Patent Document

The present application is a 371 of International application PCT/EP2010/003608, filed Jun. 16, 2010, which claims priority of DE 10 2009 033 082.8, filed Jul. 3, 2009, the priority of these applications is hereby claimed and these applications are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The invention concerns a method for automatically controlling a gas engine, in which both a fuel volume as a fraction of an air/fuel mixture is determined as a function of a set volume flow and a mixture pressure of the air/fuel mixture in the intake manifold upstream of the intake valves of the gas engine is determined as a function of a set volume flow. 
     Gas engines are often used as power plants for emergency generators, standby-ready units, or combined heat and power (CHP) installations. In these applications, the gas engine is operated at a combustion air ratio of, for example, 1.7, i.e., in a lean operation with air excess. Typically, the gas engine has a gas throttle valve for setting the gas fraction in the air/fuel mixture, a mixer for mixing the combustible gas with the air, a compressor as part of an exhaust gas turbocharger, a cooler and a mixture throttle valve. The mixture throttle valve serves to set the filling of the working cylinders and thus the torque of the gas engine. The filling of the working cylinders in turn is computed from the pressure of the air/fuel mixture with other parameters otherwise held constant, for example, at constant intake manifold temperature, at constant engine speed, and at constant combustion air ratio. 
     DE 10 2007 045 195 B3 discloses an automatic control method for a stationary gas engine with a generator, in which a speed controller uses a speed control deviation to determine a controller torque as a correcting variable. The controller torque in turn and the actual speed are used to determine a set volume flow by means of an efficiency input-output map. The set volume flow is both the input variable for controlling the gas throttle valve and the input variable for setting the mixture pressure in the intake manifold. The central element is the parallel control of the two control elements as a function of the same actuating variable, in this case, the set volume flow. The mixture pressure in the intake manifold is set via a cascade closed-loop pressure control system for the intake manifold. In this intake manifold closed-loop control system, the set intake manifold pressure represents the reference input and the measured intake manifold pressure is the controlled variable. The gas motor and the generator then constitute the controlled system. The set intake manifold pressure is computed from the set volume flow, taking into account the actual speed of the gas engine, the temperature in the intake manifold, and constants. Constant values include, for example, the combustion air ratio and a stoichiometric air requirement. The method we have been describing has been found to be effective in actual practice. However, the effect of different gas grades (volume fraction) within the same family of gases on the emission values remains critical. 
     DE 699 26 036 T2 also describes a method for automatically controlling a gas engine, in which a control signal for controlling the mixture throttle valve is computed from the speed control deviation by a PID controller. A correction value is determined, likewise as a function of the speed control deviation, and is then used to change the control signal for the gas throttle valve. However, the objective of the method is to suppress engine speed oscillations that develop after a change in the set engine speed. 
     SUMMARY OF THE INVENTION 
     Proceeding from an automatic control method with parallel control of the gas throttle valve and mixture throttle valve and cascade closed-loop pressure control system for the intake manifold, the objective of the invention is to minimize the effect of a different gas grade on the automatic control method. 
     The effect of a different gas grade is minimized by computing a deviation of the controller torque, i.e., the correcting variable of the speed controller, from the generator torque, and the set intake manifold pressure is corrected on the basis of this deviation. The deviation is a measure of the amount by which the energy content of the gas actually being used, for example, biogas, deviates from the energy content of the reference gas. The gas engine is calibrated to this reference gas on a test bench, with natural gas being used as the reference gas. For the gas being used on site, which occurs as a mixed gas comprising gases of a known family of gases, the fuel parameters must be known. These are the calorific value, the stoichiometric air requirement, and the density. The fuel parameters are then stored in the system as fixed values. The speed controller, by which the controller torque is computed, uses natural gas as the reference. The set intake manifold pressure is corrected by computing a corrected value from corrected input variables, namely a corrected set volume flow, a corrected combustion air ratio, and a corrected air requirement. 
     The corrected set volume flow is computed by multiplying the set volume flow by the square of the deviation. The corrected combustion air ratio is computed from a reference combustion air ratio and the deviation, where the reference combustion air ratio is computed by an input-output map as a function of the controller torque and the actual speed of the gas engine. The corrected air requirement is determined by a recursive method, likewise as a function of the deviation. 
     The use of the method of the invention offers the advantage that despite variation of the gas grade, the power output of the gas engine remains unchanged. Therefore, a gas engine that is being operated with, for example, biogas, has the same power output as a gas engine operated with natural gas. If the volume fraction of the combustible gas varies, the set intake manifold pressure is adjusted by the method of the invention, so that the power output remains unchanged in this case as well. Therefore, the volume fraction does not have to be known. As a consequence, the pollutant emissions are the same as for the reference gas. Since the method is based on the same sensor signals that are already being used, no modification or supplementation of the sensor technology or of the engine control unit is necessary. Therefore, gas engines that have already been delivered can be retrofitted with the method of the invention without any problem, for example, during maintenance. Compared to automatic control of the combustion air ratio, parallel control of the gas throttle valve and the mixture throttle valves as a function of the same actuating variable offers the advantage of a reduced response time and more precise transient oscillation with improved adjustability of the total system. As a result, smooth automatic control of the engine output is obtained. 
     The drawings show a preferred embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a system diagram. 
         FIG. 2  is a block diagram. 
         FIG. 3  shows the computation of the set intake manifold pressure as a block diagram. 
         FIG. 4  shows the computation of the corrected air requirement as a block diagram. 
         FIG. 5  is a program flowchart. 
         FIG. 6  is a subroutine. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a system diagram of a gas engine  1  in V configuration with a generator  5 . The generator  5  is powered by the gas engine  1  via a shaft  2 , a coupling  3 , and a shaft  4 . Electric power is generated by the generator  5  and fed into an electric network. The following mechanical components are assigned to the gas engine  1 : a gas throttle valve  6  for setting a supplied volume flow or fuel, for example, biogas, a mixer  7  for mixing air and fuel, a compressor  8  as part of an exhaust gas turbocharger, a cooler  9 , an A-side mixture throttle valve  10  in the A-side intake manifold  12 , and a B-side mixture throttle valve  11  in the B-side intake manifold  13  of the gas engine  1 . Naturally, instead of the throttle valves, i.e., the gas throttle valve  6  and the mixture throttle valves  10  and  11 , it is also possible to use other control devices, for example, a venturi mixer or a rotary valve. 
     The mode of operation of the gas engine  1  is determined by an electronic engine control unit  14  (GECU). The electronic engine control unit  14  contains the usual components of a microcomputer system, for example, a microprocessor, interface adapters, buffers and memory components (EEPROM, RAM). Operating characteristics that are relevant to the operation of the gas engine  1  are applied in the memory components in the form of input-output maps/characteristic curves. The electronic engine control unit  14  uses these to compute the output variables from the input variables. The following input variables are shown in  FIG. 1 : the A-side intake manifold pressure pRRA, a mixture temperature T 1 , a B-side intake manifold pressure pRRB, the (unfiltered) raw values of the engine speed nMOT of the gas engine  1 , a set speed nM(SL), which is preset by a system controller (not shown), and an input variable EIN. The input variable EIN is representative of the other input signals, for example, the oil temperature. The following are shown as output variables of the electronic engine control unit  14 : an adjusted set volume flow Va(SL) for controlling the gas throttle valve  6 , an A-side mixture throttle angle DKWA for controlling the A-side mixture throttle valve  10 , a B-side mixture throttle angle DKWB for controlling the B-side mixture throttle valve  11 , and a signal AUS. The signal AUS is representative of the other signals for automatically controlling the gas engine  1 . 
     The system has the following general functionality: A fuel volume flow supplied to the mixer  7  is adjusted by the position of the gas throttle valve  6 . The position of the A-side mixture throttle valve  10  defines an A-side mixture volume and thus the A-side intake manifold pressure pRRA in the A-side intake manifold  12  upstream of the intake valves of the gas engine  1 . The B-side intake manifold pressure pRRB upstream of the intake valves of the gas engine  1  is determined by the B-side mixture throttle valve  11 . 
       FIG. 2  shows a block diagram for controlling the two mixture throttle valves  10  and  11  and the gas throttle valve  6 . The system controller of the generator is identified by reference number  15 . Reference number  14  identifies the electronic engine control unit in the form of a reduced block diagram, in which the depicted elements represent the program steps of an executable program. The input variables of the electronic engine control unit  14  that are shown in this drawing are the raw values of the engine speed nMOT, the A-side intake manifold pressure pRRA, the B-side intake manifold pressure pRRB, the set speed nM(SL), and an active electric power Pwel. The set speed nM(SL) and the active electric power Pwel are supplied by the system controller  15 . The electronic engine control unit  14  computes the generator torque MGen from the active electric power Pwel. The electronic engine control unit  14  uses the raw values of the engine speed nMOT to compute the actual speed by means of a speed filter (not shown). The output variables of the electronic engine control unit  14  that are shown in  FIG. 2  are the A-side mixture throttle angle DKWA for controlling the A-side mixture throttle valve  10 , the B-side mixture throttle angle DKWB for controlling the B-side mixture throttle valve  11 , and the adjusted set volume flow Va(SL) for controlling the gas throttle valve  6 . 
     The set speed nM(SL), for example, 1500 rpm, which corresponds to a frequency of 50 Hz, is preset by the system controller  15  as the desired output. At a point A, a speed control deviation dn is computed from the set speed nM(SL) and the actual speed mM(IST). A speed controller  16  in turn uses the speed control deviation dn to compute the controller torque MR as a correcting variable. In practice, the speed controller  16  is realized as a PIDT1 controller. The controller torque MR is the first input variable of a consumption input-output map  17 . The second input variable is the actual speed nM(IST). A set volume flow V(SL) is determined by the consumption input-output map  17  as a function of the two input variables. The set volume flow V(SL) is the input variable for both a volume adjustment unit  31  and a mixture quantity unit  18 . The set volume flow V(SL) is adjusted by the volume adjustment unit  31  at least as a function of the generator torque MGen. The output variable of the volume adjustment unit  31  is an adjusted set volume flow Va(SL), which is the input variable of the gas throttle valve  6 . An electronic processing unit integrated in the gas throttle valve  6  assigns to the value of the adjusted set volume flow Va(SL) a corresponding cross-sectional area and a corresponding angle. A fuel volume flow as a gas fraction of the air/fuel mixture is set by the gas throttle valve  6 . 
     The mixture quantity unit  18  combines the computation of the set intake manifold pressure and a cascade closed-loop pressure control system for the intake manifold. The conversion of the set volume flow V(SL) in the mixture quantity unit  18  is shown in  FIG. 3  and will be described with reference to that drawing. As shown in  FIG. 2 , the other input variables of the mixture quantity unit  18  are the generator torque MGen, the controller torque MR, the actual speed nM(IST), and the two intake manifold pressures pRRA and pRRB as controlled variables of the closed-loop pressure control system for the intake manifold. The A-side mixture throttle angle DKWA and the B-side mixture throttle angle DKWB are computed by the mixture quantity unit  18 . The A-side mixture throttle angle DKWA controls the A-side mixture throttle valve  10 , by which the A-side intake manifold pressure pRRA is automatically controlled. The B-side mixture throttle angle DKWB controls the B-side mixture throttle valve  11 , by which the B-side intake manifold pressure pRRB is automatically controlled. As is clear from the figure, a central element is the parallel control of the gas throttle valve and the mixture throttle valves as a function of the same actuating variable, here the set volume flow V(SL). 
     The block diagram in  FIG. 3  shows a functional block  19  for determining the set intake manifold pressure pRR(SL), which is part of the mixture quantity unit  18 . The input variables of the functional block  19  are the controller torque MR computed by the speed controller  16  ( FIG. 2 ), the actual speed nM(IST), the generator torque MGen, and the set volume flow V(SL). The output variable is the set intake manifold pressure pRR(SL), which is then the reference input for the following intake manifold closed-loop pressure control system. An intake manifold closed-loop pressure control system of this type is described, for example, in DE 10 2007 045 195 B3. A computing unit  20  uses the controller torque MR and the generator torque MGen to determine a deviation yS by computing the quotient MR/MGen. The deviation yS is a measure of the amount by which the energy content of the fuel actually being used differs from the energy content of the reference fuel. The unit of energy content is given in kilowatt-hours per standard cubic meter of gas (kWh/Nm 3 ). The system is set to natural gas as the reference fuel by the manufacturer of the gas engine. If natural gas is used, then the quotient MR/MGen is equal to one. If, on the other hand, a gas with a lower energy content is used, for example, biogas, then the quotient can be in the range of 1.4 to 1.5, depending on the inert gas fraction. 
     A reference combustion-air ratio LAMr is assigned to the controller torque MR and the actual speed nM(IST) by an input-output map  21 . The reference combustion-air ratio LAMr is the first input variable of a correction unit  22 . The second input variable is a reference air requirement LMINr, which in the present case is constant. The reference air requirement LMINr represents the stoichiometric air requirement for the complete combustion of one cubic meter of the reference gas. The third input variable is the deviation yS. The correction unit  22  uses the following relation to compute the corrected combustion air ratio LAMk:
 
 LAMk=LAMr +[(1 −ys   2 )/ LMINr]   (1)
 
     The output variable of the correction unit  22 , i.e., the corrected combustion air ratio LAMk, is the first corrected input variable of a computing unit  25  for determining the set intake manifold pressure pRR(SL). A correction unit  23  computes a corrected air requirement LMINk as a function of the deviation yS. The correction unit  23  is shown in  FIG. 4  and will be described with reference to that drawing. The corrected air requirement LMINk is the second corrected input variable of the computing unit  25 . The third corrected input variable of the computing unit  25  is a corrected set volume flow Vk(SL), which is computed in computing unit  24  by multiplying the set volume flow V(SL) by the square of the deviation yS. The computing unit  25  computes the set intake manifold pressure pRR(SL) by the following relation:
 
 pRR ( SL )=2 ·yS   2   ·T 1 ·p 0·[1 +LAMk·LMINk]·Vk ( SL )/[ LG·VH·nM ( IST )· T 0]  (2)
 
where yS is the deviation, T 1  is the temperature measured in the intake manifold, p0 is the standard air pressure at mean sea level (1013 hPa), LAMk is the corrected combustion air ratio, LMINk is the corrected air requirement, Vk(SL) is the corrected set volume flow, LG is the volumetric efficiency, VH is the stroke volume of the cylinder, nM(IST) is the actual speed of the gas engine, and T 0  is standard temperature (273.15K). In the drawing ( FIG. 3 ), the volumetric efficiency LG, the stroke volume VH, and the standard temperature T 0  are combined under the reference letter E. As is evident from  FIG. 3 , the set intake manifold pressure pRR(SL) is corrected by using corrected input variables. The deviation yS proved to be the most useful correction variable, since it is a measure of the amount by which the energy content of the gas that is actually used, for example, biogas with a methane gas content of 45 vol. %, deviates from the energy content of the reference gas, which in the present case is natural gas.
 
       FIG. 4  is a block diagram that shows the computation of the corrected air requirement. The input variables are the deviation yS and the ignition point ZZP. The output variables are the corrected air requirement LMINk and a filtered mixing parameter xSF. The corrected air requirement LMINk is computed by means of a recursive loop  26 . A mixing parameter xS is computed by a computing unit  27  from the deviation yS and an efficiency ratio ETA. In the first pass through the recursive loop, the initial value of the efficiency ratio is ETA=1. The mixing parameter is computed by the following relation:
 
 xS={HUO−[HUr·ETA ·(1/ yS   2 )]}/[ HUO−HUU]   (3)
 
where xS is the mixing parameter. The constant HUO represents the greatest calorific value to be adopted for the fuel that is actually being used, for example, biogas. The constant HUU represents the smallest calorific value to be adopted for the fuel that is actually being used. The constant HUr represents the calorific value of the reference fuel, here: natural gas. To determine the constants HUO and HUU, it is necessary to know the provenience and the family of gases to which the fuels belong. These constants are not varied in the operation of the gas engine. The mixing parameter xS is then supplied to an efficiency input-output map  28 , which determines a new efficiency ratio ETA as a function of the mixing parameter xS and the ignition point ZZP. The efficiency ratio ETA can be obtained as the ratio of an actual efficiency to a reference efficiency determined on the test bench with the use of the reference fuel (natural gas). The new efficiency ratio ETA is then fed back to the computing unit  27 , in which the mixing parameter xS is then recomputed from the new efficiency ratio ETA by formula (3). The recursive loop is repeatedly passed through until a termination criterion is recognized. A termination criterion occurs when the recursive loop  26  has been passed through i times. Alternatively, a termination criterion is present when the difference between two recursively computed mixing parameters is less than a limit. When the termination criterion has been recognized, the last mixing parameter computed is set as the valid value. The recursive loop  26  is followed by a filter  29 , typically a PT1 filter, which filters the mixing parameter that has been set as valid. A computing unit  30  uses the filtered mixing parameter xSF and constant values K to compute the corrected air requirement LMINk, which is further processed in the functional block  19  shown in  FIG. 3 . The computation is carried out with the following relation:
 
 LMINk=xSF·LMINu +(1− xSF )· LMINo   (4)
 
where LMINu is the minimum air requirement of the fuel that is actually used and LMINo is the maximum air requirement of the fuel that is actually used, which are the constants. The mixing parameter xSF is further processed internally, for example, for adjustment of the fuel density and the ignition point.
 
       FIG. 5  is a program flowchart of the method of the invention. At S 1  the actual speed nM(IST) and the set speed nM(SL) are read in, and then at S 2  they are used to compute the speed control deviation dn. At S 3  the speed controller uses the speed control deviation dn to determine the controller torque MR as a correcting variable, for example, by means of a PIDT1 algorithm. At S 4  the consumption input-output map  17  ( FIG. 2 ) computes the set volume flow V(SL) as a function of the controller torque MR and the actual speed nM(IST). At S 5  the deviation yS is determined from the controller torque MR and the generator torque MGen by computation of the quotient MR/MGen. At S 6  a reference combustion air ratio LAMr is assigned to the controller torque MR and the actual speed nM(IST) by an input-output map  21  ( FIG. 3 ). At S 7  formula (1) is then used to determine the corrected combustion air ratio LAMk. Then at S 8  control passes to a subroutine UP 1  for computation of the corrected air requirement LMINk. The subroutine UP 1  is shown in  FIG. 6  and will be described in connection with  FIG. 5 . After control passes back to the main program, then at S 9  the corrected set volume flow Vk(SL) is computed from the set volume flow V(SL) and the deviation yS. At S 10  formula (2) is used to compute the set intake manifold pressure pRR(SL) as a function of the corrected set volume flow Vk(SL), the corrected air requirement LMINk, and the corrected combustion air ratio LAMk. The program then ends. 
       FIG. 6  shows the subroutine UP 1  for computing the corrected air requirement LMINk by a recursive method. At S 1  a running variable I and the efficiency ratio ETA are set to the initial value of one. At S 2  the mixing parameter xS is computed by formula (3). The constant HUO represents the greatest calorific value to be adopted for the fuel that is actually being used, for example, biogas. The constant HUU represents the smallest calorific value to be adopted for the fuel that is actually being used. The constant HUr represents the calorific value of the reference fuel, here: natural gas. To determine the constants HUO and HUU, it is necessary to know the provenience and the family of gases to which the fuels belong. These constants are not varied in the operation of the gas engine. Then at S 3  an input-output map  28  ( FIG. 4 ) assigns a new efficiency ratio ETA to the mixing parameter and the ignition point ZZP. The efficiency ratio ETA can be obtained as the quotient of an actual efficiency and a reference efficiency. At S 4  a new mixing parameter is determined with the previously computed new efficiency ratio ETA and set as the new mixing parameter. On the first pass through the loop, the new mixing parameter is then xS(2). The running variable i is then increased by one at S 5 , and its value is interrogated at S 6 . If the running variable i is less than four (interrogation result at S 6 : yes), then the program flow continues at S 3 . If the interrogation result at S 6  is negative, the last computed mixing parameter is set as the valid mixing parameter. The valid mixing parameters computed in this way within an interval of time are filtered at S 7 , for example, by a PT1 filter. The result is the filtered mixing parameter xSF. At S 8  the corrected air requirement LMINk is computed as a function of the filtered mixing parameter xSF. The filtered mixing parameter xSF is further processed internally, for example, for adjustment of the fuel density BD and the ignition point ZZP. Control then returns to the main program of  FIG. 5  at S 8 . 
     The invention was described with reference to a gas engine that powers a generator. Instead of a generator, a standby-ready unit or a combined heat and power (CHP) installation can also be used. In this case, the generator torque MGen then corresponds to the torque delivered by, for example, the standby-ready unit. 
     LIST OF REFERENCE NUMBERS 
     
         
           1  generator 
           2  shaft 
           3  coupling 
           4  shaft 
           5  generator 
           6  gas throttle valve 
           7  mixer 
           8  compressor 
           9  cooler 
           10  A-side mixture throttle valve 
           11  B-side mixture throttle valve 
           12  A-side intake manifold 
           13  B-side intake manifold 
           14  electronic engine control unit (GECU) 
           15  system controller 
           16  speed controller 
           17  consumption input-output map 
           18  mixture quantity unit 
           19  functional block 
           20  computing unit 
           21  input-output map 
           22  correction unit 
           23  correction unit 
           24  computing unit 
           25  computing unit 
           26  recursive loop 
           27  computing unit 
           28  efficiency input-output map 
           29  filter 
           30  computing unit 
           31  volume adjustment unit

Technology Category: 2