EP2354475B1 - Method for operating a piston expander of a steam motor - Google Patents

Description

Technical field:

The invention relates to a method for operating a piston expander, in which live steam from a steam supply is fed through an inlet valve into a cylinder chamber, the live steam introduced into the cylinder chamber is expanded in the power stroke due to a movement of a piston from top to bottom dead center, and the expanded steam is released Reaching the bottom dead center is directed from a closable outlet into a vapor discharge.

State of the art:

In order to be able to save fuel, especially in mobile internal combustion engines, such as motor vehicle internal combustion engines, two technical solutions are currently being prioritized. In addition to the use of different hybrid concepts, which are particularly suitable for city and distribution traffic due to the braking and acceleration processes that occur there, heat recovery systems are also known that use the waste heat from an internal combustion engine to provide additional drive energy. Such systems for using waste heat are suitable for mobile internal combustion engines, especially for vehicles that are used in long-distance traffic.

In such waste heat utilization systems, the waste heat occurring in the area of the internal combustion engine and/or in the exhaust gas discharge is at least partially transferred to a secondary heat circuit. A heat transfer fluid is circulated in the secondary heat circuit and is usually at least partially evaporated in an evaporator, the vapor is expanded in an expansion unit, for example in a piston expander, and finally liquefied again in a condenser. The mechanical work generated by the expansion unit is supplied to the drive system, in particular a vehicle drive system, as additional work. It is also conceivable to use the mechanical work gained by using waste heat to drive other components, such as a fan or a compressor, or to generate electrical energy.

In this context, from the DE 10 2006 043 139 A1 a heat recovery system for an internal combustion engine is known. With the aid of the system described, the vehicle is provided with additional drive energy from the waste heat from the internal combustion engine and/or the exhaust system. After expansion of the vaporous working medium in the expander, the working medium of the secondary heat cycle is pumped into a condenser, in which it is liquefied with the release of heat, so that the corresponding steam cycle process is closed.

Furthermore, from the EP 2 154 400 A2 a piston of a reciprocating piston expander is known, in which an outer diameter of the piston neck is smaller than an outer diameter of the piston head and/or the piston skirt and at the same time the length of the piston neck almost corresponds to the stroke of the piston when installed. With the technical solution described, it is possible, with the aid of relatively simple design means, to implement effective use of the energy contained in the steam and thus of the heat loss occurring in an internal combustion engine. The described design of the piston ensures a smooth start-up of a piston expander and an effective separation of the oil and steam circuits. The effective separation of the oil and steam circuits reliably prevents mutual contamination of the circuits by overflowing the respective medium.

The steam piston expanders known from the prior art are usually operated in the two-stroke process. Here, at top dead center, the live steam is fed into a cylinder of the expander unit via an inlet valve, and in the following power stroke, the steam is expanded while releasing work. Finally, the exhaust valve is opened at bottom dead center and the expanded vapor is pushed out of the cylinder during the exhaust stroke by moving the piston from bottom to top dead center. When the top dead center is reached, the exhaust valve closes and the corresponding cycle process begins again. Depending on the live steam parameters and the back pressure on the outlet side, the compression ratio must be designed in such a way that the working medium is expanded to a suitable level in the power cycle.

If the compression ratio is too low, when the exhaust valve opens, the working medium will have an overpressure compared to the back pressure in the exhaust line, which has a negative effect due to the potential possibility of achieving greater relaxation affects the efficiency of the cycle. On the other hand, if the compression ratio is designed too high, the working medium is expanded to a pressure below the back pressure in the outlet line, which makes it more difficult to expel the expanded vapor and in turn has a negative effect on the efficiency of the cycle process.

The use of steam piston expanders when using waste heat from internal combustion engines requires a complex design. In order to be able to meet all requirements regarding weight, costs, durability and necessary service, vertical valves are usually used. This measure creates a dead space at top dead center that is comparatively large and thus leads to low geometric compression ratios. This often results in the problem that not even the dead space may be sufficiently filled with live steam.

From the DE 10 2007 049 464 A1 a piston machine operated with gas pressure is known. After expansion, steam can flow out via an outlet slit.

The U.S. 2009/0056331 A1 discloses a highly efficient integrated heat engine consisting of a double compound cylinder structure, where the first cylinder is the primary combustion and/or expansion cylinder and the second cylinder is the secondary combustion and/or expansion cylinder. Power strokes driven by expansions of various working fluids such as air-fuel combustion products, steam, and compressed air are integrated into an engine block. There is, among other things, an exhaust air duct in the lower area of the cylinder.

The U.S. 1,227,836 discloses a single acting steam engine. In order to avoid an ineffective exhaust angle, the connection between the cylinder and the condenser can be drilled wholly or partially perpendicular to the cylinder axis immediately above the piston in its lowest position.

The DE 198 47 742 C1 discloses a steam engine with a device for generating steam and a piston-cylinder unit for generating torque with the help of the steam. Only in the area of bottom dead center is an exhaust valve opened and the cooled vapor expelled.

Proceeding from the known state of the art and the problem described, the invention is based on the object of specifying a method for operating a steam piston expander unit which can be operated with a comparatively high level of efficiency. The method to be specified is intended in particular to reduce the amount of live steam required to fill the piston without the efficiency of the cycle process being significantly reduced as a result.

The object described above is achieved with the aid of a method according to claim 1. Advantageous embodiments of the invention are the subject matter of the dependent claims and are explained in more detail in the following description with partial reference to the figures.

According to the invention is a method for operating a piston expander, in which live steam from a steam supply is conducted through an inlet valve into a cylinder chamber, the live steam introduced into the cylinder chamber is expanded in the power stroke due to a movement of a piston from top to bottom dead center, and the expanded steam is at least partially expanded is conducted from a closable outlet opening into a vapor discharge, has been developed in such a way that the outlet opening is opened when or after reaching bottom dead center and then closed before the piston reaches top dead center in the exhaust stroke.

According to the invention, the outlet opening is closed in the exhaust stroke in a range of a crankshaft angle of 70° to 100° after bottom dead center.

In principle, the amount of live steam per cycle of the work process can be reduced in two ways. On the one hand, it is conceivable to open the inlet valve only as long and to the extent that the live-steam pressure is not reached in the cylinder. In this case, the inlet valve throttles the live-steam to a lower pressure than the live-steam pressure. However, this reduction in pressure leads to a decisive reduction in efficiency. For this reason, the method according to the invention makes use of a second option for reducing the amount of live steam per cycle. In this case, the outlet opening is not open during the entire exhaust stroke, i.e. during the time in which the piston moves from bottom to top dead center, but the outlet opening is closed well before the piston reaches top dead center. Through this measure ensures that a significant amount of already expanded steam remains in the cylinder and is not discharged into the steam outlet.

This residual steam remaining in the cylinder is advantageously compressed in the exhaust stroke by the movement of the piston to top dead center. As a result, when the intake valve opens, the pressure is already significantly higher than the back pressure that otherwise prevails in the cylinder without compression of the residual steam. Due to the pressure inside the cylinder, which is higher than the usual back pressure, only a comparatively small quantity of live steam is introduced into the cylinder. Nevertheless, through the injection of the live steam, the mixed steam forming in the dead volume of the cylinder is brought to a pressure that corresponds to the live steam pressure. The live-steam pressure is achieved by further compression of the already pre-stressed residual steam within the dead volume of the cylinder, with the state of the mixed steam being adjusted after injection of the live steam as a function of the states of the precompressed residual steam and the live steam. This mixed vapor pressure can be achieved independently of the selection of a suitable compression ratio.

The method according to the invention for operating a piston expander advantageously achieves that the efficiency of the expander is comparatively high and at the same time a moderate increase in pressure within the cylinder is achieved. The moderate increase in pressure is due to the fact that this is realized over a longer period of time, since the residual steam is initially pre-stressed and only then is it compressed to the live-steam pressure level by injecting live steam.

The intake valve is preferably opened as soon as the piston reaches top dead center and is kept open up to a crankshaft angle of about 30° after top dead center. The great advantage of the method according to the invention is based on the fact that two vapors mix in the dead volume, namely the live steam and the pre-compressed residual steam, which are comparatively similar in terms of exergy.

When carrying out the method according to the invention, it is furthermore preferably ensured that in a period of time between the closing of the outlet opening and the following Closing the inlet valve, a mixed steam is generated by supplying the live steam into the cylinder chamber, in particular the dead volume of the cylinder chamber, the mixed steam pressure of which corresponds at least approximately to the live steam pressure. The comparatively high mixed-steam pressure The live-steam pressure is achieved by further compression of the already pre-stressed residual steam within the dead volume of the cylinder, with the condition of the mixed-steam being adjusted after injection of the live steam as a function of the states of the pre-compressed residual steam and the live steam. Mixing the live steam with the pressurized residual steam thus produces a mixed steam with an enthalpy that is higher than the usual ratios. This measure increases the efficiency of the cyclic process in a relatively simple manner.

Figur 1:

Ventilerhebungskurve der Auslassöffnung;

Figur 2:

Druckverlauf im Zylinder sowie

Figur 3:

Schemadarstellungen eines 2-Takt-Dampfexpanders.

The invention is explained in more detail below with reference to figures without restricting the general idea of the invention. Show it:

Figure 1:

exhaust port valve lift curve;

Figure 2:

Pressure curve in the cylinder as well

Figure 3:

Schematics of a 2-stroke steam expander.

In figure 1 shows the lift of a valve in an outlet port of the cylinder of a vapor expander. Here, the function curves a, b, c of the valve lift over the crankshaft angle are given in relation to three different valve controls. Reaching the bottom dead center and the top dead center is indicated by a vertical line at approximately 182° or 361° crankshaft angle. The continuous thin function curve a and the dotted curve b each show the lift of the exhaust valve in known standard methods. The third function curve c, which is designed as a thick line, shows the valve lift of the exhaust valve when using the method according to the invention for opening and closing the exhaust valve.

It can be clearly seen that with a valve lift of the exhaust valve according to the function curves a, b, the exhaust valve is opened over a comparatively wide range between the bottom and the top dead center of the piston. In comparison to the opening of the outlet valve according to the invention, which is represented by the functional curve c drawn in bold, the outlet valve is not only longer in the standard method, but also also open further. In contrast to the known methods, when the method according to the invention is used, the outlet opening is already closed again well before the top dead center is reached. As a result of the measure described, the residual steam in the cylinder at this point in time is not expelled but rather compressed due to the movement of the piston in the direction of top dead center when the exhaust valve is closed.

An embodiment not according to the invention is given in figure 1 illustrated by the function curve d. With this technical solution, a special design of the outlet opening is used. In this case, slots are provided in the cylinder wall, which create a connection between the cylinder interior and a vapor discharge as soon as the piston edge sweeps over the slot in the expansion stroke. In the exhaust stroke, the at least one slot is closed again as soon as the piston edge has once again passed over the slot due to the opposite movement of the piston. In the illustrated case, the at least one slot is opened at a crankshaft angle of approximately 20° before bottom dead center is reached and closed again at a crankshaft angle of approximately 20° after bottom dead center.

In addition to the in figure 1 illustrated course of the stroke of the exhaust valve during the movement of a piston, are shown in figure 2 the pressure histories for the three in figure 1 illustrated method for closing the outlet port shown. It can be clearly seen that in the standard methods a, b there is a very rapid increase in pressure shortly before the top dead center is reached. In comparison, the pressure rise is very gentle if the exhaust valve closes early, i.e. well before the piston reaches top dead center. This is due to the fact that if the exhaust valve closes early, the pressure in the cylinder increases continuously as a result of the compression of the residual steam, while when standard methods a, b are used, the pressure only increases very shortly before top dead center is reached, in particular about 10° crankshaft angle before it is reached the top dead center, increases.

Regarding the in connection with the function curve d in figure 1 illustrated slot control of the exhaust port, it is pointed out that the pressure within the cylinder in the exhaust stroke depending on the design of the slots, in particular their geometric shape, increases. Compared to a valve control, the pressure rise in most Cases take place less gently, the corresponding pressure profile curve thus have a somewhat steeper course in this area.

In figure 3 the structure of a steam expander is shown schematically. Since a corresponding vapor expander is usually operated in the two-stroke process, the crankshaft and camshaft speed are the same, so that the intake and exhaust valves are actuated by means of a corresponding crank web provided on the crankshaft. Such a design offers the advantage that neither an additional camshaft nor a corresponding drive is required. Of course, it is fundamentally conceivable to provide an additional camshaft in addition to the crankshaft, even in the case of a steam piston expander operated in the two-stroke process.

As the previous statements have shown, the invention relates to a method for suitably actuating inlet and outlet valves of a piston machine for vapor expansion. For this are in figure 3 schematically shown three technical options with which the actuation of the valves 4, 5 can be realized. The method according to the invention, which mainly relates to the point in time at which the valves 4, 5 are opened and closed, can be carried out with each of the three possible valve actuations.

In the figure 3 The components of a steam piston expander shown that are essential for the implementation of the method according to the invention are the crankshaft 1, the camshaft 2 with the molded cams 3, the inlet valve 4, the outlet valve 5, the position sensor 6 and an actuation unit 7. Depending on which structural design for the steam engine is selected, the valves are actuated via the crankshaft 1 ( Figure 3a ), via the camshaft 2 (3b) or the further actuating unit (7), which can be driven electrically, hydraulically or pneumatically. When using a further actuating unit 7, which is primarily characterized by the fact that there is no mechanical connection between the crankshaft 1 and the intake or exhaust valve, a position sensor 6 on the crankshaft and a control unit 10 are also provided. The instantaneous position of the crankshaft 1 is determined with the aid of the position sensor 6 and a corresponding value is sent to the control unit as an input variable. This value is processed in the control unit 10 and an output variable is generated, on the basis of which the actuation of the inlet or outlet valve 4, 5 by the actuation unit 7 takes place.

All three in figure 3 What the valve actuation mechanisms shown have in common is that live steam is conveyed to the inlet valve 4 via a steam supply 8 . The intake valve is opened either by a crank web of the crankshaft ( Figure 3a ), a cam 3 of the camshaft 2 ( Figure 3b ) or by the actuating device 7 as soon as the piston 9 is at top dead center. When the top dead center is reached, there is compressed residual steam within the remaining cylinder volume, the so-called dead volume, which has not been blown out after expansion, but has been compressed again. After the inlet valve 4 has opened, live steam flows into the dead volume, with the inflow of live steam also compressing the prestressed residual steam in the dead volume of the cylinder. Finally, the mixed steam forming in the cylinder has a pressure which at least almost corresponds to the live steam pressure in the steam supply 8 . The live-steam pressure is achieved by further compression of the already pre-stressed residual steam within the dead volume of the cylinder, with the state of the mixed steam being adjusted after injection of the live steam as a function of the states of the precompressed residual steam and the live steam.

At a crankshaft angle of approximately 30° after top dead center, intake valve 4 is closed again. Due to the compressed mixed steam in the cylinder, the piston 9 is now moved in the direction of bottom dead center in the power stroke, so that the steam is expanded. When the bottom dead center is reached, an outlet opening 5 is opened. In a variant not according to the invention, this is designed as a slot 11 in the cylinder wall, which is released as soon as the piston 9 is in the area of bottom dead center, the exhaust slot being opened by sweeping the piston edge at a crankshaft angle of about 20° before reaching the bottom dead center is released. Expanded mixed steam now escapes through the released outlet opening 5 or the outlet slot 11 .

As soon as the piston 9 moves again in the direction of top dead center, the outlet opening 5 is closed. If the outlet opening does not have an outlet valve but rather the already described non-inventive slot, which is released at about 20° before bottom dead center is reached, this is closed again due to the movement of the piston in the exhaust stroke and the resulting sweeping over of the piston edge . By closing the outlet opening 5 or the outlet slot 11 the residual steam is compressed in such a way that the residual steam pressure is only slightly lower than the live steam pressure when the top dead center is reached. The small difference in pressure between residual and live steam offers the main advantage that when the live steam is injected into the dead volume of the cylinder, vapors that are exergetically very similar are mixed with one another. In addition, the components of the cylinder, in particular the intake valve, are subjected to comparatively little stress due to the compression of the residual steam and the associated small difference between the residual steam pressure and the live steam pressure. In addition, due to the first possible design of the outlet opening 5 described, no valve is required in addition to the inlet valve 4 .

After reaching the top dead center, the intake valve 4 is opened again and the cycle process described begins again.

In a variant for carrying out the method according to the invention, a valve is also provided in the outlet opening 5 , which is actuated via a crank web of the crankshaft 1 , a cam 3 of the camshaft 2 or a further actuating unit 7 . With such a structural design of the outlet opening, the outlet valve is opened as soon as the piston has reached bottom dead center and is closed at a crankshaft angle of approximately 70° to 100° after bottom dead center. After the outlet valve 5 has been closed, the residual vapor remaining in the cylinder is again compressed by the movement of the piston, so that the advantages already mentioned are achieved in this way.

Finally, it is pointed out that due to the preferred operation of a steam piston expander in the two-stroke process, the valves can be suitably closed with the aid of a crank web ( Figure 3a ) can be pressed. The main advantage of this design is that there is no need for an additional camshaft.

Reference List

1

crankshaft

2

camshaft

3

cam

4

intake valve

5

outlet valve

6

position sensor

7

operating unit

8th

steam supply

9

Pistons

10

control unit

11

exhaust slot

Claims (6)

1. A method for operating a piston expander, in the case of which method live steam is conducted out of a steam supply (8) through an inlet valve (4) into a cylinder space, the live steam which is introduced into the cylinder space is expanded in the work cycle on account of a movement of a piston (9) from the top dead centre to the bottom dead centre, and the relieved steam is conducted out of a closable outlet opening (5; 11) into a steam discharge after the bottom dead centre is reached, the outlet opening (5; 11) being opened as soon as the piston is situated in the region of the bottom dead centre, and is subsequently closed before the piston reaches the top dead centre in the discharge stroke, characterized in that the outlet opening (5) is closed in the discharge stroke in a range of a crankshaft angle of from 70° to 100° after the bottom dead centre.
2. The method according to Claim 1, characterized in that the outlet opening (5, 11) is opened at least once at a crankshaft angle of from 20° before the bottom dead centre is reached to 20° after the bottom dead centre is reached.
3. The method according to Claim 1 or 2, characterized in that the inlet valve (4) is closed in a range of a crankshaft position of from 25° to 35° after the top dead centre is reached.
4. The method according to one of Claims 1 to 3, characterized in that a mixed steam is produced by way of the feed of the live steam into the cylinder space in a time period between the closure of the outlet opening (5; 11) and the closure of the inlet valve (4), the mixed steam pressure of which mixed steam corresponds approximately to the live steam pressure.
5. The use of the method according to one of Claims 1 to 4, in a piston expander of a steam circuit which is supplied with heat from a cooling circuit of an internal combustion engine.
6. The use of the method according to one of Claims 1 to 4, in a piston expander of a steam circuit which is supplied with heat from a cooling circuit of a motor vehicle internal combustion engine.

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