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The composition of natural gas delivered through the pipeline varies with time and location around the USA. These variations are known to affect engine performance and emissions through changes in fuel metering characteristics and knock resistance of the fuel. High output, low emissions natural gas engines are being developed that take advantage of the high knock resistance of natural gas. These optimized engines are operated close to knock-limited power where changes in fuel knock resistance can cause operational problems. Octane tests were conducted on natural gas blend fuels using a CFR octane rating engine. Two relationships between motor octane number and fuel composition were established. A correlation for motor octane number versus the reactive hydrogen-carbon ratio was developed, and octane weighting factors, which used the molar composition of the fuel to predict motor octane number, were also found.

The composition of end-use natural gas is known to vary significantly around the United States. Of primary interest for this analysis was the impact of natural gas fuel composition on fuel metering and engine operational characteristics. A fuel metering model was developed to analyze the impact of fuel composition on carbureted, fuel-injected (premixed), and direct-injected engine configurations. The change in physical properties of the fuel was found to have a profound effect on fuel metering characteristics. Fuel composition was found to affect different fuel metering configurations differently, but these variations were minor compared to the fuel property effects. Equivalence ratio variations of 12 percent lean and 6 percent rich were calculated from a median gas composition. Wobbe Index was identified as a key parameter for estimating the change in metered equivalence ratio between two fuels.

The potential for existing small natural gas-fueled engines to meet the demands of commercial cogeneration applications was demonstrated. Six small engines, ranging in size from 6 to 35 kW electrical output, modified for operation on natural gas were evaluated for performance, durability, serviceability, and reliability through extended engine operation. Initially performance of both engines and generators was measured, followed by extended durability runs. Maintenance was performed as needed, and engine and component wear were monitored. The major barriers which limit engine durability and reliability were identified. Engine improvements were made, where possible, and evaluated for their effectiveness. Results indicate that some small gas-fueled engines can achieve 4,000-hour service intervals and 20,000-hour engine life. Engine reliability and life are largely dependent on the detailed design of the engine.

Durability of a current production rotary engine was evaluated while operating on natural gas. Through regular scheduled teardown inspections, wear data was gathered on critical engine components and used to develop component wear trends. Wear trends were used to predict the usable life of the engine before requiring a major rebuild. Preliminary results indicate apex seal wear rates and trochoid wear rates low enough to achieve 20,000 hours of engine operation. Areas requiring further durability improvement include the ignition system and optimization of the oil injection. Advanced knowledge of current engine component durability will be used to accelerate the development of novel gas fueled rotary engines currently under development in the United States.