Cooling water heating via generator power losses

The present invention relates to a method and device for rapidly warming up a liquid-cooled internal combustion engine.

Internal combustion engines have substantially higher efficiency at the operating temperature than immediately after starting and thus work optimally in the warmed-up state. It may take a considerable amount of time until the engine has warmed up, especially at low ambient temperatures and with a small load. This problem plays an even more serious role in high-efficiency diesel engines.

Conventionally, in order to warm up the internal combustion engine to operating temperature as quickly as possible, additional electric heating devices heat up the cooling water after starting. A heating resistor is usually provided that is powered directly by the generator and heats up the cooling water in a short time. Conventional cooling water heating devices have a relatively complicated construction, however, and require in particular additional heating elements to heat up the engine.

It is an object of the present invention to create a cooling water heating device having a substantially simpler construction.

In accordance with an example embodiment of the present invention, the power loss of a generator, in particular of a crankshaft starter generator, is used to warm up the cooling water of an internal combustion engine. The generator is preferably a permanent magnet synchronous generator that is also connected to the cooling water circulation system of the internal combustion engine.

The generator is operated to supply a sufficient amount of heating power during a heating phase at an operating point having large, preferably maximum power loss, and at an operating temperature of the engine at an operating point having small, preferably minimum power loss. The efficiency of the generator is regulatable using either threshold values or characteristic curves. That means it is possible to adjust generator efficiency stepwise using temperature threshold values or continuously using characteristic curves as a function of the engine temperature or coolant temperature.

Generator efficiency is adjusted using a suitable device, for example using a pulse-controlled inverter, preferably by changing the phase angle between generator current and generator voltage.

Especially effective warming of the cooling water is attainable if a crankshaft starter generator (CSG) is used for this purpose.

The present invention is explained in greater detail below using the example embodiments illustrated in the drawings.

FIG. 1 shows a schematic equivalent circuit diagram of a crankshaft starter generator.

FIG. 2 shows a representation of currents and voltages in the equivalent circuit diagram of FIG. 1 at maximum generator power.

FIG. 3 shows a representation of currents and voltages in the equivalent circuit diagram of FIG. 1 at minimum and maximum generator power loss.

FIG. 1 shows the equivalent circuit diagram of a crankshaft starter generator (CSG) that is designed as a permanent magnet synchronous machine 1 and is connected to a vehicle's electrical system via a pulse-controlled inverter (not shown).

Synchronous machine 1 is fundamentally characterized by an interior voltage source 2 having synchronous generated voltage Up and inductive resistance 3 . It is possible to ignore the ohmic coil resistance for performance, but it is responsible for the losses (3*R*I 2 ).

Both variables Up and X are proportional to the frequency, i.e., the speed of machine 1 . Synchronous generated voltage Up of synchronous machine 1 is thus constant at a given speed, and terminal voltage U 1 of synchronous machine 1 is also constant, having a maximum value U 1 =U 1 max that is provided by the battery voltage or line voltage.

Synchronous machine 1 works almost in the entire generator operating range in the field-weakening range. That means that synchronous generated voltage Up is greater than the maximum terminal voltage U 1 of synchronous machine 1 .

FIG. 2 shows a current-voltage diagram in the complex plane for full-load operation (CSG 2 produces all generator power). If synchronous generated voltage Up of synchronous machine 1 is plotted on the real axis in the complex plane, then terminal voltage U 1 of synchronous machine 1 is able to travel on a circle having the radius of the maximum terminal voltage U 1 max (depending on how the electronics adjust the phase angle).

The machine current I 1 produced this way also has a circle as a locus. The center of the current circle is on the tip of short circuit current Ik (=Up/jX), and the radius of the circle is U 1 /jX.

The component of machine current I 1 creating the torque is parallel to synchronous generated voltage Up (real component of I 1 ). Points P, P′ of maximum power are thus easy to detect. FIG. 2 shows the maximum power point in generator operation (lower half of the current circle) via reference symbol P and the maximum power point in engine operation (upper half of the current circle) via reference symbol P′.

Voltage across inductive internal resistance jXI 1 is derived following Kirchhoff's Loop Rule using jXI 1 =−Up+U 1 shown in FIG. 2 as voltage vector jXI 1 between vectors Up and U 1 .

FIG. 3 shows a current-voltage diagram of synchronous machine 1 for the partial-load range (the CSG provides less than full generator power).

In the converter's block mode (U 1 =U 1 max), there are two operating points AP, AP′ where a requested power level is reached.

With a sufficiently high engine temperature, generator regulation (not shown) is normally adjusted so that it adjusts the operating point AP with index a (U 1 a , I 1 a ). At this operating point AP, the requested generator power P G =3*UP I 1 cosphi is generated having minimal machine current I 1 and thus the smallest ohmic power loss as well.

Along constant power straight lines G (here, the real component of machine current I 1 is always constant), there exists a second operating point AP′ having the same generator power but with distinctly higher machine current I 1 b . At this operating point AP, the real component (q component) of current I 1 (parallel to Up) is equal to that at first operating point AP, but the imaginary component (d component) of the current (transverse to Up) is distinctly greater. Thus, operating point AP′ here has been reached using the greatest ohmic power loss, which again is usable to heat up the cooling water and thereby warm up the engine.

If there is deviation from the converter's block mode (U 1 =U 1 max) discussed above and if smaller values are permitted for terminal voltage U 1 , then ohmic power loss and generator efficiency are adjustable in any desired way along G,G′ constant power straight lines. Copper losses in synchronous machine 1 are thus variable in any way within the two threshold values I 1 a , I 1 b.

Continuous regulation of generator efficiency as a function of engine temperature is not absolutely required for purposes of warming up cooling water. Instead, setting operating point AP′ of maximum losses is sufficient as a rule in the heating phase, and, if the engine is at operating temperature, operating at operating point AP of minimum losses is sufficient after the heating phase.

The current circle becomes smaller as the speed of generator 1 increases. This then limits the possibility of increasing the power loss. For the cooling water heating device, however, the internal combustion engine's idling (ca. 700–800 l/min.) is of special significance.

The internal combustion engine works using mostly greater loads at higher speeds, generating greater power loss, while the speed-dependent losses, such as iron losses, for example, increase in the CSG.