Patent Application
20070176025
The invention relates to a corrosion resistant magnetic component, and in particular to a magnetic component for use in a magnetically actuated fuel injection valve which operates in a corrosive environment.
Magnetically actuated devices, such as solenoid valves are used in many types of systems including automotive applications such as fuel injection, anti-lock braking and active suspension systems.
Magnetically actuated devices typically include a magnetic coil and a moving magnetic core or plunger. In a typical arrangement of a solenoid valve 10, as shown in
It is desirable that the material used to make the magnetic core have good soft magnetic properties, principally, a low coercive field strength to minimize “sticking” of the component and a high saturation induction to minimize the size and weight of the component.
The plunger is often in direct contact with the local environment such as the fluid that is being controlled. Many environments and fluids are corrosive and will corrode the plunger, which may cause the device to malfunction or the valve to leak or become inoperative. It is, therefore, desirable that the plunger be formed of a material that has good resistance to the corrosive influence of the environment in which it is to be used.
The increasingly frequent use of magnetically actuated valves in automotive technologies as fuel injection systems has created a need for a magnetic material having improved corrosion resistance. The need for better corrosion resistance is of particular importance in automotive fuel injection systems in view of the introduction of more corrosive fuels such as those containing ethanol or methanol.
It is known to use ferritic steels for the magnetic component of fuel injection valves, but the corrosion resistance has been found to be insufficient in corrosive fuel environments.
It is, therefore, an object of the invention to provide a magnetic component for a magnetically actuated fuel injection device which is suitable for use in corrosive fuel environments and, in particular, methanol-containing or ethanol-containing fuel mixtures.
It is also desirable that the magnetic component has a saturation induction, a coercive field strength and an electrical resistivity which are sufficient for future requirements, in particular, for the fine control required by future fuel injection systems in order that the engine fulfils future environmental emissions legislation.
Additionally, it is desirable that the magnetic component is easily machined so that manufacturing costs are not increased and the components can be manufactured with the required tolerances and surface finish.
According to the invention, a magnetic component for a magnetically actuated fuel injection device is provided. The magnetic component is formed of a corrosion resistant soft magnetic alloy consisting essentially of, in weight percent, 3%<Co<20%, 6%<Cr<15%, 0%≦S≦0.5%, 0%≦Mo≦3%, 0%≦Si≦3.5%, 0%≦Al≦4.5%, 0%≦Mn≦4.5%, 0%≦Me≦6%, where Me is one or more of the elements Sn, Zn, W, Ta, Nb, Zr and Ti, 0%≦V≦4.5%, 0%≦Ni≦5%, 0%≦C<0.05%, 0%≦Cu<1%, 0%≦P<0.1%, 0%≦N<0.5%, 0%≦0<0.05%, 0%≦B<0.01%, and the balance being essentially iron and the usual impurities.
The magnetic component according to the invention has excellent corrosion resistance in corrosive fuel environments and soft magnetic properties suitable for a magnetically actuated fuel injection valve, in particular a high saturation polarization, Js, low coercive field strength, Hc, and a high resistivity, ρ. The magnetic component also has good machining properties.
In this description, all compositions are given in weight percent, wt %.
In further embodiments of the invention, the Co-content of the magnetic component lies in the ranges 6%<Co <16% or 10.5%<Co<18.5%. For applications in which a high Js is desirable, a higher Co content may be provided. Since Cobalt is a relatively expensive element, it may desirable to use a lower cobalt content for applications in which it is desired to reduce the materials cost.
The alloy may contain 0.01%≦Mn≦1% and 0.005%≦S≦0.5% or 0.01%≦Mn≦0.1% and 0.005%≦S≦0.05%. In a further embodiment, the ratio of manganese to sulphur, Mn/S, is ≧1.7. The provision of manganese and sulphur additions within these ranges further improves the free machining properties of the alloy. The alloy may comprise Titanium in the place of manganese and, therefore, may contain 0.01%≦Ti≦1% by weight. Ti also improves the free machining properties of the alloy and has the additional advantage that it improves the magnetic properties and corrosions resistance of the alloy.
The sum of Cr and Mo may lie in the range 11%≦Cr+Mo≦19% and in a further embodiment, the sum of Si+1.3Al +1.3Mn+1.7Sn+1.7Zn+1.3V≦3.5%.
The polarization J of the magnetic component at a magnetic field H of 160 A/cm may be greater than 1.6 T or greater than 1.7 T. The saturation polarization Js of the magnetic component at a magnetic field H of 600 A/cm may be greater than 1.75 T or greater than 1.8 T. A high value of the saturation polarization Js enables the size and weight of the magnetic component to be reduced.
The magnetic component may have an electrical resistivity, ρ, which is greater than 0.4 μΩm or greater than 0.5 μΩm or greater than 0.58 μΩm. A higher value of resistivity, ρ, leads to a reduction in eddy currents after the magnetic field is applied or removed to the magnetic component. Damping of the eddy currents improves the responsiveness of the device. This can be advantageously used in optimization of the control of the fuel injection device at high engine revolutions.
The fuel injection device, according to the invention, may be used in a gasoline engine or a diesel engine. In this context, gasoline engine is used to denote an engine designed to operate with a gasoline fuel supply and diesel engine is used to denote an engine designed to operate with a diesel fuel supply.
The fuel injection site and the environment under which the fuel injection device operates, for example pressure and engine revolutions, is different in gasoline engines and diesel engines. The corrosiveness of the environment in which the magnetic component of the fuel injection device operates may, therefore, differ in addition to the desired magnetic and electrical properties of the magnetic component. Therefore, the composition most suitable for a fuel injection device for a gasoline engine and the composition most suitable for a fuel injection device for a diesel engine may differ although both compositions lie within the ranges of the invention. In a further embodiment, the fuel injection device is a direct fuel injection valve.
In an embodiment of the invention, the magnetic component is for use in an environment comprising a mixture of fuel and an alcohol, wherein the fuel is one of gasoline and diesel. Fuel mixtures including an alcohol are known to be extremely corrosive. These fuel mixtures may also comprise a small quantity of water in a form commonly described as corrosive water.
In further embodiments, the mixture comprises 90% gasoline and 10% alcohol or 85% gasoline and 15% alcohol or 80% gasoline and 20% alcohol.
The alcohol may comprise methanol, ethanol, propanol, butanol or a mixture of two or more of methanol, ethanol, propanol and butanol.
Fuel mixtures of gasoline and alcohol are often found to be more corrosive than fuel mixtures of diesel and alcohol. Consequently, a composition particularly suitable for use in a gasoline/alcohol fuel mixture environment and a composition particularly suitable for use in a diesel/alcohol fuel mixture environment may differ although both compositions lie within the ranges defined by the invention.
In an embodiment, the alcohol is methanol. In further embodiments, the mixture comprises 90% gasoline and 10% methanol or 85% gasoline and 15% methanol or 80% gasoline and 20% methanol.
In an embodiment, the alcohol is ethanol. In further embodiments, the mixture comprises 90% gasoline and 10% ethanol or 85% gasoline and 15% ethanol or 80% gasoline and 20% ethanol.
Similarly, fuel mixtures of gasoline and methanol or ethanol are often found to be more corrosive than fuel mixtures of diesel and methanol or ethanol. For example, a composition particularly suitable for use in a gasoline/methanol fuel mixture environment and a composition particularly suitable for use in a diesel/methanol fuel mixture environment may differ although both compositions lie within the ranges defined by the invention.
Table 1 Table showing the composition of the batches of alloys according to the invention.
Table 2 Table showing coercive field strength, Hc, as a function of annealing temperature
Table 3 Table showing the electrical resistivity, ρ, measured for samples with different Co-contents.
Table 4 Table showing a comparison of the magnetic and electrical parameters of the alloys according to the invention and commercially available alloys.
Table 5 Table showing the results of corrosion tests at 85° C. and 85% humidity.
Table 6 Table showing the results of corrosion tests in a gasoline/methanol/corrosive water solution.
Table 7 Table showing results of corrosion tests in a sulphate, nitrate and chloride-containing solution.
Five FeCrCo-based alloys of differing composition were fabricated by melting and casting 5 kg of each composition. Each alloy comprised 13 wt % chromium and the cobalt content was varied from 0 wt % to 20 wt %. The composition of each of the five batches is listed in Table 1.
Each of the cast blocks was turned to a diameter of 40 mm. The blocks were heated to a temperature of 1200° C. and then hot rolled to a diameter of approximately 12 mm. The samples were then etched in hydrochloric acid and aqua regia.
Each sample was swaged from a diameter of 12 mm to a diameter in the range of 10.47 mm to 10.66 mm. The rods were then degreased and cold-drawn to a diameter of 10 mm. From each of these rods, ten measurement samples, each with a length of 100 mm, were cut for annealing experiments and magnetic measurements. From each alloy composition, a measurement sample was annealed at a temperature between 500° C. and 1150° C. in a hydrogen atmosphere for five hours.
The coercive field strength Hc (A/cm) was measured for each of the compositions and annealing temperatures and the results are summarized in Table 2 and
A low value of Hc is desired for the magnetic component of magnetically actuated devices. Hc is inversely proportional to the permeability, μ. A high permeability leads to a reduction in the electric current required to achieve a given flux density. A low value of Hc permits rapid magnetization and demagnetization and enables the valve to be quickly opened and closed. This is particularly desirable in fuel injection systems and in particular for fuel injection systems for petrol motors where the rpm of the engine is high.
As can be seen in Table 2 and
However, the batch with a Cobalt content of 20 wt % shows a different type of behavior. For this composition, the lowest value of the coercive field strength, Hc, was reached at an annealing temperature of 550° C. For higher annealing temperatures, the coercive field strength, Hc, increases to over 30 A/cm after annealing at 700° C. and then decreases again with increasing temperature for annealing temperatures between 700° C. and 1000° C.
The polarization J for applied magnetic fields H of up to 600 A/cm was measured for samples of each of the compositions and each of the annealing temperatures. The results of these experiments are shown in FIGS. 3 to 11.
The relationship between the polarization at a measurement magnetic field of 160 A/cm (J160) and the annealing temperature is summarized in
The relationship between the saturation polarization Js at a measurement magnetic field of 600 A/cm (J600) and the annealing temperature is summarized in
A high value of Js is desirable so that the size and weight of the magnetic component may be reduced. For a magnetic field of 160 A/cm, a value of J160 of above 1.7 T is observed for the alloys with a cobalt content of 6 wt % and 9 wt % and an annealing temperature of 650° C. and 700° C.
The electrical resistivity, ρ, was also measured for each of the batches and is shown in Table 3. It is desirable that the electrical resistivity be as high as possible to dampen eddy currents and improve the responsiveness of the device. The resistivity, ρ, was measured to increase from 0.428 μΩm for the alloy containing 0 wt % cobalt to 0.768 μΩm for the alloy containing 20 wt % cobalt.
The alloy comprising 9 wt % Co, 13 wt % Cr, rest Fe showed the best soft magnetic characteristics for annealing conditions of 700° C. for five hours. The highest saturation polarization value, Js, also the polarization at a field of 160 A/cm, J160, was also attained for this composition and the coercive field strength, Hc, which lies at 1.57 A/cm is also reasonably low. The resistivity is increased to 0.582 μΩm which is advantageous for the dynamics of fuel injection valves.
Table 4 compares the values of Hc, Js, J160, μ and ρ for a composition of 13 wt % Cr, 9 wt % Co, rest Fe with the composition 0 wt % Co, 13 wt % Cr, rest Fe, commercially available pure Fe (VACOFER™ S1) and a commercially available FeCo alloy (VACOFLUX® 17) of composition 17 wt % Co, 2 wt % Cr, 1 wt % Mo, rest Fe.
As shown in Table 4, an alloy comprising 9 wt % Co, 13 wt % Cr, rest Fe has a value of saturation polarization at a field of 160 A/cm, J160, which is approximately 0.1 T higher than that observed for a binary alloy comprising 13 wt % Cr, rest Fe. The resistivity is also increased by around 0.15 μΩm over that measured for the binary alloy comprising 13 wt % Cr, rest Fe.
The composition of 9 wt % Co, 13 wt % Cr, rest Fe has a higher resistivity, but a slightly lower Hc, Js and J160 compared to pure Fe. However, as will be seen in the results from the corrosion experiments, the corrosion resistance of the 13 wt % Cr, 9 wt % Co, rest Fe is significantly improved over that of pure Fe.
The corrosion resistance of the five batches in addition to two commercially available alloys (VACOFLUX® 17 and VACOFLUX® 50 (49 wt % Co, 2 wt % V, rest Fe)) were investigated. In a first test, pieces of each batch were subjected to an environmental test at 85° C. and 85% humidity. The results of observational examination are summarized in Table 5.
After 14 days exposure, the alloys with cobalt contents of between 3 wt % and 9 wt % did not show any signs of corrosion.
The corrosion behavior of the alloys was also investigated for a gasoline/methanol/water environment. A solution comprising 84.5% gasoline, 15% methanol and 0.5% corrosive water was prepared. The corrosive water comprised 16.5 mg of sodium chloride per litre, 13.5 mg of sodium hydrogen carbonate per litre, and 14.8 mg of Formic acid. The samples were immersed in the solution for 150 hours at 130° C. The results of this test are shown in Table 6. The tests were optically observed under an optical microscope at a magnification of 16 times. Samples with 0 wt %, 3 wt % and 9 wt % cobalt respectively were not observed to show any signs of corrosion.
In a third corrosion test, samples were immersed in a sulphate, nitrate and chloride containing-solution. The solution comprises 1000 ppm sulphates, 500 ppm nitrates, 100 ppm chlorides and has a pH of 1.6. The samples were immersed in the solution for 11 days at 60° C. The results of this test are shown in Table 7.
As can be seen from Table 7, samples with 6 wt % cobalt and 9 wt % cobalt fulfilled the criterion of group 2 and are denoted as sufficiently corrosive resistant.
