Abstract:
A pressure control device for a vehicle comprises an electronic control unit and a valve device for controllably connecting a compression chamber of the vehicle&#39;s air compressor to the vehicle&#39;s compressed-air supply and storage system. The pressure control device is configured to (i) connect the compression chamber to the compressed-air supply and storage system in a compressed-air production mode to convey compressed air from the compression chamber into the compressed-air supply and storage system, (ii) connect the compression chamber to the compressed-air supply and storage system in a compressed-air expansion mode to convey compressed air from the compressed-air supply and storage system into the compression chamber, and (iii) switch from the compressed-air production mode to the compressed-air expansion mode and vice-versa by electrically actuating one or more electrically operable valves of the valve device.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a pressure control device and method for a vehicle. 
     BACKGROUND OF THE INVENTION 
     In the sector of utility vehicles, that is, motor trucks and omnibuses, it is customary to use compressed air braking installations. Vehicles of this type therefore have elements for generating compressed air and for storage, such as, for example, a compressor and a reservoir. The compressor is usually driven by the vehicle engine—permanent and sometimes also releasable couplings, for example via a controllable clutch, being provided between the compressor and the engine. 
     The proposal to use a compressor as an additional drive unit of a vehicle is disclosed in DE 10 2005 039 281 A1. In this case, it is proposed to operate the existing valves of a compressor, to be precise a high-pressure valve and a low-pressure valve, mechanically via a cam mechanism. In order to change over between the compressor mode and engine mode as operating modes, it is proposed to use a cam phaser, which is known from engine technology. A similar proposal is also disclosed in DE 10 2007 033 693 A1. 
     However, the use of a cam mechanism with a cam phaser is relatively complicated and costly in mechanical and structural terms. Also, this necessitates a reconstruction of the cylinder head of the compressor. 
     SUMMARY OF THE INVENTION 
     Generally speaking, it is an object of the present invention to provide a simplified and cost-effective implementation for operating a compressed air compressor—both for generating compressed air and as an additional drive for the vehicle. 
     The invention is based on the idea of utilizing the compressed air compressor usually present in a utility vehicle for assisting the vehicle engine. Thus, the compressed air compressor can be operated as a pneumatic engine to assist an acceleration action of the vehicle or when the vehicle is driving uphill, in that compressed air is conveyed out of a compressed air reservoir of the vehicle back into the compressor, specifically in synchronism with the movement of the compressor, such that the engine undergoes an assisting torque. As a result, in a similar way to a hybrid electric drive of a passenger car, the vehicle engine usually designed as an internal combustion engine can have a lower nominal power rating. A smaller, lighter and more cost-effective engine can therefore be used. This operating mode is also designated as a compressed air expansion mode. 
     Correspondingly, in a compressed air generation mode, compressed air compressed by the compressor is conducted from a compression space of the compressed air compressor into the compressed air supply and storage installation and, in the compressed air expansion mode, compressed air is conducted from the compressed air supply and storage installation into the compression space where the compressed air is expanded to assist the drive of the vehicle. 
     In a medium-weight utility vehicle, which nowadays is a conventional vehicle type, when a commercially available compressed air compressor is used, for example, an additional average drive torque of approximately 50 Nm can be expected by virtue of the invention, this being a considerable value, as compared with a drive torque of a diesel engine of about 700 Nm. 
     According to an embodiment of the present invention, a pressure control device is configured to change over from the compressed air generation mode into the compressed air expansion mode, and vice-versa, as a result of the electrical actuation of one or more electrically actuatable valves of a valve assembly by means of an electronic control apparatus. For this purpose, the electronic control apparatus may be provided with corresponding programming for actuating the electrically actuatable valves when a changeover demand is recognized. It is therefore advantageous that no complicated mechanical structures, such as a cam phaser, are required for changing over from the compressed air generation mode into the compressed air expansion mode and back again. Instead, electrically actuatable valves, which are actuated by the electronic control apparatus, can be provided for this purpose. The advantage of this is that the inventive embodiments can be implemented with existing commercially available compressed air compressors, either only simple, cost-effective modifications or no modifications at all being required on the compressed air compressors, depending on the configuration and arrangement of the electrically actuatable valves. In the latter case, the electrically actuatable valves may precede or follow the compressed air compressor. 
     Insofar as reference is made herein to crankshaft-dependent control, this embraces mechanical crankshaft-dependent control, for example via a camshaft, and any other type of crankshaft-dependent control, for example electrical control in which, for example, the crankshaft position is detected via a crankshaft sensor and control is carried out to some extent in a crankshaft-dependent way as a function of the sensor signal, for example the actuation of electrically actuatable valves. 
     Insofar as mechanical crankshaft-dependent control is referred to, this covers, for example, control via a camshaft, in which case cam actuation may be implemented, for example, via mechanical tappets or hydraulic tappets. 
     What is included, furthermore, is electrohydraulic actuation such that the electrically actuatable valves used are hydraulic valves, which, as a result of electrical actuation, allow a throughflow, shut-off or change of the flow of a hydraulic medium. 
     In embodiments in which mechanical crankshaft-dependent control is not provided, but purely electrically actuatable valves are used, crankshaft-dependent control takes place as a result of the correspondingly clocked actuation of these valves. In this case, a changeover between the compressed air generation mode and the compressed air expansion mode can be carried out by means of a change in the activation signals, that is, for example, a change in the clocking of these valves. For this purpose, a control program of the electronic control apparatus stores the type of actuation of electrical valves, which is necessary in the respective operating mode. 
     The possibilities explained in more detail hereinafter for implementing the invention by means of the electrically actuatable valves may be classified technically as follows. In this case, a compressed air compressor is assumed which, as is customary, for example, in piston compressors, has an inlet valve (also called an intake valve or low-pressure valve) and an outlet valve (also called an expulsion valve or high-pressure valve) for the compressed air generation mode. To distinguish this from the compressed air expansion mode, these valves may also be designated as a compression inlet valve and a compression outlet valve. 
     1. The compressed air compressor remains unchanged: 
     An inlet connection, leading to the compression inlet valve, of the compressed air compressor is preceded by an electrically actuatable valve; an outlet connection, leading to the compression outlet valve, of the compressed air compressor is followed by a further electrically actuatable valve. 
     2. The compressed air compressor is modified, the compression inlet valve and the compression outlet valve remaining unchanged: 
     An additional connection or tap is provided in the cylinder head of the compressed air compressor such that an electrically actuatable valve can be connected to the compression space via the additional connection. The connectable electrically actuatable valve can be formed from discrete individual valves for a compressed air inlet function in the compressed air expansion mode (expansion inlet valve) and for the compressed air outlet function in the compressed air expansion mode (expansion outlet valve) or else as a combined expansion inlet/outlet valve, for example in the manner of a 3/2-way valve. The expansion inlet and outlet valves may also have, in addition to electrical actuation, crankshaft-dependent mechanical actuation, for example via a camshaft. 
     3. The compressed air compressor is also modified in terms of the compression inlet valve and/or of the compression outlet valve:
         a) The compression inlet valve and the compression outlet valve of the compressed air compressor are replaced by electrically controllable inlet and outlet valves, by means of which both the compressed air generation mode and the compressed air expansion mode are implemented by electronic control.   b) Mechanically crankshaft-controlled inlet valves and outlet valves are provided, which can be locked in an open or closed position by electrical actuation. Both the compressed air generation mode and the compressed air expansion mode are implemented by means of the mechanically crankshaft-controlled inlet valves and outlet valves.   c) The compressed air compressor is provided with additional mechanically crankshaft-controlled expansion inlet and expansion outlet valves, which can then be connected to the compressed air reservoir or to the atmosphere via electrically actuatable valves.       

     It is also advantageous that combinations of the abovementioned embodiments can be implemented. Thus, in the abovementioned embodiments a) to c), for example, the compression inlet valve of the compressed air compressor may also be kept unchanged. An additional crankshaft-controlled expansion outlet valve is then added, which can be connected to the atmosphere via an electrically actuatable valve. In addition, the existing compression outlet valve of the compressed air compressor may be preserved, but may be additionally controlled in a crankshaft-dependent way. 
     Advantageously, a pressure control device detects, for example by means of an electronic control apparatus, whether there is increased demand for drive power when the vehicle is in operation, for example when the vehicle is to be accelerated or when it is driving uphill. In this case, the pressure control device ensures that the compressed air compressor is operated in a compressed air expansion mode. In the compressed air expansion mode, the engine is assisted by compressed air from the compressed air supply and storage installation by the compressed air compressor being acted upon with compressed air. Advantageously, also, the pressure control device detects overrun phases when the vehicle is in operation. An overrun phase is detected, for example, when the driver wants to decelerate the vehicle or when driving downhill. When an overrun phase is present, by means of the pressure control device, the compressed air compressor is automatically operated in a compressed air generation mode in which the compressed air compressor conveys compressed air into the compressed air supply and storage installation. Thus, during overrun phases when the vehicle is in operation, the bleeding of compressed air from the compressed air supply and storage installation, which was carried out during the phases of increased drive power demand, can be compensated for. It is thereby possible for the vehicle to be operated in an especially energy-saving way. 
     The compressed air compressor may additionally be switched by the pressure control device into an idling mode in which compressed air is neither generated nor consumed in the compressed air expansion mode. The idling mode is provided as a third operating mode in addition to the compressed air generation mode and to the compressed air expansion mode. It is known, for example, to use a connectable dead space in the case of a single-cylinder compressed air compressor, to connect the pressure spaces in the case of a multicylinder compressed air compressor or to uncouple the compressed air compressor from the engine by means of a shiftable clutch. 
     According to an embodiment of the invention, the pressure control device is designed, when a compressed air compressor separable from the vehicle engine via a shift clutch is used, for connecting this compressed air compressor automatically to the engine when the compressed air compressor is operated in the compressed air generation mode or in the compressed air expansion mode. 
     According to an embodiment of the invention, the valve assembly has a connecting valve that is configured to directly connect two compression spaces of a multicylinder compressed air compressor. The pressure control device is further configured to open the connecting valve in the idling mode, with the result that the compression spaces connected to the connecting valve are connected to one another. By the connecting valve being opened, a connection is made between the connected compression spaces, as a result of which, for example in the case of a conventional two-cylinder compressor, the air present in the compression spaces is, in each case, pressed into the other compression space during the upward stroke of the piston, and vice-versa. Advantageously, in such multicylinder compressors, the pistons are arranged contradirectionally to one another by an appropriate configuration of the crankshaft, such that, during the upward stroke of the piston, the adjacent piston executes a downward stroke. 
     According to an embodiment of the invention, all or at least some of the electrically actuatable valves are designed as piezoelectrically, electromagnetically, electromotively, electropneumatically or electrohydraulically actuatable valves. In this case, especially for those of the abovementioned embodiments in which mechanical crankshaft-dependent control is not provided at all, it is especially advantageous to use piezoelectrically actuatable valves since these make it possible to have especially short response and reaction times along with relatively large implementable passage cross sections for the compressed air. 
     According to another embodiment of the invention, all or at least some of the electrically actuatable valves are integrated into the cylinder of the compressed air compressor. The integration of the valves into the cylinder head can be especially advantageous. Also, all or at least some of the electrically actuatable valves can be integrated into the crankcase of the compressed air compressor. The advantage of this is that the compressed air compressor can be designed, together with the electrically actuatable valves, as a robust and compact unit, which can also be of a type that saves construction space. A further advantage is that the electrically actuatable valves can be arranged in the immediate vicinity of their respective place of use, so that rapid response times can be implemented because of short flow paths and small flow volumes. 
     According to an embodiment of the invention, all or at least some of the electrically actuatable valves precede the inlet connection of the compressed air compressor. Also, all or at least some of the electrically actuatable valves can follow the outlet connection of the compressed air compressor. The advantage of this is that the invention can be implemented with commercially available compressed air compressors, these not having to be modified at all or having to be modified only slightly. A simple and cost-effective possibility for retrofitting the present invention in existing compressed air installations is thereby also afforded. 
     According to another embodiment of the invention, the valve assembly is connected to an additional tap of the compression space of the compressed air compressor. This likewise permits a simple and cost-effective implementation, in combination with existing purchasable compressed air compressors, and therefore a simple possibility of retrofitting for the invention. The tap may be provided, for example, in the form of a bore on the cylinder head. The valve assembly is then connected to this bore. 
     According to a further embodiment of the invention, the volume of the compression space is variable in time. This does not mean, for example, that the compressor or part thereof, for example the cylinder, changes its form in time, but rather, for example in a piston compressor, an upward and a downward movement of the piston. In a volume reduction phase, the volume decreases in time, while in a volume enlargement phase, the volume increases in time. The pressure control device is configured, in the compressed air expansion mode, for connecting the compression space to the compressed air supply and storage installation during a volume enlargement phase and for connecting the compression space to the atmosphere during a volume reduction phase. In the compressed air expansion mode, therefore, the compression space is relieved in the volume reduction phase. The compressed air previously fed into the compression space during a volume enlargement phase can thus be discharged into the atmosphere. The compressed air compressor is consequently operated in the compressed air expansion mode “in reverse” to the compressed air generation mode. In the compressed air generation mode, compressed air is advantageously sucked in from the atmosphere during the volume enlargement phase and compressed air is dispensed to the compressed air supply and storage installation in a volume reduction phase. 
     According to an embodiment of the invention, the connection of the compression space to the compressed air supply and storage installation is made, in the compressed air expansion mode, only during part of the volume enlargement phase. As a result, the efficiency of the compressed air compressor as an assistant to the vehicle drive can be optimized in terms of energy consumption. In particular, because of this, only as much compressed air is consumed as is expedient and necessary for as economical a use as possible. 
     According to another embodiment of the invention, in the compressed air expansion mode, the connection of the compression space to the compressed air supply and storage installation is made immediately at the commencement of the volume enlargement phase. In the case of a piston compressor, this means that the compression space is connected to the compressed air supply and storage installation from the time when top dead center of the piston is reached. 
     According to a further embodiment of the invention, in the compressed air expansion mode the connection of the compression space to the compressed air supply and storage installation is made only during half the duration of the volume enlargement phase. As a result, compressed air consumption can be minimized and therefore the energy efficiency of the device can be increased. A further reduction in compressed air consumption can advantageously be achieved if the connection is made only during one quarter of the volume enlargement phase. A further reduction in compressed air consumption is advantageously possible if the connection is made only during one eighth of the volume enlargement phase. The expansion outlet valve is, in this case, kept open during one complete half revolution of the crankshaft from bottom dead center to top dead center. The control times for the crankshaft-dependent control of the expansion inlet valve and of the expansion outlet valve are set correspondingly. 
     Consistently, in another embodiment of the invention, the pressure control device is designed, in the compressed air generation mode, for connecting the compression space to the compressed air supply and storage installation during a volume reduction phase, especially when the pressure in the compression space is higher than the pressure in the compressed air supply and storage installation, and for connecting the compression space to the atmosphere during a volume enlargement phase, especially when the pressure in the compression space is lower than atmospheric pressure. To implement the compressed air generation mode, an arrangement of a compression inlet valve and a compression outlet valve, for example, in the compressor cylinder head, may advantageously be provided. The compression inlet valve and compression outlet valve may be designed as nonreturn valves, for example as lamella valves. 
     As described above, in the compressed air generation mode, the respective connection of the compression space to the atmosphere or to the compressed air supply and storage installation can be carried out purely by overpressure or underpressure control. In an advantageous embodiment of the invention, the control of these connections can take place in a crankshaft-dependent way, for example via a camshaft or by means of an electronic control apparatus. Also, the compression inlet valve can be opened during one complete half revolution between top dead center and bottom dead center of the piston. Further, the compression outlet valve may not be opened completely during half the crankshaft revolution, but only during part, in particular, during the last half of the crankshaft revolution, before top dead center is reached, or only during the last 35° of revolution before top dead center is reached. 
     According to an embodiment of the invention, in the compressed air expansion mode the quantity of compressed air can be fed into the compression space by the pressure control device synchronously with the rotation of the drive shaft of the compressed air compressor. Optimal efficiency in drive power assistance and accurate synchronicity with the rotation of an output shaft to the engine can thereby be ensured. 
     According to another embodiment of the invention, the pressure control device has a valve assembly with an aeration path and with a deaeration path for aerating and deaerating the compression space of the compressed air compressor. The aeration path and the deaeration path are connected or connectable to the compression space. The aeration path can be connected to the compressed air supply and storage installation and the deaeration path to the atmosphere. Advantageously, the valve assembly can be actuated mechanically via a camshaft connected or connectable to the output shaft of the engine. The camshaft control tried and tested in internal combustion engines can thereby also be used for controlling the compressed air compressor in the compressed air expansion mode. Camshaft controls can have a highly robust design with a long service life. The camshaft may be connected permanently to the output shaft of the engine or may be connectable temporarily to the output shaft of the engine, for example via an actuatable clutch. The camshaft may be implemented, for example, by the arrangement of cams on the crankshaft of the compressed air compressor. It is also possible to provide a separate camshaft connected to the crankshaft of the compressed air compressor or to the output shaft of the engine. 
     According to an embodiment of the invention, the valve assembly has at least one electrically actuatable solenoid valve. Moreover, an electronic control device for controlling the solenoid valve is provided. The design as a solenoid valve likewise makes it possible to have control of the compressed air compressor in the compressed air expansion mode, while tried and tested solenoid valve technology can be adopted, for example solenoid valves with a long service life, from the sector of antilock systems. The use of an electronic control device has the advantage that detection of the overrun phases and of the phases with an increased drive power demand can be integrated in a single control, together with the control of the compressed air compressor by the solenoid valve. Control may be implemented as a control program for a microprocessor provided in the electronic control device. 
     According to an embodiment of the invention, the electronic control device of the pressure control device is fed with a signal from a sensor, via which the position of the crankshaft of the compressed air compressor or, in the case of appropriate calibration regarding the position of the crankshaft of the engine, alternatively also the crankshaft position of the latter is detected. The signal may be fed directly to the electronic control device by a corresponding inductive, optical or otherwise suitable sensor. The electronic control device uses the signal indicating the position of the crankshaft for the in-phase control of the electrically actuatable valves of the valve assembly. 
     According to another embodiment of the invention, the electronic control device receives from an engine control apparatus information on the angular position of the output shaft of the vehicle engine. The engine control apparatus in this case serves for controlling the vehicle engine. Known engine control apparatuses have in any case information determined, for example, via sensors on the angular position of the output shaft of the vehicle engine. By the electronic control device being connected to the engine control apparatus, for example via a data bus provided in the vehicle, information on the angular position of the output shaft of the vehicle engine can be received cost-effectively in the electronic control device and can be used for controlling the solenoid valve in the compressed air expansion mode of the compressed air compressor. 
     Since the angular position of the output shaft of the vehicle engine with respect to the angular position of the crankshaft of the compressor may be different from vehicle to vehicle because of the type of mounting, according to an embodiment of the invention this relative position can be determined and compensated for by an electronic control device. For this purpose, the pressure peaks in the compressed air installation due to the compression action are evaluated in time via at least one pressure sensor present in the vehicle. An angle difference between the angular position of the crankshaft of the compressed air compressor and the angular position of the output shaft is determined as a correcting value from this. The electronic control device subsequently uses the calculated correcting value for the in-phase control of the valve assembly. 
     According to yet another embodiment of the invention, the pressure control device has a further valve, via which a crankcase of the compressed air compressor can be acted upon with compressed air. A further increase in the efficiency of assisting the vehicle drive is thereby possible. By the crankcase being acted upon by pressure, the assistance of the engine via an additional torque from the compressed air compressor can take place even in the volume reduction phases. A piston of the compressed air compressor can thus be acted upon with compressed air now from one side and now from the other side, depending on the operating phase (upward movement, downward movement). Thus, virtually permanent assistance of the engine by the compressed air compressor is possible during the entire revolution of the drive shaft of the compressed air compressor. The valve assembly for acting upon the compression space with compressed air is, in this case, operated complementarily in phase with the further valve assembly for acting upon the crankcase with compressed air. In other words, when the compression space is connected to the compressed air reservoir via the valve assembly, the crankcase is connected to the atmosphere. When the crankcase is acted upon with compressed air by the compressed air reservoir, the compression space is connected to the atmosphere. 
     The present invention generally relates, moreover, to a vehicle compressed air installation with a pressure control device and with a compressed air compressor of the type described above. According to one embodiment, the vehicle compressed air installation has a heat exchanger, which, on the one hand, is thermally coupled to the engine or to parts of the vehicle heated by the engine. On the other hand, the heat exchanger is thermally coupled to parts of the compressed air supply and storage installation for dispensing the heat absorbed by the engine or by parts heated by the engine, so that heat can be dispensed to the compressed air in the compressed air supply and storage installation and therefore the compressed air can be heated. To absorb the heat, the heat exchanger may be coupled directly to the engine or parts connected thereto, for example to the exhaust system or to the deconverter. The advantage of this is that the waste heat from the engine can be used for further increasing the energy efficiency of the vehicle compressed air installation. The compressed air is therefore fed with further energy, which can be utilized when the compressed air is used in the compressed air expansion mode or when the compressed air is used in another way in the compressed air installation, for example for braking the vehicle. In particular, by heat being dispensed to the compressed air, the available pressure in the compressed air reservoir or in other parts of the compressed air installation can be increased, so that compressed air at a higher level is available and can be used correspondingly. 
     According to another embodiment of the invention, the pressure control device is configured to permit dispensing of heat to the compressed air supply and storage installation only when the compressed air generation mode is not activated. The advantage of this is that, in the compressed air generation mode, the compressed air compressor does not have to convey counter to a backpressure, which is already increased by the heat supplied, which, in turn, has the advantage that less energy is used for driving the compressed air compressor and also the wear of the compressed air compressor is lower. For this purpose, the pressure control device may be equipped with a further, for example electrically actuatable, valve via which a heat fluid circuit through the heat exchanger can temporarily be shut off. Alternatively, for this purpose, a pump provided for conveying the heat fluid may also be switched off. 
     The present invention also generally relates to a compressed air compressor for such a vehicle compressed air installation. 
     Moreover, the present invention generally relates to a method for controlling pressure in a vehicle, the vehicle having at least one engine for generating vehicle drive power, a compressed air supply and storage installation and a compressed air compressor coupled or coupleable to the engine of the vehicle, having the following features: 
     a) in a compressed air generation mode, compressed air is conducted from a compression space of the compressed air compressor into the compressed air supply and storage installation, 
     b) in a compressed air expansion mode, compressed air is conducted from the compressed air supply and storage installation into the compression space, 
     c) the changeover from the compressed air generation mode into the compressed air expansion mode, and vice-versa, takes place as a result of the electrical actuation of one or more electrically actuatable valves of the valve assembly by means of an electronic control apparatus. 
     According to an embodiment of the invention, during an overrun phase when the vehicle is in operation, the compressed air compressor is operated in the compressed air generation mode and/or, in a phase with an increased drive power demand when the vehicle is in operation, the compressed air compressor is operated in the compressed air expansion mode. 
     Advantageously, the electronic control device and a pressure sensor, necessary where appropriate, may be designed as part of an EAPU. An EAPU is an electronically controlled air processing unit for a vehicle provided with a compressed air installation. In this case, in particular, the control function of the pressure control device may be implemented in the form of a program part in the control software of the EAPU. 
     Still other objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification. 
     The present invention accordingly comprises the features of construction, combination of elements, arrangement of parts, and the various steps and the relation of one or more of such steps with respect to each of the others, all as exemplified in the constructions herein set forth, and the scope of the invention will be indicated in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in more detail below by means of exemplary embodiments, with reference to the accompanying drawings in which: 
         FIG. 1  illustrates a vehicle equipped with a compressed air compressor of a general type under consideration; 
         FIGS. 2 to 8  show the compressed air components of the vehicle together with various embodiments of the pressure control device according to the present invention; and 
         FIGS. 9 and 10  show exemplary valve control times of the pressure control device according to embodiments of the present invention. 
     
    
    
     The same reference symbols are used in the figures for corresponding elements. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a vehicle  100  that has an engine  101 , for example a diesel engine, which, via a drive shaft  102  connected cardanically to the rear axle of the vehicle, can furnish drive power for driving the vehicle  100 . The engine  101  is connected via a shaft  103  to a compressed air compressor  1  of the vehicle  100 . The shaft  103  is at the same time the output shaft of the engine  101  and the drive shaft of the compressed air compressor  1 . Depending on design, step-up may also be provided between the engine and the compressed air compressor by means of a transmission. Moreover, the compressed air compressor  1  is connected via a compressed air line to a compressed air supply and storage installation of the vehicle  100 , the installation being symbolized in the drawing figures by a compressed air reservoir  12 . The compressed air supply and storage installation has, for example, a multicircuit protection valve, an air dryer and a compressed air reservoir in the form of an overpressure-resistant vessel. 
     Insofar as the electrically actuatable valves are illustrated with a symbol for a magnet, this will embrace all types of electrical actuation, in particular piezoelectric, electromagnetic, electrohydraulic, electropneumatic and electromotive actuation. 
       FIG. 2  shows a diagrammatic illustration of the compressed air compressor  1 , which may be designed as a single-cylinder or multicylinder compressed air compressor, and of the compressed air supply and storage installation, symbolized by the compressed air reservoir  12 . The compressed air compressor  1  is designed as a conventional piston compressor. The compressed air compressor  1  has a cylinder  2  and a piston  3  located therein. Below the cylinder  2 , the compressed air compressor  1  has a crankcase  10  in which a connecting rod drive for the piston  3  is provided. The connecting rod drive has a connecting rod  11  connected to the drive shaft  103  of the compressed air compressor  1  and which is connected on one side to the piston  3  and on the other side to a connecting rod pin on the drive shaft  103 . When the drive shaft  103  rotates, the piston  3  is set in cyclic upward and downward movement via the connecting rod drive. 
     The piston  3  divides the compressed air compressor  1  into a compression space  4  and a crankshaft space, which is separate from the latter on the pressure medium side and which is connected on the pressure medium side to the inner space  5  of the crankcase  10 . During a downward movement of the piston  3 , the volume of the compression space  4  is enlarged. The compressed air compressor is then in a volume enlargement phase in which air is sucked in from the atmosphere via an inlet connection  13  and passes into the compression space  4  via a compression inlet valve  16  serving as an intake valve. The compression inlet valve  16  may be designed as a nonreturn valve that opens automatically as a result of underpressure in the compression space  4 , as compared with atmospheric pressure, and closes automatically in the event of overpressure. 
     During an upward movement of the piston  3 , the volume of the compression space  4  is reduced. The air located in it is compressed. When a pressure is reached in the compression space  4  that is higher than the pressure in the connected compressed air reservoir  12 , a compression outlet valve  17 , likewise designed as a nonreturn valve, opens. The compressed air is led via the latter from the compression space  4  to an outlet connection  14  of the compressed air compressor  1  and passes via a compressed air line  15  into the compressed air reservoir  12 . In a volume enlargement phase, the compression outlet valve  17  closes automatically on account of the underpressure present in the compression space  4 , as compared with the pressure at the outlet connection  14  of the compressed air compressor  1 . 
       FIG. 2  also illustrates, as part of a pressure control device, an electronic control device designed as an electronic control apparatus  18 . The electronic control apparatus  18  is connected via a data connection  19 , for example a databus in the form of a CAN bus, to further electronic devices in the vehicle. In particular, the electronic control apparatus  18  can be connected to an engine control apparatus of the vehicle engine. The electronic control apparatus  18  is connected via an electrical line to a first electrically actuatable valve  7  designed as a solenoid valve in the form of a 2/2-way valve. The first electrically actuatable valve  7  is connected via a compressed air line  20  to a tap  33  of the compression space  4 . The tap  33  may be designed, for example, as a bore in the cylinder  2  of the compressed air compressor. The first electrically actuatable valve  7  can be switched from the shut-off position illustrated in  FIG. 2  into a passage position by the electronic control apparatus  18 . In the passage position, the first electrically actuatable valve  7  connects the compression space  4  to a first mechanically actuatable valve  9  of the illustrated valve assembly  7 ,  8 ,  9 ,  70 . In the shut-off position, the first electrically actuatable valve  7  separates this connection. Moreover, the electronic control apparatus  18  is connected via an electrical line to a second electrically actuatable valve  70  designed as a solenoid valve in the form a 2/2-way valve. The second electrically actuatable valve  70  can be switched from the shut-off position illustrated in  FIG. 2  into a passage position by the electronic control apparatus  18 . In the passage position, the second electrically actuatable valve  70  connects the inner space  5  of the crankcase  10  to a second mechanically actuatable valve  8  of the illustrated valve assembly  7 ,  8 ,  9 ,  70 . In the shut-off position, the second electrically actuatable valve  70  separates this connection. 
     The first mechanically actuatable valve  9  is designed as a mechanically actuatable 3/2-way valve. In an aeration position, which is illustrated in  FIG. 2 , the first mechanically actuatable valve  9  connects the compression space  4  via a compressed air line  20  to the compressed air reservoir  12 , insofar as the first electrically actuatable valve  7  is in the passage position. In a deaeration position, the first mechanically actuatable valve  9  connects the compression space  4  via the compressed air line  20  to the atmosphere, insofar as the first electrically actuatable valve  7  is in the passage position. 
     The second mechanically actuatable valve  8  is likewise designed as a mechanically actuatable 3/2-way valve. The second mechanically actuatable valve  8  is connected via a compressed air line  21  to the inner space  5  of the crankcase  10 . The second mechanically actuatable valve  8  has a comparable function to the first mechanically actuatable valve  9 , to be precise, a connection of the crankshaft  10  selectively to the compressed air reservoir  12  or to the atmosphere, insofar as the second electrically actuatable valve  70  is in the passage position. 
     The first and the second mechanically actuatable valves  8 ,  9  are actuated mechanically via a camshaft control. A cam  6  is illustrated symbolically on the drive shaft  103  of the compressed air compressor  1 . The camshaft control is illustrated by dashed lines from the cam  6  to mechanical actuation elements of the mechanically actuatable valves  8 ,  9 . The camshaft control is designed such that the mechanically actuatable valves  8 ,  9  are actuated complementarily in phase, that is, with the first mechanically actuatable valve  9  actuated mechanically, the second mechanically actuatable valve  8  is nonactuated. With a second mechanically actuatable valve  8  actuated mechanically, the first mechanically actuatable valve  9  is nonactuated. As a result, in the compressed air expansion mode, the compression space  4  and the inner space  5  of the crankcase  10  are acted upon alternately with compressed air from the compressed air reservoir  12 . 
     The electronic control apparatus  18  switches the first and the second electrically actuatable valves  7 ,  70  into the passage position in the compressed air expansion mode and into the shut-off position in the compressed air generation mode. 
     In the embodiment according to  FIG. 2  the pressure control device comprises the electronic control apparatus  18  and a valve assembly formed from the electrically actuatable valves  7 ,  70  and from the mechanically actuatable valves  8 ,  9 . 
       FIG. 3  shows a further embodiment of the pressure control device. The pressure control device again has the electronic control apparatus  18 . The valves  8 ,  9  actuatable mechanically via the camshaft control according to  FIG. 2  are designed according to  FIG. 3  as electrically actuatable solenoid valves, specifically as a third electrically actuatable valve  80  and as a fourth electrically actuatable valve  90 , these valves being controllable by the electronic control apparatus  18  via electrical lines. In terms of their pneumatic valve function, the third and the fourth electrically actuatable valves  80 ,  90  correspond to the mechanically actuatable valves  8 ,  9  described with reference to  FIG. 2 . The solenoid valve  7  of  FIG. 3  corresponds to the first electrically actuatable valve  7  according to  FIG. 2 , in the version according to  FIG. 3  only the first electrically actuatable valve  7  being provided as a central shut-off/passage valve arranged between the third and fourth electrically actuatable valves  80 ,  90  and the compressed air reservoir  12 . The use of two solenoid valves  7 ,  70 , as in  FIG. 2 , is basically also possible in the version according to  FIG. 3 , but is not absolutely necessary, since the third and the fourth electrically actuatable valves  80 ,  90  are not permanently camshaft-controlled, but instead can be brought by the electronic control apparatus  18 , in the compressed air generation mode, into a neutral position for compressed air generation. Thus, in the compressed air generation mode, the third electrically actuatable valve  80  can be nonactuated and the fourth electrically actuatable valve  90  can be actuated. The version according to  FIG. 3  can thereby be designed cost-effectively with only one solenoid valve  7  as a central shut-off/passage valve. 
     The control of the third and of the fourth electrically actuatable valves  80 ,  90  is carried out by the electronic control apparatus  18  synchronously with the rotation of the drive shaft  103  of the compressed air compressor  1  on the basis of information the electronic control apparatus  18  receives from an engine control apparatus via the databus  19 . Maximum efficiency in the assistance of the engine by the compressed air compressor can thereby be ensured. 
     A correcting value, required if appropriate, of the relative angular position of the crankshaft of the compressed air compressor  1  with respect to the position of the drive shaft  103  can be calculated and taken into account by the electronic control apparatus  18  on the basis of the evaluation in time of the pressure peaks via the pressure sensors present in the vehicle. 
     Alternatively, to detect the rotary angle position of the drive shaft  103 , the electronic control apparatus  18  may also be connected to an electrical sensor  22 , which directly detects the rotary angle position of the drive shaft  103  or of the crankshaft of the compressed air compressor  1 . The sensor  22  may, for example, be designed in a similar way to a sensor of an antilock system as an inductive sensor, which senses the angular position of the drive shaft  103  or of the crankshaft of the compressed air compressor  1  by detecting the position of a cam  6 . 
     As can be seen, in the embodiment according to  FIG. 3  the pressure control device comprises the electronic control apparatus  18 , the sensor  22  and a valve assembly formed from the first, the third and the fourth electrically actuatable valves  7 ,  80 ,  90 . 
     In the devices according to  FIGS. 2 and 3 , the compressed air expansion mode is activated by the electronic control apparatus  18 , in that the first electrically actuatable valve  7  and, if appropriate, the second electrically actuatable valve  70  are switched from the shut-off position into the passage position. It thereby becomes possible for the compression space or the inner space  5  of the crankcase  10  to be acted upon with corresponding pressure via the first and the second mechanically actuatable valves  8 ,  9  or the third and the fourth electrically actuatable valves  80 ,  90 . By the compression inlet valve  16  being designed as a nonreturn valve, there is no need for special control of the compression inlet valve  16  in the compressed air expansion mode. By the compression space being acted upon with compressed air via the compressed air line  20 , the compression inlet valve  16  closes automatically. Also, there is no need for special control of the compression outlet valve  17 , since this is kept closed via the pressure prevailing at the outlet connection  14  via the compressed air line  15 . 
     The embodiments according to  FIGS. 2 and 3  may also be implemented without those valves that are provided for acting with pressure upon the crankcase  10 . In this case, the invention can be implemented with fewer modifications to the compressed air compressor and with fewer components, but also without the possibility of assisting the vehicle drive in the upward phases of the piston  3 . Similarly, the embodiments described below according to  FIGS. 4 to 8  may also additionally be configured with the action of pressure upon the crankcase  10 , as described with reference to  FIGS. 2 and 3 . 
       FIG. 4  shows an advantageous embodiment of the pressure control device that can be used in conjunction with a compressed air compressor  1  of conventional construction, without modifications to the compressed air compressor being necessary. The embodiment according to  FIG. 4  is therefore suitable especially for the retrofitting of the invention on existing vehicles, in which no additional compressed air connection is to be retrofitted on the compression space  4  of the compressed air compressor  1 . The pressure control device has the electronic control apparatus  18 , which can be connected to the databus  19  or to a sensor  22 , as required. The pressure control device also has a fifth electrically actuatable valve  23  designed as a solenoid valve and a sixth electrically actuatable valve  24  designed as a solenoid valve. The fifth electrically actuatable valve  23  precedes the inlet connection  13  of the compressed air compressor  1 . The sixth electrically actuatable valve  24  follows the outlet connection  14  of the compressed air compressor and is arranged in the compressed air line  15  to the compressed air reservoir  12 . The fifth electrically actuatable valve  23  is designed as a 3/2-way valve. The sixth electrically actuatable valve  24  is likewise designed as a 3/2-way valve. However, the 3/2-way valves may also be designed as a combination of two 2/2-way valves. 
     The fifth electrically actuatable valve  23 , in one switching position, can connect the inlet connection  13  of the compressed air compressor  1  to the atmosphere, as illustrated in  FIG. 4 . In a second switching position, the fifth electrically actuatable valve  23  can connect the inlet connection  13  to the compressed air reservoir  12 . 
     In the switching position illustrated in  FIG. 4 , the sixth electrically actuatable valve  24  can connect the outlet connection  14  of the compressed air compressor to the compressed air reservoir  12 . In a second switching position, the sixth electrically actuatable valve  24  can connect the outlet connection  14  to the atmosphere. 
     The electronic control apparatus  18  controls the fifth and the sixth electrically actuatable valves  23 ,  24  such that, in the compressed air generation mode, both the fifth electrically actuatable valve  23  and the sixth electrically actuatable valve  24  are nonactuated and are therefore in their switching position illustrated in  FIG. 4 . In this case, the inlet connection  13  is connected to the atmosphere and the outlet connection  14  is connected to the compressed air supply and storage installation  12 . In the compressed air expansion mode, the electronic control apparatus  18  activates the fifth and the sixth electrically actuatable valves  23 ,  24  as a function of the rotary angle position of the drive shaft  103 , that is, depending on the presence of a volume reduction phase or volume enlargement phase. The rotary angle position is determined, for example, via the sensor  22  or by the reception of corresponding information from an engine control apparatus via the databus  19 . 
     In the compressed air expansion mode, the electronic control apparatus  18  actuates the fifth electrically actuatable valve  23  in a volume enlargement phase. The compression space  4  is thereby connected to the compressed air reservoir  12  so that the compression space  4  is acted upon with the pressure in the compressed air reservoir  12  via the automatically opening compression inlet valve  16 . The sixth electrically actuatable valve  24  is in this case nonactuated. The compression outlet valve  17  is thereby kept closed via the pressure coming from the compressed air reservoir  12 . 
     In the compressed air expansion mode, the electronic control apparatus  18  actuates the sixth electrically actuatable valve  24  in a volume reduction phase, while the fifth electrically actuatable valve  23  is nonactuated. The pressure built up in the compression space  4  is thereby broken down via the deaeration connection of the sixth electrically actuatable valve  24  by means of the automatically opening compression outlet valve  17 . As a result of the nonactuated fifth electrically actuatable valve  23 , the inlet connection  13  is connected to the atmosphere, so that compressed air cannot continue to flow from the compressed air reservoir  12  in an undesirable way. 
     As can be seen, in the embodiment according to  FIG. 4 , the pressure control device comprises the electronic control apparatus  18  and a valve assembly formed from the fifth and the sixth electrically actuatable valves  23 ,  24 . 
       FIG. 5  shows an embodiment in which the cylinder head of the compressed air compressor  1  has been modified. The cylinder head has, in addition to the compression inlet valve  16  and compression outlet valve  17  discussed above with reference to  FIGS. 2 to 4 , an expansion outlet valve  50  and an expansion inlet valve  51 . When the compressed air compressor is operating in expansion, that is, in the compressed air expansion mode, in which the compressed air compressor  1  assists the drive of the vehicle, the feed of air into the compression space  4  and its discharge are controlled via the expansion outlet valve  50  and the expansion inlet valve  51 . The expansion outlet valve  50  is connected via a separate expansion outlet connection  52  of the cylinder head to a seventh electrically actuatable valve  57  designed as a 2/2-way valve. The seventh electrically actuatable valve  57 , when actuated electrically, connects the expansion outlet connection  52  to the atmosphere or, in the nonactuated position illustrated in  FIG. 5 , shuts off the expansion outlet connection  52  with respect to the atmosphere. The expansion inlet valve  51  is connected via a separate expansion inlet connection  53  in the cylinder head to an eighth electrically actuatable valve  58  designed as a 2/2-way valve. In an actuated position, the eighth electrically actuatable valve  58  connects the expansion inlet connection  53  to the compressed air supply and storage installation  12  or shuts off this connection in the nonactuated position, which is illustrated in  FIG. 5 . The seventh and the eighth electrically actuatable valves  57 ,  58  are connected, for electrical actuation, to the electronic control apparatus  18 . 
     Moreover, the expansion outlet valve  50  and expansion inlet valve  51  are mechanically crankshaft-controlled, specifically via a camshaft  56  by means of actuating tappets  54 ,  55 , which act mechanically on the expansion outlet valve  50  and the expansion inlet valve  51 , respectively. The camshaft  56  is connected to the crankshaft of the compressed air compressor  1 , the crankshaft corresponding at the same time to the drive shaft  103 , or is connected to the output shaft of the engine  101 . 
     The electronic control apparatus  18  leaves the seventh and eighth electrically actuatable valves  57 ,  58  nonactuated in the compressed air generation mode. In this case, the compressed air compressor  1  carries out compressed air generation as a result of the normal functioning of the compression inlet valve  16  and compression outlet valve  17 . In the compressed air expansion mode, the electronic control apparatus  18  actuates the seventh and the eighth electrically actuatable valves  57 ,  58  permanently. The camshaft control of the expansion outlet valve  50  and of the expansion inlet valve  51  is thereby activated. By means of the camshaft control, the expansion inlet valve  51  is opened in a volume enlargement phase, that is, during a downward movement of the piston  3 , with the result that compressed air can flow from the compressed air reservoir  12  via the eighth electrically actuatable valve  58  into the compression space  4 . In a volume reduction phase, that is, during an upward movement of the piston  3 , the expansion inlet valve  51  is closed by the camshaft control. At the same time, the expansion outlet valve  50  is opened by the camshaft control, so that the air in the compression space  4  can escape into the atmosphere via the seventh electrically actuatable valve  57 . In this case, in the compressed air expansion mode, the compression inlet valve  16  is kept closed automatically by the overpressure located in the compression space  4 . The compression outlet valve  17  is kept closed by the pressure prevailing at the outlet connection  14  from the compressed air reservoir  12 . 
     In the embodiment according to  FIG. 5 , the pressure control device comprises the electronic control apparatus  18  and a valve assembly formed from the seventh and the eighth electrically actuatable valves  57 ,  58 . 
       FIG. 6  shows an embodiment that resembles the embodiment of  FIG. 5 , and therefore only the salient differences are addressed below. According to  FIG. 6 , only the expansion outlet valve  50  is camshaft-controlled in the same way as described above with regard to  FIG. 5 . Control takes place in a comparable way to the embodiment according to  FIG. 5 , using the seventh electrically actuatable valve  57 . 
     In contrast to  FIG. 5 , in the embodiment according to  FIG. 6  no expansion inlet valve  51  and no compression outlet valve  17  are provided. Instead, the connections  14 ,  53  illustrated in  FIG. 5  are combined into one common outlet connection  14  in the cylinder head of the compressed air compressor  1 . The outlet connection  14  is connected to the compressed air supply and storage installation  12  via a ninth electrically actuatable valve  60  designed as a 2/2-way valve. The ninth electrically actuatable valve  60  can be actuated electrically by the electronic control apparatus  18 . When actuated, the ninth electrically actuatable valve  60  connects the outlet connection  14  to the compressed air supply and storage installation  12 . In the nonactuated case, as illustrated in  FIG. 6 , this connection is shut off. 
     The electronic control apparatus  18  controls the seventh electrically actuatable valve  57  in the same way as described above with regard to  FIG. 5 . The camshaft control of the expansion outlet valve  50  also takes place in the same way. In contrast to  FIG. 5 , in the embodiment according to  FIG. 6  the ninth electrically actuatable valve  60  is actuated differently by the electronic control apparatus  18  according to programming provided in the latter, depending on the operating mode of the compressed air installation. In the compressed air generation mode, the electronic control apparatus  18  actuates the ninth electrically actuatable valve  60  as a function of the crankshaft position, which is detected via the sensor  22 , such, during a volume reduction phase, the ninth electrically actuatable valve  60  is actuated only at the end of the volume reduction phase, for example at a crankshaft angle of about 35 to 450 before top dead center is reached. This ensures that the compression space  4  is connected to the compressed air reservoir  12  only when overpressure is achieved. Advantageously, moreover, the electronic control apparatus  18  is supplied with information on the currently prevailing pressure in the compressed air reservoir  12 . The information can be supplied to the electronic control apparatus  18  either via the databus  19  or via a separate pressure sensor connected to the compressed air reservoir  12  or to other parts of the compressed air supply and storage installation. Advantageously, the commencement of actuation of the ninth electrically actuatable valve  60  and the duration of actuation are fixed by the electronic control apparatus  18  as a function of the current pressure in the compressed air reservoir  12 . For this purpose, corresponding characteristic curves or data can be stored in the electronic control apparatus  18 . 
     In the compressed air expansion mode, the electronic control apparatus  18  actuates the ninth electrically actuatable valve  60  likewise as a function of the crankshaft angle, specifically when a volume enlargement phase commences. For example, the electronic control apparatus  18  switches on the ninth electrically actuatable valve  60  just when top dead center is reached and switches the ninth electrically actuatable valve  60  off again, for example, at a crankshaft angle of 15 to 30° after top dead center. 
     As can be seen, in the embodiment according to  FIG. 6 , the pressure control device comprises the electronic control apparatus  18  and a valve assembly formed from the seventh and the ninth electrically actuatable valves  57 ,  60 . 
       FIG. 7  shows an embodiment of the invention that is optimized further in terms of component outlay. In this case, the compressed air compressor  1  is freed of all the valves illustrated in the cylinder head in the embodiment according to  FIG. 5 , to be precise, the compression inlet valve  16 , compression outlet valve  17 , expansion outlet valve  50  and expansion inlet valve  51 . Instead, the inlet connection  13  can be connected to the atmosphere via a tenth electrically actuatable valve  71  designed as a 2/2-way valve. The outlet connection  14  can be connected to the compressed air supply and storage installation  12  via the ninth electrically actuatable valve  60  discussed above with reference to  FIG. 6 . In  FIG. 7 , the ninth and the tenth electrically actuatable valves  60 ,  71  are again illustrated in the nonactuated case, that is, in the shut-off position. If the ninth or the tenth electrically actuatable valve  60 ,  71  is actuated electrically by the electronic control apparatus  18 , it is switched into the respective passage position. 
     The electronic control apparatus  18  in this case controls the ninth electrically actuatable valve  60  in the compressed air generation mode and in the compressed air expansion mode in the same way as described above with regard to  FIG. 6 . The tenth electrically actuatable valve  71  is controlled by the electronic control apparatus  18  in the compressed air generation mode as a function of the crankshaft angle such that, when underpressure is present in the compression space  4 , the connection to the atmosphere is made. In an advantageous embodiment, the tenth electrically actuatable valve  71  is opened during the entire volume enlargement phase, that is, from top dead center to bottom dead center. In one embodiment, in the compressed air expansion mode, the tenth electrically actuatable valve  71  is opened continuously by the electronic control apparatus  18  during the entire volume reduction phase, that is, from bottom dead center to top dead center. In the remaining times, the tenth electrically actuatable valve  71  is nonactuated and therefore the connection to the atmosphere is shut off. 
     In the embodiment according to  FIG. 7 , the pressure control device comprises the electronic control apparatus  18  and a valve assembly formed from the ninth and the tenth electrically actuatable valves  60 ,  71 . 
       FIG. 8  shows a further embodiment of the invention, in which, with regard to the ninth and tenth electrically actuatable valve  60 ,  71  discussed above with reference to  FIG. 7 , the same functional principle is implemented as in the embodiment of  FIG. 7 . In contrast to  FIG. 7 , the electrically actuatable valves are designed as piezoelectrically actuatable 2/2-way valves integrated directly into the cylinder head and taking the form of an eleventh electrically actuatable valve  81  and of a twelfth electrically actuatable valve  82 . The eleventh and the twelfth electrically actuatable valves  81 ,  82  likewise have again in each case an opening position assumed during actuation and a closing position. When actuated, the eleventh electrically actuatable valve  81  connects the inlet connection  13  to the compression space  4 , and in the nonactuated case this connection is shut off. When actuated, the twelfth electrically actuatable valve  82  connects the outlet connection  14  to the compression space  4 , and in the nonactuated case this connection is shut off. 
     In the embodiment according to  FIG. 3 , the pressure control device comprises the electronic control apparatus  18  and a valve assembly formed from the eleventh and the twelfth electrically actuatable valves  81 ,  82 . 
       FIG. 8  illustrates by way of example a further embodiment of the invention. A heat exchanger  83  is provided. The heat exchanger  83  has a heat absorption element  84 , which may be designed, for example, as a spiral coil. The heat absorption element  84  is coupled thermally to the engine  101  or to a part connected thermally to the latter. Moreover, the heat exchanger  83  has a first heat dispensing element  85 , which is coupled thermally to the compressed air reservoir  12 , and a second heat dispensing element  86 , which is coupled thermally to the compressed air line  15 . It is also possible that only one of the heat dispensing elements  85 ,  86  is provided. The heat dispensing elements  85 ,  86  may be constructed in a comparable way to the heat absorption element  84 . The heat absorption element  84 , the first heat dispensing element  85  and the second heat dispensing element  86  are connected to one another via corresponding pipelines. A heat transfer medium, for example water, is pumped through the pipelines. For this purpose, a, for example, electromotively driven pump  86  is arranged in one of the pipelines. The pump  86  may be controlled electrically by the electronic control apparatus  18 . The electronic control apparatus  18  can in this case switch the pump  86  on or off, as required. Owing to the heat exchanger  83  described, waste heat from the engine  101  can be utilized for heating the compressed air in the compressed air supply and storage installation  12 . 
       FIG. 9  shows control times of the various above-described valves as a function of the crankshaft angle φ of the compressed air compressor  1 .  FIG. 9  illustrates the control times when the compressed air compressor  1  is operated as a compressor, that is, in the compressed air generation mode. As can be seen, in an angular range from bottom dead center BDC to top dead center TDC (volume reduction phase), a valve connecting the compression space  4  to the compressed air supply and storage installation  12  is opened only during a comparatively short time segment  91 . The valve is closed when top dead center TDC is reached. In this period of time, compressed air can flow out of the compression space  4  into the compressed air supply and storage installation  12 . By contrast, in the volume enlargement phase, starting with top dead center TDC and going to bottom dead center BDC, a valve connecting the compression space  4  to the atmosphere is opened during a relatively long period of time  92 , which extends over the entire volume enlargement phase. 
       FIG. 10  shows actuation of the abovementioned valves when the compressed air compressor operates by expansion, that is, in the compressed air expansion mode in which the compressed air compressor serves at the same time as a drive for the vehicle. Valve actuation via the crankshaft angle φ of the compressed air compressor  1  is illustrated once again. As can be seen, during the volume reduction phase, a valve connecting the compression space  4  to the atmosphere is opened for a relatively long period of time  93 , which extends over the entire volume reduction phase from bottom dead center BDC to top dead center TDC. By contrast, after top dead center TDC is reached, another valve connecting the compression space  4  to the compressed air supply and storage installation  12  is opened immediately. However, this valve is opened only for a comparatively short period of time  94  and is closed again at once, for example about 15° after top dead center. 
     It is advantageous to configure the abovementioned valves with sufficiently large compressed air cross-sectional areas, in order to ensure rapid filling and deaeration of the compression space  4  or of the inner space  5  of the crankcase  10  in the compressed air expansion mode. 
     It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained, and since certain changes may be made without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
     It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention that, as a matter of language, might be said to fall therebetween.