The invention relates to an energy converter operat¬ ing according to the Stirling, Ericsson, or similar ther- modynamic cycle and being power-controlled by adjustment of the mean pressure of the working gas with the aid of a least one pressure tank which is filled with working gas and connected to the cylinder(s) of the energy converter and in which the gas pressure is substantially higher tha the mean pressure, thus enabling an increase of the latter; at least one gas pump which at the exhaust side communicates with the pressure tank and at the inlet side generates a negative pressure enabling a decrease of the working-gas pressure; and valves for controlling both the pressure increase and decrease.

It is well-known that such energy converters are ad¬ vantageously power-controlled by varying the mean pressur of the working gas according to the desired output of the energy converter, e.g. for a heat engine, so that, at a certain engine speed, the mean pressure is augmented when the power requirement increases and reduced when the powe requirement decreases. The pressure tank and the associ- ated valve for controlling the pressure increase ensure the increase in mean pressure without any noticeable delay, which means that the engine power can be quickly increased. The difficulty lies in reducing the mean pres¬ sure, and consequently the engine output, in the same rapid manner. The gas pump, which is used for reducing th mean pressure and returning working gas to the pressure tank, has, however, a restricted capacity, since it must generate a pressure at least equalling that in the pres¬ sure tank, and this does not permit sufficiently rapid adjustment. Usually, this difficulty has been evaded by interconnecting the two sides of the working piston by means of a so-called short-circuit valve which rapidly

SUBSTITUTE SHEET

reduces the driving pressure difference across the working piston. However, this suffers from the disadvantage that the supply of fuel (energy) to the engine cannot be re¬ duced at all, or at least not in proportion to the power decrease, which substantially reduces the efficiency of the engine. This state, during which the efficiency is reduced, lasts for as long as it takes for the gas pump to pump the requisite amount of working gas back to the pres¬ sure tank. There are other methods of rapidly reducing the out¬ put of energy converters, e.g. adjusting the dead volume, involving the connection and disconnection of ancillary spaces at a constant mean pressure. However, these methods are either too slow, too complicated or suffering from other limitations, not least as to space requirement, which render them impractical.

The object of the invention is to provide an energy converter enabling rapid adjustment of the output power without causing any substantial losses in efficiency. Fur- ther, the energy converter should be simple in construc¬ tion and, if so required, extremely compact and light.

According to the invention, this object is achieved by at least one low-pressure tank which is accommodated i the energy converter and filled with working gas and in which the gas pressure is substantially below the mean pressure and which communicates both with the inlet side of the gas pump and, via the valve(s) for controlling the decrease in mean pressure, with the cylinder(s) of the energy converter, the total volume of the low-pressure tank(s) being much larger than the stroke volume of the energy converter.

By connecting a comparatively large low-pressure tan between the valve for reducing the mean pressure and the inlet of the gas pump, as in the invention, large amounts of gas can, in an extremely short time, be drawn off from the cylinder(s) of the energy converter, independently of the capacity of the gas pump. The only requirement on the

gas pump is that it should be able to ensure that the pressure differences between the high-pressure tank and the cylinder(s) of the energy converter as well as betwee the cylinder(s) and the low-pressure tank(s) are suffi- ciently large, which in practice means that it is often possible, as compared with the prior art, to choose a slightly smaller pump which, for instance, can be accom¬ modated in the crankcase of the energy converter, in whic case it can be driven directly by the crank mechanism of the energy converter.

The most advantageous arrangement is achieved if the crankcase of the energy converter is used as low-pressure tank, since the volume of the crankcase generally is of a suitable size, which means that the dimensions of the en- ergy converter need not be increased. The relatively low pressure in the low-pressure tank (crankcase) further means that one may choose a slight material thickness favourable as to weight, and that the crank mechanism is not noticeably affected, owing to the comparatively thin atmosphere.

If the dimensions of the energy converter are to be further reduced, also the high-pressure tank is suitably arranged within the energy converter. In energy converters having a domed displacement piston, this can be achieved by using the piston as a high-pressure tank.

Several tanks or volumes having different pressures can be arranged in modulated pressure stages with gas pumps adjusting the different pressures of the volumes. This has the advantage that the volumes can be optimally pressurised in view of their strength and wall thickness, e.g. if in a fully enclosed energy converter is used a magnetic shaft coupling which requires the provision of a thin partition between the driving and driven units, or in view of the risk of working gas leaking to the sur- roundings, e.g. when the crankcase contains a through- going, rotary shaft sealed against the surroundings.

The invention is advantageously used also in energy converters in which are integrated both a driving Stirlin engine and a driven heat pump or refrigerating machine of Stirling type. The outputs of the driving and driven unit can be modulated by ensuring that the gas mean pressure i the driving unit suitably differs from the gas mean pres¬ sure in the driven unit, whereby the desired outputs are obtained. The pressure in the crankcase may simultaneousl be maintained at a comparatively low level. In the cran - case, a mechanically driven gas pump serves to pump work¬ ing gas to a high-pressure tank, from which the working gas can be rapidly distributed, either to the driving or the driven unit, or to both of them. Alternatively, the working-gas pressure in the driving or the driven unit c be rapidly reduced by returning working gas to the crank¬ case. Thus, the two integrated Stirling units can be con¬ trolled and rapidly adjusted without detracting from the efficiency.

In such an integrated machine with a driving Stirli engine and a driven Stirling heat pump or refrigerating machine, one or more electric generators may also be con nected to the crank mechanism, in which case the mean pressures in the driving and driven units can be modulat in such a manner that a desired amount of the mechanical energy generated is used for producing electric current, the remainder being used by the heat pump or the refrige rating machine. Thus, it is possible to control the powe levels in the various parts of the integrated machine in the desired manner without incurring any losses in effi- ciency.

As already mentioned, the invention is advantageous by making it possible, in fully enclosed energy converte operating according to the Stirling, Ericsson or similar thermodynamic cycle, to reduce the wall thickness in the spaces where the gas pressure is low, in order to reduce the weight of the energy converter. When the energy con¬ verter is switched off, working gas is first pumped from

the low-pressure tank to the high-pressure tank, so that the equalised gas pressure in the remainder of the energy converter will not exceed the rated working-gas pressure in e.g. the crankcase and other low-pressure spaces. When the energy converter is started, working gas is supplied to the cylinder to achieve a suitable starting pressure. The prior art, as well as preferred embodiments of the invention, will be described in more detail below, reference being had to the accompanying drawings, in whic Fig. 1 is a schematic view of a prior art energy con verter with mean-pressure control,

Fig. 2 is a schematic view of a single-cylinder Stir¬ ling engine with mean-pressure control according to the invention, Fig. 3 is a schematic view of a four-cylinder double- acting Stirling engine with mean-pressure control accord¬ ing to the invention, and

Fig. 4 is a schematic view of a four-cylinder double- acting Stirling engine connected both to a four-cylinder double-acting heat pump also operating according to the

Stirling cycle, and to integrated electric generators, the Stirling engine and the heat pump being mean-pressure con¬ trolled in accordance with the invention.

The prior art power control using a so-called short- circuit valve is illustrated in Fig. 1. In the cylinder

101 of a single-cylinder Stirling engine, the displacement piston 102 moves the working gas from the hot upper part of the cylinder via the heater 103, in which heat is sup¬ plied to the engine from an external source of energy, e.g. a combustion chamber. The supplied heat accumulates in the regenerator 104, and can thus be used at the right moment of the thermodynamic cycle. The cooler 105 removes heat not transformed into pressure energy and mechanical energy, with the aid of the working piston 106 whose upper part is subjected to the varying gas pressure and at its lower part is subjected to the pressure in the buffer 107 which is determined by a non-return valve 108. In the case

illustrated, the pressure in the buffer is close to the maximum pressure of the cycle.

To increase the output of the engine at a given engin speed, working gas is supplied from the high-pressure tank 109 by opening a valve 110, thereby rapidly increasing the mean pressure of the engine and, consequently, the engine power.

To reduce the power at a given engine speed, a valve 111 is opened, and a gas pump 112 recycles working gas to the high-pressure tank 109 at a rate determined by its capacity. However, this is a slow process, taking about 10-30 s at best, and therefore is unsatisfactory in many applications. For this reason, there is provided a so- called short-circuit valve 113 serving to rapidly equalise the pressure differences across the working piston, thereb rapidly lowering the engine output. While the short-circui valve 113 is open, such amounts of fuel and energy as cor¬ respond to the higher output must be supplied to the en¬ gine. It is not until the gas pump has returned a suffi- cient amount of working gas to the high-pressure tank 109 that the amounts of fuel and energy supplied can be re¬ duced. When a great many rapid power reductions are effected, involving frequent use of the short-circuit val 113, the fuel consumption of the engine becomes unfavour- ably high.

Fig. 2 shows a single-cylinder Stirling engine 201 with a combustion chamber 202 and an air preheater 203. T heat generated upon combustion is supplied to the engine i a heater 204. Heat is stored in a regenerator 205, and he not transformed to pressure energy or mechanical energy is diverted to a cooler 206. A displacement piston 207 moves the working gas from a hot part of the engine to a col part, and vice versa, a variation in pressure arising on the underside of the working piston 208. In a buffer spac 209, the gas mean pressure prevails with only slight vari tions, since the buffer volume is substantially larger th the stroke volume of the engine. The pressure difference

across the working piston drives a mechanism 210 in the crankcase 211, in which the gas pressure is essentially lower than in the buffer 209. The crank mechanism 210 drives two counterrotating electric generators 212, which are hermetically enclosed in the crankcase. Only an elec¬ tric cable supplying the useful electric power communicat with the surroundings. Further, the crank mechanism also drives a gas pump 213 which pumps working gas from the crankcase to a high-pressure bottle 214. Working gas can rapidly supplied to the cylinder of the engine by a valve 215, thereby rapidly increasing the engine output. Simila ly, a valve 216 can be opened to rapidly remove working g from the cylinder of the engine, in which case the engine output rapidly decreases. It is thus possible to rapidly alter the engine output in the desired manner while main¬ taining the efficiency, and thus without increasing the fuel consumption.

Fig. 3 illustrates a four-cylinder double-acting Stir ling engine 301 having a combustion chamber.302 common to all four cylinders; heating tubes 303 arranged in a circle round the zone of combustion; regenerators 304; coolers 305; and pistons 306, one in each cylinder. The cylinders are arranged in a row, and connected to the heat exchanger in accordance with the well-known double-acting Stirling principle. The movements of the pistons are transformed to rotational movements via yokes 307 by means of two counter rotating crankshafts 308 enclosed in the crankcase 309. Th crankshafts 308 are synchronised by means of gear wheels 310. In addition, each crankshaft is connected to a rotat- ing electric generator 311 integrated in the crankcase.

When there is need of increased power, the engine is power-controlled by working gas being supplied to the con¬ nection line 312 in the engine between the cooler 305 and the cold space beneath the piston 306 by opening a valve 313, resulting in that working gas of a high pressure flow from a high-pressure bottle 314. Gas is continuously pumpe from the crankcase 309 of the engine to the high-pressure

bottle 314 by four gas pumps 315, one for each cylinder, the pumps being incorporated in the crankcase and driven directly by the piston rods 316. When there is need of le power, working gas is drawn off from the engine via a val 317 and conducted to the crankcase in which the pressure considerably lower than the gas mean pressure in the en¬ gine, owing to the gas pumps 315. In this manner, it is possible to rapidly increase as well as decrease the engi power by means of the valves 313 and 317 which communicat with all four cylinders in the engine, thereby enabling similar control thereof. Alternatively, the crankcase 309 can be hermetically sealed and have integrated electric generators; be connected via magnetic couplings on the crankshafts to outer driven devices; or be equipped with sealed-off shaft lead-ins formed in the crankcase wall. Fig. 4 illustrates a four-cylinder double-acting Stirling engine 401 which, via piston rods 402 and a crank mechanism 403, drives a four-cylinder double-acting heat pump 404 operating according to the Stirling cycle. Each crankshaft 405 is connected to a rotating electric generator 406 integrated in the crankcase. In the heat pump, there is arranged a heat-absorbing heat exchanger 407, a heat-giving heat exchanger 408, and a regenerator 409. A heat-exchanging medium circulates in the heat ex¬ changer 407 through an inlet 410 and an outlet 411. In similar manner, the heat-exchanger 408 has an inlet 412 and an outlet 413 for a heat-exchanging medium. By means of the high-pressure bottle 414, the Stirling engine wit its four cylinders can be pressurised via the valve 415, while the Stirling heat pump also having four cylinders can be pressurised via the valve 416. Thus, the pressure in the crankcase 417 is substantially lower than those i the two Stirling cycles, owing to four gas pumps 418a, b, c, d (one for each cylinder) pumping working gas to the high-pressure tank. In this manner, the outputs of the Stirling engine and the Stirling heat pump can be sepa-

rately increased by supplying working gas. In analogous manner, the outputs of the two energy converters are reduced by drawing off working gas from the cycles to the crankcase via the valve 419 (the engine) and the valve 429 (the heat pump). The electric generators connected to the crankshafts can be made to supply more or less elec¬ tric current, depending on the control of the engine and heat pump power.

Alternatively, the heat pump in Fig. 4 can serve as a refrigerating machine, the cooling medium flowing through the heat exchanger 407 via the inlet 410 and the outlet 411.

The embodiments of Figs. 2-4 only illustrate con¬ ceivable energy converters operating according to the Stirling, Ericsson or s ⁱ milar thermodynamic cycle and being power-controlled ..'j modulated mean-pressure control according to the invention. It is also possible to use two or more pressure levels in the low-pressure part, e.g. by letting the crankcase work at a low-pressure level while the electric generator housing(s) work(s) at another low-pressure level, provided that several sets of gas pumps and valves are used. Several high-pressure le¬ vels can also be arranged, e.g. one level for the inte¬ rior of the displacement piston connected to the gas pump by a tubular piston rod, and another level for an exter¬ nal high-pressure bottle.