Patent Publication Number: US-2005133002-A1

Title: Hydrogen combustion engine

Description:
TECHNICAL FIELD  
      The invention relates to a power producing combustion engine for combusting a mixture of a fast burning gas and air. The engine comprises a cylinder, a piston movable between a top dead center and a bottom dead center and an injection nozzle for injecting the gas into the cylinder.  
      With such a combustion engine a crank shaft, for example, can be driven to move a vehicle or to run a machine. A piston is arranged within a cylinder. The piston is connected to a crank shaft. The piston moves between a top dead center and a bottom dead center. The volume between the piston and the inner volume of the cylinder (expansion volume) is a minimum at the top dead center. The expansion volume is a maximum at the bottom dead center.  
      The movement of the piston is caused by the expansion of the gas. A combustible fuel is injected into the cylinder before the top dead center is reached, then mixed with air and finally combusted. During this exothermal process heat and power are generated. The combusted gas is compressed very much and therefore expands. It pushes the piston in a direction out of the cylinder. This movement is transmitted to the crank shaft and can be used for the generation of mechanical power. The expanded hot exhaust gas is released in the environment at the bottom dead center. The piston then moves again in the opposite direction.  
     PRIOR ART  
      Depending on the combustion fuel, different types of engines are distinguished. Otto motors are used, amongst others, for the vehicle technology. In such Otto motors the combustion fuel, i.e. gasoline, is swirled together with air in the combustion volume. Only then the mixture is ignited with a separate spark plug. A flame front is generated propagating from the spark plug in the direction of the cylinder walls. The cylinder walls continuously remain in contact with the burning fuel-air-mixture during the combustion until the combustion is finished. The pressure generated thereby produces the desired forces on the piston.  
      In an injection Diesel engine the fuel is directly injected into the compressed air within the combustion volume at the end of the compression cycle. The compressed air in the cylinder is hot and causes the self igniting of the fuel without the need of a separate spark plug. It is known to inject the fuel in one to five phases to avoid unwanted noise and vibrations. This is effected before and after reaching the top dead center. At a pre-injection a small amount of diesel is inserted per injection cycle. It is combusted with a portion of the oxygen. In the subsequent main injection, ignition is effected by the flame present due to the pre-injection. In such a way the degree of the increase of the pressure is decreased and thereby the generation of noise and vibrations is reduced. It is further known to inject small amounts of diesel fuel again in an after-injection. This is introduced to reduce the emission of particles.  
      Furthermore it is known to provide a recess in the piston of the diesel engine which is formed to enhance the swirling of the fuel-air-mixture. It is also known to provide a recess in the piston to avoid hitting of the valves on the piston at the top dead center. These recesses are designed such that the dead volume remains as small as possible in order not to decrease the efficiency. All injection processes are effected by means of one valve with one or more openings.  
      Normally modified Otto motors are used when hydrogen is combusted. Therein the hydrogen is inserted with a nozzle into the combustion volume and ignited with a spark plug. A flame front is formed propagating from the spark plug towards the cylinder wall. The combustion velocity of hydrogen is in the range of 200 m/s and is considerably higher than the combustion velocity of the other fuels, i.e. Diesel and gasoline (about 20 m/s). Apart from hydrogen engines and natural gasoline and organic gasoline engines, the use of further combustion gases are known.  
      In such hydrogen combustion engines the energy of the combustion process is transmitted not only to the crank shaft, but to a large extent to the exhaust gas in the form of heat and to the cooling water cooling the cylinder also in the form of heat. The efficiency of the engine is increased if more energy is transmitted in the form of mechanical power to the crank shaft when the same amount of combustion fuel is used. In other words: the efficiency is increased if the heat losses are reduced.  
     DISCLOSURE OF THE INVENTION  
      It is an object of the invention to provide a gas combustion engine with an increased efficiency. According to the invention this object is achieved in that the cylinder and the piston form an additional cavity at the top dead center in which hydrogen or any other fast burning gas is injectable the shape of the cavity being adapted to the shape of the flame of the combustion process and having a size which is large enough that no combustion reactions occur at the wall of the cavity.  
      Contrary to the technical development aiming at a small dead volume to increase the efficiency, the dead volume is here increased by an additional cavity. The disadvantages involved with this step are compensated by a reduction of heat dissipation.  
      The invention is based on the surprising realization that the efficiency can be increased for fast burning fuels, such as, for example, hydrogen, if a cavity is provided around the injection nozzle although additional dead volume is formed thereby, not contributing to the power generation. The formation of such an additional cavity with certain geometric characteristics decreases the heat transmission between the flame of the combusted gas and the cylinder and piston wall and thereby reduces the heat loss. Thereby the efficiency is increased. The effect is enhanced if the air is—contrary to the known Otto and Diesel engines—swirled either only very little or in such a way that the heat generating combustion process occurs in the center of the expansion volume and not at the cylinder wall.  
      Due to the high combustion velocity the fuel is combusted already before it reaches the cylinder wall. The heat transmission to the piston and the cylinder is reduced.  
      The combustion gas is preferably hydrogen. Hydrogen has a high burning velocity, is environmentally friendly and can be produced in large quantities. Further combustion gases with high burning velocity, such as, for example, propane, ethanol or acetylene, however, my also be used under suitable conditions.  
      Preferably at least one igniting device is provided. Such an igniting device is, for example, a spark plug. The ignition is then effected by independent ignition. The ignition of the combustion gas in the cylinder of the engine may also be effected by self-ignition.  
      Preferably the piston is provided with a concave cavity. The injection nozzle can then be arranged in the center of the cylinder so that the distance between the injection nozzle and the cavity wall is about the same in all directions.  
      The cylinder head may also be provided with a concave cavity forming the additional cavity together with the concave cavity of the piston. The additional cavity may be essentially spherical and the end of the injection nozzle may be arranged in the center of the cavity. In this case the injection nozzle is fully separated from the wall of the expansion volume with a distance already at the top dead center.  
      Instead of one igniting device several igniting devices may be provided. It can be reasonable to provide more than one additional cavities.  
      In a particularly preferred embodiment of the invention the injection nozzle is provided with a plurality of openings. The contact surfaces of the combustion gas jets injected by each injection nozzle to the combustion air thereby is larger and the fuel-air-mixture can be combusted particularly fast. Preferably these openings are directed such that they have the largest possible distance from the cylinder and piston wall, respectively.  
      The injection nozzle may be made from high temperature resistant material at its end, especially from ceramics. In this case this provides the option that the nozzle protrudes a little into the cavity.  
      In a particularly preferred embodiment of the invention the injection nozzle is provided with means for fast controlling of the injection process. The injection process may then be controlled in the time frame around the top dead center or up to the top dead center in such a way that the combustion process is concentrated in the central range of the cavity or the expansion volume.  
      The injection of the combustion gas can be effected in a series of several injection processes. The injection process is interrupted in very fast cycles until the respective gas portion is practically completely combusted. Thereafter the next partial injection is activated. In such a way the fuel is individually combusted for each individual injection. Thereby, the formation of a flame front propagating to the wall of the expansion volume is impeded. Contrary to a pre-injection of a diesel engine the fuel is almost completely combusted and swirling is not desired. The main injection of a diesel engine, however, is effected into the combustion of the pre-injection. The pulsed injection enhances the effect of a combustion without wall contact.  
      The injection can be effected with a large number of injection processes, for example up to 50 processes per operating cycle. Preferably the injection processes are carried out exclusively during the time before the top dead center is reached or around the top dead center and its duration maximally lasts for a tenth of an operating cycle.  
      Further modifications of the invention are subject matter of the sub-claims. A preferred embodiment is described below in greater detail with reference to the accompanying drawings. While the invention is described for a specific embodiment advantages and modifications are possible without deviating from the idea of the invention and they are not meant to limit the scope of the invention, which is set forth in the accompanying claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross section through a hydrogen combustion engine schematically shown with a hemispherical cavity at the top dead center.  
       FIG. 2  is a cross section through a hydrogen combustion engine schematically shown with hemispherical cavity at the top dead center.  
       FIG. 3  shows the injected volumes for the respective crank angles  
       FIG. 4  is a cross section through a hydrogen combustion engine schematically shown with two additional circular cavities at the top dead center injection which has a positive effect on the flows. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       FIG. 1  shows a cross section through a schematically represented cylinder  10  and a piston  12  of a hydrogen combustion engine generally denoted with numeral  14 . The piston  12  is arranged in the cylinder and moves in the direction of an arrow denoted with numeral  16 . In  FIG. 1  the piston  12  is at the top dead center. This means that the expansion volume is at a minimum. From this position the piston  12  moves downwards in  FIG. 1  to the bottom dead center. This is shown by a dashed line  18 . The expansion volume, shown as a hatched area, is then at a maximum. The expansion volume, therefore, is variable with the movement of the piston.  
      A nozzle is arranged at the top end of the cylinder  10 . The nozzle  20  ends at the wall  22  of the cylinder  10 . Hydrogen emerges from the nozzle in all directions, and is controlled by a valve control (not shown). This is represented by arrows  26 . The hydrogen enters the cavity  24  which is formed by a recess in the piston  12 . Contrary to the expansion volume, the additional cavity  24  has a constant volume. The cavity  24  is formed by the upper cylinder wall  22  and the wall of the recess  30 . Spark plugs  32  and  33  are provided at the nozzle  20 . The combustion process is started with such spark plugs  32  and  33 . The hydrogen and the oxygen comprised in the air in the cavity react in an exothermal chemical reaction. This causes a pressure wave pushing the piston  12  downwards in  FIG. 1 . The reaction takes place very quickly and is essentially finished before it reaches the wall of the cylinder or the piston, respectively. The essential portion of the reaction occurs in the area of the central starting point for combustion at the spark plugs or the nozzle.  
       FIG. 2  shows an alternative embodiment of the invention. There is also a piston  34  provided in a cylinder  36 . In this embodiment, however, the cavity  38  is spherical. The lower half of the sphere is formed by a concave recess  40  in the piston. The upper half is formed by a concave recess  42  in the cylinder head  36 . The recesses  40  and  42  are arranged to join each other. A high temperature resistant injection nozzle  44 , made of ceramics extends to the center of the sphere  46 . The end  48  of the injection nozzle is also spherically shaped and provided with a plurality of openings. Hydrogen is injected in all directions into the spherical cavity through such openings. The openings are distributed in such a way that a maximum distance to all walls of the cavity is achieved. Correspondingly the combustions essentially takes place in the center in a range around the nozzle as represented by a dotted line  50 . The combustion is essentially finished here also before heat is transmitted to the walls. The ignition of the combustion gas in the cavity  38  is effected in this embodiment by self-ignition. However, spark plugs may also be used. For optimizing the combustion process a very fast controllable injector is used. Such an injector is described in DE 102 34 50 31 A1 which is incorporated herein by reference. With such an injector it is possible to very accurately control the injection process. The injection process is started shortly before the top dead center is reached. This is shown in  FIG. 3 . The injection volume is represented depending on the crank angle. In  FIG. 3  two cycles are shown. The injection process is interrupted several times to provide pulse-like injections of the fuel. In  FIG. 3  five injection processes  52  are shown per cycle. However, depending on the layout of the cylinder, the piston and the cavity, more or less injection processes per cycle may be more suitable. The pulse frequency is very high. In such a way almost a complete combustion of the hydrogen may be achieved near the openings of the nozzle  24  or  50 , respectively. By the pulsed injection effected under very high pressure the flame burns in a pulsed manner. The size of the flame varies during the injections depending on the frequency and the duration of the injection pulses. These variations avoid the propagation of the flame front to the cylinder and piston wall. In total the same amount of exhaust gas is produced as with a continuous injection.  
      A further advantage of the pulsed injection is the avoiding of high temperatures. Furthermore the flame front can not be formed at the cylinder wall. Thereby the heat loss is considerably reduced. With low exhaust temperatures the efficiency is improved and the formation of damaging nitrogen oxides NO x  is avoided.  
      In  FIG. 4  yet another embodiment is shown wherein the piston  60  and the cylinder head  62  form cavities  68  with circular cross sections. Such cavities can form two spheres, two lying cylinders or a torus. The tangential injection or the injection from the side  64  is carried out in a pulsed manner as described above. Thereby a varying combustion flame  66  is produced not filling the entire cavity  68  and not having direct contact with the piston walls  70 . Due to the circular cross section of the cavity  68  the flame generates a turbulence  72  in the cavity which in turn feeds the flame with combustion air. This supports the stable generation of a pulsed flame near the nozzle.  
      Whereas the invention is here illustrated and described with reference to embodiments thereof presently contemplated as the best mode of carrying out the invention in actual practice, it is to be understood that various changes may be made in adapting the invention to different embodiments without departing from the broader inventive concepts disclosed herein and comprehended by the claims that follow.