Understanding types of emissions, control startegies and aftertreatment modelling in SI and CI engines.
Objective :
Understanding types of emissions, control startegies and aftertreatment modelling in SI and CI engines.
Pollutants in IC engines :
1. CO : Carbon monoxide (CO) is a colorless, odorless, and tasteless flammable gas that is slightly less dense than air. It is toxic to animals that use HB as an oxygen carrier (both invertebrate and vertebrate) when encountered in concentrations above about 35 ppm , although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal biological functions. In the atmosphere, it is spatially variable and short lived, having a role in the formation of ground-level zone.
Carbon monoxide consists of one carbon atom and one oxygenatom, connected by a triple bond that consists of two covalent bonds as well as one dative covalent bond. It is the simplest oxocarbon and is isoelectronic with other triply-bonded diatomic molecules having ten valence electrons, including the cyanide anion, the nitrosonium cation and molecular nitrogen.
2. Nitrogen Oxides ( NOx) : Nitrogen dioxide is an irritant gas, which at high concentrations causes inflammation of the airways.When nitrogen is released during fuel combustion it combines with oxygen atoms to create nitric oxide (NO). This further combines with oxygen to create nitrogen dioxide (NO2). Nitric oxide is not considered to be hazardous to health at typical ambient concentrations, but nitrogen dioxide can be. Nitrogen dioxide and nitric oxide are referred to together as oxides of nitrogen (NOx).NOx gases react to form smog and acid rain as well as being central to the formation of fine particles (PM) and ground level ozone, both of which are associated with adverse health effects.
3. Hydro-carbons (HC) : A class of burned or partially burned fuel, hydrocarbons are toxins. Hydrocarbons are a major contributor to smog, which can be a major problem in urban areas. Prolonged exposure to hydrocarbons contributes to asthma, liver and lung diseases, and cancer. Regulations governing hydrocarbons vary according to type of engine; in some cases, "non-methane hydrocarbons" are regulated, while in other cases, "total hydrocarbons" are regulated. Technology for one application (to meet a non-methane hydrocarbon standard) may not be suitable for use in an application that has to meet a total hydrocarbon standard. Methane is not directly toxic, but is more difficult to break down in fuel vent lines and a charcoal canister is meant to collect and contain fuel vapors and route them either back to the fuel tank or, after the engine is started and warmed up, into the air intake to be burned in the engine.
4. Particulate matter : Soot or smoke made up of particles in the micrometer size range: Particulate matter causes negative health effects, including but not limited to respiratory disease and cancer. Very fine particulate matter has been linked to cardiovascular disease.
5. Sulfur oxide (SOx) : A general term for oxides of sulfur, which are emitted from motor vehicles burning fuel containing sulfur. Reducing the level of fuel sulfur reduces the level of Sulfur oxide emitted from the tailpipe.
6. Volatile organic compounds (VOCs) : Organic compounds which typically have a boiling point less than or equal to 250 °C; for example chlorofluorocarbons (CFCs) and formaldehyde. Volatile organic compounds are a subsection of Hydrocarbons that are mentioned separately because of their dangers to public health.
Aftertreatment devices and catalyst materials :
Spark ignition engines :
1. Three-way catalytic convertor
The most effective aftertreatment for reducing engine emissions is the catalytic converter.
Catalytic converters are chambers mounted in the flow system through which the exhaust
gases pass through. These chambers contain catalytic material, which promotes the
oxidation of the emissions contained in the exhaust flow.
Generally, they are called three way converters because they are used to reduce the
concentration of CO, HC and NOx in the exhaust. It is basically a stainless steel container
and inside the container is a porous ceramic structure through which the exhaust gas
flows. Aluminium oxide (Alumina) is the base ceramic material used for most catalytic
materials. In most converters, the ceramic is a single honeycomb structure with many
flow passages.
The surface of the ceramic passages contains small embedded particles of catalytic
material that promote the oxidation reactions in the exhaust gas as it passes.
The catalyst materials most used are Platinum, Palladium and Rhodium. Palladium and
platinum promote the oxidation of CO and HC, with platinum especially active in the
hydrocarbon reaction. Rhodium promotes the reaction of NOx.
The efficiency of a catalytic converter is very much dependent on Temperature. When a
converter is working at a fully warmed temperature of 400 degree celsius or above, it will
remove 98-99% of CO, 95% of NOx, and more than 95% of HC from exhaust flow
emissions.
Compression Ignition Engines :
1. Diesel Oxidation Catalyst (DOC) :
It is a honeycomb ceramic filter coated with Platinum and Palladium, which are two main
catalyst materials used to promote the removal of 30- 80% of the gaseous HC and 40-
90% of the CO in the exhaust. The catalysts have little effect on solid carbon soot but do
remove 30-60% of the total particulate mass by oxidizing a large percent of the HC
absorbed on the carbon particles.
2. Diesel Particulate Filter (DPF) :
CI engines are equipped with particulate traps which are filter like systems often made of
ceramic in the form of a monolith or mat, or else made of metal wire mesh.
As traps catch the soot particles they slowly fill up with the particulates, which increase
the back pressure of the engine. To reduce this flow restriction, particulate traps are
regenerated when they begin to saturate.
Regeneration consists of combusting the particulates in the excess Oxygen contained in
the exhaust of the lean operating CI Engine. Carbon Soot ignites at about 550-650 degree
celsius, while CI engine exhaust is 150- 300 degree celsius at normal operating
conditions. As the particulate trap fills with soot and restricts flow, the exhaust temperature
rises but is still not enough to ignite the soot and regenerate the trap. In some systems,
automatic flame igniters are used which start combustion in the carbon when the pressure
drop across the trap reaches a pre-determined value.
Another way of regenerating the traps is by using catalyst which lowers the ignition
temperature of the carbon soot.
3. Selectice Catalytic Reduction (SCR) :
Selective Catalytic Reduction (SCR) is an advanced active emissions control technology
system that injects a liquid-reductant agent through a special catalyst into the exhaust
stream of a diesel engine. The reductant source is usually automotive-grade urea,
otherwise known as Diesel Exhaust Fluid (DEF). The DEF sets off a chemical reaction
that converts nitrogen oxides into nitrogen, water and tiny amounts of carbon dioxide
(CO2), natural components of the air we breathe, which is then expelled through the
vehicle tailpipe.
SCR technology is designed to permit nitrogen oxide (NOx) reduction reactions to take
place in an oxidizing atmosphere. It is called "selective" because it reduces levels of NOx
using ammonia as a reductant within a catalyst system. The chemical reaction is known
as "reduction" where the DEF is the reducing agent that reacts with NOx to convert the
pollutants into nitrogen, water and tiny amounts of CO2. The DEF can be rapidly broken
down to produce the oxidizing ammonia in the exhaust stream. SCR technology alone can
achieve NOx reductions up to 90 percent.
Strategies for tailpipe emission reduction :
Emission control options can be grouped into three categories:
(1) Engine design techniques,
(2) Fuel and lubricant related technologies,
(3) Exhaust gas aftertreatment.
(4) Powertrain integration and control technologies
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 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. |
| 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. |
| 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. |
| 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. |
| 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. |
Three Way Catalyst (TWC) Model :
Chemical Reactions :
Selectice Catalytic Reduction (SCR ) Model :
Chemical Reactions :
Aftertreatment DOC model Sampara and Bisset :
Following is the model for DOC, where exhaust gasses pass through a inlet cone which is diverging in nature, flow through the catalytic convertor, and passes through the outlet cone to the outlet cone.
The catalytic convertor has the following honey comb like structure, and which can be modified according to the requirements,
Following is the species related data accesed from the main tab of the DOC.
Further site elements are tabulated as follows :
Coverages and Reactions involved in the model are as follows :
Case setup was set for 2 cases :
Results after post-processing :
Conversion efficiency for case 1
Conversion efficiency for case 2 :