Patent Publication Number: US-2004058211-A1

Title: Fuel cell system in a vehicle with an internal combustion engine and method for the operation thereof

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     [0001] The present application is a continuation of International Patent Application No. PCT/EP01/13313, filed Nov. 17, 2001, designating the United States of America and published in German as WO 02/049131, the entire disclosure of which is incorporated herein by reference. Priority is claimed based on Federal Republic of Germany Patent Application No. 100 62 965.2, filed Dec. 16, 2000. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The invention relates to a fuel cell system in a vehicle with an internal combustion engine and a method for the operation thereof.  
       BACKGROUND AND SUMMARY OF THE INVENTION  
       [0003] It is known to use a fuel cell to generate electric power in a vehicle. In a special embodiment a conventional internal combustion engine, which drives a crankshaft by way of a piston by burning a combustion gas, is used as the primary drive of the vehicle. Furthermore, a fuel cell for the operation of a vehicle electrical system is used for an engine-independent power supply. The fuel cell is operated, for example, with hydrogen or a combustion gas, produced in a reformer.  
       [0004] The maximum power output of a fuel cell is a function of the volumetric flow rate of a combustion gas. If the fuel cell is supplied with more combustion gas than the electric power requirement demands, then the unconsumed combustion gas is usually released to the environment. In the case of such a method, especially with the use of a reformer conversion, fuel utilization is usually less than 80%.  
       [0005] Furthermore, to achieve a sudden change in the fuel cell&#39;s load as in the case of a battery, an adequate amount of a reaction gas must always be available. This feature results conventionally in specifying the delivery of reaction gas to the fuel cell in a buffer container or by means of a complicated regulating system. It is also known to provide a higher volumetric flow rate than necessary. Otherwise, the electric load can be increased only as a function of the load alternation behavior of the reactant supply.  
       [0006] When the electric load is suddenly removed, as stated above, the combustion gas passes unutilized through the fuel cell until the volumetric flow rate can be reduced with a delay to the necessary quantity.  
       [0007] The object of the present invention is to provide a fuel cell system and a method for operating such a system, in which the combustion gas, used for operating the fuel cell, is not needlessly wasted, and the overall efficiency of the system is increased. Yet an adequate electric power output is always provided.  
       [0008] This problem is solved by the embodiments of the invention as described and claimed hereinafter.  
       [0009] A core idea of the present invention consists of providing always an adequate volumetric flow rate of fuel to the fuel cell, so that the maximum electric power output demands can be met. In order not to have to release any existing excess combustion gas from the fuel cell to the environment and to be able to further utilize it, the fuel cell can be connected, according to various embodiments, to either the internal combustion engine and/or to an after-burner. Then the excess combustion gas is fed to these systems, connected in series, and utilized therein.  
       [0010] Altogether the fuel cell is operated in essence at a relatively constant volumetric flow rate of the combustion gas while the internal combustion engine is running. In the medium term this volumetric flow rate is supposed to be designed in such a manner that the maximum electric power output, required instantaneously or in the future, can be generated. If the excess combustion gas is utilized then in the internal combustion engine, it serves the additional drive of the vehicle. When excess combustion gas is utilized in an auxiliary heating device (after-burner), the cooling water for the internal combustion engine can be preheated for example. This feature enables not only lower friction losses of the engine during startup, but also an auxiliary heating function.  
       [0011] When the internal combustion engine is switched off, the quantity of combustion gas is, for example, a function of the main loads, for example an electric air conditioning system or a light.  
       [0012] In order to be able to control the flow of the excess combustion gas to the internal combustion engine or to the auxiliary heating device, a controllable valve is disposed preferably in the appropriate direction of flow. Should this valve have to be closed completely even when the fuel cell is in operation, one should also provide the possibility to ventilate to the environment.  
       [0013] According to an especially preferred embodiment, the excess combustion gas can be fed, and in particular depending on the requirements, selectively to an auxiliary heating device and/or to the internal combustion engine. Appropriate constructive designs must then be provided.  
       [0014] Of course, the fuel cell can always be operated at a maximum volumetric flow rate. According to an especially preferred embodiment, however, the volumetric flow rate is set as a function of the operation. In so doing, a controller evaluates the available input signals, for example the necessary electric power output of different load, and selects a volumetric flow rate for the combustion gas that meets these requirements. In addition to the current power consumption of the active electric devices, such as the air conditioning system, light, energy supply of the controllers, etc., or a foreseeable future power consumption of such devices, other information, such as the additional vehicle operating data and/or the environmental data, can be fed as the input signals to the controller. For example, at low ambient temperatures one can conclude that it is necessary to preheat a vehicle. On the other hand, at high external temperatures, one can conclude that an air conditioning system will be switched on. Should, moreover, so-called brake-by-wire systems be used, then in addition, one must observe, for example, at high speeds, that adequate operating power for the brakes is always available.  
       [0015] A single embodiment of the present invention is explained in detail below with reference to a single drawing. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
     [0016] The FIGURE shows an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT  
     [0017] The drawing shows a fuel cell  12 , which receives hydrogen from a hydrogen tank  10 . In the fluid connection between the hydrogen tank  10  and the fuel cell  12  there is a volumetric flow regulator  32  (valve/pump), which is actuated by a controller  30 . The controller  30  receives input data E 1 , E 2 , on the basis of which it calculates the volumetric flow rate for the maximum power output needed and adjusts the volumetric flow rate with the volumetric flow regulator  32 . The fuel cell  12  produces electric energy U and provides it, as a function of the demand, to the vehicle electric system, which is not depicted in detail. If now a larger volumetric flow is sent through the fuel cell  12  than is necessary to generate the electric power, then the volumetric flow goes by way of a fuel cell outlet to an internal combustion engine  16  or to an after-burner  18 . In so doing, the control of the excess combustion gas is assumed by a valve element  14 , which draws off, as a function of the demand, the excess combustion gas to the internal combustion engine  16 , the after-burner  18 , or, if necessary, to the environment (outlet  15 ). In the internal combustion engine  16  the excess combustion gas is admixed in an appropriate manner with the rest of the combustion gas. In this respect the particulars are not discussed in detail here. If, in contrast, the excess combustion gas is fed to the after-burner  18 , then the cooling water in the cooling circulation  20  of the internal combustion engine  16  is heated by a heat exchanger  22 . The resulting exhaust gases from the internal combustion engine  16  or the after-burner  18  respectively are exhausted over the respective outlets  24  and  26  to the environment. If hydrogen is used as the combustion gas, then only water is produced as the exhaust gas.  
     [0018] The present invention offers the simple possibility of operating optimally and altogether efficiently a fuel cell without having to waive the possibility of realizing sudden load changes when supplying power in a vehicle. If in the internal combustion engine a different combustion gas than hydrogen were also to be used, then the cold start emission could also be improved by preheating. In addition, an exhaust gas catalyst could be heated, a feature that also has a positive effect on the emission behavior in the cold start area. Moreover, owing to the use of the combustion gas, an auxiliary heater is available for heating the cooling water; and the engine can be operated with little friction losses at startup. In addition, the system does not need any excessively complicated regulating system and fewer components, a feature that has altogether a positive effect on the costs, the unit volume and the overall weight of the system.  
     [0019] The foregoing description and examples have been set forth merely to illustrate the invention and are not intended to be limiting. Since modifications of the described embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed broadly to include all variations falling within the scope of the appended claims and equivalents thereof.