METHOD FOR CONTROLLING SPARK IGNITION IN INTERNAL COMBUSTION ENGINES

Abstract: Method for controlling the spark ignition in an Otto engine, which comprises an ignition voltage-generating ignition system with a spark plug electrode (13) fixedly arranged in the combustion chamber (2) of the engine and with an earth electrode (14) co-operating movably therewith and joined fixedly to the engine piston (3) in question. Engines with two-part ignition arrangements of the above-mentioned type are previously known. In these the ignition has generally been effected by relatively simple ignition systems and the ignition moment has occurred more or less randomly when the electrodes have come sufficiently close to one another. For the purpose of producing a controlled ignition moment control in such engines the present invention is characterized essentially by the fact that the ignition moment is controlled so as to occur later and with shorter sparking distance when the engine load increases from light load to whole load, wherein at the same time the ignition voltage necessary for spark formation is controlled so as to vary within relatively narrow limits over essentially the whole load range.

Description: - 1 - Method for controlling spark ign tion in internal com¬ bus ion engines The present invention relates to a method for con¬ trolling spark gnition of a fuel/air mixture contained in the combustion chamber of an Otto engine, which engine is equipped with an ignition voltage-generating ignition system comprising at least one spark plug electrode for spark ignition fixedly arranged in each combustion chamber and with an earth electrode co-operating ovably there¬ with and joined fixedly to the engine piston in question. Two-part ignition arrangements of the type mentioned in the introduction are previously known in a large number of embodiments, for example by American Patent Specifications 1,623,432 and 2,253,204. ' In the -known solutions, as a" result of the division of the ignition arrangement into two parts, simple ignition systems are used for generating the ignition voltage to the ignition arrangement in que ' stion. However, the moment of the ignition spark formation occurs in an uncontrol led manner by virtue of the fact that ignition takes place when the electrodes are sufficiently close to one another to cause spark formation under the prevailing pressure and tem¬ perature conditions. Solutions of this type may be adequate for engine appl cations where demands on fuel consumption, exhaust gas emissions and performance are comparatively small and/or where the engine runs with small variations in the operating conditions. However, the demands placed on modern engines for operating passenger cars cannot be satisfied ' by solutions of this type. The object of the present invention is to make possible a method, when using two-part ignition arrangements modern Otto engines for motor vehicle operation, for con¬ trolling the ignition such that a well-controlled combustion rs: achieved under widely differing operating conditions. I t-tris connection the invention involves the ignition mom¬ ent being controlled in relation to the sparking distance such that, an gnition spark necessary for satisfactory combustion of the fuel/air mixture is obta ned indepen¬ dently of whether the engine is running at low or high load. For this purpose the nvention is characterized in that, for each engine speed, ignition voltage is generated for spark formation at a first ignition moment, which at a first engine load value corresponds to a first sparking distance, while at a second engine load value, which is higher than the first, ignition voltage is generated at a second ignition moment which corresponds to a second sparking d stance shorter than the first, wherein the ignition moment is controlled over essentially the whole load range for the engine in ^uch a way that the ignition voltage level for spark formation, which in a manner known per se depends on the engine load and the sparking distanc varies within relatively narrow limits. The invention makes possible an advantageous app¬ lication of two-part ignition arrangements in modern Otto engines which run under considerably varying loads and w th high demands placed on performance, fuel consum- ptioπ and exhaust gas emissions, such as is the case in operation of cars. For the said type of engines, the sol¬ ution according to the invention ensures, at each engine speed, spark formation over a predetermined long sparking distance when the eng ne is running at a low load and over a successively shorter sparking distance as the load in¬ creases. The ignition voltage and with it the spark energy can thus be maintained at a high level over the whole load range, which ensures satisfactory ignition and combustion of the fuel/air mixture within the whole range mentioned. The risk of incomplete combustion with, as a consequence. - 3 - worse 1 ewh ist gas emissions and impaired exhaust gas cat¬ alyst function is thus reduced, and at the same time the engine runs economically in terms of fuel and achieves a smooth run which is desirable from the point of view of comfort. In an advantageous embodiment of the invention the ignition moment is controlled such that the ignition volt¬ age essentially varies by less than 20 percent about a mean value defined by the limits. This means in practice that the ignition voltage level, and the spark energy which is essen¬ tially in proportion thereto in every respect, is considei— ably raised particularly at low engine load compared with the levels which occur in known engines, the ignition ai— range ents of which have a fixed sparking distance. In this way the risks of unsatisfactory ignition of the fuel/air mixture at low engine load can be considerably reduced. Other features characterizing the invention emerge from the attached patent claims and the following descrip¬ tion of an exemplary embodiment of the invention. In the description reference is made to the figures in which Figure 1 shows schematically an internal combustion en¬ gine with an ignition system comprising a two- part ignition arrangement to which the method according to the invention can be applied, Figure 2 shows the relationship of the sparking distance to the crankshaft angle. Figure 3a shows the principal dependence of the ignition voltage on the sparking distance, Figure 3b shows the principal dependence of the ignition voltage on the engine load at a certain fixed sparking distance and Figure 4 shows the position of the ignition moment ex- pressed as sparking distance depending on the engine load and also the ignit on voltage re¬ quirement as this is obtained from its depen- ' ' deπce on both load and sparking distance. Figure 1 shows schematically a combustion chamber 2 in a mul t icyl inder Otto engine 1. In conventional manner the combustion chamber 2 is delimited by a piston 3, a cylinder 4 and a cylinder head 5 with valves 6 included in it for controlling the entry and outlet of the fuel/air mixture relative to the combustion chamber. In the manner known for the 4-stroke principle, the control of the- valves 6 is effected in dependence on the forward and backward move ¬ ment of the piston 3. This is determined by the crankshaft 7 of the engine and by a connecting rod 8 connected to the piston and the crankshaft. The ignition of the fuel/air mixture (hereinafter referred to as the gas mixture) effected during the compres¬ sion stroke is achieved by means of a two-part ignition arrangement 10, which is included in an ignition-control ignition system 11. The ignition arrangement comprises a spark plug 12 attached to the cylinder head 5 with a centre electrode 13 for supplying ignition voltage and with an earth electrode 14 arranged on the piston 3 and following the movement of the latter. The ignition is controlled by the ignition system 11 which, in the present invention, is advantageously of the capacitive type. In this connection the ignition sys¬ tem 11 includes in known manner a low-voltage source 16, - 5 - a charging circuit 17, a discharging circuit 18, an ig¬ nition circuit 19 and an ignition pulse trigger unit 20. The low-voltage source 16 is expediently of the 12V battery type and the charging circuit 17 transforms this low voltage up to about 400V for charging a charging capacitor (not shown). After release of the trigger unit 20, the discharging circuit 18 provides for the discharg¬ ing of the charging capacitor via the primary winding in one of several ignition coils (not shown) to each one of which is assigned an ignition arrangement of an engine cy¬ linder. In the ignition circuit 19 an ignition voltage is then generated in the secondary winding of the ignition coil, which voltage is supplied to the spark plug electrode 13 in question via a wire 21. The ignition pulse trigger unit 20 controls, by means of a microprocessor incorporated therein, the ignition moment, i.e. the moment of discharging of the charging capaci ' tor, for supplying ignition voltage to the respective ignition arrangement. This is effected on the basis of incoming data on a wire bundle 22 in respect of en- gine speed, engine load, engine temperature and, possibly, further signals in respect of, for example, exhaust gas emissions, fuel/air ratio, knocking etc. The use of microprocessor technology for detei— mining the ignition moment on the basis of the said in¬ coming data is well known in internal combustion engine technology and does not constitute part of the present invention. Thus, this is not confined to any particular processor solution for controlling the ignition moment, but can be used together with any solut on of this type available on the market. In the present invention the ignition moment is controlled such that, for each speed, the ignition occurs at a different sparking distance depending on - 6 - whether the engine is running at low or gh load. In t is context the expression low engine load does not include idling load, since, when idling, special demands regai— ding emissions and the li e affect the choice of igni ion moment and thus, on application of the present invention, also the spark ng distance. Figure 2 shows a curve of how the spar ing distance varies depending on the value of the crankshaft angle at the top dead centre (TDC) of the piston. Thesparking distance S depends on the fol¬ lowing equation: where r s the crankshaft radius, I is the connecting rod length and d is the crankshaft, angle. The parameters ment ' ioned ars defined in Figue 1. For an eng ne th r =» 40 mm and a ratio l/r «* 3.4 the following approximate spar ing distances are obtained within the crankshaft angle range generally used in modern Otto eng nes at different engine loads for gni¬ on of the gas mi ture before the p ston has reached the T 30° before TDC ca. 6.8 mm 25° 4.8 20° 3.2 15° 1.8 10° 0.8 5° 0.2 0° 0 The table values show that the sparking distance is 0 mm at the piston TDC, somet ing w ch can of course be simply avoided by des gning the electrodes such that they overlap one another by a certain distance in a crank¬ shaft angle range of, for example ± 5° of the piston TDC. Overlapping of this type also reduces the sparking dis¬ tance at upper crankshaft angle values which can thus be adjusted to values suitable for spark formation without the ratio l/r having to be affected. Figures 3a and b show how the ignition voltage depends on the sparking distance and the engine load. With otherwise unchanged conditions in respect of engine speed, engine load, fuel/air ratio etc. a higher ignition voltage is required for spark formation to take place the greater the distance between the electrodes. As emerges from Figure 3a, the connection is essentially linear. In a corresponding manner Figure 3b shows how the ignition voltage requirement for reliable spark formation increases essentially linearly w th increased engine load where there are otherwise unchanged conditions, including a fixed sparking distance. The increased engine load cor¬ responds to a higher pressure in the combustion chamber during the compression stroke and the increased pressure makes spark formation difficult. In engines with a fixed sparking distance the latter must therefore be selected so short that the ignition voltage which is generated is able reliably to trigger spark formation. However, at the same time the operation of the engine at low load requires a sufficiently long sparking distance for the ignition voltage and with it the spark energy to be so high that the gas i ture which is relatively difficult to ignite at low load can reliably be ignited. The said problem of selecting a suitable fixed sparking distance is made worse if the engine is super¬ charged and thus runs over a considerably greater pressure - 8 - range than a conventional aspirating engine. In the case of a supercharged engine, at the moment of ig ition the compression pressure at full load can indeed amount to about 2,000 kPa whereas in an aspirating engine it usually reaches only about 1,500 kPa. The method obtained according to the present inven¬ tion for controlling the spark formation so as to occur at different sparking distances is therefore of particular ad- t)I vantage for a supercharged Otto engine. Figure 4 shows a con tinuous line which shows how the ignition moment in the case of the present invention occurs at large sparking distances when the engine load is low, whereas it is controlled, with successively increasing engine load, so as to occur at suc- cessively shorter sparking distances. The change in distance with changed engine load can be affected, by selecting a suit able ratio between the crank-shaft radius r and the coπnect- ng rod Length I, in such a way that the spark formation takes place at an ignition voltage which ex ibits co para- 0 tively small variations over the whole load range of the en¬ gine. This is shown in Figure 4 by the sectioned area be¬ tween two horizontal broken lines. Independent of the engine load, the ignition voltage occurs in the range mentioned. 5 By comparatively small ignition voltage variations there are meant variations which are considerably less than s the case in a conventional Otto engine with fixed spark¬ ing distance. In such an engine the gnition voltage at low load can indeed be in the order of magnitude of 5 kV, where- 0 as at high Load it can amount to essentially over 20 kV. The differences relative to a mean value defined by the said limits are by a good margin over 50 percent. In an engine with a two-part ignition arrangement 5 tα which the method according to the invention is applied, - 9 - the variation in ignition voltage can in contrast be main¬ tained within ± 20 percent from such a mean value. In this connection the engine should have an above-mentioned ratio l/r between 3.3 and 3.8. In the case of a supercharged Otto engine with the ratio l/r between 3.4 and 3.5 and equip¬ ped with a capacitive ignition system CIS with two-part igni¬ tion arrangement, the ignition voltage has deviated by a max¬ imum of 5 kV from a mean value of 25 kV, i.e. varied between and 30 kV. It has thus emerged that the sparking distance at 25° crankshaft angle before the TDC is advantageously less th 5.5 mm for the said limited variation in igni ion voltage to be obtained. By measuring and establishing, on reference engines for the engine type in question, the ignition voltage requirement under different conditions such as these are repr sented by the incoming signals on the wire bundle 22 to the i nition pulse trigger unit 20, the ignition moment-controlling processor can then operate, on the individual engine samples, with programmed values of the sparking distance which results in an essentially constant ignition voltage in all operating condi t ions . An important advantage is the distinctly increased ignition voltage and with it the spark energy which, in the method according to the invention, is available for igniting the gas mixture at low engine load. This makes possible re¬ liable ignition and smooth running also under those oper¬ ating conditions of the engine. In contrast to engines with fixed sparking distance, an engine to which the invention is applied can thus exhibit essentially the same ignition volt- age at full and at low engine load, for which reason essent¬ ially the same demands are placed on the ignition system in¬ dependent of the operating condition of the engine. The variable sparking distance in a two-part ignition arrangement, which the present invention uses, is particularly advantageous in motor vehicle engines equip¬ ped with csptac i tive ignition systems, since, in these systems, the spark time is extremely short, which aggra¬ vates the problem of ignition of the gas mixture at fixed short sparking distances and low engine load. In this con¬ nection it is difficult for a sufficient amount of the gas volume Located in the combustion chamber to come into contact with the spark. In nductive ignition systems this is solved by the burning time of the spark being ex- tended to values which it is difficult to obtain with capacitive ignition systems CIS. However, by varying the sparking d stance, as s the case in the present invention the spark can acquire a greater length at low loads where the ignition problem is greatest. By this means a larger amount of the gas mixture comes into contact with the spark, which results in a more reliable ignition of the latter. Contributing to this is also th'e fact that the ' division of the ignition arrangement into two parts res¬ ults in a position ng and design of the spark plug elec- trode and earth electrode which means that the spark formation can occur centrally in the combustion chamber, which further facilitates the contacting of the gas mix¬ ture wi th the spark. With the method according to the invention ther is also the advantage that the spark plug in the two-part ignition arrangement comprises only one centre electrode. In order to insulate this relative to the material an in¬ sulator is required, and the possibilities of doing this effectively and durably at the end extending into the com¬ bustion chamber increase considerably when the spark plug does not comprise an earth electrode. The high ignition voltages - in certain cases up to 40 kV - in a CIS can thu be reliably trnas itted via the said spark plug. The invention can be modified in a number of ways within the scope of the subsequent claims, inter alia by the ignition moment being controlled such that the ignit¬ ion voltage varies wi in said relatively narrow limits over an engine load range which does not include idling Load and/or other special load cases such as, for example, the range around full engine load. In these excluded Load cases ignition voltage levels outside the li its may be per itted without deviating from the scope of the present invent i on .

Claims: - - C LA I MS 1. Method for controlling spark ignition of a fuel/air mixture contained in a combustion chamber (2) of an Otto engine, which engine is equipped with an ig¬ nition voL tage-geπera ing ignition system (11) co pri- sing at least one spark plug electrode (13) for spark ig¬ nition fixedly arranged in each combustion chamber and with an earth electrode (14) co-operating movably there¬ with and joined fixedly to the engine piston (3) in ques¬ tion, characterized in that, for each engine speed, igniti voltage is generated for spark formation at a first igni¬ tion moment, which at a first engine Load value corres¬ ponds to a first sparking distance, while at a second en¬ gine load value, which is higher than the first, ignition voltage is generated at a second ignition moment which corresponds to a second sparking distance shorter than the first, wherein the ignition moment is controlled over essentially the whole load range for the engine in such a way that the ignition voltage Level for spark formation, which in a manner known per se depends on the engine load and the sparking distance, varies within relatively narrow Li its. 2. Method accord ng to Claim 1, characterized in that the ignition moment is controlled such that the ig¬ nition voltage essentially varies by less than 20 percent about a mean value defined by the limits. 3. Method according to Claim 1 or 2, charac¬ terized in that the engine has a ratio between the con¬ necting rod length (I) and the crankshaft radius (r) which is within the values 3.3 and 3.8. 4. Method according to Claim 1, 2 or 3, charac- terized in that the ignition moment is controlled such that the ignition voltage essentially varies within the limits 20 kV and 30 kV. 5. Method according to Claim 4, characterized in that the ignition system ( 11) is of the capaciti e type and the ratio between the connecting rod length (I) and the crankshaft radius (r) is between 3.4 and 3.5. 6. Method according to any one of the preceding claims, characterized in that the sparking distance at 25 crankshaft angle degrees before the top dead centre of the piston (3) is Less than 5.5 mm.

Inventors: GILLBRAND PER SUNE, JOHANSSON SVEN HANS, NYTOMT JAN GUNNAR, GILLBRAND, PER, SUNE, JOHANSSON, SVEN, HANS, NYTOMT, JAN, GUNNAR
Applicants: SAAB SCANIA AB, SAAB-SCANIA AKTIEBOLAG
Application year: 1986
Main classification: F02P15
IPC codes: F02B1, F02P9, F02P15