Method and apparatus for utilizing the waste heat energy of an internal combustion engine

A method and apparatus for utilizing the waste heat energy of an internal combustion engine in which a turbine is driven by the exhaust gases from the engine and drives an electrical generator. In order to permit a direct coupling between the turbine and the generator and to utilize completely the electrical energy recoverable from the waste heat energy, the generator is a synchronous machine with a non-wound rotor, the generator being connected via an electrical converter to an electric motor which is drivingly coupled to the internal combustion engine to relieve the load therein.

FIELD OF THE INVENTION 
The present invention relates to a method and apparatus for utilizing the 
waste heat energy of an internal combustion engine in which a 
thermodynamic expansion machine, the operating fluid of which is heated by 
the waste heat energy of the internal combustion engine and is separated 
physically from the exhaust gases of the internal combustion machine, 
drives an electrical power generator to produce electrical current. 
PRIOR ART 
One such method is known from West German Unexamined patent application OS 
No. 26 18 584. In this method, steam is produced by means of the waste 
heat energy of an internal combustion engine, the steam being expanded in 
a steam turbine or a steam engine coupled to a generator. Furthermore, in 
the known method it is proposed to have the steam engine also apply input 
to the shaft of the internal combustion engine. Such a coupling of the 
steam engine back to the internal combustion engine, as a rule, requires 
an adaptor transmission due to the different speeds of rotation of the two 
types of machines. Further difficulties occur when the internal combustion 
machine is itself operated at different speeds of rotation as is the case 
in internal combustion engines installed in vehicles. If the steam engine 
only drives a generator then, for a stationary internal combustion engine 
there are no difficulties in feeding the electrical energy obtained by the 
generator into an output circuit. On the other hand, if the waste heat of 
a movably arranged internal combustion engine is to be utilized, 
considerable problems arise with respect to the further use of the 
recoverable electrical energy. 
At a relatively low power of production such as for automobile internal 
combustion engines, small turbines of very high speeds of rotation must be 
used in order to obtain good efficiency. Since conventional electrical 
machines having a wound rotor can not be operated at the customary high 
speeds of rotation of turbines of relatively small power output, for 
example, 120,000 rpm, difficulties arise with respect to the coupling of 
such a generator to such a turbine. The generator can be connected to the 
turbine via a transmission which reduces the speed of rotation of the 
turbine to a speed which is permissible for the generator. Due to the high 
speed of rotation of the turbine, such a transmission requires very high 
precision in manufacture and, in addition, produces mechanical losses. In 
order to avoid this expense, the possibility of direct coupling of the 
generator to the turbine can be contemplated. As a result of the fact that 
the speed of rotation of a generator having a wound rotor is limited due 
to considerations of strength, direct coupling is possible only with a 
correspondingly reduced speed of rotation of the turbine. However, since 
the efficiency of a turbine decreases rapidly for substantial reduction of 
its speed of rotation below the optimum operating speed, the yield of the 
waste-heat energy decreases greatly in the same proportion. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method and apparatus in 
which direct coupling of the thermodynamic expansion engine to the 
electric generator is effected without reduction of the speed of rotation, 
without great structural expense and in which furthermore electrical 
energy obtained from the waste heat energy can always be fully utilized 
also without great technical expense, particularly in the case of an 
internal combustion machine which operates in fixed or movable condition. 
The above and other objects are achieved, in accordance with the invention, 
by the use of a generator, as a synchronous machine with a non-wound rotor 
and at least one electrical converter connected after the synchronous 
machine and at least one electrical motor connected to said one electrical 
converter and applying input power to said internal combustion engine. The 
non-wound rotor of the synchronous machine can be constructed in such a 
manner with respect to its mechanical strength that no reduction in speed 
of rotation is necessary even with direct coupling with the expansion 
machine. The expansion machine can therefore operate at its optimum speed. 
The rotor may be either a so-called external rotor which supports 
permanent magnets instead of the customary excitation winding or it may be 
constructed as the rotor of a homopolar reluctance machine. Such a 
machine, whose electrical or permanent magnetic excitation as well as the 
operating winding is arranged in the stator, may contain a rotor which 
consists only of solid-iron shaped elements connected with the shaft and 
thereby capable of withstanding the stresses at extremely high speeds of 
rotation. By means of the electrical converter, the voltage produced by 
the synchronous machine is adapted in respect of its magnitude and/or 
frequency to the specific requirements of the associated motor. 
The invention therefore makes it possible to optimize the utilization of 
the energy of the waste heat by operating the turbine in its region of 
maximum efficiency and transmitting the mechanical energy produced by the 
turbine to the electrical generator and converting the mechanical energy 
into electrical energy with the smallest possible windage losses. The 
utilization of the electrical energy is always assured by relieving the 
load of the internal combustion engine load by feedback from one or more 
electrical motors driven from the generator. 
Due to its high operating speed, the synchronous generator can be made 
smaller and lighter than a machine with a wound rotor which is limited in 
its speed of rotation. Furthermore, the elimination of a transmission 
achieves a further saving of space and weight. As a result, the invention 
enables the use of a substantially smaller and lighter system as compared 
to the known systems operating with intermediate transmissions, thereby 
assuring optimum adaptation for use in vehicles along with the robustness 
of a synchronous machine having a non-wound rotor. 
Another advantage of the invention is that it provides a strong connection 
or a one-piece shaft between turbine and generator with optimum 
utilization of the turbine and no additional loads can be imposed on the 
shaft bearings because of possible inaccurate alignment of the two shafts. 
The synchronous machine, when constructed as a homopolar machine, can be 
excited electrically and the excitation be used, if needed, for voltage 
regulation. A part of the electrical energy recovered from the waste heat 
can be used as a source of energy for the excitation. 
The excitation of the synchronous machine can, however, also be effected by 
permanent magnets as a result of which, due to the elimination of energy 
losses, the efficiency is improved and the removal of heat from the 
machine is facilitated. 
If the electrical energy produced is to be utilized for DC supply, then at 
least a part of the output of the synchronous generator can be fed to the 
electrical converter which can be a rectifier. In order to adapt the DC 
voltage to a value required by DC mains, the rectifier is constructed so 
that it can be, at least partically controlled, by a small DC regulator. 
In accordance with a further feature of the invention, a buffer battery is 
connected in parallel to the consumer line in the DC circuit. The buffer 
battery can serve for starting the internal combustion engine or for 
supplying electrical energy to the consumer line, at least temporarily, 
for example, when the internal combustion engine is at rest. In this way, 
the customary dynamo in a vehicle can be replaced by the method of the 
invention. 
However, the invention also contemplates rectifying the entire output from 
the synchronous machine by means of a rectifier and using the output to 
drive one or more DC motors which supply input power to the internal 
combustion engine and thereby relieve the load thereon. For this purpose, 
the DC motor or motors are adapted to the instantaneous speed of rotation 
of the internal combustion engine by regulation of their excitation and/or 
their terminal voltage. 
Instead of a DC motor a stronger and less expensive asynchronous or 
synchronous motor can also be used for the mechanical relief of the load 
on the internal combustion engine. This is possible due to the fact that 
the entire power available at each instant from the synchronous generator 
is used, via the converter, for feeding corresponding AC motors. The 
operating frequency of the AC motor, which corresponds instantaneously to 
the speed of rotation of the internal combustion engine, is obtained by 
the converter, which can be an intermediate-circuit or a direct converter. 
The converter can also adapt the instantaneous voltage fed to the AC motor 
to the frequency or speed thereof.

DETAILED DESCRIPTION 
Referring to FIG. 1, heated cooling water 11 flowing from an internal 
combustion engine 10 is evaporated in an evaporator 13 by the exhaust 
gases 12 from the engine. The exhaust gases 12 and the cooling water 11 
being fully separated. Steam 14 produced in evaporator 13 drives a 
thermodynamic expansion machine 15 constituted as a turbine which is 
connected via a shaft 16 with a non-wound rotor 17 of a synchronous 
machine 18 which operates as a generator and by which a major portion of 
the thermal energy contained in the cooling water 11 and the exhaust gases 
12 is converted into electrical energy. 
The generator output is supplied to a DC circuit 20 and/or an AC circuit 21 
for associated DC mains 22 and AC mains 23 respectively. The rectification 
of the AC voltage produced by the synchronous generator 18 is effected by 
a rectifier 25 followed by a DC regulator 26 for adapting the current to 
the specific requirement. The branched AC circuit 21 includes a frequency 
converter 28 constructed as a direct converter or an intermediate-circuit 
converter for adapting the AC voltage to the AC mains 23. 
An electrical motor means is provided for converting the electrical energy 
into mechanical energy which is supplied to the drive shaft 30 of the 
internal combustion engine 10 in order to save primary energy. The motor 
means can be a DC motor 31 which is connected to the DC circuit 20 or an 
AC motor constructed as an asynchronous or synchronous motor. 
In order to be able to meet the demand for electrical power at the DC mains 
even when the internal combustion engine 10 is not operating, a battery 33 
is connected in the DC circuit 20 in parallel with the mains 22. 
The expansion machine 15 and the synchronous generator 18 are combined into 
a compact structural unit 37 by a common one-piece shaft 16, one end of 
which carries the impeller wheel 39 of the expansion machine 15 and the 
other end of which carries the rotor 17 of the synchronous generator 18. 
In order to reduce windage losses of the rotor 17 which are considerable 
in view of the high speed of rotation, a vacuum is produced within the 
stator cavity of the synchronous generator 18. The vacuum is maintained by 
labyrinth seals 40 arranged at the ends of the rotor 17, said seals 40 
also preventing penetration of particles or foreign substances into the 
generator. In this way there is complete freedom in the selection of 
bearings 41 for the shaft 16. The use of oil-lubricated bearings, 
therefore, poses no problems. 
A portion of the coolant which has been condensed and further cooled in a 
condenser 45 can advantageously be diverted through the stator 46 of the 
synchronous generator 18 to cool the same and be returned to the coolant 
going to the internal combustion engine 10. The diversion circuit is shown 
by dotted lines 47 and 48. 
A homopolar machine is particularly well suited for coupling with the 
expansion machine 15. While such a homopolar machine, due to the 
construction of its rotor, can be designed for very high speeds of 
rotation, the space required by it is greater due to its principle of 
operation as compared to an alternating pole machine of the same power and 
speed of rotation. A synchronous generator which is particularly suitable 
for high speeds of rotation is characterized by a construction comprising 
an alternating pole machine with an external rotor. 
In the synchronous machine 18 shown in FIG. 2, a bell-shaped iron part 51 
of the rotor forms the magnetic flux yoke and is cantilevered on the shaft 
end 52 of expansion machine 15. Mounted on the inner side of part 51 are 
cup-shaped permanent magnets 53 (upper half of the section) and 54 (lower 
half) which are magnetized radially with alternating polarity. The magnets 
include associated laminated pole pieces 55 and 56. In order to minimize 
windage losses and at least partially to secure the permanent magnets 53 
and 54 and the pole pieces 55 and 56 respectively, intervening spaces are 
filled with a preferably elastic potting composition 57 such that the 
inner surface of the rotor forms a smooth hollow cylinder. 
The centrifugal forces of the permanent magnets, the pole pieces and the 
potting composition are resisted by the part 51, which enables high speed 
operation. In order to make the mechanical stresses and elongation of the 
part 51 uniform, its wall is strengthened at the opening. 
The stator consists of a laminated core 58 seated on a stationary mandrel 
59 and carries a stator winding 60 anchored in slots by wedges 61. The 
ends 62 of the winding which face the drive side are bent radially inward 
in order to minimize their overhang and thus the axial length of the part 
51. 
The permanent magnets 53 (upper half of the section) are longer axially 
than the core 58. The associated axially, parallel laminated pole pieces 
55 concentrate the flux from the permanent magnets to the core 58. In the 
lower half of the section, the pole pieces 56 are laminated normal to the 
axis and extend, in the same way as the corresponding permanent magnets 
54, only over the length of the core 58 but can, however, also achieve a 
concentration of flux. Furthermore, by the use of metallic and therefore 
electrically conductive magnets with laminated pole pieces, surface losses 
caused by the stator slots will be reduced to a fraction of the value that 
would result without pole pieces. 
The invention provides a construction which is particularly compact in the 
axial direction which is very advantageous at high speed of rotation. 
There is furthermore obtained a strong, operationally reliable 
construction which is relatively inexpensive.