Given the current abundant supply and cheap price of natural gas (NG) in the United States, there is a significant opportunity to use this fuel for transportation. However, there are few fueling stations for consumers. To address this need, a dual-mode engine has been developed where in one mode all engine cylinders fire normally, providing locomotion for the vehicle. In the other mode, one cylinder of the engine is used to compress low pressure residential NG, in multiple stages, to a standard U.S. compressed natural gas (CNG) vehicle storage tank pressure of 248 bar [3600 psi]. This allows a natural gas vehicle (NGV) to be refueled anywhere there is access to the natural gas distribution network. A detailed numerical model was developed as a design aid to simulate the compressor capability of the engine. Additionally, an original first order time response filling model for the compressor and storage tanks was developed in order to draw insight into and optimize the refueling rate of the system. Using this model, it was determined that the fastest refueling rate occurred at the lowest pressures and that switching between intermediate staging tanks more frequently led to an increase in the overall refueling rate. It also became apparent that the low pressure residential NG (1.7 kPa [1/4 psi]) source was the most limiting factor of the design, as the peak pressure at the end of a compression stroke is dictated by the inlet pressure. Boosting the inlet pressure a small amount before the initial stage of compression would dramatically improve the refueling rate.
Experimental studies were conducted on this prototype engine at the Oregon State University Energy Systems Laboratory located in Bend, OR. Knowledge gained from applying the aforementioned models combined with empirical results led to the realization of a self-refueling natural gas vehicle. The integral compressor had an approximate refueling efficiency of 70%, with an electrical-equivalent parasitic load of 12%. Idling of non-compression cylinders and the distance between the compressor and the 3-way valves used to control the compression staging were significant sources of inefficiency within the system. This fully functional vehicle was developed and operated during the course of this thesis and driven over 161 km [100 miles] using self-compressed natural gas as a fitting conclusion to this effort.