FR2826291A1 - PROCESS AND DEVICE FOR THE DEULFURATION OF A CATALYST THAT ACCUMULATES NITROGEN OXIDES - Google Patents

Abstract

The invention relates to a method of desulfurizing the NOx oxide accumulation catalyst disposed in a gas exhaust channel of an internal combustion engine capable of operating with a lean mixture, in which the catalyst receives at recurring intervals a rich atmosphere. exhaust gas with lambda values less than unity and a variable desulfurization temperature which is greater than or equal to a minimum desulfurization temperature (Tmin), as well as a device for carrying out the process. provided that at least 70% of the desulfurization intervals (DEa) which are initiated by active measures to raise the temperature of the catalyst be performed as desulfurization (DEa, L) at low temperature at a first temperature (TL) of the gases of exhaust upstream of the catalyst (18) for accumulation of NOx oxides which is less than or equal to a limit temperature (TL, max) which exceeds by a maximum of 50 K a temperature (T70) at which e, in three minutes, a maximum of 70% of a maximum mass of sulfur accumulable in the catalyst (18) is discharged. The measures in accordance with the invention constitute an optimized technique for carrying out the desulphurization, on the one hand, with care for the catalyst and with low consumption and, on the other hand, with sufficient depth.

Description

confectionery. The invention relates to a process for the desulfurization of a

  nitrogen oxidation accumulation catalyst NOX, disposed in an exhaust gas exhaust duct of an internal combustion engine capable of operating with a lean mixture, and a device for the implementation of

the work of this process.

  Internal combustion engines which, for the purpose of optimizing consumption, operate at least temporarily in a lean regime, that is to say with an oxygen-rich exhaust gas of value greater than 1, produce even in low nitrogen oxides NOX diet,

  although specifically less than stoichiometric.

  In a lean regime, the disadvantage is that during a catalytic oxidation reaction of unburned HC hydrocarbons and carbon monoxide CO with conventional trifunctional catalysts, NOX nitrogen oxides can not be converted to neutral nitrogen. to the environment. To remedy this, it is known to have in the exhaust pipes of internal combustion engines NOX oxide accumulation catalysts which in themselves contain the components of trifunctional catalysts and NOX oxide accumulation catalysts. Under the effect of a discontinuous operation of the internal combustion engine in poor alternating phases (A> 1) and rich (\ <1), the storage of nitrogen oxides in the form of nitrates takes place during the poor phases, while a regeneration of the NOX oxides of the accumulator takes place during the shorter rich phases. Due to the presence of a high reducing agent mass flow in the exhaust gas during the rich phase, the nitrogen oxides accumulated in the nitrate form are desorbed and catalytically react at the oxide storage catalyst. NOX with simultaneous oxidation of

CO and HC.

  The sulfur contained in the current fuels which is stored in a lean state almost completely in the form of sulphate in the NOX accumulator, is a known problem of the NOx accumulation catalysts operating in this way. This results in a slow poisoning of the catalyst by sulfur, which leads to a weakening of the NOX oxide accumulation capacity. In order to restore an initial catalyst accumulation capacity, it is known to conduct at relatively large intervals desulphurations in which the catalyst is exposed to an atmosphere rich in exhaust gas at temperatures above 600 C. temperature, as well as a significant contribution of reducing agent present in the rich exhaust gases, allow the desorption and the reduction of the sulphate mainly in sulfur dioxide SO2, partly also in sulfur compounds with a degree of oxidation negative. The desulphurization rate is then accelerated by higher temperatures and richer ratios of air to fuel, expressed by the relationship <1, and thus a more complete desulphurization is achieved.

  deeper layers of the catalyst coating.

  However, this has the disadvantage that such stringent desulphurization conditions cause an increase in fuel consumption, which reduces the consumption gain achieved by the lean-cycle operating capacity of the internal combustion engine. In addition, the high temperatures lead to a high thermal load of the entire catalyst system, particularly a pre-catalyst often installed upstream, this charge can lead to irreversible deterioration and a reduction in the life of the catalysts . On the other hand, if the NOX oxide accumulation catalyst is desulphurized only at relatively low temperatures and in poor atmospheres of exhaust gas, only the sulfur accumulated on the surface is removed and gets a low desulphurization rate. As a result, the desulphurizations must be performed relatively frequently and during longer desulfurization times. DE 198 55 090 A1 takes account of this tension which exists between high fuel consumption and catalyst deterioration, on the one hand, and incomplete desulfurization, on the other hand, by describing a desulfurization process in which Desulfurations are carried out alternately at desulphurization temperatures of different levels. However, no indication is given regarding the weighting of the high and low temperature desulfurization intervals.

  The problem underlying the present invention is.

  therefore, to achieve a desulfurization process of a NOX oxide storage catalyst of the kind defined in the preamble, which is optimized with respect to a heat load as low as possible of the catalyst and an increase in consumption as low as possible fuel and at the same time ensures a regeneration of sulfur as complete as possible. This problem is solved by a process for the desulphurization of a NOX oxide accumulation catalyst arranged in an exhaust gas channel of an internal combustion engine capable of operating with a lean mixture, the catalyst for the accumulation of oxides. NOX being supplied at regular intervals with a rich atmosphere of exhaust gas in an air ratio: fuel <1 and at a variable desulphurization temperature which is greater than or equal to a minimum desulfurization temperature, characterized in that at least 70 % of the desulphurization intervals which are initiated by active measures to raise the catalyst temperature are carried out as low temperature desulfurization at a first exhaust temperature upstream of the N x oxide accumnlation catalyst, which is less than 10%. a maximum temperature of at least 30% of the lagoon temperature, in a period of mum of a maximum accumulable sulfur mass in the

  NOX oxide accumulation catalyst are discharged.

  The problem is also solved by a device I for the implementation of a desulfurization process of a NOX oxide accumulation catalyst disposed in an exhaust channel of an internal combustion fuel that can operate with a poor mixture. wherein the N0 oxide-storage catalyst has a recurrence of an exhaust gas rich melange with a value of less than unity and a variable desulturation temperature which is greater than or equal to a minimum of demperature. @sulfuraLion, caracL @ ris in that means are provided to allow an exmultion of at least 70 desulphurization intervals that are primed by active meGureG of the catalyGGlower coating, as desulfurization low temperature a temperature of ga in addition to the NOx oxide adsorption analyzer which is less than or equal to a limit temperature which exceeds a maximum of 50 lagoon temperature in three minutes at least 70 hours. a maximum accumulated goufre mass in the catalyGeur

  Accumulation of oxides Ox OxLchargAG.

  In accordance with the invention, it is anticipated that at least the desulphurization ranges that are undertaken by active measures to lift the NOx oxide catalyst bed, be accomplished as desulfurization at room temperature for a first gas release. in addition to the NOx oxidation accumulator, which is less than or equal to a limit temperature which is at most 50 K over a lagoon temperature, at a time of 3 minutes, 70% at more than one mass of sulfur which can be stored at most in the NOX oxide storage catalyst is discharged. According to a particularly advantageous embodiment of the process, the limiting temperature exceeds at most 30 K and preferably at most 20 K this temperature. Therefore, since a major portion of the actively initiated desulphurization range is accomplished in a predetermined low temperature range that extends from a minimum desulphurization temperature to the predetermined limit temperature, the catalyst is The accumulation of NOX oxides is subjected throughout its service life to a mainly gentle treatment, whereby irreversible thermal damage to the catalyst is largely avoided. At the same time, a consumption gain achieved by the lean-operating capacity of the internal combustion engine is

  attenuated only to the extent necessary.

  In order to be able to allow the NOX oxidation catalyst, if necessary, an almost total desulphurization which removes the sulfur accumulated in the deep layers of the accumulator, it is provided according to a preferred form of implementation of the process of at least 0.1%, in particular at least 2% of all the desulfurization intervals as high temperature desulphurization at

  a second desulfurization temperature which is

  above the temperature limit. High temperature desulphurization parameters are then chosen, in particular a desulphurization time and / or an air: carEurizing ratio during the desulphurization so that at least 90%, in particular at least 95% of the maximum sulfur accumulable in the catalyst. accumulation of NOX oxides are eliminated. This desulphurization at high temperature can then be carried out both as active desulfurization and as so-called passive desulfurization, in which - because of an effective point of operation of the internal combustion engine - a sufficient catalyst temperature exists even without appropriate measures are taken for the raising of the temperature. High temperature desulfurization is then covered as widely as possible by the passive desulfurization interval, as long as

  o a predominant use of the vehicle allows it.

  Thus, if the vehicle is used regularly or frequently under full load conditions, for example for highway driving, passive high-temperature desulphurisation often occurs, so that actively initiated high-temperature desulphurizations do not occur. are needed only rarely, never in the extreme case. If, on the other hand, a vehicle is used mainly at partial load or low load in city driving, for example, active desulphurization at high temperature is more frequently required. The method according to the invention therefore allows optimal adaptation to the beach

  typical use of a vehicle.

  Various methods are known which make it possible to know the necessity of an actively initiated desulfurization. In the context of the process according to the invention, the need for desulphurisation is preferably established according to a calculated introduction of sulfur into the NOX oxide accumulation catalyst. The calculation can then rely in particular on a mass of fuel actually injected, an average proportion of sulfur in the fuel and / or a modeled or measured catalyst temperature. It is furthermore possible to establish the need for desulfurization as a function of an effective activity of the NOX accumulation catalyst, which is for example obtained by means of a NOX sensor disposed downstream of the oxide accumulation catalyst. NOx. The two processes mentioned can very advantageously be combined with each other. When the catalyst is charged with sulfur more rapidly than calculated, for example as a result of a sulfur content of the fuel higher than the expected content, the need for desulphurization can be recognized from the decrease in catalyst activity

  NOx accumulation detected by the NOx sensor.

  According to another advantageous embodiment of the process, an active desulphurization can be undertaken without a desulphurization northerence being recognized, when certain operating conditions are favorable for desulfurization. This is particularly so when the temperature of the exhaust gas, that is to say the catalyst temperature has exceeded a predetermined temperature threshold, but the minimum desulfurization temperature is not yet reached. In this case, the temperature of the catalyst must be high only relatively slowly to initiate the desulfurization, so that the fuel consumption to be applied to this heating measurement is relatively low. In the sense of a positive cost-utilization balance, desulfurization in the absence of necessity should only be performed when the mass of sulfur actually accumulated amounts to less than 10% of the mass of sulfur.

maximum accumulable.

  It is considered as a measure to raise the temperature of the catalyst any known interventions of the engine that raise a combustion temperature and therefore the temperature of the exhaust gas or increase a fraction of unburned hydrocarbons in the exhaust gas and raise the temperature of the exhaust gas and / or catalyst by catalytic post-combustion of these hydrocarbons. Measurements are known as a rational imbalance of the air-fuel ratio of certain cylinders of the internal combustion engine, for

  which the cylinders operate with air-ratios

  different fuel (lamed extension); delayed movement of the ignition angle; a post-injection of i; fuel before, during or after an end of combustion of a cylinder and an injection of fuel into the exhaust gas channel. The measurements can also be used in combination with each other. Also known is a method of direct heating, by

electric example, of the catalyst.

  The device according to the invention comprises means allowing the implementation of the method. These means preferably comprise a control unit in which an algorithm is recorded in numerical form for carrying out the method. The control unit can advantageously be integrated into a motor control unit. The advantages of the process are particularly apparent in the case of internal combustion engines with direct ignition with spark ignition. They are particularly advantageously operated in a layered working mode in which particularly poor air-fuel ratios and high proportions of poor phases can be achieved throughout the entire operation and hence at a higher efficiency. Such internal combustion engines are preferably equipped with a load movement register disposed in the suction channel, by the position of which a switch is

  possible between the layer mode and the homogeneous mode.

  The invention is illustrated in detail by the examples

  following description of which is described in

  see the attached drawings. In the drawings: FIG. 1 is a block diagram of an internal combustion engine equipped with an exhaust system; Figure 2 is a variation curve of the desulphurization as a function of temperature; and; FIG. 3 is a simplified representation of the time course of an exhaust gas temperature upstream of a NOX oxide accumulation catalyst according to FIG.

desulfurization intervals.

  Figure 1 shows an internal combustion engine which is associated with an exhaust system generally designated 12, comprising a channel 14 for exhaust gases. The gas exhaust channel 14 houses a catalyst system which comprises a low volume precatalyst 16 disposed near the engine, which is in particular a trifunctional catalyst, and a NOX nitrogen oxide storage catalyst 18 installed in such a manner. classic under the box. A detector system with which the internal combustion engine 10 as well as an exhaust gas cleaning is controlled, is further disposed in the exhaust gas channel 14. The sensor system comprises a lamed probe 20 disposed as close as possible to the engine, for the purpose of equating an air-fuel mixture supplied to the internal combustion engine. Downstream of the NOx oxide accumulation catalyst 18 is a NOX detector 22 which measures the nitrogen oxide content of the exhaust gases and which serves to equilibrate the NOX regeneration intervals as well as indirectly to the NOX regeneration intervals. adjusting the desulfurization of the NOX accumulation catalyst 18. In variants, the function of the NOX detector 22 can also be assumed or supplemented by a downstream lamin probe. A temperature sensor 24, furthermore installed upstream of the NOX oxide accumulation catalyst 18, measures a temperature of the exhaust gases and thus allows returns to a temperature of the NOX accumulation catalyst 18. Because of the exothermic reactions that take place at the accumulation catalyst 18, the catalyst temperature exceeds by about 10 to 100 K the temperature of the exhaust gas measured with the temperature sensor 24. The effective temperature of the catalyst can to be determined in a particularly precise manner from a difference between the exhaust gas temperatures upstream and downstream of the catalyst 18. The temperature existing downstream of the NOx accumulation catalyst 18 can also be measured directly with a sensor temperature or it can be obtained from an internal temperature sensor signal 22 NOX oxides. Each of the signals available in the sensors 20, 22, 24 has its input into a motor control apparatus 26 which

  performs digitization and signal processing.

  A control unit 28 integrated in the motor control apparatus 26 includes an algorithm for controlling the process according to the invention for the desulfurization of the NOX oxide storage catalyst 18. The motor control unit 26 and the control unit 28 act on various operating parameters of the internal combustion engine 10 according to the incoming signals. The desulfurization intervals of the NOX oxide accumulation catalyst 18, in particular, are controlled according to the algorithm recorded in the control unit 28, a

  way that still needs to be explained.

  As already mentioned in the preamble, NOX oxide accumulation catalysts receive for their desulfurization a high thermal load and an exhaust gas atmosphere which is rich, ie it contains a reducer. The rate of desulfurization

  then increases with the high temperatures of the catalyst.

  This relation is represented in FIG. 2, which shows the variation of the percentage of sulfur removal as a function of the temperature of the exhaust gases measured upstream of a NOx oxide accumulation catalyst 18 according to FIG. In order to determine the measuring points, a constant volume exhaust gas of constant compositions and constant volumetric flows was set at a constant temperature by means of a cooling device installed upstream of the NOX oxide accumulation catalyst 18 and it was determined the cumulative percentage of sulfur removal for three minutes in each case by quantitative mass spectroscopy. A duration of one to three minutes corresponds to a normal desulfurization time. It can be seen from the diagram of FIG. 2 that the removal of sulfur at a temperature of the exhaust gases between 500 and 600 ° C. upstream of the catalyst 18 (TVNSK) has an approximately linear temperature dependence. Above TVNSK = 600 C, the curve flattens more and more and reaches a saturation zone above TVNSK = 700 C. At TVNSK = 700 C, sulfur elimination is observed in three minutes. virtually 100%. The catalyst must not be in

  contact with exhaust fumes at temperatures above

  above TVNSK = 700 C, because from 850 C, a reaction of the solid bodies of the accumulation material in the catalyst is initiated with other components of what is called Washcoats. The difference in temperature between the exhaust gases upstream of the catalyst 18 as well as in the catalyst 18 must be taken into consideration because of the exothermic reactions which take place in the catalyst 18. In the example shown, the minimum temperature of The desulfurization above which a sufficient desulphurisation rate can be noted is at about 600 ° C upstream of the NOX oxide accumulation catalyst 18. In accordance with the process of the invention, a large portion (at least 70%) of the actively initiated desulphurization intervals are performed at temperatures which are at most K above a T70 temperature at which three minutes at most 70% of the accumulable maximum sulfur mass in the NOX accumulation catalyst 18 are discharged. The temperature T70 is also included in the diagram of FIG. 2. In the present example, it is approximately 610 C. The temperature T70 taken into consideration in the present invention can be determined by means of a profile of temperature in accordance with Figure 2 for any NOX oxide storage catalyst. Practical adaptations are needed 12; depending on the circumstances, to compensate for dispersions in serum. FIG. 3 shows, by way of example, the simplified curve of an exhaust gas temperature TVNSK measured upstream of the NOX oxide accumulation catalyst 18 over a total of eight DEs. For a better overview, the intermediate areas of operation; in which the temperature T is below the minimum desulfurization temperature Tmin, have been compressed. In addition to the minimum desulphurization temperature Tmin which is in the region of 600 ° C., the maximum desulphurization temperature TmaX, which is around 700 ° C., is indicated as the temperature T70 and a limit temperature T.sub.Max, which exceeds 50 K. the temperature T70. For a NOX oxide storage catalyst 18 having the temperature characteristic shown in FIG. 2, the temperature T70 is in the vicinity

  of 610 C and the temperature T, maX is around 660 C.

  A low temperature range T (shaded area) extends between the minimum desulphurization temperature Tmin and the limiting temperature TmaX. Desulphurizations which are carried out at a temperature of the exhaust gas of the accumulation catalyst 18 in this zone are referred to as low temperature DE desulfurations within the scope of the present invention. The high temperature range TH is connected above T, maX, and desulfurations in this range are referred to as DEH high temperature desulphurization. In accordance with the invention, at least 70% of the depressurization intervals with active primer are conducted as DEa desulfurization at low temperature. Preferably, at least one desulphurization per thousand, in particular one in fifty, is carried out as desulphurization DEH at high temperature. In the present period of time, six of the eight desulphurization intervals are low-temperature, active-initiated DEa desurcations. One of the most important dewatering units is a so-called passive DE where the exhaust gas temperature spe- cifically exceeds the maximum permissible temperature of the temperature rise. Here, the last disulphurization is a so-called passive passive phase sulphurization, which is achieved by driving at the Imin temperature and for which heating values are no longer required to a low degree. Normally, 10% of desulfurization, which is expected to occur, will occur during the full load operation, eg 10% of the pipeline on the auger, for the vehicle the vehicle will run for a long time. h. In the case of heavy-duty operation, the internal combustion engine must be able to operate, because of strong load demand, a sluggish regime with the result that the exhaust gas temperature in general drops below the limit temperature. in the measurement of the vehicle is used GUFFIGGENGENGELEN FULL CHARGE AND D OGUFUCALIAGO PAGGIVEG DEB BAULE THE LUMBURETH its effectives with sufficient suffiGen, we can waL waLallly the desulfurions of the lemp6raLue baitLs acLive. In this case, LouLes 1eG dssulfuraLionG amlogging aclif - as in the time period of Lemps - carried out in the range T, low temperatures. If, in the other extreme case, it is seldom observed that dehydrogenation is occurring at a very low temperature, then occasionally high-temperature deposition should be carried out at once. in accordance with the invention, not more often than 30

  all the disulfurations active amlogging.

  The measurement of the invention for the derivatization of oxide accumulation catalysts NOx constiLuenL a technique oplimise to accoplir the desulfurion, a parL, a way dargnanL the caLalyGeur eL consaLerAL peaLL, auLre parL, in a manner that is sufficient. The process continues in cooperation with the conductor's own vehicle ducts.

  peal glue made without the defense of inglumenLG.

R E P E R T O I R E

  Internal combustion engine 12 Exhaust system 14 Exhaust gas channel 16 Precatalyst 18 NOx oxide storage catalyst Laminated probe 22 NOx sensor 24 Temperature sensor 26 Motorized control unit 28 Control unit DEa Desulphurization with elevation Active temperature control DEp Passive desulphurization without active temperature rise DE Low temperature desulphurization DEH High temperature desulphurization TVNSK Exhaust gas temperature upstream of the NOX oxide catalyst T70 Exhaust gas temperature upstream of the catalytic converter accumulation of oxides NOx, at which at most 70 of the mass of accumulable sulfur are discharged in three minutes T, max Temperature limit TH High desulfurization temperature T Low desulphurization temperature TmaX Maximum desulfurization temperature Tmin Minimum desulfurization temperature

Claims (15)

  1. A process for desulphurizing a NOX oxide storage catalyst disposed in an exhaust gas channel of an internal combustion engine capable of operating with a lean mixture, the NOx oxide accumulation catalyst being fed to recurring intervals with a rich atmosphere of exhaust gas in an air: fuel ratio 1 and at a variable desulfurization temperature which is greater than or equal to a minimum desulphurization temperature (Tmin), characterized in that at least 70% of the Desulfurization intervals (DEa) which are initiated by active measures to raise the temperature of the catalyst are carried out as low temperature desulphurization (DEa,) at a first temperature (T) of the exhaust gas upstream of the catalyst (18). NOX oxide accumulation, which is less than or equal to a limit temperature (Tmax) that exceeds a temperature (T70) of more than 50 K at which a period of three minutes, a maximum of 70% of a maximum accumulable sulfur mass in the NOX oxide accumulation catalyst (18) is discharged. 2. Process according to claim 1, characterized in that at least 0.1%, especially 2%, of all the desulfurization intervals (DEa, DEp) are carried out as high temperature desulphurization (DEH) at a second temperature (TH). desulfurization that is greater than the limit temperature (Tmax).   3. Process according to claim 2, characterized in that the desulfurization parameters, in particular a desulfurization time and / or the air: fuel ratio of the desulphurization (DEH) at high temperature, are chosen so that at least 90 %, in particular at least 95% of the maximum accumulable sulfur in the catalyst (18)   accumulation of NOx oxides are eliminated. D   4. Process according to one of the claims   previous, characterized in that the limiting temperature (TmaX) exceeds at most 30 K, in particular at most K, the temperature (T70) at which, in three minutes, at most 70% of the mass of accumulable sulfur are discharged.   5. Method according to one of the claims   characterized in that the nobility of an active-initiated desulfurization (DEa) is established as a function of a calculated supply of sulfur in the NOX oxide accumulation catalyst (18) and / or as a function of NOX oxide accumulation activity of the catalyst (18). 6. A method according to claim 5, characterized in that the calculation of the supply of sulfur to the catalyst (18) for the accumulation of NOX oxides is carried out as a function of a fuel injected mass, a proportion of sulfur in fuel and / or modeled temperature or measured catalyst.   7. A process according to claim 5 or 6, characterized in that a desulphurization (DEa) is initiated without a need for desulphurization being recognized when the temperature of the exhaust gas exceeds a predetermined temperature threshold which is lower than   minimum temperature (Tmin) of desulfurization.   8. Process according to claim 7, characterized in that a desulphurization (DEa) is initiated in the absence of a need for desulfurization only when the mass of sulfur actually accumulated amounts to at least 10% of the maximum sulfur mass that can be accumulated in the   catalyst (18) for the accumulation of NOX oxides.   9. Process according to one of the claims   prior art, characterized in that the active measures for raising the temperature of the catalyst include an adequate imbalance of the air: fuel ratio of certain cylinders of the internal combustion engine (10); retardation of the ignition angle; a post-injection of fuel before, during or after an end of combustion and / or a fuel injection in the exhaust channel (14).   10. Process according to one of the claims   preceding, characterized in that the temperature of the exhaust gas is measured by means of a temperature sensor (20) arranged upstream of the NOX oxide accumulation catalyst (18) or is modeled by means of appropriate algorithms.   11. Device for the implementation of a desulfurization process of an NO oxide accumulation catalyst arranged in an exhaust channel of an internal combustion engine that can operate with a lean mixture, in which the catalyst of NOX accumulation receives at recurrent intervals a rich exhaust gas atmosphere with a value less than unity and a variable desulfurization temperature which is greater than or equal to a minimum desulphurization temperature (Tmin), characterized in that means are provided to allow at least 70% of the desulphurization intervals (DEa) which are initiated by active catalyst temperature rise measures, such as desulfurization (Dea), to be performed at a low temperature at a low temperature. temperature (T) of the exhaust gases upstream of the NOX oxide accumulation catalyst (18) which is less than or equal to a limiting temperature (T, max) which exceeds a temperature (T70) of not more than 50 K at which, in at least three minutes, at least 70% of a maximum accumulable mass of sulfur in the accumulation catalyst (18) NOX oxides are discharged.   12. Device according to claim 11, characterized in that the means comprise a control unit (24) in which an algorithm for the implementation   The work of the process is recorded in numerical form.   lg Device according to Claim 12, characterized in that the control unit (24) is integrated   in a motor control apparatus (26).   14. Device according to one of claims 11 to   13, characterized in that the internal combustion engine (10) is an internal combustion engine with direct injection and / or spark ignition, which can operate with a poor mixture.   Device according to claim 14, characterized in that the internal combustion engine (10) is operable at least at intervals in the low-pressure mode.