Modern Diesel Engines Research Articles

Progression of Soot Cake Layer Properties During the Systematic Regeneration of Diesel Particulate Filters Measured with Neutron Tomography

Although particulate filters (PFs) have been a key component of the emission control system for modern diesel engines, there remain significant questions about the basic regeneration behavior of the filters and how it changes with accumulation of increasing soot layers. This effort describes a systematic deposition and regeneration of particulate matter in 25-mm diameter × 76-mm long wall-flow PFs composed of silicon carbide (SiC) material. The initial soot distributions were analyzed for soot cake thickness using a nondestructive neutron imaging technique. With the PFs intact, it was then possible to sequentially regenerate the samples and reanalyze them, which was performed after nominal 20, 50, and 70 % regenerations. The loaded samples show a relatively uniform distribution of particulate with an increasing soot cake thickness and nearly identical initial density of 70 mg/cm3. During regeneration, the soot cake thickness initially decreases significantly while the density increases to 80–90 mg/cm3. After ∼50 % regeneration, the soot cake thickness stays relatively constant, but instead, the density decreases as pores open up in the layer (∼35 mg/cm3 at 70 % regeneration). Complete regeneration initially occurs at the rear of the PF channels. With this information, a conceptual model of the regeneration is proposed.

Characterization of Exhaust Gas Recirculation for Diesel Low Temperature Combustion

Exhaust Gas Recirculation (EGR) is common on most modern diesel engines resulting in significant reduction of NOx emissions. Heavy EGR application is an enabling technique for the advanced combustion modes operating in the low temperature combustion (LTC) regime, wherein simultaneous NOx and soot emission reduction can be attained. The primary effect of EGR is the dilution of the intake charge following the displacement of fresh air by the combustion products. However, the correlation between EGR and its effectiveness is non-linear due to the lean burn nature of boosted diesel engines. This correlation is further complicated when oxygenated fuels are used for combustion in the LTC mode. In this work, the intake oxygen concentration [O2]int is selected as a representative of EGR and its effectiveness in emission abatement is shown using an array of experimental results. An EGR characterization model is developed to quantify the dynamic interaction between [O2]int and engine operating variables, namely the intake boost, exhaust gas recirculation (EGR) amount, the fueling quantity and the fuel type. The model is validated on the research engine platform in steady state and transient tests. Finally, the control of EGR effectiveness is experimentally demonstrated to achieve ultra-low NOx emissions at different engine operating points.

APPROVAL TESTS AND EVALUATION OF EMISSION PROPERTIES OF VEHICLE

Operation of vehicles equipped with combustion engines is a source of pollution. That is why the emissions from this source are restricted. The emissions reduction system consists of several steps, whose basic component is a typeapproval of vehicle in terms of emissions level and admission to the registration of vehicles that meets the requirements for emissions. It is assumed, that the gradual reduction of emissions limits will improve the emission properties of the vehicles, and gradual withdrawal of used vehicles, will reduce of emissions growth. The basis for issuing opinions on a vehicle's exhaust emissions is the measurement of emissions performed in the laboratory on a chassis dynamometer in reproducible conditions of preparing the vehicle for testing, test replication, fuel for the engine, and the procedure for collecting the exhaust gas analysis. Motor Transport Institute conducts the type approval tests, as well as the research on emissions from vehicles in the real traffic conditions, which are used to determine the actual emission from the vehicle population. Emission tests for applications, that go beyond the application of type approval show, that there are vehicles whose emissions in operation is substantially different from the emissions measured during type approval. It is a natural phenomenon in the case of used vehicles and vehicles with engine malfunctions or exhaust aftertreatment system defect. However, there are vehicles whose emissions under specific operating traffic conditions are significantly different from the values obtained in the course of type approval tests, despite the good properties of emission measured under the conditions provided by the type-approval procedure. The article discusses the results of the emissions from vehicles, obtained in the tests used to determine the emission factors for vehicles used in real traffic conditions, thus taking into account emissions from vehicles, made under different conditions than the type approval tests, which show that, in terms of driving cycles, which are not covered by the emission test procedure, can multiply and exceed the approved values.. This is particularly true for modern diesel engines.

IMPACT ANALYSIS OF INTERNAL CATALYST CONVERTER ON OPERATING PARAMETERS OF VW 1.9 TDI ENGINE

The largest amount of air pollutants is produced by the combustion of fossil fuels (coal, lignite, petroleum, natural gas, etc.). Such combustion releases huge amounts of greenhouse gases into the atmosphere (including carbon oxides (CO, CO2), sulphur oxides (SO2, SO3), nitrogen oxides (NO, NOx)) and dust, ashes and soot. Transportation is a significant share of the air pollution, mainly wheel-based transportation. The combustion process in diesel engine is very complex physico-chemically and varies in time. The process is accompanied by simultaneous exchange of heat and mass and chemical reactions. The development of internal combustion engines is made towards meeting of the increasingly stringent requirements for toxic exhaust emissions, reduction of fuel consumption, and preservation of Earth's natural resources. The problem to solve in modern diesel engines is the emission of toxic compounds. One way to reduce emissions is to use a catalytic converter positioned directly in the combustion chamber of an internal combustion engine. The aim of this analysis is assessment of toxic emissions of a diesel engine with an internal catalyst. Qualitative assessment of engine emissions was determined by comparing test results of an engine with ceramic-ally coated valves against one with just conventional valves (commercial valves without a ceramic coating). The study utilized a fourcylinder 1.9 TDI VW engine. It was expected that the use of glow plugs with an active coating would reduce the release of toxic compounds.

The Dynamics of Second Ring Flutter and Collapse in Modern Diesel Engines

Second ring fluttering and radial ring collapse are recognized as having significant influences on engine blowby and oil consumption. As the gas flow is coupled with the piston ring motion, understanding the ring dynamics is important for understanding not only the engine blowby mechanism, but also oil consumption mechanisms and how to control them. Only second ring flutter and collapse that occurs around the top dead center (TDC) firing conditions is examined in this paper based on a modern heavy-duty diesel engine. However, the principles described are equally applicable to all engines. First, the authors describe the fundamental mechanisms of how second ring fluttering and radial ring collapse occur. This is described by examining the forces that are acting on the second ring. Then, two cases are shown. One case shows second ring flutter and the other case shows stable second ring motion. The reasons for these two different cases are explained, including the effect of static twist and the end gaps of the rings. A sensitivity study was performed to evaluate the effect of changing the top and second ring end gaps on ring lift. It was shown how the gaps could affect the second ring flutter and ring collapse. It is concluded that the second ring will be more likely to flutter or collapse if it has a negative static twist, if the second ring end gap is large, and/or if the top ring end gap is small. If the second ring does not flutter, it may still be possible to design the ring pack such that there is not any reverse blowby. However, this must be carefully studied and controlled or the second land pressures will be too high, resulting in reverse blowby and/or top ring lifting.

Improvements in thermal efficiency of premixed diesel combustion with optimization of combustion-related parameters and fuel volatilities

The influence of combustion-related parameters and fuel volatility on the premixed diesel combustion was experimentally investigated in a modern direct injection diesel engine, and the evaporation process of the fuel spray in the combustion chamber was analyzed with computational fluid dynamics simulation. By optimizing the fuel injection timing and the intake oxygen content, ultralow NOx and smokeless premixed diesel combustion with high thermal efficiency and acceptable levels of CO and total hydrocarbon emissions is possible with both diesel fuel and normal heptane. The optimum fuel injection timing for the indicated thermal efficiency is obtained when the fuel spray does not enter the squish area, maintaining the 50% heat release crank angle at around top dead center. The indicated thermal efficiencies reach the maximum at around 12% intake oxygen concentration. The indicated thermal efficiency in the premixed diesel combustion with normal heptane is slightly higher than with diesel fuel and is very similar to the conventional diesel combustion in a wide indicated mean effective pressure range of below 0.8 MPa. The indicated thermal efficiency decreases when advancing injection timings mainly due to the deterioration in the combustion efficiency when the fuel is injected to the outside of the piston cavity. The degree of decrease in the indicated thermal efficiency with advancing injection timings is more significant with diesel fuel than with normal heptane due to the wall wetting.

Computational fluid dynamics modeling of combustion in heavy-duty diesel engines

The design and optimization stages of combustion systems for modern heavy-duty diesel engines must be supported by reliable computational fluid dynamics tools for the definition of the chamber geometry and injection strategy. To be fully predictive in terms of in-cylinder thermodynamics and flame structure, the employed combustion models must account for complex chemistry and turbulence–kinetics interactions. Within this context, the authors have implemented into an open-source code a model based on the multiple representative interactive flamelets approach and applied it to diesel combustion simulations. New numerical techniques were integrated in the proposed multiple representative interactive flamelets model in order to speed up the central processing unit time in integrating chemistry and the β-probability density function of the chemical species to compute composition in the computational fluid dynamics domain. A parallel validation was performed both with constant-volume and heavy-duty diesel engine experiments, selecting similar operating conditions. In such way, both flame structure and heat release rate predictions are analyzed and the model capabilities with respect to its setup and mesh structure are assessed.

Development of an electrically driven diesel fuel pump for commercial vehicles using HILS

Common-rail fuel injection systems on modern diesel engines have fuel pumps that are mechanically driven by the crankshaft. The pumps actually house two pumping elements: a low pressure element and a high pressure element. Part of the fuel compressed by the low pressure element returns to the tank in the process of maintaining the pressure in the common-rail at adequate levels for injection. Since the returning fuel represents pumping energy loss, fuel economy improves if the returned fuel can be reduced by using a properly controlled electrical fuel pump. As the first step in developing an electrical fuel pump the fuel supply system on a six-liter diesel engine was modeled with AMESim to analyze the workload and the fuel feed rate of the injection pump; the results served as the basis for selecting a suitable servo-motor to drive the pump. A motor controller was built using a DSP and a program which controls the common-rail pressure using a proportional control method based on the target fuel pressure information from the engine ECU. Test equipment was also fabricated to evaluate the performance of the electrically driven fuel pump. In a simulated vehicle test during the first 200 seconds of the NEDC driving schedule, the electric pump satisfied the fuel pressure and flow demand of the engine, consuming only 48.3% of the energy that an engine-driven pump would require.

Development of PN emission factors for the real world urban driving conditions of a hybrid city bus

The effects of basic engine operating parameters on particle emissions from a hybrid city bus were examined under urban driving conditions involving both constant speed and frequent acceleration. To perform the examinations, real-time data for the parameters and solid Particle Number (PN) emissions were synchronously collected. The driving conditions were specifically chosen because, depending on the driving conditions, PN emissions from a city bus may change by an order of magnitude. A PN measurement system highly sensitive to engine transients and background emissions was specifically chosen, because the particle emission levels typically found in modern diesel engines are relatively low.The measurements indicated that while operating over a flat route, the PN emissions of the bus were maximized at full-load accelerations and minimized at a constant speed of 70km/h, which is the maximum speed. Because the combustion process in a diesel engine is relatively stable under constant speed driving conditions, the particle formation was minimized. During acceleration, however, the particle formation is accelerated due to well-known turbo-lag phenomena that limit the intake air flow rate. For these reasons, there is no simple linear correlation between the PN concentration and the engine operating parameters for driving conditions that involve both acceleration and constant speed driving. However, it was observed that there is a strong correlation between the cumulative PN emissions and cumulative brake engine energy (BEE). The R2 values of the linear regression lines were almost 1.0 when the acceleration and constant speed operating conditions of the bus were separately evaluated.Because a city bus operates under real-world conditions involving both accelerating and constant speed driving conditions, a real world PN emission factor was suggested as a function of the maximum, minimum PN emissions (#/kWh) of the bus and a traffic intensity factor.

Direct injection of neat n-butanol for enabling clean low temperature combustion in a modern diesel engine

This study investigates the effects of neat n-butanol replacing conventional diesel fuels to enable clean combustion on a modern common-rail diesel engine. Systematic engine experiments are conducted to examine the combustion characteristics and exhaust emissions in correlation to n-butanol’s relatively high oxygen content and high volatility but low ignitability, and control strategies are thereafter developed for enabling clean and efficient combustion of neat n-butanol. Compared to its diesel counterpart, the single-shot injection of neat n-butanol offers substantially reduced NOx emissions without the use of EGR and near-zero soot emissions, but the applicable injection timing window is narrower for n-butanol limited by high maximum rates of pressure rise and/or unstable combustion. EGR is effective to reduce the combustion roughness, but it further narrows the applicable injection timing window and deteriorates the HC and CO emissions. A control strategy that deploys multi-shot injections combined with moderate use of EGR is developed and applied to improve the combustion controllability and exhaust emissions while minimizing the penalties in the engine efficiency.

Evaluation of the Effect of Fuel Properties on the Fuel Spray and Jet Characteristics in a HGV DI Diesel Engine Operated by Used Cooking Oils

Fuel injection systems in modern diesel engines are designed and built to comply with very stringent environmental standards. They should also meet the highest level of fuel economy. Drivability, rapid response and easy and accurate control are a common demand. Changing the fuel characteristics could affect the performance of the fuel injection system. This study focuses on the evaluation of fuel spray characteristics of straight used cooking oil (SUCO) and its blends with petroleum diesel (PD) as a surrogate for pure PD. Used cooking oil blends have quite different physical properties from those of pure PD. Data for the lower heating value (LHV), density and viscosity were obtained from laboratory analysis. These data were merged with the physical and thermodynamic conditions of the diesel engine of interest to evaluate the dynamic behaviour of the fuel jet in 360° of crank rotation namely, the compression stroke, and the power stroke including the injection process. Engine operational conditions were calculated using a diesel dual thermodynamic cycle taking into account fuel injection adjustment at three different speeds, namely, idle speed, maximum torque speed and rated power speed. The results showed that fuel jet characteristics vary with SUCO content in the fuel blend. Two ranges of SUCO content in the blends were distinguished, 0 – 80% SUCO content and 80 – 100% SUCO content. Both showed a constant rate of change of jet characters per 10% increase in SUCO content in the fuel blend. Lower rates of change of fuel characters were observed at 0-80% SUCO content. The higher the temperature, the lower the rate of change of fuel jet characteristics.

A study on the 0D phenomenological model for diesel engine simulation: Application to combustion of Neem methyl esther biodiesel

The design and monitoring of modern diesel engines running on alternative fuels require reliable models that can validly substitute experimental tests and predict their operating characteristics under different load conditions. Although there exists a multitude of models for diesel engines, 0D phenomenological models present the advantages of giving fast and accurate computed results. These models are useful for predicting fuel spray characteristics and instantaneous gas state. However, there are few reported studies on the application of 0D phenomenological models on biodiesel fuel combustion in diesel engines.This work reports the elaboration, validation and application on Neem methyl ester biodiesel (NMEB) combustion of a 0D phenomenological model for diesel engine simulation.The model addresses some specific aspects of diesel engine modeling found in previous studies such as the compromise between computers cost, accurateness and model simplicity, the reduction of the number of empirical fitting constant, the prediction of combustion kinetics with reduction of the need of experimental curve fitting, the ability to simultaneously predict under various loads engine thermodynamic and spray parameters as well as emission characteristics and finally the ability to simulate diesel engine parameters when fueled by alternative fuels.The proposed model predicts fuel spray behavior, in cylinder combustion and nitric oxides (NOx) emissions. The model is implemented through a Matlab code. The model is mainly based on Razlejtsev’s spray evaporation model, Watson’s double Wiebe function and Ferguson’s tabulated chemistry thermodynamic cycle algorithm. NOx emissions are implemented in the model using the extended Zeldovich mechanism, heat transfer and ignition delay are also taken into account in the model.Following the validation in different operating points of the model on a four stroke 1 – cylinder direct injection diesel engine, the operating characteristics were predicted with an average error accuracy of about 15%. The relative error is, respectively, 10% for Sauter diameter, 12% for maximum penetration, 9.8% for peak pressure, 7.38% for peak temperature, 10.7% for NOx emission and 13.7% for engine power output. The model predicts higher Sauter mean diameter, higher fuel spray length and NOx emission for NMEB as compared to conventional diesel fuel which is in accordance with experimental results obtained on biodiesel fuels in previous researches. Methyl butyrate and methyl butanoate thermochemical data were used as surrogates for NMEB, the results obtained from the model were close, independently of the surrogate used for biodiesel. The average simulation time for a full cycle simulation was about 94.22s, which gives to the model applicability for diesel engine optimization purposes. The results obtained were found satisfactory in terms of accuracy, algorithm simplicity and computer cost.

Influence of fuel molecular structure on the volatility and oxidative potential of biodiesel particulate matter.

We have studied the effect of chemical composition of biodiesel fuel on the physical (volatility) and chemical (reactive oxygenated species concentration) properties of nano particles emitted from a modern common-rail diesel engine. Particle emissions from the combustion of four biodiesels with controlled chemical compositions and different varying unsaturation degrees and carbon-chain lengths, together with a commercial diesel, were tested and compared in terms of volatility of particles and the amount of reactive oxygenated species carried by particles. Different blends of biodiesel and petro diesel were tested at several engine loads and speeds. We have observed that more saturated fuels with shorter carbon chain lengths result in lower particle mass but produce particles that are more volatile and also have higher levels of Reactive Oxygen Species. This highlights the importance of taking into account metrics that are relevant from the health effects point of view when assessing emissions from new fuel types.

Control-Oriented Gas Exchange Model for Diesel Engines Utilizing Variable Intake Valve Actuation

Modeling and control of the gas exchange process in modern diesel engines is critical for the promotion and control of advanced combustion strategies. However, most modeling efforts to date use complex stand-alone simulation packages that are not easily integrated into, or amenable for the synthesis of, engine control systems. Simpler control-oriented models have been developed; however, in many cases, they do not directly capture the complete dynamic interaction of air handling system components and flows in multicylinder diesel engines with variable geometry turbocharging (VGT), high pressure exhaust gas recirculation (EGR), and flexible intake valve actuation. Flexibility in the valvetrain directly impacts the gas exchange process not only through the effect on volumetric efficiency but also through the combustion process and resulting exhaust gas enthalpy utilized to drive the turbomachinery. This paper describes a low-order, five state model of the air handling system for a multicylinder variable geometry turbocharged diesel engine with cooled EGR and flexible intake valve actuation, validated against 286 steady state and 62 transient engine operating points. The model utilizes engine speed, engine fueling, EGR valve position, VGT nozzle position, and intake valve closing (IVC) time as inputs to the model. The model outputs include calculation of the engine flows as well as the exhaust temperature exiting the cylinders. The gas exchange model captures the dynamic effects of the not only the standard air handling actuators (EGR valve position and VGT position) but also IVC timing, exercised over their useful operating ranges. The model's capabilities are enabled through the use of analytical functions to describe the performance of the turbocharger, eliminating the need to use look-up maps; a physically based control-oriented exhaust gas enthalpy submodel and a physically based volumetric efficiency submodel.

Analysis of Fuel Injection Parameter on Biodiesel and Diesel Spray Characteristics Using Common Rail System

Precise control of fuel injection is essential in modern diesel engines especially in controlling the precise injection quantity, flexible injection timing, flexible rate of injection with multiple injections and high injection pressures. It was known that the fuel-air mixing is mainly influenced by the fuel injection system and injector nozzle characteristics. Thus, mixture formation during ignition process associated with the exhaust emissions. The purpose of this study is to investigate the influence of spray characteristics on the mixture formation. In this study, common rail injector systems with different model of injector were used to simulate the actual mixture formation inside the engine chamber. The optical visualization system was constructed with a digital video camera in order to investigate the detailed behavior of mixture formation. This method can capture spray penetration length, spray angle, spray evaporation and mixture formation process clearly. The spray characteristic such as the penetration length, spray angle and spray area are increasing when the injection pressure increased. The mixture formation can be improved effectively by increasing the injection pressure.

Turbocharger Map Reduction for Control-Oriented Modeling

Models of the gas exchange process in modern diesel engines typically use manufacturer-provided maps to describe mass flows through, and efficiencies of, the turbine and compressor based on pressure ratios across the turbine and compressor, as well as the turbocharger shaft speed, and in the case of variable-geometry turbochargers, the nozzle position. These look-up maps require multiple interpolations to produce the necessary information for turbocharger performance, and are undesirable when modeling for estimation and control. There have been several previous efforts to reduce dependence on maps with general success, yet many of these approaches remain complex and are not easily integrated into engine control systems. The focus of this paper is the reduction of turbomachinery maps to analytical functions that are amenable to estimator and control design, and have been validated against manufacturer-provided turbomachinery data.

Exhaust gas recirculation for advanced diesel combustion cycles

Modern diesel engines tend to utilize significantly large quantities of exhaust gas recirculation (EGR) and high intake pressures across the engine load range to meet NOx targets. At such high EGR rates, the combustion process and exhaust emissions tend to exhibit a marked sensitivity to small changes in the EGR quantity, resulting in unintended deviations from the desired engine performance characteristics (energy efficiency, emissions, stability). An accurate estimation of EGR and its effect on the intake dilution are, therefore, necessary to enable its application during transient engine operation or unstable combustion regimes. In this research, a detailed analysis that includes estimation of the transient (cycle-by-cycle) build-up of EGR and the time (engine cycles) required to reach the steady-state EGR operation has been carried out. One-step global equations to calculate the transient and steady-state gas concentrations in the intake and exhaust are proposed. The effects of engine load and intake pressure on EGR have been examined and explained in terms of intake charge dilution and in-cylinder excess-air ratio. The EGR analysis is validated against a wide range of empirical data that include low temperature combustion cycles, intake pressure and load sweeps. This research intends to not only formulate a clear understanding of EGR application for advanced diesel combustion but also to set forth guidelines for transient analysis of EGR.

Air-fraction modeling for simultaneous Diesel engine NOx and PM emissions control during active DPF regenerations

This study develops a control-oriented air-fraction model which can accurately predict the air-fraction dynamics for a Diesel engine and the coupled aftertreatment system (mainly Diesel oxidation catalyst (DOC)) during active DPF regenerations. NOx and particulate matter (PM) emissions are the two main concerns for modern Diesel engine development. Diesel particulate filters (DPFs) have become a standard device for reducing the PM emissions from Diesel engine powertrains. To maintain high filtration efficiencies and avoid high back pressures, active DPF regenerations have to be implemented periodically by significantly elevating the exhaust gas temperatures. However, during active DPF regenerations, the selective catalytic reduction (SCR) deNOx performance may be significantly decreased, while the high temperature durability and total hydrocarbon (THC) poisoning of SCRs may be potential issues. To simultaneously meet the NOx emission standard, high-pressure loop EGR needs to be applied. Since in-cylinder NOx reduction relies heavily on the in-cylinder oxygen content, an accurate air-fraction model is necessary, and the effects of recycled THC and CO emissions should be considered. The developed air-fraction model has been experimentally validated to show its capability of capturing the actual air fraction dynamics accurately. Such a model can be instrumental in simultaneous NOx and PM emission control during active DPF regenerations and in advanced combustion mode control.

HILS기반 상용차 디젤엔진용 연료펌프의 전기구동 시스템 적용에 관한 연구

Fuel injection pressure has steadily increased in diesel engines for the purpose of improving fuel efficiency and cleaning exhaust gas, but it has now reached a point, where the cost for higher pressure does not warrant additional gains. Common rail systems on modern diesel engines have fuel pumps that are mechanically driven by crankshaft. The pumps actually house two pumping module inside: a low pressure pump component and a high pressure pump component. Part of the fuel compressed by the low pressure component returns to the tank in the process of maintaining the pressure in the common rail. Since the returning fuel represents pumping loss, fuel economy improves if the returned fuel can be eliminated by using a properly controled electrical fuel pump. As the first step in developing an electrical fuel pump the fuel supply system on a 6 liter diesel engine was modeled with AMESim to analyze the workload and the fuel feed rate of the injection pump, and the results served as basis for selecting a suitable servo motor and a reducer to drive the pump. A motor controller was built using a DSP and a program which controls the common rail pressure using a proportional control method based on the target fuel pressure information from the engine ECU. A test rig to evaluate performance of the fuel pump is implemented and used to show that the newly developed electrically driven fuel pump can satisfy the fuel flow demand of the engine under various operating conditions when the rotational speed of the pump is adequately controlled.

An Equivalent Emission Minimization Strategy for Causal Optimal Control of Diesel Engines

One of the main challenges during the development of operating strategies for modern diesel engines is the reduction of the CO2 emissions, while complying with ever more stringent limits for the pollutant emissions. The inherent trade-off between the emissions of CO2 and pollutants renders a simultaneous reduction difficult. Therefore, an optimal operating strategy is sought that yields minimal CO2 emissions, while holding the cumulative pollutant emissions at the allowed level. Such an operating strategy can be obtained offline by solving a constrained optimal control problem. However, the final-value constraint on the cumulated pollutant emissions prevents this approach from being adopted for causal control. This paper proposes a framework for causal optimal control of diesel engines. The optimization problem can be solved online when the constrained minimization of the CO2 emissions is reformulated as an unconstrained minimization of the CO2 emissions and the weighted pollutant emissions (i.e., equivalent emissions). However, the weighting factors are not known a priori. A method for the online calculation of these weighting factors is proposed. It is based on the Hamilton–Jacobi–Bellman (HJB) equation and a physically motivated approximation of the optimal cost-to-go. A case study shows that the causal control strategy defined by the online calculation of the equivalence factor and the minimization of the equivalent emissions is only slightly inferior to the non-causal offline optimization, while being applicable to online control.