The present invention relates generally to a heat generator for motor vehicles and primarily to one for reducing emissions from the internal combustion engine of a motor vehicle.
U.S. Pat. No. 4,484,049 discloses a water-cooled heat generator for the passenger compartment of a motor vehicle. The heat generator comprises a shaft driven by the vehicle engine, which shaft is common to the rotor in an electric generator and a rotor in the heat generator itself. Alternating current drawn from the stator winding of the electric generator is rectified and fed as magnetising current to the rotor in the heat generator. The latter also has a laminated stator with armature bars connected between two short-circuit rings, the bars like the short-circuit rings being hollow. The armature bars, in which the rotor of the heat generator generates induction currents as it rotates, are, like the short-circuit rings, cooled by water that circulates through them. The water thus heated is in turn used for heating the vehicle passenger compartment.
The said heat generator is bulky, complicated and also has poor efficiency.
U.S. Pat. No. 5,573,184 discloses a heat generator based on friction heating. This heat generator, too, is bulky owing to a high volume/output ratio, a relatively high-viscosity operating liquid moreover being required.
PCT/SE99/00283 describes a heat generator with a permanently magnetised, disc-shaped rotor, a stator, which is axially separated from the rotor and in which the rotor induces electric currents as it rotates, which generate heat in the stator, and adjoining the stator a cooling duct for a flowing liquid in order to dissipate the heat generated in the stator.
This heat generator is compact and provides very efficient heating of the cooling liquid, which is intended for use in heating the coolant in an internal combustion engine in an initial phase after starting the internal combustion engine.
Because of the highly efficient conversion from mechanical work, which drives the heat generator, to generated heat in the cooling liquid, it may be necessary in many applications to control the generated heat output according to a varying demand. This might obviously be done by corresponding variation of the mechanical power delivery, which drives the heat generator, by adjusting the speed of rotation of the rotor of the heat generator. Such a speed variation is not possible in all applications, however, since other requirements may be crucial for the magnitude of the mechanical power output, for example the power needed to propel a vehicle. In such a case the desired heat output can nevertheless be produced by connecting the heat generator intermittently in such a way that the mean value of the generated heat output corresponds to the desired heat output. This intermittent connection, however, presupposes a connecting arrangement, which requires space and may mean that the heat generator becomes complicated and/or undesirably expensive.
An object of the present invention is to provide a heat generator for motor vehicles, which is compact but still permits easy adjustment of the heat generated.
A heat generator according to the invention has a permanently magnetised, disc-shaped rotor; a stator, which is axially separated from the rotor and in which the rotor induces electric currents as it rotates, which generate heat in the stator; and adjoining the stator a cooling duct for a flowing cooling liquid in order to dissipate the heat generated in the stator. The rotor and the stator are furthermore axially moveable in relation to one another, in order to adjust the distance between them and thereby to adjust the heat generated in the stator or the heat output. This makes use of the fact that an adjustment of the distance between the rotor and the stator affects the magnitude of the currents. that the rotor induces in the stator and hence the magnitude of the heat generated by the said currents. Finally, means are arranged for determining the heat output generated in the stator by acting axially on the rotor in the direction of the stator with a variable force against the action of a repelling force, generated by the currents induced in the stator. The rotor can thereby be set to an axial position in relation to the stator depending on the heat output to be generated in the stator.
The stator preferably comprises two metallic layers, which define a narrow, substantially radial gap, which constitutes a part of the cooling duct designed for radial flow. In this way the distance over which the liquid in the cooling duct is heated is rendered relatively short, which is a pre-requisite for rapid heating utilising a high output.
In addition, the cooling duct suitably comprises two annular spaces, which adjoin the radial gap on that side thereof remote from the rotor and are designed for a circumferential flow of the cooling liquid. This design means that the cooling liquid follows intersecting paths on both sides of one of the two metallic layers, represented by the layer furthest from the rotor. Because of these intersecting flows of cooling liquid, such phenomena as evaporation and film boiling, which might otherwise occur as a result of the rapid heating and lead to cavitation and overheating, can be prevented.
The means for acting axially on the rotor in the direction of the stator, that is to say for adjusting the distance between the rotor and the stator depending, for example, on the desired heating of the flowing liquid, may be designed in many different ways, but they suitably comprise a soft magnetic material, which constitutes part of the stator, so that the magnetic field of the rotor is closed and a magnetic attraction force acts between the permanently magnetised rotor and the stator.
Of the two metallic layers, one nearest the rotor may comprise an electrically conductive, preferably non-magnetic material, and one furthest away from the rotor may comprise the soft magnetic material.
The force generated in the stator, which has a repelling action on the rotor, may be dimensioned so that at a predetermined speed of rotation it moves the rotor away from the stator against the action of the magnetic attraction force between the permanently magnetised rotor and the stator. This is achieved, for example, through the choice of stator magnetic material and through the choice of its volume and distance from the rotor.
The means for acting axially on the rotor may comprise spring means, pneumatic or hydraulic piston and cylinder units and/or electrically operated units. The said means may strengthen and/or weaken the magnetic attraction force or the repelling force in order to permit the achievement of a predetermined output/speed profile.
The means for acting axially on the rotor may furthermore comprise an override means for positively shifting the rotor to a desired output position or locking the rotor in a lower output position, for example when the entire output of the car engine is required for acceleration.
The means acting axially on the rotor may furthermore be controlled by an override signal from control electronics, which control the internal combustion engine, for limiting the heat output generated in the stator by positively shifting the rotor in relation to the stator, and achieving a position with limited output. Such an override signal might be generated by the control electronics in connection with acceleration, for example from stationary or in excess of a predetermined limit, the rotor being shifted to and locked in a lower output position.
By giving the means for applying force to the rotor in the direction of the stator a suitable force/distance profile, the heat output generated in the stator can thus take on a predetermined output/speed profile.
The currents that the rotor induces in the stator increase normally with increasing speed. In order to take account of this increased current, the distance of the rotor from the stator can also be suitably increased as the speed of the rotor increases. This increase may be used, for example, to achieve the desired heat output/speed profile for an adjusted rotor speed. In particular, the increase in the distance between the rotor and the stator may be initiated only in excess of a predetermined speed, in order thereby to limit the maximum output.
In another embodiment, the means for applying force to the rotor in the direction of the stator comprise a pneumatic piston, the force of which may be produced by negative pressure or excess pressure. If produced by negative pressure, the pneumatic piston may advantageously be combined with spring means, which likewise exert force on the rotor, and more specifically a diminishing force as the distance between the rotor and the stator increases. It is especially advantageous if the force characteristic of the said spring means is designed so that, at a constant rotor speed, an increasing negative pressure in the pneumatic piston moves the rotor closer to the stator.
As a safeguard against excessive heating of the cooling liquid, adjusting means may be provided alongside the cooling duct, which means are sensitive to the temperature of the cooling liquid and have the ability, when a predetermined temperature is reached, to act largely instantaneously on the rotor and to increase its distance from the stator to a value at which the heat generated in the stator is negligible. In this way, heating of the liquid flowing in the cooling duct in excess of a predetermined temperature value can be prevented, for example, where water is used as cooling medium, heating in excess of 100xc2x0 C. can be prevented, thereby preventing evaporation at normal pressure.
The design of the cooling duct is critical with regard to the high output (with an order of magnitude of tens of kilowatts) that a heat generator according to the invention can develop. According to the invention, short flow paths are produced in the most active heating zone closest to the rotor by designing the stator with a first disc extending radially along the disc-shaped rotor and with a second disc situated along the said first disc and on the opposite side to the rotor, in order to form a radial gap between them. The cooling liquid can be made to flow radially through the said gap in that it is connected to openings around its outer and its inner circumference.
In a further embodiment of the heat generator, the rotor comprises two axially separated rotor discs and the stator two stator discs, which are arranged between the two rotor discs and adjacent to a respective one of these. In this case distance adjustments are produced in that the rotor discs are axially moveable away from one another and from the respective stator disc, which is fixed in relation to the axial movement.
On each rotor disc the permanently magnetised, disc-shaped rotor may have a plurality of permanent magnets, and may for the rest consist of a soft magnetic material, and each stator disc may consist of an electrically conductive material. As the rotor rotates, the magnetic field generated by the permanent magnets gives rise to currents in the stator discs, which currents in addition to generating heat in the stator discs in turn also give rise to magnetic fields that endeavour to counteract the magnetic field generated by the rotor magnets. The magnetic fields counteracting one another give rise to repelling forces, which act axially between the stator and the rotor, especially each pair of stator discs and each pair of rotor discs. These repelling forcing increase as the rotor speed increases and decrease as the distance between rotor and stator increases.
In the case with the two stator discs and two rotor discs, the cooling duct comprises two radial gaps, which are formed between each stator disc and a disc of magnetic material fixed alongside this. The said fixed disc of magnetic material is annular and closes the flux path for the magnetic field from the rotor through the stator. An attraction force thereby also occurs between. each rotor disc and the associated fixed disc, which force endeavours to move the rotor discs towards the respective stator disc. This attraction force is largely dependent only on the distance.