EP0156664B1 - Method of starting and running at a low load a diesel engine, and diesel engine applying this method - Google Patents

Description

The present invention relates to a method for starting and running at low load of diesel engines in which the injection of fuel and the opening of the valves is controlled by an electronic control system.

Currently, many engines are already equipped with electronically controlled injection systems and it has also been proposed, for example in French patent No. 2339748, to control the intake and exhaust valves, no longer from the cams of a camshaft, but by means of electro-pneumatic or electro-hydraulic actuators receiving orders prepared by a computer into which the engine operating parameters are introduced.

An electro-hydraulic valve control system for an electronically controlled diesel engine has been described in the European patent application published under the number EP-A-139 566, in the name of the applicant.

It is well known now that these electronic control systems allow automatic or controlled adjustment of optimal combinations of the working parameters of an engine, whereas, up to now, the operating conditions have been determined, without possible adjustment, by the unchanging profile of the engine cams. It is thus possible, among other advantages, to reduce the fuel consumption of the engine and the toxicity of the exhaust gases, thanks to the choice of the instant and the volume of the fuel injection and thanks to the choice of the instant. and the duration of valve lift.

However, motors with electronic control pose the same problem as traditional motors with cam control, for starting and operating at low loads, in particular in the case of supercharged motors known as “with reduced volumetric ratio and, more generally, in all cases where the air compressed in the cylinder by the compression stroke does not reach the ignition temperature.

It suffices to recall that these conditions occur, at start-up and at low load: when the ambient air is very cold; when the turbo-compressor does not deliver or has only a low flow rate, so that the engine supply air is not heated by compression in the turbo-compressor; and when the engine, by construction and with the aim of increasing its power under normal load and at full load, has only a reduced volumetric ratio (for example 9 or 10, whereas a similar engine would have a volumetric ratio 12 or 13).

To solve this problem, at start-up and at low load, due to the fact that the air in the cylinders does not naturally reach the ignition temperature at the end of compression, we must have recourse to devices making it possible to raise this temperature.

Thus it has been proposed to make the volumetric ratio variable, so as to be able to increase this ratio at start-up, but the practical difficulties of implementation have caused such a system to be abandoned.

Another device consists in creating a restriction of the exhaust section, for example at the outlet of the exhaust turbine. However, this system is ineffective and only provides insufficient heating, since the volume of air to be heated, comprised between the intake manifold and the shut-off valve on the exhaust circuit, is very large.

In practice, the only device used is to heat the supply air (for example up to 70 ° C), before it is introduced into the cylinders. This reheating can be done for example by means of auxiliary burners in the intake manifold, or alternatively heat exchangers, supplied by an external heat source, on the air circuit of the engine.

We are therefore forced to provide additional auxiliary equipment, external to the engine itself (as well as the means of starting and stopping this equipment), which increase the price of the engines and which, in fact, are not used. only during a small proportion of the actual engine running time.

The object of the present invention is to remedy these drawbacks and to allow starting and running at low load of electronically controlled diesel engines, in particular engines with reduced volumetric ratio supercharged by turbo-compressor, without having to resort to means. auxiliary air heating.

The method according to the invention for starting and running at low load of a diesel engine with electronic control of the injector control and of the valve control is characterized in that it consists in adjusting said controls to keep closed the intake and exhaust valves and for cutting off fuel injection, on at least some of the engine cylinders, during several successive compression / expansion cycles, whereby the air trapped in these cylinders by the previous stroke d admission of the piston is heated by several successive compressions until reaching the ignition temperature on the first fuel injection.

The succession of several compressions / detents, whose yields are different, ensures the heating of the air, since there is no flow and the volume of air is limited to that of the cylinder. The air in the cylinder can thus reach the ignition temperature and the fuel injection is then controlled, which causes the combustion / detent stroke.

This is how, in a four-stroke engine. if an additional compression / expansion cycle is planned before the first ignition. the engine will operate on a six-stroke cycle. If two successive compression / expansion cycles are planned before the first ignition. the engine will operate on an eight-stroke cycle, and so on.

Of course, at the time of launch, the compression energy is supplied by the starter, but this energy is partially recovered during expansion.

After the launch, and during the low load operation, the engine can be run for a shorter number of times (six instead of eight for example) and one can limit (in duration or in lifting) the opening of the exhaust valve, to recompress the gases contained in the cylinder several times in succession in order to raise the temperature to a sufficient level.

The invention also relates to a diesel engine in which is applied, at start-up and under low load, the method according to the invention. Such a motor comprises: at least one sensor for the angular position and the speed of the motor shaft; actuators for the intake and exhaust valves and for the injectors; and at least one processing unit receiving signals from the sensor and generating the control signals for the actuators, said motor being characterized in that it comprises at least one cycle modifier circuit, associated with the processing unit and suitable for suppressing the opening signals of the valves and of the injectors, for at least some of the engine cylinders, during several successive cycles of compression / expansion of the engine pistons in these cylinders.

• La figure 1 est une représentation schématique des phases successives du cycle de fonctionnement d'un des cylindres d'un moteur suivant l'invention.
• La figure 2 représente de façon détaillée divers modes de mise en oeuvre de la phase d'échappement.
• La figure 3 montre une variante de la phase d'admission.
• La figure 4 est une représentation schématique d'un moteur Diesel suivant l'invention.

The invention will be better understood on reading the detailed description which follows and on examining the appended drawings which represent, by way of nonlimiting examples, various embodiments of the method according to the invention.

• Figure 1 is a schematic representation of the successive phases of the operating cycle of one of the cylinders of an engine according to the invention.
• Figure 2 shows in detail various modes of implementation of the exhaust phase.
• Figure 3 shows a variant of the admission phase.
• Figure 4 is a schematic representation of a diesel engine according to the invention.

Schematically shown in Figure 1, the cylinder 12, the piston 14, the connecting rod 16, the crankshaft 18, the intake valves A and exhaust E and the fuel injector 20 of a diesel engine, preferably a supercharged Diesel engine with reduced volumetric ratio.

We will now describe the engine launch phase. The starter drives the motor shaft, the piston 14 performs its normal intake stroke, the intake valve A being normally open (1-Figure 1). The piston then performs (2-Figure 1) its normal compression stroke, valves closed and it is at the end of this compression stroke that, in a conventional engine, the opening of the fuel injection would be controlled.

However, in the absence of a system for heating the supply air (system which the invention proposes to eliminate) the compression stroke does not allow the air to reach the temperature necessary for ignition, especially in the case of a Diesel engine with “Reduced Volumetric Ratio”.

According to the invention, the injector 20 is kept closed as well as the valves A and E during at least one expansion / compression cycle (3 and 4-Figure 1) and for example, as shown in Figure 1, for another two expansion / compression cycles (3-4-5 and 6-figure 1).

This succession of several compression strokes (three in the example above) ensures the heating of the air enclosed in the cylinder to a temperature allowing the ignition of the fuel. The fuel is then injected at the end of this last compression (6-figure 1), which conventionally produces the combustion / expansion stroke (7-figure 1) then the exhaust stroke (8-figure 1), with the exhaust valve E open.

Figure 1 therefore illustrates well the operation of an engine following an eight-stroke cycle, instead of four-stroke.

Of course we could also, if the air heating is sufficient, perform the first injection after the second compression only (4-Figure 1) which would give a six-stroke operation.

More generally, it is possible to carry out a succession of n additional compression / expansion cycles, with injection and closed valves, which gives operation at (4 + 2n) time.

The value of n is limited by the number of engine cylinders and can vary with various engine parameters (power, speed, temperature, etc.).

After the exhaust stroke (8-figure 1), the cycle starts again at the intake stroke (1- figure 1).

We can therefore say that the problem of launching an engine is solved, according to the invention. by preventing the valves from opening and by preventing the injection of fuel, during the first turns of the engine.

Once the first ignition has been made, the engine must still maintain its rotation without using the starter. The temperature at the end of compression, after the first ignition, is already higher than that at the start. because of the contacts with the slightly warmed walls and because of the higher speed of rotation ensuring a more efficient compression (reduction of the relative value of the piston / cylinder leaks). However, in the case of an engine with a “Reduced Volumetric Ratio” or in the case where the intake air is too cold, it is possible that certain cylinders of the engine will extinguish or burn poorly, which produces exhaust fumes. repulsive toxic tees, if the normal four-stroke operating cycle is resumed from the first ignition.

This is why an operation cycle similar to that shown in FIG. 1 is maintained, after launching and during idling or at low cold load, when necessary by reducing the number n of additional compression / expansion cycles. compared to the number chosen for the launch itself. This is how we can perform the launch on a ten-stroke operation, then gradually switch, according to one of the engine parameters, to the eight and six stroke cycles to arrive at the normal four-stroke operation when the engine is hot and when the turbo-compressor flows normally.

There is shown (8-Figure 1) the exhaust stroke, with the exhaust valve E open to full opening during substantially all of this stroke.

However, it may be advantageous, in the process according to the invention, to brake the exhaust of the gases in order to keep part of the hot gases in the cylinder and thus increase the temperature reached at the end of the following compressions, after the first ignition, when idling and at low load.

Various means have been shown in FIG. 2 for obtaining such a result. In the case of the figure 2-a, the exhaust valve E, opened in the vicinity of the PMB (in the opening zone O) closes before the TDC (closing zone F).

In the case of the figure 2-b, the exhaust valve E is closed in the vicinity of the PMB and does not open until late, for example in the vicinity of the half-stroke of the piston (opening zone 0 ') for close around TDC (closing zone F '). Thanks to the microprocessor (or similar processing unit) processing the valve opening signals, it is easy to choose the angular positions of the crankshaft for which the opening orders. or closing are given.

In the case of figures 2-a and 2-b one uses the total lifting of the valve of exhaust, but the duration of the period of exhaust is reduced. Some electronic valve opening control systems allow you to control not only the time of opening but also the amount of valve lift. It is what one represented on the figure 2-c, where the exhaust valve opens in a traditional way in the vicinity of the PMB (opening zone O ") to close again in the vicinity of the TDC (zone of closing F "), but with only partial lifting which produces a rolling of the gases and causes part of the hot gases to remain in the cylinder.

Of course, the three solutions a-b-c described above can be combined.

The intake stroke in the engine launch phase with the intake valve A open and the exhaust valve E closed is shown in diagram 1 of FIG. 1. However, it may be advantageous to perform the intake stroke with cleaning of the exhaust gases. This allows, after the first or the first ignitions, and during idling or low load, to raise the initial temperature of the gases introduced into the cylinder, before the successive compressions which allow to reach the ignition temperature .

This is what has been represented in FIG. 3 in which the first diagram (8) corresponds to the exhaust stroke (8) in FIG. 1. The second diagram shows the piston 14 arrived at TDC and the two valves closed. The three diagrams 1 (1) -1 (2) -1 (3) are a breakdown of the intake stroke shown in (1) in Figure 1 and show that during part 1 (2) of this stroke the valve The exhaust is temporarily reopened to swallow some of the hot exhaust gases which mix, to heat them, with the cold air introduced by the intake valve. Of course, here again, the lifting of the exhaust valve can be total or partial.

The low-load starting and running method according to the invention is preferably applied to all of the cylinders of an engine, but it can be applied to only a part of them.

It is well known that thermal engines in general, when they operate at low load or speed get dirty. The oil flows carried by the pistons enter the intake and exhaust pipes, oxidize and in the long run coke. This fouling reduces the sections and compromises the proper functioning.

In load, on the other hand, the high combustion or exhaust temperature causes combustion of any oil spills.

The process according to the invention makes it possible to eliminate this fouling at low load thanks to the fact that the successions of compressions (with injection and closed valves) artificially raise the temperatures of the cycle to an exhaust temperature sufficient to burn the oil and prevent fouling, this increase in temperature being further improved by the delayed or partial opening of the exhaust valves which has been described with reference to FIGS. 2 and 3.

We will now describe a diesel engine, for example with a reduced volumetric ratio and supercharged by a turbo-compressor, implementing the method according to the invention. The electronic control system, which is not part of the invention, will only be described in its essential parts.

This system comprises, in a known manner, a sensor 21-22 of the angular position and of the speed of the shaft 18 of the motor.

This sensor applies its signals to three processing units 24-26-28. Unit 24 manages the opening and closing function of the intake valves A. Unit 28 manages the opening and closing function of the exhaust valves E. Unit 26 manages the opening and closing function of the injector 20.

The valves A and E are controlled by electro-hydraulic actuators 30. 32 (for example of the type described in the aforementioned European patent application EP-A-139566) or electro-pneumatic actuated by electromagnets 34. 36. L injector 20 is controlled by an actuator 37 controlled by an electromagnet 38. The control signals developed and conditioned by the units 24-26-28 are transmitted respectively to the electromagnets 34-38-36 via power interfaces 40-42-44. As is known, the units 24-26-28 receive other information 45 relating to other operating parameters of the motor and necessary for the regulation function. The units 24-26-28 are established and programmed to deliver their control signals according to the conventional sequence of the four-stroke cycle.

In accordance with the invention, an electronic cycle modifier system is added to the electronic control system. This system can consist of three signal inhibitor circuits 46-48-50 connected to the processing units 24-26-28 and periodically canceling the valve opening control signals and the injection control signals normally produced by these processing units based on a given signal from sensor 21-22. If the inhibitor circuit is set for only one cancellation of signals per cycle, the engine will operate at six times (two successive compressions), if it is set for two cancellations per cycle, the engine will operate at eight times (three successive compressions, such as in Figure 1) and so on. An appropriate means of adjustment on the inhibitor circuit will make it possible to choose the operating cycle of the engine which is suitable for the periods of launching, idle running, running at low load.

The specific types of opening of the exhaust valve (delayed opening, partial or with facelift), which have been described with reference to FIGS. 2 and 3, can be developed in the processing unit 28, on the basis of information also supplied by the inhibitor circuit 50 or supplied directly by the regulation circuit 45.

Of course, in the case where a central processing unit replaces the three individual units 24-26-28 which have been shown, a central cycle modifier circuit could replace the three individual circuits 46-48-50.

It appears from the above that. thanks to the succession of several compression / expansion. the invention makes it possible to produce an internal combustion engine with internal heating of the supply air.

The invention has above all been described, in the foregoing, with regard to a four-stroke supercharged Diesel engine with a reduced volumetric ratio, but the invention could also apply to two-stroke engines, provided that the circulation of gas is controlled at least by electronically controlled opening exhaust valves.

Claims (11)

1. A process for the starting and low-load running of a diesel engine with electronic monitoring of the control of injectors and the control of the valves, characterized in that it consists in adjusting the said controls to keep the inlet and exhaust valves closed and to cut off fuel injection, in at least some of the cylinders of the engine, during several successive compression/expansion cycles, whereby the air contained in these cylinders by the preceding intake stroke of the piston is heated as a result of several successive compressions, until the ignition temperature is reached at the first fuel injection.

2. A process as claimed in claim 1, characterized in that it consists, for the starting of the engine in adjusting the control of the valves and the control of the injectors to prevent the opening of the valves and fuel injection during the first starting revolutions of the engine.

3. A process as claimed in one of claims 1 or 2 for the starting and low-load running of a four-stroke diesel engine, wherein the valves and injectors are kept closed during n additional compression/expansion cycles before first ignition, whereby the engine operates according to a cycle of (4 + 2n) strokes during starting and low-load running.

4. A process as claimed in claim 3, wherein the control of the valves and of the injectors is adjusted in order to operate the engine according to one of the cycles of six, eight,eten or twelve strokes.

5. A process as claimed in one of claims 3 or 4, wherein, after the starting of the engine, the control of the valves and of the injectors is adjusted to reduce the number n of additional compression/expansion cycles.

6. A process as claimed in any one of the preceding claims, wherein the control of the exhaust valves is adjusted so as to cause only a limited opening of the said valves during the exhaust stroke.

7. A process as claimed in any one of the preceding claims, wherein the control of the exhaust valves is adjusted in order to cause a limited opening of the said valves during the intake stroke.

8. A diesel engine for carrying out the process as claimed in one of the preceding claims, which comprises : at least one sensor (21-22) for sensing the angular position and speed of the shaft (18) of the engine ; actuators (30, 32, 37) for the inlet and exhaust valves (A and E) and for the injectors (20) ; and at least one processing unit (24, 26 and 28) receiving signals from the sensor (21-22) and preparing control signals for the actuators (30, 32 and 37), the said engine being characterized in that it comprises at least one cycle-modifying circuit (46, 48 and 50) associated with the processing unit (24, 26 and 28) and designed to eliminate the opening signals of the valves and injectors, in at least some of the cylinders (12) of the engine during several successive compression/expansion cycles of the pistons (14) of the engine in these cylinders (12).

9. A diesel engine as claimed in claim 8, wherein the cycle-modifying circuit (24, 26 and 28) is adjustable so as to vary the number n of successive compression/expansion cycles.

10. A four-stroke diesel engine as claimed in one of claims 8 or 9, wherein the cycle-modifying circuit (24, 26 and 28) can be adjusted in order to operate the engine, during starting and under low load, according to operating cycles of between six and twelve strokes.

11. A diesel engine as claimed in claim 10, which is supercharged and the compression ratio of which is reduced.

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