Title: ENVE 4003
1ENVE 4003
- MOBILE SOURCES
- Types of emissions, control technologies and
trends, inspection and maintenance programs.
2Motor Vehicles
- Internal combustion (IC) engines
- Spark ignition (SI) - gasoline, propane, natural
gas, ethanol - 4-stroke vs 2-stroke
- Compression ignition (CI) - diesel, biodiesel
3Figure 13.1 de Nevers
- Schematic of piston and cylinder in IC engine
4Figure 18.2 Cooper Alley
- Schematic of four stroke IC engine
5Figure A3.1.5 Faiz, Weaver Walsh
- Two stroke motorcycle engine
6Figure A3.2.1 Faiz, Weaver Walsh
7MOTOR VEHICLE EMISSIONS
- Regulated (criteria pollutants)
- CO, NOx, NMHC, PM
- Non-regulated
- Individual (speciated) HCs
- carbonyl compounds (alcohols, aldehydes, ketones)
- Air toxics, e.g. benzene, toluene, ethylbenzene,
1,3,butadiene, formaldehyde, acetaldehyde - CO2 (i.e. fuel economy)
8Table 13.1 de Nevers
- Contribution of motor vehicles to U.S. national
emissions
9MOTOR VEHICLE EMISSIONS
- Exhaust (tailpipe) (CO, NOx, VOC, PM)
- Evaporative (VOC)
- Resting
- Diurnal heat build
- Hot soak
- Running
- Refuelling
10COMBUSTION IN IC ENGINES
- Air/Fuel ratio, mass of air per mass of fuel,
15 - Normalized A/F ratio,
- ? (A/F) actual / (A/F) stoichiometric
- Equivalence ratio
- ? (A/F)stoichiometric / (A/F) actual
- ? ? 1 for gasoline engines most of the time,
- ? gt 1 (fuel rich) during high power demand and
start - ? lt 1 (fuel lean) for diesel most of the time,
- ignition - combustion - extinction
- sequence repeated 102 103 times a minute
unsteady combustion
11Figure (13.2) de Nevers
- Emissions and fuel consumption vs lambda
12Figure 10.16 (7.5) de Nevers
- Effect of air-fuel ratio and quality of mixing on
composition of combustion gases
13Table 13.3 de Nevers
- Equivalence or A/F ratios
14POLLUTANT FORMATION MECHANISMS - SI ENGINES
- HC
- Rich Fuel/Air mixture, oxygen deficit
- Flame quenching at walls, crevices, quench
zone - CO
- Rich Fuel/Air mixture, oxygen deficit
- incomplete reaction, even with sufficient
oxygen - NO, Thermal
- ?T compression 600 F
- ?T combustion 3600 F
- Short times but high peak temperatures
15DIESEL COMBUSTION CHARACTERISTICS
- Only air is compressed during compression stroke,
reaching 700-900 C - Fuel is injected into hot air just before top
of compression stroke - A fuel-air mixture forms around the periphery of
the fuel jet and ignites after an ignition delay.
This premixed combustion phase accounts for only
a fraction of the fuel and causes a pressure peak - The remainder of the fuel burns under mixing
controlled combustion causing a more gradual
pressure increase, and then decline with expansion
16DIESEL NOx FORMATION CHARACTERISTICS
- Most NOx formed during the high T and P premixed
combustion phase - NOx formation can be reduced effectively by
reducing flame temperature - delay combustion into the expansion phase
- cool the air charge going into the cylinder
- exhaust gas recirculation (EGR)
17PARAMETERS AFFECTING DIESEL PM AND HC EMISSIONS
- Air/Fuel ratio, generally lean overall, to allow
for complete combustion within limited time
available for mixing - Minimum ?? 1.5 for smoke point, smoke
increases dramatically below this limit - Rate of air-fuel mixing, can be enhanced by
imparting a swirl to the injected fuel - fuel injection timing
- compression ratio
- temperature and composition of charge in the
cylinder
18DIESEL VISIBLE SMOKE
- Black smoke from soot
- White, blue or gray smoke condensed hydrocarbon
droplets in the exhaust - Blue or gray generally due to vaporized lubricant
- White due to cold start
- Sulfur in the fuel forms sulfuric acid which is
later sampled as PM
19Table 13.5 de Nevers
- Comparison of gasoline and diesel engines
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21VEHICLE EMISSION CONTROL
- Control technology is aimed at reducing the
second term fuels, engines, vehicles etc. - Urban and transportation planning addresses the
first term housing density, location,
transportation infrastructure - the second term is relatively insensitive to the
number of passengers in the vehicle - Increasing vehicle occupancy helps reduce
emissions mass transit, car pooling etc.
22CONTROL TECHNOLOGY - SI
- Air/Fuel ratio. CO and HC emissions increase as
mixture gets richer in fuel (start and high power
conditions), NOx emissions peak near
stoichiometric ratio - Fuel metering systems carburetors and fuel
injectors (throttle body TBI, multi-port PFI,
simultaneous or sequential) - Electronic Control Systems adjust the air/fuel
ratio based on the signal from an oxygen sensor
in the exhaust
23EXHAUST GAS RECIRCULATION (EGR) - SI AND CI
ENGINES
- Dilutes Air/Fuel mixture with exhaust gases
thereby reducing peak combustion temperatures and
NOx formation - There are limits to how lean an air-fuel-exhaust
gas mixture can be for ignition - Ignition systems (spark plugs etc.) and
combustion chambers can be designed to improve
performance with these lean mixtures
24EXHAUST AFTERTREATMENT SI ENGINES
- Air injection - thermal oxidation of residual CO
and HC with excess air introduced after the
engine into the exhaust system, very temperature
sensitive Minumum 600 C for HC, 700 C for CO - Catalytic convertors can achieve conversion at
lower temperatures 350 C - Oxidation (two-way) catalyst - for HC and CO
- Oxidation-reduction (three-way) catalyst (TWC)
for HC, CO, and NOx according to
25CATALYTIC CONVERTORS SI ENGINES
- Pellet and monolith types
- Require near stoichiometric combustion for
effective conversion of all three pollutants, CO
and HC conversion efficieny drop for rich
mixtures, NOx conversion efficiency drops for
lean mixtures - Exhaust gas oxygen sensor (Zirconia, ZrO2 based)
essential to keeping the Air/fuel ratio in window
of optimum conversion efficiency for all three
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27TWC picture from ICT-Umicore CD
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29EVAPORATIVE EMISSION CONTROL SI ENGINES
- Blowby and Crankcase emissions - fuel and partial
combustion product molecules pass by the piston
into the crankcase - recycled back to air intake
manifold by Positive Crankcase Ventilation (PCV) - Charcoal canister for capturing fuel tank,
carburetor and miscellanous evaporative
emissions. Adsorption during hot-soak, diurnal
heat build (breathing), refuelling periods,
desorption into the air intake during engine
operation (regeneration)
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31CONTROL TECHNOLOGY - CI
- PM and NOx more important in diesel exhaust than
CO and HC, relative to gasoline exhaust - A general trade-off between PM and NOx exists
although reductions in absolute levels of both
emissions have been achieved - Emissions more strongly dependent on engine
design - most emission reductions so far have
been achieved through combustion modifications
rather than exhaust aftertreatment in contrast
to gasoline engine emissions
32DIESEL PM FORMATION CHARACTERISTICS
- Particulate Matter forms in fuel rich zones
primarily during the mixing controlled combustion
phase - mostly an aggregate chain carbon core (soot)
- adsorbed hydrocarbons (aliphatic and
polyaromatic) soluble organic fraction (SOF) - significant fraction of SOF may come from
lubricating oil - Most of the PM formed during combustion is
subsequently burned during the expansion stroke,
the unburned part forms the emissions - Sulfur in the fuel forms sulfuric acid which is
later sampled as PM
33DIESEL EXHAUST AFTERTREATMENT
- Flow through oxidation catalyst (two-way
catalytic convertor) for reduction of CO and VOC
(80), and PM SOF (20-30), does not retain PM - Trap oxidizer (Diesel particulate filter), reduce
PM by 95, filter oxidation (regeneration)
functions - active and passive regeneration types
- Passive regeneration catalyst coated onto trap
or added to fuel bring regeneration temperature
down to 400-450 C which can be achieved in diesel
exhaust - Active regeneration monitors PM build-up on the
trap and triggers regeneration by diesel fuel
burning, electric heating, catalyst injection
34DPF from ICT-Umicore CD
35DPF detail from M.Walsh
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40EXHAUST EMISSION MEASUREMENT
- Simulated driving conditions
- Mass Emission rates in g/km for light duty
vehicles (LDV) on a chassis dynamometer - Mass Emission rates in g/kWh for heavy duty (HD,
diesel) engines on an engine dynamometer - Actual driving conditions
- On-board measurement systems
- Tunnel studies
- Remote sensing, g/L of fuel burned
41EVAPORATIVE EMISSION MEASUREMENT
- SHED Test, Sealed Housing Evaporative
Determination - Carbon canisters attached to various points on
vehicle to adsorb HC vapors
42DRIVING OR OPERATING CYCLES
- Actual vs Synthesized
- Transient, steady state, multi-mode
- Modal analysis
- Acceleration
- Cruise
- Deceleration
- Idle
43EMISSION FACTORS
- Amount of pollutant emitted per unit activity
- g/km, (distance travelled)
- g/kWh, (mechanical energy delivered)
- g/L, (quantity of fuel burned)
- For a single vehicle with given engine and
emission control technology, the factors that
influence the emission factor are speed,
acceleration/deceleration, trip length, ambient
temperature - Vehicles with similar size, engine, and emission
technology may be expected to show similar
emission behaviour
44EMISSION FACTORS AND EMISSION MODELLING
- Regulated emissions from new vehicles vs
emissions from in-use vehicles - Emissions surveillance program to test emissions
from thousands of in-use vehicles at different
ages in the U.S. - Emission modelling from motor vehicles involves
the consideration of different types of vehicles
and their driving conditions to arrive at a grand
total
45INSPECTION AND MAINTENANCE PROGRAMS
- Field studies suggest that more than 50 of motor
vehicle pollution may come from less than 10 of
vehicles which have poorly maintained or
malfunctioning emission control devices - Inspection and maintenance (I/M) programs aimed
at identifying such gross-emitter vehicles and
ensuring the repair of their emission control
systems are becoming more important in the face
of reduced emission regulations for new vehicles - Remote sensing of CO, HC, and NOx, along with CO2
offers both I/M and fuel-based emission inventory
advantages
46I/M PROGRAMS
- Emission control technology for LDGV very
effective 90--95 reduction compared with no
controls - Emission control system performance deteriorates
with vehicle age but only gradually - Emissions from a small fraction of vehicles with
malfunctioning control systems erode the benefits
of emission reductions from a large number of
vehicles - I/M programs aim to maintain control system
efficiency for the entire fleet, over the useful
life of vehicles
47I/M PROGRAMS
- Objectives
- Identify and repair vehicles with maladjustments
or control system malfunctions - Discourage willful tampering with control systems
- Modes
- Periodic checks of all vehicles
- Identification and repair of high emitting
vehicles, - Identification and exemption of low emitting
vehicles, - clean screening
48I/M PROCEDURES
- SI Engines
- Exhaust concentrations measurement CO, HC, NOx
- No load, idle/2500 rpm
- Loaded dynamometer tests
- ASM, Acceleration simulation mode
- (AMS2525, 25 mph, 25 maximum FTP acceleration)
- IM240, first 240 seconds of FTP (Federal Test
Procedure) - Visual inspection of control system components
- Pressure/purge tests for evaporative emission
control systems - CI Engines
- Bosch method for smoke pull measured amount of
exhaust through filter paper, check light
transmission of filter - Opacity meter check light attenuation directly
across exhaust path under snap acceleration
conditions
49I/M PROGRAMS
- Institutional setting
- Centralized - inspection
- Decentralized - test and repair
- Frequency
- Vehicle age at first test, 1-4 years
- Subsequent tests every 1-2 years
- Costs
- Program operating costs
- Repair costs
- Cost/benefit ratio
- Improvement in ambient air quality vs I/M costs
50I/M PROGRAMS - COMPLEMENTS
- Remote sensing
- Clean screening, high emitter profiling
- On-board diagnostics (OBD)
- Sensing and monitoring devices to detect
malfunctions - Light indicator
- Stored computer codes for malfunctioning
components - catalyst
- oxygen sensor
- engine misfire
- evaporative system integrity
51Pb, S, and Transportation fuels
- Pb used to be added to gasoline (tetra-ethyl lead
TEL) as an octane enhancer. - Phased out in most countries and being phased out
in others - permanent poisoning of TWC using leaded
gasoline in a vehicle with TWC once is sufficient
to make the TWC useless - Neuro-toxic health effects on children
52Pb, S, and Transportation fuels
- S is a natural component of crude oil. Can be
removed effectively by hydrodesulfurization. - Adverse (though reversible) effect on efficiency
of TWC and DPF. Low sulfur fuel increases
efficiency of modern TWC and makes it possible to
use advanced diesel exhaust after-treatment like
DPF - contribution to PM emissions as sulfate
- contribution to gaseous Sox emissions
- Current trends coming down to 15 ppm (ULSD
ultra low sulfur diesel), from 300-500 ppm.