The present invention relates to a method of controlling the fuel injection of a direct-injection internal-combustion engine, notably a compression-ignition engine, and to an engine using same.
It more particularly relates to a method for an engine usable in the air transport or road sector, or in the field of stationary equipments such as engine generators.
This type of engine generally comprises at least a cylinder, a piston provided with a teat arranged in a concave bowl and sliding in this cylinder in a reciprocating rectilinear motion, intake means for an oxidizer, burnt gas exhaust means, a combustion chamber and injection means for injecting a fuel into the combustion chamber.
As it is generally admitted, upon design of an engine, the performance, pollutant emission and mechanical strength constraints of the combustion chamber are increasingly high whereas the means for meeting them are quite the opposite.
Thus, performance increase generally leads to an increase in emissions and to higher mechanical stresses.
It is therefore necessary to overcome these stresses so as to guarantee limited pollutant emissions and satisfactory mechanical strength over the entire operating range of the engine, in particular at very high load. In particular for pollutant emissions, using all of the oxidizer present in the combustion chamber, for example an oxidizer comprising air at ambient pressure, supercharged air or a mixture of air (supercharged or not) and of recirculated burnt gas, is of great importance.
Indeed, the fuel mixture (oxidizer/fuel) in the combustion chamber needs to be as homogeneous as possible.
In practice, the fuel remains confined in the bowl and it cannot mix with the oxidizer contained notably in the squish area, i.e. in the volume located in the upper part of the combustion chamber delimited by the cylinder wall and the face of the cylinder head opposite the piston.
This involves the drawback of creating high richness areas in the combustion chamber, generating a high production of soots, carbon oxide (CO) and unburnt hydrocarbons (HC) upon combustion of this fuel mixture.
Furthermore, to come back to the mechanical strength problem, the thermal load is focused on the re-entrant part of the piston, i.e. the bowl neck or diameter restriction that marks the transition between the piston bowl and the upper zone encompassing the squish area, which may be limiting in terms of mechanical strength at very high loads.
To overcome these drawbacks, and as better described in French patent application No. 13/60,426 filed by the applicant, an internal-combustion engine comprising fuel injection means with jets having at least two sheet angles and a piston comprising a bowl provided with a teat with two combustion zone volumes and internal aerodynamics substantially improving the combustion quality is provided.
This allows to use a larger amount of oxidizer compared to conventional engines, and to distribute the thermal load over a larger surface area of the combustion chamber.
In this type of engines, mixing of the injected fuel and of the oxidizer, such as air at ambient pressure or supercharged air or a mixture of air (supercharged or not) and of recirculated exhaust gas, admitted to the combustion chamber occurs in two stages.
First, upon fuel injection, the oxidizer located on the periphery of the fuel jet is carried along by this jet. Small-scale mixing due to the turbulence generated by this entrainment occurs then.
In order to improve this fuel/oxidizer mixing, a swirling motion of the oxidizer, referred to as swirl, which provides large-scale “stirring” of the unmixed fuel, is used in a second stage. This swirl can be seen as a rotating motion of the oxidizer about an axis substantially parallel to or merged with that of the combustion chamber. This swirl can be obtained by means of a particular oxidizer intake, such as a specific intake line geometry.
In this configuration, it should however be noted that, while the small-scale mixing performed in the gaseous spray is very quick, the large-scale mixing associated with the swirling motion occurs more slowly.
The performances of the engine, the fuel consumption thereof or the discharge of pollutants such as soots, carbon monoxide or unburnt hydrocarbons greatly depend on the capacity for quick mixing of the fuel with the oxidizer admitted.
Optimization of the injection system and of the swirl level is therefore generally performed in order to optimize the engine performances.
One solution consists in using a relatively high swirl number Ns, of the order of 2 to 3, this number being equal to the ratio of the rotation speed of the swirling motion of the oxidizer to that of the crankshaft.
One drawback of this solution is that, for some engine operating points, in particular when the fuel injection pressure is not high enough, or when a large amount of fuel is injected, the fuel jets can be excessively diverted circumferentially, thus causing interaction or even superposition between the various jets.
This phenomenon can significantly increase soot and unburnt hydrocarbon emissions while degrading the combustion efficiency, and therefore the power and the consumption.
The present invention aims to overcome the aforementioned drawbacks by means of a method allowing to obtain better mixing of the oxidizer (gaseous fluid) while enabling to use a fuel injection system with at least two sheet angles and a piston whose profile allows the combustion chamber to comprise at least two combustion zones.
The invention therefore relates to a fuel injection method for a compression-ignition direct-injection internal-combustion engine comprising at least a cylinder, a cylinder head carrying fuel injection means, a piston sliding in this cylinder, a combustion chamber delimited on one side by the upper face of the piston comprising a teat extending in the direction of the cylinder head and arranged in the centre of a concave bowl with at least two mixing zones, said injection means projecting fuel in at least two fuel jet sheets with different sheet angles, a lower sheet of jet axis C1 for zone Z1 and an upper sheet of jet axis C2 for zone Z2, characterized in that it consists in injecting into the combustion chamber the fuel jets of one of the sheets in a radial direction forming a non-zero angle b2 with the radial direction C2 of the fuel jets of the other sheet and in admitting the oxidizer in a swirling motion with a swirl number less than or equal to 1.5.
The method can consist in injecting the fuel jets with an angular offset, between two neighbouring jets belonging to different sheets, substantially equal to the half angle between two jets of the same sheet.
The method can consist in injecting the fuel in a number n of jets related to the swirl number Ns by the correlation: −4·Ns+16≦n≦−4·Ns+18.
The method can consist in injecting the fuel in at least two fuel jet sheets positioned axially one above the other, with each a different sheet angle.
The method can consist in injecting the fuel with a different fuel flow rate in each sheet.
The invention also relates to a compression-ignition direct-injection internal-combustion engine comprising at least a cylinder, a cylinder head carrying fuel injection means, a piston sliding in this cylinder, a combustion chamber delimited on one side by the upper face of the piston comprising a teat extending in the direction of the cylinder head and arranged in the centre of a concave bowl, said method consisting in injecting the fuel in at least two fuel jet sheets with different sheet angles, a lower sheet of jet axis C1 and an upper sheet of jet axis C2, characterized in that it comprises fuel injection means for injecting the fuel jets of one of the sheets in a radial direction forming a non-zero angle with the radial direction of the fuel jets of the other sheet and means for admitting the oxidizer in a swirling motion with a swirl number less than or equal to 1.5.
Other features and advantages of the invention will be clear from reading the description hereafter, given by way of non limitative example, with reference to the accompanying figures wherein:
With reference to
Fuel is understood to be a liquid fuel such as diesel fuel, kerosene or any other fuel with the physicochemical characteristics allowing operation of an engine of compression ignition type including a direct injection system for this fuel.
This engine also comprises a burnt gas exhaust means 18 with at least one exhaust pipe 20 whose opening can be controlled by any means such as an exhaust valve 22 for example, and an intake means 24 for an oxidizer with at least one intake pipe 26 whose opening can be controlled by any means such as an intake valve 28 for example.
The intake means are designed for admitting the oxidizer with a predetermined swirl ratio. The intake means can therefore comprise at least one throttling means and the engine can comprise at least one control means for actuating the throttling means so as to obtain the predetermined swirl ratio. These intake means can also comprise a specific geometry of intake pipe 26.
The injection means comprise at least one fuel injector 30, preferably arranged along axis XX′ of the piston, whose nozzle 32 comprises a multiplicity of orifices 33 through which the fuel is sprayed and projected in the direction of combustion chamber 34 of the engine.
It is from these injection means that the projected fuel forms at least two fuel jet sheets, here two sheets 36 and 38 of fuel jets 40 and 42, which, in the example shown, have a general axis merged with that of piston 16 while being axially positioned one above the other.
More precisely, sheet 36 that is the closer to piston 16 is referred to as lower sheet in the description hereafter, while sheet 38 that is further away from this piston is referred to as upper sheet.
As can be seen in
Advantageously, sheet angle A1 of the lower sheet is at most equal to 130°, preferably ranging between 105° and 130°, while sheet angle A2 of the upper sheet is at most equal to 180°, preferably ranging between 155° and 180°.
For simplification reasons, in the rest of the description, angle a1 corresponds to A1/2 and angle a2 corresponds to A2/2 (see
Preferably, the difference between angle A1 and angle A2 is greater than or equal to 25°. This thus allows to limit fuel jet overlaps between the two sheets and therefore formation of pollutants such as soots.
Of course, it is possible for the injection means not to be arranged along axis XX′, but in this case the general axis of the fuel jet sheets from the fuel injector is at least substantially parallel to this axis XX′.
Similarly, it is possible for each sheet to be carried by a distinct injector (single-sheet injector) with dedicated targeting in distinct zones of the combustion chamber.
Combustion chamber 34 is delimited by the inner face of cylinder head 12 opposite the piston, the circular inner wall of cylinder 10 and upper face 44 of piston 16.
This upper face of the piston comprises a concave bowl 46, whose axis is here merged with that of the cylinder, whose concavity is directed towards the cylinder head and which houses a teat 48 arranged substantially in the centre of the bowl, which rises towards cylinder head 12, while being preferably coaxial with the axis of the fuel sheets from injector 30.
Of course, it is possible for the axis of the bowl not to be coaxial with that of the cylinder, but the main thing is the layout according to which the axis of the fuel jet sheet, the axis of the teat and the axis of the bowl are preferably merged.
Furthermore, with reference to
Of course, without departing from the scope of the invention, inclined surface 52 can be nonexistent (zero length) and inclined flank 54 then connects the top of the teat to the bottom of the bowl.
In the example of
The two rounded surfaces 58 and 60 thus delimit the lower part of a toroidal volume, here a torus of substantially cylindrical section 64 and of centre B whose purpose is described in the rest of the description.
Lateral wall 62 is extended, still while moving away from axis XX, by a convex rounded surface 66 in form of an arc of a circle with radius R3, referred to as re-entrant, leading to an inclined plane 68 linked to a concave inflection surface 69 connected to a substantially plane surface 70. This plane surface is continued by an outer convex surface 72 in form of an arc of a circle with radius R5 that leads to a plane surface 74 extending up to the vicinity of the cylinder wall.
The combustion chamber thus comprises two distinct zones Z1 and Z2 where mixing of the oxidizer they contain (air, supercharged or not, or mixture of air and recirculated burnt gas) with the fuel coming from the injector, as well as combustion of the fuel mixture thus formed, occurs.
Zone Z1, delimited by teat 48, torus 64 at the bowl bottom, wall 62 and convex rounded surface 66, forms the lower zone of the combustion chamber associated with lower sheet 36 of fuel jets of axis C1. Zone Z2, delimited by inclined plane 68, concave surface 69, substantially plane surface 70, convex surface 72, plane surface 74, the peripheral inner wall of the cylinder and cylinder head 12, forms the upper zone of this chamber associated with upper sheet 38 of fuel jets of axis C2.
In this configuration, the bowl comprises, for a piston position close to the top dead centre:
All these parameters are appreciated for a position of piston 16 in the vicinity of the top dead centre that corresponds to a distance D considered between point M and the origin T2 of axis C2 of jets 42.
More precisely, this distance D is equal to the sum of height L4 and height C, height C corresponding to the axial height between origin T2 and point P. This height corresponds to formula ID1/tan(a2).
Thus, the dimension and angle parameters of this bowl meet at least one of the following conditions:
Furthermore,
Thus, by means of this bowl parametrization, the fuel jets of lower sheet 36 directly target torus 64 and they do not directly impact re-entrant 66.
Therefore, combustion of the lower fuel/oxidizer mixture occurs essentially in the torus volume, whereas combustion of the upper fuel/oxidizer mixture occurs essentially in the squish area and above the piston.
Furthermore, the interaction of the upper sheet jets with the lower sheet jets is limited, which allows the fuel/oxidizer mixture to be homogenized while meeting mechanical strength constraints at high load.
We consider now
As already mentioned, injector 30 carries, in the region of nozzle 32 thereof, injection orifices 33 from which the fuel jets extend radially (see
In this configuration, radial injection of the fuel jets occurs in a radial direction from the injector to the walls of the combustion chamber corresponding to axes C1 and C2.
Of course, without departing from the scope of the invention, the diameters of orifices 33a and 33b can be different. By way of example, the diameter of orifices 33a can be larger than the diameter of orifices 33b. Since the injection pressure is identical in the region of the injector nozzle, this results in two fuel jet sheets with different flow rates.
Similarly, the number of orifices between the lower sheet and the upper sheet can be different and correlated as a function of the swirl number as explained below.
Finally, the number n of orifices that can be provided for the whole of the two sheets can be related to the swirl number Ns by the correlation: −4·Ns+16≦n≦−4·Ns+18.
The limiting case of a number of orifices where n=18 corresponds to a zero swirl number. Indeed, the angle formed by the fuel jets being generally close to 20°, in the absence of swirl, the number of jets has to be less than or equal to 18 so as to avoid direct interaction between the jets. Furthermore, in this limiting case, the interaction of the jets of the lower sheet with the bowl bottom causing widening of the head of the fuel jets, upon upflow thereof towards the squish area, they may interact with the jets of the upper sheet and disturb the mixing process.
This generally results in a soot production increase and combustion efficiency decrease. The number of jets is therefore preferably slightly reduced to 17 or even 16 so as to ensure that no interaction is possible between the jets.
When the swirl is not zero, simulations carried out by the applicant with an identical number of orifices on each sheet allowed to show that the number of jets should not exceed 14 when the swirl number is 1, otherwise the production of soot might increase substantially.
In general terms, the goal being to minimize the role of the swirling motion in the mixing process and to perform mixing by multiplying the number of jets, it is recommended not to go below 10 injection ports, i.e. 5 ports per sheet, and beyond a swirl number of 1.5.
With reference to
Furthermore, orifices 33a of the lower sheet and orifices 33b of the upper sheet must not be positioned one above the other; the axes of two neighbouring jets belonging to different sheets must have an angular offset, denoted by b2, which is here substantially equal to the half angle between two jets of the same sheet.
This allows to prevent interaction between the two sheets when the fuel from the lower sheet leaves the bowl zone to pass into the squish area.
It is thus possible to use a large and equal number of ports for each sheet with a low swirl number, ideally below 1.5, so as to mix the fuel and the oxidizer as quickly as possible, predominantly during the injection process.
Mixing is then mainly achieved through entrainment of the gaseous oxidizer by the fuel jets, the contribution related to the swirling motion remaining low and being kept only to complete the mixing process with large-scale stirring upon expansion of the piston.
Thus, during fuel injection, the oxidizer is admitted to combustion chamber 34 with a swirling motion S and a swirl number of 1.5.
There are 12 fuel jets evenly distributed among the two sheets (6 jets for the lower sheet and 6 jets for the upper sheet) and angle b2 is 30°.
Fuel jets 40 of the lower sheet are sent towards the bottom of bowl 46 in zone Z1 (
During the final injection phase, it can be observed that, despite swirl S, the fuel jets of the two sheets do not overlap (
Number | Date | Country | Kind |
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1452119 | Mar 2014 | FR | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2015/052442 | 2/5/2015 | WO | 00 |