Diesel Engine Emissions
PM/soor in exhaust gas are removed by trapping PM/soot using
diesel PM filter (DPF)
DPF can tolerate exhaust temperatures up to
600 degrees C
carbon atom sizw
70 pm
the diameter of soot primary particle is about __________ times that of carbon atoms
700
for lean operation, HC and CO can be oxidized because there is O2 present in ______________________________
diesel engine exhaust
deisel engine after-treatment system
diesel oxidation catalyst (DOC)
when can oxidation of soot occur
during all four steps of nuclei formation
high combustion temperatures ___________________ the dissociation of CO2
enhance
the PM/soot particles that got oxidized in the cylinder are the "permeable" or "less condensed" ones which allows O2 to enter the inside of the PM particle and got oxidized ____________________
from the inside and outside
the increased back pressure of the DPF will impact engine performance, especially
fuel economy
configuration of DPF
honeycomb design
liquid diesel fuel components are _________________ with H/C ratio of about 2. PM consists of mainly carbon with majority of H lost
hydrocarbons
active regeneration system post-engine combustion system
increasing exhaust temperature by post-engine diesel combustion: -diesel is injected into diesel engine exhaust system prior to DOC -there are excess O2 available in diesel engine exhaust gas -DOC help to further oxidize unburned HC is survived the combustion process -the burning of diesel fuel and oxidation of unburned HC further increase exhaust temperature
why doesn't TWC work for diesel engine?
it operates at globally lean mixture other than stoichiometric mixture
some HC survives the combustion process in local rich mixture due to ___________________, and the majority is burned later once more O2 is present.
lack of O2
The CO emissions from diesel engine originate from diesel fuel injection in ____________________
late phase
diesel engines emits much __________ HC than gasoline engine
less
the CO emissions from diesel engine are much ___________ than gasoline engine
less
CO is formed in ______________
local rich mixture
how to inject diesel fuel more than once in a cycle
multi-pulse fuel injection technology
three mode
nuclei mode, accumulation mode, coarse mode
selective catalytic converter (SCR)
only reduces NOx
most CO formed is later _______________ to CO2 once more O2 is made available
oxidized
CO in ___________ mixtures have the opportunity to further oxidize to CO2 once more O2 is present
rich
PM is mainly formed in local _____________ mixture at high temperature in the center line of diesel spray plume
rich
PM/soot is formed in
rich mixture
a _________ portion of PM/soot particles formed during combustion process emits from engine as PM/soot emissions
small
ultra-fine particles
small percentage in mass, large percentage in number, major concerns for health
reduce PM emissions by avoiding rich mixture
smaller droplets through better atomization, higher injection pressure (over 2500-3000 bar,) smaller injection hole and more injection holes
how to increase diesel injection pressure
smaller droplets, higher injection pressure, smaller injection hole and multi-hole injector
PM 1 and ultra-fine PM
smaller than 1 microns in diameter, smallest PM, most dangerous to health because they can easily pass through the lung vesicles and reach the entire body; may even end up in the brain
PM 10
smaller than 10 microns in diameter
active regeneration
some special activities are performed for the purpose of DPF regeneration
four steps of bonding nuclei
surface growth, coagulation, aggregation, absorption of volatile HC
what leads to the formation of the first recognizable soot particle noted as nuclei
the condensation of gas-phase species
how are soot nuclei formed
through the condensation of PAH directly
the HC survived in local rich mixture, O2-rich mixture entrainment into spray, and the post-combustion HC oxidation process eventually exits the engine as _____________________
unburned HC
most-likely precursors for the formation of soot
unsaturated HC, polycyclic aromatic hydrocarbon
the tendency of HCs molecular in producing PM or soot:
unsaturated HCs, polycyclic aromatic hydrocarbons
PM contains mainly carbon with ___________ absorbed on the surface of the particles
volatile HC
fuel-lean mixture is always present in diesel engine. the ______________ movement will bring oxidizing species to soot containing region. with these present, soot can be oxidized
bulk-gas
DPF material
ceramic: cordierite, silicon, carbide, aluminum, titanate
DPF regeneration process
cleaning of DPF since PM buildup will impact engine performance
most of HC emitted from diesel engine originates from the late _____________ process of the diesel fuel injected into cylinder at the end of the injection process
combustion
EGR decreases the ______________________ of O2 in intake mixture, which increases the emissions of PM/soot from diesel engine
concentration
each chain of PM consists of numerous nano-scale soot primary particles about _______________ nm in diameter
30-50
without the presence of catalyst, the temperature needed for PM/soot to be oxidized is about
550-650 degrees C
passive regeneration
-DPF is regenerated at normal operation temperatures as a by-product of engine operation, which is not performed specifically for the regeneration of DPF -need other approaches to assist the oxidation of PM/soot
factors impacting NOx formation
-availability of O2 after combustion -combustion temperature (minimum or 1900)
factors impacting NOx formation
-availability of O2 after combustion: technologies capable of decreasing O2 concentration -decrease combustion temperature
Why does the EGR decrease NOx emissions?
-exhaust gas contains less O2 than air -the "replacement" of air by "exhaust gas" decreased the O2 concentration in engine and decreases the formation of NOx -EGR decreases combustion temperature by absorbing thermal energy and slowing down the combustion process, decreasing the heat release rate
diesel engine operation features
-globally burn lean -there is always excess O2 available in cylinder supporting the formation of NOx in diesel engine -high combustion temperature -diesels operate at a low speed
CO is formed through three mechanisms
-incomplete combustion of HC in local fuel-rich mixture -CO2 dissociation -incomplete oxidation of HC during post-combustion oxidation process
HC emissions resources
-intake gas of diesel engine is air only as diesel fuel is injected into cylinder -diesel fuel is ignited through compression ignition without the need of spark ignition and flame propagation, so diesel misfire seldom occurs with exception of cold start -there presents local rich mixture, which is the main contributor to HC emissions
passive regeneration at low temperature
-oxidize PM at normal engine p=operation temperature with the presence of NO2 -temperature needed is 300-400 degrees C -catalyst coated to the surface of DPF -heating exhaust gas using electrical heater
losing H in HC in local rich mixture under high temperature causes
-pyrolysis-thermal decomposition of organic molecule -dehydrogenation-removal of H from an organic molecular -partial oxidation-H is oxidized but C is left
ways to reduce NOx emissions
-reduce compression ration: less compression decreases the compression temperature, and increasing the volume of combustion chamber also decreases the combustion temperature -retard spark timing: shift peak heat release to avoid concurrence of peak temperature caused by compression and combustion, and it will decrease the temperature and efficiency of engine
how does SCR work
-reductant agent or reducer is needed for the reduction of NOx to N2 and O2 -HC and CO can not be used as reducer for NOx in the presence of O2 as it is much easier for HC and CO to react with O2 than NOx -the reducing agent used for reducing NOx is diesel exhaust fluid (DEF) which is a mixture of purified water and urea -NH3 is the reductant reducing NOx to N2 under the presence of catalyst
PM 2.5
-smaller than 2.5 microns in diameter or 1/20th of a human hair -examples: bacteria, combustion particles, and organic compounds
products of pyrolysis, dehydrogenation, and partial oxidation
-unsaturated HC, particularly ethyne and its higher analogues -polycyclic aromatic hydrocarbon
each soot primary particle contains about 10^5 carbon atoms with H/C ratio of
0.1
diesel engine emissions
CO, HC, NOx, PM
SCR system components
DEF tank, DEF heater and temperature control system, DEF fluid level sensor-detecting the DEF level in tank, DEF injector, DEF dosing control unit, exhaust temperature, SCR catalyst reactor, NOx sensor for closed loop control urea dosing control, ammonia sensor
NOx is reduced to
N2 and O2
pressure transducers are installed before and after DPF to monitor the pressure drop and estimate ___________________________
PM accumulation in DPF
why is the pressure drop through the DPF gradually increased
PM/soot accumulates in DPF
active regeneration system post-combustion diesel injection system
an extra post-combustion diesel injection process can significantly increase exhaust temperature: increased exhaust temperature helps to oxidize the PM accumulated in DPF
reduce PM emissions by avoiding rich mixture
better mixing of diesel fuel vapor with air by bulk gas movement, intake swirl, and squishing flow into and out of the bowl in combustion chamber
PM emissions
black smoke, worst case is usually observed during acceleration/gear change, impacts vision and causes safety problems, and is a pollution problem
small nuclei ____________ together and grow into large particles
bond
aggregation
bonding of nuclei: the PM primary particle aggregates together and form chains of primary particles. the aggregation of the soot chains forms larger soot particles noted as a "cluster"
coagulation
bonding of nuclei: the big nuclei collide and coalesce to form bigger particle, which decreases the number of particle without increasing the total mass of soot. the coagulated soot particle is also noted as primary particle in the size of 30-50 mm
surface growth
bonding of nuclei: the gas-phased species is combined with the small nuclei, which makes the small nuclei to grow into a bigger one, which increases the mass/size of soot particles without increasing the number of soot particles
