WO1992012332A1

WO1992012332A1 - Two-stroke engine with delayed cylinder scavenging

Info

Publication number: WO1992012332A1
Application number: PCT/FR1991/001035
Authority: WO
Inventors: Gaétan Monnier
Original assignee: Institut Français Du Petrole
Priority date: 1990-12-28
Filing date: 1991-12-19
Publication date: 1992-07-23
Also published as: EP0517887A1; FR2671137A1

Abstract

Two-stroke engine including at least one cylinder (1) in which a piston moves delimiting a combustion chamber (4), a crankcase-pump (5) located at one of the cylinder ends and separated from the combustion chamber (4) by said piston (3), at least one transfer port in the combustion chamber, at least one transfer conduit (11) linking said combustion chamber (4) to the crankcase-pump (5), at least one exhaust port (12) for evacuating combustion gases from the combustion chamber (4), and means (14, 63) located in the region of the transfer conduit (11) and controlled by the rotational movement of the crankshaft (6) for blanking off said conduit (11) at certain moments of the cycle, in order to delay scavenging in the combustion chamber (4).

Description

TWO-STROKE DELAYED CYLINDER SCAN ENGINE

The invention relates to a two-stroke engine, comprising one or more cylinders, in each of which the scanning is delayed by means related to the rotation of the crankshaft.

Two-stroke engines conventionally comprise at least one cylinder in which a piston delimiting a combustion chamber moves, a pump casing situated at one end of said cylinder and separated from said combustion chamber by said piston, at at least one transfer opening in the combustion chamber, at least one transfer duct connecting said combustion chamber to said pump casing, and at least one opening for exhausting the burnt gases from the combustion chamber.

With this type of engine, when the piston reaches the end of its travel near the bottom dead center, it discovers the exhaust lights (or openings) thus letting escape the combustion gases from the combustion chamber, then the piston discovers the transfer lights (or openings) located at a different level from the exhaust lights so that fresh air compressed by the piston in the pump housing can then escape from the pump cartel, and be introduced into the combustion chamber via the transfer conduits.

The introduction of fresh air or fuel mixture from the pump housing into the combustion chamber is therefore directly linked to the opening by the piston of the transfer lights. The operations taking place in the cylinder are essentially the introduction of air or a fuel mixture into the cylinder, the ignition, the combustion and then the discharge of the burnt gases out of the cylinder where combustion took place. All these operations must take place in a precise order, in extremely short times which vary according to the power at which the engine operates.

Sweeping, that is to say the movement of gases in the combustion chamber When replacing the burnt gases with fresh gases, is therefore very important since its smooth running ensures good combustion of the gases, i.e. -to say a good performance of the engine.

In addition, good sweeping ensures a reduction in the discharge of unburnt hydrocarbons to the atmosphere.

Many solutions have already been proposed in order to improve scanning in a two-stroke engine.

It is known for example to play on the difference in height between the exhaust and transfer lights which determines the difference between the start of the moments of exhaust of the used gases and introduction of fresh air into the combustion chamber. . It is also known from the patent application

IN. 90 / 07.472, filed in the name of the applicant, to direct the gases in the combustion chamber optimally, by an appropriate form of the upper part of the piston.

In order to improve sweeping and combustion, it is also known, in particular by the document SAE 810,922 published in September 1981 during an SAE congress and by patent EP-246,877 filed by The Honda company, to place at the level of the conduits exhaust systems intended to modify the timing of the start of exhaust, or systems intended to modify the flow of exhaust gases.

On the other hand, in order to reduce the discharge of unburnt hydrocarbons, catalysts have been placed on certain two-stroke engines, downstream of the combustion chambers. These additional parts, in addition to the fact that they raise the cost of the engine, are more fragile that the engine at least thermally, so it is difficult to use a rich mixture if such a catalyst is present in an engine. Therefore, the specific power, that is to say the power related to the engine displacement will be lower for an engine equipped with a catalyst.

The aforementioned prior art makes it possible to improve the combustion and / or the discharge of hydrocarbons by intervening in the sweep.

However, the known devices most often require the addition of parts, which increases the cost, complicates the operation and thereby increases the risk of engine failure.

The present invention aims to provide a 2-stroke engine of the type defined at the head of the description, of simple design therefore reliable and which, because of its simplicity, is a very reasonable cost price, while ensuring high efficiency and by limiting the discharge of hydrocarbons to the atmosphere as much as possible.

The motor according to the invention allows an improvement in the sweeping efficiency, in particular at low speeds. In fact, in low revs, that is to say for engine rotation speeds of less than 4,000 revolutions per minute, the sweeping time in the combustion chamber is relatively long so that all of the gas dynamics carrying out in the first part of the combustion cycle, part of the fresh gases leaves towards the exhaust without having been burned. This phenomenon is also called "direct short circuiting of unburnt fuel at the exhaust".

By delaying according to the invention the introduction of fresh gases into the combustion chamber, it is also possible to delay the exhaust of the burnt gases, so that the effect of the pressure of the gases in the combustion chamber is better exploited. . In other words, this pressure is used for a longer time, hence a gain in terms of gas expansion, that is to say in terms of engine efficiency.

However, the start of the sweep should not be delayed too long because at high speeds. The sweep is done in a very short period of time, if all the fresh gases arrive too late, the chamber will contain less fresh gas, the engine will have a lower power, which is not desirable.

The present invention therefore relates to an engine of the type defined at the head of the description and which also comprises means placed at the level of the transfer duct, driven in rotation by the crankshaft and intended to close said channel at certain moments of the cycle, so to delay The sweep in the combustion chamber. According to a first embodiment of the invention, the means for periodically closing the transfer duct consist of at least one coaxial flange and fixed to the crankshaft, the means being provided with at least one cutout at the periphery, the cutout allowing at certain moments in the cycle, the exhaust of fresh gases from the pump housing to the combustion chamber via the transfer duct. According to another embodiment, the closure means consist of at least one valve linked to the crankshaft and provided with cutouts at the periphery.

Advantageously, in order to be able to maintain good performance both at low speeds and in high speeds, the engine according to the invention may include means intended to modify the shape of the cutting of the flange or of the valve according to the speed of rotation of the flange.

More precisely. The means for modifying the shape of the peripheral cutout consist of at least one annular sector (s) movable relative to the flange and which react to the action of the centrifugal force exerted on the flange.

According to yet another embodiment of the invention, the means for selectively closing the transfer duct consist of a rotary plug placed in the transfer duct. More precisely. The rotary valve is placed near a transfer opening.

The present invention relates to both "loop scanning" type motors and "transverse scanning" type motors, the definition of which will be given below. The invention will be better understood on reading the description which follows, given by way of indication and in no way limiting, with reference to the appended drawings, according to which:

FIG. 1 is a vertical section view of a two-stroke engine according to one of the embodiments of the invention, showing the piston near bottom dead center,

- Figure 2 is a vertical sectional view of a two-stroke engine according to one of the embodiments of the invention, showing the piston at the dead top, "Figure 3 is a front view of a flange according to the invention,

FIG. 4 is a front view of another embodiment of the flange according to the invention,

FIG. 5 is a front view of a device allowing variable opening of the transfer lights as a function of the engine speed,

FIG. 6 shows in section a transverse scanning motor equipped with a shutter means according to the invention, and

- Figure 7 is a diagram showing the opening law of the transfer lights according to the invention and conventionally, around the bottom dead center of the piston.

Figures 1 and 2 show in vertical section a first embodiment of the invention, where only appear the parts essential to understanding the invention.

The cylinder 1 is closed in the upper part by a cylinder head 2. A piston 3 moves inside the cylinder between the top dead center as shown in FIG. 2 and the bottom dead center as shown in FIG. 1.

The piston 3 thus delimits a combustion chamber 4 of variable volume, between its upper surface 31 and the lower surface of the cylinder head 21.

Cooling fins 10 can be provided on the outside face of the cylinder 1. Such fins can also be made on the outside face of the cylinder head 2. Any other cooling means, such as cooling by a liquid, can of course be provided without departing from the scope of the invention.

A pump housing 5 is arranged in the extension of Chamber 4, below the piston 3. The pump housing 5 is crossed by the axis of the crankshaft 6 to which is linked for example a flywheel 7. A connecting rod 8 connects the piston 3 to the flywheel 7.

The wall of the cylinder 1 is traversed, preferably in its lower part, by openings or lights 9 called "transfer" which each communicate via a conduit or transfer channel 11 with the interior volume of the pump housing 5 which also comprises appropriate openings 51.

The wall of the cylinder 1 also comprises so-called exhaust openings 12 intended for the evacuation of the gases burned in the combustion chamber 4.

The motor illustrated in FIGS. 1 and 2 is of the loop scanning type because the transfer openings 9 create, by their orientation, a movement of the fresh gases in the cylinder. Thereby. The scanning is carried out as indicated by the arrows 13 in FIG. 1, that is to say in the form of a loop.

The transfer 9 and exhaust 12 openings are conventionally closed and uncovered by the piston at different times of the operating cycle. According to Figures 1 and 2, the start of the exhaust will take place before the start of the injection of fresh air or fuel mixture since the upper level of the exhaust lights 12 is located above the upper level of the lights 9. The difference in the heights of the aforementioned lights allows per¬ to intervene on the moments of exhaust and intake.

The piston is, in known manner, the only element controlling the opening of the transfer lights 9 and the exhaust lights 12.

According to a first embodiment of the invention, flanges or valves 14 are connected, by any means known per se, to the axis of the crankshaft 6. In the case of single-cylinder engines, the flanges of the crankshaft are cut to obtain disc shapes cut at the periphery.

In the case of multi-cylinder engines, a part is preferably attached to the crankshaft 6. This part has the form of a disc provided with cutouts at the periphery and cooperates with the opening 51 to form a valve.

All that will be said below concerning an embodiment comprising a flange is, of course, valid for a multi-cylinder engine comprising valves. FIG. 3 shows an exemplary embodiment of a closure means according to the invention. According to this example, a cut is made in the form of a ring portion on the periphery of the flange. The width of the ring portion here is substantially equal to the height H of the opening 51 of the transfer duct on the pump housing side, so that at certain times of the cycle, the flange discovers the opening 51 thus authorizing the passage of non-carburetted air in the transfer duct 11. In FIG. 3, only the left half of the transfer light 51 is uncovered.

The opening of the transfer duct is therefore not controlled only by the piston 3 but also by the flange 14.

More precisely, the cut in the flange defines the moment of opening of the transfer duct. In addition, the shape of the cutout can define how the opening will be made. FIG. 4 shows another possible form of the cut, according to which the opening of the transfer light 51 will take place gradually since the flange first discovers the outermost zone of La then the entire height H of the light.

FIG. 7 shows the curves defined by the opening of the transfer lights as a function of the angle of rotation of the crankshaft when the piston is near the bottom dead center.

Curve 70 in solid lines shows the law of opening the transfer lights obtained in a known manner, that is to say when only the piston controls the opening of the lights.

Curves 71 and 72 drawn respectively in lines mixed and dotted each show a law of opening of the lights, obtained according to the invention.

Curve 71 was obtained for a first setting, that is to say a first law of opening of the flange or of the valve while curve 72 relates to a second setting different from the first.

These characteristic curves show that the opening of the transfer ports can be delayed by the presence of an appropriate element preferably placed near the transfer duct. Indeed, with the first setting, we can for example start the opening of the lights for a crankshaft rotation angle of 50 before bottom dead center, while with a second setting. The opening of the Lights will begin for an angle of 40 before the bottom dead center. The fresh gases therefore escaping later from the pump housing 5, they will arrive later in the combustion chamber 4, that is to say when more burnt gases have already left this combustion chamber. Combustion will therefore take place with more fresh gas, resulting in better engine performance. Another advantage of the invention resides in that the direct rejection of fresh gases into the exhaust is greatly reduced, due to the delay in the arrival of these gases in the combustion chamber, hence better engine efficiency.

To further improve the efficiency of the engine, it is possible, according to the invention, to produce a different opening of the transfer lights according to the engine speed.

Indeed, the duration of the sweep in the combustion chamber is different depending on the engine speed. Thereby. The constraints at the sweeping level are different, even contradictory depending on the speed of rotation and the load of the motor.

At high speeds for example, you should not delay the sweep too much because otherwise, the sweep time being very short, the amount of gas to burn would decrease, thus lowering the engine power. To solve this problem, it is proposed according to the invention to produce a variable opening as a function of the engine speed, by fixing on the flanges 14 one or more elements such as annular sectors 20 movable relative to the flange. These sectors modify the shape of the flange since, depending on the centrifugal force exerted, they overlap all or part of the flange 14. FIG. 5 shows two annular sectors 20 completely overlapping the flange 14. In this case, the flange will have a maximum cutting surface of so that the scanning will be slightly delayed which is desirable in the high regimes.

FIG. 5 illustrates a possible embodiment of such a system, according to which each annular sector 20 has an arm 23 and cooperates with a return spring 22. The centrifugal force thus tends to counteract the action of the return spring 22, it that is to say to bring the annular sectors 20 on the uncut surface of the flange, as shown in dotted lines.

This device is a nonlimiting example, known per se, which will therefore not be described further here. Any other means capable of varying the opening angle of the flange as a function of the engine speed is of course conceivable without departing from the scope of the invention.

FIG. 6 illustrates another embodiment of the invention applicable to so-called transverse scanning motors.

These engines, well known to those skilled in the art, differ from loop scanning engines by the embodiment of the scanning in the combustion chamber and by the shape of the piston.

As shown in FIG. 6, the exhaust duct 61 opens into the combustion chamber 4 in a zone diametrically opposite to the transfer lumen 9. Therefore. The scanning is carried out in the direction indicated by the arrows 62, that is to say by crossing the combustion chamber 4 along a diametrical plane.

The means intended to close off The transfer duct at certain moments of the cycle consists, for example, of a rotating plug 63 placed in the transfer channel 11 near the transfer lumen 9 located on the side of the combustion chamber.

Without departing from the scope of the invention, the plug 63 can be placed near the transfer light 51 on the pump housing side.

The rotary plug 63 is mechanically connected to the rotation of the crankshaft 7 by a chain or a toothed belt 64 in order to transmit the rotation of the engine to the rotary plug, thereby making it possible to close the transfer duct 11 as a function of the rotation of the crankshaft.

Of course, any other means intended to control the rotation of the crankshaft to the rotation of the means for periodically closing the transfer duct can be imagined by a person skilled in the art without departing from the scope of the invention.

This device therefore performs the same function as the flanges provided with cutouts described above in the case of loop scanning motors. The advantages brought to the engine have therefore already been mentioned above.

Of course, those skilled in the art will be able to imagine, from the engine whose description has just been given by way of illustration and in no way limiting, various variants, modifications and additions that are not outside the scope of the invention.

- * - * - * -

Claims

1. - Two-stroke engine comprising at least one cylinder (1) in which moves a piston (3) delimiting a combustion chamber (4), a pump casing (5) located at one of the ends of said cylinder and separated from said combustion chamber (4) by said piston (3), at least one transfer opening (9) in the combustion chamber, at least one transfer duct (11) connecting said combustion chamber (4 ^{) to} said casing -pump (5), and at least one exhaust opening (12) for the burnt gases from the combustion chamber (4), characterized in that it further comprises means (14; 63) placed at said duct transfer (11), subject to the rotation of the crankshaft (6) and intended to close off said channel (11) at certain times of the cycle, so as to delay scanning in the combustion chamber (4).

2. - Engine according to claim 1, characterized in that said means for periodically closing the transfer duct consist of at least one flange (14) coaxial and fixed to the crankshaft (6), said means being provided with at least one cutting at the periphery, said cutting authorizing at certain moments of the cycle, the escape of gases from the pump housing (5) towards the combustion chamber (4) via said transfer duct (11).

3. - Engine according to claim 1, characterized in that said closure means consist of at least one valve connected to the crankshaft and provided with cutouts at the periphery.

4. - Motor according to any one of the preceding claims, characterized in that it further comprises means (20,

22, 23) intended to modify the shape of said cutout as a function of the speed of rotation of said flanges.

5. - Motor according to claim 4, characterized in that said means for modifying the shape of the peripheral cut consist of at least one sector (s) annular (s) (20) movable relative to the flange (14) and which react to the action of the centrifugal force exerted on the flange.

6. - seLon engine Any one of the preceding claims, characterized in that the sweeping in the combustion chamber (4) is of the loop type.

7. - Motor according to claim 1, characterized in that said means for selectively closing the transfer duct consist of a rotary plug (63) placed in said transfer duct (11).

8. - Motor according to claim 7, characterized in that Said rotary valve (63) is placed near a transfer opening (9, 51).

9. - Engine according to any one of claims 7 or 8, characterized in that the rotary plug (63) is connected to the crankshaft (6) by a flexible transmission element (64) in closed loop.

10. - Engine according to any one of claims 7 or 9, characterized in that the scanning is transverse in the combustion chamber.