WO1996024764A1

WO1996024764A1 - Fluid proportioning and atomizing device

Info

Publication number: WO1996024764A1
Application number: PCT/DE1996/000154
Authority: WO
Inventors: Andreas Kappel; Randolf Mock; Hans Meixner
Original assignee: Siemens Aktiengesellschaft
Priority date: 1995-02-07
Filing date: 1996-02-01
Publication date: 1996-08-15
Also published as: EP0808419A1; DE19504175C2; EP0808419B1; DE59609753D1; DE19504175A1

Abstract

The invention concerns a fluid proportioning and atomizing device which should be easy to manufacture and have a long useful life. High demands are also placed on the proportioning accuracy and quality of atomization. The device of the invention comprises a fluid-filled chamber (K) in which a heating element (H) for evaporating the fluid is disposed. If the vapour pressure in the chamber (K) rises above a specific threshold value, a spray valve (AV) opens the chamber (K) and the fluid is injected through a spray nozzle (ED).

Description

description

Fluid metering and atomizing device

The invention relates to a device for metering and atomizing fluid. A device of this type is known from injection valve technology.

Stricter exhaust gas values place increasingly higher demands on the accuracy of fuel metering and mixture formation. Modern engine management concepts can contribute to a higher environmental compatibility of the motor vehicles. The fuel injection valves play a central role here, since the improvement in the combustion engine is decisively determined by their performance characteristics.

Since the amount of fuel injected per work cycle is controlled via the opening time of the fuel injection valve, an injection valve is required for accurate and reproducible spraying of even small amounts of fuel, in which the opening or closing process takes very little time compared to the opening time. This is of great importance, especially in the part-load and idle mode of the engine. Because of the low load on the engine, relatively small amounts of fuel have to be dosed very precisely per work cycle.

An electrothermal fuel injection system for measuring fuel is known from the prior art US Pat. No. 5,165,373. Pulsed thermal energy produces a change in the volume of the fuel. This change in volume is used to deliver the fuel via a fuel valve. A computer and sensors are provided to determine the pulse width, height and the number of pulses taking into account the motor properties. O 96/24764

The object of the invention is to provide a device for metering and atomizing fluid, which has few moving parts and whose manufacture places low demands on the dimensional accuracy of the individual parts.

The object is achieved by a device according to patent claim 1.

Advantageous configurations result from the dependent claims.

A heating element according to claim 2 is particularly suitable because of the large active surface and the low thermal time constant.

In order to keep heat losses during the heating process as low as possible and to protect the heating element, a structure according to claims 3 and 4 has proven to be advantageous.

Furthermore, the heating element described according to claims 2, 3 and 4 has the advantage that the pressure in the chamber is largely constant after a threshold value of the control voltage is exceeded and is independent of the operating voltage.

In order to be able to meter the amount of liquid to be sprayed even more precisely, the heating element can be designed as specified in claim 5.

With a heating element dimensioned according to claim 11, the thermal time constant and thus the speed of the device can be further improved.

With an additional heating element according to claim 12, the metering accuracy can be further improved. The invention is explained in more detail below with reference to several figures.

Figure 1 shows a possible embodiment of the device according to the invention.

Figure 2 shows the same device as in Figure 1, but during the filling phase.

Figure 3 shows the same device as in Figure 1, but during the spraying phase.

FIG. 4 shows the pressure curve in the chamber of the device according to FIGS. 1-3.

Figure 5 shows a further possible embodiment of the device according to the invention.

Figure 6 shows the same device as in Figure 5, but during the spraying phase.

Figure 7 shows the layer structure of a heating element.

Figure 8 shows a special embodiment of the heating element.

FIG. 9 shows the electrical equivalent circuit diagram corresponding to the heating element shown in FIG. 5.

Figure 10 shows two pressure diagrams.

Figure 11 shows a pressure control voltage characteristic of a heating element.

The device for metering and atomizing fluid shown in FIG. 1 has a chamber K which is provided with an inlet Z and an outlet A. In the chamber K is a Backflow valve RV arranged, which closes or opens the inlet Z. A discharge valve AV is provided in sequence A. In the idle state (FIG. 1), the backflow valve RV locks the inlet Z and the spray valve AV the outlet A. In the following, the term closable inlet is used for the backflow valve RV in connection with the inlet Z. In the idle state, both valves RV and AV are pressed against the valve seats VS via springs F1, F2. Both springs Fl, F2 are supported on a fixed bearing. Furthermore, a heating element H is arranged in the chamber K, which is connected to a control electronics (not shown) via two connecting lines AL.

The device works as follows:

A fluid is pressed into the chamber K via the inlet Z, as shown in FIG. 2, with the backflow valve RV open. The filling pressure is determined as p _z - PR, where p _{z is} the inflow pressure and PR is the opening pressure of the backflow valve RV. The pressure PR, which is necessary to open the backflow valve RV, can be adapted to the inflow pressure p _z via the spring hardness and the spring preload of the spring F2 in the backflow valve RV, as well as via the geometry of the backflow valve RV. The spray valve AV remains closed due to the stronger preload force of the spring F1. If the chamber K is completely filled with fluid, the backflow valve RV closes the chamber K.

In order to prevent gas, for example air, from being drawn in from the outside through the spray valve AV into the chamber K, the opening pressure p _{a of} the spray valve AV is above the opening pressure PR of the backflow valve RV. This ensures that when the pressure in the chamber K drops, the spray valve AV closes first.

As shown in FIG. 3, an electrical control pulse I is applied to the heating element H via the connecting lines AL. sets. As a result, parts of the liquid which are in contact with the surface of the heating element become extremely overheated within a very short time. The liquid evaporates, an expanding vapor bubble EDB is created, which briefly creates a high excess pressure (based on the chamber pressure p _z - PR) in the closed liquid volume in the chamber K, which is greater than the restoring force (the opening pressure p _a ) the spray valve is AV and this opens it. Part of the fluid in the chamber K is sprayed off through the injection nozzle ED.

If the vapor bubble collapses again, the pressure in the chamber K is reduced and if the discharge valve opening pressure p _a is undershot, the discharge valve AV closes automatically. The missing amount of liquid sprayed off in the chamber K leads to a negative pressure (in relation to the stationary chamber pressure level p _z - PR) when the vapor bubble collapses further, which causes the opening of the backflow valve RV and the refilling of the chamber K. After refilling, the process described above can be triggered again.

The parts of the liquid which are in contact with the heating surface are overheated within a very short time, the physical limit of the overheatability being the temperature of the liquid at the critical point.

4 shows in the upper part a pressure / time diagram in which the internal pressure Pj is plotted on the ordinate and the time t is plotted on the abscissa. I is an electrical control pulse. A control voltage / time diagram is shown in the lower part of FIG. 4, in which the control voltage U is plotted on the ordinate and the time t is plotted on the abscissa. During the evaporation process - there is a phase transition from liquid to vapor - a high overpressure occurs briefly in the closed chamber K, which leads to the liquid being sprayed out of the spring-loaded spray valve AV. soon the pressure p ^ in the chamber K exceeds the discharge valve opening pressure p _a . The evaporation process leads to the formation of the vapor bubble EDB expanding under high chamber pressure pjζ. It is fed by the amount of heat that is stored in the thin, overheated liquid layer above the heating element. The expansion of the vapor bubble continues as long as the heat of vaporization required for further vaporization of liquid at the phase interface of the vapor bubble can be brought about from parts of the previously overheated thin liquid layer by heat conduction. The upper hatching in the diagram indicates the spraying process AV, while the lower hatching represents the filling process BV.

The peak pressure (= chamber pressure pjς) can reach some 100 bar. As a result, the fluid is injected in the form of a much finer aerosol than is the case with the electromagnetic injection valves operating at 2 to 3 bar injection pressure.

Typical values are for:

Spray valve opening pressure p _a : 5 bar inlet pressure p _z : 3 bar backflow valve opening pressure PR: 1 bar

FIG. 5 shows a further embodiment of the device according to the invention. The chamber K here has an inlet Z, a return R and an outlet A. The inlet Z can be hermetically separated from the chamber K by a first shut-off valve AVI. The return R can be hermetically separated from the chamber K by a second shut-off valve AV2. The sequence A is designed here in the same way as shown in FIG. 1 and already described above. Just as in FIG. 1, there is a heating element H in FIG. 5 in the chamber K, which is connected to control electronics (not shown) via connecting lines AL. The AVI and AV2 can be driven electrically, hydraulically, pneumatically or mechanically.

The device according to FIG. 5 works as follows:

During the rinsing phase, which is synonymous with the filling phase, the two shut-off valves AVI and AV2 are open, i.e. the liquid flows from the inlet Z through the chamber K into the return R.

When the flushing process has ended, compare FIG. 6, the two shut-off valves AVI and AV2 are closed, ie the connection between inlet Z and chamber K and the connection between chamber K and return R is interrupted. A voltage applied to the heating element H means that the liquid on the heating element surface evaporates very quickly and the pressure in the chamber increases correspondingly quickly. When the spray valve opening pressure p _{a is} exceeded, the spray valve AV opens and the liquid is sprayed through the injection nozzle ED. As soon as the pressure in the chamber K falls below the spray valve opening pressure p _a , the spray valve AV closes automatically. The two shut-off valves AVI and AV2 can be opened and the process described above can start again.

During the rinsing phase, the liquid flows through the chamber K to refill and expel residual vapor bubbles.

The state of severe overheating can only be maintained in the liquid for a very short time, since the slightest unavoidable disturbances, such as thermal fluctuations, induce the phase transition from liquid to vapor. Such a condition is called metastable.

It follows that the liquid must overheat within a period of time that is less than the decay rate. θ time for this metastable aggregate state. A planar thin-film or thick-film heating element is particularly well suited for this purpose. Despite a large effective surface, it has a very small thermal time constant τ _therm (0.1μs <T hβrm <100ms).

The thermal time constant τ _thβrm of such a thin-film heating element, as shown in FIG. 7, is determined by the heat capacity of the resistance layer and the sum of the heat capacities of the electrical, chemical and mechanical protective and passivation layers DS, DF and WB, in which the heating element H must be embedded to increase the service life.

As shown in FIG. 7, the heating element H has several layers. In addition to the protective function mentioned, the layer system has to perform a differentiated thermal function. In order to keep heat losses low during the heating phase, the heat source WQ, also referred to as the heating layer, is made of the thermally highly conductive Si substrate S, also referred to as the heat sink (specific thermal conductivity λ Si = 150W / mK) by a µm the factor 100 worse heat-conducting heat barrier WB (specific thermal conductivity λ Si02 ⁼ lf5W / mK) of thickness d is separated from SiO 2. The penetration depth δ of the heat source WQ, which is in the form of a

Resistance layer is formed, within the heating time Δt propagating heat wave can be approximated using the formula

δ = 1.28 ^■ Yes • Δt

can be calculated, where a is the thermal conductivity and is derived from

can be determined, λ is the specific thermal conductivity, p the density and c the specific heat capacity. In order to decouple the heating layer WQ from the silicon substrate S in a thermally effective manner during the heating-up time Δt, the thickness d of the heat barrier WB must be greater than the penetration depth δ, ie the thickness d of the heat barrier WB must be chosen such that the the heat wave propagating from the heat source WQ does not reach the Si substrate S within the heating time Δt. For typical heating times of Δt = 6μs, for example, the minimum required thickness d of the heat barrier WB is d> 3μm, the temperature conductivity being agi _Q 2 = 0.874-10 ^{~ 6} m ² / s. For a new actuation process, the heat source WQ and the rest of the layer structure must have previously cooled to the lower ambient temperature below the boiling point of the liquid F. It is therefore not expedient to increase the thickness d of the heat barrier WB, since firstly the energy requirement cannot be reduced further and secondly the repetition frequency decreases due to the longer cooling pauses.

In order to achieve a stable mode of operation and a high degree of efficiency, the heat generated in the heat source WQ must be transferred into the liquid as quickly and without loss as possible. This can be achieved by a cover layer material with a high temperature conductivity and a small thickness. Since the cover layer DS, also referred to as a heat-conducting layer, additionally has an electrical and chemical insulation function, dielectrics or materials with low electrical, deposited by means of plasma deposition processes (CVD, PECVD etc.) or sputtering are preferably suitable but as high a thermal conductivity as possible, such as Si3N4, SiC and limited Siθ2-

To protect the heating element surface from cavitation effects that occur when the vapor bubble collapses (= collapse), the cover layer DS can additionally be coated with a thin, preferably metallic thin film DF (= protective layer) Example tantalum can be covered. Although the thermal behavior deteriorates somewhat as a result, the service life of the heating element H increases considerably as a result of this measure.

The structure of the cover layer DS and the thin film DF must be individually matched to the liquid F used.

The heating element H can be produced, for example, by sputtering or vapor deposition of high-melting metals or their compounds and structured using phototechnical processes. The borides of the transition metals, in particular HfB2 and ZrB2, have proven to be particularly suitable materials for the heating resistance layer WQ. Because of its very high melting point T _m ≡ 3600 ° C, its very low temperature coefficient of electrical resistance Tjς = -70ppm and the very good adhesive strength on Siθ2, the HfB2 connection is particularly well suited for use as a heat source WQ (electrothermal transducer).

The adhesive strength is of great importance for the service life of the heating element H, since the very fast heating and cooling processes in the layer system result in very high thermomechanical stresses.

The following dimensions and materials have proven to be particularly suitable for the heating element H:

Layer Thickness Material Function / remark

Thin film DF 0.6 μm tantalum cavitation protection

Cover layer DS 1.0 μm Si3N4 passivation

Heat source WQ 0.2 μm HfB2 heating resistor

Thermal barrier WB 3.0 μm Si02 passivation

Heat sink S 560 μm Si substrate 1 - 10mm AI mechanical carrier

The amount of liquid sprayed is related both to the duration of action and to the level of the amount acting in the liquid Pressure proportional. The effective area of the heating element H can thus be used to influence the amount of liquid sprayed off. A particularly advantageous arrangement for heating is shown in FIG. 8. Here, the entire heating element H is subdivided into individual separately controllable individual heating elements EH, the areas of the individual heating elements EH being, for example, binary-weighted, ie if F _{Q is} the area of the smallest individual heating element EH is, the areas of the following individual heating elements EH are in ascending order 2 ¹ -FQ, 2 -FQ, ... 2 ⁿ -FQ, where n represents the number of individual heating elements EH.

In Figure 8, five individual heating elements EH are shown. This enables 32 pressure levels to be generated. If the heating surfaces of the individual heating elements EH each have the same width 1 and the same thickness D, a voltage control results in a constant area power density P _n / ^F n ⁼ const for all individual heating elements EH. , where the power P _{n of} the nth individual heating element EH

P _n = n • U / R ₀

and R "= —p _u ^• 1 results.

⁰ D • b

With a heating element H structured according to this method, individual control of the individual heating elements EH, see electrical equivalent circuit diagram in FIG. 9, can thus produce 2 ⁿ equidistant switching states, that is to say pressure stages. The entire heating element H thus represents a digital-to-analog converter with which it is possible to convert digital electronic voltage signals into equivalent analog pressure signals. Due to the proportionality between pressure amplitude / duration of action and injection quantity, the injection quantity can be metered in 256 steps, for example, in the case of a heating element H weighted eight times according to the above method. Since the spray nozzle opening pressure p _a must first be overcome in order to spray the liquid, it is advantageous to begin the linear working range of the voltage / pressure converter working according to the digital / analog converter principle only in the vicinity of the opening pressure p _{a of} the spray valve AV to leave. For this purpose, a pressure / heating surface diagram without base pressure and on the right with base pressure is shown in FIG. 10. This can be generated with the help of an additional individual heating element of the area Fg = k -FQ, where k is a constant adapted to the boundary conditions of the device. This base heating element, which is also operated in each case, is used to generate a base pressure which is generally somewhat lower than the opening pressure p _{a of} the spray valve AV. Depending on the design, this can either increase the pressure metering accuracy or increase the dynamic range.

An essential advantage of the pressure generation method described is that the pressure pulse is almost constant over a wide range and independent of the operating voltage after a threshold value of the control voltage U has been exceeded. This behavior, shown in FIG. 11, is based on the thermodynamics and kinetics of the active principle. The working range AB of such a thermodynamic heating element H is characterized by the appearance of a plateau in the pressure / control voltage characteristic. Therefore, a complicated and expensive adjustment procedure can be omitted. A working point near the left plateau area is preferred for practical operation. Since this is on the one hand sufficiently far from the unstable rising branch of the characteristic curve and on the other hand an unnecessary thermal overload of the heating element H is avoided. The thermal destruction of the heating element H occurs at point TZ of the characteristic curve.

The device according to the invention is designed as a fuel injection valve with an electrothermal liquid evaporator pressure-generating drive element suitable. A very precise dosing of even the smallest injection quantities at precisely defined times with a time resolution of a few μs is possible.

Additional advantages of the device are the good aerosol quality, the possibility of a simple (digital) electronic power control, the low wear and a long service life (few mechanically moving parts), the simple and inexpensive manufacturability of the device, which ultimately also one Actuator represents, with batch methods of thin film process technology, the small size of the actuator (a few mm ² actuator area) and the high working frequency.

The backflow valve RV, the spray valve AV and the shut-off valves AVI, AV2 do not have to be designed as shown in the figures. A possible embodiment is shown in the figures. However, the entire range of known valves is available. H. J. Matthies described in "Introduction to Oil Hydraulics", Teubner-Verlag, Stuttgart,

1991 in the chapter "Elements and devices for energy control and regulation (valves)" valves which can be used for the above-mentioned device.

Claims

claims

1. device for metering and atomizing fluid,

- in which a chamber (K) which can be filled with the fluid is provided, - in which a planar heating element (H) is provided in the chamber (K) for heating the fluid,

- in which the chamber (K) has a closable inlet (RV, Z; AVI, Z), which is closed during the heating process, and - in which the chamber (K) for delivering the fluid to the environment closable drain (A; AV2).

2. Device according to claim 1,

- In which the heating element (H) is a planar thin-film or thick-film heating element.

3. Device according to claim 1 or 2,

in which the heating element (H) has a heat-conducting substrate (S) over which a heat barrier (WB) is arranged,

- in which a heat source (WQ) is arranged above the heat barrier (WB),

- in which a heat-conducting layer (DS) is arranged above the heat source (WQ),

- In which a protective layer (DF) is arranged over the heat-conducting layer (DS).

4. The device according to claim 3,

where the thickness d of the thermal barrier (WB) of the inequality d> 1.28 • -v / a • Δt is sufficient,

- where a is the thermal conductivity of the heat barrier (WB) and Δt is the heating time of the heat source (WQ).

5. Device according to one of claims 1-4,

- in which the heating element (H) has several individual heating elements (EH),

- In which the individual heating elements (EH) have different sized effective heating surfaces.

6. Device according to one of claims 1-5,

- In which an outflow (A) is followed by an injection nozzle (ED).

7. Device according to one of claims 1-6, - in which the chamber (K) has a closable return (AV2) which is closed during the heating process.

8. Device according to one of claims 1-7,

- In which the drain (A) is a spring-loaded or electrically controllable valve.

9. Device according to one of claims 1-8,

- in which the closable inlet (Z) is a spring-loaded or electrically controllable valve.

10. The device according to claim 7, 8 or 9, - in which the closable return (R) is a federbelaste¬ tes or electrically controllable valve.

11. The device according to any one of claims 1-10,

- in which the heating element (H) has the following features:

Layer Thickness Material Function / remark

Thin film DF 0.6 μm tantalum cavitation protection

Cover layer DS 1.0 μm Si3N4 passivation

Heat source WQ 0.2 μm HfB2 heating resistor

Thermal barrier WB 3.0 μm Si02 passivation

Heat sink S 560 μm Si substrate 1 - 10mm AI mechanical carrier

12. The device according to one of claims 1-11,

- In which an additional heating element (H) is provided for generating a base pressure.

13. Use of the device according to one of claims 1-12, for metering and atomizing fuel.