Tracer Gas Measurements of Trapped A/F Ratio and Scavenging Characteristics in a Large Bore Natural Gas Engine (Research)
The tracer gas method is investigated as a means to evaluate trapped A/F ratio and scavenging characteristics in large bore 2-stroke cycle engines. Currently there is no standardized method for measuring trapped A/F ratio, a parameter that has a dramatic impact on emissions, in large bore natural gas engines. Scavenging efficiency is important because it indicates the fraction of air trapped in the cylinder. The experimental setup is external to the engine, requiring no major engine modifications, making it suitable for field applications. The investigation is performed on a Cooper-Bessemer GMV-4TF natural gas engine, with a 14-inch bore and a 14-inch stroke. Measurements with the tracer gas method are compared with in-cylinder sampling techniques to evaluate method accuracy. Two different tracers are evaluated, monomethylamine and nitrous oxide. Monomethylamine is investigated because of its common use historically as a tracer gas. Nitrous oxide is a new tracer gas that overcomes many of the difficulties experienced with monomethylamine. The tracer gas method with nitrous oxide is determined to be very accurate for evaluating scavenging efficiency and trapped A/F ratio in comparison to the in-cylinder sampling techniques implemented, often characterized as direct measurements. The tracer gas method using nitrous oxide as the tracer gas is implemented to study scavenging characteristics and evaluate the trapped A/F ratio for various operating conditions. Engine operating conditions are altered through variations in boost pressure, speed, backpressure, and intake port restriction. For the GMV-4TF, tracer gas method measurements are nearly coincident with predictions of scavenging characteristics with the perfect mixing model, often used to evaluate scavenging characteristics analytically. Backpressure variations are shown to have the largest influence over scavenging characteristics. A potential field application for the tracer gas method, the detection of carbon dep
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