This paper describes a fuel management known as engine air control as opposed to the more conventional management technique, or engine fuel control. When applied to an internal combustion engine, conventional fuel management systems utilize fuel modulation, where the human is in direct control of the intake air-throttling valve. Here, the throttle indirectly controls the airflow into the engine. The fuel management controller determines the air flowing into the engine and based on the desired air fuel ratio, calculates the appropriate amount of fuel necessary, based on the desired performance index (minimize fuel, maximize power, etc.). In an air control system, the human is in control of fuel flow as opposed to airflow. The engine management system interprets the human input as a desired fuel flow rate, and must calculate the proper amount of air necessary to obtain the desired air fuel ratio. The controller must also deliver the correct amount of fuel to the engine. Thus, the control system becomes “Drive by Wire”, with no direct connection between the human and the engine itself. The fuel is delivered by electronic fuel injectors, while a servomotor connected to the air-throttling valve regulates airflow into the engine. Engine air control systems have been shown to allow for leaner operation, lower fuel consumption, and better transient response.
A non-linear model of an internal combustion engine and actuators was developed. Using the conventional fuel control strategy, a set of initial conditions was obtained, to be used later for the air control system. The plant was linearized, resulting in a two input, single output system. The fuel transfer function was already stable, although additional control could have been added to further optimize the system. The air transfer function was marginally stable, with a pole at the origin resulting from the use of the servomotor controlling the position of the throttling valve. The pole at the origin made the system type one with respect to a step input, a desirable characteristic for internal combustion engines undergoing acceleration transients, thus it was decided to leave the controlled system type one as well. This, along with the uncertainty of the exact location of some of the poles in the transfer function, led to the use of a simple proportional controller as a first attempt at actual control. The resulting control strategy was applied to the non-linear model, which yielded excellent results. A variety of gains were simulated, with the results indicating that severely under-damped control affected air fuel ratio, but engine speed showed little oscillation. A BMW k100 four cylinder spark ignition engine was mounted on a water brake dynamometer, and a digital controller was fabricated allowing for control of both the fuel injection system as well as the throttle servomotor. Conventional fuel injectors were used while a brushed DC servomotor connected to the throttle valve through a gearbox controlled airflow. The system was given a series of fuel step inputs while both the gain and desired air fuel ratio were varied. Test results indeed verified the results of the non-linear model. Figures shown in this paper indicate a close correlation between actual and predicted data, under a variety of operating conditions. Future work will include a more sophisticated air control strategy and fuel minimization will be considered.


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