Thermal power engine and its operating method

A method for improving the total production of useful energy in an energy utilization system of a thermal power engine (1) that is liquid-cooled and is used for the production of thermal energy as well as mechanical energy. In the energy utilization system, thermal energy is taken from the coolant of the engine cooling system. At least a part of the coolant from the engine is led to a vaporization space (5) where, either by lowering the pressure or by increasing the amount of thermal energy within that space (5), a part of the coolant is transformed to vapor. The vapor is used within the energy utilization system for energy transport and/or as a medium for recovering energy.

BACKGROUND OF THE INVENTION 
The invention relates to a method for improving the total production of 
useful energy of an internal combustion engine producing heat and 
mechanical energy, herein sometimes referred to as a "thermal power 
engine," and to an engine intended for the application of this method. 
In operation of a thermal power engine, the energy content of fuel supplied 
to the engine is converted partly to mechanical energy, which is made 
available at a mechanical power output of the engine, and partly to 
thermal energy, of which a portion is removed from the engine by the 
engine's cooling system. In order to maximize the overall efficiency of 
the engine, it is desirable that the thermal energy be applied to a useful 
purpose and not discharged as waste heat. 
In known large internal combustion engines, such as for example large 
diesel engines, the cooling system of the engine often raises the 
temperature of the coolant to a level that is less than 100.degree. C., 
usually in the range 80.degree. C. to 85.degree. C. It is difficult to 
find a useful application for waste heat at such low temperature values, 
and consequently a significant part of the energy content of the fuel 
consumed will not be used for useful purposes. In some rather large plants 
employing thermal power engines the thermal energy of the coolant has 
however been used, by means of heat exchangers, to produce, for example, 
warm water (e.g. warm process water) or for district heating applications. 
A thermal power plant may comprise several thermal power engines, whereby 
the total power output of the plant may be of the order of magnitude of 
100 MW. The greater is the power of a thermal power engine or a power 
plant containing such engines, the more important for energy saving it is 
to find a useful application for its waste heat and the better are the 
chances that the investments made for waste heat recovery will prove to be 
profitable. 
SUMMARY OF THE INVENTION 
An aim of this invention is to create a thermal power engine, for example a 
diesel engine, in which it is possible to utilize the heat energy produced 
in a more efficient manner. 
By vaporizing, for example, some of the coolant used in a diesel engine, a 
part of its thermal energy is absorbed as heat of vaporization. Vapor is a 
medium, the contained thermal energy of which can be quite effectively 
utilized by known methods. The partial vaporization is obtained either by 
lowering the pressure or by supplying additional thermal energy to a 
vaporization space. The vaporization takes place preferably in a separate 
vaporization space outside the thermal power engine, but vaporization may 
also theoretically take place by using heat produced by the engine in the 
cooling system of the engine, although this method of operating requires 
that the vaporization process takes place in such a part of the engine and 
under such good control that it is not harmful or dangerous to the 
functioning of the engine. 
It is of advantage to build thermal power engines intended to operate 
according to the invention as rather large units, having normally an 
output power of the order of magnitude of some megawatts, even some tens 
of megawatts. 
In a thermal power engine operating according to the invention, it is of 
advantage for thermotechnical reasons to operate with a coolant 
temperature above the normal range of 80.degree. C. to 85.degree. C. This 
requires, however, that the thermal power engine be able to withstand 
coolant temperatures above normal in its cooling system and that the 
temperature of the coolant is properly supervised and controlled. 
By superheating the vapor that is produced in accordance with the invention 
by using different waste heat sources associated with a thermal power 
engine, for example by means of the exhaust gases, the usefulness of the 
vapor for practical purposes is improved. Vapor, in particular superheated 
vapor, may be used for many purposes, for example as a process vapor in 
different machines and devices or for the production, e.g. in a steam 
turbine, of mechanical energy which can easily be converted to electric 
energy. 
It is known to improve the utilization of vapor by condensing the vapor at 
very low pressure in a condenser and returning it, in liquid state, to a 
vaporization circuit. In accordance with the invention, this known 
technique is applied to the cooling system of a thermal power engine. By 
preheating the returning condensate, with waste heat taken from the 
thermal power engine, a favorable condition is created either for its 
revaporization before return to the cooling system or for use of its 
remaining thermal energy by feeding the condensate into the coolant 
circuit from where the thermal energy may be utilized as described. 
A part of the cooling medium returned to liquid phase may be led, before 
its return to the cooling system, to a second circuit where a fresh 
vaporization is accomplished using the heat content of the flow of some 
hot medium available from the thermal power engine. In this way at least a 
part of the vapor produced may be fed into the vapor flow emanating from 
the first vaporization phase before any possible superheating of the vapor 
has been undertaken. A part of the vapor of the second circuit may also 
form a separate circuit passing through a turbine and a condenser. These 
types of circuits open up the possibility of utilizing a heretofore unused 
part of the waste heat of a thermal power engine in a coolant circuit 
according to the invention. 
By keeping the pressure of the cooling system of a thermal power engine at 
a level of more than 2 bar, and preferably more than 5 bar, it will be 
possible, when using pure water as a coolant, to raise its temperature 
considerably over 100.degree. C. without causing boiling. For applying the 
invention, it is believed that pure water is the best coolant. If its 
pressure is kept so high that the temperature in the cooling system may 
exceed 140.degree. C., good conditions prevail for utilizing, according to 
the invention, the thermal energy of the coolant. 
The invention may, with advantage, be applied to a large supercharged 
diesel engine, in which some high temperature medium flow of the 
supercharging arrangement of the engine can be used for superheating vapor 
received from the coolant of the engine and/or for revaporizing condensed 
coolant. Thereby a substantial part of the waste heat of the supercharging 
arrangement can also be recovered. The high temperature charge air of the 
supercharging arrangement may be used for superheating vapor and/or for 
vaporizing condensate, whereby at the same time cooling of the charge air 
is achieved, which allows the operating efficiency of the engine to be 
optimized. 
If, in an arrangement according to the invention, the energy required for 
the vaporization of the coolant largely corresponds to the amount of 
thermal energy transferred to the coolant from the thermal power engine, a 
thermally balanced arrangement is achieved and at the same time the waste 
heat of the cooling arrangement of the engine is effectively utilized.

DETAILED DESCRIPTION 
In FIG. 1, 1 indicates a thermal power engine, for example a large 
water-cooled supercharged diesel engine. The cooling system of the engine 
1 includes a pipe 2 which has a pressure and temperature monitoring 
control device 3 operating a valve 4 for controlling the pressure in the 
coolant (cooling water). The pressure of coolant is maintained by a pump 
7, supplying coolant through a pipe 8. The cooling water is led to a tank 
5, in which the pressure is so much lower than in the coolant passages in 
the engine 1, that a part of the cooling water (typically under 5 percent) 
is transformed into steam. That part of the cooling water (usually over 95 
percent), which does not become vaporized, is, by means of the pump 7, 
pumped through the pipe 8 back to the coolant passages of the engine 1. 
The level of liquid in the tank 5 is monitored by a control device 9, 
which controls the back flow of water in a pipe 21 by means of a throttle 
valve 6. 
The pressure in the cooling system of the engine 1 and in the tank 5 is 
preferably so selected that the pressure of the steam generated is from 4 
to 8 bar, in the embodiment shown about 6 bar, the temperature being 
correspondingly about 159.degree. C. The steam is led through a pipe 10 to 
a superheater 11 in the exhaust line 23 of the engine 1, in which its 
temperature rises to about 300.degree. C. From the superheater 11 the 
steam is led through a pipe 12 to a steam turbine 13, in which the steam 
is used to generate mechanical energy, which, as illustrated, is used for 
producing electricity in a generator 14. 
From the turbine 13, low pressure steam (.ltoreq.0.1 bar) flows through a 
pipe 15 to a condenser 16, from which the condensate water is led, through 
a pipe 17, to a pump 18. The pump 18 raises the pressure of the water to 
such a level that the water, after passing through a preheater 22 also 
provided on the exhaust line 23, the pipe 21 and the throttle valve 6, is 
able to join the water flow in the pipe 8 on the suction side of the pump 
7. 
Possible leakage losses are replaced by water fed through a pipe 19 
upstream of the pump 18. In the superheater 11, in the preheater 22 and in 
an exhaust gas vaporizer 31, thermal energy is obtained from the hot 
exhaust gases of the engine 1, which are led away from the engine through 
the exhaust gas line 23. 
Downstream of the heat exchanger 22, the temperature of the water is about 
159.degree. C. A part of the preheated water is, in the embodiment 
according to FIG. 1, led through a pipe 25 to a tank 26 which functions as 
a vapor separator and from which the water separated out is led through a 
pipe 27 to two branches 28 and 34. Branch 28 passes, via a pump 29, to the 
exhaust gas vaporizer 31 and branch 34 passes, via a pump 35, to a charge 
air vaporizer 37 heated by compressed charge air flowing to the engine 1 
through a pipe 39. The branches 28 and 34 unite again in the tank 26, from 
which the steam separated out is led back to the pipe 10. 
The supercharging arrangement of the engine 1 comprises an exhaust gas 
turbine 38 and a charge air compressor 40 driven by the turbine 38. The 
temperature of the supercharged air may rise close to 250.degree. C. With 
respect to efficiency considerations of engine operation, it is important 
to cool the charge air and the required cooling may at least partly take 
place as shown using the vaporizer 37. Further air cooling in a cooler 42 
is usually required, which lowers the temperature of the charge air to a 
desired centigrade value. 
FIG. 2 has much in common with FIG. 1 and like reference numerals have the 
same meaning in both figures. In the embodiment according to FIG. 2, the 
superheater 11 has been replaced by a twin superheater 11a, which has two 
separate steam flow ducts. The circulation path through the twin 
superheater 11a, which includes the pipe 12, the steam turbine 13, the 
condenser 16 and the preheater 22, corresponds to the arrangement shown in 
FIG. 1. 
Downstream of the preheater 22, a part of the preheated water is led 
through the pipe 25 to two separate tanks 26a and 26b, each of which 
operates as a steam separator. From the tank 26a, the water is led through 
the pipe 34 and the pump 35 to the charge air vaporizer 37 from which the 
generated steam-water mixture is led back to the tank 26a and there, 
further, to the pipe 10 just as shown in the embodiment of FIG. 1. From 
the tank 26b, the water is led through the pipe 27 and the pump 29 to the 
exhaust gas vaporizer 31, from which the steam-water mixture generated, as 
in the embodiment according to FIG. 1, is led back, but now to the tank 
26b, from which the steam flows through the second flow duct of the twin 
superheater 11a and through a pipe 12a to a second steam turbine 13a and 
there produces mechanical energy. The low pressure steam (.ltoreq.0.1 
bar), received from the steam turbine 13a flows through a pipe 15a to the 
condenser 16, in which it unites with the low pressure steam flow coming 
from the steam turbine 13. 
The use of two separate steam turbines is justifiable, because the 
temperature and the pressure of the steam received from the exhaust gas 
vaporizer 31 are notably higher than the temperature and pressure of the 
steam received both from the charge air vaporizer 37 and from the tank 5. 
In the graph of FIG. 3, curve A represents, as a function of the steam 
pressure p (in bar), the shaft power p (in kilowatts) of a steam turbine 
utilizing the waste heat of a diesel engine when the steam turbine 
recovers thermal energy only from the exhaust gases of the engine. 
Curve B in FIG. 3 represents the shaft power of the turbine as a function 
of the steam pressure, when a part of the thermal energy of the liquid 
coolant circulating in the engine is vaporized and used according to the 
invention for useful purposes with the arrangement shown in FIG. 1 but 
without the charge air vaporizer 37 and its circulation arrangement (i.e. 
without using the circuit 26, 27, 34, 35, 37, and 26). Curve B shows that 
the shaft power of the steam turbine rises some 30 to 40 percent compared 
to the application represented by curve A. However, the relationship 
between the power of the steam turbine and the steam pressure is not a 
convenient one in curve B because it is difficult to achieve the high 
steam pressures (greater than about 10 bar) which correspond to maximum 
power output. Thus, the most advantageous areas of curve B will not easily 
be attainable in practice. 
Curve C corresponds to an application according to FIG. 1 but using all the 
heat utilization arrangements shown. It is considerably more advantageous 
than that represented by curve B, because now the peak power occurs within 
the pressure range 4 to 6 bar, which is a realistic pressure range for 
utilizing the invention. 
The arrangement of FIG. 2 gives a curve similar to curve C. 
The invention is not restricted to the embodiments illustrated, since 
several modifications are feasible within the scope of the following 
claims.