Naturally, there are complex interactions among internal combustion engine parameters such as in-cylinder pressure, emissions, speed, and load. These basic relationships are studied in a naturally aspirated, spark-ignited, two-stroke large bore natural gas engine. The typical application for such an engine is in the oil and gas industry, operating heavy machinery such as large compressors and oil field pump jacks.

Cylinder pressure measurements averaged over 300 cycles are captured for speeds of 350 and 525 RPM and load ratings of 50%, 75%, and 100% of rated torque at each respective speed. Non-sequential individual cycle pressure curves are also captured to depict cycle-to-cycle variation within the engine at each operating point. Emissions data are captured and presented for each operating point. At another 8 test conditions, pressure measurements averaged from 100 cycles are taken to specifically investigate disagreement among compression pressure curves; these conditions include 350, 400, 440, and 470 RPM at 50% and 85% of the rated load.

It is found that low load cycle-to-cycle variation is extreme, having COV of IMEP values over 30%. Such engines are not designed to operate at low-load conditions. Cyclic variation is shown to decrease with increasing load and decreasing speed. As expected, peak pressures increase with increasing load and decrease with increasing speed. Emissions of THC and CO decrease with increasing load, while emissions of NO are highest at low speed and high load. It is also shown that compression pressure behavior after exhaust port closure but before ignition is different among different speeds and loads. This may be characteristic of scavenging and heat transfer behavior.

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