Evaluation of Universal Exhaust Gas Oxygen Sensor for Trapped Equivalence Ratio in Large-Bore, Two-Stroke Gas Engines, 03-R9493Printer Friendly Version
Inclusive Dates: 07/01/04 09/01/06
Background - The gas transmission industry operates more than 4,000 integral engine compressors, the majority being two-stroke, with a median age of 45 years and a median size of 2,000 horsepower. These engines have historically exhibited poor performance and high emissions, due in part to poor engine control. The end results are misfires and partial burns that lead to increased fuel usage and exhaust emissions. Recent investigations in several SwRI projects have acquired data indicating a significant spread in the actual air/fuel ratio (or equivalence ratio) between cylinders in these engines. This spread of equivalence ratios limits the range of operation, which prevents optimal settings of spark timing and overall air/fuel ratio for peak efficiency and NOx emissions. Therefore, to achieve optimal efficiency and emissions with a spark-ignited engine, the fuel-air equivalence ratio must be measured in real-time and adjustments made by the control system to each cylinders fueling to maintain the optimal setting.
Approach - The objective was to develop and demonstrate a new concept for advanced controls for two-stroke integral engines, using universal exhaust gas oxygen (UEGO) sensors in the exhaust port of each cylinder to feedback trapped equivalence ratio for cylinder-based control. Individual cylinder fuel trim valves were installed and controlled by the UEGO sensor feedback and control algorithms were developed and confirmed that the air-to-fuel ratio from cylinder to cylinder can be better balanced, thus minimizing the spread across the engine.
Accomplishments - An enabling technology was investigated using a Cooper GMVH-6 test engine and has demonstrated improved control of trapped equivalence ratio for integral two-stroke compressor engines. Data correlated with predicted trapped equivalence ratio. UEGO measurements in the exhaust runner indicated that the left and right engine banks were operating at different equivalence ratios, which correlated with computational modeling that showed bank-to-bank differences in trapped air mass due to the articulated design of the connecting rods. A control system was configured to adjust individual cylinder fueling based on the processed UEGO signals. Control valves were installed in each cylinders fuel lines and were controlled as a function of UEGO feedback. This technique shows promise for accurate closed-loop air/fuel ratio control in two-stroke integral engine compressors. Plans to continue development on a commercial project have been proposed to an engine manufacturer.