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IC Engine Exhaust Emissions

VIEWS: 39 PAGES: 35

									IC Engine Exhaust
    Emissions

    Section 7


                    1
                  Pollutant Formation and Control

•  All IC engines produce undesirable emissions as a result of combustion
including hydrogen fuelled engines.

•  The emissions of concern are: unburned hydrocarbons (UHC),
         carbon monoxide (CO), nitric oxide and nitrogen dioxide (NOx),
         sulfur dioxide (SO2), and solid carbon particulates.

•  These emissions pollute the environment and contribute to acid rain, smog,
and respiratory and other health problems.

•  HC emissions from gasoline-powered vehicles include a number of toxic
substances such as benzene, polycyclic aromatic hydrocarbons (PAHs),
1,3-butadiene and three aldehydes (formaldehyde, acetaldehyde, acrolein).

•  Carbon dioxide (CO2) is an emission that is not regulated but is the primary
greenhouse gas responsible for global warming.


                                                                          2
                        Historical Perspective


•  During the 1940s air pollution as a problem was first recognized in the Los
Angeles basin.

•  Two causes of this were the large population density and the natural weather
conditions. Smoke and other pollutants combined with fog to form smog.

•  In 1966 HC and CO emission limits were introduced in California.

•  All of North America usually follows California’s lead (all US in 1968).

•  By making more fuel efficient engines and with the use of exhaust after
treatment, emissions per vehicle of HC, CO, and NOx were reduced by
about 95% during the 1970s and 1980s.

•  Automobiles are more fuel efficient now (2x compared to 1970) but there are
more of them and the trend has been towards larger SUVs (e.g. Suburban,
Navigator, Escalade) as a result fuel usage is unchanged over this period.
                                                                              3
                     Ontario Drive Clean Program

In Ontario every vehicle must undergo a tail pipe emission test every other
year to check compliance with regulation:

•  Nitrogen Oxide – 984 ppm @ 3000 rpm

•  Carbon Monoxide – 0.48% @ 3000 rpm and 1.0% @ 800 rpm

•  Unburned hydrocarbons – 86 ppm @ 3000 rpm and 200 ppm @ 800 rpm

•  Particulates (diesels only at present) – 30% opacity

•  Evaporative emissions from gas refuelling cap (SI only at present)




                                                                        4
                 Ontario Drive Clean Program Stats



Test results between 1999 and March 2004

Light-Duty Program*:      14.6% failed test

Heavy-Duty Diesel**:      4% failed test

Heavy-Duty Non-Diesel**: 27.3% failed test


* 6 million vehicles (automobiles, vans, SUVs, pick-ups) in program
** 200,000 vehicles in program




                                                                      5
                      Nitrogen Oxides

•  NOx includes nitric oxide (NO) and nitrogen dioxide (NO2)

•  In SI engines the dominant component of NOx is NO

•  Forms as a result of dissociation of molecular nitrogen and oxygen.

•  Since the activation energy of the critical elementary reaction O+N2→NO+N
is high the reaction rate is very temperature dependent, ω''′ ~ exp (-E/RT)

•  Therefore NO is only formed at high temperatures and the reaction rate is
relatively slow.

•  At temperatures below 2000K the reaction rate is extremely slow, so NO
formation not important.




                                                                          6
                        SI Engine In-cylinder NO Formation

•  Since the cylinder temperature changes throughout the cycle the NO
reaction rate also changes.

•  Each fluid element burns to its AFT based on its initial temperature,
elements that burn first near the spark plug achieve a higher temperature.

•  Since the chemistry is not fast enough the actual NO concentration tends
toward but never achieves the equilibrium value.
     If NO concentration is lower than equilibrium value – NO forms
     If NO concentration is higher than equilibrium value – NO decomposes

•  Once the element temperature cools to 2000K the reaction rate becomes
so slow that the NO concentration effectively freezes at a value greater than
the equilibrium value.

•  The total amount of NO that appears in the exhaust is calculated by
summing the frozen mass fractions for all the fluid elements:

                                     1
                              xNO = ∫0 xNO dx                                7
x=0


x=1
                 -15o (x = 0)     25o (x = 1)
              (assuming no mixing of fluid elements)




                  Equilibrium concentration:
      x=0    based on the local temperature, pressure,
             equivalence ratio, residual fraction

       x=1       Actual NO concentration:
             based on kinetics
                                                8
              Effect of Equivalence Ratio on NO Concentration

One would expect the peak NO concentrations to coincide with highest AFT.

Typically peak NO concentrations occur for slightly lean mixtures – that
corresponds to lower AFT but higher oxygen concentration.




                                                                           9
              Effect of Various Parameters on NO Concentration

Increased spark advance and intake manifold pressure both result in higher
cylinder temperatures and thus higher NO concentrations in the exhaust gas


                Pi= 658 mm Hg    Pi= 354 mm Hg




                      φ = 0.97




                                  φ = 0.96




                 φ = 1.31

                                      φ = 1.27




                                                                      10
                Exhaust NO Concentration Reduction

Since the formation of NO is highly dependent on cylinder gas temperature
any measures taken to reduce the AFT are effective:
    •  increased residual gas raction
    •  exhaust gas recirculation (EGR)
    •  moisture in the inlet air

In CI engines the cylinder gas temperature is governed by the load and
injection timing



                                                IDI/NA – indirect injection
                                                         naturally aspirated

                                                DI/NA – direct injection
                                                        naturally aspirated




                                                                               11
                            Hydrocarbons

Hydrocarbon emissions result from the presence of unburned fuel in the
engine exhaust.

However, some of the exhaust hydrocarbons are not found in the fuel, but are
hydrocarbons derived from the fuel whose structure was altered due to
chemical reaction that did not go to completion. For example: acetaldehyde,
formaldehyde, 1,3 butadiene, and benzene all classified as toxic emissions.

About 9% of the fuel supplied to the engine is not burned during the normal
combustion phase of the expansion stroke.

Only 2% ends up in the exhaust the rest is consumed during the other
three strokes.

As a consequence hydrocarbon emissions cause a decrease in the thermal
efficiency, as well as being an air pollutant.



                                                                         12
            Hydrocarbon Emission Sources for SI Engines

There are six primary mechanisms believed to be responsible for
hydrocarbon emissions:

                        % fuel escaping
Source                  normal combustion       % HC emissions

Crevices                        5.2                     38
Oil layers                      1.0                     16
Deposits                        1.0                     16
Liquid fuel                     1.2                     20
Flame quench                    0.5                      5
Exhaust valve leakage           0.1                      5

Total                           9.0                     100




                                                                  13
                    Hydrocarbon Emission Sources

Crevices – these are narrow regions in the combustion chamber into which
the flame cannot propagate because it is smaller than the quenching distance.

Crevices are located around the piston, head gasket, spark plug and valve
seats and represent about 1 to 2% of the clearance volume.

The crevice around the piston is by far the largest, during compression the fuel
air mixture is forced into the crevice (density higher than cylinder gas since gas
is cooler near walls) and released during expansion.




                     Crevice                 Piston ring




                                                                           14
                    Hydrocarbon Emission Sources
Oil layers - Since the piston ring is not 100% effective in preventing oil
migration into the cylinder above the piston, an oil layer exists within the
combustion chamber that traps fuel.

Deposits – Carbon deposits build up on the valves, cylinder and piston
crown. These deposits are porous with pore sizes smaller than the
quenching distance so trapped fuel cannot burn.

Liquid fuel – For some fuel injection systems there is a possibility that liquid
fuel is introduced into the cylinder past an open intake valve. The less volatile
fuel constituents may not vaporize (especially during engine warm-up) and be
absorbed by the crevices or carbon deposits.

Flame quenching – It has been shown that the flame does not burn
completely to the internal surfaces, the flame extinguishes at a small but
finite distance from the wall.



                                                                               15
                  Hydrocarbon Exhaust Process

When the exhaust valve opens the large rush of gas escaping the cylinder
drags with it some of the hydrocarbons released from the crevices, oil layer
and deposits.

During the exhaust stroke the piston rolls the hydrocarbons distributed along
the walls into a large vortex that ultimately becomes large enough that a
portion of it is exhausted.




                                 Blowdown        Exhaust
                                 (near BC)       Stroke                 16
                  Hydrocarbon Exhaust Process

The first peak is due to blowdown and the second peak is due to vortex roll
up and exhaust (vortex reaches exhaust valve at roughly 290o)




               Exhaust                           Exhaust
               valve                             valve
               opens                             closes




                         BC                       TC                  17
               Hydrocarbon Emission Sources for CI Engines

Crevices - Fuel trapped along the wall by crevices, deposits, or oil due to
impingement by the fuel spray (not as important as in SI engines).

Undermixing of fuel and air - Fuel leaving the injector nozzle at low velocity,
at the end of the injection process cannot completely mix with air and burn.

Overmixing of fuel and air - During the ignition delay period evaporated fuel
mixes with the air, regions of fuel-air mixture are produced that are too lean to
burn. Some of this fuel makes its way out the exhaust. Longer ignition delay
more fuel becomes overmixed.




                                                Exhaust HC, ppm C




                                                                            18
Note for the direct injection diesel the hydrocarbon emission are the worse at
light load (long ignition delay)




                                                                         19
                          Particulates

A high concentration of particulate matter (PM) is manifested as visible
smoke in the exhaust gases.

Particulates are any substance other than water that can be collected by
filtering the exhaust, classified as:
          1) solid carbon material or soot
          2) condensed hydrocarbons and their partial oxidation products

Diesel particulates consist of solid carbon (soot) at exhaust gas temperatures
below 500oC, HC compounds become absorbed on the surface.


In a properly adjusted SI engines soot is not usually a problem

Particulate can arise if leaded fuel or overly rich fuel-air mixture are used.
Burning crankcase oil will also produce smoke especially during engine warm
up where the HC condense in the exhaust gas.

                                                                           20
                          Particulates (soot)

Most particulate material results from incomplete combustion of fuel HC for
fuel rich mixtures.

Based on equilibrium the composition of the fuel-oxidizer mixture soot
formation occurs when x ≥ 2a (or x/2a ≥ 1) in the following reaction:

                                        y
                  C x H y + aO2 → 2aCO + H 2 + ( x − 2a)C ( s)
                                        2
i.e. when the (C/O) ratio exceeds 1. Experimentally it is found that the critical
C/O ratio for onset of soot formation is between 0.5 and 0.8

The CO, H2, and C(s) are subsequently oxidized in the diffusion flame to
CO2 and H2O via the following second stage
            1                                            1
        CO + O2 → CO2         C ( s) + O2 → CO2     H 2 + O2 → H 2O
            2                                            2
Any carbon not oxidized in the cylinder ends up as soot in the exhaust!
                                                                           21
                        Particulates and CI Engines
Particulates are a major emissions problem for CI engines.

Exhaust smoke limits the full load overall equivalence ratio to about 0.7




                                φ = 0.7




                                 φ = 0.5   One technique for measuring particulate
                                           involves diluting the exhaust gas with
                                 φ = 0.3   cool air to freeze the chemistry before
                                           measurements



In order to reduce NOx one wants to reduce the AFT but that has the
adverse effect of decreasing the amount of soot oxidized and thus
increases the amount of soot in the exhaust.                                22
                      Particulates and CI Engines

An example of this dilemma is changing the start of injection, e.g., increasing
the advance increases the AFT




                                          Crank angle bTC for
                                          start of injection




                                                                         23
                            Carbon Monoxide

•  Carbon monoxide appears in the exhaust of fuel rich running engines.

•  For fuel rich mixtures there is insufficient oxygen to convert all the carbon
in the fuel to carbon dioxide.


                                                        C8H18-air




                                                                             24
                          Carbon Monoxide

The C-O-H system is more or less at equilibrium during combustion and
expansion.

Late in the expansion stroke when the cylinder temperature gets down to
around 1700K the chemistry in the C-O-H system becomes rate limited and
starts to deviate from equilibrium.

In practice it is often assumed that the C-O-H system is in equilibrium until
the exhaust valve opens at which time it freezes instantaneously.

The highest CO emission occurs during engine start up (warm up) when the
engine is run fuel rich to compensate for poor fuel evaporation.

Since CI engines run lean overall, emission of CO is generally low and not
considered a problem.




                                                                         25
                          Emission Control

The current emission limits for HC, CO and NOx have been reduced to 4%,
4% and 10% of the uncontrolled pre-1968 values, respectively.

Three basic methods used to control engine emissions:

1)  Engineering of combustion process - advances in fuel injectors, oxygen
sensors, and engine control unit (ECU).

2) Optimizing the choice of operating parameters - two NOx control measures
that have been used in automobile engines since 1970s are spark retard and
EGR.

3) After treatment devices in the exhaust system - catalytic converter




                                                                         26
                       Catalytic Converter

All catalytic converters are built in a honeycomb or pellet geometry to expose
the exhaust gases to a large surface made of one or more noble metals:
platinum, palladium and rhodium.

Rhodium used to remove NO and platinum used to remove HC and CO.




Lead and sulfur in the exhaust gas severely inhibit the operation of a catalytic
converter (poison).                                                       27
                        Three-way Catalytic Converter

A catalyst forces a reaction at a temperature lower than normally occurs.

As the exhaust gases flow through the catalyst, the NO reacts with the CO,
HC and H2 via a reduction reaction on the catalyst surface.
e.g., NO+CO→½N2+CO2 , NO+H2 → ½N2+H2O, and others

The remaining CO and HC are removed through an oxidation reaction forming
CO2 and H2O products (air added to exhaust after exhaust valve).

A three-way catalysts will function correctly only if the exhaust gas composition
corresponds to nearly (±1%) stoichiometric combustion.

        If the exhaust is too lean – NO is not destroyed
        If the exhaust is too rich – CO and HC are not destroyed

A closed-loop control system with an oxygen sensor in the exhaust is used to
A/F ratio and used to adjust the fuel injector so that the A/F ratio is near
stoichiometric.
                                                                            28
                       Effect of Mixture Composition




Since thermal efficiency is highest for slightly lean conditions it may seem that
the use of a catalytic converter is a rather severe constraint.

The same high efficiency can be achieved using a near stoichiometric mixture
and diluting with EGR to reduce NOx                                   29
                            Effect of Temperature
The temperature at which the converter becomes 50% efficient is referred to
as the light-off temperature.

The converter is not very effective during the warm up period of the engine




                                                                         30
                      Catalytic Converter for Diesels

For Diesel engines catalytic converters are used to control HC and CO, but
reduction of NO emissions is poor because the engine runs lean in order to
avoid excess smoke.

The NO is controlled by retarding the fuel injection from 20o to 5o before TC
in order to reduce the peak combustion temperature.

This has a slight negative impact on thermal efficiency since it reduces the
combustion temperature  increases fuel consumption by about 15%.




                                                                         31
                                  IC Engine Fuels

Crude oil contains a large number of hydrocarbon compounds (25,000).

The purpose of refining is to separate crude oil into various fractions via a
distillation process, and then chemically process the fractions into fuels and
other products.

A still is used to heat a sample, preferentially boiling off lighter components
which are then condensed and recovered.

The group of compounds that boil off between two temperatures are referred
to as fractions.

The order of the fractions as they leave the still are naptha, distillate, gas oil,
and residual oil. These are further subdivided using adjectives light, middle,
and heavy.

The adjectives virgin or straight run are often used to signify that no chemical
processing has been performed to a fraction.
                                                                               32
Distillation Process




Refining Process




                       33
                             Gasoline

Light virgin (or straight run) naptha can be used as gasoline.

Gasoline fuel is a blend of hydrocarbon distillates with a range of boiling
points between 25 and 225oC (for diesel fuel between 180 and 360oC)

Chemical processing is used to:

•  Produce gasoline from a fraction other than light virgin, or

•  Upgrade a given fraction (e.g., Alkylation increases the MW and octane
number of fuel: produce isooctane by reacting butene with isobutane in the
presence of a catalyst.




                                                                              34
                          Reformulated Gasoline

In order to reduce CO and HC the oxygen content of gasoline is increased to
about 3% by weight (U.S. oxygenated fuels program, winter only).

The U.S. reformulated gasoline program is a year-round program used
to reduce ozone by requiring a minimum oxygen content of 2% by weight and
maximum benzene content of 1%.

The primary oxygenates are MTBE (CH3)OC(CH3)3 and ethanol (C2H5OH)

Also as part of the reformulated gasoline program sulfur is restricted to 31 ppm

Note: gasoline with 10% ethanol by volume also marketed as gasohol




                                                                          35

								
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