EP1009923A1

EP1009923A1 - Device and method for reduction of harmful emissions from a combustion engine

Info

Publication number: EP1009923A1
Application number: EP98934110A
Authority: EP
Inventors: Göran WIRMARK; Ingemar Gottberg; Jan Roger Linna; Lennart Andersson
Original assignee: Volvo Car Corp
Current assignee: Volvo Car Corp
Priority date: 1997-07-18
Filing date: 1998-07-17
Publication date: 2000-06-21
Also published as: SE9702757L; WO1999004145A1; SE509787C2; JP2001510260A; SE9702757D0

Abstract

The invention relates to an arrangement for reducing harmful emissions from an internal combustion engine (1) by rapid heating-up of an exhaust gas catalyst (9) which is arranged in an exhaust system (8) belonging to said engine (1), comprising means (11, 16) of supplying hydrogen and means (21; 11, 13) of supplying air to the exhaust system (8) upstream of said catalyst (9), the mixture of hydrogen and air giving rise to spontaneous exothermic combustion in association with the catalyst (9), and also a control unit (5) for controlling the functioning of said means (11, 13, 16, 21). The invention is characterized in that said control unit (5) is adapted to control the supply of air and hydrogen to the exhaust system in connection with starting the engine (1) for a period of time (t) that lasts at least until the exhaust gas catalyst (9) has reached a limit temperature at which its functioning is not hampered by CO or HC poisoning. The invention also relates to a procedure for such reduction in harmful emissions. A short heating-up time for the catalyst (9) is achieved by means of the invention.

Description

TITLE Device and method for reduction of harmful emissions from a combustion engine.

TECHNICAL FIELD

The present invention relates to an arrangement for reducing harmful emissions from an internal combustion engine according to the pre-characterizing clause of Patent Claim 1 below. The invention is intended in particular for use in connection with engines adapted for supplying hydrogen gas to the inlet side and/or exhaust gas side of the engine. The invention also relates to a procedure for such reduction in harmful emissions according to the pre-characterizing clause of Patent Claim 14 below.

STATE OF THE ART

In connection with vehicles that are operated with the aid of internal combustion engines, there is a general requirement for low emissions of harmful substances in the exhaust gases from the engine. These substances consist primarily of impurities in the form of nitrogen oxide impurities (NO_::) , hydrocarbon impurities (HC) and carbon monoxide (CO) . As far as modern petrol engines are concerned, the exhaust gases are normally cleaned with the aid an exhaust gas catalyst belonging to the exhaust system, through which the exhaust gases are guided. In a three-way catalyst of known type, the majority of the abovementioned harmful impurities are eliminated by means of known catalytic reactions.

In order to optimize the functioning of the catalyst so that it provides as great a cleaning effect as possible for NO;,, HC and CO, the engine is in most operating cases operated with a stoichiometric air/fuel mixture, that is to say a mixture in which the quantity of fuel is adjusted to the quantity of air in such a manner that all the oxygen is consumed during combustion and no surplus oxygen appears in the exhaust gas flow. In connection with internal combustion engines, this is often represented by 1=1.

Although modern three-way catalysts normally achieve a high degree of cleaning which greatly limits emissions of harmful impurities into the atmosphere, there are requirements today for further reductions in such emissions. These requirements originate from inter alia increasingly strict legislation in various countries, with requirements for extremely low emissions of NO_::, CO and HC impurities.

One disadvantage connected with modern three-way catalysts relates to the fact that they must be heated up to a given ignition temperature (the light-off temperature) at which they can provide an optimum cleaning function. In particular, exhaust gas emissions are relatively high during the initial heating-up stage of an internal combustion engine after starting. At temperatures below the light-off temperature, the necessary catalytic reactions do not, therefore, take place to a sufficiently great extent. The light-off temperature can be defined as the temperature at which the catalyst provides a 50?> conversion rate. Modern catalysts operate with light-off temperatures of around 200°C to 300°C.

In order to reduce the quantity of harmful emissions during the initial heating-up stage, a number of solutions have been proposed. Many of these solutions are based on the fact that the time that passes before the light-off temperature is reached is shortened by increasing the temperature in the catalyst as rapidly as possible. One way of rapidly heating up a three-way catalyst is to use an electrically heatable catalyst that can be adapted so that it is heated up by current supplied from a battery. A major disadvantage of this method is that it requires a very great current supply. This is true especially in the starting cycle of the internal combustion engine when the vehicle of course has an especially high current demand (in order that, for example, the starter motor and electrically heated windows and seats of the vehicle can be activated) . Normally, a particularly powerfully rated power supply system is therefore required in the vehicle if it is to be possible for such an electrically heatable catalyst to be used. A further problem connected with an electrically heatable catalyst is that there is a risk that its life will be limited. Moreover, the high power requirement leads to a relatively great load on the engine, which may result in increased emissions and increased fuel consumption.

Another way of shortening the time that passes before the three-way catalyst reaches its light-off temperature is to use a separate, smaller primary catalyst, which is positioned upstream of the ordinary three-way catalyst. With suitable positioning and design of the primary catalyst, an increased temperature of the exhaust gases that flow through the three-way catalyst positioned downstream is obtained (on account of the exothermic reaction) . Moreover, the primary catalyst contributes to reduced emissions from the catalyst system as a whole. A disadvantage of this method, however, is that the primary catalyst positioned upstream may have a negative influence on the external gas exchange of the engine. Moreover, such a primary catalyst is normally arranged relatively close to the engine, which may lead to problems relating to ageing as a result of the high temperatures that normally prevail in the vicinity of the engine .

A further method of shortening the light-off time of the three-way catalyst is to delay the moment of ignition of the engine, which produces an increase in the exhaust gas temperature and a reduction in emissions. A disadvantage of this method is that it gives rise to relatively high fuel consumption. Moreover, there is a risk of partial isfirings in the engine if combustion does not take place in a sufficiently rapid and robust manner. This may in turn lead to impaired drivability and increased emissions .

A further method of shortening the light-off time of the three-way catalyst is to have a combustible gas flow towards the three-way catalyst and be ignited with the aid of a separate ignition arrangement. In this way, effective and rapid heating-up of the catalyst is achieved. A disadvantage of this method is that it requires a separate ignition arrangement, which in turn creates problems relating to reliability and adds an extra cost in the manufacture of the vehicle. Moreover, this method leads to a very high thermal load on the catalytic coating (thermal shock) , which may lead to a limited life of the catalyst.

Furthermore, the three-way catalyst may also be brought to its light-off temperature rapidly by allowing a gas to be combusted spontaneously (that is to say without any separate ignition arrangement) in close association with the three-way catalyst. A system that applies this principle is disclosed in patent specification WO 96/11330. According to this system, hydrogen gas is produced on board the vehicle with the aid of a separate electrolysis arrangement. The hydrogen gas is supplied to a point upstream of the three-way catalyst together with added air fed from a secondary air pump. By allowing the hydrogen to be combusted spontaneously together with air in close association with the three-way catalyst, rapid heating-up of the three-way catalyst can be achieved.

Although the system according to WO 96/11330 in principle provides satisfactory functioning, it nevertheless suffers from certain disadvantages. Firstly, it may be pointed out that this known system does not take account of a number of different parameters of the three-way catalyst, and the engine system in general must be optimized if it is to be possible to achieve satisfactory functioning on heating up the three-way catalyst, that is to say in order for it to be possible for its light-off temperature to be reached rapidly and under all the existing circumstances. Moreover, this known arrangement involves a certain risk of flame formation in connection with the catalyst. A further disadvantage is that air supply with the aid of a separate secondary air pump is a prerequisite.

A further problem that may arise in connection with exhaust gas cleaning with the aid of a catalyst is that, in the case of a certain type of gas composition, the presence of HC, CO and certain other substances leads to poisoning of the active surfaces of the catalyst. To a great extent, this poisoning controls the light-off temperature which, as mentioned above, is of the order of 200-300°C, which applies in the case of a normal hydrogen content in the exhaust gases (of the order of < 1%), while the light-off temperature is roughly 100-130°C in the case of an increased hydrogen content (at least roughly 4%) .

DESCRIPTION OF THE INVENTION

The aim of the present invention is to provide an improved arrangement for reducing harmful emissions from an internal combustion engine, which brings about in an optimum manner a shortening of the time until a three-way catalyst reaches its light-off temperature. This aim is achieved by means of an arrangement, the characterizing features of which emerge from Patent Claim 1 below. Said aim is also achieved by means of a procedure, the characterizing features of which emerge from Patent Claim 14 below.

The invention is intended for reducing harmful emissions from an internal combustion engine by rapid heating-up of an exhaust gas catalyst which is arranged in an exhaust system belonging to said engine. The invention comprises means of supplying hydrogen and means of supplying air to the exhaust system upstream of said catalyst, the mixture of hydrogen and air giving rise to spontaneous exothermic combustion in association with the catalyst. The invention also comprises a control unit for controlling the functioning of said means. The invention is characterized in that said control unit is adapted to control the supply of air and hydrogen to the exhaust system in connection with starting the engine for a period of time that lasts at least until the exhaust gas catalyst has reached a limit temperature at which its functioning is not hampered by CO or HC poisoning. A short heating-up time for the catalyst is achieved by accurate optimization of the invention.

A further aim of the invention is to provide a stable starting cycle of an engine while the emissions of CO and HC impurities are essentially eliminated. According to a preferred embodiment of the invention, this aim can be achieved by providing a hydrogen supply on the inlet side of the engine during the starting cycle, that is to say before any liquid fuel is supplied to the engine. The term "liquid fuel" means the normal fuel for the engine, which at present in most markets consists of petrol or alcohol/petrol mixtures.

Advantageous embodiments of the invention emerge from the dependent patent claims that follow.

DESCRIPTION OF THE FIGURES

The invention is explained in greater detail below with reference to a preferred exemplary embodiment and the appended drawings, in which

Figure 1 shows a basic diagram of an internal combustion engine arrangement, in which the present invention can be used, and Figure 2 shows an alternative embodiment of the invention.

PREFERRED EMBODIMENTS Figure 1 shows diagrammatically an arrangement according to the present invention. According to a preferred embodiment, the invention is arranged in association with an internal combustion engine 1 in the form of a conventional petrol engine. The engine 1 is fed in the usual manner with inflowing air via an air inlet 2. The engine 1 is also provided with a number of (for example four) cylinders 3 as well as a corresponding number of injection arrangements 4 for fuel. Each injection arrangement 4 is electrically connected to a central control unit 5. The quantity of air that is supplied to the engine 1 is regulated in a known manner with the aid of a gas throttle 6.

The control unit 5 is preferably computer-based and is adapted to control in a known manner the fuel supply to each injector arrangement 4 with fuel from a fuel tank

(not shown) so that a constantly adjusted air/fuel mixture is fed to the engine 1. The engine 1 according to the embodiment is of the multi-point injection type, in which the correct quantity of fuel for the engine 1 can in a known manner be supplied individually to each cylinder 3.

The exhaust gases from the engine 1 are led out from the cylinders 3 via a branch pipe 7 and onward to an exhaust pipe 8 connected to the branch pipe 7. Arranged further downstream along the exhaust pipe 8 is an exhaust gas catalyst 9 which consists of a conventional three-way catalyst for reducing NO_;: and HC impurities as well as CO, which takes place by means of known catalytic reactions .

While the engine 1 is running, the control unit 5 is adapted to control the air/fuel mixture to the engine 1 so that it is constantly adapted to the current operating conditions. For this purpose, the system comprises at least one sensor for detecting the oxygen concentration in the exhaust gases. In the figure, a sensor 10 is shown, which is preferably of the lambda-probe type and is connected to the control unit 5 via an electrical connection. The sensor 10 is preferably positioned in the exhaust pipe 8, upstream of the catalyst 9. Control of the engine 1 takes place in an essentially known manner depending on various parameters which reflect the operating conditions of the engine 1 and the vehicle concerned. For example, the engine control may take place depending on the acceleration applied, the engine speed, the quantity of air to the engine and the oxygen concentration in the exhaust gases.

The engine 1 shown in the figure is of the four-cylinder type. It is to be noted, however, that the invention may be used in engines with different numbers of cylinders and different cylinder configurations. Moreover, the invention may in principle also be used in the case of single-point injection, where a single fuel-injection arrangement is positioned in the inlet pipe of the engine .

According to the invention, the engine 1 can be provided with hydrogen gas from a hydrogen gas container 11. Supply of hydrogen to the inlet side of the engine takes place via a first hydrogen gas line 12 which opens into the air inlet 2 and is controlled with the aid of a first valve 13 which can preferably be controlled electrically and is for this purpose connected to the control unit 5. Furthermore, the exhaust gas side of the engine 1 can also be fed with hydrogen gas from the container 11. This takes place via a second hydrogen gas line 15 which opens into the exhaust pipe 8. The hydrogen supply to the exhaust gas side is controlled with the aid of a second valve 16 which can also preferably be controlled electrically and is therefore connected to the control unit 5.

The hydrogen that is to be supplied to the air inlet 2 and/or the exhaust pipe 8 is preferably produced on the vehicle with the aid of a separate electrolysis arrangement 17 which is connected to the hydrogen gas container 11 via a further line 18. A container 19 for water is also connected to the electrolysis arrangement 17 via a further line 20.

The arrangement for producing hydrogen gas by electrolysis is shown only diagrammatically in the figure. Such production of hydrogen is nevertheless previously known per se, for example from patent specification WO 96/11330, and is therefore not described in detail here. It may be stated, however, that hydrogen gas under pressure is produced with the aid of the electrolysis arrangement 17 and is stored in the container 11 until it is fed via one of the valves 13, 16 to the inlet and/or exhaust system of the engine 1.

While the engine 1 is running, the electrolysis arrangement 17 is preferably activated, which means that hydrogen will be produced and stored under pressure in the container 11. The control unit 5 is also adapted so as, under certain predetermined operating conditions of the engine 1 (preferably when the engine is cold and the catalyst therefore needs to be heated to its light-off temperature as rapidly as possible) , to open the valve 16 and feed hydrogen to a point upstream of the catalyst 9. The invention is also adapted so as to supply air to the exhaust system 8 for mixing with said hydrogen so that a combustible gas mixture is formed. For this purpose, the control unit 5 is adapted to control the functioning of the engine 1 so that a given quantity of surplus oxygen is produced in the exhaust gas mixture that flows through the catalyst 9. One way of achieving this air supply is to operate the engine 1 using essentially only hydrogen, which then preferably takes place for a given period of time in connection with starting the engine 1. According to combustion technology that is known per se, the hydrogen supply allows the combustion to be controlled towards lean operation, a lean fuel/air mixture being combusted so that an oxygen surplus arises on the exhaust gas side, more specifically an exhaust gas mixture with an oxygen content of 4% or more.

Another way of bringing about a supply of air to the exhaust system is to use a separate secondary air pump 21 which is then adapted to feed air to a point along the hydrogen line 15 via an air line 22. The air pump 21 is electrically connected to the control unit 5. According to the detailed description below, the control unit 5 is adapted so as to activate the air pump 21 under certain operating conditions. Furthermore, the air line 22 is adapted to open into the hydrogen gas line 15 at a point which preferably lies immediately downstream of the valve 16. In this way, an initial intermixing of air and hydrogen can take place before the gas mixture reaches the exhaust pipe 8, which results in a homogeneous gas mixture .

The air that is supplied to the exhaust system (either by hydrogen-assisted lean operation of the engine or with the aid of the air pump 21) will be mixed with the hydrogen that has been supplied via the line 15. This in turn gives rise to spontaneous exothermic combustion in association with the catalyst 9. This leads to heating-up which in turn results in the catalyst 9 being heated up rapidly to its light-off temperature.

As mentioned above, the arrangement according to the invention is preferably adapted so that hydrogen gas can be fed to the inlet side of the engine 1. More specifically, such hydrogen supply can take place during the first seconds after starting the engine 1. In this stage, therefore, preferably only hydrogen is supplied, that is to say without any simultaneous supply of liquid fuel (which usually consists of petrol or a suitable alcohol/petrol mixture) . In this way, a number of advantages are obtained. Firstly, it may be pointed out that emissions of CO, CO and HC impurities from the engine 1 can be virtually eliminated during this stage. By means of the hydrogen gas supply, a very stable starting cycle of the engine is also brought about, not least during cold starts. Moreover, lambda regulation is simple because the quantity of oxygen supplied does not need to be regulated with particularly great accuracy.

In the event that the engine 1 is operated using only hydrogen gas during the starting cycle itself, the control unit 5 is preferably adapted in such a manner that liquid fuel is supplied gradually while the supply of hydrogen is gradually throttled. This phasing-in of the liquid fuel is activated at a given moment after starting the engine 1.

The functioning of the invention will now be described in detail. On starting the engine 1, when the catalyst 9 is cold, it is of utmost importance that the catalyst 9 is heated up as rapidly as possible so that in this way it reaches its light-off temperature rapidly. According to the invention, this rapid heating-up can be achieved in several ways. According to a first embodiment, the invention can be used during a "pre-crank" cycle, that is to say according to a cycle that is initiated before the engine 1 has been started. This pre-crank cycle can preferably be initiated with the aid of a separate proximity sensor, for example in the form of a circuit breaker 23 which senses the presence of the driver in the vehicle in which the invention is used and which for this purpose can be arranged in a door (not shown) in the vehicle. This circuit breaker 23 is then electrically connected to the control unit 5. When the driver of the vehicle comes to start the car and opens the door in question, a signal will be issued from the circuit breaker 23 to the control unit 5. This leads to the control unit 5 initiating the feed of hydrogen from the hydrogen container 11. At the same time, the air pump 21 is also activated by the control unit 5 to feed air. By virtue of the fact that the control unit 5 also sets the valve 16 in an open position, hydrogen and air are mixed in the downstream part of the hydrogen line 15. This gas mixture is combusted and heat is generated. The supply of hydrogen gas to the exhaust gas side is preferably controlled in such a manner that it amounts to 0-28% of the total quantity of gas (as a percentage by volume) , preferably 3-18%. It has emerged that such a proportion of hydrogen in the gas mixture in the exhaust pipe 8 leads to substantial heating-up of the upstream end section of the catalyst 9, which makes possible rapid heating-up of the catalyst 9.

Alternatively, other types of proximity sensor can be used in order to detect the presence of the driver of the vehicle and for initiating the pre-crank cycle when presence is detected. For example, a capacitive proximity sensor may be used, which is a type of sensor in which conductive elements are arranged in association with the driver's seat and are adapted so that they form a given capacitance, the value of which can be detected by a separate measuring unit. In this way, given predetermined values of said capacitance may correspond to presence or non-presence respectively in the seat. When presence is detected, the pre-crank cycle according to the above can then be activated.

During the pre-crank cycle, the mixture of hydrogen and air is therefore combusted in association with the catalyst 9 before the engine 1 is started, the result of which is that the catalyst 9 can be heated up rapidly. As no liquid fuel (and therefore no hydrocarbons) have as yet been fed to the engine 1 during this cycle, no carbon monoxide (CO) will be fed through the exhaust system. This means that there is no risk of CO poisoning of the catalyst 9, which otherwise can influence the light-off temperature to a great extent, as has been explained above. CO poisoning can therefore be eliminated if the catalyst is heated according to said pre-crank cycle at least to a limit temperature T at which the active surfaces are not poisoned. For catalysts of the type that is normally used in vehicles, this limit temperature T_G is of the order of 100-130°C.

According to the embodiment, the air and hydrogen mixture is fed to the exhaust pipe 8 for a given period of time t that lasts at least until said limit temperature T has been reached. When the catalyst has been heated up to the limit temperature T_G, the risk of CO poisoning in the hydrogen-rich environment has therefore been eliminated. The engine 1 can then be started in the conventional manner. When the engine 1 has been started, carbon monoxide is fed through the catalyst 9 (on account of incomplete combustion of the hydrocarbons contained in the engine fuel) , but this carbon monoxide does not give rise to any CO poisoning because the catalyst 9 has been heated to the limit temperature T . Subsequently, the catalyst 9 therefore continues to be heated by the hydrogen/air mixture supplied in the hydrogen-rich environment until it reaches its normal light-off temperature T_L which is usually of the order of 200- 300°C. The engine system can then be controlled with a normal exhaust gas environment, that is to say without hydrogen being supplied.

The period of time for which the pre-crank cycle is active (which is normally the period of time that passes before the engine 1 is started) can be defined by predetermined factors, for example the anticipated time required before said limit temperature T_G has been reached. In such a case, the control unit 5 is adapted to activate the pre-crank cycle during this predetermined period of time. Alternatively, this period of time may depend on, for example, the temperature of the catalyst 9 or the external temperature. In such a case, such a temperature value can be detected by a temperature sensor (not shown) , and the control unit 5 can be adapted to interrupt the pre-crank cycle and start the engine 1 when a predetermined limit value has been reached.

In the same way as CO poisoning can occur in the engine described above, HC poisoning can also occur in diesel engines. The invention can therefore be used for supplying air and hydrogen to an exhaust system in connection with starting an engine, which takes place for a period of time that lasts at least until the catalyst has reached a limit temperature at which its functioning is not hampered by either CO poisoning or HC poisoning.

A further way of reaching the light-off temperature of the catalyst rapidly is to use a "post-crank" cycle, that is to say a cycle that is initiated essentially at the same time as the engine is started and continues for a given time after the engine 1 has been started. In this connection, the necessary air supply on the exhaust gas side can be achieved with the aid of the abovementioned air pump 21 or - according to an alternative embodiment - by operating the engine with essentially only hydrogen gas for a given time. In particular, the control unit 5 is then adapted to control the supply of hydrogen gas to the engine so that an oxygen surplus is created in the exhaust gases of the engine. The engine is preferably controlled with a hydrogen gas supply on the inlet side so that an oxygen surplus of roughly 4% is produced in the exhaust gases. In the same way as in the pre-crank cycle, the hydrogen that is supplied to the exhaust system will react catalytically with the oxygen in the exhaust gas even at low temperatures if CO or HC is not present in the exhaust gas. The engine must therefore be operated with only hydrogen and air for a period of time until the catalyst has reached the limit temperature T_G. According to the embodiment, the quantity of hydrogen gas supplied on the exhaust gas side is then controlled so that at least stoichiometry in relation to the quantity of oxygen is achieved, that is to say so that the ratio of hydrogen gas to oxygen is at least 2:1. This leads to a reduction in NO._: impurities at temperatures above the limit value T .

According to an alternative embodiment, the air pump 21 can be operated parallel with the hydrogen gas supply to produce the oxygen surplus . The control unit 5 can then be adapted so that a given quantity of hydrogen is fed to the inlet 2 of the engine, the control unit 5 controlling the air pump 21 so that it delivers any surplus that may be required in order for the desired hydrogen/air mixture to be obtained in the exhaust pipe 8.

After the catalyst has reached the limit temperature T_G, operation of the engine 1 can continue with liquid fuel, that is to say petrol or an alcohol/petrol mixture. According to what was stated above, the control unit 5 is preferably adapted so that the liquid fuel is supplied gradually while the supply of hydrogen is gradually throttled.

According to a further variant of said post-crank cycle, in which the engine is therefore operated by hydrogen gas immediately after starting, a small quantity of liquid fuel can be supplied via the ordinary fuel injection of the system of the engine parallel with the hydrogen supply. In this way, the engine is started on hydrogen- enriched fuel and not pure hydrogen gas, the risk of undesirable self-ignition and flame propagation on the inlet side of the engine being reduced in a known manner.

The quantity of energy that is delivered to the engine in the form of liquid fuel and hydrogen gas respectively can then be varied during the starting cycle and is suitably controlled by the control unit 5.

In order to bring about optimum functioning of the system according to the invention, in particular with regard to minimizing heat losses in the catalyst 9, a number of different measures can be taken, as will be explained in detail below.

According to a preferred embodiment of the invention, it is designed with means of minimizing heat losses, which are in turn adapted to limit any flame formation in the catalyst 9. According to the invention, this can be brought about by a special flame extinguisher (not shown) which, in a known manner, consists of a tube-like element with a number of duct-like or tube-like elements running through, through which the exhaust gases from the engine flow. The flame extinguisher is positioned in the upstream end portion of the catalyst 9. The flame extinguisher can be designed as a separate component positioned next to the catalyst 9 or as an integral part of the catalyst 9. With the aid of the flame extinguisher, propagation of any flames occurring in the catalyst 9 is prevented, which could otherwise lead to thermal shock in the catalyst 9. The flame extinguisher is preferably designed so that the so-called hydraulic diameter of its duct-like or tube-like elements is of the order of 0.5-1.2 mm, which may vary, however, depending on the hydrogen concentration, the material selected etc. Effective flame-limiting can be achieved by means of an accurately adjusted hydraulic diameter.

In order to reduce any pressure drop in the exhaust system, a greater hydraulic diameter than that indicated above may be preferred. This in turn requires a lower hydrogen concentration in the exhaust system.

An alternative way of preventing flame formation in the catalyst 9 (and therefore achieving reduced heat losses) is to design the upstream end portion of the catalyst 9 without any catalytic coating.

Furthermore, the supporting structure of the catalyst 9 is preferably made of a ceramic material, suitably cordierite. This ceramic material has a certain porosity and can in this way absorb the water that is formed during combustion of the hydrogen in the washcoat layer of the catalyst. This water evaporates during operation as a result of heat generation in the catalyst. This represents an advantage over supporting structures made of, for example, metal which do not have the same porosity or therefore the same capacity to absorb water. There is then instead a risk of water poisoning of the catalyst.

In order further to improve the characteristics of the invention in relation to low heat losses, the valve 16 can be controlled so that a pulsed supply of hydrogen is brought about, that is to say so that a periodic activation and deactivation of the valve 16 takes place. Such a control is possible because the valve 16 can be controlled electrically and is connected to the control unit 5. In this way, the control unit 5 can be adapted to switch the valve 16 on and off alternately. For example, the valve can be controlled so that it is open for a tenth of a second and then closed for a corresponding time.

As a result of this pulsed hydrogen supply, the

(upstream) front surface of the catalyst 9 is cooled down periodically, so that any tendencies towards flame formation are reduced, which reduces heat losses in the system. If a flame nevertheless appears in the catalyst 9, its propagation is severely restricted by the pulsation.

In the event that the air that is required for combustion with hydrogen is supplied by a separate secondary air pump, the injection point (along the hydrogen gas line 15) for the air from the air pump is selected so that the hydrogen and the air are mixed well and also are supplied to the exhaust pipe 8 at a gas velocity that exceeds the flame velocity of the hydrogen/air mixture. In this way, the risk of a flame being propagated "backwards" in the line 15 is eliminated.

According to a further embodiment, shown in Fig. 2, an exhaust gas catalyst 9' can be combined with an HC adsorbent 2 . Such an HC adsorbent is known per se and is used for adsorption of HC impurities in the exhaust gases from the engine. After a given time, which corresponds to the HC adsorbent 24 being heated up and reaching a predetermined temperature, the adsorbed HC impurities are given off and in this way flow through the catalyst 9'. This means that the catalyst 9' then cleans the HC impurities. As can be seen from Figure 2 , the HC adsorbent 24 is positioned next to the exhaust pipe 8 Between the HC adsorbent 24 and the catalyst 9', a volume 25 is formed, into which a mixture of hydrogen and air is fed via a line 26. In a similar manner to that indicated above, the air is preferably supplied from a secondary air pump and is mixed in the line 26 with hydrogen, preferably from an electrolysis arrangement. The mixture is supplied to the volume 25 where combustion takes place. In this way, the catalyst 9 can be heated up to its light-off temperature rapidly.

The embodiment according to Figure 2 can also be combined with means for reducing heat losses, for example by flame limiting, by analogy with what was explained above with reference to Figure 1.

The invention is not limited to the exemplary embodiments described above and shown in the drawings but can be varied within the scope of the patent claims below. For example, the abovementioned pre-crank and post-crank cycles can be combined, that is to say they can be initiated in a sequence one after the other during the starting sequence of the engine. It is also true that the invention is effective for preventing both CO and HC poisoning of a catalyst. In this respect, it may be pointed out that HC poisoning may occur mainly in diesel engines .

According to the above, hydrogen is produced on board the vehicle by an electrolysis process. Alternatively, the invention may use an exchangeable hydrogen gas container (for storing hydrogen under pressure) which is mounted in the vehicle and exchanged or refilled after it has been emptied.

Claims

PATENT CLAIMS

1. Arrangement for reducing harmful emissions from an internal combustion engine (1) by rapid heating-up of an exhaust gas catalyst (9) which is arranged in an exhaust system (8) belonging to said engine (1), comprising means (11, 16) of supplying hydrogen and means

(21; 11, 13) of supplying air to the exhaust system (8) upstream of said catalyst (9) , the mixture of hydrogen and air giving rise to spontaneous exothermic combustion in association with the catalyst (9), and also a control unit (5) for controlling the functioning of said means (11, 13, 16, 21), c h a r a c t e r i z e d i n that said control unit (5) is adapted to control the supply of air and hydrogen to the exhaust system in connection with starting the engine (1) for a period of time (t) that lasts at least until the exhaust gas catalyst (9) has reached a limit temperature at which its functioning is not hampered by CO or HC poisoning.

2. Arrangement according to Patent Claim 1, c h a r a c t e r i z e d i n that said means of supplying air to the exhaust system (8) comprise an air pump (21) which is adapted to be activated for a predetermined period of time before the engine (1) is started.

3. Arrangement according to Patent Claim 1 or 2, c h a r a c t e r i z e d i n that said means of supplying hydrogen to the exhaust system (8) comprise a hydrogen container (11), a hydrogen line (12) connected between the hydrogen container (11) and the inlet (2) of the engine (1) , and also a controllable valve (13) arranged in the hydrogen line (12) , the control unit (5) being adapted to supply hydrogen to the engine (1) so that lean operation of the engine (1) can be brought about, an oxygen surplus being produced in the exhaust gases of the engine (1) .

4. Arrangement according to Patent Claim 3, c h a r a c t e r i z e d i n that said control unit (5) is adapted to feed essentially only hydrogen gas to the engine (1) for a given period of time immediately after starting.

5. Arrangement according to Patent Claim 4, c h a r a c t e r i z e d i n that the control unit (5) is adapted to gradually increase the supply of liquid fuel to the engine (1) and gradually reduce the quantity of hydrogen supplied when the catalyst (9) has reached a predetermined temperature.

6. Arrangement according to any one of Patent Claims 3-5, c h a r a c t e r i z e d i n that the control unit (5) is adapted to supply hydrogen and liquid fuel in a parallel manner to the engine (1) during starting and for a given period of time immediately after starting.

7. Arrangement according to Patent Claim 2, c h a r a c t e r i z e d i n that the control unit (5) is adapted to feed a quantity of hydrogen gas to the exhaust system (8) amounting to 0-28% of the total quantity of hydrogen and air, preferably 3-18%.

8. Arrangement according to Patent Claim 1, c h a r a c t e r i z e d i n that it comprises an arrangement for preventing flame formation in the exhaust system (8) or the catalyst (9) .

9. Arrangement according to Patent Claim 8, c h a r a c t e r i z e d i n that said arrangement for preventing flame formation consists of a flame extinguisher designed with duct-like or tube-like elements running through, the hydraulic diameter of which is of the order of 0.5-1.2 mm.

10. Arrangement according to Patent Claim 8, c h a r a c t e r i z e d i n that said arrangement for preventing flame formation consists of at least one section of the catalyst (9) , which is designed without catalytic coating to limit any flame formation.

11. Arrangement according to any one of the preceding patent claims, c h a r a c t e r i z e d i n that said control unit (11) is adapted to periodically activate and deactivate said supply of hydrogen to the exhaust system

12. Arrangement according to any one of the preceding patent claims, c h a r a c t e r i z e d i n that it comprises an HC adsorbent (24) arranged upstream of said catalyst (9) , said supply of air and hydrogen to the exhaust system (8) taking place to a volume (25) defined by the space between the HC adsorbent (24) and the catalyst (9) .

13. Arrangement according to any one of the preceding patent claims, c h a r a c t e r i z e d i n that it comprises an electrolytic arrangement associated with the engine (1), for producing said hydrogen.

14. Procedure for reducing harmful emissions from an internal combustion engine (1) by rapid heating-up of an exhaust gas catalyst (9) which is arranged in an exhaust system (8) belonging to said engine (1) , comprising supply of hydrogen and air to the exhaust system (8) at a point upstream of said catalyst (9), the mixture of hydrogen and air giving rise to spontaneous exothermic combustion in association with the catalyst (9) , and also control of the functioning of said means (11, 13, 16, 21), c h a r a c t e r i z e d i n that air and hydrogen are supplied to the exhaust system (8) in connection with starting the engine (1) for a period of time (t) that lasts at least until the exhaust gas catalyst (9) has reached a limit temperature at which its functioning is not hampered by CO or HC poisoning.

15. Procedure according to Patent Claim 14, c h a r a c t e r i z e d i n that said supply of air to the exhaust system (8) takes place for a predetermined period of time before the engine (1) is started.

16. Procedure according to Patent Claim 14 or 15, c h a r a c t e r i z e d i n that said supply of air to the exhaust system (8) takes place by feeding hydrogen to the engine (1) during operation of the same, an oxygen surplus being produced in the exhaust gases of the engine (1) ΓÇó

17. Procedure according to Patent Claim 14, c h a r a c t e r i z e d i n that essentially only hydrogen gas is fed to the engine (1) for a given period of time immediately after starting.

18. Procedure according to Patent Claim 17, c h a r a c t e r i z e d i n that it comprises a gradual increase in the quantity of liquid fuel to the engine (1) and a gradual reduction in the quantity of hydrogen to the engine (1) when the catalyst (9) has reached a predetermined temperature.

19. Procedure according to any one of Patent Claims 16-18, c h a r a c t e r i z e d i n that hydrogen and liquid fuel are supplied to the engine (1) in a parallel manner during starting and for a given period of time immediately after starting.