No abstract

Air-fuel ratio control of large-bore, two-stroke, natural gas engines, typically used in the oil and gas field, is critically important to maintain stable operation and emission compliance. Many two-stroke applications rely on reed valves to control air and gas induction, which can involve complicated gas flow behavior; standard gas dynamic relationships are typically insufficient to characterize such behavior. Computational fluid dynamic simulations offer the needed complexity, but even so the computational fluid dynamic models, as shown in this work, must also capture the dynamic behavior of the valves themselves. The current work reports on a computational fluid dynamics-based model representing this type of large-bore, two-stroke, natural gas engine using commercially available computational fluid dynamic software. The engine under study is an AJAX E-565 with rated power of 30 kW (40 HP), a bore of 216 mm (8½$), and a stroke of 254 mm (10$). The large engine geometry makes a relatively large solution domain, hence requiring an intense, timeconsuming numerical investigation. This large-bore engine works at a rated speed of 525 RPM with a compression ratio of 6 to 1. Two approaches to modeling the reed valve are investigated: (1) a pressure difference-based user-defined function and (2) a fluid-structure interaction user-defined function. The pressure difference-based user-defined function captures reed valve behavior in a simple, binary fashion (i.e. valves are either open or closed based on the pressure difference between the intake pipe and the engine's stuffing box). The fluid-structure interaction user-defined function, however, predicts the motion of the reed valve strips based on fluid and body motions; although a more complex solution, the fluid-structure interaction user-defined function accurately predicts the engine's gas exchange process. In this article, the results of each method are presented and validated to show that the added complexity is necessary to properly predict (and thus eventually improve) the engine's air-fuel ratio control.

The natural gas industry has long depended on large bore, two-stroke cycle, spark-ignited, gas-powered, reciprocating engines to move gas from the well to the pipeline and downstream. As regulations governing the pollutant emissions from these engines are tightened the industry is turning to the engine OEMs for a solution. The challenge of further reducing engine emissions is not a new task to the industry. However, as the requirements placed on the engines are further restricted, the technology required to achieve these goals becomes more advanced, along with the required tools and technology to create it. New predictive tools have been created and have become more powerful and capable as computer software and hardware becomes more advanced, enabling engineers to create more complex designs and to do so quickly and at lower cost, all of which may not have been possible previously. This paper investigates methods used in designing the Ajax 2800 series, which is a large bore, two-stroke cycle, gas-powered, reciprocating engine and the improvements in emissions that resulted from the application of these methods.. Solutions to overcoming the challenges encountered during the process will also be presented.

Large bore, spark ignited, reciprocating, gas engines have been the workhorse of the pipeline industry for many years when it comes to transmission of gas. Recently, the US government has released an update to the NSPS and RICE NESHAP regulating emissions from many of these engines to [1]: • 1gbhp·hgbhp-hrNOX • 2gbhp·hCO • 0.7gbhp·hVOCs (Non-methane, non-ethane hydrocarbons) This new rule leaves many of these legacy engines out of compliance with the standard. Because of this, engine operators are left with the choice of decommissioning these engines as they come due for compliance based retrofit or working with engine OEMs to implement an emissions reduction strategy. For many years, the traditional methods and tools used for engine design were more than enough to create a successful engine. But with tightening restrictions and higher expectations for customers and the amount of improvement that can be extracted from design changes shrinking with every redesign, these methods by themselves are no longer sufficient. This paper will examine modern design methods and tools available to an engine designer as well as their integration with more traditional testing methods. A comparison of results for the redesigned COOPER BESSEMER® GMVH-6C3 will also be presented for analysis.