Aftertreatment integrated SI and CI engine modelling using GT suite
Objective:
To explore aftertreatment integrated SI and CI engine model in GT suite and study the results.
Emissions:
1. Unburnt hydrocarbons (HC)
2. oxides of carbon (COx)
3. oxides of nitrogen (NOx)
4. oxides of sulphur (SOx)
5. Solid carbon particles (Soot)
Generally, HC, CO and NOx are dominant emissions in gasoline engines and NOx and soot partciles are dominant emissions in diesel engines.
Hydro carbon emissions (HC):
Hydro carbon emissions are a strong function of air fuel ratio.
Causes for hydro carbon emissions from SI engines are:
1. Incomplete combustion: Even when the fuel and air entering an engine are at the ideal stoichiometric condition, perfect combustion does not occur and HC emissions ends up in the exhaust. The reason is homogenous mixture of air and fuel is almost impossible practically.
2. Crevice volumes and flow in crevices: The volume between the piston, piston rings and cylinder wall are known as crevice volumes. During compression and early combustion stroke, air and fuel are compressed into the crevice volume at high pressure. Almost 3.5% of fuel is trapped in crevice volume. Later during expansion stroke, pressure in the cylinder reduces below crevice volume pressure ,therfore fuel and air will flow back to combustion chamber. Location of spark plug relative to top compression ring gap will affect the amount of HC in engine exhaust. The farther the spark plug is from ring gap, the greater is the HC in the exhaust.Because more fuel will be forced into the gap before flame front passes.
3. Valve overlap: A well designed engine minimize fresh air fuel mixture escaping through exhaust valves.
4. Deposit on wallls: Gas particles including those of fuel vapour are absorbed by the deposit on walls of combustion chamber. The amount of absorption is a function of gas pressure. High swirl help in keep wall deposits to minimum. When leaded gasoline is burnt,the lead components make the walls harder and less porous to gas absorption. When unleaded gasoline is used HC emissions from wall deposits become more severe.
Q. Why HC emission increases as engine ages?
As engine ages,the clearance between piston rings and cylinder walls becomes greater and a thicker film of oil is left on walls.Some of this oil gets scrapped off the walls during the compression stroke and gets burned during combustion. Oil is high molecular weight hydro carbon compound that does not burn as readily as gasoline.Some of it comes out as HC emissions also increases oil consumption.
HC emissions in CI engines: CI engines operate with overall fuel lean equivalence ratio.Therefore they have only about one-fiveth the HC emissions of SI engines.
Q. Why CI engines have more soot formation?
The components in diesel fuel have high molecular weights on average than those in gasoline blend and this results in higher boiling and condensing temperatures.Therefore soot formation is more in CI engines.
Carbon monoxide(CO) emission:
Carbon monoxide is a colorless and odorless but a poisoness gas. It is generated in an engine when operated with fuel- rich equivalence ratio. When there is not enough oxygen to convert all carbon to CO2, some fuel does not get burned and some carbon ends up as CO. Maximum CO is generated when an engine runs rich.Rich mixtures is required during starting or when accelerating under load. Even when air-fuel mixtures is stoichiometric or lean, some CO will be generated in engine.Poor mixing and local rich regions and incomplete combustions will also be source for CO emissions. Not only CO is considered as undesirable emission,but it also represents lost chemical energy.
Oxides of nitrogen(NOx) emission:
Realeased NOx reacts in the atmosphere to form ozone and is one of the major causes of photochemical smog. At low temperatures, atmospheric nitrogen exists as stable diatomic molecule. At very high temperature(during combustion), some diatomic nitrogen breaks down into monoatomic nitrogen. Maximum NOx is formed at a slightly less equivalence ratio of about 0.95. At this condition flame temperature is till very high, and in addition,there is excess of oxygen that can combine with the nitrogen to form various oxides.
Soot particles:
What are soot particles and how are they generated?
Soot particles are clusters of solid carbon spheres. Carbon spheres are generated in the combustion chamber in the fuel rich zones where there is not enough oxygen to convert all carbon to CO2. Maximum density of particulate emissions occurs when the engine is under load at WOT.At this condition maximum fuel is injected to supply maximum power,resulting in rich mixture and poor fuel economy. This can be seen in the heavy exhaust smoke emitted when a truck on railroad locomotive accelerates up a hill or from a stop. If CI engines are made to operate with an overall stoichiometric air fuel mixture, instead of overall lean as they do, particulate emissions in the exhaust would far exceed acceptable levels.
What is soluble organic fraction(SOF)?
Because of high compression ratio of CI engine, a large expansion occurs during the power stroke.This makes the gases within the cylinder cooled to a relatively low temperature.This causes the remaining high boiling points components found in the fuel and lubricating oil to condense on the surface of the carbon soot particles.This absorbed portion of soot particles is called soluble organic fraction(SOF) and the amount is highly dependent on cylinder temperature.
If combustion time is extended by combustion chamber design and timing control, particulate amounts in the exhaust can be reduced. Soot particules originally generarted will have a great time to be mixed with oxygen and combusted to CO2. However a longer combustion the means a high cylinder temperature and more NOx generation. Dilution with EGR lowers NOx emissions but increases particulates and HC emissions.Higher injection pressure gives finer droplet size. This will reduce HC and particulate emissions but increases cylinder temperature and NOx emissions.
Thermal Converters:
Thermal converters are high temperature chambers through which the exhaust gas flows.The promote oxidation of CO and HC which remain in the exhaust.Even though HC and CO emissions can be reduced by oxidation but NOx emissions cannot be reduced using a thermal converter. It is necessary for a thermal converter to be effective, it operate at higher temperature(above 700 C). It should be large enough to provide adequate residence time of the exhaust gases to promote the occurence of secondary reaction.
Catalytic converters/ Three way cat con:
Catalytic converters are chambers which contain catalytic materials which promotes the oxidation of the emissions contained in the exhaust flow.Generally they are called three way converters becasue they are used to reduce the concentration of CO,HC and NOx in the exhaust. A catalyst is a substance that accelerates a chemical reaction by lowering the energy needed for it to proceed.The catalyst is not consumed in the reaction and so functions indefinitely unless degraded by heat,age,containments on other factors. Inside the container is a porous ceramic structure through which the exhaust gas flows.In most converters, the ceramic is a single honeycomb structure with many flow passages. Some converters use loose granular ceramic with the gas passing between packed spheres.
Aluminium oxide(alumina) is the base ceramic material used for most catalytic converters. Alumina can withstand the high temperatures,it remains chemically neutral, it has very low thermal expansion and it does not thermally degrade with age.
The catalytic materials used are platinum, palladium and rhodium. Palladium and platinum promote the oxidation of CO and HC with platinum expecially active in hydrocarbon reaction. Rhodium promotes the reaction of NOx.
Allows reduction of NOx (nitrogen oxides) to N2 (nitrogen) and O2 (oxygen). Allows oxidation of CO (carbon monoxide) to less-harmful CO2 (carbon dioxide). Allows oxidation of HC (unburned hydrocarbons) to CO2 (carbon dioxide) and H2O (water).
Why leaded gasoline cannot be used in engines equipped with catalytic converters?
Leaded gasoline cannot be used in engines equipped with catalytic converters because leaded gasoline filled two times would completely poison a catalytic converter and make it totally useless.
Selective 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.
What is DEF?
Diesel Exhaust Fluid (DEF) is a non-toxic fluid composed of purified water and automotive grade aqueous urea. DEF is available with a variety of storage and dispensing methods.
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.
Detailed CI engine+ aftertreatment Model:
The above model is CI engine integrated with after-treatment model(i.e. Diesel oxidation catalyst(DOC), Diesel particulate filter(DPF) and selective catalytic reduction).The aftertreatment model is linked to a shadow model where the chemical reactions of aftertreament are captured and processed. The engine model is solved using explicit solver with automatic controlled time step. The exhaust system is duplicated such that one copy uses the explicit flow solver and the second copy uses the quasi-steady (QS) flow solver with larger, 0.1 second time step.
Engine:
The engine used is Turbocharged 4 cylinder CI engine with waste gate controller.
Urea control stratergy:
Urea controller limits the amount of urea entering the SCR according to the amount of NO and NO2 entering the SCR and temperature of gases entering SCR. The appropriate amount of urea is necessary because urea vaporises to NH3 , and NH3 is used for reduction of NO and NO2. If amount of NH3 is higher than required, then NH3 is considered as emission which is known as NH3 slip. Therefore urea controller takes in NO and NO2 concentration from orifice, exhaust temperature from SCR inlet pipe and NH3 slip from SCR outlet pipe. The controller limits the ammonia sip to 15 ppm thrshold, at which point it cuts the urea dosing. The controller also restricts urea injection below 200 c temperature.
DOC monitor:
The DOC monitor calculates the conversion efficiency of CO,HC and NO by measuring the difference in concentration of CO,HC and NO from DOC outlet and DOC inlet.
Assembly 24:
Assembly 24 helps in converting the necessary species information from engine model( Explicit steady solver) to aftertreatment model( Quasi-static solver). Since engine model is solved periodic, cycled averaged information to aftertreatment transient model. The information passed on from exhaust pipe to aftertreatment model are mass flow rate, temperature, species mole fraction of CO2,H2,NO,soot,H20,O2, liquid fuel and vapour fuels.
SCR monitor:
SCR monitor calculates the concentration of NH3, NO,NO2 at SCR inlet and SCR outlet. It also calculates the amount of urea inlet and HNCO inlet.
Case setup:
Flow circuit solver definition:
DOC Reactions:
DPF reactions:
SCR reactions:
Case 1 results:
1. DOC conversion efficiency:
The diesel oxidation catalyst (DOC) owes its name to its ability to promote oxidation of exhaust gas components by oxygen, which is present in ample quantities in diesel exhaust. When passed over an oxidation catalyst, carbon monoxide (CO), gas phase hydrocarbons (HC), the organic fraction of diesel particulates (OF), as well as non-regulated emissions such as aldehydes or PAHs can be oxidized to harmless products, and thus can be controlled using the DOC. In modern diesel aftertreatment systems, an important function of the DOC is to oxidize nitric oxide (NO) to nitrogen dioxide (NO2)—a gas needed to support the performance of diesel particulate filters and SCR catalysts used for NOx reduction.
From above figure, we can observe sudden increase in conversion efficiency of CO, HC and NO as the temperature increases from 0 to 780K. Further temperature increase leads to decrease in conversion efficiency and at 800K The conversion efficiency of CO,HC and NO are 13,9 and 5 respectively.
2. DPF soot mass retained:
From above figure, we can observe that at 2000 rpm, soot mass retained increases continuously as active regeneration is not modelled.
3. SCR monitor:
From above figure, we can observe that NOx out is lower than NOx in as NOx is being converted. we can also observe that as NH3 out crosses the threshold value of 15pm , NH3 inlet is stopped and again NH3 slip is decreased beyond certain value NH3 is supplied.
4. NH3 slip monitor:
As the NH3 is supplied in periodic manner, we can also observe that NH3 slip also fluctuates similarly.
5. DPF pressure drop:
regeneration of DPF is activated (in case of active) by monitoring the pressure drop across the DPF, as the pressure drop increases beyond a certain value, regeneration is activated.
Case 2: SI engine + aftertreatment model:
SI engine combined with a detailed TWC in a secondary aftertreatment circuit. The original base model SI_4cyl_EGRController was combined with the aftertreatment example model TWC_Ramanathan_and_Sharma, which uses a published reaction mechanism for a passenger car TWC. A secondary circuit is used to solve the reactions using the quasi-steady (QS) flow solver, which is able to use a large master time step so that it does not restrict the computation time. The back pressure from the explicit engine circuit is passed to the QS circuit and actuated as the downstream pressure boundary, and the outlet gas temperature from the QS circuit is passed back to the explicit engine circuit and actuated as the TWC wall temperature so that the exothermic heat release is captured for acoustics.
The engine model used is EGR controlled 4 cylinder engine and the exhaust pipe is connected too aftertreatment model.
The various monitors show the TWC conversion efficiency, the exhaust lambda, the oxygen storage, the temperature upstream and downstream of the TWC, and the deltaT across the TWC.
TWC monitor:
The TWC monitor calculates the conversion efficiency of CO,HC,NO,H2 by calculating the difference in mole fraction at inlet and outlet of appropriate species.In order to calculate the convertion efficiency of CO, it takes in value of CO in and CO out. In order to calculate the value of HC, it takes in the value of C3H6(in and out) and C3H8(in and out). It also takes in the value of inlet gas temperature from inlet cone and wall temperature of catalyst .
Case setup:
There are three cases in Case Setup for three air to fuel ratios for rich, stoich, and lean conditions. The engine speed is set to 3000 rpm, and the throttle angle is set to 45° for a part load cruising situation. The burned fuel fraction in the SIWiebe combustion model is set to 0.995 to generate unburned liquid and vapor indolene fuel as the hydrocarbons, which are then converted to C3H6 and C3H8 by a control system for the TWC model.
Results:
1. EGR fraction monitor:
Case 1:
Case 2:
Case 3:
2. TWC conversion efficiency:
Case 1:
Case 2:
Case 3:
From TWC efficiency plots,the following observations are observed:
1. As the air fuel mixture increases fromn rich to lean mixture(14 to 15), we can observe that conversion efficiency of CO increases. Maximum conversion efficiency is obtained at lean mixture(AF=15) condition as due to sufficient availability of oxygen ,more CO can be converted.Therefore more conversion efficiency of CO is obtained at lean mixture.
2. As the air fuel mixture increases fromn rich to lean mixture(14 to 15), we can observe that conversion efficiency of HC increases. Maximum conversion efficiency is obtained at lean mixture(AF=15) condition due to decrease in amount of fuel. As amount of fuel mixed with air decreses, amount of HC decreases hence also results higher conversion efficiency.
3. As the air fuel mixture increases fromn rich to lean mixture(14 to 15), we can observe that conversion efficiency of NO decreases. Minimum conversion efficiency is obtained at lean mixture(AF=15) because as the amount of air in the mixture increases, amount of NO produced in the combustion increases, therefore the conversion efficiency of NO also decreases.
4. The conversion efficiency of H2 remains similar in all three cases.
3. Lamda Monitor:
Case 1:
Case 2:
Case 3:
In all three cases we can observe that lamda of explicit engine more is more fluctuating than lamda of quasis static aftertreatment model.
4. TWC temperature:
Case 1:
Case 2:
Case 3:
Engine performance prediction:
Key cylinder 1 predictions:
| Case 1: AF ratio= 14 | Case 2: AF ratio= 14.5 | Case 3: AF ratio= 15 | |
| Burned residual mass(SOC) % | 12.9 | 12.9 | 12.8 |
| EGR % | 9.8 | 9.9 | 9.8 |
| Lamda effective | 0.946 | 0.982 | 1.019 |
| IMEP(bar) | 10.14 | 10.11 | 10.02 |
| Max pressure(bar) | 51.32 | 50.74 | 50.23 |
| Max temperature(K) | 2502 | 2497 | 2482 |
| NOx(ppm) | 1667.66 | 2276.89 | 2810.25 |
| HC(ppm) | 76.66 | 74.75 | 72.72 |
| CO(ppm) | 21666.8 | 11237.3 | 1841.74 |
| CO2(ppm) | 117120 | 124071 | 129667 |
From above tables the following are observed,
1. Brake power and brake torque is maximum at fuel rich mixture and least at fuel lean mixture.
2. Indicated mean effective pressure(IMEP) and BSFC(brake specific fuel consumption) decreases as fuel turns rich to lean. And max value of IMEP and BSFC is obtained at fuel rich mixture.
3. Max temperature is highest in fuel rich mixture since high pressure created in cylinder than other cases.
4. Though max temperture is highest in fuel rich mixture, NOx generated is highest in fuel lean mixture because due to more amount of air in mixture also leads to higher NOx.
5. hydro carbon(HC) emission is generated highest in case of fuel rich mixture beacause fuel is main source of HC emission.
6. CO emission generated is highest in case of fuel rich mixture because in fuel rich mixture less amount of oxygen is available in order complete the combustion process to CO2. In other words due of inavalability of sufficient amount of oxygen leads to incomplete combustion in local rich zones which leads to generation of CO.
7. Where as in case 3(fuel lean mixyture)as there are more availability of oxygen, more amount of CO2 is produced in case 3 than in case 1.