Gasoline Engine Emissions

fuel slips through engine during valve overlap

-HC slips through engine combustion chamber during valve overlap, and emits engine as unburned HC -the contribution of valve overlap to HC emissions from a well designed SI gasoline engine is very small

NO2 formation mechanisms

-NO is formed during or after combustion process, prior to the formation of NO2 -NO2 is formed in the engine through NO+HO2 = NO2+OH -HO2 is formed during the oxidation process of hydrocarbon fuel

equivalence ratio impact on NOx emissions (rich mixture)

-NOx emissions are dominated by the availability of O2 and combustion temperature -NOx emissions observed at mixture richer than stoichiometric is much lower than that observed at stoichiometric mixture -at rich operation, there is shortage of O2, thus less NOx emissions -lean mixture has extra O2, so more NOx emissions are created, but lean mixtures decrease the combustion temperature max NOx emissions are observed at ER=0.88, where the O2 and combustion temperature balance each other

how to reduce NOx emissions

-NOx emissions are dominated by the availability of O2 and combustion temperature -modern engines operate on stoichiometric mixture, so combustion temperature is dominating factor -decrease combustion temperature

key components of controlling air-fuel ratio

-O2 sensor -ECU -injector

how to reduce HC emissions from gasoline engine

-avoid operating a rich mixture -eliminate misfire and flame extinction during cold start process -minimize the crevice of combustion chamber eliminate HC loss due to slip through during valve overlap

how to reduce CO emissions from gasoline engine

-avoid operation at rich mixture --minimize local rick area by improving the evaporation of gasoline fuel and its mixing with air -minimize the HC that survives the combustion process

causes of high combustion temprature

-compression due to piston movement, which is dominated by compression ratio -compression due to combustion or flame propagation -fast combustion process around TDC or before TDC, which combine the impact of compression and combustion

how to decrease combustion temperature

-decrease compression ratio -retard spark timing, which retards the combustion phasing and avoid the concurrence of compression and combustion process -the application of exhaust gas recirculation (EGR.) the exhaust gas is cooled before recirculated back to engine, which is noted as cooled EGR

oil film absorption and desorption

-during intake stroke, HC (gasoline) fuel is absorbed by lubrication oil film in cylinder liner -the lubrication oil film cannot be burned during combustion process. The HC absorbed in oil film is released into bulk gas, and emits the engine as unburned HC

equivalence ratio impact on NOx emissions (lean mixture)

-further leaning the gasoline-air mixture beyond peak NOx emissions eventually decreases NOx emissions

combustion-chamber crevice

-gasoline-air mixture enters the crevices through the narrow entrance during compression and early combustion process when cylinder pressure is increasing. In general, crevices serve as traps of gasoline-air mixture -flame propagates through gasoline-air mixture but not able to pass through the narrow entrance and eventually extinguished -HX trapped in crevice cannot be burned during combustion process the gasoline-air mixture trapped in crevices survives the combustion process eventually exits the crevice and mix with the bulk gas and eventually exits the cylinder as unburned HC -crevice is one of the major contributors to HC emissions under normal combustion

factors dominating NOx formation

-high temperature -availability of O2 or O after combustion -reaction time available for NO formation reactions to occur

how does the engine ensure stoichiometric mixture?

-if lean, the engine increases the gasoline fuel t be injected by elongating the fuel injection pulse -if rich, the engine decreases the gasoline fuel to be injected by shortening the injection pulse

when does misfire occur

-misfire occurs during cold start especially at the first few cycles, gasoline has been delivered into cylinder but has not been ignited. All gasoline delivered into cylinder exits engine without participating in combustion -failure of spark system -flame extinction during propagation process, so a portion of gasoline survives the combustion process and exits the engine as unburned HC

boundary layer

-the HC boundary layer cannot be burned under normal operation -the absorption of HC fuel into oil layer and its release into cylinder after combustion process contributes to HC emissions -HC emissions due to lack of oxygen (rich mixture)

equivalence ratio impact on combustion temperature

-the maximum combustion temperature is observed at about stoichiometric mixture, slightly rich side (ER=1.05) -for lean mixture, there are less fuel burned for the same amount of air, so the combustion temperature is lower than that observed at stoichiometric. the leaner (lower ER) the gasoline-air mixture, the lower the combustion temperature -for rich mixture, there is less air available to support the complete combustion, a portion of gasoline cannot be burned completely, so the combustion temperature is also lower than Tmax. The richer the gasoline-fuel mixture, the lower the combustion temperature

stoichiometric mixture voltage

0.45-0.5 V

rich mixture voltage

0.8 volts

CO formation mechanisms

CO is formed during the combustion process through: -incomplete combustion of HC in rick mixture -incomplete combustion of HC during combustion process due to the presence of local rich mixture -CO formation through dissociation of carbon dioxide -incomplete oxidation of the HC survived combustion process mixes with the hot combustion products

toxic gases/disease causing gases

CO, hydrocarbons

gases that are not harmful unless mixed together (main reason for sunlight smog)

HC + NOx

thermal NOx mechanism

NO needs to break up to nitrogen and oxygen. this requires a temperature of 1900 K, which can only occur during or after combustion. NO must be formed during or after the combustion process

catalysts coated on _________ surface play a major role in decreasing HC, CO, and NOx emissions

TWC

catalyst

a substance that increases the rate of chemical reaction without itself undergoing any permanent chemical change

HC emissions mechanisms

abnormal combustion, combustion-chamber crevice, boundary layer, oil film absorption and desorption, fuel slips through engine during valve overlap

spark ignition gasoline engine emissions

carbon monoxide, hydrocarbon, NOx

the emissions control system operates as a

closed-loop system

increasing temperature improves the oxidation _______________ efficiency

conversion

the catalyst converter is very effective in ________ HC (>95%,) CO (over 95%,), and NOx (85-90%) emissions

decreasing

ECU input signals

engine speed, engine load, intake temperature, intake pressure, air flow rate, coolant temperature, lubrication oil temperature, lubrication oil pressure, gasoline fuel pressure, O2 concentration, knock sensor signal, exhaust temperature

why cooled EGR decreases combustion temperature

exhaust gas consists of mainly N2, CO2, and H2O without fuel, which are inert gases -the blending of cooled exhaust gas to stoichiometric mixture decreases the combustion temperature for the following reasons: N2, CO2, and H2O are inter gases, which will absorb hear produced by combustion and decrease the combustion temperature, and they slow down flame propagation

chemical reactions occur ____________ in the presence of a catalyst because the catalyst provides an alternative pathway with a lower activation energy than the non-catalyzed mechanism.

faster

ECU output signals for engine operation control

gasoline fuel injection pulse width or gasoline fuel rate, spark ignition timing, EGR flow rate, idle air-valve control signal

oxidation catalyst

helps to oxidize HC and CO at lower temperature

what does the three-way catalyst look like

honey comb structure, catalyst coated on surface

the composition of gasoline-fuel mixture prior to combustion

hydrocarbon, oxygen, nitrogen

the conversion efficiencies of HC and CO are very high over a wide range of R but NOx efficiency is low

lean operation

lean mixture voltage

less than 0.1 volts

lean mixture

lots of O2 in exhaust gas

abnormal combustion

misfire or failed to form a flame kernel, all HC or gasoline exits gasoline engine as unburned HC

composition of the three-way catalyst converter

noble metal

at low temperature, the ________________ efficiency of HC and CO is very low

oxidation

the conversion efficiency of nox is high but the conversion efficiency of CO and HC are low

rich operation

most catalysts for exhaust gas are noble metal, such as

silver, gold, platinum, rhodium, iridium, palladium, ruthenium, and osmium

the ER window achieving high efficiency for all three pollutants is very narrow, and is achieved around ________________________

stoichiometric mixture

O2 sensor only works when

temperature is high (over 250 degrees C)

engine control unit (ECU)

the component receiving the O2 sensor signal, justifying if the gasoline-air mixture is lean or rich and tells the injector to inject more/less fuel

light-off temperature

the temperature achieving 50% oxidation efficiency

source of HC

there is HC in intake mixture. The HC in exhaust gas may originate from the intake fuel

stoichiometric mixture

there is almost no O2 in exhaust gas

source of CO

there is no CO, so CO in exhaust gas is formed during the combustion process

source of NOx

there is no NOx, so NOx in exhaust gas is formed during combustion

rich mixture

there is no O2 in exhaust gas

where is the catalyst converter installed

very close to the engine exhaust manifold

the output of the oxygen sensor is measured in

volts

when does TWC work perfectly

when gasoline-air mixture is stoichiometric

when do catalysts play an active role

when the reactants can actually contact the catalyst