Modeling and Control of a High Pressure Combined Air/fuel Injection System

A high pressure combined air-fuel injection system is designed and tested for an experimental free liquid-piston engine compressor. The application discussed utilizes available high pressure air from the compressor’s reservoir, and high pressure fuel to mix and then inject into a combustion chamber. This paper addresses the modeling, design and control for this particular high-pressure air-fuel injection system, which features an electronically controlled air/fuel ratio control scheme. This system consists of a fuel line and an air line, whose mass flow rates are restricted by metering valves. These two lines are connected to a common downstream tube where air and fuel are mixed. By controlling the upstream pressures and the orifice areas of the metering valves, desired A/F ratios can be achieved. The effectiveness of the proposed system is demonstrated by a lumped-parameter model in simulation and validated by experiments. INTRODUCTION In the conventional spark-ignition (SI) engine research community, high-pressure direct injection has been increasingly attracting researchers’ attention for its more complete combustion, high power output and low emissions of substances that affect the environment [3]. Under a high pressure, direct injection systems are able to supply finely atomized fuel to the combustion chamber. Conventional direct injection concerns only the fuel. In the application studied in this paper, a free-piston engine compressor, high pressure air is also always on hand. Such a system presents the opportunity of combining the high pressure injection of fuel with high pressure air. Such a scheme allows an engine to bypass the conventional intake and compression strokes; the simultaneous introduction of high pressure air and fuel presents a high pressure mixture directly before combustion that is equivalent to the end of a conventional compression stroke. Utilizing high pressure for both air and fuel “combined injection” is proposed in this paper for small scale free-piston engines [1, 2] such as that shown in Fig. 1. Although the proposed approach is developed for this particular configuration, it can be applied to any engine application where both high pressure air and high pressure fuel are metered into the combustion chamber. FIGURE 1. FREE-PISTON ENGINE COMPRESSOR The proposed injection system has two separate air and fuel lines that can be metered and then combined to supply a controlled air/fuel mixture. If the driving pressure of the air and fuel is considered as arbitrary, and the downstream pressure of the mixture is also considered arbitrary, the proposed combined injection system shares many similarities to the air/fuel injection scheme used in conventional SI engines. However, the proposed combined air/fuel injection ratio control strategy is functional different. Generally speaking, conventional air/fuel ratio control regulates the fuel flow as a certain ratio of the air flow where the air flow is typically directly commanded by the driver in automobile engines. An illustration of the typical air/fuel injection system is shown in Fig. 2. The air flow is regulated by throttling and fuel flow is then appropriately controlled. For the specialized application considered here, the system is still applicable but upstream and downstream pressures are somewhat different. A conventional IC engine with fuel injection inducts atmospheric pressure air a P and injects high pressure fuel f P into a downstream pressure d P . For the proposed application here, the air and fuel pressures will always be significantly higher than the downstream pressure. Hence, this particular configuration requires a different control strategy. Conventional air/fuel ratio control for internal combustion engines commonly includes an inner-loop controller based on the deviation of the estimated three-way catalyst stored oxygen state [4][5]. This scheme must account for the delay between the controlled input and the sensor reading. The proposed system directly injects a high pressure air and fuel mixture to the combustion chamber through separately metered air and fuel lines, assisted by high upstream driving pressures. The control strategy will be to regulate the fuel pressure to match that of the air pressure. A local feedback control scheme based on pressure is used before the mixture enters the intake manifold. This pressure based control approach eliminates the delay problem introduced in conventional IC engines that measure exhaust products. FIGURE 2. GENERALIZED AIR/FUEL RATIO CONTROL The proposed combined injection system is designed for an experimental free liquid-piston engine air compressor, as shown in Fig. 1, which, by virtue of being an air compressor, has a high pressure air reservoir available for high pressure air injection. The “high pressure” mentioned here is between 300kPa and 700kPa. The pressure based control scheme will control the fuel pressure to match that of the air pressure. Once the two driving pressures are equal, air and fuel will flow and mix simultaneously into a common downstream pressure. The ratio of the two mass flows will be determined not by each flow’s duration, as is the case in conventional engines, but by the ratio of the flow restrictions of the two lines subject to the same pressure drop. The specifics of this strategy will be discussed in detail in the remainder of the paper. Therefore, pressure is used for feedback control in this approach. As a matter of fact, pressure has been used by engineers to control various applications in internal combustion engines [6][7], made possible by pressure sensors that can provide fast, accurate, and convenient real-time pressure for examining the state of the system. This paper presents modeling, design and control of the combined air/fuel injection system for our free liquid-piston engine compressor. Since this injection system is designed to be simple and compact, the control law is accordingly designed to be simple so that it can be easily transferred to other small scale SI engine applications. The design of this system will be first introduced, followed by a lumped-parameter dynamic model. Simulation results will be shown and compared with experimental data. PRESSURE-BASED MASS FLOW CONTROL High-Pressure Combined Air/Fuel Injection System The conventional approach to the control of fuel flow uses a high speed on/off injection valve and PWM control. The proposed approach here will control the fuel flow relative to air flow by controlling the upstream pressure to a fixed but known flow restriction. Therefore, the configuration in Fig. 2 can be extended to a more specific configuration shown in Fig. 3. The proposed system primarily consists of two supply lines: the air line and the fuel line, respectively. The air used for mixture injection comes from an external air reservoir. On the other line, the fuel source is a 0.5kg bottle of propane, which at room temperature has a vapor pressure of about 1 MPa (154 psia). The mass flow rates of air and fuel, through the air line and the fuel line respectively, are controlled by the metering valves to achieve the stoichiometric ratio, based on the proposed pressure-based mass flow control method. Air and fuel are mixed in the downstream tube before entering the combustion chamber. Note that there is a fuel buffer cylinder between the fuel valve and the metering valve in Fig. 3. Its function will be discussed in the control section.