TECHNICAL FIELD: The present invention relates to an internal combustion engine and a method for operating a multi-stroke combustion engine, which is provided with indi- vidually variable controlled inlet and outlet valves in each cylinder.

BACKGROUND OF THE INVENTION: Most of the current standard production car engines all use an identical princi- ple of operation, known as four-stroke operation. The four strokes are known as compression, expansion, exhaust and intake stroke. The principle of two-stroke operation and six-stroke operation are also known, but restricted in their fre- quency of usage. An internal combustion engine, which can operate under sev- eral stroke-modes, is defined as a multi-stroke engine In the document EP, B 1,0 352 861 the two-, four-and six-stroke operation of an internal combustion engine is described. The six-stroke operation is only de- scribed in combination with engine start and warm-up.

In internal combustion engines the decrease in frequency of the combustions reduces the maximum output, which can be described as the frequency of the combustions times the maximum output per combustion. And the maximum output per combustion is determined by the geometry of the engine.

A combination of a number of stroke operation modes can eliminate the de- scribed restriction. However previously considerations concerning the demand for high performance, have prevented these more efficient engines with a higher number of strokes than four to become more common. The complexity of the required system, which allows for a combination of a number of stroke

operation modes, has been extremely high. This complexity has made mass production not cost-worthy and/or feasible.

The complexity of the system is caused by the requirements a combustion cycle sets in combination with the degree of freedom required for multi-stroke op- eration. An implicit implication is that with multi-stroke operation, changes between two or more stroke-modes have to be performed. A smooth transition between two such stroke-modes puts high demands on the degrees of freedom of the system.

For non-transition between stroke-modes the following can be said. A normal combustion requires all valves in the specific cylinder, where the combustion will take place, to be closed under a certain period of the combustion. To achieve combustion with a normal efficiency, the combustion should be located in the vicinity of top dead centre (TDC), i. e. close to the crank angle degree where the piston in the specific cylinder reaches the highest position. These two criteria alone are not problematic to achieve. For a standard production engine, the camshaft and crankshaft are constructed in such a way that this is guaran- teed for at four-stroke operation. However under multi-stroke operation the in- terval, or as described before the frequency of the combustions changes. The change of interval is a restriction, which is posed by driveability criteria. A non-equidistantly fired engine shows especially at lower speeds and/or high loads a very unstable or undrivable character. The combination of restrictions for a combustion of equidistantly fired engine, TDC and closed valves, which are achieved for four-stroke operation, also have to be met for these different intervals.

An appropriate transition is restricted in its possibilities, again considering the requirements of TDC and closed valves. An additional restriction in this case is the necessity of having ignitable mixture in the cylinder at firing conditions.

This implies that a number of cylinders are excluded from participation in this

transition, since they either contain burned gas, or have to be prepared for fired operation the next cycle. This latter restriction prohibits fired operation, be- cause this will have the implication that the next cycle has burned gas in the cylinder. Pressurized filling methods such as turbo charging, compressor charging and other methods, allow for exhaust and intake in only two strokes.

SUMMARY OF THE INVENTION: A main object of the invention is to overcome the above-mentioned disadvan- tages and to provide a method for transition between different stroke-modes throughout the entire operating range for an internal combustion engine.

A further object of the invention is to increase the efficiency of the internal combustion engine and thereby reduce the fuel consumption of the engine.

A further object of the invention is to reduce the emissions from an internal combustion engine.

The above-mentioned objects are accomplished by a method comprising the steps of : controlling the inlet and outlet valves, so that the opening and closing of the valves are adapted to a second stroke-mode, which is different from a first stroke-mode in which the engine currently running, controlling the injec- tion of fuel into the cylinders, so that fuel is injected prior to an expansion stroke, and transition from the first stroke-mode to the second stroke-mode independently of the operation condition of the engine, throughout the entire operation range of the engine.

Under constant conditions, i. e. constant power demand, an increase of per- formed work per combustion can be achieved by reducing the frequency of the combustions. The increased amount of work performed per combustion in- creases the efficiency and thereby reduces the fuel consumption.

Since an internal combustion engine operates under different loads and speeds it is essential that the transition between different stroke-modes can be per- formed independently of the operation condition of the engine, throughout the entire operation range of the engine.

The previously described higher amount of performed work at a lower fre- quency, improves the combustion conditions in such a way that the emissions are reduced under active catalyst conditions.

A further object of the invention is to achieve a smooth and fast transition be- tween the different stroke-modes.

This object is accomplished by an internal combustion engine, which is pro- vided with individually variable controlled inlet and outlet valves in each cyl- inder and a control device for controlling the ignition. The control device is adapted to change the ignition order of the cylinders when the operation of the engine is converted from a first stroke-mode to a second stroke-mode.

The changing of the ignition order leads to a smooth and fast transition between the different stroke-modes. This implies that an extra degree of freedom in the system has to be present to achieve a transition with consideration for drive- ability.

With the introduction of electrically controlled valve mechanisms, such as hy- draulically, pneumatic, electromagnetic and piezo-electrical, a possibility of adaptation for mass production of an engine, which can be operated in a num- ber of stroke-modes, has arisen. The electronic control unit can meet the closed valve restriction independently of any engine state.

BRIEF DESCRIPTION OF THE DRAWINGS:

The invention will now be described in greater detail by way of example only and with reference to a particular embodiment illustrated in the drawings, in which: Fig. 1 shows a five-cylinder internal combustion engine, Fig. 2 shows a table on feasible combinations regarding the amount of cylinders and ignition order for internal combustion engines with different amount and configuration of cylinders, Fig. 3 A shows a graphical display of the piston movement of a five- cylinder engine, which operates in four-stroke mode, Fig. 3B shows a graphical display of the piston movement of a five- cylinder engine, which operates in six-stroke mode, and Fig. 4 illustrates the transition from four-stroke to six-stroke operation.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT: Fig. 1 shows an internal combustion engine 1, which is provided with five cyl- inders 2 arranged in-line. Each cylinder 2 has a number. The first cylinder 2, disclosed as the uppermost cylinder 2 in fig. 1 is number one, the next cylinder 2 is number two etc. All cylinders 2 are connected to a crankshaft 3. Preferably each cylinder 2 is provided with two inlet valves 4 and two outlet valves 5. Ac- cording to the invention the valves 4,5 are individually variable controlled by a control-unit 6. The control-unit 6 also controls the ignition timing and the in- jection of fuel into the cylinders 2. As will be described later in the text the control-unit 6 also controls the firing or ignition order of the cylinders 2. A typically firing order for a five-cylinder engine in four-stroke operation is

3 with the respect to the number of the cylinders 2. The internal com- bustion engine 1 is provided with an exhaust system 7, which comprises a catalyst 8. It is also possible to arrange an integrated starter generator (ISG) 9 at the engine 1, which can transform power to the engine 1, as will be described later in the text. The method and internal combustion engine is not restricted to a five-cylinder engine.

Fig. 2 shows a table explaining how the firing order will change for combustion engines provided with different amount of cylinders when transit from one fir- ing order to another. The first column shows the amount of cylinders, the sec- ond column the firing interval in crank angle degrees for four-stroke operation.

The last four columns show the firing order, when it is feasible geometrically, by giving the order of ignition, when an engine is taken to be designed with the firing order which is assumed in the column for four-stroke operation. Different firing orders are fully possible and require merely a reassigning of the cylinder numbers, but the principle of as shown with this table on how to alter between the modes is still valid.

The first six lines in the table of fig. 2 relates to in-line engines comprising up to six cylinders. The following lines relates to V-engines provided with six, eight, ten or twelve cylinders and to a boxer engine provided with ten cylinders.

Some cells in the table contain a double asterisk (**). This means that equidis- tant ignition can be achieved by means of cylinder deactivation. In this kind of mode some of the cylinders are deactivated and do not generate any positive work of the engine. A special case exists for the six-cylinder in-line engine, which allows for both cylinder deactivation and eight-stroke operation.

Cells which contain a triple asterisk (***) are cases where eight-stroke opera- tion can be achieved by deactivation of one of the cylinder banks. For the spe- cific case in fig. 2, the first bank comprising the low cylinders numbers are ac-

tive. It is however possible to deactivate the first bank and to activate only the second bank.

The boxer engine with ten cylinders (B 10) is only described by the ignition in one side of the engine. Cylinder number 6 is ignited simultaneously with cylin- der number one, cylinder number 7 is ignited simultaneously with cylinder number two etc.

W-engines can be constructed in such a way that they allow for different stroke-modes as well. This is however not described in detail.

A specific example will be given in relation to figures 3A, 3B and 4, which de- scribe the implementation of six-and four-stroke operation and the transition between the two modes for a five-cylinder engine with firing order 1,2,4,5,3 in four-stroke operation and with respect to the number of the cylinders. Such an engine 1 has been described in connection to fig. 1.

By definition a revolution of the crank shaft exist of two strokes, which are 180° crank angle (CA) in length. For a five-cylinder engine and equidistant ig- nition or firing order and four-stroke operation, an interval of 144° CA is mathematically correct (720°CA/5=144°CA). Under the same conditions but for six-stroke operation, the interval becomes (1080°CA/5) =216°CA.

Four-stroke operation disclosed in fig. 3A is not explained herein since this op- eration-mode is known for the skilled person.

Six-stroke operation, which is illustrated in fig. 3B requires 216° CA between the firing of the individual cylinders 2. Starting with cylinder number one at 0° CA, the next ignition must take place at 216° CA. This can be achieved by ig- niting cylinder number three, which is illustrated with a dot on the sinusoidal line in fig. 2B. It can be calculated that the ignition would take place at (4* 144°

CA) =576° CA, but igniting 360° earlier, which also is a top dead centre event, the 216° CA criteria is achieved. The next ignition must take place after 432°, which can be achieved by igniting cylinder number five. It can be calculated that (3 * 144° CA) =432° CA is the angle when ignition takes place. The rest of the steps consist of the same algorithm: every second firing event, a non- synchronous firing event, i. e. the required criteria are achieved for a cylinder located 360° forward or backward as can be seen in figure 3B, can be moved these 360°, and every other second firing event, a synchronous firing event, i. e. the required criteria are achieved for a cylinder at the exact same crank angle as the firing event, can be used.

The relocation of non-synchronous firing events is achieved by the engine con- trol-unit 6, which relocates the ignition, injection and the valve events. The concept of synchronous and non-synchronous events has to be seen as a six- stroke from a four-stroke point of view. For a six-stroke point of view, all events in six-stroke operation are synchronous, and four-stroke events are both non-synchronous and synchronous.

In case of transition from one mode to another mode, a discontinuity occurs in the ignition interval. For six-and four-stroke operation transitions, a transition from 144° to 216° CA or vice versa occurs. A requirement of fresh mixture of air and fuel in the cylinders restricts the choice of possible cylinders 2 in such a way that an intermediate mode of only one combustion has to be applied. The jump for both transitions between four-and six-stroke mode has this intermedi- ate interval of 288°CA to the previous combustion, and the required interval depending on the direction of the jump to the next combustion.

The graph shown in fig. 4 describes the transition from four-stroke to six-stroke operation of the five-cylinder combustion engine 1, with the upper bar giving the relative crank angle degrees. The lightning symbol points to the ignition event, while the black vertical bars point to the TDC events. The numbers on

the left designate the cylinders 2. The upper half of the graph represents the four-stroke process, while the lower half of the graph represents the six-stroke process. The arrows indicate how the original four-stroke ignition order has to be changed to achieve the six-stroke ignition order.

As can be seen the ignition order for the four-stroke operation mode is 1,2,4,5,3 with an ignition interval of 144° CA. To achieve six-stroke operation, the verti- cal arrows only point out the location where the ignition should occur in the cases where the ignition interval equals the required 216° CA. In certain cases there are horizontal arrows. These indicate that the obtained TDC is not the re- quested TDC, since the distance to a previous ignition is not equal to 216°CA.

In practice this means that a subsequent TDC has to be used. This step requires that besides the correct conditions for the mixture preparation, the valves have to be closed. This last condition can be fulfilled using a completely independent valve actuation system. The ignition order for the six-stroke operation mode for the five-cylinder engine 1 after transition from the four-stroke mode is 1,3,5,4,2 with an ignition interval of 216° CA.

The transition between stroke modes disclosed in fig. 2, other than four-stroke and six-stroke modes, is achieved in a similar manner as the transition between the four-stroke and six-stroke modes, which is described above. The transition between the stroke-modes can takes place independently of the operation con- dition of the engine 1, throughout the entire operation range of the engine 1.

Hence, a transition between different stroke-modes can be made irrespective of the load, temperature and speed of the engine 1. It can be desirable to manually control the engine 1 to run in only one single stroke-mode in some operation conditions of the engine 1. To achieve this, a switch 10 (fig. 1) is connected to the control unit 6. When the switch 10 is pressed, the engine 1 is set to run in only one single stroke-mode.

The transition between the different stroke-modes described above is smooth and fast, since the firing order of the engine 1 is changed. However, a number of strategies are possible to make the transition between the different stroke- modes even more smoother. Active methods of intervention could be amongst others temporary integrated starter generator (ISG) 9 utilisation and output ad- aptations, which smoothen the transition. The ISG 9 works as a combined starter and generator for the internal combustion engine 1. If there is a power reduction from the engine 1 under the transition, the ISG 9 can work as an electrical machine and thereby transform power to the engine 1. In fig. 1 the ISG 9 is connected directly to the crank shaft 3 of the engine.

The six-cycle mode of operation has extra compression and expansion strokes in relation to a four-stroke mode. The extra strokes can be used for multiple purposes, such as early induction of the mixture, which mode increases the amount of time the mixture is contained within the cylinder 2. This means that the mixture will be subject to a longer and more intense mixture preparation, which results in improved combustion conditions. Also, the heat transfer from the cylinder walls to the mixture increases, thereby improving combustion con- ditions.

The catalyst 8 only reduces emissions in the exhaust gases from the engine 1 when the temperature of the catalyst 8 has reached a predetermined tempera- ture, the so-called"light-off temperature". Therefore, it is of interest to reach this predetermined temperature as fast as possible under warm-up conditions of the engine 1. A method according to the invention is to control the operation of the engine during cold starting in a manner so as to obtain a relatively high concentration of hydrogen in the exhaust gas. The air/fuel mixture to the engine 1 is controlled, so that the engine 1 is given an excess of fuel which, according to known principles, generates a certain amount of hydrogen and carbon mon- oxide in the exhaust gas. If additional air is added to the exhaust gas, so that a gas mixture comprising exhaust gas and said added secondary air, an increased

oxidation of combustible components in the exhaust gas is provided. The oxi- dation of the combustible components in the exhaust gas leads to an increase of the temperature in the exhaust system 7 and thereby in the catalyst 8. Hence, a rapid catalyst light-off temperature is achieved.

The secondary air is added into an outlet channel 10 of the engine 1 during the extra strokes under six or higher stroke modes. Under the extra strokes the outlet valves 5 are opened under a short period, so that air is added to the ex- haust gas in the outlet channel 10. As a result an oxidation of the combustible components in the exhaust gas is provided, which leads to an increase of the temperature in the exhaust system 7. When the light-off temperature of the catalyst has been reached the air/fuel mixture to the engine 1 is set to normal values and no additional air is added to the exhaust gas under the extra strokes.

At this stage however, the working temperature of the cooling liquid of the en- gine has not yet been reached.

When the light-off temperature of the catalyst has been reached the extra strokes under six or higher stroke modes can under a warming up period of the cooling liquid of the engine be moved to take place after the expansion stroke.

Under the extra stokes the exhaust gas is captured in the cylinders, so that the high temperature of the exhaust gas warm up the cylinder walls and thereby the cooling liquid. When the cooling liquid has reached the working temperature the extra strokes are moved to take place before the expansion stroke, to achieve improving combustion conditions as mentioned above.

Another possibility to achieve a rapid catalyst light-off is to open the outlet valves 5 early under the expansion stroke. Hence, a part of the expansion will take place in the exhaust system 7, which leads to an essentially increase of the temperature in the catalyst. When the catalyst has reached the light-off tem- perature, the outlet valves 5 is set to work under normal conditions.

The above described method can be implemented straightforward to achieve any combination of internal combustion engine, firing order and stroke modes.