U.S. patent application number 12/532599 was filed with the patent office on 2010-03-04 for low temperature urea injection method.
This patent application is currently assigned to PEUGEOT CITROEN AUTOMOBILES S.A.. Invention is credited to Gabriel Crehan.
Application Number | 20100050597 12/532599 |
Document ID | / |
Family ID | 38669690 |
Filed Date | 2010-03-04 |
United States Patent
Application |
20100050597 |
Kind Code |
A1 |
Crehan; Gabriel |
March 4, 2010 |
LOW TEMPERATURE UREA INJECTION METHOD
Abstract
The invention relates to a method for injecting urea in an
exhaust line of an engine (1), the urea containing ammonia which is
used for chemically reducing, during a selective catalytic
reduction reaction, or SCR reaction, the nitrogen oxides produced
by the engine, the injection being performed upstream of a catalyst
(3) in which the reaction takes place, the process comprising the
following steps: the temperature in the exhaust line of the engine
is measured (7); an amount of urea to be injected is determined
(5), from a known relation between the temperature and the amount
of urea to be injected for all possible operating temperature
values; a determined urea amount is injected (6) in the engine
exhaust line.
Inventors: |
Crehan; Gabriel; (Paris,
FR) |
Correspondence
Address: |
NICOLAS E. SECKEL;Patent Attorney
1250 Connecticut Avenue, NW Suite 700
WASHINGTON
DC
20036
US
|
Assignee: |
PEUGEOT CITROEN AUTOMOBILES
S.A.
Velizy Villacoublay
FR
|
Family ID: |
38669690 |
Appl. No.: |
12/532599 |
Filed: |
March 10, 2008 |
PCT Filed: |
March 10, 2008 |
PCT NO: |
PCT/FR08/50397 |
371 Date: |
September 22, 2009 |
Current U.S.
Class: |
60/274 ;
60/287 |
Current CPC
Class: |
Y02T 10/47 20130101;
F01N 13/009 20140601; Y02T 10/40 20130101; F01N 3/035 20130101;
F01N 2560/026 20130101; Y02T 10/12 20130101; Y02T 10/24 20130101;
F01N 11/002 20130101; F01N 2610/02 20130101; F01N 3/208
20130101 |
Class at
Publication: |
60/274 ;
60/287 |
International
Class: |
F01N 3/20 20060101
F01N003/20; F01N 3/00 20060101 F01N003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 22, 2007 |
FR |
0753990 |
Claims
1. Method of injecting urea into an exhaust line of an engine, the
urea containing ammonia to be used in a selective catalytic
reduction reaction, or SCR reaction, to chemically reduce nitrogen
oxides discharged by the engine, the injection taking place
upstream of a catalyst in which the reaction takes place, the
method comprising the following steps: measuring the temperature in
the engine exhaust line, determining, based on this temperature, a
maximum quantity of urea that can be broken down into ammonia and
thus be injected into the engine exhaust line with no risk of
polymerization, and injecting the quantity of urea thus determined
into the engine exhaust line.
2. Method according to claim 1, comprising the step of measuring
the quantity of nitrogen oxides entering the catalyst, and using
this measurement to determine the quantity of urea to inject.
3. Method according to claim 1, comprising the step of measuring
the quantity of ammonia exiting the catalyst, and using this
measurement to determine the quantity of urea to inject.
4. Method according to claim 1, comprising the step of calculating
the quantity of ammonia present in the SCR catalyst, and using this
calculation to determine the quantity of urea to inject.
5. Method according to claim 1, wherein to determine the quantity
of urea to inject, predetermined data are used, which are recorded
in a memory of a processor used to carry out the method.
6. System for injecting urea into an engine exhaust line, the urea
containing ammonia to be used in a selective catalytic reduction
reaction, or SCR reaction, to chemically reduce nitrogen oxides
discharged by the engine, the system comprising: a catalyst which
is the site of the reduction reaction, a device for measuring the
temperature of the gases in the exhaust line, and a device for
calculating, based on a measured temperature, the maximum quantity
of urea that can be broken down into ammonia and thus be injected
into the engine exhaust line with no risk of polymerization, and a
urea tank and a urea injector located upstream of the catalyst.
7. Method according to claim 2, comprising the step of measuring
the quantity of ammonia exiting the catalyst, and using this
measurement to determine the quantity of urea to inject.
8. Method according to claim 2, comprising the step of calculating
the quantity of ammonia present in the SCR catalyst, and using this
calculation to determine the quantity of urea to inject.
9. Method according to claim 3, comprising the step of calculating
the quantity of ammonia present in the SCR catalyst, and using this
calculation to determine the quantity of urea to inject.
10. Method according to claim 7, comprising the step of calculating
the quantity of ammonia present in the SCR catalyst, and using this
calculation to determine the quantity of urea to inject.
11. Method according to claim 2, wherein to determine the quantity
of urea to inject, predetermined data are used, which are recorded
in a memory of a processor used to carry out the method.
12. Method according to claim 3, wherein to determine the quantity
of urea to inject, predetermined data are used, which are recorded
in a memory of a processor used to carry out the method.
13. Method according to claim 4, wherein to determine the quantity
of urea to inject, predetermined data are used, which are recorded
in a memory of a processor used to carry out the method.
14. Method according to claim 7, comprising the step of calculating
the quantity of ammonia present in the SCR catalyst, and using this
calculation to determine the quantity of urea to inject.
15. Method according to claim 8, wherein to determine the quantity
of urea to inject, predetermined data are used, which are recorded
in a memory of a processor used to carry out the method.
16. Method according to claim 9, wherein to determine the quantity
of urea to inject, predetermined data are used, which are recorded
in a memory of a processor used to carry out the method.
17. Method according to claim 10, wherein to determine the quantity
of urea to inject, predetermined data are used, which are recorded
in a memory of a processor used to carry out the method.
Description
[0001] The present invention relates to a strategy for injecting
urea into the exhaust line of an engine, and more particularly, of
an engine installed in a diesel-type motor vehicle.
[0002] In diesel-type vehicle engines, fuel combustion results in
the creation of gases such as nitrogen monoxide (NO), nitrogen
dioxide (NO2), and nitrous oxide (N2O).
[0003] These gases, which are generally known by the name nitrogen
oxides (NOx), pose a hazard, firstly to human health, and secondly
to the environment, since they help to create smog in cities and
contribute to global warming by increasing the greenhouse effect.
Consequently, we must come up with solutions to destroy these gases
internally in vehicles before they are released into the
atmosphere. The treatment of these gases in vehicles is strictly
regulated, moreover, by various standards.
[0004] In order to chemically destroy these nitrogen oxides before
they are discharged into the atmosphere, it has been envisioned to
use a reduction process of the type known as SCR, or selective
catalytic reduction.
[0005] Various reductants can be used for this purpose.
[0006] A first possibility, which is to use hydrocarbons as a
reductant, has a double disadvantage in that it is both costly, due
to the current price of fuel, and polluting, since it produces an
increase in carbon dioxide emissions exiting the vehicle
engine.
[0007] To remedy these disadvantages, numerous solutions have been
proposed using urea as a reductant. That is, urea contains ammonia,
which reacts with the nitrogen oxides in an SCR catalyst to form
completely harmless dinitrogen. Most of these solutions involve
injecting urea in liquid form. It is injected into the exhaust line
at a temperature greater than 180.degree. C.
[0008] At this temperature, the breakdown of urea into ammonia is
complete and practically instantaneous, making it possible to use a
relatively high injection rate, e.g., around 20 g/h. This high
decomposition rate is partly due to the thermodynamic stability of
urea and the size of the injected urea droplets.
[0009] The reason for using urea rather than pure ammonia is that
ammonia is a toxic, corrosive gas, and therefore it is costly and
complicated to provide a tank for storing a gas of this kind safely
in a standard vehicle. Urea, on the other hand, may be stored as an
aqueous solution, making it easier to store and inject into the
exhaust line.
[0010] The reduction of nitrogen oxides using liquid urea involves
several successive chemical reactions.
[0011] When a urea solution is injected into an exhaust line, first
the water evaporates, thereby causing solid urea to form as tiny
particles.
[0012] This reaction is expressed by the following chemical
equation:
NH2-CO--NH2.sub.(aqueous)->NH2-CO--NH2.sub.(solid). (Eq 1)
[0013] Generally, once evaporation takes place, the solid urea
undergoes thermolysis in the surrounding high-temperature gases, at
180.degree. C. and up. This thermolysis produces gaseous ammonia
and isocyanic acid through the following reaction:
NH2-CO--NH2.sub.(solid)->NH3.sub.(gas)+HNCO.sub.(gas) (Eq 2)
[0014] The last step of the reduction process is hydrolysis of the
isocyanic acid to form gaseous ammonia and carbon dioxide:
HNCO.sub.(gas)+H20.sub.(gas)->NH3.sub.(gas)+C02.sub.(gas) (Eq
3)
[0015] Below 180.degree. C., the breakdown of the urea particles by
thermolysis occurs at a slower rate than the partial polymerization
of urea. As a result, the urea particles turn into biuret. The
biuret breaks down quickly by sublimation above 180.degree. C., or
below 180.degree. C. on any catalytic surface containing acid
sites. The two forms of biuret decomposition may be comprehensively
described by the following reaction:
H2NCONHCONH2.sub.(solid)+2.times.H2O.sub.(gas)->NH3.sub.(gas)+CO2.sub-
.(gas) (Eq 4)
[0016] Now, it has been observed that in light vehicles, the
temperature in the exhaust line is commonly below 180.degree. C.,
in particular because of deceleration or frequent stopping of the
vehicle, or prolonged low-speed city driving.
[0017] When the temperature of the urea injected into the exhaust
line becomes less than 180.degree. C., decomposition is no longer
complete, and then complete polymerization of the
high-concentration urea begins to occur, and a white solid forms on
the surface of the SCR catalyst or the wall of the exhaust
line.
[0018] As previously explained, the thermolysis of the solid urea,
represented by equation 2, actually occurs because of the high
temperature of the exhaust gases. When the temperature is lower, a
solid polymer, cyamelide, is formed:
NH2-CO--NH2.sub.(solid)->HNCOX.sub.(solid polymer) (Eq 5)
[0019] This reaction occurs only with part of the urea that builds
up at high concentrations on the surface of the catalyst, while the
other part undergoes thermolysis as previously described. The
formation of the solid polymer occurs at the moment where urea
decomposition by thermolysis in the gaseous phase or by catalytic
decomposition on the surface of the catalyst becomes slower than
the reaction in which urea accumulates on the surface of the
catalyst.
[0020] In this case, the surface of the catalyst is coated with a
polymer, and the ammonia no longer comes in contact with the
nitrogen oxides, which makes it impossible to reduce the latter.
The decomposition rate of biuret decreases as a function of the
temperature. The rate of urea polymerization, on the other hand,
depends on the local concentration of urea or biuret molecules.
That is, the higher the concentration, the greater the risk of
polymerization, especially at low temperatures.
[0021] In addition, to destroy the polymerized solid deposited on
the various elements of the exhaust line, the temperature must be
raised to a value of about 450.degree. C. But bringing the catalyst
to such a high temperature on a regular basis is likely to destroy
the active zones of the catalyst, which are indispensable for
carrying out the chemical reactions of reduction.
[0022] A solution for limiting the accumulation of urea on the
surface of the SCR catalyst and reducing the risk of polymerization
would therefore be to control the quantity of urea injected into
the exhaust line as a function of temperature.
[0023] In this way, the invention aims to remedy these
disadvantages by proposing a urea injection method that can be used
at any temperature, and particularly at low temperatures.
[0024] More precisely, the invention relates to a method for
injecting urea into an exhaust line of an engine, the urea
containing ammonia to be used in a selective catalytic reduction
reaction--or SCR reaction--to chemically reduce nitrogen oxides
discharged by the engine. The urea is injected upstream of a
catalyst in which the reaction takes place, and the method
comprises the following steps: [0025] measuring the temperature in
the engine exhaust line, [0026] determining a quantity of urea to
inject from a known relation between the temperature and a quantity
of urea to inject, for all possible operating temperature values.
[0027] injecting the quantity of urea thus determined into the
engine exhaust line.
[0028] Such a method makes it possible, based on the temperature of
the exhaust gases, to determine the quantity of urea that can be
injected safely, i.e., with no risk of polymerization, which can
result in poor removal of nitrogen oxides.
[0029] Thus, when the temperature is less than 180.degree. C., we
can choose to inject urea anyway, but at a lower concentration so
that all of the urea injected will break down into ammonia, and
there will be no polymerization.
[0030] A method in accordance with the invention makes it possible
to determine a quantity of urea that can be injected into the
exhaust line with no risk of polymerization, regardless of the
temperature in the exhaust line. This quantity of urea is
determined by using a relation that links the exhaust gas
temperature to a quantity of urea over a range of values covering
all of the temperatures that can be detected in the exhaust
line.
[0031] This relation can be expressed as a mathematical relation,
such as a polynomial equation, a graph, or a correspondence
table.
[0032] These data are recorded in the memory of a processor, for
example, to which the method refers in order to perform the various
steps.
[0033] Preferably, data of this kind are determined experimentally,
since they vary from one exhaust line to another. That is, they
depend on physical characteristics of the various elements that
make up this exhaust line, such as the engine type, the type of
technology used for injection, and the catalyst type and size.
[0034] Even though the data used in calculating the quantity of
urea are determined experimentally, the calculation can end up
being skewed by parameters that vary in the exhaust line.
Consequently, it can be useful at times to have the calculated
quantities corrected during the process.
[0035] To this end, in some embodiments the method comprises the
step of measuring the quantity of nitrogen oxides entering the
catalyst, and using this measurement in the step where the final
quantity of urea is determined.
[0036] Similarly, in some embodiments the method comprises the step
of measuring the quantity of ammonia exiting the catalyst, and
using this measurement to determine the quantity of urea to inject.
That is, if there is too high a quantity of urea observed to be
exiting the catalyst, there must be immediate intervention to avoid
risking a noxious discharge of gases from the vehicle.
[0037] As a variant, a method in accordance with the invention also
comprises one or more of the following steps: [0038] the step of
determining a maximum quantity of urea that can be injected, based
on the temperature, and determining the quantity to inject as a
function of this maximum quantity and at least one other parameter
that is calculated or measured in the exhaust line, [0039] the step
of calculating the quantity of ammonia present in the SCR catalyst,
and using this calculation to determine the quantity of urea to
inject, and [0040] the step of determining the quantity of urea to
inject using predetermined data, such as a data plot, recorded in a
memory of a processor used to carry out the method.
[0041] The invention also relates to a system for injecting urea
into an engine exhaust line, the urea containing ammonia to be used
in a selective catalytic reduction reaction--or SCR reaction--to
chemically reduce nitrogen oxides discharged by the engine, the
system comprising: [0042] a catalyst which is the site of the
reduction reaction, [0043] a device for measuring the temperature
of the gases in the exhaust line, and [0044] a device for
calculating the quantity of urea to inject into the catalyst, using
a known relation between the temperature and a quantity of urea to
inject, for all possible operating temperature values, [0045] a
urea tank and a urea injector located upstream of the catalyst, and
intended for injecting the quantity of urea determined.
[0046] Several embodiments of the method will now be described in
order to highlight other advantages and characteristics thereof.
This description is given on a non-limiting basis, using the
following figures:
[0047] FIG. 1 shows the change in certain parameters present in a
vehicle engine over a known operating cycle, i.e., a MVEG
cycle.
[0048] FIG. 2 shows a vehicle exhaust line equipped with a liquid
urea injector,
[0049] FIG. 3 is a graph of a polynomial relation between a
quantity of urea and a temperature,
[0050] FIG. 4 shows an operational diagram of various urea
injection strategies, and
[0051] FIG. 5 shows the nitrogen oxide conversion rate as a
function of temperature.
[0052] FIG. 1 shows various parameters measured in a motor vehicle
engine over a standardized MVEG operating cycle. This cycle is
currently used for vehicle certification in Europe.
[0053] FIG. 1 shows three curves representing the change in the
vehicle speed (curve 11), the change in the quantity of nitrogen
oxides discharged by the engine (curve 12) and the change in
temperature (curve 13), respectively, as a function of time.
[0054] Operating the vehicle according to the MVEG cycle involves a
succession of vehicle accelerations and decelerations. Thus,
although the temperature in the engine would tend to increase over
time, this increase is curbed by the frequent decelerations.
[0055] It can be observed that during the first 900 seconds of the
cycle, the temperature does not exceed 180.degree. C. except
intermittently. Now, it has been previously explained that such a
temperature is necessary for the urea to break down completely into
ammonia with no polymerization.
[0056] Consequently, it seems obvious from this graph that it is
not feasible to use a standard method of reducing nitrogen oxides
with urea on light vehicles. Therefore, a method in accordance with
the invention allowing low-temperature urea injection must be
used.
[0057] An advantageous implementation of this method is used in an
exhaust line as shown in FIG. 2.
[0058] In this figure, there is a vehicle engine 1 that releases
nitrogen monoxide NO and nitrogen dioxide NO.sub.2. At this
engine's output there is an oxidation catalyst 2, used to increase
the NO.sub.2/NO ratio in the exhaust gases, thereby enabling better
reduction of the nitrogen oxides subsequently in the selective
reduction catalyst 3.
[0059] Lastly, the treated exhaust gases pass through a particulate
filter 4 before being discharged into the atmosphere.
[0060] In order to make it possible to carry out the method, the
various elements in the exhaust line are managed by the vehicle's
onboard computer 5. For example, from the gas temperature reading
taken by the device 7, the computer 5 is able to determine the
quantity of urea that must be injected into the exhaust line by the
injector 6, using experimental data recorded in a memory. The urea
is stored in a tank 8.
[0061] In addition, the exhaust line is equipped with two gas
detectors 9 and 10 to measure the quantities of gas present
upstream and downstream, respectively, of the SCR catalyst 3. In an
example, the computer 5 uses the measurements provided by these two
detectors to control the injection of urea into the system.
[0062] The predetermined data contained in a memory of the computer
5 can be in plotted or table form, or any other data set.
[0063] For example, the relation between the quantity of urea to
inject and the temperature can be modeled by a polynomial relation
equal to:
Y=7E.sup.-06x.sup.3+0.0082x.sup.2-2.5934x+182.2
[0064] This relation is shown graphically in FIG. 3 for
temperatures varying from 100 to 200.degree. C. This way, with the
temperature value for the gases in the exhaust line, a vehicle
computer can use this relation to determine the quantity of urea to
inject, e.g., in the form of a liquid additive such as AdBlue. This
quantity is expressed here in milliliters per hour.
[0065] To gain a better understanding of an injection method
according to the invention, it will be described for three
operating phases of the vehicle, which are distinguished here
according to the exhaust gas temperature.
[0066] Range A corresponds to temperatures less than 120.degree. C.
It has been observed that it is useless to inject urea into the
exhaust line at these temperatures, for several reasons: [0067]
firstly, at these temperatures, urea evaporates relatively poorly,
making it difficult to set off the first necessary chemical
reaction, which normally leads to urea particle formation, [0068]
in addition, SCR catalysts are generally such that they cannot be
active at less than 120.degree. C., and thus cannot serve as the
site for urea to break down into ammonia. Besides, even if the SCR
catalyst already contains ammonia stored in its micro-pores,
nitrogen oxide reduction is impossible at temperatures this
low.
[0069] Consequently, in this vehicle operating range, urea
injection is not undertaken.
[0070] Range C corresponds to temperatures greater than 180.degree.
C. It was explained above that at these temperatures, there is a
complete and near-instantaneous breakdown of urea into ammonia.
Consequently, in this operating range, it is possible to inject as
much urea into the exhaust line as is necessary to reduce the
nitrogen oxides released by the engine.
[0071] The specificity of the method according to the invention is
evident in operating range B, which corresponds to temperatures
between 120.degree. C. and 180.degree. C. Actually, in this
temperature range, the hydrolysis reaction (Eq 3), which
corresponds to a breakdown into the gaseous phase, cannot take
place. Consequently, the breakdown of urea into ammonia is not
complete, and thus it is advisable to limit the concentration of
ammonia being injected.
[0072] The relation by which this concentration can be calculated
is shown in an area B of FIG. 3. It must be determined for each
type of engine, since it depends on a number of engine components
such as injectors, the exhaust line, and the catalysts, e.g., the
pollution control catalysts.
[0073] For an advantageous implementation of the invention, a
processor may be used, installed for example in an onboard computer
of a motor vehicle. This processor can use various parameters,
which are predetermined, calculated or measured, in order to
determine the quantity of urea that must be injected into the
exhaust line. The role of these various parameters is illustrated
in FIG. 4.
[0074] In this figure, a processor 20 is shown. This processor is
in communication with a temperature measuring device 21. In an
advantageous embodiment, this temperature device 21 is a
thermocouple, i.e., a device comprising two metals connected by two
junctions, and which generates a difference in potential that
depends on the difference in temperature between the two junctions.
In order to link the difference in potential to a temperature
difference, one must know the thermocouple response as a function
of temperature. This response is kept in a memory of the processor,
for example, so that it can be used during a step of the
method.
[0075] Thus, using the measured temperature and data 22 recorded in
a memory of the processor, it is possible first of all to determine
a maximum quantity of urea that can be injected safely into the
exhaust line, as a function of temperature. The data 22 are in the
form of one or more data plots, for example.
[0076] Next we determine the quantity of urea that actually needs
to be injected in order to destroy the nitrogen oxides produced by
the engine, which is a function of the maximum quantity of urea,
the quantity of ammonia already stored in the SCR catalyst, the
quantity of ammonia actually consumed by the nitrogen oxide
reduction reaction, the temperature, and the speed.
[0077] Ammonia quantities are calculated by a calculator 23, as a
function of the quantity of nitrogen oxides produced by the engine,
and are measured by a detector 24.
[0078] To this end, the calculator uses the ammonia consumption
rate, expressed by the formula:
Rate=kA(-AE/RT)[A].sup.x[B].sup.y[C].sup.z where: [0079] K is a
constant [0080] A is an exponential factor, [0081] AE is the
activation energy, [0082] R is the ideal gas constant, [0083] T is
the temperature, [0084] A, B and C are the concentrations of the
species in the reaction, whose respective orders are x, y, and
z.
[0085] The final quantity of urea thus determined is injected into
the exhaust line by the injector 25.
[0086] FIG. 5 illustrates the change in this ammonia consumption in
an SCR catalyst by showing the percentage of nitrogen oxide
conversion as a function of the temperature and the ratio of
nitrogen dioxide to nitrogen monoxide.
[0087] The quantity of ammonia already present in the catalyst is
calculated based on parameters such as the quantity of urea
previously injected, the quantity of urea consumed by the reduction
reaction, and the storage capacity of the catalyst.
[0088] This storage capacity is for example determined
experimentally based on the age of the catalyst. It also depends on
the type of catalyst, i.e., whether one is using a catalyst with a
micro-porous structure, a high-capacity catalyst, or a non-porous
catalyst.
[0089] The ammonia used for nitrogen oxide reduction can be in any
phase--liquid, gas or solid. However, as described in the present
application, an additive such as AdBlue is preferably used, i.e.,
an aqueous solution containing 32.5% urea. For a standard diesel
vehicle with a high capacity tank, the AdBlue injection rate is
generally 20 to 40 liters per hour. The urea injection frequency
can vary from 1 to 100 Hz, and a frequency of 10 Hz is preferably
used.
* * * * *