Gas engine
TECHNICAL FIELD 
     This invention relates to an internal combustion engine which burns a combustible gas. 
     PRIOR ART STATEMENT 
     Gas engines consist of, among other components, a mixture line to the combustion spaces in the cylinders, a mixer being arranged in the mixture line, which mixer mixes air into the gas. A metering valve is provided for the metering of the gas delivery. 
     In order that the emission standards imposed by the legislature can be complied with, it is the state of the art to control the metering valve via a controller in dependence on the pollutant emission of the engine. For this purpose, an engine parameter measuring the pollutant emission, namely the oxygen content in the exhaust gas, and an engine power signal is supplied to the controller as control variable. In the controller, these values are compared with a setpoint, and the metering valve is accordingly controlled in such a fashion that the predetermined setpoint is attained. 
     A disadvantage herein is that, upon a fluctuation in gas quality, for example if propane gas is mixed into the natural gas, the pollutant emission increases while the air ratio in the exhaust gas remains constant. 
     OBJECTS AND BRIEF DESCRIPTION OF THE INVENTION 
     It is therefore an object of the invention to improve a gas engine in accordance with the preamble of claim 1 in such a fashion that the pollutant emission is low even under conditions of fluctuating gas quality and fluctuating intake air temperature. 
     In accordance with the invention, this object is achieved by virtue of the fact that the engine parameter is the mean temperature in the combustion space. As this temperature depends on the gas quality and the intake air temperature, this measured value is optimally suited to the control of the air ratio and thus to the abatement of pollution. This mean temperature and the associated power setting of the engine are supplied to the controller. The two values are then compared with a setpoint in the controller. Next, the metering valve is adjusted in such a fashion that the gas-air ratio takes on an optimal value for pollutant emissions. 
     The mean temperature is advantageously picked off or monitored via sensors, for example thermocouples. 
     It is sufficient to measure the mean temperature of even one combustion space. It is advantageous, however, to determine the mean temperatures of all cylinders and form a mean value from these. 
     In one embodiment of the invention, the power setting of the gas engine is determined via the throttle valve in the mixture line and supplied to the controller. The power setting can be monitored by a throttle position sensor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The single FIGURE of the drawings schematically illustrates an engine incorporating one embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE DRAWING 
     The FIGURE shows a gas engine 1 in accordance with the invention. Gas is delivered to the gas engine 1 via a mixture line 7. A mixer 5, which mixes air into the gas, is arranged in the said mixture line 7. Ahead of the mixer 5 there is arranged a metering valve 8 for the metering of the gas delivery. A controller 4 is provided for the control of the gas-air ratio, which controller controls the metering valve 8 in such a fashion that the gas-air ratio satisfies the requirements for low pollutant emissions. 
     The mean temperature of the combustion spaces or, respectively, of the cylinders is supplied to the controller 4 as control variable. These temperature values are monitored via sensors 3. The power setting of the engine is monitored by a sensor sensing the position of the throttle valve 6 in the mixture line 7, the setting of this throttle valve 6, for example, being transmitted to the controller 4. The controller compares these two values, the mean temperature and the associated power, with a stored setpoint value and accordingly controls the metering valve 8 until the measured actual values correspond to the setpoint. By this means, pollutant emissions are minimal. Since the mean temperature of the combustion spaces is input as control variable, low pollutant emissions are attained even under conditions of fluctuating gas quality and fluctuating intake air temperature.