WO2009115682A2

WO2009115682A2 - Method and device for measuring the opacity level of exhaust gases from a diesel engine

Info

Publication number: WO2009115682A2
Application number: PCT/FR2009/000176
Authority: WO
Inventors: Thierry Lepage
Original assignee: Ifp
Priority date: 2008-02-21
Filing date: 2009-02-17
Publication date: 2009-09-24
Also published as: WO2009115682A3; FR2928000B1; FR2928000A1

Abstract

The invention relates to a method and a device for measuring the opacity level of exhaust gases from a diesel engine, wherein the method comprises using an opacimeter for collecting the exhaust gases, emitting a light beam through the collected gases, and acquiring a signal relative to the light intensity of the light beams that passed through the gases. The method further comprises extracting from the signal a first light intensity measurement during an engine acceleration phase, and extracting from the signal a second light intensity measurement during an engine idle phase. The two measurements are sufficiently close in time to provide equivalent measurement conditions. Finally, the method comprises deriving, from said measures and by a relative opacity calculation, the opacity level of the gases by calculating the gas absorption coefficient during the acceleration phase.

Description

METHOD AND DEVICE FOR MEASURING THE EXHAUST GAS OPACITY LEVEL ISSUED BY A DIESEL ENGINE

The present invention relates to the field of controlling the pollution level of a combustion engine, especially diesel engines.

More particularly, the invention relates to a method and a device for measuring the level of opacity of the exhaust gases emitted by diesel engines, the opacity constituting a known criterion of the pollution level of this type of engine.

State of the art

It is known to perform these measurements of the pollution level from a partial flow opacimeter free acceleration, that is to say that the engine is subjected to acceleration without load (gearbox in neutral), and a part of the exhaust gas is analyzed.

In France, for example, cars are regularly checked in technical centers where they undergo various tests, including a pollution test. To carry out this test, the technical control centers use opacimeters that meet the standard

NF R 10-025-2: 1996. Such an opacimeter is described for example in the patent application FR 2 703 460.

The control procedure is defined by standard NF-R-10-025-3. The opacity of the exhaust gases is measured by their absorption coefficient, according to the law of Béer Lambert. According to this law, the luminous intensity decreases exponentially in a homogeneous medium thus: I = Io * e (-k * L) with:

/ = light intensity received by a photocell

Io = initial luminous intensity emitted by a light bulb of the opacimeter k = absorption coefficient of the medium (the exhaust gases) L = length of the measuring chamber

From the previous equation, we obtain k (expressed in m ^"1 ):

-Ln (If Io) L

From this absorption coefficient, it is possible to define the pollution level of the investigated engine, using, for example, the decision to accept paragraph 7.3 of the NF-R 10-025-3: 1996 standard. must not exceed the value specified by the manufacturer when it exists, or the following values:

- 2.5 m ^"1 in the case of naturally aspirated diesel engines;

- 3.0 m ^"1 in the case of turbocharged diesel engines;

- 1.5 m ^"1 for all vehicles registered or put into circulation as of ¹ July 2008.

The application of Béer Lambert's law to the measurement of the opacity of the exhaust gases of diesel engines assumes that, on the one hand, that the initial light intensity emitted by the source

(Io) is stable throughout the measurement, and secondly, that the degradation of the state of the measuring chamber does not alter the transmission of this light. Several phenomena call into question these imperatives:

The evolution of temperature: the yields of light sources are often dependent on temperature. This is particularly the case for the diodes used by some opacimeters.

Fouling of the cell caused by soot deposition: during the measurement, soot may be deposited on the optical elements, which causes a parasitic weakening of the light beam.

The water vapor contained in the exhaust gas can condense on surfaces if they are not maintained at a sufficient temperature, and alter the optical beam if these surfaces are optical components (glass plate). To overcome these disturbances, it is known (standard NF R 10-025-1: 1996): to maintain a temperature in the measuring cell of the order of 70 ° C to prevent the condensation of water vapor; to establish air flows to ensure that soot does not settle on the measuring elements.

These techniques require the use of a complex opacimeter, bulky and expensive.

The invention relates to an alternative method for measuring the level of opacity of the exhaust gases of diesel engines, reliably, taking into account the requirements mentioned above, from a simple opacimeter and having a low cost.

The invention also relates to such an opacimeter.

The device and the method according to the invention An object of the invention relates to a device for measuring a level of opacity of exhaust gas produced by a diesel engine. The device comprises: - a gas flow chamber extending over a predetermined length and comprising at a first end a gas inlet port, at a second end a gas outlet port; means for emitting light beams through the circulation chamber placed at one end of the circulation chamber; means for capturing and transforming light beams emitted and passing through the circulation chamber into an electrical signal proportional to the light intensity sensed; an electronic processing unit comprising a control module for the emission means and a processing module for the electrical signal coming from the sensor means. The signal processing module comprises: means for extracting from the signal a first measurement of luminous intensity during an acceleration phase of the engine, and a second measurement of luminous intensity during an idle phase of the engine, these means are designed so as to extract these two measurements over a time interval sufficiently small to ensure equivalent measurement conditions; and means for calculating a light absorption coefficient of the gases in the acceleration phase from the extracted measurements, to define the level of opacity of the gases.

The light beam emission means and the sensor means may be placed at the same end of the circulation chamber, a concave mirror placed at the other end providing a reflection of the light beams to the sensor means. According to this configuration, the light beam emission means can advantageously be placed in a transmission chamber, and the sensor means placed in a measurement chamber, so that no light beam emitted reaches the chamber directly. without having crossed the circulation chamber.

According to the invention, the emission chamber may also comprise a photodiode positioned to capture light beams emitted directly by the emission means and light beams reflected by a transparent wall separating the circulation chamber from the emission means. , in order to detect condensation of water vapor within the circulation chamber.

The device may also include:

means for heating the circulation chamber, in order to limit condensation of the gases within the circulation chamber.

means for measuring the temperature within the device. means for compensating for drifts related to the dark current of the sensor means.

Finally, according to the invention, the electronic unit for processing the device can be designed to control a constant current source, so as to drive the transmission means so that it generates a current draw of a duration chosen at regular intervals.

Another object of the invention relates to a method for measuring a level of opacity of exhaust gas emitted by a diesel engine, in which the exhaust gases are collected, a beam of light is emitted through the collected gases, and acquiring a signal relating to the light intensity of light beams having passed through the gases. The method comprises the following steps: - A first light intensity measurement is extracted from the signal during an acceleration phase of the motor;

a second light intensity measurement is extracted from the signal during an idle phase of the engine, the two measurements being sufficiently close in time to ensure equivalent measurement conditions;

the opacity level of the gases is deduced by calculating a gas absorption coefficient during the acceleration phase from the measurements.

According to one embodiment, the first measurement corresponds to a peak amplitude of the signal, and the second measurement corresponds to the light intensity received during the idle phase preceding the acceleration phase. According to this embodiment, it is possible to calculate the absorption coefficient of the acceleration phase from Lambert's Beer Law, and considering that a gas absorption coefficient during the idling phase is negligible. . One can thus, for example, deduce the absorption coefficient by the following relation:

where kp = absorption coefficient of amplitude peak Is = luminous intensity received during idle phase Ip = luminous intensity received at maximum of amplitude peak

L = length of a cell in which the gases are collected.

According to another embodiment, it is possible to measure a level of opacity of exhaust gas emitted by a diesel engine, from a process in which several cycles of acceleration / idling of said engine are carried out from a opacimeter comprising:

a gas circulation chamber comprising at a first end a gas inlet orifice, and at a second end a gas outlet orifice,

a transmitting chamber comprising an emission diode for emitting a light beam, and a reference photodiode measuring a beam light intensity light emitted directly by the emission diode and light beams reflected by a transparent wall separating the circulation chamber from the emission chamber,

a measurement chamber comprising a measurement photodiode for acquiring a signal relating to the light intensity of light beams having passed through the gases, and in which,

amplitude peaks are detected on the signal, each peak corresponding to an acceleration phase;

for each peak detected, a condensation of water vapor is detected within the circulation chamber; for each peak detected for which no condensation is detected, the temperature is expected to increase by a few degrees from the moment when such a peak is detected; then

the opacity level of said gases is deduced by calculating a gas absorption coefficient for each detected peak for which no condensation is detected, starting from the process described above.

According to this embodiment, a condensation of water vapor can be detected by detecting an increase in light intensity in the emission chamber, from the light intensity measured by the reference photodiode. It is also possible to estimate an evolution of temperature within the circulation chamber, starting from the signal. It is also possible to validate the opacity measurement, by performing the following steps: for each cycle, an opacity value corresponding to an amplitude peak is estimated during the acceleration phase, and the measurements are validated of opacity when the opacity values of four successive peaks are within a chosen range of values. According to the invention, it is also possible to perform a verification phase, before any opacity measurement, so as to check that the initial level of fouling of said opacimeter is below a fixed threshold, by comparing the light intensity measured by the reference photodiode and the light intensity measured by the photodiode measurement.

Advantageously, a condensation of water vapor can be minimized by using the heat of the exhaust gas to heat the circulation chamber. The emission diode can be controlled by current pulses, so as to emit a high intensity light beam while maintaining a low average consumption, and the measurements are acquired during the current draw.

Finally, according to the invention, it is possible to compensate a drift of the dark current of the photodiodes by also acquiring the measurements after the current draw, the luminous intensity received by the photodiode corresponding to the difference between an average of the measurements acquired during the draws and averages the measurements acquired after the puise.

Other features and advantages of the method and the device according to the invention will appear on reading the following description of nonlimiting examples of embodiments, with reference to the appended figures and described below.

BRIEF DESCRIPTION OF THE FIGURES FIG. 1 is a diagram of the opacimeter according to the invention. FIG. 2 illustrates an example of the electronic processing unit (10) of the device according to the invention. FIG. 3 presents a timing diagram of the measurement acquisition sequence according to the method of the invention.

FIG. 4 represents an opacity signal obtained on an acceleration. - Figure 5 shows a possible mechanical implantation of the opacimeter according to the invention on a vehicle exhaust pipe.

Figure 6 shows a detail of the emission chamber of the device.

Figure 7 shows opacity level measurement results for a series of acceleration cycles. FIG. 8 shows the evolution of the electrical reference (ReJ) and measurement (Mes) signals during two acceleration cycles.

FIG. 9 illustrates the evolution of the temperature (T), in parallel with a signal (ε _Ref ) coming from the reference photodiode. FIG. 10 represents steps of the method, including the various embodiments making it possible to obtain a precise and reliable value of the level of opacity of the exhaust gases coming from a diesel engine.

Detailed Description of the Invention The device

Figure 1 illustrates the popacimeter according to the invention. This device for measuring the level of opacity of exhaust gas produced by a diesel engine comprises the following elements: a gas circulation chamber (2), in which the exhaust gases emitted by the vehicle are collected and circulating. It extends over a predetermined length, and may be cylindrical in shape. At one end, an orifice (1) allows the introduction of the exhaust gas, while another orifice (3), located at the other end of the chamber, allows its extraction. Two light-ray-transparent walls (5 and 6), such as glass plates, are fixed at each end, delimiting the length of the circulation chamber (2). These plates make it possible to let the light beams through while protecting the means of the device outside the circulation chamber from fouling due to gases (soot deposit, etc.).

means (4) for emitting light through the circulation chamber, placed at one end of said chamber. These means may correspond to a light bulb or a light emitting diode (LED). It is possible to choose a diode whose emission spectrum is between 550 nm and 570 nm in accordance with standard NF R 10-025-1.

means (8) for capturing and transforming the light emitted and passing through the circulation chamber, into an electric current whose intensity is proportional to the intensity of the light captured. These means may correspond to a measurement photodiode. A photodiode is a semiconductor component having the ability to detect radiation from the optical domain and transform it into an electrical signal.

an electronic processing unit (10), comprising:

a control module for the light-emitting diode (SRCE, MC); a module for acquiring the electrical signal coming from the photodiode (AF, CAN, MC) - A module for processing the electrical signal from the photodiode (MC). This module makes it possible to extract from the signal a first measurement of luminous intensity during an acceleration phase of the engine, and a second measurement of luminous intensity during an idle phase of the engine. The module is designed to extract these two measurements over a short period of time (for example, a few seconds).

Thus, the two measurements are sufficiently close in time to ensure equivalent measurement conditions within the device. To extract a measurement during an acceleration phase, this module may include means for detecting large variations in signal amplitudes, such as peaks. Thus, when the electrical signal processing module detects a peak (voltage for example), it extracts the maximum value of the peak. A peak then corresponds to a peak of luminous intensity (this peak is negative if there is absorption of light, Fig. 8, signal Mes)

This module then makes it possible to calculate the opacity of the gases during the acceleration phase. This calculation is based on a relative measure of opacity, based on the Beer-Lambert law, and described hereinafter in the method according to the invention. According to the invention, the opacity level corresponds to an absorption coefficient of light beams by the exhaust gases.

a module for transmitting the results to the user of the device (MC, HMI).

Figure 2 illustrates an example of the electronic processing unit (10). According to this arrangement, it consists of:

- a power supply (ALIM) which supplies the necessary current to the various constituents of the unit and the device. It is obtained either from the battery voltage of the vehicle to be tested (via the cigarette lighter socket for example) or from batteries or accumulators. - a constant current source (SRCE). This source makes it possible to control the diode (4).

- A module (AF) for amplifying and filtering the current from the photodiode (8).

- a Digital Analog Converter (ADC) to convert the output voltages of the amplification and filtering module into digital signals.

- a microcontroller (MC). It performs the following functions: control of the current source (SRCE), acquisition of measurement signals via the CAN, calculation of the opacity, and management of the Human Machine Interface (HMI). - A Human Machine Interface (HMI) to communicate with the end user. It can be rudimentary (consisting of some signaling LEDs), or evolved. In the latter case, it consists of an LCD display and some control buttons. It can be integrated into the system or deported to facilitate the reading of information while the rest of the system is close to the muffler. It can also be integrated in a complex remote system. In this case, it is connected to the opacimeter via a digital link, such as an RS232, RS 485, Bus Can, or Ethernet link.

According to one embodiment, the light beam emission means and the sensor means are placed at two different ends of the circulation chamber.

According to a preferred embodiment (Figure 1), a concave mirror (7), located behind the transparent wall (6), reflects the light beam coming from the transmission head. This mirror can also replace the transparent wall (6), thus delimiting the length of the circulation chamber (2). In one or other of these configurations, the light beam emission means and the sensor means are placed at the same end of the circulation chamber, and the concave mirror is placed at the other end. This concave mirror ensures reflection of light beams to the sensor means. Thus, this embodiment makes it possible to limit the length of the circulation chamber, and therefore of the device. Preferably, to improve the quality of the measurements, the dimensional characteristics of the concave mirror (7) and the measurement photodiode (8) are such that the photodiode captures the entire beam reflected by the mirror. According to this configuration, the light beam emission means are placed in a transmission chamber (11), and the sensor means are placed in a measurement chamber (12), so that no light beam emitted reaches directly the measuring chamber (12) without passing through the circulation chamber (2): no luminous flux can go directly from the emission chamber to the measuring chamber.

According to this configuration, a light beam (dotted arrow in FIG. 1) is emitted by the diode (4). The beam passes through the transparent wall (5), the circulation chamber

(2), the second transparent wall (6) is reflected towards the measuring chamber by the mirror concave (7). It crosses the measuring chamber (2) and the transparent walls (5 and 6) before reaching the measuring photodiode (8).

The method There are two methods for performing an opacity measurement of gas emitted by a motor.

The engine can be placed in conditions of determined speed and load, the exhaust gases then have a constant opacity rate. The values obtained depend on engine speed, load and condition. You can also not run the engine and cause rapid acceleration, from idle to maximum speed. The opacity level then goes through a peak during the acceleration phase. The method according to the invention uses the second method.

According to the invention, the Beer-Lambert law is used to perform a measure of relative opacity, the principle of which is presented below.

Let: II = light intensity received by the photodiode at a time Tl

12 = light intensity received by the photodiode at a time T2 can be written

where Io ₍ τi) and Io ( _T2 ) respectively correspond to the reference signal (initial) at times T1 and T2. It is assumed that these two instants are sufficiently close so that the values I _{0 (T} i) and I ₀ (T ₂ ) are equal. This amounts to saying that the intensity of the light beam and the measurement cell state have not varied between these two instants.

We can then write: / 1 = I ₀ * e (-k \ * L)

I2 = Io * e (-k2 * L)

We then have:

/ 1h * e (-kl * L)

= e ((k2 - k \) * L)

12 I ₀ * e (-k2 * L) We deduce k _{2 =} Ln (IV 12) _{+ kl}

In conclusion, if two opacity measurements are close to each other, we can express one with respect to the other as a function of the ratio of the luminous intensity received for these two measurements.

The opacity measurement method is based on the detection of pollution peaks during free acceleration cycles. The measurements made in free accelerations are materialized by a series of peaks of opacity. For each peak, two levels of opacity can be defined: - kp (k peak), the maximum opacity level measured on acceleration.

- ks (k threshold), the opacity level corresponding to the idle speed preceding the acceleration.

Figure 4 shows the opacity signal obtained on an acceleration. The y-axis represents the opacity value, VOP, the x-axis the sample number (of the measure), NE. As these two values, ks and kp, are very close to each other (a few seconds), we can consider that the measurement conditions have not changed for these two measurements.

We can thus write:

. Ln (IsIIp). kp = - - + ks

In addition, a diesel engine, in good condition and operating at idle, emits exhaust gases with a very low opacity, of the order of 0.05 m ^"1. The standard NF R 10-025-2: 1996 specifies that the accuracy of the opacity measurement is 0.25 m- ¹ for a measurement range of 1.5 to 3 m- ^{1, which means} that the level of pollution of a diesel engine at idle is very low and negligible compared to the accuracy of the anti-pollution test, and even assuming that a diesel engine, because it is poorly tuned, has such a high level of pollution at idle speed as to jeopardize this assumption. a motor has this type of defect, it is always accompanied by combustion problems during acceleration, and therefore very high pollution peaks. relative is minimal compared to the amplitude of pollution peaks and the result remains the same: the engine has a pollution level higher than the threshold defined by the standard.

We can thus finally express the level of opacity of the peak of pollution by the following formula:

With: kp = value of the opacity of a signal amplitude peak

Is = luminous intensity received during the idle phase preceding the cycle

Ip = luminous intensity received at the maximum amplitude peak L = cumulative length of the circulation chamber, the emission chamber and the measuring chamber?

Thus, the method according to the invention for measuring the level of opacity of exhaust gas emitted by a diesel engine comprises the following steps: the exhaust gases are collected in a circulation chamber;

a beam of light is emitted through the collected gases.

Then, by performing at least one free acceleration for the stability of the measurement conditions, the value of an absorption coefficient corresponding to a measurement of the relative opacity of the exhaust gases is calculated: a signal relating to the the luminous intensity of the light beams passing through the gases;

- A first light intensity measurement is extracted from the signal during an acceleration phase of the motor;

a second light intensity measurement is extracted from the signal during an idle phase of the engine. Both measurements are sufficiently close in time to ensure equivalent measurement conditions; an absorption coefficient of the accelerating gases is deduced from these measurements, neglecting the level of pollution of the diesel engine at idle, and using the following formula:

Ln (Is Up) kp =

According to a preferred mode, the first measurement corresponds to a peak of luminous intensity (luminous intensity received at the maximum of the amplitude peak of the signal), the second measurement corresponds to the luminous intensity received during the idle phase preceding the phase of light. 'acceleration.

Acquisition phase

The diode (4) is controlled by the constant current source (SRCE) via the microcontroller (MC). Every 10 ms, the diode (4) is traversed by a constant current draw with a duration of 500 μs. The fact that the control of the diode (4) is performed by current draws, has the advantage of allowing a large diode current, and therefore a high intensity light beam while maintaining a low average consumption. This arrangement also has the advantage of compensating for measurement errors that may be due to the dark current of the photodiodes. Figure 3 shows the timing diagram of the acquisition sequence. The signal at the top corresponds to the command (CDE) of the current source (SRCE). The bottom signal corresponds to the voltage (TEN) of the amplification and filtering module (AF). For each current draw, a voltage draw is obtained. The microcontroller (MC) controls the acquisition of data (represented by the vertical arrows) during the current draw, but also after the tap, advantageously. The actual measurement of the level of light received by the photodiode corresponds to the difference between the average of the data acquired during the tap and the average of the data acquired after the tap. This compensates for the drift of the dark current of the photodiodes.

The device and the method according to the invention therefore make it possible to perform a relative measurement of the opacity of the exhaust gases emitted by a diesel engine. This relative measurement, when it involves two measurements close in time, makes it possible to overcome the fouling by the soot. The invention thus makes it possible to simplify and limit the size of the device, since it is no longer necessary to complete the opacimeter by ventilation means.

variants

According to other embodiments, the device and the method are completed to answer the problems related to the use of an opacimeter.

Measuring the temperature difference

The light output of a diode is a function of the temperature. This means that for a constant current flowing through the diode, the light intensity, and therefore the signal of the reference photodiode, depends on the temperature of the diode.

For this purpose, the device may comprise means for measuring the temperature in the gas circulation chamber, or directly in the emission chamber according to the embodiment.

To reduce costs, and further simplify the device, it may include a reference photodiode (9), which measures the level of the signal at the level of the emission chamber. Figure 6 shows a detail of the emission chamber. The reference photodiode (9) is placed against the emission diode (4). It therefore captures some of the light emitted by the diode, but also a portion of the light in the emitting chamber (11). The received signal level is independent of the nature of the gases in the circulation chamber (2). This photodiode is connected to the electronic processing unit similarly to the measurement photodiode (8): it is connected to a module (AF) for amplifying and filtering the current coming from the photodiode (9), itself even connected to

Analogue Digital Converter (CAN).

Figure 9 shows two signals. The lower (smooth) signal corresponds to a temperature measurement (T) made by a temperature sensor placed on the body of the circulation chamber. The upper signal (noisy) corresponds to the result of a processing performed on the reference signal (ε _Ref ). Both signals are normalized at the point of origin (sample

(NE) 2500). It can be seen that these two signals are perfectly correlated.

It is therefore possible to complete the process according to the invention by estimating the temperature evolution of the emission chamber from the signal derived from the reference photodiode (9). To achieve this step, the device therefore comprises a reference photodiode (9) and a microcontroller (MC) adapted to evaluate the temperature as a function of the signal from the reference photodiode (9).

The variations of the reference signal related to the condensation of the water vapor are eliminated by the treatment, because they are of higher frequencies, and of rather low amplitude.

Treatment of water vapor

Engine exhaust contains water vapor that can condense on cold surfaces. In order to reduce manufacturing costs, the device does not have a heating unit. It uses, according to a preferred embodiment, the heat of the exhaust gas to rise in temperature. Being small, it can be placed near the muffler. It then arrives quickly at an operating temperature.

Figure 5 shows a possible mechanical implantation on a vehicle exhaust pipe. The opacimeter (Op) is held in the vehicle exhaust (Po) by means of a clamp (Pi). This assembly is carried out so as to ensure a minimum distance between the exit of the muffler and the circulation chamber (2).

In addition, according to the invention, the device can be provided with a module for detecting measurements that are tainted by error due to the condensation of water in the measuring chamber. The method then comprises a step of detecting the condensation.

As long as the opacimeter has not reached a sufficient temperature, a part of the water vapor contained in the exhaust gas will condense, making the measurement incorrect. The device has the ability to detect the presence of water vapor in two different ways.

Measurement of the emission beam

Figure 6 shows the detail of the emission chamber. The reference photodiode

(9) captures a portion of the light emitted by the diode, but also a portion of the light in the emitting chamber (11). The latter being largely due to the light reflected by the glass plate (5) ensuring the isolation between the circulation chamber (2) and the emission and measurement chambers. When water vapor is deposited on the glass plate (5), the level of light reflected by the glass plate increases, and therefore the level of the signal from the reference photo diode (9).

It is therefore possible to complete the method according to the invention by measuring the luminous intensity of the emission chamber (light beams emitted directly by the emission means and the light beams reflected by the transparent wall), so as to detect the presence condensed water inside the circulation chamber.

Monitoring opacity peak level When the system has not reached its operating temperature, the water vapor condenses during the acceleration phase and then evaporates during the idle phase. Figure 7 shows a series of acceleration cycles. The first peaks have a very large amplitude compared to the following ones. They served to raise the system in temperature conditions. The standard NF Rl 0-025-3: 1996 defines that, for the determination of the opacity value, the system is stabilized when the opacity values of four successive peaks lie in a range equal to 0.25 m ^-1. The examination of Figure 7 shows that by applying this criterion, error-prone measurements due to condensation of water vapor are rejected.

Thus, according to a particular embodiment, the method includes a validation phase of the opacity measurement, by performing the following steps: several cycles of acceleration / idling are carried out. For each cycle, an opacity value is estimated, corresponding to a peak during the acceleration phase. The opacity measurements are validated when the opacity values of four successive peaks are within a chosen value range. We can use the range defined by the standard: 0.25 m ^"1 .

Temperature monitoring

This information is used to complete the principle of water vapor detection.

We have shown in the previous paragraph that it is possible to detect the presence of water vapor from the signal from the reference photodiode. To be certain that the circulation chamber no longer contains condensed water, we set the following rule: there is no risk of error related to the condensation of water vapor when we wait until the system heats a temperature jump set by default (of the order of a few ° C) after finishing detecting water vapor on the reference signal.

Clogging The device allows a relative measurement to be made between an opacity value (ks) and a peak opacity value (kp). It is therefore sensitive only to the fouling that could occur between these two measures. These two measurements being very close, this fouling is negligible.

On the other hand, the gradual fouling of the circulation chamber causes a regular weakening of the measurement signal. This may eventually cause errors because of a signal that is difficult to exploit.

FIG. 8 shows the evolution of the reference (ReJ) and measurement (Mes) signals (ε) during two acceleration cycles. The bottom curve represents the signal from the measurement photodiode (Mes). The top curve represents the reference signal. This figure shows that it is possible to set a threshold below which the ratio (measurement signal / reference signal) must not fall. This threshold is a factory datum determined from the mechanical and optical characteristics of the system. This threshold is used to determine that the level of fouling does not disturb the measurement, but also to verify that the alignment conditions of the optical part have been met during manufacture (control of the opacimeter).

FIG. 10 represents the steps of the method, including the various embodiments making it possible to obtain an accurate and reliable value of the opacity level of the exhaust gases originating from a diesel engine. The value Im corresponds to the luminous intensity measured by the photodiode of measurement, and the value Ir corresponds to the luminous intensity measured by the reference photodiode.

The device according to the invention provides reliable information of the opacity measurement at a much lower cost than that of the devices used in the technical control centers. It is intended for individuals or small garages that seek to know the status of an engine as part of a purchase, sale, preparation for technical control.

Claims

A device for measuring a level of opacity of exhaust gas produced by a diesel engine, comprising: - a gas flow chamber extending over a predetermined length and comprising at a first end a gas inlet orifice at a second end a gas outlet; means for emitting light beams through said circulation chamber placed at one end of said circulation chamber; means for capturing and transforming light beams emitted and passing through said circulation chamber into an electrical signal proportional to the light intensity sensed; an electronic processing unit comprising a control module for the emission means and a module for processing the electrical signal from the sensor means, characterized in that the signal processing module comprises:

means for extracting from the signal a first measurement of luminous intensity during an acceleration phase of the engine, and a second measurement of luminous intensity during an idle phase of the engine, these means are designed to in order to extract these two measurements over a time interval sufficiently small to ensure equivalent measurement conditions; and

means for calculating a light absorption coefficient of the gases in the acceleration phase from the extracted measurements, to define the level of opacity of the gases.

2. Device according to claim 1, characterized in that the light beam emission means and the sensor means are placed at the same end of the circulation chamber, a concave mirror placed at the other end providing reflection of the beams. bright towards the sensor means.

3. Device according to claim 2, characterized in that the light beam emission means are placed in an emission chamber and the sensor means are placed in a measurement chamber, so that no light beam emitted does not reach the measuring chamber directly without having crossed the circulation chamber.

4. Device according to claim 3, characterized in that the emission chamber comprises a photodiode positioned to capture light beams emitted directly by the emission means and light beams reflected by a transparent wall separating the circulation chamber. emission means, for detecting a condensation of water vapor within said circulation chamber.

5. Device according to one of the preceding claims, characterized in that it comprises means for heating the circulation chamber, to limit a condensation of gas within said circulation chamber.

6. Device according to one of the preceding claims, characterized in that it comprises means for measuring the temperature within the device.

7. Device according to one of the preceding claims, characterized in that it comprises means for compensating for drifts related to the dark current of the sensor means.

8. Device according to one of the preceding claims, characterized in that the electronic processing unit is designed to control a constant current source (3) so as to drive the transmission means to generate a drawing of current of a chosen duration at regular time interval.

9. A method for measuring a level of opacity of exhaust gas emitted by a diesel engine, in which the exhaust gases are collected, a beam of light is emitted through the collected gases, and a signal relating to the luminous intensity of light beams having passed through said gases, characterized in that the method comprises the following steps:

- A first light intensity measurement is extracted from the signal during an acceleration phase of the motor; a second light intensity measurement is extracted from the signal during an idle phase of the engine, the two measurements being sufficiently close in time to ensure equivalent measurement conditions;

the opacity level of said gases is deduced by calculating a gas absorption coefficient during the acceleration phase from said measurements.

10. The method of claim 9, wherein the first measurement corresponds to a peak amplitude of said signal, and the second measurement corresponds to the light intensity received during the idle phase preceding the acceleration phase.

11. A method according to claim 10, wherein the absorption coefficient of the acceleration phase is calculated from the Lambert Beer Law, and considering that a gas absorption coefficient during the phase of idle is negligible.

12. Process according to claim 11, in which the absorption coefficient is deduced by the following relation:

Ln (Is / Ip)

where kp = absorption coefficient of amplitude peak

Is = luminous intensity received during the idle phase

Ip = luminous intensity received at maximum amplitude peak

L = length of a cell in which the gases are collected.

13. A method for measuring a level of opacity of exhaust gas emitted by a diesel engine, in which several cycles of acceleration / idling of said engine are carried out from an opacimeter comprising: a gas circulation chamber comprising a first end of a gas inlet port, and at a second end a gas outlet port, a transmission chamber having an emission diode for emitting a light beam, and a reference photodiode measuring a light intensity. of light beams emitted directly by the emission diode and light beams reflected by a transparent wall separating the circulation chamber from the emission chamber, a measurement chamber comprising a measuring photodiode for acquiring a signal relative to the luminous intensity of light beams having passed through said gases, and wherein, amplitude peaks are detected on said signal, each peak corresponding to an acceleration phase;

the opacity level of said gases is deduced by calculating a gas absorption coefficient for each detected peak for which no condensation is detected, from the method according to claims 9 to 12.

14. The method of claim 13, wherein a condensation of water vapor is detected by detecting an increase in light intensity in the emission chamber, from the light intensity measured by the reference photodiode.

15. Method according to one of claims 13 to 14, wherein it is estimated a change in temperature within the flow chamber from said signal.

16. Method according to one of claims 13 to 15, wherein the opacity measurement is validated, performing the following steps: for each cycle, an opacity value corresponding to a peak amplitude is estimated during the acceleration phase, and the opacity measurements are validated when the opacity values of four successive peaks are within a chosen range of values.

17. Method according to one of claims 13 to 16, wherein performs a verification phase, before any opacity measurement, so as to control that the initial level of fouling of said opacimeter is below a fixed threshold, in comparing the light intensity measured by the reference photodiode and the light intensity measured by the measurement photodiode.

18. Method according to one of claims 13 to 17, wherein a condensation of water vapor is minimized by using the heat of the exhaust gas to heat said circulation chamber.

19. Method according to one of claims 13 to 18, wherein the emission diode is controlled by current pulses, so as to emit a strong light beam. intensity while maintaining a low average consumption, and the measurements are acquired during the current draw.

20. The method of claim 19, wherein a drift of the dark current of the photodiodes is compensated by also acquiring the measurements after the current draw, the light intensity received by the photodiode corresponding to the difference between an average of the measurements acquired. during the puise and an average of the measurements acquired after the puise.