DE10323245A1 - Process for cleaning the exhaust gas of a diesel engine using a diesel oxidation catalytic converter - Google Patents

Abstract

The invention relates to a method and a device for cleaning the exhaust gas of a modern diesel engine with the aid of a diesel oxidation catalyst. In the low load range, such diesel engines have an exhaust gas temperature that can be below the light-off temperature of the oxidation catalytic converter. This leads to an insufficient conversion of pollutants during operating phases of the diesel engine with low loads and to the sooting of the catalytic converter. The method according to the invention solves the problem, inter alia, that during operating phases of the engine with a low load and low exhaust gas temperature, which is less than a minimum temperature, the catalyst temperature is increased to at least this minimum temperature, the minimum temperature of the catalyst ensuring a sufficient conversion of pollutants.

Description

The The present invention relates to a method for exhaust gas purification a diesel engine with the help of a diesel oxidation catalyst.

The primary pollutants of diesel engines are in addition to the very small amounts of hydrocarbons (HC) and carbon monoxide (CO), nitrogen oxides (NOx) and particles (PM). The particles consist of one in organic solvents soluble and insoluble Component together. The soluble Share consists of a variety of different hydrocarbons, which condenses on the particle core, or ad- or absorbed are. The insoluble Ingredient is sulfur trioxide or sulfate, carbon, metal abrasion (for Example iron and nickel) and in small quantities other than oxides Subsequent products of additives in lubricating oil and fuel (for example Zinc, calcium, phosphorus). Sulfur trioxide is created by oxidation of sulfur dioxide on the catalyst depending on temperature, precious metal loading and exhaust gas flow. A special characteristic of diesel engines is their high Oxygen content in the exhaust gas. While the exhaust gas from stoichiometric powered petrol engines can contain only about 0.7 vol .-% oxygen the exhaust gas of diesel engines between 6 and 15 vol .-% oxygen exhibit.

The ratio The different pollutants of the diesel exhaust gas depend on each other the type of diesel engine and its mode of operation. in principle the information given applies to both stationary diesel engines and diesel engines in motor vehicles for light and heavy applications.

The permissible Emissions from diesel engines are due to legal upper limits established. To comply with these limit values, depending on the type of Diesel engine and its mode of operation pursues different concepts.

For diesel engines of lower power in passenger cars, it is often sufficient to pass the exhaust gas over a so-called diesel oxidation catalyst, which burns the emitted hydrocarbons, carbon monoxide and in part also the soluble organic compounds adsorbed on the soot particles. The oxidation function of the diesel oxidation catalytic converters is designed so that although it oxidizes the organic compounds and carbon monoxide, it does not convert the nitrogen oxides and sulfur dioxide to even more highly oxidized species. Together with the rest of the particles, the nitrogen oxides and sulfur dioxide leave the catalyst almost unchanged. A typical representative of these catalysts is in the DE 39 40 758 A1 ( US 5,157,007 ) described.

The Implementation of pollutants by such catalysts is strong depending on the temperature. For carbon monoxide and hydrocarbons the conversion of pollutants increases with increasing exhaust gas temperature to. The temperature at which a pollutant becomes a given Percentage, ordinary 50%, is implemented as the light-off temperature of the catalyst for the Implementation of this pollutant. It is an important one Parameter for the description of catalytic activity of the catalyst.

In dependence on the mode of operation of the vehicle and thus the speed of the engine and the load, the amounts of exhaust gas, the exhaust gas composition and the exhaust gas temperature. The development of diesel engines has by optimizing combustion to reduce exhaust gas temperatures guided. For example, the exhaust gas temperature of modern diesel engines is only 100 to 200 ° C in the low load range. It only increases at full load Exhaust gas temperature above 300 ° C.

The Low exhaust gas temperatures pose a major problem in exhaust gas cleaning because they are partially below the light-off temperature of the catalyst lie. So does the exhaust gas temperature of the diesel engine decrease during the Operation due to the current low load below the light-off temperature of the catalytic converter, it "goes out", that is to say in the The catalytic converter no longer removes pollutants implemented and are thus released to the environment. Only at again increasing load on the engine the exhaust gas temperature, the light-off temperature of the catalyst and the pollutant conversion starts again. Such operating states occur especially with diesel cars in city traffic and contribute significantly the remaining emissions from road traffic.

On Another problem due to the low exhaust gas temperatures is Sooting the catalyst. As a result of the low exhaust gas temperatures unburned hydrocarbons, volatile organic compounds are stored and particles on the catalytic coating and reduce their catalytic activity. About that there is also a risk that the flammable components deposited on the catalyst suddenly releasing large amounts of heat burn if the exhaust gas temperature in the case of random operating modes with a high load exceeds the appropriate ignition temperature for this Components increases. This can result in thermal damage to the catalytic activity of the catalyst.

These problems can be countered in part by making the catalyst as close as possible is arranged on the engine, which is often not possible due to space constraints. Furthermore, the starting temperature of the catalyst can be reduced by increasing the noble metal loading. Precious metal loads of more than 100 grams per liter of the catalyst volume are not uncommon for this and are associated with correspondingly high costs.

The The present invention is intended to address the disadvantages described (poor Pollutant conversion with low load on the diesel engine; Sooting the catalyst; high precious metal loads) by specifying a suitable one Eliminate exhaust gas cleaning procedure.

This Task is achieved by a method for purifying the exhaust gas Diesel engine solved with the help of a diesel oxidation catalyst, the Diesel engine a raw emission of hydrocarbons and carbon monoxide and the exhaust gas has an exhaust gas temperature and the catalytic converter has one Have catalyst temperature. The process is characterized by that during operational phases of the Engine with low load and low exhaust gas temperature, the lower than a minimum temperature, the catalyst temperature to at least this minimum temperature increases becomes.

The The catalyst is heated by the exhaust gas. His temperature on The entrance follows the thermal inertia of the Exhaust gas temperature. Through the conversion of pollutants there is heat on the catalytic converter released, which heats up both the catalytic converter and the exhaust gas.

How already explained, there is a big problem with exhaust gas cleaning of diesel engines in that the Exhaust gas temperature at low load below the light-off temperature of the catalyst for the Oxidation of carbon monoxide and hydrocarbons can fall that in modern diesel oxidation catalysts in the area between 100 and 200 ° C lie.

According to the inventive method can the pollutant conversion on the catalyst over a driving cycle clearly be improved if through measures to be discussed the catalyst temperature during Operating phases with low load, i.e. during operating phases with an exhaust gas temperature below the light-off temperature of the catalyst, at least the minimum temperature is raised, taking this minimum temperature equal to or greater than that Starting temperature for The oxidation of carbon monoxide will choose to be sufficient Ensure sales of carbon monoxide. The minimum temperature is preferred 10 to 30 ° C chosen higher than the light off temperature. With conventional oxidation catalysts the minimum temperature is preferably in the interval between 150 and 180 ° C.

The proposed procedure ensures that the Oxidation catalyst during of the entire operation of the diesel engine (with the exception of the cold start phase) above the light-off temperature for the implementation of the pollutants is operated, which leads to a reduction in pollutant emissions over the Driving cycle leads.

simultaneously the process prevents the catalyst from soaking, this means it is prevented because of too low exhaust gas temperature there are large amounts of hydrocarbons deposit on the catalyst without being reacted. Much more the process is continuous Combustion of hydrocarbons. High temperature peaks when suddenly sudden Combustion of hydrocarbons accumulated on the catalyst can therefore occur do not occur.

Also allowed the process, the precious metal loading of the catalyst clearly to reduce. While according to the status The technology is attempting the oxidation catalyst through high precious metal loads To equip with a low light-off temperature, according to the present Process the temperature of the catalyst to its light-off temperature raised. The height the precious metal loading is therefore no longer required by the Pollutant conversion conditional, but can largely be based on economic Aspects are optimized.

Various strategies are proposed for increasing the temperature of the catalyst:
According to a first embodiment of the invention, the catalyst temperature is increased in that the exhaust gas temperature is raised to the predetermined minimum temperature before entering the oxidation catalyst. The exhaust gas temperature should preferably be raised to at least 10 ° C. and in particular to at least 20 ° C. above the light-off temperature for the pollutants.

The Exhaust gas temperature can be reduced by injecting fuel into the Cylinder of the diesel engine, by setting a late combustion position on Engine or be raised by multi-stage combustion. As a general rule the exhaust gas temperature of a diesel engine can be increased that engine tuning and thus the efficiency of the engine is deteriorated.

Modern diesel engines are equipped with an electronic engine control. In normal operation, the engine control ensures that the diesel engine is optimally tuned, i.e. with op efficiency is operated. This leads to correspondingly low exhaust gas temperatures.

The transition from normal operation of the engine to operation with increased exhaust gas temperature be initiated by the electronic engine control. In the storage room the electronic engine control are usually the so-called engine maps deposited. At each operating point of the engine given by speed and load condition, that is Torque, the exhaust gas temperature can be determined in advance. In order to those operating points are known at which the exhaust gas temperature is below the light-off temperature of the oxidation catalyst. The control program of the engine control can therefore be based on the Operating data of the engine the entry into operating phases with low load (low exhaust gas temperature) and the exhaust gas temperature during the Duration of these operating phases by one of the aforementioned measures increase at least the minimum temperature. When changing to operating phases with sufficiently high exhaust gas temperature, these measures will be taken again reversed.

alternative this can occur during operating phases with low exhaust gas temperatures also directly by measuring the exhaust gas temperature upstream of the catalytic converter be determined. For this purpose, a temperature sensor is placed in the exhaust pipe attached to the catalyst and its output signal supplied to the engine control.

For increasing the Exhaust gas temperature will increase fuel consumption slightly Taken purchase. However, this additional consumption is low because the exhaust gas temperature only marginally elevated must become, by a significantly increased Pollutant conversion above the light-off temperature of the catalyst to achieve.

According to one second embodiment In the invention, the temperature of the oxidation catalyst can low load can be increased by electrical heating of the catalyst. This ensures that the Catalyst also during of these operating phases remains active.

The invention is now based on the 1 and 4 explained in more detail. Show it

1 : A diesel engine with an exhaust system with oxidation catalytic converter suitable for the exhaust gas purification process

2 : Exemplary course of the exhaust gas temperature during the operation of a diesel engine

3 : Real course of the exhaust gas temperature of a vehicle with a 2.0 l diesel engine during a NEDC cycle (NEDC = New European Driving Cycle) and temperature course corrected according to the invention

4 : Calculated CO and HC emissions of the diesel vehicle behind a diesel oxidation catalytic converter for the temperature profile of 3 and for 3 changes in temperature according to the invention

1 shows a diesel engine ( 1 ) with an exhaust pipe ( 2 ). Just behind the diesel engine is a diesel oxidation catalyst ( 3 ) arranged in the exhaust pipe. ( 4 ) denotes a silencer. ( 5 ) denotes the electronic engine control of the diesel engine. Via data lines ( 6 ) Measurement signals are transmitted to the engine control system via the operating state of the engine and, in return, control signals to the engine. ( 7 ) denotes a temperature sensor for the temperature of the exhaust gas before entering the oxidation catalytic converter ( 3 ). The temperature sensor ( 7 ) is over the line ( 8th ) connected to the engine control.

2 shows schematically the temperature curve ( 10 ) of the exhaust gas upstream of the oxidation catalytic converter during a driving cycle with changing loads. Boundary line ( 30 ) indicates the light-off temperature of the catalyst for the oxidation of hydrocarbons. As can be seen from the diagram, the exhaust gas temperature falls below the light-off temperature of the catalytic converter during operating phases of the engine with low loads. During these operating phases, the catalytic converter "goes out". It is no longer able to convert the pollutants. According to the invention, suitable measures on the engine raise the exhaust gas temperature to a temperature (minimum temperature) equal to or above the light-off temperature during the duration Δt of these operating phases. The temperature is preferably raised by a value ΔT of 10 to 30 ° C. above the light-off temperature.

to temperature increase through late Combustion situation becomes the focus of combustion from 7 ° after TDC (top dead center) moved backwards at a later time (for example 14 ° after OT). This happens when the exhaust valves are still closed. The combustion then sits until after opening the exhaust valves continued what an increase of the exhaust gas temperatures.

at Multi-stage combustion becomes fuel injection made in several stages. If only three levels are provided, this is called pre, main and post injection. The exhaust gas temperature can also be increased by this procedure.

Further motor measures to increase Exhaust gas temperature are conceivable, such as the opening of the turbine blades of the turbocharger. The boost pressure drops and the efficiency drops. This in turn leads to an increase in the exhaust gas temperature, since more fuel has to be burned for the same output. EGR (exhaust gas recirculation) can also influence the exhaust gas temperature.

By The proposed method can extend the life of a diesel oxidation catalyst significantly extended become. However, it is not applied to diesel oxidation catalysts limited. Rather, it can be at all for the exhaust gas purification of diesel engines proposed catalysts use that have an oxidation function. Acting for example, SCR catalysts, hydrolysis catalysts, HC DeNOx catalysts or four-way catalysts.

SCR catalysts are used, for example, in European patents EP 0 376 025 B1 ( US 5,116,586 ) and EP 0 385 164 B1 ( US 5,198,403 ) described. According to the EP 0 376 025 B1 For the selective catalytic reduction, catalysts based on acid-resistant zeolites can be used, which can be exchanged with the metals iron, copper, cerium and molybdenum. In contrast, the EP 0 385 164 B1 Selective catalytic reduction catalysts mainly consisting of titanium oxide, which is used in admixture with various other oxides. According to the cited patent, the reduction catalysts contain titanium oxide and at least one oxide of tungsten silicon, boron, aluminum, phosphorus, zirconium, barium, yttrium, lanthanum and cerium and as additional components at least one oxide of vanadium, niobium, molybdenum, iron and copper. These additional components in particular give the catalyst good reduction properties in oxygen-containing exhaust gases.

Oxides with solid acid properties, which contain titanium dioxide, aluminum oxide, silicon dioxide or their mixed phases and compounds with one another as matrix oxide, are usually used as the hydrolysis catalyst, the acid properties by adding oxides of pentavalent and hexavalent elements, such as SO ₃ and WO ₃ , as stabilizers and for reinforcing them Activity. Suitable active components of a hydrolysis catalyst are in the DE 42 03 807 A1 Mixtures of aluminum oxide with titanium oxide, silicon dioxide, zirconium dioxide and / or H-zeolites in the weight ratio between aluminum oxide and the other oxides from 90:10 to 10:90.

An HC-DeNOx catalytic converter is a catalytic converter that is able in the lean exhaust gas of an internal combustion engine to convert nitrogen oxides into nitrogen, water and carbon dioxide using unburned hydrocarbons contained in the exhaust gas. A suitable catalyst for this is for example in the DE 196 14 540 A1 ( US 5,928,981 ) described. As a catalytically active component, it contains at least one platinum group metal, preferably platinum, which is deposited on a high-surface aluminum silicate as the carrier material. In addition, the catalyst also contains various zeolites which, due to their acidic surface properties, are able to crack even long-chain organic molecules which are adsorbed on soot particles. This catalyst is also called a four-way catalyst because, in addition to the conversion of carbon monoxide, hydrocarbons and nitrogen oxides as the fourth component, it can also reduce the amount of soot particles.

example

It were model calculations for the pollutant conversion in the NEDC test on a real drive system consisting of a 2.2 liter diesel engine and a double flow Exhaust gas cleaning system made with two oxidation catalysts.

in the NEDC test (New European Driving Cycle) is the pollutant implementation for specified driving conditions with inner-city and out of town Proportions determined. The total test duration is approximately 1200 seconds. The thereby covered distance are 11.4 km.

The diesel engine has a raw emission in the NEDC test of 1.5 g / km carbon monoxide and 0.32 g / km hydrocarbons. The oxidation catalysts of this drive system are two honeycomb catalysts with a volume of 1.1 l each, a cell density of 62 cm ^-2 (400 cpsi) and a platinum loading of 3.18 g / l (90 g / ft ³ ).

The above-mentioned diesel engine has a total distance of 11.4 km during the NEDC 3 shown course of the exhaust gas temperature depending on the driving time. The first 800 seconds of this test cycle simulate inner-city traffic after a cold start.

The carbon monoxide and hydrocarbon emissions were calculated for four different cases in the model calculations. In the first case, it was assumed that the exhaust gas was passed over the catalytic converter without additional measures. In the other cases it was assumed that between about 180 seconds and 800 seconds by appropriate motor measures or external heating is prevented that the exhaust gas temperature temperature drops below a minimum temperature of 150 ° C, 160 ° C or 170 ° C.

4 shows that by ensuring an exhaust gas temperature of at least 170 ° C during the inner city part of the NEDC test, the carbon monoxide emission can be reduced from 8.3 to 6.5 g per test, that is to say by approximately 22%. In this case, the HC emission is reduced from 1.3 to 1.2 g per test.

Claims (6)

Process for cleaning the exhaust gas of a diesel engine with the aid of a diesel oxidation catalyst, the diesel engine having a raw emission of hydrocarbons and carbon monoxide and the exhaust gas having an exhaust gas temperature and the catalyst having a catalyst temperature, characterized in that during operating phases of the engine with a low load and a low exhaust gas temperature , which is lower than a minimum temperature, the catalyst temperature is increased to at least this minimum temperature. A method according to claim 1, characterized in that the Minimum temperature selected is from the interval between 150 and 180 ° C. A method according to claim 1 or 2, characterized in that that for increase the catalytic converter temperature, the exhaust gas temperature before entering the oxidation catalytic converter is raised to at least the minimum temperature. A method according to claim 3, characterized in that the Exhaust gas temperature by injecting fuel into the cylinders of the diesel engine, by setting a late combustion position on the engine or is raised by multi-stage combustion. A method according to claim 1 or 2, characterized in that that the Temperature of the oxidation catalytic converter due to electrical heating is raised to at least the minimum temperature. Device for cleaning the exhaust gas of a diesel engine with a diesel oxidation catalyst, especially for performing a method according to any one of the preceding claims, wherein the Diesel engine a raw emission of hydrocarbons and carbon monoxide and the exhaust gas has an exhaust gas temperature and the catalytic converter has one Have catalyst temperature, characterized by, a device to increase the catalyst temperature to at least a minimum temperature while Operating phases of the engine with low load and low exhaust gas temperature, which is lower than the minimum temperature.