A Stirling engine is a known external combustion engine in which heat supplied from the combustion process is transferred by a primary heat exchanger or the like to a pressurized working fluid in a drive system of the engine. Mechanical work is produced by fluid expansion during an isothermal expansion cycle phase when heat is transferred to the working fluid. To maintain maximum work output from the engine, the temperature of the working fluid should be maintained at a constant level at an upper limit determined by the metallurgical composition of the engine's primary heat exchanger.
Unlike internal combustion engines, in which the expansion of gases due to combustion moves the piston, the heat output of the combustion of the air-fuel mixture is varied to maintain the high temperature of the working fluid in the drive system of the Stirling engine. Thus, temperature sensing of this working fluid comprises the primary indicia for control of the air-fuel ratio in previous Stirling engine controls. Typically, an air compressor is controlled in response to the change in temperature and the air passes through a throttle body with a fuel inlet. Such control of the air-fuel ratio delivered to the combustion chamber does not maintain the proper air-fuel which optimizes the efficiency of combustion and reduces the release of harmful exhaust products. Furthermore, it produces hot spots in the combustion chamber due to incomplete mixing of the air and the fuel.
While the air-fuel mixture may be varied to adjust the output of both internal and external combustion engines, the conditions under which the air-fuel ratio must be adjusted are substantially different. In particular, it will be appreciated that the air-fuel ratio in previously known Stirling engines may be substantially higher than the air-fuel ratios commonly encountered in internal combustion engines, and the higher level of air controls heat transfer to the working fluid. Moreover, the combustion chamber does not support reciprocating pistons and may be constructed of less durable materials such as sheet metal. However, such material is more vulnerable to uneven heating problems. The problem of uneven heating has been evident when control of the air-fuel ratio has been provided by adjusting air-flow through a throttle body. As a result, previously known apparatus and methods for adjusting the air-fuel ratio in internal combustion engines are not readily applicable to the air-fuel mixture controls in Stirling engine systems.
U.S. Pat. No. 4,231,222 discloses an air-fuel control system for Stirling engines adapted to overcome the problem of controlling previously known fuel injection devices throughout a wide range of air-fuel ratios required. A temperature sensor generates a signal in response to deviation of working fluid temperature from its desired limit to control an air flow throttle valve. Variations in air flow of the combustion circuit is then sensed by a vortex shedding device which delivers a DC electrical signal to control one or more solenoid type fuel injectors feeding a common manifold leading to the fuel nozzle for the combustion circuit. Exhaust gas recirculation is controlled by a valve which also affects the air-fuel ratio input to the combustion chamber.
U.S. Pat. No. 3,956,892 to Nystrom discloses a Stirling engine which utilizes a closed loop fuel control system regulated by a temperature sensor. The system delivers a constant amount of fuel and air per unit time in an amount which is less than necessary for idling when the sensed temperature is above a predetermined level, and delivers a constant amount of fuel and air which is more than necessary to generate the heat required for maximum engine output. The duration of the delivery of those higher and lower amounts of fuel and air is varied depending upon engine load. As a result, the sensed temperature does not affect the air-to-fuel ratio and provides a simple apparatus for control of the combustion in the Stirling engine.
U.S. Pat. Nos. 4,083,342; 4,007,718; 4,023,357; 4,146,000; 4,052,968; 3,977,375; and 3,931,710 disclose carburetor controls for internal combustion engines in which exhaust gas constituents are sensed to provide a signal that controls the introduction of bypass or secondary air downstream of the air-fuel mixing throat of the carburetor.
U.S. Pat. No. 4,096,839 discloses an air-fuel ratio control system for internal combustion engines in which an oxygen sensor is used to maintain the primary intake air-fuel ratio at a predetermined level whereas the second intake is controlled in response to exhaust pressure to control the amount of fuel fed to the internal combustion chamber.
U.S. Pat. No. 4,191,149 discloses an air-fuel control for internal combustion engines which provides an increased range of pressure to the carburetor float chamber which may be beyond the levels of the compressor source pressure and atmospheric pressure.
U.S. Pat. No. 4,291,659 discloses a three-stage control in which only the third stage of operation adjusts the air pressure in the fuel passages opened to the venturi nozzle and the bypass port in the carburetor for an internal engine combustion.
U.S. Pat. No. 3,911,884 discloses a fuel injection system for internal combustion engines with a fuel metering control and having a fuel pressure regulator responsive to the magnitude of the sensor signal detecting the presence of oxygen in the exhaust gases providing fuel to the metering control.
U.S. Pat. No. 3,952,710 discloses an air-fuel ratio control system for internal combustion engines in which the oxygen sensor alternately controls the injection of air or the injection of fuel depending upon whether the concentration of oxygen in the exhaust gases is higher or lower than a predetermined concentration or air-fuel ratio.
U.S. Pat. No. 4,043,305 discloses an internal combustion engine in which an exhaust gas sensor controls an electrical valve within an exhaust gas recirculating duct. The control of the air into the intake passage is responsive to the pressure within the exhaust outlet.
The patents relating to internal combustion engine control devices do not describe how such controls can be effectively applied to external combustion engines. In particular, they do not teach or suggest the adjustment of both air flow and fuel pressure in response to a combination of primary heat exchanger temperature and exhaust gas composition.