Engine-out Nitrogen Oxides Research Articles

A New Control-Oriented Semi-Empirical Approach to Predict Engine-Out NOx Emissions in a Euro VI 3.0 L Diesel Engine

The present study is focused on the development of a new control-oriented semi-empirical model to predict nitrogen oxide (NOx) emissions in a light-duty diesel engine under both steady-state and transient conditions. The model is based on the estimation of the deviations of NOx emissions, with respect to the nominal engine-calibration map values, as a function of the deviations of the intake oxygen concentration and of the combustion phasing. The model also takes into account the effects of engine speed, total injected quantity, and ambient temperature and humidity. The approach has been developed and assessed on an Fiat Powertrain Technologies (FPT) Euro VI 3.0 L diesel engine for light-duty applications, in the frame of a research project in collaboration with FPT Industrial. The model has also been tested on a rapid prototyping device, and it was shown that it requires a very short computational time, thus being suitable for implementation on the Engine Control Unit (ECU) for real-time NOx control tasks.

Exploitation of injection fusion strategies in diesel engines equipped with solenoid injectors

Pilot–main injection strategies are investigated in the short dwell-time range at medium load and speed conditions for a low-temperature combustion diesel engine endowed with indirect-acting solenoid injectors. Dwell-time sweeps have been carried out for pilot injections with different energizing times under steady-state working conditions and constant combustion phasing. The experimental results on the engine are combined with in-cylinder analyses of pressure, heat release rate, temperature and emission time histories. As the electric dwell time is reduced toward zero, combustion noise reaches a minimum at a dwell time of 120–140 µs, and a reduction of 1–4.5 dB is possible compared to levels at dwell time = 500 µs. This decrease in combustion noise is obtained in conjunction with a local reduction in engine-out nitrogen oxide emissions, but penalties occur for engine-out soot emissions and brake-specific fuel consumption. The pilot and main injection shots feature fusion phenomena at the shortest dwell-time values, thus giving rise to rate-shaped single injections, which resemble the boot-shaped profiles in direct-acting piezoelectric injectors. The increase in the velocity of the needle, which occurs during the nozzle opening phase pertaining to the main injection for closely coupled injections as dwell time is reduced, is shown to be not the primary cause responsible for the observed trends in combustion noise and soot emissions as dwell-time changes. The combination of simulation and experimental data supports the theory that the main injection interacts with the pilot mixture field: the relative phasing between pilot and main combustion and the in-cylinder equivalent ratio distribution are the dominant aspects in the definition of the engine performance in the short dwell-time range.

A numerical investigation of the effects of combustion parameters on the performance of a compression ignition engine toward NOx emission reduction

In this paper, a detailed multi-dimensional simulation based on fuel chemical reactions has been presented to investigate the effects of the most important parameters on performance and emission of a reactivity controlled compression ignition (RCCI) engine. The different characteristics of combustion phasing such as start of combustion, combustion duration, and 10%–90% of mass fraction burned (MFB) for iso-octane/n-heptane fuels have been studied. The effects of each important parameter such as amount of fuel delivered by varying fuels ratio, engine load and speed, injection timing, equivalence ratio, exhaust gas recirculation (EGR) application, boost pressure, injection pressure as well as combination of these different parameters and improved fuel octane number at higher compression ratios were evaluated to discover the most effective parameter on performance and emission of an RCCI engine. Distribution and coefficient of variation (COV) of equivalence ratio were examined in base cases to evaluate the influence of high reactivity fuel on distribution points. In addition, to acquiring a comprehensive perception, the results highlighted that by concurrently applying different parameters with diverse effects, new corridors to significantly abate nitrogen oxides (NOx) pollutant can be achievable depending on the specific situations. Generally, the parameters that prolong the combustion duration demonstrated positive effects on combustion process leading to cleaner combustion with lower engine-out NOx.

Impact of propane energy fraction on diesel-ignited propane dual fuel low temperature combustion

This paper focuses on quantifying the effects of percent energy substitution of propane on Diesel-ignited Propane Dual Fuel Low Temperature Combustion (DPDFLTC) performed in a single cylinder research engine (SCRE). All the experiments were performed with the engine load and speed held constant at 1500rev/min and 3.3bar BMEP, respectively. The intake pressure and temperature were held constant at 150kPa and 30°C, respectively, the diesel injection timing and injection pressure were maintained at 310 CAD (50deg. BTDC) and 500bar, while the propane energy fraction (PEF) was varied throughout the experiment. The results indicate that the maximum PEF was limited by to 90% due to high cyclic combustion variability (COV imep∼11%), and the minimum PEF was limited to 53% due to onset of engine knock (MPRR∼10.5bar/deg). Additionally, the engine-out unburned hydrocarbon emissions decreased from nearly 40g/kWh at 90%PEF to about 10g/kWh at 53% PEF, the engine-out carbon monoxide emissions decreased from nearly 30g/kWh 90% PEF to about 5g/kWh at 53% PEF and the engine-out nitrogen oxide emissions increased from 0.1g/kWh 90% PEF to 1.5g/kWh at 53% PEF; whereas, engine-out smoke emissions remained low throughout the experimental program.

Impact of diesel pilot distribution on the ignition process of a dual fuel medium speed marine engine

Recent emission legislation in the marine sector has emphasized the need to reduce nitrogen oxides (NOx) emissions as well as sulphur emissions. The fulfilment of emission legislation limits with conventional marine diesel oil (MDO) requires complex and expensive aftertreatment systems and in this framework lean burn pilot ignited dual fuel (diesel and natural gas) is revealed as one of the most suitable engine platforms to decrease pollutant formations at its source and therefore to mitigate aftertreatment system requirements. For this reason, an experimental study has been carried out in an 8.8l dual fuel single cylinder Wärtsilä 20DF engine in order to evaluate different diesel equivalence ratio distributions in the combustion chamber and to get a deeper insight into the interaction between the high reactivity (diesel) and the low reactivity (natural gas) fuels during the ignition process. Engine testing has been complemented with diesel spray pattern simulations for a better understanding of local combustion conditions. Results show the importance of local pilot fuel distribution as a way to control combustion phasing and consequently its impact on combustion instability, emissions and knock conditions. Stable combustion with engine-out NOx levels below legislation have been achieved without the need of after-treatment system using appropriate high reactivity fuel (HRF) distribution control.

Evaluation of the real-time de-NOx performance characteristics of a LNT-equipped Euro-6 diesel passenger car with various vehicle emissions certification cycles

Advanced nitrogen oxides (NOx) after-treatment systems for diesel cars are effectively reducing tailpipe NOx emissions in laboratory test cycles; however, some de-NOx systems showed limitation for passing environmental standards during real-world driving conditions. In this study, the NOx concentration of a lean NOx trap (LNT)-equipped diesel engine were investigated over various vehicle certification cycles. The LNT performance was compared to the new European driving cycle (NEDC), world-harmonized light-duty vehicle test cycle (WLTC), federal test procedure (FTP)-75, highway fuel economy test (HWFET), and US06. The real-time NOx concentration behaviors were tracked using NOx sensors at the engine-out and downstream of the LNT to determine the NOx storage and regeneration phase. The NOx conversion efficiencies were 36.3–71.7% of which reflecting the mode severity and cycle duration of the diesel engine. The tailpipe NOx emissions were 0.059 g/km during the NEDC which was within the Euro-6 emissions regulations. The NOx emissions in the WLTC, FTP-75, and US06 modes were approximately 1.9, 1.5, and 6.6 times higher than the NEDC due to the higher frequency of LNT purge and higher engine-out NOx formation. During the diesel particulate filter (DPF) regenerating stage in WLTC mode, tailpipe NOx emissions substantially increased by more than 8.8-fold. The exhaust gas recirculation (EGR) supply and lambda control scheme were closely related with strong NOx increment at de-NOx and de-PM processes.

Real-World NO X Emissions of Transit Buses Equipped with Diesel Exhaust Aftertreatment Systems

In this work, we compare nitrogen oxide (NOX) emissions from vehicles equipped with 2013 and 2015 model year (MY) diesel engines and exhaust aftertreatment, both certified to the same emissions standards, over a variety of real-world drive cycles. Our study concludes that 2013MY and 2015MY buses achieved real-world NOX conversion efficiencies of 75 and 95%, respectively. Engine-out NOX levels remained unchanged between the two busses when driven over the same transit bus routes. Therefore, emissions reductions are attributed to greater NOX conversion efficiency by the exhaust aftertreatment system, especially in low catalyst temperature and transient response scenarios. Although it is likely that other transit buses and vocational vehicles will express different characteristics based on their specific operating conditions and powertrain setup; our results show that improvements to aftertreatment systems, in the areas of catalyst activity, aftertreatment controls, and urea dosing strategy can significantly reduce NOX emissions over the investigated real-world drive cycles.

Fuel formulation effects on the soot morphology and diesel particulate filter regeneration in a future optimized high-efficiency combustion system

The focus of research is shifting toward development of new engine fuels for optimized combustion systems. The altered chemical structure of these new fuels may impact their thermal decomposition chemistry during the ignition process and hence the in-cylinder conditions for particulate formation and post oxidation. This work fundamentally focuses on the influence of the fuel properties on particulate matter morphology and, thereby, the regeneration behavior of diesel particulate filters. The experiments for particulate analysis were conducted with a single-cylinder diesel research engine designed for future passenger car applications. A detailed analysis of soot characteristics and its consequences on diesel particulate filter behavior were studied at part-load engine operation at EU 6 engine-out nitrogen oxide level. Next to standard EN590 diesel, a paraffinic fuel was investigated as non-oxygenated biofuel candidate. A blend of the 2-methyltetrahydrofuran and di-n-butylether was studied as tailor-made oxygenated biomass-derived fuel candidate. With all fuels, samples of state-of-the-art diesel particulate filter were loaded at the research engine. In succession, the regeneration of the filters was investigated at a laboratory gas bench. Furthermore, the primary particle size, the total number concentration, and size-based number distribution were investigated in detail by means of a transmission electron microscope, condensation particle counter, and Engine Exhaust Particle Sizer™, respectively. Furthermore, the graphitic character of the soot structure was analyzed by optical measurements such as absorption coefficient. It was found that the soot oxidation temperature was decreased by ∼10 °C and ∼65 °C with the paraffinic fuel and the blend of 2-methyltetrahydrofuran and di-n-butylether, respectively, compared to conventional diesel fuel. Overall, the results indicate that with specific tailored fuels not only the total particle mass and number could be reduced but, with altering the soot structure and composition, also the energy requirement for diesel particulate filter regeneration can be reduced.

Emission Characteristics of a Natural Gas Engine Operating in Lean-Burn and Stoichiometric Modes

AbstractTo comply with the stringent Euro VI emission standards, two techniques have been proposed for natural gas engines, namely, lean-burn combustion with oxidation catalyst (OC) and stoichiometric combustion with three-way catalyst (TWC). In this paper, comparisons are drawn between the two techniques in terms of carbon monoxide (CO), nitrogen oxides (NOx), and methane (CH4) emissions and fuel economy. The results indicated that the postcatalyst CO emissions of lean-burn combustion with OC are lower, whereas the postcatalyst NOx and CH4 emissions are higher than those of stoichiometric combustion with TWC. This can be explained by the combined effects of lower engine-out CO emissions and higher engine-out NOx and CH4 emissions as well as lower overall conversion efficiency for all emissions with the adoption of lean-burn combustion with OC. Thus, it can be concluded that although the fuel economy of stoichiometric combustion with TWC is inferior to lean-burn combustion with OC, it is still considered ...

Computational Fluid Dynamics Simulation of Gasoline Compression Ignition

Gasoline compression ignition (GCI) is a low temperature combustion (LTC) concept that has been gaining increasing interest over the recent years owing to its potential to achieve diesel-like thermal efficiencies with significantly reduced engine-out nitrogen oxides (NOx) and soot emissions compared to diesel engines. In this work, closed-cycle computational fluid dynamics (CFD) simulations are performed of this combustion mode using a sector mesh in an effort to understand effects of model settings on simulation results. One goal of this work is to provide recommendations for grid resolution, combustion model, chemical kinetic mechanism, and turbulence model to accurately capture experimental combustion characteristics. Grid resolutions ranging from 0.7 mm to 0.1 mm minimum cell sizes were evaluated in conjunction with both Reynolds averaged Navier–Stokes (RANS) and large eddy simulation (LES) based turbulence models. Solution of chemical kinetics using the multizone approach is evaluated against the detailed approach of solving chemistry in every cell. The relatively small primary reference fuel (PRF) mechanism (48 species) used in this study is also evaluated against a larger 312-species gasoline mechanism. Based on these studies, the following model settings are chosen keeping in mind both accuracy and computation costs—0.175 mm minimum cell size grid, RANS turbulence model, 48-species PRF mechanism, and multizone chemistry solution with bin limits of 5 K in temperature and 0.05 in equivalence ratio. With these settings, the performance of the CFD model is evaluated against experimental results corresponding to a low load start of injection (SOI) timing sweep. The model is then exercised to investigate the effect of SOI on combustion phasing with constant intake valve closing (IVC) conditions and fueling over a range of SOI timings to isolate the impact of SOI on charge preparation and ignition. Simulation results indicate that there is an optimum SOI timing, in this case −30 deg aTDC (after top dead center), which results in the most stable combustion. Advancing injection with respect to this point leads to significant fuel mass burning in the colder squish region, leading to retarded phasing and ultimately misfire for SOI timings earlier than −42 deg aTDC. On the other hand, retarding injection beyond this optimum timing results in reduced residence time available for gasoline ignition kinetics, and also leads to retarded phasing, with misfire at SOI timings later than −15 deg aTDC.

Disturbance Rejection in Diesel Engines for Low Emissions and High Fuel Efficiency

This brief presents a novel and time-efficient control design for modern heavy-duty diesel engines using a variable geometry turbine and an exhaust gas recirculation valve. The goal is to simultaneously and robustly achieve low fuel consumption and low emissions of nitrogen oxides (NOx) and particulate matter (PM). A new combination of three controlled outputs is used: 1) specific engine-out NOx emissions; 2) air-fuel equivalence ratio; and 3) the pressure difference between intake and exhaust manifold, which reflect NOx and PM emissions and fuel efficiency, respectively. It is shown that this combination allows for effective disturbance rejection and results in a well-conditioned system. An underactuated input-output system is formed, for which a linear feedback controller is designed. In addition to this feedback controller, a feedforward controller is implemented, which improves the torque response and lowers the PM emissions during fast changes in torque demand. The combined control system is suitable for the full range of speed and load variations. This new controller is tested experimentally on a modern heavy-duty engine running a hot world harmonized transient cycle and compared with a baseline controller. The new controller reduces the NOx and PM emissions by 3.9% and 11.7%, respectively, without a fuel penalty.

Urea-SCR Temperature Investigation for NOx Control of Diesel Engine

SCR (selective catalytic reduction) system is continuously being analyzed by many researchers worldwide on various concerns due to the stringent nitrogen oxides (NOx) emissions legislation for heavy-duty diesel engines. Urea-SCR includes AdBlue as urea source, which subsequently decomposes to NH3 (ammonia) being the reducing agent. Reaction temperature is a key factor for the performance of urea-SCR system, as urea decomposition rate is sensitive to a specific temperature range. This particular study was directed to investigate the temperature of the SCR system in diesel engine with the objective to confirm that whether the appropriate temperature is attained for occurrence of urea based catalytic reduction or otherwise and how the system performs on the prescribed temperature range. Diesel engine fitted with urea-SCR exhaust system has been operated on European standard cycle for emission testing to monitor the temperature and corresponding nitrogen oxides (NOx) values on specified points. Moreover, mathematical expressions for approximation of reaction temperature are also proposed which are derived by applying energy conservation principal and gas laws. Results of the investigation have shown that during the whole testing cycle system temperature has remained in the range where urea-SCR can take place with best optimum rate and the system performance on account of NOx reduction was exemplary as excellent NOx conversion rate is achieved. It has also been confirmed that selective catalytic reduction (SCR) is the best suitable technology for automotive engine-out NOx control.

Modelling the effect of injection pressure on heat release parameters and nitrogen oxides in direct injection diesel engines

Investigation and modelling the effect of injection pressure on heat release parameters and engine-out nitrogen oxides are the main aim of this study. A zero-dimensional and multi-zone cylinder model was developed for estimation of the effect of injection pressure rise on performance parameters of diesel engine. Double-Wiebe rate of heat release global model was used to describe fuel combustion. extended Zeldovich mechanism and partial equilibrium approach were used for modelling the formation of nitrogen oxides. Single cylinder, high pressure direct injection, electronically controlled, research engine bench was used for model calibration. 1000 and 1200 bars of fuel injection pressure were investigated while injection advance, injected fuel quantity and engine speed kept constant. The ignition delay of injected fuel reduced 0.4 crank angle with 1200 bars of injection pressure and similar effect observed in premixed combustion phase duration which reduced 0.2 crank angle. Rate of heat release of premixed combustion phase increased 1.75 % with 1200 bar injection pressure. Multi-zone cylinder model showed good agreement with experimental in-cylinder pressure data. Also it was seen that the NOx formation model greatly predicted the engine-out NOx emissions for both of the operation modes.

The influence of cylinder deactivation on the emissions and fuel economy of a four-cylinder direct-injection diesel engine

The potential benefits and limitations of deactivating two of four cylinders by cam switching to disable the intake and exhaust valve lift were investigated experimentally on a turbocharged four-cylinder common-rail direct-injection diesel engine. When running on two firing cylinders, at light engine loads (a brake mean effective pressure of 2 bar, based on four-cylinder operation), the brake specific fuel consumption at given engine-out nitrogen oxide levels is comparable with or marginally better than when the engine is running on four cylinders. Cylinder deactivation allowed higher fuel rail pressures to be used to reduce the soot emissions while maintaining the advantages of lower carbon monoxide emissions and lower hydrocarbon emissions. At engine loads with a brake mean effective pressure of up to 3 bar on four cylinders, cylinder deactivation lowered the carbon monoxide and hydrocarbon emissions and raised the exhaust gas temperature by around 120 °C but, at higher loads, the fuel economy deteriorated and the soot and nitrogen oxide emissions increased markedly. The benefits of cylinder deactivation are therefore limited to light-load operating conditions, where the fuel economy is improved, the hydrocarbon and carbon monoxide emissions are reduced and the exhaust gas temperature is raised.

Lean NOx trap supplemental energy savings with a long breathing strategy

Current and upcoming diesel engine emission regulations in the USA and in Europe stipulate significant reductions of nitrogen oxide emissions. To satisfy these emission regulations and to maintain high fuel efficiency, energy efficient diesel after-treatment to remove nitrogen oxides is required. In this study, a long breathing (long adsorption) strategy was investigated for the reduction of supplemental energy consumption of a diesel lean nitrogen oxide trap. The long breathing strategy would be enabled by moderate exhaust gas recirculation to reduce the engine-out nitrogen oxide levels. With reduced feed gas nitrogen oxide levels, the adsorption time of the lean nitrogen oxide trap could be extended, leading to less frequent fuel-rich regeneration of the lean nitrogen oxide trap. Proof of concept studies were undertaken on a diesel engine to demonstrate the enabling of the long breathing lean nitrogen oxide trap strategy, while further tests were undertaken on a flow bench set-up to demonstrate the potential energy savings with the long breathing lean nitrogen oxide trap strategy. The test results indicated that, at the selected operating conditions, the long breathing strategy could be enabled by reducing the engine-out nitrogen oxide from 110 ppm to 50 ppm via moderate exhaust gas recirculation. The flow bench test results indicated that the adsorption time of the lean nitrogen oxide trap increased exponentially when the feed gas nitrogen oxide level was reduced. The longer adsorption led to supplemental energy savings in excess of 60% when the feed gas nitrogen oxide level was reduced from 110 ppm to 50 ppm. Furthermore, it was calculated that the long breathing lean nitrogen oxide trap strategy enabled a higher overall indicated efficiency of 36.4% compared to 35.9% with a conventional lean nitrogen oxide trap strategy.

Diesel pressure departure ratio algorithm for combustion feedback and control

The pursuit for higher efficiency and ultra-low exhaust emissions from diesel engines requires the combustion process to be precisely controlled so as to minimize departures from the intended engine operation. The combustion control system must be able to perform corrective actions on a cycle-by-cycle basis, with a robust feedback on the combustion process. The combustion phasing, commonly represented by the crank angle of 50% heat release and derived from the measured cylinder pressure data, shows a strong correlation to the efficiency and the engine-out nitrogen oxide emissions. To accurately estimate the combustion phasing from the derived heat-release rate, the authors previously introduced and experimentally validated a computationally efficient diesel pressure departure ratio algorithm, against selected cases of boost, engine load and exhaust gas recirculation. In this work, the formulation of the pressure departure ratio algorithm is presented in detail along with its implementation to enable combustion control during both transient and steady-state engine operations. Engine tests demonstrate that the algorithm was effective in stabilizing the combustion process on a cycle-by-cycle basis for a range of engine speeds, load and exhaust gas recirculation, which included conventional and low-temperature diesel combustion modes.

Reactivity controlled compression ignition combustion on a multi-cylinder light-duty diesel engine

Reactivity controlled compression ignition is a low-temperature combustion technique that has been shown, both in computational fluid dynamics modeling and single-cylinder experiments, to obtain diesel-like efficiency or better with ultra-low nitrogen oxide and soot emissions, while operating primarily on gasoline-like fuels. This paper investigates reactivity controlled compression ignition operation on a four-cylinder light-duty diesel engine with production-viable hardware using conventional gasoline and diesel fuel. Experimental results are presented over a wide speed and load range using a systematic approach for achieving successful steady-state reactivity controlled compression ignition combustion. The results demonstrated diesel-like efficiency or better over the operating range explored with low engine-out nitrogen oxide and soot emissions. A peak brake thermal efficiency of 39.0% was demonstrated for 2600 r/min and 6.9 bar brake mean effective pressure with nitrogen oxide emissions reduced by an order of magnitude compared to conventional diesel combustion operation. Reactivity controlled compression ignition emissions and efficiency results are compared to conventional diesel combustion operation on the same engine.

Enhancing the NO 2/NOx ratio in compression ignition engines by hydrogen and reformate combustion, for improved aftertreatment performance

Enhanced NO 2 production (without raising total NOx) in a diesel engine combustion chamber can improve the performance of several catalytic aftertreatment systems. Thus this can facilitate a further reduction in key regulated emissions such as nitrogen oxides (NOx) and particulate matter (PM) emissions. The oxidation of NO to NO 2 is an important intermediate step involved in all current aftertreatment systems that are designed for NOx and PM catalytic removal. The performance of both NOx control systems and catalysed particulate filters depend highly on the NO 2 concentration. In this work we have examined the influence of using hydrogen (H 2) and simulated reformate (H 2, CO and EGR gases) as a supplement to diesel fuel on NO 2 production. In actual engine applications a reformer will be integrated within the engine EGR system. This will not only provide the engine with recirculated exhaust gas (i.e. EGR), but will enrich it with H 2 and CO. The effects of adding H 2 or reformate results in a significant decrease in total engine-out NOx emissions, as well as an increase in both NO 2 concentration and NO 2/NOx ratio. The influence of the simulated reformate combustion on the NO 2 production is dependent on the engine load and in-cylinder conditions. It was observed that both reformate composition and concentration significantly influence the NO 2/NOx ratio of the exhaust gas. Air/fuel ratio, combustion efficiency and in-cylinder temperatures were the most influential parameters in this study. The NO 2 production was dependent on the EGR addition and air/fuel ratio.

Influence of Early Fuel Injection Timings on Premixing and Combustion in a Diesel Engine

The influence of fuel injection timing on precombustion mixing of diesel fuel and air, combustion, and emissions at early-injection conditions similar to homogeneous charge compression−ignition (HCCI) engine conditions was investigated experimentally in an automotive-size compression−ignition engine and a constant-volume vessel. The injection timing was controlled by electronic fuel injection equipment. In-cylinder pressure measurements, engine-out emission measurements, and imaging of the spray development were used to analyze the ignition delay period and fuel distribution. The ignition delay period was measured over a wide range of injection timings as well as at various compression ratios and engine speeds. With advancing fuel injection timing, the ignition delay increased and the engine-out nitrogen oxides (NOx) decreased, suggesting that increased premixing time results in a lean mixture and low flame temperature. It was also found that the ignition delay period to decrease NOx emissions to a neglig...