Engine-out NOx emissions characteristics of heavy-duty diesel trucks under real-world dynamic load conditions

<div class="section abstract"><div class="htmlview paragraph">In this work a quasi-dimensional multi-zone combustion diagnostic tool for homogeneous charge Spark Ignition (SI) engines is analytically developed for the evaluation of heat release, flame propagation, combustion velocities as well as engine-out NOx and CO emissions, based on in-cylinder pressure data analysis. The tool can be used to assess the effects of fuel, design and operating parameters on the SI engine combustion and NOx and CO emissions formation processes.</div><div class="htmlview paragraph">Certain novel features are included in the presently developed combustion diagnostic tool. Firstly, combustion chambers of any shape and spark plug position can be considered due to an advanced model for the calculation of the geometric interaction between a spherically expanding flame and a general combustion chamber geometry. Also, the temperature stratification of the burned gas developed during the combustion phase, which has to be captured for the theoretically realistic estimation of the in-cylinder formation of pollutant emissions, is taken into account by a multi-zone thermodynamic treatment. According to this, multiple spherically allocated burned zones are sequentially generated at specified (user-defined) crank angle intervals, forming overlapping shells. Moreover, complex chemical equilibrium compositions are computed, which can include any user-defined mixture of species in the combustion products, using an advanced modular method based on the minimization of Gibbs energy. Furthermore, NOx and CO engine-out emissions are calculated based on novel in-cylinder formation models presented by the authors in the past. Specifically, NOx emissions are evaluated by a global algebraic NOx emissions formation model, accounting for both thermal and N<sub>2</sub>O pathway NO formation, based on the calculation of the characteristic timescales of the relevant NO formation mechanisms. CO emissions are assessed using a new kinetics-based model, consisting of a single Ordinary Differential Equation (ODE) that can be analytically integrated. The CO emissions model is derived considering the dynamics of a representative pool of active radicals in post-flame gases and explicitly describes the CO oxidation quenching process.</div><div class="htmlview paragraph">The combustion diagnostic tool is applied to a lean burn gas engine at various engine speeds under full load conditions and constant lambda value. Various combustion and NOx and CO emissions related results are presented and discussed to illustrate the capabilities of the tool for combustion diagnosis, while at the same time calculated engine-out NOx and CO emissions are found to be in satisfactory agreement with measured ones.</div></div>

<div class="section abstract"><div class="htmlview paragraph">While significant progress has been made in recent years to develop hybrid and battery electric vehicles for passenger car and light-duty applications to meet future fuel economy targets, the application of hybrid powertrains to heavy-duty truck applications has been very limited. The relatively lower energy and power density of batteries in comparison to diesel fuel and the operating profiles of most heavy-duty trucks, combine to make the application of hybrid powertrain for these applications more challenging. The high torque and power requirements of heavy-duty trucks over a long operating range, the majority of which is at constant cruise point, along with a high payback period, complexity, cost, weight and range anxiety, make the hybrid and battery electric solution less attractive than a conventional powertrain. However, certain heavy-duty applications, such as Class 6-7 urban vocational trucks, can benefit from hybridization due to their transient operating profiles and relatively lower vehicle weight. While many studies have quantified the fuel consumption benefits of hybridization in this segment, very few studies have outlined the arduous process of selection and sizing of hybrid powertrain components based on the trade-offs between fuel consumption, payback period, cost, weight, packaging, emissions and aftertreatment temperature.</div><div class="htmlview paragraph">To investigate the potential for electrification in heavy-duty applications, FEV has developed a system level approach for the selection and sizing of heavy-duty diesel hybrid powertrain components using GT-SUITE. The approach has been applied for a Class 6-7 urban vocational truck, which typically experiences low speed driving with frequent start-stops. A dynamic model for the baseline vehicle was developed and calibrated to test data that included, fuel efficiency, engine-out NOx, engine-out PM and aftertreatment system temperature. The model was then updated with hybrid powertrain components and evaluated over cycles developed for chassis dynamometer testing of heavy-duty vehicles, specifically the Heavy Heavy-Duty Diesel Truck (HHDDT) schedule and EPA Urban Dynamometer Driving Schedule (HDUDDS). In the evaluation, key trade-offs were identified between fuel consumption, initial cost, payback period, package size, emissions and vehicle weight. The trade-off analysis demonstrated that similar fuel consumption benefits with an identical payback period could be achieved with multiple hybrid powertrain configurations, however package size, initial cost and weight considerations determined the final optimum solution. The final hybrid powertrain configuration for a Class 6-7 urban vocational truck proposed from this study demonstrates a 20.7% fuel consumption reduction when comparing to the baseline vehicle and applying a two year payback period. In addition, the diesel hybrid powertrain configuration provides an 11% reduction in engine-out NOx emissions and an 86% reduction in engine-out PM emissions, while maintaining aftertreatment temperature of the baseline configuration.</div></div>

<div class="section abstract"><div class="htmlview paragraph">Greenhouse gas regulations and global economic growth are expected to drive a future demand shift towards diesel fuel in the transportation sector. This may create a market opportunity for cost-effective fuels in the light distillate range if they can be burned as efficiently and cleanly as diesel fuel. In this study, the emission performance of a low cetane number, low research octane number naphtha (CN 34, RON 56) was examined on a production 6-cylinder heavy-duty on-highway truck engine and aftertreatment system. Using only production hardware, both the engine-out and tailpipe emissions were examined during the heavy-duty emission testing cycles using naphtha and ultra-low-sulfur diesel (ULSD) fuels. Without any modifications to the hardware and software, the tailpipe emissions were comparable when using either naphtha or ULSD on the heavy duty test cycles. Overall lower CO<sub>2</sub> emissions and fuel consumption were measured for naphtha due in part to its higher heating value and higher hydrogen to carbon ratio. Engine-out and tailpipe NOx emissions were lower for naphtha, and measured PM emissions were also lower due to naphtha’s higher volatility and lower aromatic content compared to ULSD. To help assess the potential impact on diesel particulate filter design and operation, engine-out PM samples were collected and characterized at a steady-state mid-speed, mid-load operating point. A significant reduction in elemental carbon in PM samples was observed for naphtha fuel, and similar oxidation rates and peak oxidation temperatures were measured for the PM from both fuels.</div></div>

The pros and the cons of lean-burn, compression ignition (CI), direct injection (DI) internal combustion engines (ICE) are reviewed for transport applications. Fueling options considered include diesel only and dual-fuel applications with diesel and a gaseous fuel (CNG, LNG and LPG). CIDI ICEs have higher fuel conversion efficiencies than stoichiometric, spark ignition (SI) ICEs, whether DI or port fuel injected (PFI). However, diesel-fueled CIDI ICEs have higher particulate matter (PM) and NOx engine-out emissions. The tail-pipe NOx emissions in real-world driving of diesel-powered vehicles have been, in the past, above the limits requested over the simplified cold start driving cycles used for certification. This issue has recently been resolved. The newest diesel-powered vehicles are now compliant with new laboratory test cycles and real-world-driving schedules and have no disadvantages in terms of criteria air pollutants compared to older diesel vehicles, while delivering improvements in fuel economy and CO2 emissions. Dual-fuel CIDI ICEs offer the opportunity for enhanced environmental friendliness. Dual-fuel CIDI ICEs have lower engine-out NOx and PM emissions compared to diesel-only CIDI ICEs. The latest diesel-only vehicles and vehicles with dual-fuel ICEs deliver dramatic reductions in tail-pipe PM emissions compared to older diesel-only vehicles. Moreover, they deliver tail-pipe PM emissions well below the ambient conditions in most city areas that are highly polluted, thereby helping to clean the air. The diesel-fueled CIDI ICEs may be further improved to deliver better fuel economy and further reduced tail-pipe emissions. The dual-fuel CIDI ICE has more room for improvement to produce similar or better steady state and transient performance in terms of torque, power output and fuel conversion efficiency compared to diesel-fueled CIDI ICEs, while drastically reducing CO2 and PM tail-pipe emissions, and improving NOx tail-pipe emissions. This is due to the ability to modulate the premixed and diffusion phases of combustion with a second fuel that is much easier to vaporize and is less prone to auto-ignition. Further development of the fuel injection system for the second fuel will lead to novel dual-fuel CIDI ICE designs with better performance.

<div class="htmlview paragraph">The emission of excessive quantities of NOx when the automobile air conditioner is turned on has received a fair amount of attention in recent years. Since NOx is a smog precursor, it is important to understand the reasons for this jump in emissions especially on hot sunny days when air conditioner usage is at a maximum. A simple thermodynamic model is used to demonstrate how the torque from a typical air conditioner compressor is mainly related to the ambient temperature. The compressor's on-off cycling patterns are also characterized. Since the compressor significantly loads the engine, it affects fuel economy and emissions. The key independent variable that we employ to represent engine load is fuel rate. The correlations between engine-out NOx emissions and fuel rate are shown for a number of light duty vehicles and trucks. From these, a physical model for engine-out NOx emissions (with and without air conditioning) is presented. The non-linear relationship between engine-out NOx and load does not by itself explain how a 25% increase in fuel rate results in an observed 70% increase in NOx emissions. It is likely that the significant idle and low load excess emissions are due to the added air (and fuel) to the combustion without a corresponding increase in engine speed when the air conditioner is engaged. With slight modifications, the model is simple enough to be used in conjunction with other modal emissions models. Comparisons are made with other modeling efforts such as MOBILE6. Finally, suggestions are offered to vehicle designers which may decrease emissions and improve fuel economy by employing existing automobile air conditioner technology in a more efficient manner.</div>

The increased production of natural gas harvested from unconventional sources, such as shale, has led to fluctuations in the species composition of natural gas moving through pipelines. These variations alter the chemical properties of the bulk gas mixture and, consequently, affect the operation of pipeline compressor engines which use the gas as fuel. Among several possible ramifications of these variations is that of unacceptably high engine-out NOx emissions. Therefore, engine controller enhancements which can account for fuel variability are necessary for maintaining emissions compliance. Having the means to predict NOx emissions from a field engine can inform the development of such control schemes. There are several types of compressor engines; however, this study considers a large bore, lean-burn, two-stroke, integral compressor engine. This class of engine has unique operating conditions which make the formation of engine-out NOx different from typical automotive spark-ignited engines. For this reason, automotive-based methods for predicting NOx emissions are not sufficiently accurate. In this study, an investigation is performed on the possible NO and NO2 formation pathways which could be contributing to exhaust emissions. Additionally, a modeling method is proposed to predict engine-out NOx emissions using a 0-D/1-D model of a Cooper-Bessemer GMWH-10C compressor engine. Predictions are achieved with GRI-Mech3.0, a natural gas combustion mechanism, which allows for simulated formation of NOx species. The implemented technique is tuned using experimental data from a field engine to better predict emissions over a range of engine operating conditions. Tuning the model led to acceptable agreement across operating points varying in both load and trapped equivalence ratio.

Maintaining a stable voltage level at the DC bus is crucial to safeguard equipment, ensure a consistent power supply, and enhance system efficiency. A combination of battery storage and photovoltaic (PV) systems is often employed to maintain an equilibrium between energy supply and demand and keep the DC bus voltage within optimal levels. Nonetheless, frequent and rapid charging and discharging of battery storage in dynamic conditions can negatively impact its lifespan. To address this issue, this study utilizes the application of a frequency‐based current controller in battery storage that facilitates smooth charging and discharging operations in transient conditions. The study evaluates the performance of a grid‐tied PV‐based nanogrid (GT‐PVN) system with three distinct configurations: (a) PV system without storage, (b) PV system with battery storage, and (c) PV system with hybrid energy storage system (HESS) under varying dynamic load and irradiance conditions. The results show that the transient performance on DC bus voltage with battery integration is efficiently improved by 0.3%‐0.5% under dynamic irradiance conditions and 0.5%‐1.17% under dynamic load conditions. Further, with HESS installation, the DC bus voltage transients are improved by 0.85%‐1.25% under dynamic irradiance conditions and 0.5%‐1.5% under dynamic load conditions. Moreover, energy storage technology implementation has the potential to decrease energy procurement from the grid by up to 80%. These findings highlight the potential of energy storage technology in mitigating the intermittency and dynamic operational challenges faced by PV systems.

In 2027, the fully phased-in EPA/NHTSA Phase-2 greenhouse gas (GHG) emission legislation for heavy-duty (HD) diesel engines will mandate a 5.1% reduction in fuel consumption for MY2017 tractor engines and a 4.2% reduction in fuel consumption for MY2017 vocational engines. Along with improvements in engine efficiency, manufacturers are likely to face a simultaneous challenge to achieve a significant reduction in tailpipe NOx emissions, as the ARB is expected to implement an ultra-low NOx emission standard in the 2024–27 timeframe. With this consideration, technology solutions for Phase-2 GHG will have to be NOx neutral or provide additional reduction in NOx emissions which is typically contrary to a reduction in fuel consumption. In this study, various advanced engine technologies — such as engine downsizing and downspeeding, variable compression ratio, cylinder deactivation and turbocompounding — have been evaluated to improve engine efficiency with a goal to reach Phase-2 GHG engine requirements. Simultaneously, the impact of these technologies on engine-out NOx emission and aftertreatment inlet temperature has also been evaluated. The technologies were evaluated with a GT-Power model of a 7.7 liter medium HD diesel engine applied in vocational vehicles at steady-state operating conditions as well as over transient operating profiles. Significant fuel consumption reductions were observed with engine downsizing and engine downspeeding at the same engine-out NOx emissions as the baseline engine. Cylinder deactivation showed a moderate impact on fuel consumption while variable compression ratio and turbocompounding had a much lower impact on fuel consumption. In general, exhaust gas temperatures decreased with a reduction in fuel consumption, except in the case of cylinder deactivation where significant increase in exhaust gas temperatures was observed. The results of the study show that engine efficiency improvements beyond what has been mandated by the Phase-2 GHG regulations are possible without increasing the engine-out NOx emissions of a Phase-1 GHG compliant engine. However, if an ultra-low NOx emission standard is implemented as expected, some of the efficiency gains demonstrated in this study will need to be offset to achieve higher exhaust gas temperatures and lower engine-out NOx emissions.

High-efficiency and low-emissions heavy-duty (HD) internal combustion engines (ICEs) offer significant GHG reduction potential. Mild hybridization via regenerative braking and enabling the use of an electric heater component (EHC) for the aftertreatment system (ATS) warm-up extends these benefits, which can mitigate tailpipe GHG and NOx emissions simultaneously. Understanding such integrated hybrid powertrains is essential for the system optimization of real-world driving conditions. In the present work, the potential of a low engine-out NOx (1.5–2.5 g/kWh range) ‘Low-NOx’ HD diesel engine and EHCs were analyzed in a 48V P1 mild-hybrid system for a class 8 commercial vehicle concept and compared with those in an EPA-2010-certified HD diesel truck as a baseline under real-world driving cycles, including those from the US, Europe, India, China, as well as the world harmonized vehicle cycle (WHVC). For analysis, an integrated 1-D vehicle model was utilized that consisted of models of the ‘Low-NOx’ HD engine, the stock ATS, and a production EHC. For the real driving cycles, ‘GT-RealDrive’-based vehicle speed profiles were generated for busy trucking routes for different markets. For each cycle, the effects of the Low-NOx and EHC performances were quantified in terms of the ATS warm-up time, engine-out NOx emissions, and net fuel consumption. Depending on the driving route, the regenerative braking fully or partly neutralized the EHC power penalty without a significant impact on the ATS thermal performance. For a two-EHC system, the fueling penalty associated with every second reduction in the warm-up time FCEHC (g/s) was several-fold higher for the real driving routes compared with the WHVC. Overall, while a multi-EHC setup accelerated the ATS warm-up, a single EHC integrated at the SCR inlet showed minimized EHC heating power, leading to a minimized fueling penalty. Finally, for the India and China routes, being highly transient, the P1 hybridization proved inadequate for GHG reduction due to the limited energy recuperation. A stronger hybridization was desirable for such driving cycles.

Gasoline compression ignition operation on a multi-cylinder heavy duty diesel engine

<div class="section abstract"><div class="htmlview paragraph">In recent years, the stronger push for reducing GHG and NOx emissions has challenged vehicle manufacturers globally. In USA, Multi-Pollutant Emissions Standards for Model Years 2027 and Later Light Duty and Medium-Duty Vehicles released by EPA in April 2023 aims to reduce the CO2 emissions by 56% and 44%, respectively, for light and medium duty vehicles by 2032 from 2026 levels. It also includes the NMOG+ NOx standards, which require a 60 – 76% reduction by 2032 from 2026 levels for light to medium-duty vehicles. Europe also aims to reduce CO2 emissions by 55% by 2030 from 1990 levels and 100% by 2035.</div><div class="htmlview paragraph">To achieve such low levels of CO2 emissions, especially in the near-term scenario of limited EV sales, hybridization of conventional powertrains has found renewed interest. While hybrid powertrains add complexity, if optimized well for the application, they can offer best tradeoff between upfront cost, range, payload, performance, emissions and off-ambient operation. This study investigates the benefits and challenges of various hybrid architectures suitable for a pickup truck application using a model-based approach. First, a baseline vehicle model of a conventional powertrain pickup truck was developed using GT-SUITE and correlated to test data for fuel economy, and engine-out emissions over EPA regulatory cycles. Thereafter, the model was extended to represent various electrified powertrains such as P2, P3, P1P2, P1P3, range extender and Battery Electric Vehicle (BEV) architecture. The component sizes and energy management strategy for each hybrid architecture was then optimized using a genetic algorithm-based optimization approach to maximize fuel efficiency. The optimized powertrains were finally compared against each other on performance, fuel efficiency, added curb weight, added cost and cost of ownership.</div><div class="htmlview paragraph">In comparison to the baseline vehicle, the optimized P1P2 and P1P3 parallel hybrid configurations showed a 29% and 32% increase in fuel economy over the regulatory cycles in charge sustaining mode, respectively. The range extender concept (referred to as hybrid BEV architecture in the study) with a dedicated hybrid engine showed the highest potential of 46% increase in fuel economy along with 75% reduction in engine-out NOx emissions. The hybrid BEV architecture also showed the lowest Total Cost of Ownership (TCO) among the other electrified powertrains.</div></div>

Small diesel engines are widely used for commercial vehicle and passenger car applications due to their higher torque requirements, fuel economy, and better thermal efficiency. These engines are exposed to different operating and environmental conditions and hence emissions from these engines are erratic. Strategies are required to enhance performance and reduce engine-out emissions considering environmental pollution and regulations. The main objective of this experimental study is to develop strategies for performance improvement and emission reduction for naturally aspirated engines, which can further be used for emission reduction of the multicylinder engine. Experimental work has been carried out on a single-cylinder naturally aspirated diesel engine to study the impact of engine operating parameters like valve timing, swirl ratio, and injection pressure on engine performance and emissions. Parameters considered for the study are: three intake valve opening timings, two fuel injection pump pressures, two-cylinder head swirls, and three start of injection timings. Results showed improvement in performance, lower exhaust gas temperature, and reduction of engine-out emission. Exhaust gas temperature was reduced by 5-18% with advanced valve opening and lower cylinder head swirl option. NOx emission was reduced by 5-50% at advanced intake valve opening (IVO) options with retarded start of injection (SOI) and lower swirl cylinder head. This has a penalty on CO and HC emissions since the availability of fresh air is less due to higher internal exhaust gas recirculation (EGR). Higher pressure fuel injection pump helps in improving engine torque with an adverse effect on engine-out NOx emission. As these engines are of low power capacity segment and are used in few countries, research on these engines is limited. All research work has been carried out in the field of intake valve closing timings, swirl ratio and injection timings; however, very limited research is available for the effect of intake valve opening timings due to practical limitations of the lower valve to piston clearance in diesel engines.

Characteristics of NOx emission of light-duty diesel vehicle with LNT and SCR system by season and RDE phase

The applications of diesel engines in ground vehicles have attracted much attention over the past decade for the reasons of outstanding fuel economy, power capability, and reliability. With the increasing demand of less greenhouse gas emissions, the current diesel engine fuel efficiency remains unsatisfactory partially due to the conflict between the engine fuel efficiency and engine-out NOx emissions. While advanced aftertreatment systems, such as selective catalytic reduction (SCR) systems or lean NOx trap, have been integrated to diesel engines for reducing the tailpipe NOx emissions, the integrated controls for coordinating diesel engine and SCR system to achieve high engine efficiency and low tailpipe emissions are still limited. The purpose of this study is to develop such an integrated diesel engine and SCR system control method using nonlinear model predictive control (NMPC) approach with both start of injection (SOI) timing and urea solution injection rate as the control inputs. Control-oriented engine models were developed to quantify the influences of SOI timing on engine efficiency and engine-out NOx emissions. Simulation results under US06 driving cycle demonstrate that, given the same catalyst size in total, the proposed controllers are capable of reducing total engine fuel consumption over the driving cycle by 9.36% and 9.50%, respectively, for lumped SCR system and two-cell SCR system, while maintaining high NOx conversion efficiencies and low tailpipe ammonia slip.

In support of the Daimler SuperTruck I team’s 55% brake thermal efficiency (BTE) pathway goal, researchers at Oak Ridge National Laboratory performed an experimental investigation of the potential efficiency and emissions benefits of dual-fuel advanced combustion approaches on a modified heavy-duty 15-L Detroit™ DD15 engine. For this work, a natural gas port fuel injection system with an independent injection control for each cylinder was added to the DD15 engine. For the dual-fuel strategies investigated, 65%–90% of the total fuel energy was supplied through the added port fuel injection natural gas (NG) fueling system. The remaining fuel energy was supplied by one or more direct injections of diesel fuel using the production high pressure diesel fueling system. The production DD15 air handling system and combustion geometry were unmodified for this study. Efficiency and emissions with dual-fuel strategies including both low temperature combustion (LTC) and non-LTC approaches such as dual fuel direct-injection were investigated along with control authority over combustion phasing. Parametric studies of dual-fuel NG/diesel advanced combustion were conducted in order to experimentally investigate the potential of high-efficiency, dual-fuel combustion strategies to improve BTE in a multi-cylinder engine, understand the potential reductions in engine-out emissions, and characterize the range of combustion phasing controllability. Characterization of mode transitions from mixing-controlled diesel pilot ignition to kinetically controlled ignition is presented. Key findings from this study included a reproducible demonstration of BTE approaching 48% at up to a 13-bar brake mean effective pressure with significant reductions in engine-out NOx and soot emissions. Additional results from investigating load transients in dual-fuel mode and initial characterization of particle size distribution during dual-fuel operation are presented.