GB2518009A

GB2518009A - System layout for an exhaust recirculation gas cooler

Info

Publication number: GB2518009A
Application number: GB1316135.1A
Authority: GB
Inventors: Alberto Vassallo; Alok Warey; Anil Singh Bika
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2013-09-10
Filing date: 2013-09-10
Publication date: 2015-03-11
Also published as: GB201316135D0

Abstract

Disclosed is a heat exchanger system layout 305 comprising a heat exchanger 310 and an oxidation catalyst, such as a Diesel oxidation catalyst 285, located upstream of the heat exchanger. The heat exchanger comprises a low temperature coolant inlet 312 and a low temperature coolant outlet 314 the heat exchanger exchanging heat at least between water vapour and a low temperature coolant. A temperature in the low temperature coolant inlet 312 is lower than 40C so that the amount of the exchanged heat is sufficient to generate water vapour condensation. This results in only dry soot flowing from the heat exchanger outlet and helps to prevent fouling of the heat exchanger. The heat exchanger system forms part of the exhaust gas recirculation system in an internal combustion engine.

Description

s SYSTEM LAYOUT FOR AN EXHAUST RECIRCULATION GAS COOLER

TECHNICAL FIELD

The present disclosure relates to an exhaust recirculation gas (EGR) system of an internal combustion engine. More particularly, the disclosure relates to diesel engines provided with an LOR system comprising an EGR cooler upstream the EGR valve.

BACKGROUND

An internal combustion engine, particularly a highly efficient diesel engine is normally provided with an exhaust gas after-treatment system, for degrading and/or removing the pollutants from the exhaust gas emitted by the Diesel engine, before discharging it in the environment.

The after-treatment system generally comprises an exhaust line for leading the exhaust gas from the Diesel engine to the environment, a Diesel Oxidation Catalyst (DOC) located in the exhaust line, for oxidizing hydrocarbon (HG) and carbon monoxides (CO) into carbon dioxide (C02) and water (R2O), a Lean NOx Trap (LNT), which is provided for trapping nitrogen oxides (NOx) contained in the exhaust gas and is located in the exhaust line and a Diesel Particulate Filter (DPF) located in the exhaust line downstream the DOC, for removing diesel particulate matter or soot from the exhaust gas.

To further reduce the emissions content, in particular NOx emissions, normally Diesel engines include an exhaust gas recirculation (EGR) system coupled between the exhaust manifold and the intake manifold. As known, the EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. In a diesel engine, the S exhaust gas replaces some of the excess oxygen in the pre-combustion mixture.

Because NOx forms primarily when a mixture of nitrogen and oxygen is subjected to high temperature, the lower combustion chamber temperatures caused by EGR reduces the amount of NOx the combustion generates.

Normally, an EGR system also comprises an EGR cooler. This is a heat exchanger installed in the EGR circuit. The EGR system recirculates exhaust back to the engine in order to reduce NOx emissions. The cooler simply cools the exhaust gas prior to gas being reintroduced into the engine. By cooling the gas the combustion temperature is reduced and NOx also, as NOx is formed at higher temperatures. Various cooler (heat exchanger) technologies are used by manufacturers, although tube and shell is the most common type used currently. Coolers will vary in size depending on the engine size and the emission standard the engine must comply with.

On one side, cooled EGR is an increasingly important NOx reduction technology for current and future diesel engines. On the other side, higher EGR flow rates and cooling levels, required by future emissions regulations, exacerbate fouling or the deposition of soot and HO exhaust constituents, degrading EGR cooler performance.

Document DE102004042454A1 discloses an exhaust gas heat exchanger (EGR cooler) for exhaust gas recirculation in motor vehicles, connected to an oxidation catalyst arranged upstream of the heat exchanger. This invention aims to improve an exhaust gas heat exchanger, in particular an EGR cooler, avoiding or at least reducing carbon deposits inside it. This is realized reducing the hydrocarbon amount by means of the oxidation catalyst. Even if it improves the situation, such invention does not solve the S problem, since the mechanism of soot deposition is more complex and some other parameters should be taken into account.

Therefore a need exists for a new solution for the EGR cooler, in order to avoid soot deposition inside the cooler or, better, to obtain an effective soot removal from its surfaces.

An object of an embodiment of the invention is a new EGR system layout comprising an EGR cooler and configured to solve the above problem.

These objects are achieved by an EGR system and by an internal combustion engine having the features recited in the independent claims.

The dependent claims delineate preferred and/or especially advantageous aspects.

SUMMARY

An embodiment of the disclosure provides a heat exchanger system comprising a heat exchanger and an oxidation catalyst, located before the heat exchanger, said heat exchanger also comprising a low temperature coolant inlet and a low temperature coolant outlet, the heat exchanger exchanging heat at least between water vapor and a low temperature coolant, wherein a temperature in the low temperature coolant inlet is lower than 40°C, so that the amount of the exchanged heat is sufficient to generate water vapor condensation.

An advantage of this embodiment is that the new heat exchanger system allows the heat s exchanger not to suffer any more for fouling. This is realized by imposing two conditions, which are a low hydrocarbon concentration in the gas stream and a low coolant temperature for the EGR circuit. In particular, with the use of a diesel oxidation catalyst no sludge like deposit will be observed at the heat exchanger outlet and with the combination of dry soot only deposit and water vapor condensation the mechanism of the soot layer removal has been obtained. On the other side, a coolant temperature lower than 40°C is essential to ensure the subtraction of enough heat at the water vapor, so that water condensation occurs.

According to a preferred embodiment, an exhaust gas recirculation system of an internal combustion engine is disclosed, comprising an EGR valve and a heat exchanger system according to the previous embodiment, wherein the heat exchanger is an EGR cooler and the oxidation catalyst is a diesel oxidation catalyst.

An advantage of this embodiment is to improve both the EGR cooler efficiency at now and its stability over time, thanks to a better resistance to fouling. This objective is achieved through the enabling of a powerful passive regenerative mechanisms that help maintaining clean heat exchange surfaces.

According to another embodiment, said diesel oxidation catalyst has a volume ranging between 100 and 200 cm3.

An advantage of this embodiment is that since the exhaust gas flowing in the oxidation catalyst has a lower amount than the one of the exhaust gas flowing through the aftertreatment system, the oxidation catalyst can be smaller than the standard Diesel oxidation catalysts used as aftertreatment system.

According to a further embodiment, said diesel oxidation catalyst has a porosity ranging between 350-450 cells per square inch.

An advantage of this embodiment is that, due to less severe operating conditions and required performances, the oxidation catalysts can have a lower porosity than the one used as after treatment system.

According to still another embodiment, said diesel oxidation catalyst has a precious metal load equal to 3 gIl or lower.

An advantage of this embodiment is that, due to less severe operating conditions and required performances, the requirement of precious metal load for such oxidation catalyst are much less severe.

Another embodiment of the disclosure provides an internal combustion engine at least comprising a cylinder block, a cylinder head, an intake manifold, an exhaust manifold, wherein the engine also comprises an EGR system according to any of the preceding embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The various embodiments will now be described, by way of example, with reference to the accompanying drawings, in which:

S

Figure 1 is a simplified scheme of an internal combustion engine also comprising an EGR system.

Figure 2 is a section of the internal combustion engine Figure 3 schematizes a soot removal mechanism Figure 4 is a graph showing the behavior of the surface condensation flux of water vapor.

DETAILED DESCRIPTION OF THE DRAWINGS

Some embodiments may include, as shown in Figures 1 and 2, an internal combustion engine (ICE) 110 having an engine block 120 defining at least one cylinder 125 having a piston 140 coupled to rotate a crankshaft 145. A cylinder head 130 cooperates with the piston 140 to define a combustion chamber 150.

A fuel and air mixture (not shown) is disposed in the combustion chamber 150 and ignited, resulting in hot expanding exhaust gasses causing reciprocal movement of the piston 140. The fuel is provided by at least one fuel injector (not shown) and the air through at least one intake port 210. Each of the cylinders 125 has at least two valves 215, actuated by a camshaft 135 rotating in time with the crankshaft 145. The valves 215 selectively allow air into the combustion chamber 150 from the port 210 and alternately allow exhaust gases to exit through a port 220. In some examples, a cam phaser 155 may selectively vary the timing between the camshaft 135 and the crankshaft 145.

S The air may be distributed to the air intake port(s) 210 through an intake manifold 200.

An air intake duct 205 may provide air from the ambient environment to the intake manifold 200. In other embodiments, a throttle body 330 may be provided to regulate the flow of air into the manifold 200. In still other embodiments, a forced air system such as a turbocharger 230, having a compressor 240 rotationally coupled to a turbine 250, may be provided. Rotation of the compressor 240 increases the pressure and temperature of the air in the duct 205 and manifold 200. An intercooler 260 disposed in the duct 205 may reduce the temperature of the air. The turbine 250 rotates by receiving exhaust gases from an exhaust manifold 225 that directs exhaust gases from the exhaust ports 220 and through a series of vanes prior to expansion through the turbine 250. The exhaust gases exit the turbine 250 and are directed into an exhaust system 270.

The exhaust system 270 may include an exhaust pipe 275 having one or more exhaust aftertreatment devices 280. The aftertreatment devices may be any device configured to change the composition of the exhaust gases. Some examples of aftertreatment devices 280 include, but are not limited to, catalytic converters (two and three way), oxidation catalysts, lean NOx traps, hydrocarbon adsorbers, selective catalytic reduction (8CR) systems, and particulate filters.

Other embodiments may include an exhaust gas recirculation (EGR) system 300 coupled between the exhaust manifold 225 and the intake manifold 200. The EGR system 300 may include an EGR cooler 310 to reduce the temperature of the exhaust gases in the EGR system 300. An EGR valve 320 regulates a flow of exhaust gases in the EGR system 300. Such EGR valve can be located after the EGR cooler.

S The main goal of the invention is to improve both the EGR cooler efficiency at new and its stability over time, thanks to a better resistance to fouling. This objective is achieved by enabling a powerful passive regenerative mechanisms that help maintaining clean heat exchange surfaces. The mechanism requires availability of conditions with low HG/soot content and water vapor content in the recirculated gases close or below the dew point.

In fact, from one side, it has been demonstrated the strong effect that hydrocarbon (HC) condensation can have on the deposit layer. For example, a comparison between soot deposition on an EGR cooler has been carried out. In the first case a mostly soot only deposit has been created on the EGR cooler in the following conditions: 25°C as coolant temperature and a smoke number of 2.0, while keeping the HG concentration at 40 ppm.

In the second case, soot deposition on the E3R cooler was created under the same conditions, except the HG concentration, which was increased to 250 ppm. Due to hydrocarbon condensation, which is more significant at the cooler outlet due to lower gas temperatures, the deposit changes from a dry porous layer to a shiny sludge or lacquer like deposit.

Keeping under control the hydrocarbon concentration is a necessary condition but it is unfortunately not sufficient. Indeed, necessary conditions for an effective soot removal are both a low hydrocarbon concentration in the EGR gas stream and a low coolant temperature for the EGR circuit.

The necessity that both conditions work together is explained by the soot removal mechanism shown in Fig. 3. In both pictures (Fig. 3a and Fig.3b) a generic waIl 400 of a s heat exchanger (for example an EGR cooler) is shown. Through it, an exhaust gas stream 410 flows. In both cases, attached to the wall, there is a carbon deposition, which in case of Fig. Sa is a soot only deposit layer 420 while in case of Fig. 3b is a deposit layer composed of soot and hydrocarbons 430. In both cases, the coolant temperature, better the wall temperature Twa, is lower than the water dew point temperature Td In the first case (Fig. 3a), water vapor 440 diffuses through the porous soot layer and comes in contact with the cold cooler wall 400 resulting in condensation and consequent formation of water droplet 450 on the wall surface. Since soot is hydrophobic, the water droplets disrupts the soot layer, which is removed by the gas shear force.

In the second case (Fig-3b), for deposit layer made up soot and hydrocarbons, the mechanism is pretty different: the condensed hydrocarbons fill up the voids, reducing the porosity of the layer and increasing its thermal conductivity. Due to the reduced porosity and the deposit surface temperature being close to the wall temperature Twall, condensation occurs on the deposit surface rather than underneath the deposit layer. In such case the disrupting action of the water droplets does not occur and consequently no soot removal occurs as well.

From the comprehension of such soot removal mechanism and the two necessary conditions (low hydrocarbon concentration in the exhaust gas stream and low coolant temperature for the EGR circuit) a new heat exchanger system 305 (see again Fig. 1) has been defined. Such system comprises a heat exchanger 310, which according to a preferred embodiment is an EGR cooler 310, and an oxidation catalyst 285 located before the heat exchanger. Such oxidation catalyst, according the same preferred embodiment is a diesel oxidation catalyst 285, similar to the one which is used as aftertreatment system 280 in the exhaust line 270 of the internal combustion engine 110.

The new heat exchanger system is characterized by the fact that the heat exchanger exchanges heat between a gas stream and a coolant. The gas stream should contain at least water vapor and normally, for applications according to the preferred embodiment, will be an exhaust gas stream.

The coolant should have a low temperature and therefore cannot be the same coolant which is used for the ICE cooling circuit. The coolant should have a separate cooling circuit or at least a special routing of engine coolant (both shown as a black box and referenced as 316 in Fig. 1), a low temperature coolant inlet 312 and a low temperature coolant outlet 314. The heat exchanger or the EGR cooler will allow to exchange heat at least between the water vapor (or, more in general, the exhaust gas stream comprising water vapor) and the low temperature coolant. The thermal dimensioning of the heat exchanger and the thermal characteristics of the fluids exchanging heat should ensure that water vapor condensation will occur.

As mentioned, a preferred embodiment of the present invention allows to define a new exhaust gas recirculation system 300 of an internal combustion engine 110. Such EGR system will comprise an EGR valve 320 and a heat exchanger system 305 as previously defined, having an EGR cooler 310 as heat exchanger. The EGR cooler will be provided with a low temperature coolant inlet 312 and a low temperature coolant outlet 314 and the low temperature coolant will have a separate circuit or a special routing of the engine coolant circuit. The EGR system will also be provided with a small diesel oxidation catalyst 285 upstream the cooler itself. The heat exchanger system will be active, S whenever the EGR cooler is active. Otherwise, the whole heat exchanger system will be simply by-passed by the exhaust gas stream. Therefore, a remarkable advantage of such system is that it does not need any additional control.

The use of a conventional Diesel oxidation catalyst can provide the hydrocarbon reduction efficiency that is needed to enable the soot removal mechanisms under most of the engine operating conditions. According to a preferred embodiment, the exhaust gas flowing in such oxidation catalyst is the amount of the recirculated exhaust gas and therefore a lower amount than the one of the exhaust gas flowing through the aftertreatment system. This means that the oxidation catalyst 285 can be smaller than the standard Diesel oxidation catalysts used as aftertreatment system. In fact, the volume of the diesel oxidation catalyst 285 can range between 100 and 200 cm3, preferably 150 cm3.

For the same reasons, smaller gas flowrate but also less severe operating conditions (lower temperatures than in the standard oxidation catalysts) and required performances, the oxidation catalysts can have a lower porosity. A good range for the porosity can be between 350-450 cells per square inch, preferably 400 cells per square inch. For the same reasons, also the requirement of precious metal load are much less severe: a precious metal load not overcoming 3 gIl is sufficient. As a reference, the standard oxidation catalysts, installed along the exhaust line as aftertreatment system, have a

II

precious metal load which can reach 100 gIl, since the catalyst must operate at much more higher temperature, S As mentioned, the coolant used in the EGR cooler must work at lower temperature than the standard engine coolant. Reason for this is that at a higher temperature the coolant would not subtract enough heat to generate water condensation. In Fig. 4 a graph representing the behavior of the mass condensation flux of water vapor (measured in mg/m2s) as a function of the coolant temperature (°C) is shown. As can be observed, a remarkable flux value of the condensed water vapor can be obtained up to 40°C, while with higher temperature the flux of condensed water vapor becomes negligible.

Therefore a temperature lower than 40°C is an essential feature of the system and the threshold of 40°C has to be the maximum allowed temperature of the coolant, at least at the EGR cooler inlet 312.

Summarizing the new heat exchanger system allows the heat exchanger, and in particular the EGR cooler, not to suffer any more for fouling. This is realized by imposing both conditions, which are a low hydrocarbon concentration in the EGR gas stream and a low coolant temperature for the EGR circuit. In particular, with the use of a diesel oxidation catalyst above light off temperature, no sludge like deposit has been observed at the cooler outlet; with the combination of dry soot only deposit (due to use of the oxidation catalyst) and water vapor condensation the mechanism of the soot layer removal has been successfully satisfied.

While at least one exemplary embodiment has been presented in the foregoing summary and detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing at least one exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents.

REFERENCE NUMBERS

internal combustion engine engine block 125 cylinder cylinder head camshaft piston crankshaft 150 combustion chamber cam phaser intake manifold 205 air intake pipe 210 intake port iS 215 valves 220 port 225 exhaust manifold 230 turbocharger 240 compressor 250 turbine 260 intercooler 270 exhaust system 275 exhaust pipe 280 aftertreatment devices 285 oxidation catalyst, diesel oxidation catalyst 300 exhaust gas recirculation (EGR) system 305 heat exchanger system 310 heat exchanger, EGR cooler 312 low temperature coolant inlet 314 low temperature coolant outlet 316 low temperature coolant circuit 320 EGR valve 330 throttle body 400 wall of a heat exchanger, wall of an EGR cooler 410 exhaust gas stream 420 soot only deposit layer 430 soot and hydrocarbons deposit layer 440 water vapor 450 water droplet Twan wall temperature TdeW water dew point temperature deposit surface temperature