Design of a Compression Pressurized Air Blast Direct Injection System for Small Displacement Two-Stroke Engines

Abstract

In two-stroke engines replacement of carburetors with direct fuel injection systems greatly reduces engine emissions and fuel consumption by eliminating fuel short-circuiting. Air-blast direct fuel injection using a dedicated air pump has been successfully applied to both two- and four-stroke engines. In this study we re-examine the design of a low cost compression pressurized direct injection system. This system uses gases extracted from the combustion chamber during the compression stroke to supply pressure for the air blast injection, thus eliminating the air pump [1,2]. Gases, predominantly scavenging air, are transferred to a mixing cavity from the combustion chamber via a small (5mm diameter) solenoid poppet valve as the piston rises during the compression stroke. Proper functioning of the system requires careful optimization of the mixing cavity size and the blast valve timing to ensure adequate mixing cavity pressure and fuel atomization. To assist in the optimization of these design parameters a one-dimensional fluid dynamics model has been developed. Parameter sensitivity studies were carried out using the model to determine the optimum cavity size, blast valve timing, and fuel injection duration. These parameters were optimized over a wide range of engine speeds and throttle settings. Results show that a mixing cavity pressure of 500 kPa is attainable over the range of 1000 to 6000 rpm, from closed throttle to wide open throttle (WOT) without cavity pressurization encroaching into the ignition regime. Fuel maps and valve timings are presented and results are contrasted with the carbureted case, showing improved fuel efficiency and emissions for the direct injection system. These data will be used in the design of a physical demonstration engine.