EP1857664A1 - Pipe of an internal combustion engine of an automobile, intended to carry exhaust gases coming from the engine to a mixing junction - Google Patents

Abstract

The pipe has an element (4) integrated to a heat exchanger (5), where the length of the element is chosen such that intake and exhaust gas flows are in phase with a junction at which an exhaust gas to be recycled is mixed with air for constituting an intake mixture of a heat engine e.g. DW12BTED4 oil engine, for an operating point of the engine. The element is arranged to increase the length before being traveled by the gas for reaching the junction. The exchanger has tubes (6) that extend parallel to each other, where an outer wall of the exchanger is in contact with a liquid cooling circuit.

Description

The invention relates to a driving of a combustion engine of a motor vehicle, intended to convey the exhaust gas of the engine from an engine outlet point to a junction at which these gases are mixed with air to form an intake mixture for the engine.

Such a pipe, intended to form an EGR (Exhaust Gas Recycling) system, allows a portion of the gas to be recycled by the engine to reduce pollutant emissions during engine operation.

To optimize the effect of the exhaust gas recirculation, it is necessary that these gases distribute homogeneously with the intake air at the inlet of the cylinder.

In the example described by the US patent US2002 / 0088443 the junction comprises a frustoconical section defining a complex transverse section with a star-shaped contour, making it possible to orient the air and the exhaust gases in opposite radial directions at the junction, in order to promote their mixing upstream of the junction; engine intake.

This solution does not ensure that the distribution of exhaust gas is sufficiently uniform between the cylinders.

The present invention aims to overcome this disadvantage by providing a simple solution, inexpensive and easily integrated with an existing pipe, to further improve the homogeneity of the air / gas mixture.

For this purpose, the invention relates to a driving of a thermal engine of a motor vehicle, intended to convey exhaust gases to a junction where these gases are mixed with air to form a mixture of inlet for supplying a combustion chamber of the engine.

According to the invention, the pipe comprises an additional element intended to increase the length to be traversed by the exhaust gases to reach the junction.

The length of this element is chosen so that the gas flow rate and the intake flow rate are in phase at the junction for a given operating point of the engine.

In this way, at the moment when the intake flow rate is maximum at the junction, the flow of exhaust gas discharged in this junction is also. Similarly, when the intake flow is minimum at the junction, the flow of exhaust gas is too.

An air / gas mixture is thus obtained having intake air and exhaust gas contents at the junction which are substantially constant during the entire operating cycle of the engine.

According to another characteristic, the additional element is integrated in a heat exchanger composed of at least one tube traversed by the exhaust gas and whose outer wall is in contact with a cooling circuit.

According to yet another characteristic, the exchanger comprises several tubes extending parallel to each other and being connected in series.

Preferably, the tubes are arranged side by side along a first assembly plane.

Advantageously, the tubes are further arranged along a second assembly plane perpendicular to the first plane.

According to another characteristic, one of the tubes of the exchanger opens directly into the junction.

According to yet another characteristic, the pipe comprises an exhaust gas control valve located upstream of the exchanger.

• la figure 1 est un graphique représentant respectivement les débits de gaz recyclé (traits pleins) et de l'air aspiré (traits interrompus) au niveau d'une jonction dans laquelle débouche une conduite de recirculation de type connu, et du mélange gaz d'échappement/air d'admission aspiré par le cylindre (traits pointillés), en fonction de la position du vilebrequin ;
• la figure 2 est un graphique analogue au graphique de la figure 1, réalisé avec une conduite selon l'invention ;
• la figure 3 est un histogramme illustrant la dispersion des gaz à recycler au sein des différents cylindres de trois répartiteurs différents, pour la conduite connue de la figure 1 (hachures) et pour la conduite selon l'invention (points) ;
• la figure 4 représente un échangeur thermique intégré à une conduite de recirculation de gaz selon l'invention.

The invention will be better understood, and other objects and advantages thereof will appear more clearly in the following description with reference to the appended figures, among which:

• FIG. 1 is a graph respectively showing the flow rates of recycled gas (solid lines) and of the sucked-up air (broken lines) at a junction into which a known type of recirculation pipe opens, and the gas mixture of exhaust / inlet air sucked by the cylinder (dashed lines), depending on the position of the crankshaft;
• Figure 2 is a graph similar to the graph of Figure 1, made with a pipe according to the invention;
• FIG. 3 is a histogram illustrating the dispersion of the gases to be recycled in the different cylinders of three different distributors, for the known pipe of FIG. 1 (hatching) and for the pipe according to the invention (dots);
• FIG. 4 represents a heat exchanger integrated in a gas recirculation pipe according to the invention.

The invention is based on the finding that for a heat engine in which a part of the exhaust gas is recycled by mixing these gases with intake air, the flow rates of air and gas to be recycled are out of phase with each other at the junction where they are mixed.

Thus, the mixture leaving the junction has contents that vary cyclically according to a period conditioned by the operating point of the engine. More particularly, the mixture is a mixture of high gas content and low air content, and mixture of low gas content and high air content.

Indeed, in a four-stroke heat engine, each cylinder describes a cycle comprising successively admission, compression, expansion, and exhaust. A complete cycle takes place on two rounds of piston, that is to say two turns of crankshaft, so that the exhaust takes place a turn and a half after admission, for the same cylinder.

In the case of a four-stroke four-cylinder engine, the cylinder cycles are shifted one half-turn crankshaft relative to each other. At each crank turn there are two inlets and two exhausts: during the first half-turn, an intake and an exhaust take place, and during the second half-turn, another intake and another exhaust take place.

Thus, the gas flow rates and the intake mixture both have amplitudes that vary with a frequency equal to twice the rotational speed of the engine. But the intake flow turns out to be out of phase with the exhaust flow.

Indeed, the maximum exhaust flow is reached at the beginning of the exhaust phase, because this maximum corresponds substantially to the opening of an exhaust valve which has the effect of releasing a high overpressure in the cylinder, this valve being open from the beginning of the escape phase.

On the contrary, the maximum flow rate of admission is reached in the middle of the intake range, that is to say a quarter turn of the crankshaft after the beginning of admission, because it corresponds to the moment when the suction effect is maximum, ie when the piston speed is the highest, which occurs when this piston is halfway through the cylinder.

It follows that in a four-stroke four-cylinder engine, the intake flow maxima are 90 ° out of phase with the intake flow maxima, which is to say that the intake flow is out of phase. 90 ° with respect to the exhaust flow.

An illustration of this shift is highlighted in Figure 1.

At the instant when the intake flow 1 is at a maximum, for example at the intake of the first cylinder, the gas flow 2 to be recycled is almost zero, so that the mixture created between the air and the gases has a negative high air content and low gas content to recycle.

When the air flow 1 is minimum, for example at the end of the first cylinder suction and before the suction of the second cylinder, the flow of gas to be recycled 2 is maximum and these gases enter the distributor, without being really mixed to air, which results in the junction, an air / gas mixture having a low air content and a high gas content to recycle.

Thus, air / gas mixtures of different contents are admitted into the different cylinders of the engine, a cylinder being able to suck up a gas / air mixture with a high gas content at one instant of the cycle, the adjacent cylinder being able to suck up an air / gas mixture at low gas content, which results in a dispersion of the distribution gases to be recycled in the various cylinders, as illustrated in FIG.

The solution proposed by the invention consists in integrating into the recirculation line an element 4 intended to increase the length to be traversed by the exhaust gases between the engine outlet point and the junction, so that the flow of gas to recycling in the duct and the flow of the mixture formed by these gases and the air and sucked by the cylinders, otherwise called admission flow of the mixture, are in phase at the junction for at least one operating point of the engine.

As illustrated in Figure 2, the integration of this element to the recirculation pipe allows the phase of the inlet flow of the mixture 1 with the flow of gas to be recycled 2 at the junction. Thus, at this junction, when the intake flow 1 is maximum, the gas flow 2 is also and when the inlet flow 1 is minimal, the gas flow 2 to be recycled is very low.

The intake air and exhaust gas contents of the mixture formed at the junction are substantially constant during the engine cycle.

Thus, as illustrated in Figure 3, it avoids too much dispersion of the gas distribution in the different cylinders in contrast to the use of a pipe of known type. The air / gas mixtures to be recycled have comparable contents from one cylinder to another (the dispersion of the gases in the different cylinders of the same distributor remains less than 6%).

This element 4 has according to the invention a length of the order of one meter, variable approximately between 1 and 4 meters depending on the operating point of the engine chosen as the objective (and engine considered). This operating point is the one for which we want that the intake flow is in phase with the flow of gas at the junction.

The length added to the recirculation line by this additional element can be determined empirically for the operating point or engine speed, chosen as a reference.

The length of the pipe with the additional element, such as the gas flow rate is in phase with the suction flow rate at the junction, depends mainly on the engine speed and the speed of circulation of the gases in the pipe at this point. regime, which condition the wavelength of the gas flow.

With the additional element, the length to be traveled by the exhaust gases between the point where they are discharged and the mixing junction must be of the order of a quarter of the wavelength of the gas flow to create a 90 ° crankshaft delay (ie a quarter turn) to cancel the phase shift at the junction point.

The determination of the length can be carried out empirically by measuring the phase difference between the exhaust gases upstream of the junction and the suction flow rate, to adjust this length so as to cancel the phase shift at a given engine speed. .

Practically, this length is between one and four meters, and it turns out that the length of the pipe must be all the more important that the chosen engine speed is low.

For example, for an operating point of a diesel engine DW12BTED4 of about 1600 rpm; 4.7 bar effective mean pressure, the length of the element, necessary to phase the intake flow rate with the flow of gas to be recycled at the junction is about 4 meters.

This additional length of pipe is advantageously integrated in a heat exchanger 5 of the recirculation pipe, between a gas inlet valve to be recycled (not shown) and the junction. This exchanger 5 cools the gases before they are introduced into the air supply tube.

This heat exchanger 5 is in contact with a liquid cooling circuit not shown.

According to the embodiment of the exchanger 5 shown in FIG. 4, this exchanger 5 consists of the series assembly of nine identical tubes 6 having a constant section (diameter 24 millimeters), a length of 444 millimeters and which are juxtaposed with each other, the gases to be recycled, thus traveling 4.5 round trips.

More specifically, these nine tubes 6 are arranged by trio in three XOY planes superimposed in the Z direction, being spaced 27.3 millimeters in the direction X and in the direction Z, so as to define a lateral space of 82 millimeters on 82 millimeters. Connecting members 7 are provided to allow the flow of exhaust gas from a tube to the adjacent tube.

One of the three tubes of the first level communicates directly with one end of the recirculation pipe and serves as a gas inlet tube 6a in the exchanger, one of the three tubes of the last level acting as an outlet tube 6b exhaust gas directly into the junction or into another end of pipe.

To reduce the total length of the exchanger 5 while keeping the additional length of 4 meters, it is possible to use more tubes 6 than for the example of FIG. 4, like eleven tubes 6 with a length of 363 millimeters each, defining a Lateral dimensions of 82 millimeters by 110 millimeters, the gases to recycle through the exchanger 5.5 round trips.

Another solution is to use thirteen tubes with a length of 307 millimeters each, defining a lateral space requirement of 110 millimeters by 110 millimeters and 6.5 round trips to be traversed by the gases to be recycled.

Soot storage areas (not shown) are preferably provided inside the exchanger 5 at the end of the inlet tube 6a of the exchanger 5 and within the preceding tube the outlet tube 6b of exchanger 5, to limit fouling.

Advantageously, means (not shown) may be used to allow short-circuit at least one tube of the exchanger to reduce the length of the path traveled by the exhaust gas within the exchanger up to 1 meter for example, to allow phase matching of flows also for higher regimes than the reference regime.

• la formation d'une réelle distribution des gaz à recycler entre les cylindres,
• l'intégration aisée de la longueur de conduite supplémentaire au sein d'un échangeur,
• la possibilité de favoriser l'échange calorifique entre l'échangeur et le circuit de refroidissement qui l'entoure, par l'augmentation de la surface de contact,
• un effet acoustique intéressant (gain en remplissage moteur) sur la pleine charge pour le point de fonctionnement choisi.

The invention as described above has various advantages, among which.

• the formation of a real distribution of the gases to be recycled between the cylinders,
• the easy integration of the additional pipe length into an exchanger,
• the possibility of promoting the heat exchange between the exchanger and the cooling circuit which surrounds it, by increasing the contact surface,
• an interesting acoustic effect (gain in motor filling) on the full load for the chosen operating point.

Claims (7)

Driving a thermal engine of a motor vehicle, intended to convey exhaust gases from the engine to a junction where these gases are mixed with air to form a mixture of engine intake, characterized in that it comprises an element (4) whose length of this element is chosen so that the gas flow and the intake flow rate are in phase at the junction for a given operating point of the engine, this element (4) intended to increase the length to be traversed by the exhaust gas to reach the junction. Pipe according to claim 1, wherein the element (4) is integrated in a heat exchanger (5) composed of at least one tube (6) traversed by the exhaust gas and whose outer wall is in contact with a circuit cooling. A conduit according to claim 2, wherein the exchanger (4) comprises a plurality of tubes (6) extending parallel to each other and connected in series. Pipe according to claim 3, wherein the tubes (6) are arranged side by side in a first assembly plane. Pipe according to claim 4, wherein the tubes (6) are further arranged in a second assembly plane advantageously perpendicular to the first plane. Conduit according to one of claims 2 to 5, wherein one of the tubes (6) of the exchanger (4) opens directly into the junction. Pipe according to one of claims 2 to 6, comprising an exhaust gas control valve located upstream of the exchanger (5).

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