Computational Study of Direct Fuel Injection in the Rotax 914 Engine

Direct injection spark ignition (DISI) is a fuel delivery method in which the fuel is introduced directly into the combustion chamber of an internal combustion engine. Although direct fuel injection was first pioneered in the early 1920\u27s, it has only recently become a reliable option due to advances made in control systems and injection technology. Direct injection enables increased fuel efficiency and higher power output than a conventional Port Fuel Injection (PFI) system. By delivering pressurized fuel directly into the cylinder, the degree of fuel atomization and the fuel vaporization rate are increased. Hence, the air/fuel mixture can be more precisely maintained, benefiting both fuel economy and emissions. In addition, the cooling effect of fuel droplets changing to vapor inside the combustion chamber facilitates a higher compression ratio and lessens the likelihood of knock. DISI has witnessed a resurrected interest in the automotive industry due to its promise of better fuel economy, additional power, reduced emissions and the ability to operate on multiple fuels. The aviation industry, on the other hand, has largely forgotten about the internal combustion engine subsequent to the invention of the jet engine. However, the introduction of unmanned aerial systems (UAS) has encouraged a renewed interest in small internal combustion engines such as the Rotax 914. Although, these engines provide a cheap power plant, they lack the power and efficiency required for their application. Consequently, by employing DISI in UAS engines, it affords flexibility with regards to fuel choice while also providing longer flight times and more power with less weight. As with any new application of technology, DISI in these smaller engines must first be tested and refined until it can seamlessly replace PFI. Experimental testing can be costly and time consuming, but computational fluid dynamics (CFD) can help speed the design process by performing parametric analysis to determine an optimum configuration to begin testing. For this thesis, a model of the Rotax 914 engine was developed to computationally model the effects of direct injection on the engine. Gambit was adopted for geometry generation and meshing, while Fluent was used for fluid motion and combustion simulation. A PFI version of the computational model was validated against experimental results of a Rotax 914 engine in order to add fidelity to the model. DISI was then applied to the model and a study was performed to determine operation capabilities under different operating conditions