Patent Application
20180274508
The invention relates to an injection nozzle for fuels, such as finds application, for example, for injecting fuel into combustion chambers of internal-combustion engines.
Injection nozzles for fuels, in particular for injecting fuel under high pressure into combustion chambers of internal-combustion engines, have long been known from the state of the art. Accordingly, a fuel injector with an injection nozzle is known from DE 199 36 668 A1, wherein the injection nozzle has a nozzle body with a pressure chamber formed therein. Arranged in longitudinally displaceable manner in the pressure chamber is a piston-shaped nozzle needle which has a sealing surface at one end, with which it interacts with a nozzle seat formed in the nozzle body for the purpose of opening and closing at least one injection port. For the purpose of controlling the longitudinal motion of the nozzle needle, at the end situated opposite the nozzle seat a control chamber has been formed which can be filled with fuel under high pressure and in which, via a control valve, a variable fuel pressure can be set by which a closing force can be exerted on the nozzle needle in the direction of the nozzle seat. The pressure chamber is connected to a fuel reservoir in which fuel is held under high pressure, in order to supply the pressure chamber with fuel under constant high pressure at all times.
The sealing of the injection ports by the resting of the nozzle needle on the nozzle seat represents the closed state of the injection nozzle. If fuel is to be injected into a combustion chamber, the nozzle needle is moved away from the nozzle seat in the longitudinal direction, by the hydraulic pressure in the control chamber being lowered. The hydraulic forces in the pressure chamber thereupon move the nozzle needle away from the nozzle seat, and the injection ports are released from the nozzle needle, so that fuel is ejected from the pressure chamber through the injection ports. In this process it is important for a clean injection that the nozzle needle moves away from the nozzle seat very rapidly. If it does so only slowly, a throttle gap forms between the sealing surface of the nozzle needle and the nozzle seat, through which fuel flows out of the pressure chamber to the injection ports only with reduced pressure, so that this fuel is only inadequately atomized when it emerges from the injection ports. Accordingly, this so-called seat-throttle region has to be kept as short as possible by a rapid movement of the nozzle needle, in order to increase the effective injection pressure at the injection ports rapidly to the level within the pressure chamber in order to obtain a good atomization of the fuel. Insufficiently atomized fuel otherwise results in insufficient combustion within the combustion chamber, and hence in increased hydrocarbon emissions of the internal-combustion engine.
For the purpose of increasing the needle-opening speed, the pressure in the control chamber can be lowered as rapidly as possible. This can be obtained by the outflow throttle, via which the fuel can flow away out of the control chamber, being configured with a large cross-section of flow in relation to the inflow throttle via which the control chamber is filled with fuel under high pressure. If the control chamber is additionally also filled via the outflow throttle, by the outflow throttle being connected to the high pressure with the control valve closed, any enlargement of the throttles results in a faster build-up of pressure or reduction of pressure. However, a rapid drop in pressure or build-up of pressure impairs the capability of the injection valve to handle extremely small amounts, since as a result the injected quantity of fuel reacts very sensitively to the actuation-time of the control valve. This entails a large stroke-to-stroke scatter—that is to say, a greater stochastic scattering of the injected quantity around the desired value from injection to injection.
Furthermore, a certain limit is set to the speed of the drop in pressure within the control chamber by virtue of the fact that in many applications the nozzle needle is operated in the so-called ballistic mode in which the nozzle needle does not reach a mechanical stroke stop but is retarded prior to reaching a stroke stop by renewed rise in pressure within the control chamber and is accelerated back in the direction of the nozzle seat. However, if the pressure in the control chamber drops too rapidly, this ballistic mode can no longer be realized, since the nozzle needle reaches the mechanical stroke stop prematurely by reason of its high opening speed.
The injection nozzle according to the invention has the advantage, in contrast, that the injection of fuel is effected by a rapid opening and rapid closing of the nozzle needle at the start and end, respectively, of the injection of fuel with high pressure at all times and hence with good atomization of the fuel, and consequently lowers the noxious emissions of the internal-combustion engine. For this purpose, the injection nozzle has a nozzle body in which a pressure chamber has been formed which can be filled with fuel under high pressure and in which a piston-shaped nozzle needle is arranged in longitudinally mobile manner. The nozzle needle has a sealing surface at one end and an end surface at its opposite end, said sealing surface of said nozzle needle interacting with a nozzle seat for the purpose of opening and closing at least one injection port. Moreover, a control chamber is present which can be filled with fuel under high pressure, in which a variable pressure can be set, and which the nozzle needle delimits with the end surface, so that a force can be exerted in the direction toward the nozzle seat by the hydraulic pressure on the end surface of the nozzle needle. The nozzle needle has an elastic longitudinal portion which has a longitudinal stiffness of less than 40,000 N/mm.
By virtue of the design of the elastic longitudinal portion, the effective opening speed of the nozzle needle can be crucially improved. By reason of the axial compression of the nozzle needle caused by the high pressure in the control chamber, the elastic longitudinal portion results in a so-called snap-action effect of the nozzle needle, which increases the actual opening speed and in this way has the result that at the start of the opening motion the sealing surface of the nozzle needle moves away from the nozzle seat more quickly in comparison with a known nozzle needle. The same effect also arises in the course of the closing motion nozzle needle, so that the speed of the sealing surface also increases in the course of the approach of the nozzle needle to the nozzle seat, and hence the seat-throttle region is passed through more quickly. For more detailed explanatory remarks on this effect, reference is made to the description.
In an advantageous configuration, the longitudinal stiffness of the elastic portion is less than 20,000 N/mm, particularly preferably 12,000 N/mm to 16,000 N/mm. Within these ranges of longitudinal rigidity the maximum effect is obtained, without the stability of the nozzle needle and the manufacturability of the nozzle needle becoming technically problematic.
In a further advantageous configuration, the longitudinally elastic portion takes the form of a circular cylinder, the material of the nozzle needle preferentially being steel. In this case the longitudinally elastic, circular cylindrical portion preferably has a diameter from 1.3 mm to 2.0 mm, preferably between 1.4 mm and 1.6 mm. In this case the modulus of elasticity of the steel preferably has a value from 200,000 N/mm2 to 230,000 N/mm2, preferentially 210,000 N/mm2.
In a further advantageous configuration, the cylindrical elastic longitudinal portion has a length from 20 mm to 30 mm, preferentially 25 mm to 27 mm. Such a length can be accommodated without difficulty in the normal injection nozzles such as are preferentially used for fuel injectors, without the construction space of the nozzle having to be increased in comparison with the models known hitherto.
In a further advantageous configuration, the sealing surface of the nozzle needle has an annular sealing line with which in the closed state of the injection nozzle it rests on the nozzle seat and seals the pressure chamber against the injection ports. This sealing line has the same diameter as the diameter of the longitudinally elastic portion, so that in this region of the nozzle needle no resultant hydraulic forces are exerted on the nozzle needle in the longitudinal direction by the fuel pressure within the pressure chamber.
In a further advantageous configuration, guide portions are located on the nozzle needle respectively upstream and downstream of the elastic longitudinal portion, with which the nozzle needle is guided in the radial direction in the pressure chamber. These guide portions are constituted, for example, by diameter extensions, in which case passages, which ensure a throttle-free flow of fuel to the injection ports within the pressure chamber, have been formed on the guide portions.
In a further advantageous configuration, the nozzle needle with its end facing away from the sealing surface is accommodated in a sleeve which radially delimits the control chamber. In this case a closing spring under initial compressive tension, which exerts a closing force on the nozzle needle in the direction toward the nozzle seat, is advantageously arranged between the sleeve and the nozzle needle. The closing spring ensures that the nozzle needle remains in abutment on the nozzle seat even when the internal-combustion engine has been switched off, and in this way a dripping of fuel into the combustion chamber does not happen, even in the absence of pressure in the control chamber.
Advantageously, a fuel injector for injecting fuel into a combustion chamber of an internal-combustion engine is equipped with an injection nozzle according to the invention.
An injection nozzle according to the invention is represented in the drawing. Shown are:
In
A piston-shaped nozzle needle 3, which is represented here in highly schematic manner, is arranged in longitudinally displaceable manner in the pressure chamber 4. The nozzle needle 3 has a longitudinally elastic portion 25 which is symbolized here by a spring but consists, for example, of a tapered cylindrical portion of the nozzle needle 3. The nozzle needle 3 has a sealing surface 6 with which the nozzle needle 3 interacts with the nozzle seat 5 which has been formed at the combustion-chamber end of the nozzle body 2, so that when the sealing surface 6 is in abutment on the nozzle seat 5 one or more injection ports 8 which have been formed in the nozzle body 2 are sealed against the pressure chamber 4. If the nozzle needle 3 lifts off from the nozzle seat 5 in the longitudinal direction, fuel from the pressure chamber 4 flows through between the sealing surface 6 and the nozzle seat 5 to the injection ports 8 and is ejected through them.
The end of the nozzle needle 3 facing away from the sealing surface 6 has an end surface 9 which delimits a control chamber 10. The control chamber 10 can be filled with fuel under high pressure via an inflow throttle 13 which branches off from the high-pressure line 21. Furthermore, the control chamber 10 has been connected to an outflow throttle 14 which can be connected to a low-pressure line 20 via a control valve 18, said low-pressure line 20 leading back into the fuel tank 7. If the control valve 18 is in its opening position, as represented in
The function of the elastic portion 25 is as follows and will also be explained in the following with reference to
The high fuel pressure in the control chamber 10, which in modern injection systems may amount to more than 2000 bar, brings about a hydraulic force FS1 on the end surface 9 of the nozzle needle, which is symbolized in
If the nozzle needle 3 now lifts away from the nozzle seat 5, the sealing surface 6 of the nozzle needle 3 is infiltrated by the fuel pressure of the pressure chamber 3, so that an increased hydraulic force Fd3 is now also acting on the sealing surface 6, as represented in
In the course of the closing motion of the nozzle needle 3 toward the nozzle seat 5, the sealing surface 6 comes into the vicinity of the nozzle seat 5, throttling the flow of fuel and hence the fuel pressure in the region of the sealing surface 6, so that the hydraulic force Fd4 distinctly decreases, as represented in
The represented cyclic axial compression and relaxation of the nozzle needle 3 in the longitudinal direction by virtue of the elastic portion 25 brings about an additional acceleration of the sealing surface 6 in the course of lifting away from the nozzle seat 5. In this regard, in
At time t3 the nozzle needle 3 reaches a position at which the seat throttling between the sealing surface 6 and the nozzle seat 5 results in a marked lessening of the hydraulic force on the sealing surface 6. As a result, the nozzle needle 3 lengthens again, resulting in an increase in the relative change in length Δl again to the value Δl2 up until time t4, as represented in
In comparison with a known nozzle needle and its opening-stroke motion, which is determined exclusively by the hydraulic pressure within the control chamber, ultimately the following effect arises: as soon as the nozzle needle 3 begins its opening-stroke motion—that is to say, lifts away from the nozzle seat 5—the pressure infiltration of the sealing surface 6 commences and axially compresses the nozzle needle 3, this occurring in
The effect according to the invention can also be explained and quantified as follows: once the pressure in the control chamber 10 falls, the end surface 9 of the nozzle needle 3 moves into the control chamber, without the sealing surface 6 moving for the time being. In the case of a longitudinal stiffness of the elastic portion of the nozzle needle of, for example, 15,000 N/mm, this effect amounts to approximately 30 μm if the nozzle needle consists of a customary steel with a modulus of elasticity of approximately 210,000 N/mm2 and the diameter of the elastic portion amounts to 1.5 mm in the case of a length of 26 mm, the longitudinally elastic portion being of circular cylindrical design. As soon as the lengthening of the nozzle needle 3 has been concluded, the sealing surface 6 moves away from the nozzle seat 5 at a certain opening speed.
By virtue of the pressure infiltration of the sealing surface 6, the nozzle needle 3 is now axially compressed again, so that the elastic deformation of the nozzle needle 3 is added to the speed of motion of the nozzle needle 3. The sealing surface 6 accordingly moves away from the nozzle seat 5 more rapidly than it would do without the elastic portion 25.
The longitudinal stiffness is defined as follows:
In general, for the strain εx in the longitudinal direction of the nozzle needle (here: the x-direction) it holds that
εx=E−1·[σx−v·(σy+σz)]
Here, σx, σy and σz are the stresses in the respective direction in space, v is Poisson's ratio, and E is the modulus of elasticity. For the following consideration, however, the strain contribution by the hydrostatic pressure in the pressure chamber (stresses σy and σz) can be neglected, since this contribution remains practically unchanged during the entire injection cycle. The above relationship is then simplified, in a manner analogous to a unidirectional load, to give
σ=E·ε
In the following consideration, a longitudinally elastic portion will be assumed which consists of a solid cylindrical portion of the nozzle needle, with a diameter d, a cross-section A and a length L. If the stresses σ of the above equation are replaced by F/A, there results
F/A=E·ε
The strain ε is given as the quotient of the relative change in length ΔL and the overall length L of the portion—that is to say, ε=ΔL/L. If the two are substituted into one another, there results
F/A=E·ΔL/L
or
F=E·A/L·ΔL
The proportionality factor between the force F and the relative change in length ΔL is designated as the longitudinal stiffness c, which is therefore given by the following relationship:
c=F/ΔL=E·A/L
If the value of E=210,000 N/mm2 which is typical of steel is inserted, and a diameter d of the longitudinally elastic portion 25 of 1.5 mm and a length L of 26 mm, a longitudinal stiffness arises of
c=210,000 N/mm2·π/4·(1.5 mm)2/26 mm≈14,300 N/mm
But good effects are already obtained also in the case of a higher longitudinal stiffness c, though the longitudinal stiffness c should be less than 40,000 N/mm, in order that an effect in an injection nozzle is to be observed.
In
In the embodiment represented, between a guide portion 103 of the nozzle needle 3 and the shoulder 36 there is located a further elastic longitudinal portion 26 of the nozzle needle 3, which has a diameter dj which corresponds at least approximately to the diameter d of the elastic longitudinal portion 25. By virtue of the further elastic longitudinal portion 26, the overall stiffness of the nozzle needle 3 can be lowered further if, for example, the elastic longitudinal portion 25 cannot be manufactured in the necessary length for reasons of space.
The overall longitudinal stiffness cges of the elastic longitudinal portions then amounts to
c
ges=1/(c1−1+c2−1)
if c1 and c2 are the longitudinal stiffnesses of the two elastic portions 25, 26. The overall longitudinal stiffness cges preferentially lies below 20,000 N/mm.
For the purpose of ensuring the flow of fuel within the pressure chamber 4 in the direction of the injection ports 8, on the first guide portion 30 and on the second guide portion 31 one or more polished sections 33 and 34, respectively, have been attached to the outside of the guide portions 30, 31, so that an unthrottled flow of fuel can occur past the guide portions 30, 31 in the direction of the injection ports 8.
In addition to the design of the elastic longitudinal portion 25 in the form of a circular cylinder with reduced diameter, it is also possible to construct this elastic longitudinal portion in a different manner, for example by a higher longitudinal elasticity being obtained by virtue of recesses in the nozzle needle. However, the design by virtue of a reduction of diameter is the simplest way to construct such a longitudinally elastic portion without the costs of manufacture of the nozzle needle rising appreciably as a result.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2014 226 407.3 | Dec 2014 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/074892 | 10/27/2015 | WO | 00 |
