Review of Advances in Engine Efficiency and After-Treatment Technologies

Ameya Joshi,Timothy V. Johnson

(康宁公司，纽约州康宁市 14831，美国)

Abstract： This is a comprehensive review of some of the major advances in the last couple of years to reduce vehicular tailpipe emissions of greenhouse gases and criteria pollutants. Discussed are both new and upcoming regulations, and technologies being developed for improving engines and after-treatment systems. We include developments in major global markets and address regulations and technologies relevant to both light- and heavy-duty segments.

Key words： internal combustion engine; efficiency; after-treatment; emission

0 Introduction

A growing emphasis on improving urban air quality and reducing atmospheric CO2levels has led to increasingly stringent emissions regulations for the mobility sector. Despite decades of advances in improving engines and after-treatment systems, future regulations in advanced markets are mandating further reductions in tailpipe emissions from modern passenger car and heavy-duty vehicles to near-zero levels. In this paper, some of these regulations and the major advances in engine efficiency and emissions reduction in the past several years are reviewed. This builds upon previous annual reviews of advances in technology and regulations[1-3]. Despite the ongoing trend towards electrification of the powertrain, internal combustion engines(ICEs) are expected to play a role in transportation sector for several decades and improving efficiency remains a critical goal. Furthermore, global demand for passenger and freight transport is expected to grow at a compounded rate of about 3% per year through 2050[4], with much of the growth expected in markets yet to adopt the most stringent regulations, which will be reviewed in the first section.

1 Regulations

Modern ICE vehicles have tailpipe limits on both CO2emissions as well as criteria pollutants — carbon monoxide(CO), hydrocarbons(HC), nitrogen oxides(NOx) and particulates — either through a limit on particle mass(PM) or number(PN). Major markets in the world are following either the US or the European regulatory framework. Future regulations in both light- and heavy-duty sectors are aiming to further limit the criteria pollutants to near zero levels(figure 1).Dpin the figure represents the diameter of particle. Note that the US does not have a particle number limit but only a particle mass limit.

Figure 1 Tailpipe emission limits are tightening for bothlight-duty LD and heavy-duty HD engines

Some countries have announced bans on the ICE in the next two decades. These dictates might be misguided, as modern ICE vehicles are exceptionally clean and under certain conditions can have tailpipe emissions lower than the ambient concentrations[5]. The following sections summarize important recent regulatory changes related to both light- and heavy-duty sectors.

1.1 Light-Duty Criteria Pollutant Regulations

As shown in figure 1, while the US have the tightest NOx+HC(hydrocarbon) tailpipe standards in the world, China and Europe have the tightest particulate standards due to a particle number limit of 6×1011#/km. Other major markets are following these advanced regulations as well. Figure 2 provides a summary. The filled cells are confirmed timings while the shaded cells are speculation on future regulations.

Figure 2 Summary of global regulations

The following are some details of regulations in the US, Europe and China.

1.1.1 US

California’s LEV3(low emission vehicle) NOx+non-methane organic gases, ozone precursors(NMOG) regulations are the world’s most stringent and have been steadily phasing down from a fleet average of about 100 mg/mi (1 mi=1.609 344 km,1 mg/mi=0.621 371 mg/km) to 30 mg/mi from 2015 to 2025. The US EPA Tier 3 regulation is the same but started phase down in 2017. While the US EPA limits PM to 3 mg/mi, California tightens this further to 1 mg/mi between 2025 and 2028. However, as noted earlier, the US does not have PN limits. The PN limit of 6×1011#/km in Europe and China is roughly equivalent to 0.5 mg/mi, half of the California PM regulation as shown in figure 3.

Figure 3 Correlation of engine out particle number and mass emissions

Also, while China and Europe have helpful, well-defined real-world driving emission(RDE) on-road test protocols, the US regulations(no RDE) are still more demanding than China or Europe(except for particulates), as it is expected in the US that the emissions in all driving conditions should be “similar” to those on the dynamometer, unless declared otherwise.

The California Air Resource Board(CARB) is focusing on in-use(or off-cycle) emission reductions, such as reducing high emissions associated with warm start conditions when the engine is not hot enough to trigger cold start strategies and the after-treatment is not above light-off conditions either[16].

1.1.2 Europe

The Euro Ⅵ regulations are now fully implemented, with Euro Ⅵ-D adopted in 2020. Other than tightening limits, a major change is the introduction of on-road RDE testing protocols. Emission limits on the RDE cycles are higher than those on the lab certification worldwide harmonized light duty test cycle(WLTC) to account for the measurement limitations of the portable emissions measurement systems(PEMS) required for such tests. The WLTC limits are multiplied by so-called conformity factors(CF) to arrive at the RDE limits.

Elements of the next stage of regulations are being discussed in Europe. “Post Euro Ⅵ” regulations will likely include the following：(1) Fuel and technology neutral standards which will result in same tailpipe limits for diesel and gasoline vehicles and extend PN standard to port-fuel injected engines(like in China). (2) Further reductions of tailpipe emissions are expected, and China 6b limits set a possible target, reducing NOxby a further 50%. (3) Particles down to 10 nm will be included in the test protocol, resulting in a much higher recorded engine out concentration compared to the current 23 nm cut-off. Significant progress has been made to demonstrate that measurement down to 10 nm is possible[17]. (4) The regulations may also include limits for previously unregulated species such as NH3, NO2, N2O, and include CH4as a greenhouse gas. (5) RDE limit on carbon monoxide will be set through an appropriate conformity factor, which will effectively limit fuel enrichment during high-load operation. (6) Durability requirements increased from 160 000 to 200 000 km(similar to China 6b).

It is expected that conformity factors will continue to be periodically reviewed and reduced with the improvement in PEMS capabilities.

1.1.3 China

China 6a light-duty standard is being phased in nationwide although the PN standard has been delayed to the start of 2021. As in Europe, the PN limit is reduced by an order of magnitude for gasoline vehicles, enforcing GPFs across most of the new car fleet. Key urban areas are already beginning to implement China 6b ahead of the national timeline in 2023. RDE is implemented to start China 6b and the conformity factors are expected to be 1.5 for both PN and NOx.

A few key similarities and differences in the regulations in some of the advanced automotive markets are listed in table 1 and figure 4[18]. Figure 4 shows the ambient boundary conditions under which an RDE test is considered valid.

1.2 Heavy-Duty Criteria Pollutant Regulations

As shown in figure 1, heavy duty tailpipe limits for NOxand particulates have also been continuously revised downward over the past several years. This section discussed further reductions that are being considered as part of next regulations. Figure 5 shows the regulatory road-map for major markets globally.It should be noted that the timing of the introduction of next regulations in most countries is still tentative.

Table 1 Summary of in-use LD testing requirements in five key markets[18]

Figure 4 Ambient boundary conditions

1.2.1 US

California is planning for significant reductions in in-use NOxemissions from heavy-duty trucks to meet the ambient air ozone standard of 70×10-9.

California’s low NOxomnibus proposal and the nationwide US EPA Cleaner Trucks Initiative(CTI) aim to reduce NOxemissions from heavy-duty trucks by 90%, while also increasing durability and warranty. Here are the key elements of California’s current proposal[19]：(1) MY 2022—2023：Minor modifications to NTE(Not to Exceed) in-use testing. (2) MY 2024—2026：NOxlimit on the transient FTP cycle reduced by 75% from the reference limit of 0.2 g/(bhp·h) (1 bhp=0.745 7 kW, 1 g/(bhp·h)=1.341 0 g/(kW·h)) today to 0.05 g/(bhp·h). A new low-load test cycle will be introduced with a corresponding limit set at 1～3×FTP. In-use testing procedure changes from the current “not-to-exceed”(NTE) to the European moving average window(MAW) method. PM limit is also reduced by 50% to 0.005 g/(bhp·h). (3) MY 2027—2030：NOxlimit on FTP further reduced to 0.02 g/(bhp·h) starting MY 2027(a 90% reduction compared to the limits today). In-use testing is further tightened to include cold-start emissions(similar to Euro Ⅵ-E). Also introduced in the use of telematics for in-use monitoring. The Full Useful Life(FUL) durability requirements, that is the period or mileage in which the engine needs to meet the regulations, is also increased at this stage. The actual increase depends on the class, and as example, the FUL for Class 8 trucks increases from 435 kmi today to 600 kmi. (4) MY 2031+：All of the above, along with a further increased FUL(800k mi for Class 8 trucks) and reduced allowable deterioration.

Figure 5 Summary of criteria pollutant and fuel consumption/CO2 regulations for on-road heavy-duty vehicles in major markets around the world

Final approval for the above proposal is expected in 2020, while a proposal from the EPA is expected in 2021.

1.2.2 Europe

Euro Ⅵ regulations are implemented in various steps. The Euro Ⅵ-E will be implemented starting January 2021 for new type and January 2022 for all vehicles, and will require inclusion of cold start emissions and solid PN measurements using PEMS for type approval and in-use compliance. The conformity factor(CF) is initially set at 1.63 for PN.

Similar to light-duty, post Euro Ⅵ options are

being evaluated for heavy-duty as well. Several of the considerations listed in the light-duty section apply, however the actual reductions in tailpipe limits are still under discussion. Given the emphasis on NOxreduction by California and the relevant demonstration programs, a further reduction of 50%～75% in Europe may be reasonable.

1.2.3 China

China 6a(comparable to Euro Ⅵ) is beginning nationwide implementation in July 2020, with China 6b implemented in major cities earlier and nationwide in July 2023. RDE PEMS testing is added for gaseous and PN emissions, with RDE boundary conditions of -7 ℃ to 38 ℃and 2 400 m(vs 1 700 m in Euro Ⅵ) altitude requirements. As additional requirement is the reporting of engine-out NOx. Vanadia SCR cannot exceed temperatures above 550 ℃ under any conditions. On-board monitoring of key operating parameters and emissions, and real-time telematic transmission to the regulatory authorities is required. This is already happening in major cities and the quality of the NOxsensor based OBM data is shown to be within about 15% of data acquired using PEMS[20].

1.3 Non-Road

Figure 6 shows the global regulations for the non-road sector. This category covers engines used in agriculture, construction, and other similar machinery, and gensets.

Figure 6 Non-road criteria pollutant regulations in some of the major markets around the world

The European Stage V regulations include a DPF-forcing PN emission limit of 1×1012#/(kW·h). In most of the market covering 56～560 kW engines, the NOxlimit is 0.40 g/(kW·h) and PM limit is 20 mg/(kW·h). China is implementing Stage 4 non-road regulations starting 2020. While the NOxlimits are about 5 times those of the EU and US, the DPF forcing PN limit of 5×1012#/(kW·h) is included for 37～560 kW engines. Manufacturers are encouraged to bring their engines to Euro Stage Ⅴ standards.

India is implementing EU Stage Ⅳ regulations in October 2020, and has already announced the move to Stage Ⅴ with a PN regulation in 2024.

Like the on-road sector, California is beginning discussions on tightening of emissions from off-road engines. Research projects are underway to explore the potential for further reductions by using advanced aftertreatment technologies.

1.4 LD CO2 or Fuel Consumption

Figure 7 is an updated summary of the CO2or fuel consumption targets in major markets. These include the latest revisions by the US EPA and the National Highway Traffic Safety Administration(NHTSA), which require a 1.5% reduction in fuel consumption per year for model years 2021—2026 passenger cars and light trucks. The CO2emission targets of NEDC for Euorpe in 2021, 2025 and 2030 are 95 g/km, 81 g/km and 59 g/km, respectively, while the 2025 target for China is not final and the CO2value is calculated based on a target by the MIIT.

Figure 7 Summary of CO2 targets for light-duty passenger cars in major markets

As is clear from the figure, the targets are most stringent in Europe. In 2019, the average CO2emissions from new cars was 123 g/km[21], which is significantly higher than the target of 95 g/km in 2021. More-over, further reduction of 37.5% is required through the end of the decade, and an annual improvement of 5.6% per year will be required. This is very aggressive, especially considering the continuing decline in the diesel share(from 36% in 2018 to 30.5% in 2019), and the increase in popularity of SUVs. OEMs are looking at significant penalties due to the stipulated fine of95 per g/km of CO2exceedance, and significant electrification is expected to meet these GHG regulations.

China’s Ministry of Industry and Information Technology(MIIT) published the Stage 5 passenger vehicle fuel economy regulations. The rule will require average fuel consumption of 4.0 L/100 km in 2025 and targets 3.2 L/100 km in 2030. Credit targets for new energy vehicles(NEVs) extended to 14%, 16% and 18% for 2021, 2022 and 2023, respectively. The subsidies for NEVs have been extended through 2022[22]. Fuel efficient vehicles(FEVs), defined as conventional ICE vehicles with fuel consumption less than 3.2 L/100 km can earn credits, like NEVs and is expected to encourage hybridization. For 2021—2025, base credits decrease from 2 to 1 for NEVs and from 1.4 to 1.0 for FEVs. Methanol fueled vehicles are included in the list of conventional vehicles. The government is targeting 25% NEV sales by 2025.

1.5 Heavy-Duty Greenhouse Gas Regulations

HD GHG regulations are now adopted by six major markets：US, China, Japan, India, Canada and Europe. Europe approves the first CO2standards for new trucks, requiring 15% reduction by 2025 and 30% by 2030, compared to a 2019 baseline. China’s Stage 3 standards require about 15% lower fuel consumption(11%～18% depending on specific vehicle type and weight). The US and Canada require 15%～27% GHG reductions by 2027 compared to a 2017 baseline, depending on the gross vehicle weight(GVW) and configuration. HD pickup trucks require reductions of 16%. Japan has finalized their phase 2 standards, which call for about 12% average reduction in fuel consumption over 2015 baseline, starting in 2025.

The first electrification mandate for heavy-duty vehicles has been proposed by CARB, titled the “Advanced Clean Trucks” regulation[23]. The proposal requires 5% of annual sales to be ZEVs starting from 2024 and ramping up to 30% for Class 2B～3 and class 7～8 tractors by 2030. For class 4～8 vocational, the requirements start at 9% and ramp up to 50% by 2030.

2 Light Duty Engine Technology

Technologies aimed at reducing fuel consumption and tailpipe GHG emissions are rapidly advancing, and fleet-wide reductions will require a concerted approach of improved IC engine efficiency, fuels and electrification[24-25].

Several recent studies show the significant potential for IC engine improvements alone to deliver 20%～30% CO2reductions. In a benchmarking study, Kargul et al.[26]measured 39.8% peak brake thermal efficiency(BTE) on Toyota’s 2018 4-cylinder 2.5 L naturally aspirated engine, using Atkinson cycle engine, dual port- and direct-fuel injection and cooled exhaust gas recirculation(c-EGR). This is representative of the best peak BTE for non-hybridized spark ignited engines today. Simulations done for a mid-size vehicle showed the possibility of 35% reduction in GHG without any level of electrification. Technologies included were 8-speed transmission, start-stop, fixed or dynamic cylinder deactivation technologies, and reduced curb weight(7.5%), and reductions in both aerodynamic drag and rolling resistance(10%). Deeper reductions in GHG can be obtained by using renewable fuels. Sellers et al.[27]demonstrated 45% BTE on a single cylinder engine through a combination of technologies such as smaller bore/stroke ratio, high CR(17), Miller cycle with early intake valve closure(EIVC), lean homogeneous combustion(λ>2) and direct water injection. As an example of incremental improvements towards exceeding 45% BTE, figure 8 shows the approach by Kapus et al.[28]. Simulations further show the possibility of exceeding 50% BTE using lean combustion.

Figure 8 Pathway for development of gasoline engines to exceed 50% BTE[18]

Table 2 shows some of the leading engine technologies for improving fuel economy. While some of these are already commercialized, there are others which are in the early to late stages of development.

It is not within scope to describe all of the technologies indetail, however it is worth highlighting some of the leading ones. For gasoline direct injection compression ignition(GDCI), Sellnau et al.[29]showed pathway to 48% BTE using high CR(17), advanced fuel injection and turbocharger systems, intake temperature management, reduced thermal and friction losses. Combined with mild hybridization, fuel savings of up to 44% can be obtained. Mazda has commercialized the spark assisted compression ignition technology which, combined with a high CR(16.3) and improved combustion chamber design offers larger than 10% improvement in CO2and torque each.

Technologies for addressing knock and mitigating the need for fuel enrichment are also being developed. Water injection has been studied by various groups and offers 5%～7% improvement in fuel consumption on average, with much higher reductions seen under high load conditions(where fuel enrichment is used). Another technology is pre-chamber ignition, in which radicals from the pre-chamber introduced in the main chamber act as multiple ignition sites to reduce knock. Both passive and active pre-chamber systems are being developed, and for the latter, Bunce et al.[30]showed 42.3% peak BTE for a 3-cylinder 1.5 L GDI engine. Octane on demand relied on an on-

Table 2 CO2 reduction engine technologies for light-duty vehicles

board module which separates fuel into high and low-octane components, with the former being used under high load operating conditions for reduced knock[31-33].

Of course, reductions in CO2are not limited to engine improvements alone. The combination of advanced ICE technologies, renewable fuels and electrification is projected to offer 70%～90% reduction in CO2[34-35]. The DoE sponsored Co-Optima program[36]is identifying synergies between advanced combustion and tailored fuel properties for reducing GHG and criteria emissions. As an example of the progress made, isopropanol blended at 30% is estimated to improve lifecycle GHG by 16% annually by 2050.

Various powertrain projections are converging, and it is expected that in 2030, 70%～80% of new vehicles sold will have an IC engine. However, in the short-medium term, hybrids(mild to full) are expected to increase in market share driven by the CO2targets. As an OEM example, Kimura[35]of Honda showed that by 2030, every other new vehicle sold will be hybrid or plug-in hybrid.

Normalized to battery size, hybrids offer the highest CO2savings, an important consideration for limited battery raw materials. Fleet testing under real-world driving conditions has shown that mild and full hybrids can offer about 30% CO2reduction with an average battery size of 1.2 kW·h[37], without requiring any additional investment in charging infrastructure. Through simulations, Abdul-Manan et al.[38]showed that hybridized GCI engines can offer 26%～55% lower well-to-wheel GHG compared to a conventional gasoline vehicle. On a per kW·h battery basis, over 50% of the GHG reductions were obtained through mild hybridization alone, with diminishing returns with added battery size. Mild hybrids, with 48 V battery systems are accordingly also expected to gain significant market share, offering CO2reduction through idling savings, and engine-off coasting and energy recuperation. Various architectures are being considered(P0～P4, depending on location of the generator/motor). Alt et al.[39]estimated the CO2reductions at 8%～12% for P1, 12%～16% for P2 and 14%～19% for P3 layout. Incremental cost for mild hybridization was in the range of 40～80/(g·km-1) of CO2improvement, substantial less than the EU penalty of 95/(g·km-1). Full hybrid 48 V systems(30 kW peak power) offer enhanced energy recuperation in braking, and limited all-electric traction. CO2reduction improves about 20% for the full hybrid system[40].

3 Heavy-Duty Engine Developments

The stringent CO2reduction targets in Europe and the US will require rapid adoption of pathways for reducing fuel consumption. It is expected that the 2025 targets can be met with advanced ICE technologies, while meeting subsequent targets will require a combination of electrification and low carbon fuels. The table below lists some of the technologies available for CO2reductions. The National Academies of Sciences, Engineering and Medicine has published a detailed report[41]on the potential of these technologies. For Class 8 trucks, these technologies can deliver up to 30% CO2reductions, and the use of low carbon fuels such as biodiesel and hydrogenation derived renewable diesel can further reduce wells-to-wheel CO2up to 80%. Table 3 provides a summary.

Table 3 CO2 reduction engine technologies for heavy-duty vehicles

Shown in figure 9 is a projection by Pischinger et al.[42], where simulations showed the possibility to reduce fuel consumption(FC) by up to 18% through a combination of various ICE technologies, mild hybridization and waste-heat recovery.

The US Department of Energy Super Truck Ⅱ is targeting 55% BTE during highway cruise, and doubling freight hauling efficiency((mi/gallon)·t) versus a 2009 baseline. Various teams are participating, and common elements include weight reduction, improved aerodynamics, lower rolling resistance, improved combustion and air handling(Miller optimization, improved turbochargers), waste heat recovery(with about 4% BTE improvement targeted), mild hybridization, and improved after-treatment systems. Table 4 provides a summary of various technology approaches being pursued specifically for achieving 55% BTE[43]. A common element for improved after-treatment systems is the use of close-coupled SCR with dual injection for lower NOx.

Dynamic cylinder deactivation is seen as a key enabler to meet the upcoming California Ultra-Low NOxregulation. The technology delivers both lower low-load fuel consumption and higher exhaust temperatures, the latter being critical for improved de NOx. A recent study[44]using a 15 L 6-cylinder diesel engine showed reduction in fuel consumption of up to 20% during idling and 1.5%～3.7% CO2reduction(simulated) on the FTP and low load cycles, while reducing engine out NOxby 45%～66%.

Gasoline compression ignition was mentioned in the light-duty section, and this is also being evaluated for heavy-duty applications.Sim et al.[45]paired a 6 L medium duty diesel engine with high reactivity gasoline fuel and found an improvement in fuel efficiency of 4.3%. NOxemissions increased however, optimization of fuel injection strategy had to be pursued to lower NOx. Opposed-piston engine technology is also progressing, and 45.1% BTE was demonstrated on a 10.6 L 3-cylinder 450 HP engine[46].

Figure 9 Pathway for fuel consumption reduction, simulated for a long-haul cycle[42]

We now provide an overview of advances in emissions control technologies in the next few sections.

4 Gasoline Emission Control

Figure 10 shows the evolution of light-duty gasoline after-treatment systems. Modern after-treatment systems have clearly advanced beyond three-way catalysts and now include gasoline particulate filters, especially in Europe and China. Moreover, with upcoming post Euro Ⅵ regulations it is expected that additional content could be added

Table 4 Various approaches being pursued by participants of the Super Truck Ⅱ program to meet the 55% BTE target

to address previously unregulated species(such as ammonia) and to reduce cold-start emissions.

Figure 10 Evolution of light-duty gasoline after-treatment systems

4.1 Three-Way Catalysts(TWC)

The three-way catalytic converter is a mature technology, ubiquitous on cars around the world for more than 35 years. Modern gasoline after-treatment systems remove larger than 99% of HC, and NOx, and yet improvements of the TWC are still being sought to meet further tightening gas emission regulations. Emphasis of most research today is on achieving earlier catalyst light-off to reduce cold start emissions, and various strategies are being pursued:(1) Moving the catalyst closer to the engine, improved insulation, increased catalyst volume and PGM loading. For recently introduced mild hybrids, moving from compliance with EuroⅥ/China 6 to SULEV standards required a combination of 50% increase in catalyst volume and moving the close-coupled TWC to a turbo-mounted position[47]. However, adding PGM alone is not going to address cold start emissions. For instance, Kim et al.[48]found that the NOxlight-off temperature only improved from 229 ℃ to 211 ℃for the highest PGM loading on four SULEV30 vehicles with PGM loadings ranging from 2.9 g/L to 6.9 g/L. Similarly, Zhang et al.[49]showed that increasing front catalyst PGM loading from 0.7 g/L to 1.4 g/L decreased the total HC emissions to below the China 6b target, but higher loadings showed no improvements. (2) Improvements in catalyst formulations. One example is the work done by Theis et al.[50], who reported greatly improved activity using a silica-stabilized alumina support modified using zirconium and titanium monolayers. The aged catalyst(50 h with 10% water at 950 ℃) achieved 90% conversion below 300 ℃ for CO, HC, and NOx. Optimized catalysts using 0.5% Rh-8.0% titanium and 0.6% Rh-15.0% zirconium formulations dropped the T90 temperatures well below those obtained using commercial catalysts, as shown in figure 11(Leftmost data was measured on commercial catalysts, reactor testing done at space velocity(SV)=30 000 h-1with aged powder catalysts). (3) Adding or modifying storage functionality. Oxygen storage capability is critical for TWC activity under varying lambda conditions. One example of innovation in this area is the work done by Chinzei et al.[51], who discussed the development of a pyrochlore ceria-zirconia OSC material. The new material has lower specific surface area(compared to conventional OSC), which helped lower oxidation of Rh. Also, pore connectivity was improved leading to better transport properties. NOxconversion was improved and led to 33% lower PGM content. Another concept being explored is adding HC trap functionality to the TWC substrate[52]. In one example[53], HC traps were installed in the underfloor location, using a combination of large pore zeolites below the TWC layer. With some catalyst optimization, 50% reduction in HC emissions was obtained on the FTP cycle. In another example, Choi et al.[54]added NOxstorage functionality to a TWC in the underfloor position on a ULEV70 vehicle with a 2.4 L 4-cylinder engine to address NOxemissions during lean deceleration fuel cuts. On the FTP-75 cycle, NOxwas below the SULEV30 emission levels, while still allowing about 100 s of additional fuel cut decelerations and enabling the associated fuel efficiency benefits. (4) Active thermal management including pre-heating of the catalyst before drive-off. Latest studies are exploring the possibility of reaching near-zero emissions, well below the tightest SULEV30 limits. These vehicles will in fact be at negative emission levels when accounting for lower tailpipe concentrations compared to the intake ambient air. Kawaguchi et al.[55]demonstrated larger than 90% reduction over SULEV30 limits on a modified MY2016 Prius Prime plug-in hybrid. The first catalyst was preheated for ～55 s during which time the vehicle was propelled using the motor. Thewes et al.[56]pre-heated the catalyst for about 77 s to reach catalyst temperatures larger than 500 ℃, and added a HC trap to minimize emissions during the first few seconds following the engine start. However, such active heating measured are expected to result in a fuel penalty. In the latter study, a 4.3% fuel penalty was calculated compared to the Euro Ⅵ-D-TEMP baseline condition.

Figure 11 Temperatures for 50%(T50) and 90%(T90) conversion on various TWC formulations[48]

As shown in figure 10, new components are being added to the gasoline after-treatment system. Gasoline particulate filters will be discussed in the next section. To address the potential limits on NH3emissions starting with Euro Ⅶ, the use of passive SCR technology is also being studied. Schmitz, et al.[57]replaced an underfloor TWC with either an SCR or ASC(ammonia slip catalyst). While both approaches reduced NH3emissions to about 1 mg/km or less, the SCR also resulted in an additional about 25% reduction in NOxcompared to the TWC-only system.

4.2 Gasoline Particulate Filters

Numerous studies have documented the harmful effects of elevated ambient particulate concentrations on human health.Recently, a comprehensive study was done by researchers at the Harvard T.H. Chan School of Public Health to discern the impact of PM2.5on human health[58]. The team drew upon data spanning over 16 years from larger than 68 million Medicare enrollees in the US. It concluded that 143 257 lives could have been saved if the US would have lowered the PM2.5annual standard from 12 mg/m3to the WHO guideline of 10 mg/m3. It provides the most robust evidence on the causal linkage between PM2.5and mortality and shows that there is no lower “safe” limit for PM2.5ambient concentrations. Another study found that an increase in PM2.5concentrations by 10 μg/m3is associated with a 0.55%～0.74% increase in cardiovascular and respiratory mortality[59]. Considering that several major cities in the world routinely have ambient concentrations above 100～150 mg/m3, there is clearly a need to adopt technologies to limit particulate emissions. Particulate filters are a proven technology for diesels and more recently introduced for gasoline vehicles. GPF technology has been reviewed in detail in some recent publications[60-61].

GDI engines have gained significant market share in the past few years in major markets of the world, due to the superior combustion it offers compared to PFI engines. However, the resulting insufficient time for air-fuel mixing and the fuel impingement on piston and wall surfaces leads to increased particulate formation, which has led to both tighter particulate regulations as well as advanced mitigation strategies. Injection systems are improving, and studies show that the trend to higher injection pressures can potentially reduce the total PN count as well as suppress the sub-23 particles[62]. Still, the use of a GPF is seen necessary to meet the tightening standards, under dynamic driving conditions and over the lifetime of the vehicle. Pauer et al.[63]measured the PN emissions from a mid-size vehicle with a 2 L turbocharged GDI under aggressive RDE urban driving conditions at -7 ℃. While the tailpipe emissions were reduced by 65% when switching from a 250 bar(1 bar=0.1 MPa) to a 500 bar injection system, the emissions were still larger than 2 times of that of the regulated limit. A high filtration GPF was used which offered a further PN reduction of 97% to bring the emission substantially below the limit.

Post Euro Ⅵ regulations will likely require compliance with the PN limit at -7 ℃. Figure 12 is a summary of testing data for vehicles sold over 15 years[64]. It is seen that at such low temperatures even PFI vehicles emit very high emissions and GPFs are expected to be required. Moreover, the required filtration efficiency of future GPFs is expected to increase.

Figure 12 Particle number emissions increase significantly when tested at sub-zero ambient temperatures

GPFs can be either uncoated or incorporated a TWC coating. The general trend has the EU favoring uncoated filters, while China prefers coated filters, likely aiming for more efficient utilization of the underfloor space to meet upcoming China 6b gas regulations. Coated GPFs can provide similar or better gas emissions performance compared to the flow-through TWC substrates they replace. Through testing on the US FTP-75 cycle, Craig et al.[65]found that while most of the NOxreductions occur in the close-coupled TWC, the GPF added 18%～30% additional reductions. The durability of coated GPF was also demonstrated on a 1.4 L GDI engine in China, where about 15 ℃ increase in light-off temperature was found after 160 000 km of testing[66]. Remarkably, it has also been shown that GPFs offer lower deterioration of the oxygen storage capacity(compared to a conventional TWC) due to separation of ash on the walls, avoiding contact with and hence poisoning of the catalyst[67].

Several studies have shown that the accumulation of ash and formation of a membrane on the inlet channel is in fact beneficial for improved filtration efficiency, although at a pressure drop penalty. In one example Liu et al.[68]tested a bare GPF at ash loadings of 0.5～1.5 g/L, corresponding to 5 000～7 000 mi(8 000～11 200 km) of driving. The filtration efficiency increased about 30% with 1.2 g/L ash. GPFs are therefore seen as one of the few technologies which improve with vehicle aging!

Contrary to DPFs, there are very few conditions when GPFs need to be actively regenerated. The combination of higher exhaust temperatures and lean fuel cuts upon deceleration can suffice to provide passive soot regeneration.Nicolin et al.[69]loaded an uncoated GPF with 6 g of Printex-U(a surrogate of soot) and measured the remaining soot after various real-world driving conditions. It was found that the soot regenerated continuously, even at temperatures below 450 ℃, assisted by fuel cuts. Using fuel cut procedures on an engine bench, Boger at al.[70]evaluated the sensitivity of soot oxidation to various engine-out exhaust and soot loading parameters. At lower temperatures, the regeneration is limited by reaction kinetics while at higher temperatures, by the soot loads. Models have been developed and the understanding is now sufficiently advanced to make recommendations on the use and suppression of fuel cuts to ensure adequate temperature management.

Hybrids, despite their improved fuel economy, are known to suffer from higher particulate emissions due to frequent engine re-starts. Yang et al.[71]measured RDE emissions from a 2 L GDI and a 1.8 L PFI China 6 hybrid vehicles. The hybrids were found to have PN emissions exceeding the stage 6 PN limits in Europe and China, and perhaps more concerningly, higher than their conventional vehicle counterparts. Under urban driving, the PFI hybrid emitted higher particulates than the GDI. This points to the need for improved strategies for managing the battery state of charge which are currently optimized for fuel economy and not criteria pollutant emissions.

Other than the low temperature testing mentioned above, another factor increasing the engine out particle count is the proposed inclusion of sub -23 nm particles in Europe. This is certainly expected to push GPF adoption on PFI engines as well.Andersson[17]has presented a comprehensive summary of a measurement campaign looking at sub-23 nm particles from a wide range of vehicles(including motorcycles and heavy-duty vehicles), fuels(including CNG) and test cycles. The work done to date shows that measurement of particles down to 10 nm looks feasible and that PFI and even CNG vehicles could exceed the regulatory limits when the sub-23 nm particles are counted. The filtration efficiency of GPFs(filters in general) increase with decreasing particle size so that capturing these sub-23 nm particles is not seen to be an issue. For a Euro Ⅵ-B compliant 1.4 L GDI vehicle equipped with a GPF, Suarez-Bertoa[72]found PN emissions were an order of magnitude below the limit, even when including sub-23 nm particles and low temperature(-7 ℃) testing on WLTP and RDE.

GPFs have other benefits extending beyond meeting the regulated particle limits. Gasoline exhaust is known to carry polyaromatic hydrocarbons(PAHs), which are likely or proven toxic carcinogens. These can be adsorbed on soot particles, and therefore removed by GPFs. In one study[73], a GDI pick-up was found to emit 14 times of more total PM-bound PAH emissions than a similarly powered PFI truck. In another study, Yang et al.[74]detected several carcinogenic gas, particle and nitrated PAHs in the exhaust from two MY2016 GDI vehicles. A catalyzed GPF replaced the TWC and led to particle-phase PAHs being reduced by 97%～99%, gas-phase PAHs by 54%～61%, and nitrated PAHs by 56%～92%, as shown in figure 13.

Figure 13 Toxic PAHs are reduced over 97% when replacing an underfloor TWC with a catalyzed GPF[74]

4.3 Lean Burn Gasoline NOx Control

As mentioned in the earlier section on engine efficiency, Mazda has commercialized the spark assisted compression ignition, which runs the 2.0 L engine lean burning in some part of the operating map and switches to stoichiometric burning at higher loads[75]. Very high direct injection pressure of 700 MPa is used which presumably leads to both improved combustion and lower particulates. Yet a GPF is employed to meet the particulate regulations. The after-treatment is still stoichiometric(TWC only) and EGR and air dilution are used to meet NOxregulations. However, for greater fuel savings, the lean burning operation will have to be extended and the engine paired with lean NOxafter-treatment.

Several teams are pursuing research on lean-burn gasoline engines and addressing the challenge of unlocking the fuel savings while managing the lean after-treatment. Oak Ridge National Labs has been working on passive-SCR approach[76], in which the ammonia is generated on the upstream TWC during periodic rich operating conditions. Lean conditions(λ=1.4～2.2) are run up to 4 500 r/min and 75% load while at higher loads the 2.0 L 4-cylinder lean GDI engine switches to stoichiometric. Fuel economy improved by 8.3% relative to baseline stoichiometric operation. The TWC is Pd-only, along with a NOxstorage and NH3generation catalyst, and is followed by a GPF and Cu-zeolite passive SCR. The system was found to meet the Tier 3 Bin 30 limit on a pseudo-transient 6-mode cycle. A cleanup oxygen-storing slip catalyst was also added in the later stages of development to reduce CO to the 1 g/mi limit, but it also converts some NH3back to NOx, so more development work is needed.

Osborne et al.[27]have outlined their approach to the development of a 2.0 L turbocharged GDI operating with lean-homogeneous combustion at part loads. The goal is to achieve 15% reduction in fuel consumption. It uses a variable geometry turbocharger and a 48 V electric supercharger to achieve high boost pressures, and variable cam timing and lift to enable Miller cycle operation. The NOxcontrol system included a close-coupled LNT, underfloor Fe-based SCR with urea injection, and low pressure EGR. The cooler exhaust temperatures motivated the use of a 3 kW EHC, while a GPF was added for particulate control.

Perhaps the challenge for lean burn systems is best shown as figure 14. GOC=Gasoline oxidation catalyst, NSC=NOxstorage catalyst, adapted from the work of Battiston[77]. In this case an active urea dosing system was used along with other components shown. The authors noted that the exhaust temperature was lower than 300 ℃ over much of the engine map and required the use of various engine thermal management measures, but which also lead to a fuel penalty. Moreover, the use of an active urea dosing raises the cost of the system such that on a $/CO2basis, lean combustion engines are at the same level as hybrids.

Figure 14 The challeage for lean burn systems

5 Light-Duty Diesel NOx Systems

Despite losing significant market share, one in every three new cars sold in Europe is a diesel(2019 figure). Diesels, especially in the larger vehicle categories, still offer reduced CO2emissions compared to gasoline, and a decline in diesel market share makes meeting the CO2regulations that much harder. Moreover, recent Euro Ⅵ and RDE regulations have mostly solved the issue of high on-road NOxemissions from diesels. In fact, as seen in figure 15, modern diesels are now emitting 90%～99% less NOxcompared to pre-Euro Ⅵ-D regulations limits, and far exceeding the requirements[56].

Figure 15 Post Euro Ⅵ-D -TEMP diesels are emitting up to 99% lower NOx emissions compared to pre-Euro Ⅵ-D -temp levels[56]

LD diesel after-treatment has evolved over the years to a rather complex system, and figure 16 shows how each regulatory tightening led to new and advanced components. Post Euro Ⅵ regulations are expected to include dual urea dosing for the twin SCR systems, and a combination of NOxstorage at low temperature through LNT and/or the early activation of catalysts using thermal management and electrically heated catalysts.

Figure 16 Evolution of light-duty diesel after-treatment systems[78]

Various groups are already demonstrating that diesels can meet potential Euro Ⅵ regulations. Krüger et al.[79]reported incremental improvements to a compact car with a 1.7 L engine and an after-treatment system comprised of a DOC, close-coupled SCR on filter, and an underfloor SCR with an ammonia slip catalyst(ASC). Engine out emissions were reduced via a combination of improved fuel injection and calibration, while variable geometry turbocharger increased exhaust heat during cold start. Transient and high-speed NOxslip were improved by increasing the underfloor SCR and ASC volume by 2.6 times and 2.2 times, respectively; and dual dosing and mixers were applied to the SCR filter and main SCR. Average NOxemissions over repeated RDE testing was about 13 mg/km.

Pre-turbo catalysts are being considered, to avail of high engine out temperatures and improve cold start emissions. Christmann et al.[80]studied the emissions from a 2.0 L 48 V mild hybrid diesel fitted with a pre-turbo DOC, SCR and SCR on filter, followed by an underfloor SCR with twin dosing. Enthalpy loss due to pre-turbo catalysts was about 4% but was overcomed by using an 11 kW ele-ctrical supercharger. An EHC and engine heating methods were used to achieve NOx<35 mg/km in continuous low-load city driving. The pre-turbo emissions system resulted in CO2savings of 6%～19% due to reduced active thermal management.

Cylinder deactivation is gaining attention as a fuel saving and emission reduction technology in both light- and heavy-duty applications. Cylinders shut off during low load, allowing the remaining cylinders to operate more efficiently and with increased exhaust temperatures for improved SCR conversion. Scassa et al.[81]simulated a typical aftertreatment system with a DOC, SCR filter, and underfloor SCR on a C-segment SUV with a 2 L engine and high- and low-pressure EGR. Exhaust temperatures increased from 25～130 ℃ depending on operating condition, 2%～4% fuel consumption reduction and up to 14% lower NOxon the RDE cycle were estimated.

Other than the above system solutions, there is of course considerable innovations happening at the component level and we briefly touch upon these next.

5.1 Diesel Oxidation Catalysts

There is a focus on improving low temperature conversion for all catalysts, and DOCs are no exception. Toops et al.[82]explored an improved DOC for achieving 90% conversion(T90) at 150 ℃. A core-shell catalyst of ZrO2on SiO2was explored, with Pt/Pd supported on the ZrO2shell. After hydrothermal aging(800 ℃ for 10 h), T90 for CO was measured at 175 ℃ and for HC at about 250 ℃.

Hybrid catalysts combining ammonia slip catalyst(ASC) and DOC functionality into the same component are being developed. The catalyst combines an inner layer of NO oxidation catalyst and an outer SCR coating for NOxconversion. The key requirement is to produce high NO2concentration for fast SCR, without generating N2O. Geisselmann et al.[83]showed this trade-off, measuring about 40% NO2/NOxratio at the low ammonia to NOxratio(ANR) of 0.6, but only 25% NO2at an ANR of 1.0. Newman et al.[84]reported on a catalyst that can generate upwards of 30% NO2at 300 ℃ in the presence of ammonia with no impact on N2O selectivity.

5.2 Selective Catalytic Reduction(SCR)

Low temperature SCR conversion is the biggest challenge for meeting the tighter NOxregulations. SCR activity is limited both by reaction kinetics at low temperatures and the inability to dose urea at temperatures below 180～200 ℃. However, with increased durability requirements and the possibility of adding SCR in the close-coupled location for heavy-duty low NOx, there is also an emphasis on improved durability and sulfur tolerance.

Newman et al.[84]made formulation adjustments and reported NOxconversion improvement from 65% to about 73% at 175 ℃ for a catalyst aged at 650 ℃ for 50 h. Conversion at high temperatures also improved from about 96% to 99% at 600 ℃, dropping these emissions by 75%. Figure 17 shows that durability is also impressive, with NOxconversion performance maintained over a wide operating window even after aging to 900 ℃. Also shown in that figure is work reported by Geisselmann et al.[83], where NO conversion was doubled from about 40% to 80% at 175 ℃ in the lab, while also improving high temperature conversion. Almost full conversion is observed over 250～550 ℃ for the “standard” SCR reaction with no NO2. These new Cu-SCR catalysts also respond better to sudden increases in NOxflux caused by increasing load, sometimes with lower oxygen and NO2levels. Under such conditions, NOxconversion of Cu SCR catalysts was found to be superior compared to iron and vanadium based catalysts.

Figure 17 Cu-zeolite SCR catalysts are showing improved conversion as well as high temperature durability for the standard SCR reaction(NO only)

Novel and hybrid catalyst formulations are being tested. Work has recently been reported on Cu/LTA catalysts showing improved hydrothermal stability under aggressive aging(900 ℃, 10% H2O for 12 h) compared to commercial Cu/SSZ-13 catalysts. However, Kim et al.[85]found that under milder aging conditions(650 ℃, 24 h), the Cu/SSZ-13 performed better. With Cu/LTA catalyst in front to provide the low-temperature activity and a Cu/SSZ-13 behind it to extend hydrothermal stability, the benefits of both catalysts was realized. With a view to reducing N2O emissions, Pauly et al.[86]showed that a 25% Fe/75% Cu catalyst mixture provides a good solution. During WHTC testing on a 6.7 L engine, the catalyst was found to reduce N2O emissions by half, while improve NOxconversion slightly, compared to Cu zeolite alone.

As mentioned at the start of this section, the other factor limiting low-temperature SCR conversion is the inability of urea dosing at temperatures less than 180 ℃ to avoid deposit formation. Various novel solutions are being proposed to address this issue. Hartley et al.[87]investigated adding ammonium titanyl oxalate to urea to promote urea decomposition, along with a surfactant to facilitate water evaporation. The combination was shown to reduce deposits by up to 72% at 180 ℃. In another approach, Wilson et al.[88]demonstrated the use of an on-board reactor using exhaust waste heat after the SCR to partially convert urea to ammonium carbamate solution. The solution is then injected before the SCR and decomposes to ammonia when temperature is larger than 60 ℃. Vehicle testing showed significant NOxreductions despite SCR inlet temperatures at or below 200 ℃. Urea heating approaches using glow plugs(Okada 2019) during injection or heating rods in the urea tank(Larsson 2019) to promote urea decomposition at low injection temperatures also show promise in low load engine tests.

The integration of SCR catalyst with a filter is now common in light-duty systems and is being investigated in heavy-duty systems to enable early warm-up to temperatures where the urea dosing can be started(about 180～200 ℃), and in non-road applications, reduce system size. George et al.[89]showed that this technology allowed for 90～140 s early urea dosing based on the reference DPF thermal mass. As shown in figure 18, testing was done on a US EPA 2017 on-road engine, and using the non-road transient cycle(NRTC) with the aim of evaluating the concept also for the Stage 5 non-road regulations. Δtis the time to get from room temperature to 200 ℃ or 180 ℃.

Figure 18 The combination of SCR with filter enables early urea dosing and improved cold start emissions[89]

The challenge for integrated DPF-SCR systems is managing the competition for NO2for SCR and passive soot regeneration. Robb et al.[90]performed simulations for a 7.75 L engine, with a DOC, SCR filter, and another SCR for a total SCR volume 3.6 times of the engine displacement. They explored sensitivity of the NOxperformance and soot regeneration to various inlet conditions. The soot balance point temperature, in which soot accumulation rate and oxidation rate are in balance, was found to be most sensitive to the soot：NOxratio, followed by the NO2/NOxratio, and then the incoming exhaust temperature. NOxemissions were below regulated limits for all evaluated test cycles, while also managing the soot loads on the filter.

5.3 NOx Adsorbers and Hydrocarbon Traps

Lean NOxtraps(LNT), passive NOxadsorbers(PNA), and hydrocarbon traps(HCT) can serve as important cold start emission mitigation tools, but also add complexity of calibration, durability, and fuel penalty.

Pd/zeolite catalysts have been the leading candidate for PNA, with SSZ-13 zeolite becoming the favorite choice. Szanyi, et al.[91]found a Si/Al ratio of 6 in the zeolite combined with atomic dispersion of 2%(by weight) Pd was found to be optimal with uptake of one NO molecule per Pd site. However, it lost less than 20% of performance when aged at 750 ℃ for 16 h with 10% water vapor.

Toops[92]probed the details of NOxuptake degradation on a 1% Pd/SSZ-13 PNA catalyst through repeated adsorption-desorption runs. It was found that about 20% of NO uptake capacity was lost in 10 repeat runs when CO was present in the feed, and that there was no loss in the presence of only HCs.

Harold et al.[93]have shown feasibility of an integrated PNA with a HCT. A dual layer catalyst with an inner Pd/SSZ-13 and outer Pt/Pd/BEA layer was evaluated by using a feed of combined HC and NO. The NO was found to be released over about 200～500 ℃(suitable for SCR conversion) and was found to partly oxidize to NO2due to the Pt content from the HCT. The desorption of NO was not inhibited by the HC species, and the overall performance was found to be similar to that of a PNA + HCT system using the catalysts in series.

5.4 Diesel Particulate Filters(DPFs)

DPFs are a mature technology and are known to deliver particulate reductions that far exceed the requirements set by regulated limits. Focus of future regulations, such as the periodic technical inspection in Europe[94], is mostly ensuring in-use compliance and ensuring against tampering and malfunctioning of filters. In a fleet study in Europe[95], it was found that about 10% of the high emitting cars were responsible for about 85% of the overall fleet emissions, highlighting the need for ensuring full in-use compliance. Majority of the cars were found to emit particles at concentrations below 104cm-3, or about 10 mg/m3, which is equivalent to the WHO annual mean guideline for PM2.5in the ambient air. Considering that some of the major cities including highly populated ones in China and India routinely have PM concentrations which exceed 100 mg/m3, it can be argued that a DPF-equipped vehicle can be effectively cleaning the ambient air and delivering a “negative emissions” vehicle!

Zone coating the DPF with a DOC catalyst is helping with system downsizing and moving the SCR upstream for improving NOxconversion. Sethuraman et al.[96]demonstrated 40% lower volume compared to a reference system, along with improved passive soot oxidation due to increased PGM — soot contact. Simulations showed that the faster heat up of the downstream SCR allowed about 60 s earlier urea injection and 33% lower NOxon the WHTC, compared to the baseline.

Starting with Euro Ⅵ-E regulations for heavy-duty trucks, particle number emissions will also have to meet in-service conformity testing(with a conformity factor of 1.63). The upcoming heavy duty ultra-low NOxregulations in California also require a reduction in tailpipe PM by 50%. Accordingly, there is a requirement for continued improvement in DPF filtration efficiency. Using a 13 L Euro Ⅵ-D engine, Petersson[97]showed that PN emissions from a soot-loaded filter increased during some periods of highway driving. The higher exhaust temperatures oxidized in-wall soot, reducing the filtration efficiency. The effect was not observed with clean filters, where the in-wall soot is continuously replenished. This data was used to highlight the need for filters with smaller pores for enhanced filtration.

Viswanathan et al.[98]discuss various advanced DPF technologies for meeting the future regulations. Higher open frontal area(OFA) designs are being advanced to improve the ash capacity. New asymmetric cell designs were shown to reduce pressure drop by 30% at 70 g/L ash load. A combination of thin walls and novel cell designs was shown to further reduce the pressure drop, while improvements in microstructure promise to improve filtration efficiency by over 95%.

Fundamental studies continue to look into improving the understanding of DPF pressure drop and filtration. Wang et al.[99]studied the increase in pressure drop due to ash clogging in mid-axial channel location, especially with high soot∶ash ratios. A model reasonably predicted the trends in pressure drop with different soot∶ash ratios. Sappok, et al.[100-101]advanced the use of Radio frequency(RF) sensing technology to diagnose change in filtration efficiency or filter integrity. By removing filter end plugs, soot slip could be varied by four-fold. The RF signal detected even small changes in filtration efficiency and accumulated soot. With neural networks, the technology can also monitor ammonia storage on soot-loaded SCR filters within ±0.25 g/L. Cooper et al.[102]used magnetic resonance imaging(MRI) to directly measure the flow field across a bare and coated filter. Measurements of the inlet/outlet contraction and expansion pressure losses and the flow velocity within the inlet and outlet channels provided insights into non-uniformities in the wall permeability and might lead to better catalyst coatings.

6 Systems for Emerging HD Low NOx Regulations

We end with a description of how some of the above technologies will come together to address one of the most stringent regulatory hurdles coming up.As described in the regulatory section, California and the US EPA are proposing 90% NOxreduction for heavyduty vehicles, while also strengthening the test framework, adding new certification test cycles and greatly increasing durability requirements. Significant advances in engine and aftertreatment technologies will be needed in the coming decade to meet these requirements. According to one estimate[103], NOxconversion efficiencies greater than 97% is required for the cold start FTP cycle, while greater than 99.3% is needed for the hot start cycle, in addition to reducing engine out NOx.

The Manufacturers of Emission Controls Association(MECA) summarized technology options for meeting the ultra-low NOxstandards in a comprehensive white paper[104]. Figure 19, taken from that paper, shows EPA certification data for HD engines going back to 2002. It is seen that both NOxand CO2have been reducing concurrently with the introduction of SCR systems and the accompanying engine and after-treatment improvements. Even today, many engines emit NOxless than 0.1 g/(bhp·h), 50% below the current certification limit, while also meeting the MY 2027 Phase 2 GHG regulated limit for vocational engines, 503～552 g/(bhp·h) CO2(HHD-LHD). While these engines are by no means mainstream, they show the possibility of future progress.

Figure 19 CO2 and NOx EPA certification data show some engines can attain 60 mg/(bhp·h) NOx while also delivering low fuel consumption

Here are some of the technology options for meeting the anticipated HD Low NOxemissions levels, as listed in the MECA white paper：(1) Substrates with thinner webs, higher cell density, and lower thermal mass. (2) Catalysts with increased activity, selectivity, and durability. (3) System architecture, such as a light-off SCR catalyst with independent urea dosing, and/or a single zone-coated filter with DOC and soot burning catalyst; packaging design for improved heat retention and faster catalyst light-off. (4) Low temperature ammonia/urea dosing using auxiliary gaseous ammonia or heated urea dosing. (5) Turbocharger and EGR bypass for better heating of the aftertreatment system at low speed operation. (6) Cylinder deactivation(CDA) to enable more efficient engine operation while delivering hotter exhaust. (7) 48 V mild hybridization to allow brake energy recuperation and efficient electrification of engine ancillaries.

Accepting that meeting all parts of the new regulations will be a challenge, hot FTP NOxemissions are already close to 0.02 g/(bhp·h) on current systems without any of the above options. Further, today N2O emissions are as low as about 0.015 g/(bhp·h)(Phase 2 GHG limit is 0.1 g/(bhp·h)), so there’s room for increases that might come with aggressive NOxreductions, or they might be traded for compliance with the CO2standards. Modeling shows that increasing SCR volume with the use of an advanced ammonia slip catalyst can reduce the composite FTP tailpipe NOxvalues to about 0.02 g/(bhp·h), indicating that meeting the MY 2024 proposed target of 0.05 g/(bhp·h) might be done with the proven systems of today. This helps compliance with the increased durability and warranty requirements.

A technology demonstration program is ongoing and nearing completion at the Southwest Research Institute, using a 2017 Cummins X15 engine[105]. Engine calibration and the use of cylinder deactivation and EGR cooler bypass helped to elevate engine out exhaust temperatures and improve cold start emissions. Cylinder deactivation is a key technology being considered for these low NOxsystems, due to the ability to raise exhaust heat while also reducing CO2emissions. We have covered some details in the section earlier on HD engine efficiency. On the cold FTP cycle, turbine out temperatures reached 250 ℃ within about 50 s. An advanced after-treatment configuration was used, including a light-off SCR zone coated with ASC, a zone-coated DOC+CSF(catalyzed soot filter), and two parallel banks of SCR and ASC. This is shown in figure 20, as are the resulting emissions. On the composite FTP cycle and the low load cycle, larger than 99% conversion is being achieved with little to no CO2penalty. The long-term durability demonstration is still in progress, but the project clearly shows that achieving the low NOxtargets is possible, especially given a few more years of development time ahead.

By no means is the after-treatment layout shown above the only possible solution. Stephenson[106]listed more than 20 combinations, including the use of a DOC ahead of the first SCR, which is helpful for generating exotherms for desulfating of the SCR catalyst, generating NO2for fast SCR, and protecting the SCR from ash. Without this DOC, SCR desulfation might be done with increasing exhaust temperatures using engine methods but with a fuel penalty cost. Given the significant increase in durability being proposed as part of the low NOxrule(see the regulations section), understanding the sulfation mechanism and de SOxstrategies is critical. Xi et al.[107]analyzed the de NOxperformance of a Cu/SSZ-13 SCR catalyst exposed to high S diesel fuel. Both physical and chemical poisoning were found as mechanisms which degrade the deNOxperformance, the former due to ammonium bisulfate formation and the latter due to interaction between S and Cu ions. Significant SCR activity was shown to be recovered via thermal treatment up to 550 ℃.

Figure 20 After-treatment architecture and test results on various cycles for NOx emissions from the low HD NOx demonstration program at SWRI

The two-stroke opposed piston engines offer an alternative approach to achieving the low NOxlimits[108]. A 10.6 L 3-cylinder engine is in a Class 8 line-haul demonstration program to aggressively reduce CO2emissions to 10% lower than 2027 HD Phase 2 GHG target while achieving 0.02 g/(bhp·h) NOxemissions. The chosen after-treatment system chooses a close-coupled light-off SCR, DOC, SCR on filter, another SCR and an ammonia slip catalyst. Simulations predict 0.01 g/(bhp·h) NOxemissions on the composite FTP cycle enabled early hot exhaust for the engine.

Natural gas engines certified to the ultra-low NOxstandard are already on the market and promise further improvements in fuel economy and emissions. Studies have shown that these engines also suffer from high particle number emissions which can exceed the regulated limits in Europe and China and will benefit from the use of filters. Another challenge is methane slip, given the difficult in converting methane on conventional three-way catalysts. There is much work being done on developing new catalysts to lower the light-off temperature and reduce precious metal usage to lower cost.

7 Summary/Conclusions

We have provided a comprehensive review of the leading and recent literature in the field of engine efficiency and emissions control. Given the concern with global warming and ambient air quality, ambitious targets for reducing CO2and criteria pollutant emissions are being mandated in major markets across the world. Europe is setting the toughest CO2standards along with fines for exceeding these. Various engine and electrification technologies are rapidly advancing for both light- and heavy-duty vehicles and have been described. On criteria pollutants, while the US continues to require the lowest tailpipe gas emissions, Europe and China have set the most stringent, filter-enforcing particle number standards. For gasoline vehicles, this is resulting in the widespread adoption of GPFs, and post Euro Ⅵ regulations are expected to drive higher filtration requirements. Diesels have shown remarkable improvements in addressing the in-use NOxemissions, with modern diesels emitting less than 10% of NOxcompared to pre-Euro Ⅵ-D levels. Overall on the light-duty side, we are beginning to reach near-zero emission levels such that under certain driving and ambient conditions the tailpipe concentrations are lower than those entering the engine, resulting in “negative emission vehicles”. California has proposed perhaps the last major regulation for criteria pollutant emissions for heavy-duty vehicles, requiring a 90% reduction in NOxalong with increased durability, through the end of this decade. Many technologies and system layouts have been discussed, showing that meeting this regulation may be possible while maintaining or even improving fuel consumption.