Engine Emission Control

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Abstract: An increased diesel engine population has created pressures on controlling diesel PM and NOx emissions. The initial progress in diesel emission control was achieved through engine technologies, including changes in the combustion chamber design, improved fuel systems, charge air cooling, and special attention to lube oil consumption. Emission standards implemented in the 2005-2010 timeframe additionally require the use of exhaust aftertreatment methods on new diesel engines. These methods include diesel particulate filters, urea-SCR catalysts, and NOx adsorbers.

Introduction

Pollutant Emissions

In modern internal combustion engines, two primary systems are responsible for the formation and reduction of pollutants:

The combustion system includes the combustion chamber, its shape and characteristics such as charge composition, charge motion, and fuel distribution. This is where pollutants such as NOx, CO and PM are created as well as where incomplete oxidation of fuel occurs. What happens in the combustion system is greatly influenced by other engine systems such as the intake charge management system and the fuel injection system. In fact, the primary purpose of these secondary systems is to influence what happens during the combustion process. Numerous options are available to limit the formation of pollutants resulting from the combustion system. Once exhaust gas leaves the combustion system, its composition is essentially frozen until it reaches the emission aftertreatment system (ATS, also abbreviated EAT or EATS) where further reductions in pollutants can be realized and also where secondary emissions such as N₂O, NO₂ and NH₃ can originate.

The aftertreatment system consists of catalytic reactors that attempt to further lower pollutants. In some cases, such as stoichiometric spark ignition (SI) engines, a single three-way catalyst (TWC) is sufficient to achieve very significant reductions in pollutants. In other cases such as lean burn diesel engines, a number of catalytic devices are required. Secondary systems are required to ensure the ATS works as intended. These include: control of exhaust gas composition through control of exhaust stoichiometry or supply of additional reactants not normally found in exhaust gas or not present in sufficient quantity (e.g., urea, additional HCs, additional air or O₂), thermal management to ensure the catalysts operate within the required temperature window, systems to ensure contaminants and pollutants that might accumulate are removed (regeneration of filters, sulfur management, urea deposits,) and systems to minimize the formation of secondary pollutants such as the ammonia slip catalyst (ASC).

It would be a mistake to consider the combustion system and the ATS as separate systems. In order maximize their effectiveness, a high degree of integration is required. A classic example is air-to-fuel ratio (AFR) in SI engines where a very high level of control precision is required to ensure the TWC performance is maximized. Thermal management of the ATS can be carried out by adjustments within the engine to affect the temperature of the exhaust gas leaving the cylinder. In some cases, additional fuel required by the ATS (e.g., for thermal management) can be supplied by the engine’s fuel injectors.

It is important to realize that the objective of engine optimization is not to minimize the pollutant emissions from the combustion system or maximize the reduction of pollutants in the ATS. Rather the objective is to achieve a target level of emissions from the entire system. The target is generally sufficiently below the regulatory limit to allow for production variability. Doing so may require the emission of some pollutants from the combustion system to increase if ATS performance is sufficiently high to still allow design target to be met. For example, NOx emissions from engines equipped with a urea-SCR catalyst can be allowed to increase to minimize GHG emissions (due to the NOx-BSFC trade-off) if high NOx conversion in the SCR catalyst is achieved.

Fuels and lubricants are an important “partner” in the combined engine and aftertreatment system. Low emissions over the life of the engine would not be possible unless fuel contaminants such as sulfur and some inorganic minerals are controlled to very low levels.

The above technologies, discussed further in the following sections, are applicable to new (OEM) internal combustion engines. Some of these technologies may also be used to reduce emissions and/or improve efficiency of existing, in-use engines. There is also a group of technologies developed specifically for in-use applications, that are normally not used in new engines. These technologies are discussed in more detail under Emission Control from In-Use Engines

GHG Emissions and Fuel Economy

GHG emission limits and fuel efficiency standards have created opportunities for a wide range of technologies to be incorporated into engines and vehicles. The search for increased fuel efficiency is focusing attention on at least three key areas:

As powertrain efficiency has a direct impact on fuel consumption, it is an obvious choice for improving fuel efficiency. Important approaches include improved engine efficiency, kinetic energy recovery (such as through regenerative braking), waste heat recovery, and reduction of parasitic losses from ancillary devices such as pumps. Among vehicle technologies, improved vehicle aerodynamics and reduced rolling friction are two obvious factors that affect fuel economy. Other factors include vehicle weight and power used by non-powertrain auxiliaries such as air-conditioning. Last but not least, vehicle operational parameters such as driving patterns and route selection can also be used to gain significant improvements in fuel economy [1376]. These technologies were discussed under Efficiency Technologies.

Emission Control Technologies

Emission control options can be grouped into three categories: (1) engine design techniques, (2) fuel and lubricant related technologies, and (3) exhaust gas aftertreatment. Each of these approaches can be divided into sub-categories, as shown in the following tables. In addition, powertrain integration and control technologies play a very important role in reducing emissions and improving the engine and vehicle efficiency. Some of the methods discussed below are implemented in today’s engines, others—still under development—show promise for future applications.

**Table 1**
Engine design technologies for emission reduction
Technology	Emission Impact	Significance
Compression Ignition (Diesel) Engines
Fuel injection	Capabilities have evolved significantly. Significant improvements in injection technology started in the 1990s with widespread implementation of systems capable of variable injection timing through the use of electronic controls. Engines with EGR place the highest demand on fuel injection pressure. Light-duty vehicles use the most demanding multiple injection strategies.
injection timing	Primarily used to limit NOx emissions	Injection timing affects combustion phasing; retarding the combustion phasing can be used to limit NOx emissions.
injection pressure	Primarily used to limit soot (PM) emissions	Higher injection pressure can lower soot emissions; especially important when combined with NOx control technologies such as EGR that would otherwise increase soot emissions.
multiple injections	Various	Multiple injections strategies have been developed to lower NOx, soot, HC and CO emissions.
Exhaust gas recirculation (EGR)	In diesel engines, primary application is to control NOx emissions	Commonly used in many light- and heavy-duty diesel engines. High pressure EGR delivery can introduce a fuel consumption penalty through higher pumping losses. Low pressure EGR has lower pumping losses but is more difficult to control during transient operation. Other measures to limit potential increases in soot and possibly HC and CO can be required.
Intake boosting	Primary emissions impact is to lower soot (PM) production. Also important for efficiency gains.	Higher intake pressure increases air/fuel ratio for given fuel injection amount and lowers soot production.Can be an important measure to offset unwanted decreases in performance and increased emissions with NOx control measures such as EGR. Often accompanied by improved intake charge cooling capabilities. Enables engine downsizing for efficiency gains. Introduces challenges such as turbocharger lag that can require complex solutions.
Intake temperature management	Most direct impact on NOx emissions. Can lower soot emissions as well.	Increased boosting and/or EGR can increase intake manifold temperature. Intake charge cooling capability improvements are required to limit intake charge temperature and minimize associated NOx emission increases, reductions in air-fuel ratio and losses in power density.
Combustion chamber design	Important soot control measure	Combustion chamber design changes are commonly used to offset increases in soot emissions when measures are taken to limit NOx emissions. In many cases, improvements enhance mixing late in the combustion process to improve soot burn-out.
Positive Ignition (SI) Engines
Fuel injection	Fuel consumption and particulate emissions	The shift from port injection to gasoline direct injection (GDI) was driven by the use of engine downsizing to meet fuel consumption and CO₂ requirements. GDI engines have a higher tendency to produce small particle emissions that can be partially offset by refinements in fuel injection system design.
Intake boosting	Fuel consumption	Enabler for engine downsizing and reduced fuel consumption and CO₂ emissions.
Variable valve actuation	Various	Some examples include: variable valve timing is an important measure to reduce cold start HCs. Variable valve lift enables throttleless operation and improved efficiency. Cylinder deactivation reduces part load pumping losses and improves efficiency. Variable valve timing enables Miller cycle operation for reduced pumping losses.
Lean burn	Fuel consumption	Lean burn can reduce pumping losses, heat transfer and improve working fluid characteristics to provide higher efficiency. Introduces the need for expensive NOx aftertreatment technologies.
Combustion	Fuel consumption	Advanced combustion concepts can improve efficiency through faster combustion and lower heat losses.
EGR	At one time used to limit NOx emissions. Modern approaches focus mainly on reducing fuel consumption.	In SI engines, EGR is an alternative to fuel enrichment at high loads to reduce knock propensity and lower exhaust temperature at high power. At part load conditions, it can reduce pumping losses.

**Table 2**
Fuel & lubricant technologies
Technology	Emission Impact	Significance
Lubricating oil	Important to reduce fuel consumption	Low viscosity lubricants are important for fuel consumption/CO₂ reductions but require other changes to ensure engine wear levels do not increase. Limiting the content of catalyst poisons (e.g., sulfur, inorganic ash, phosphorus) is a key enabler for ensuring durability and performance of catalytic exhaust emission control technologies.
Alternative fuels	Primary impact is life-cycle CO₂ emissions	Limited criteria emission reduction potential from modern engines with full range of aftertreatment for NOx and PM. Some effect on criteria pollutants (PM, NOx, SOx) is possible in applications without aftertreatment (e.g., marine). In some cases, lower operating cost is a major consideration (e.g., natural gas). Demand can often be driven by government incentives or mandates.
Fuel additives	Various	Small direct emission effect with modern engines and high quality fuels. Important to maintain long term stable operation of emission control technologies. For examples, cetane additives help ensure consistent and reliable ignition quality of modern diesel fuels to ensure reliable and predictable performance; injector cleanliness additives and lubricity additives are intended to keep fuel injection system components clean and reduce wear to ensure long term durability and consistent performance of fuel injection systems; some diesel particulate filter systems use fuel additives to assist particulate filter regeneration.

**Table 3**
Exhaust aftertreatment technologies
Technology	Emission Impact	Significance
Compression Ignition (Diesel) Engines
Diesel oxidation catalyst (DOC)	High reduction of HC/CO emissions, small to moderate PM conversion. The oxidation of NO to NO₂ enhances the performance of SCR/DPF systems.	Widely used on Euro 2/3 cars and on some US1994 and later heavy- and medium-duty diesel engines. In modern engines, used as an auxiliary catalyst in SCR/DPF aftertreatment systems (NO₂ generation, ammonia slip control).
Particle oxidation catalysts	Up to ~50% PM emission reduction	Limited commercial application in selected (EGR-equipped) Euro IV heavy-duty truck engines, as well as in some light-duty and nonroad engines.
Diesel particulate filters (DPF)	90%+ PM emission reduction	Mainstream technology used on all Euro 5 and US Tier 2 and later light-duty diesels; in all US2007 and Euro VI and later heavy-duty engines; in all Stage V nonroad engines; in retrofit programs worldwide.
Urea-SCR catalysts	90%+ NOx reduction	Mainstream technology used in US2010, Euro V and later heavy-duty engines; in US Tier 2 and Euro 5/6 and later light-duty diesel vehicles; in nonroad, marine, and stationary engines.
NOx adsorber catalysts	Up to ~70-90% NOx reduction, depends on the drive cycle	Used as a stand-alone NOx reduction catalyst in some US Tier 2 and Euro 5/6 light-duty vehicles. Used as a cold-start NOx reduction catalyst on some Euro 6 vehicles with SCR.
Lean NOx catalysts (HC-SCR)	NOx reduction potential of ~10-20% in passive systems, up to 50% in active systems	Limited OEM and retrofit commercial application, mostly in the 2000s.
Positive Ignition (SI) Engines
Oxidation catalyst (OC)	90%+ HC and CO emission reduction	Used in older gasoline cars (circa 1980-1990).
Three-way catalyst (TWC)	90%+ NOx, HC and CO emission reduction	The most important gasoline engine emission control technology. Widely used on stoichiometric SI engines worldwide.
NOx adsorber catalysts	~70-90% NOx reduction	Used in lean-burn (stratified charge) gasoline direct injection (GDI) light-duty vehicles that were common in Europe in the 2000s.
Gasoline particulate filters (GPF)	~90% PN emission reduction	Increasing use in Euro 6 light-duty GDI vehicles. Expected to be widely used in China 6 light-duty vehicles.

**Table 4**
Control, diagnostics & powertrain technologies
Technology	Emission Impact	Significance
Hybridization	Primarily to reduce fuel consumption	Hybridization with battery electric drive can enable the engine to operate longer in regions of higher thermal efficiency and less at low efficiency points such as idle and low load. Electric motor boost enables the use of efficiency technologies that might otherwise not be practical because of detrimental performance impacts.
Diagnostics	OBD ensures long term emissions compliance.	Intended to detect malfunctions that would cause emissions over the certification test to increase beyond a defined threshold.
Controls	Electronic controls ensure accurate control of numerous emissions and powertrain control components can be maintained over the life of the vehicle. Variations in ambient conditions, system integration and system aging effects can be accommodated.	Diesel engine controls include: EGR control, intake boost pressure control, fuel injection timing control and combustion control. Aftertreatment system controls include: urea dosing, temperature management to ensure high emission reduction efficiency, regeneration control to ensure accumulated materials such as soot, sulfur and urea deposits are regularly removed. Integrated system controls: Some control functions require a strongly integrated approach to ensure the engine and aftertreatment system work together. Examples include the NOx adsorber catalyst that require the engine’s air/fuel ratio to be enriched regularly to remove accumulated NOx; adjustment of engine parameters such as fuel injection timing to raise exhaust temperature for keeping the aftertreatment system efficiency high; and DPF regeneration which may require the engine operation to be strictly controlled to avoid damaging the DPF.
Controls		SI engine controls include: Air/fuel ratio control, spark timing control, idle speed control. Aftertreatment system controls include: thermal management to ensure rapid warm-up and high emission reduction efficiency; and air/fuel ratio control to ensure maximum conversion of the TWC. Integrated system controls: The need to accurately control the air/fuel ratio is driven by the very narrow air/fuel ratio window where high conversion of NOx, HC and CO is possible in a TWC.