller soot particles, increases the concentration of soot particles, and a high EGR rate, which lowers the temperature in the cylinder, increases the soluble organic fraction (SOF). In this study, we evaluated the effects of these changes in PM characteristics on EGR cooler fouling. Instead of an engine-based experiment, in which a parametric study is nearly impossible, a laboratory experiment was performed to separate variables. A soot generator was used to make model exhaust gas because the variables could be controlled separately (e.g., mean particle size and concentration of soot particles) and this improved the reproducibility of the experiments. Additionally, n-dodecane, a model compound representing diesel fuel, was vaporized and injected into the exhaust gas to test the effects of SOF on cooler fouling. For particle sizes ranging from 41 to 190 nm in diameter, the deposition fraction was inversely proportional to particle size. Thus, smaller soot particles in the exhaust gas were more likely to cause formation of thermophoretic deposits on the wall of the EGR cooler. At an EGR gas temperature of 350 °C, the deposition fraction was greatest (84%) for the smallest particle size of 41 nm, whereas the deposition fraction was least (7%) for the largest particle size, 190 nm. The performance degradation of the EGR cooler showed a similar trend to the measured deposit mass. As coolant temperature decreased, the effect of n-dodecane injection on the growth of PM deposits increased, which significantly reduced the overall thermal conductivity of the EGR cooler. This result showed that ‘wet soot’ caused greater contamination of the EGR cooler.

Effects of different cetane number enhancement strategies on HCCI combustion and emissions

Cetane number is the accepted indicator for quantifying the autoignition characteristics of diesel fuels in compression ignition engines. Diesel fuel specifications typically require a minimum cetane number to achieve satisfactory combustion behaviour in conventional diesel engines. In contrast, a high cetane number fuel may not be beneficial for implementing high efficiency, clean combustion strategies such as homogeneous charge compression ignition (HCCI). The purpose of this study was to investigate cetane number effects on HCCI combustion and emissions. The experiments were conducted in a single-cylinder, variable compression ratio, cooperative fuel research engine operated in HCCI combustion mode. The fuels were finely atomized and partially vaporized in the intake manifold. The base fuel was a low cetane refining stream derived from oil sands sources. Three different methods were employed to increase the base fuel cetane number, namely hydroprocessing, cetane improver addition, and blending with a renewable fuel component. Results show that the three methods of cetane number enhancement produce significantly different HCCI combustion behaviour. The hydroprocessed fuels exhibited more stable and complete combustion than the base fuel, which resulted in a wider operating region, reduced carbon monoxide, unburned hydrocarbon, and nitrogen oxide (NO x) emissions, and lower indicated specific fuel consumption (ISFC). The main disadvantages of the hydroprocessed fuels were the higher exhaust gas recirculation rates required to retard the combustion phasing, which limits the maximum indicated mean effective pressure for a given intake pressure, and increased knock intensity due to a faster combustion process. In comparison, the other two methods of fuel cetane enhancement increased ISFC compared to the base fuel. The addition of nitrate cetane improver resulted in higher NO x emissions, while blending with a renewable fuel component increased hydrocarbon emissions. The experimental data provide evidence that the magnitude and phasing of low temperature heat release, as well as fuel volatility, play important roles in HCCI combustion.

Effects of Compression Ratio and Simulated EGR on Combustion Characteristics and Exhaust Emissions of a Diesel PCCI Engine

The effects of compression ratio and simulated exhaust-gas recirculation (EGR) on combustion characteristics and exhaust emissions of a diesel PCCI engine were investigated using a single-cylinder test engine. Tests were carried out under constant speed with various compression ratios and EGR rates. Exhaust emissions and in-cylinder pressure were measured for all experimental conditions. Analyses based on engine performance and exhaust emissions were conducted. An optimum compression ratio that provided better indicated thermal efficiency and IMEP while yielding lower emissions of smoke, HC, and CO, and reasonable NOx without EGR was identified. High rates of EGR led to the simultaneous reduction of NOx and soot emissions due to a lower combustion temperature compared with conventional diesel combustion, with a slight penalty in HC and CO.

Transient Control of Combustion Phasing and Lambda in a Six-Cylinder Port-Injected Natural-Gas Engine

Fuel economy and emissions are the two central parameters in heavy duty engines. High exhaust gas recirculation rates combined with turbocharging has been identified as a promising way to increase the maximum load and efficiency of heavy duty spark ignition engines. With stoichiometric conditions, a three way catalyst can be used, which keeps the regulated emissions at very low levels. The Lambda window, which results in very low emissions, is very narrow. This issue is more complex with transient operation, resulting in losing brake efficiency and also catalyst converting efficiency. This paper presents different control strategies to maximize the reliability for maintaining efficiency and emissions levels under transient conditions. Different controllers are developed and tested successfully on a heavy duty six-cylinder port injected natural gas engine. Model predictive control was used to control lambda, which was modeled using system identification. Furthermore, a proportional integral regulator combined with a feedforward map for obtaining maximum brake torque timing was applied. The results show that excellent steady-state and transient performance can be achieved.

Numerical simulation of diesel combustion with a high exhaust gas recirculation rate

A series calculation method from injector nozzle internal flow to in-cylinder combustion in diesel engines has been developed. In this study, the calculation results of in-cylinder pressure, heat release rate, and NO concentration are compared with experimental results for conventional combustion conditions as well as an advanced combustion condition, which combines a high exhaust gas recirculation (EGR) rate, high boost pressure, and high pressure injection. The calculated pressure and heat release rate histories are in reasonable agreement with those of experiments, but the NO concentration is underestimated especially on the higher EGR rate condition. Therefore, an investigation to improve the quantitative accuracy of NO concentration is performed. The probability density function (PDF) method is one way to estimate the accurate mean reaction rate containing the reaction rate fluctuation by turbulence. In this study, the PDF method only applied for NO calculation is developed to balance the calculation accuracy and the computational cost. The result shows that the reaction rate fluctuation, on NO formation, has a small effect on conventional combustion conditions, but increases with decreasing combustion temperature.

Effects of ethyl tert-butyl ether addition to diesel fuel on characteristics of combustion and exhaust emissions of diesel engines

The effects of ethyl tert-butyl ether (ETBE) addition to diesel fuel on the characteristics of combustion and exhaust emissions of a common rail direct injection diesel engine with high rates of cooled exhaust gas recirculation (EGR) were investigated. Test fuels were prepared by blending 0, 10, 20, 30 and 40 vol% ETBE to a commercial diesel fuel. Increasing ETBE fraction in the fuel helps to suppress the smoke emission increasing with EGR, but a too high fraction of ETBE leads to misfiring at higher EGR rates. While the combustion noise and NO x emissions increase with increases in ETBE fraction at relatively low EGR rates, they can be suppressed to low levels by increasing EGR. Though there are no significant increases in THC and CO emissions due to ETBE addition to diesel fuel in a wide range of EGR rates, the ETBE blended fuel results in higher aldehyde emissions than the pure diesel fuel at relatively low EGR rates. With the 30% ETBE blended fuel, the operating load range of smokeless, ultra-low NO x (<0.5 g/kWi h), and efficient diesel combustion with high rates of cooled EGR is extended to higher loads than with the pure diesel fuel.

Numerical investigation of soot reduction potentials with diesel homogeneous charge compression ignition combustion by an improved phenomenological soot model

An improved phenomenological soot model coupled with a reduced n-heptane chemical mechanism was implemented into KIVA-3V code to describe soot formation and oxidation processes in diesel homogeneous charge compression ignition (HCCI) combustion. This model was first validated by the shock tube experiments with a rich n-heptane mixture over wide temperature and pressure ranges. The computational results demonstrate that the phenomenological soot model is capable of predicting the soot yield, particle diameter, and number density with satisfactory accuracy. Then the model was applied to investigate the influence of the orifice diameter and injection pressure on soot emissions in a constant-volume combustion vessel under typical diesel combustion conditions. The predictions showed qualitative agreement with the measurements on the soot volume fraction distribution. The results also indicate that the soot formation can almost be suppressed as the local equivalence ratio is kept lower than 2.0. Finally, the model was used to explore the potentials of soot reduction with HCCI combustion for diesel engines. The overall trend of soot with the variations in the start of injection timing was well reproduced by the model. With the help of an equivalence ratio—temperature map, it was found that nitrogen oxide emissions could be markedly reduced by applying a high exhaust gas recirculation rate and relative low compression ratio for diesel HCCI engines. However, the mixture preparation by using a multi-hole injector with early injection strategy remains a limitation for further reduction in soot emissions.

Laser-assisted homogeneous charge ignition in a constant volume combustion chamber

Homogeneous charge compression ignition (HCCI) is a very promising future combustion concept for internal combustion engines. There are several technical difficulties associated with this concept, and precisely controlling the start of auto-ignition is the most prominent of them. In this paper, a novel concept to control the start of auto-ignition is presented. The concept is based on the fact that most HCCI engines are operated with high exhaust gas recirculation (EGR) rates in order to slow-down the fast combustion processes. Recirculated exhaust gas contains combustion products including moisture, which has a relative peak of the absorption coefficient around 3 μm. These water molecules absorb the incident erbium laser radiations ( λ=2.79 μm) and get heated up to expedite ignition. In the present experimental work, auto-ignition conditions are locally attained in an experimental constant volume combustion chamber under simulated EGR conditions. Taking advantage of this feature, the time when the mixture is thought to “auto-ignite” could be adjusted/controlled by the laser pulse width optimisation, followed by its resonant absorption by water molecules present in recirculated exhaust gas.

Influence of high rates of supplemental cooled EGR on NOxand PM emissions of an automotive HSDI diesel engine using an LP EGR loop

Previous experimental studies on diesel engine have demonstrated the potential of exhaust gas recirculation (EGR) as an in-cylinder NOx control method. Although an increase in EGR at constant boost pressure (substitution EGR) is accompanied with an increase in particulate matter (PM) emissions in the conventional diesel high-temperature combustion (HTC), the recirculation of exhaust gases supplementary to air inlet gas (supplemental EGR) by increasing the boost pressure has been suggested as a way to reduce NOx emissions while limiting the negative impact of EGR on PM emissions. In the present work, a low-pressure (LP) EGR loop is implemented on a standard 2.0 l automotive high-speed direct injection (HSDI) turbocharged diesel engine to study the influence of high rates of supplemental cooled EGR on NOx and PM emissions. Contrary to initial high-pressure (HP) EGR loop, the gas flow through the turbine is unchanged while varying the EGR rate. Thus, by closing the variable geometry turbine (VGT) vanes, higher boost pressure can be reached, allowing the use of high rates of supplemental EGR. Furthermore, recirculated exhaust gases are cooled under 50°C and water vapour is condensed and taken off from the recirculated gases. An increase in the boost pressure at a given inlet temperature and dilution ratio (DR) results in most cases an increase in NOx emissions and a decrease in PM emissions. The result of NOx–PM trade-off, while varying the EGR rate at fixed inlet temperature and boost pressure depends on the operating point: it deteriorates at low load conditions, but improves at higher loads. Further improvement can be obtained by increasing the injection pressure. A decrease by approximately 50% of NOx emissions while maintaining PM emission level, and brake specific fuel consumption can be obtained with supplemental cooled EGR owing to an LP EGR loop, compared with the initial engine configuration (HP moderately cooled EGR). Copyright © 2008 John Wiley & Sons, Ltd.

Hydrogen–natural gas blends fuelling passenger car engines: Combustion, emissions and well-to-wheels assessment

In this study, a state of the art passenger car natural gas engine was optimized for hydrogen–natural gas mixtures and high exhaust gas recirculation (EGR) rates in the major part of the engine map. The investigations involved stoichiometric combustion. With optimal combinations of spark timing and EGR rate, the achievements are efficiency increase with substantially lower engine-out NO x while total unburned hydrocarbons or CO-engine-out emissions are only modestly affected. The efficiency is increased by 3% in the low load and by more than 5% in the medium-load domain. Increasing hydrogen content of the fuel accelerates combustion leading to the efficiency improvements. Combustion analysis showed that the increasing burning rates mainly affected the initial combustion phase (duration for 5% mass-fraction burned). Nevertheless, increase of the hydrogen fraction in the fuel over a certain threshold did not result in any efficiency increase in the medium loads. Loss analysis identified high wall heat losses as the main reason. Dedicated combustion chamber design may be able to avoid these losses and lead to additional efficiency benefits. Well-to-wheel analysis revealed paths for the production of the fuel blends still having overall energy requirements slightly higher than a diesel benchmark vehicle but reducing by 7% overall green house gas emissions.

HCCI Diesel Engine Control Design using Advanced Simulation with Real Time Capabilities

An engine model is a very flexible tool to perform a large amount of tests over a wide range of operating conditions and to access to more detailed information than those available with experiments. Therefore, the use of the numerical engine simulation in the control development process can significantly reduce the test time needed on real hardware while providing more information for understanding the system physics. Based on high-performance physical models such as HCCI combustion model, the virtual engine simulator is able to support the control design from controller development to real-time calibration. The aim of this communication is to show the relevance of using such a model-based approach for current control issues. The demonstration is achieved through the challenging case of Diesel HCCI combustion engine. As an alternative to expensive and complex after-treatment solutions, Diesel HCCI engine takes benefit of high exhaust gas recirculation rate to limit in-cylinder pollutant production. Therefore, it is recognized as one of the most promising ways for new generation of CI engines but also one of the most complex engine technologies to control due to the sensitiveness of HCCI combustion mode to small perturbations.

A methodology to identify the intake charge cylinder-to-cylinder distribution in turbocharged direct injection Diesel engines

Exhaust gas recirculation (EGR) is currently the most important NOx emission control system. During the last few years the EGR rate has increased progressively as pollutant emission regulations have become more restrictive. High EGR rate levels have given the effect of the unsuitable EGR and air distribution between cylinders away, which causes undesirable engine behavior. In this sense, the study of the EGR distribution between cylinders achieves high importance. However, despite the fact that the EGR is continuously under study, not many studies have been undertaken to approach its distribution between cylinders. In concordance with the aspects outlined before, the aim of this paper is to propose a methodology that permits us to identify the EGR cylinder-to-cylinder dispersion in a commercial engine. In order to achieve this objective, experimental tests have been combined with both one-dimensional and three-dimensional fluid dynamic models.

Influence of Pre Turbo Catalyst Design on Diesel Engine Performance, Emissions and Fuel Economy

Influence of Pre Turbo Catalyst Design on Diesel Engine Performance, Emissions and Fuel Economy

Optical Diagnostics of Late-Injection Low-Temperature Combustion in a Heavy-Duty Diesel Engine

A late-injection, high exhaust-gas recirculation rate, low-temperature combustion strategy is investigated in a heavy-duty diesel engine using a suite of optical diagnostics: chemiluminescence for visualization of ignition and combustion, laser Mie scattering for liquid-fuel imaging, planar laser-induced fluorescence (PLIF) for both OH and vapor-fuel imagings, and laser-induced incandescence for soot imaging. Fuel is injected at top dead center when the in-cylinder gases are hot and dense. Consequently, the maximum liquid-fuel penetration is 27 mm, which is short enough to avoid wall impingement. The cool flame starts 4.5 crank angle degrees (CAD) after the start of injection (ASI), midway between the injector and bowl rim, and likely helps fuel to vaporize. Within a few CAD, the cool-flame combustion reaches the bowl rim. A large premixed combustion occurs near 9 CAD ASI, close to the bowl rim. Soot is visible shortly afterward, along the walls, typically between two adjacent jets. OH PLIF indicates that premixed combustion first occurs within the jet and then spreads along the bowl rim in a thin layer, surrounding soot pockets at the start of the mixing-controlled combustion phase near 17 CAD ASI. During the mixing-controlled phase, soot is not fully oxidized and is still present near the bowl rim late in the cycle. At the end of combustion near 27 CAD ASI, averaged PLIF images indicate two separate zones. OH PLIF appears near the bowl rim, while broadband PLIF persists late in the cycle near the injector. The most likely source of broadband PLIF is unburned fuel, which indicates that the near-injector region is a potential source of unburned hydrocarbons.

Effects of exhaust gas recirculation on diesel particulate matter morphology and NOx emissions

Diesel particulate morphology and nitrogen oxides (NO x) emissions were investigated in detail to reveal the effects of exhaust gas recirculation (EGR). The different rates of EGR were precisely controlled by using a customized engine control unit in a 1.7 l turbocharged common-rail direct-injection diesel engine. The tests, which combined two different EGR modes (i.e. constant boost pressure (CBP) and constant oxygen-to-fuel ratio (COFR)), were designed to decouple the effects of EGR thermal and dilution processes. Particulate samples were collected directly from the raw engine exhaust by using a novel thermophoretic soot-sampling system. The samples were examined and imaged with a high-resolution transmission electron microscope and quantitatively analysed by using a customized image-processing/data-acquisition system. Results showed that the particulate dimensions, number density of primary particles, and soot yield all changed significantly under various EGR rates. The NO x emissions also varied significantly as the EGR rate changed, showing a typical trade-off with respect to the data measured for particulate emissions. At low EGR rates, the thermal effect was the dominant phenomenon that affected the changes of the measured morphological characters, while at higher EGR rates the dilution effect became more important. However, the fractal geometry of diesel particulates did not change significantly between the two EGR modes, suggesting that the influence of EGR dilution was less than that of the thermal process. EGR operation providing a COFR at the same EGR rate yielded a significant benefit in particulate emissions and engine power output, while still maintaining the reduction of NO x emissions at a satisfactory level.