METHOD OF OPERATING AN INTERNAL-COMBUSTION ENGINE, AND INTERNAL-COMBUSTION ENGINE

Abstract: In an internal-combustion engine (1) of the Otto type, in which exhaust gases are used in order to drive a turbocompressor (14, 15) for supercharging the engine, the engine is designed in such a manner that it can, at high load, be run with a lean mixture during supercharging, the air/fuel ratio being at least 19:1. The lean mixture is brought about by virtue of the fact that the charging pressure increases while the fuel quantity is maintained. The engine is advantageously provided with a divided exhaust-gas discharge in order to improve the operating characteristics.

Description: Method of operating an internal-combustion engine, and internal-combustion engine Technical field The invention relates to a method of operating an internal-combustion engine according to the precharacterizing clause of Patent Claim 1, and also an internal-combustion engine according to the precharacterizing clause of Patent Claim 8. State of the art In a conventional turbo-engine, that is to say a supercharged engine, which has a geometrical compression ratio which is acceptable from the point of view of fuel consumption, it is always necessary at high load to set ignition relatively late so as to avoid knocking. This means that the effective expansion ratio decreases and that less work can be extracted from the combusted gas . A high exhaust-gas temperature at high engine speed also results. Moreover, if the turbo-engine has a relatively small turbine in order to improve torque and response at low engine speeds, a further increased exhaust-gas temperature is obtained as a result of reduced mass flow through the turbine. The usual method of lowering the exhaust-gas temperature is to run the engine with an extremely rich mixture (with an air/fuel ratio of the order of 11:1) at high engine speeds. This results in the temperature increase during combustion being reduced, which results in cooler exhaust gases. At the same time, however, there is a great increase in emissions of uncombusted hydrocarbons and of carbon monoxide. Conversion in the catalyst is insufficient as a result of a lack of oxygen in the exhaust gases . A further disadvantage is that the fuel consumption increases by roughly 30%. An advantage, however, is that the quantity of nitrogen oxides decreases in comparison with the case of a stoichiometric mixture. A transition to stoichiometric operation would improve the situation as far as emissions are concerned but would on the other hand cause a decrease in power of the order of 25% as a result of the need to set ignition earlier in order to limit the increase in the exhaust-gas temperature which otherwise occurs. However, ignition being set earlier results in a greatly increased maximum pressure in the cylinder with the attendant risk of knocking. This in turn leads to a significant reduction of the charging pressure being necessary. Overall, this would result in great problems for turbo-engines . The object of the invention The aim of the invention is to improve the characteristics of a turbo-engine, in particular at high load. Another aim is to bring about a cost- effective solution. Description of the invention The aim of the invention is achieved on the one hand by using an operating method with features according to Patent Claim 1 and on the other hand by means of an embodiment of the engine with the features indicated in Patent Claim 8. By making it possible to operate the engine with a lean mixture during supercharging and at high load, it is possible to reduce to a significant extent the previously common high emissions of uncombusted hydrocarbons and of carbon monoxide during running of a turbo-engine at high load. Moreover, a lower fuel consumption is achieved. The lean mixture is brought about by increasing the charging pressure while maintaining the fuel quantity. Operation and engine are especially advantageous when the technique using a divided exhaust-gas period is used in order to obtain efficient turbine operation and efficient cylinder emptying during the exhaust stroke. Further features and advantages of solutions according to the invention emerge from the description and the other patent claims. The invention will be described in greater detail below with reference to exemplary embodiments shown in the appended drawing. Description of the figures In the drawing, Fig. 1 shows an internal-combustion engine according to the invention provided with a turbocompressor, and Fig. 2 shows an engine similar to that in Fig. 1 but in an embodiment for a divided exhaust-gas period. Description of exemplary embodiments Fig. 1 shows diagrammatically a multicylinder internal- combustion engine 1 of the Otto type made according to the invention. In this case, the cylinders of the engine each have two exhaust valves 2 and 3, via which exhaust gases are conducted out to an exhaust manifold 6 common to the cylinders. The manifold is connected via a first exhaust pipe 11 to an exhaust-gas turbine 14 which drives a compressor 15 for supercharging the engine. The charge air is in this case cooled in an intercooler 16 before it is taken into the engine in a manner which is not shown in greater detail. Arranged downstream of the exhaust-gas turbine 14 is a catalyst 10 and one or more silencers (not shown) . The engine 1 shown in Fig. 2 differs from the engine in Fig. 1 in that the exhaust system is made differently. In this case, a first exhaust valve 2 in each cylinder is connected to a first exhaust manifold 4 and a second exhaust valve 3 in each cylinder is connected to a second exhaust manifold 5. Only the first exhaust manifold 4 leads to the exhaust-gas turbine 14 while the second exhaust manifold 5 opens, via a second exhaust pipe 12, downstream of the exhaust-gas turbine 14, before the catalyst 10. The exhaust valves 2 and 3 have different open times, so that the first, strong part of each exhaust-gas pulse is conducted to the exhaust-gas turbine 14, while the remaining weak part of each exhaust-gas pulse is conducted past the exhaust-gas turbine 14. This makes possible better emptying and charging of the cylinders and in this way provides a more efficient engine. The principle of a divided exhaust-gas period is known from GB 2 185 286. According to the invention, the engine is operated in such a manner that, at high load, a lean mixture is produced instead of the previously usual rich mixture. By means of the compressor 15, such supercharging is generated that an air/fuel ratio of at least 19:1, suitably 20:1, is achieved. By keeping the fuel quantity constant at a level which corresponds to stoichiometric combustion at normal charging pressure, the air/fuel ratio increases at the same rate as the charging pressure. The lean mixture results in the knocking tendency not increasing with the charging pressure. It has been shown that the increase in the maximum pressure in the cylinder relative to the charging pressure amounts to only roughly 1/3 compared with if the air/fuel ratio is kept constant while the charging pressure increases . When the running situation of a vehicle changes, that is to say in transient states of the engine, it is desirable to use stoichiometric operation and normal charging pressure. In those cases when the load remains high after transition from a lower load, it is desirable, after a few seconds, to increase the charging pressure suddenly, so that the air/fuel ratio is changed rapidly from the stoichiometric ratio 14.6 : 1 to roughly 20:1. The fuel quantity is kept constant or somewhat reduced during this pressure stage, which results in the torque not being affected. The purpose of this pressure increase is to avoid operation at an air/fuel ratio in the range of roughly 15 - 18:1, where the content of nitrogen oxides is high. As mentioned, use of a lean mixture at high load (full throttle) requires increased charging pressure in order to maintain the power. This can result in an increase in the negative low-pressure cycle in connection with exhaust-gas discharge from the engine, which can jeopardize the possibility of maintaining the power. There is also a risk that a lean mixture at high load can lead to excessive cycle-to-cycle variations. These problems are reduced by using the principle of the divided exhaust-gas period, according to Fig. 2. By virtue of the fact that the exhaust valves in each cylinder open and close differently, the first, strong part of an exhaust-gas pulse can be conducted via the exhaust-gas turbine, while the remaining, weaker exhaust-gas pulse can be conducted past the exhaust-gas turbine. This makes possible better emptying of the cylinder, so that residual gases are almost entirely eliminated, which to a great extent reduces the knocking tendency in the next phase. This in turn makes it possible to select a more favourable ignition time. Operation with a lean mixture makes possible a greatly improved fuel economy in the load/engine speed range above 1500 kPa indicated effective mean pressure/3500 rpm and reduces the quantity of uncombusted hydrocarbons and carbon monoxide in the exhaust gases by at least one order of magnitude in this load/engine speed range. In combination with the principle of the divided exhaust-gas period, a fuel- saving potential of at least roughly 30% is achieved at high load. As mentioned, the fuel/air ratio in the lean mixture should be at least roughly 19:1 but can advantageously be 20:1. If appropriate, the mixture can be made even leaner, depending on the stability of the combustion system. The oxygen surplus thus brought about in the exhaust gases imposes different requirements with regard to the catalyst and emission control but also brings the advantage that the quantity of soot in the exhaust gases can be reduced.

Claims: PATENT CLAIMS 1. Method of operating an internal-combustion engine of the Otto type, in which exhaust gases are used to drive a turbocompressor (14, 15) for supercharging the engine, characterized in that at high load the engine is run with a lean mixture during supercharging, the air/fuel ratio being at least 19:1. 2. Method according to Claim 1, characterized in that the air/fuel ratio is preferably 20:1. 3. Method according to Claim 1 or 2 , characterized in that a lean mixture is brought about by increasing the charging pressure while maintaining the fuel quantity. 4. Method according to Claim 3, characterized in that the fuel quantity is kept at a level corresponding to stoichiometric conditions at normal charging pressure. 5. Method according to any one of Claims 1-4, characterized in that under transient conditions the engine is operated stoichiometrically, with normal charging pressure. 6. Method according to Claim 5, characterized in that when high load is maintained after transition from low load the charging pressure is increased in a single stage so that the air/fuel ratio thus increases in one stage to at least 19:1 for the purpose of lowering the exhaust-gas temperature. 7. Method according to any one of Claims 1-6, characterized in that the operation of the turbocompressor (14, 15) is effected by divided exhaust-gas discharge (4, 5), in which a first part of an exhaust-gas pulse is conducted to the turbine (14) of the turbocompressor while a subsequent part of the same exhaust-gas pulse is conducted past the turbine of the turbocompressor. 8. Internal-combustion engine of the Otto type, provided with an exhaust-driven turbocompressor (14, 15) for supercharging the engine, characterized in that the turbocompressor (14, 15) is arranged so as, at high engine load, to increase the charging pressure so that an air/fuel ratio of at least 19:1 is obtained. 9. Internal-combustion engine according to Claim 8, characterized in that there are at least two exhaust valves (2, 3) per cylinder, where at least one exhaust valve (2) in each cylinder is connected to a first exhaust manifold (4) and a second exhaust valve (3) is connected to a second exhaust manifold (5) , the first exhaust manifold (4) being connected to the inlet on the exhaust-driven turbine (14) in order to conduct a first part of an exhaust-gas pulse to the latter, and the second exhaust manifold (5) opening downstream of the turbine in order to conduct a subsequent part of said exhaust-gas pulse past the turbine.

Inventors: OLOFSSON ERIC, OLOFSSON, ERIC
Applicants: OLOFSSON ERIC, SAAB AUTOMOBILE, OLOFSSON, ERIC, SAAB AUTOMOBILE AB
Application year: 1999
Main classification: F02D23
IPC codes: F01N7, F02B37, F02D23, F02D41