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