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.