Comparison of In-Cylinder Pressure Measurement Methods in a Small Spark Ignition Engine 2014-32-0007
In this work, in-cylinder pressure was measured in a 55 cc single cylinder, 4.4 kW, two stroke, spark ignition engine. In cylinder pressure measurements were taken using two different pressure transducers to determine if the performance differences between the two transducers are discernible in a small, spark ignition engine. A Kistler brand measuring spark plug was compared to a Kistler brand flush mount high temperature pressure sensor. Both sensors employ piezo-electric pressure sensing elements and were designed to measure indicated mean effective pressure as well as to detect knock at high temperature engine conditions. The pressure sensors were installed and adjusted to ensure cylinder volume after sensor installation matched the engine's original configuration within reasonable manufacturing tolerances.
A series of tests at four throttle settings ensued to determine if either device altered the combustion volume or the engine's performance. Performance measurements were obtained over a range of engine speeds from 4000 rpm to 6000 rpm. Run to run variation from randomness in the control variables results in a 2% to 5% variation in indicated mean effective pressure. The performance difference between the two spark plugs is less than that variation. Thus, the results indicate that, as a spark plug, the Kistler measuring spark plug is not statistically different from the stock spark plug at the 95% confidence level. The results do indicate a statistically significant difference at the 95% confidence level between the pressure transducer in the measuring spark plug and the flush mount transducer. This difference, which amounts to 4% of indicated mean effective pressure, is the result of differences in the sensor linearity, likely arising from the factory calibrations. Finally, the results reveal a statistically significant difference in performance between the modified and unmodified engine heads above 6000 rpm at the 95% confidence level. This verifies the suppositions of other researchers who note that even a small modification to the combustion chamber of an engine this size could significantly impact the performance, measured in this instance as IMEP.