A gaseous emissions analysis of commercial aircraft engines during test-cell run

Abstract

This study presents an analysis of the seventeen complete measurements of gaseous emissions of eleven different CFM56-7B26 turbofan engines during the regular test-cell runs. A novel measurement setup is established in which the sample probe is located immediately behind the low pressure turbine rotor. Recording a total of 49,190 data points for 26 raw variables (obtained from both the emission measurement and engine test systems), emissions are characterized with stable engine power settings representing main flight phases, as well as fuel flow of the engine during the entire run. Carbon monoxide and total hydrocarbons concentrations are found to be a strong function of fuel flow at low power settings, and nitrogen oxides emissions at high power settings. In comparison to emission indices provided by the International Civil Aviation Organization, the current study results in considerably higher carbon monoxide and hydrocarbons under idle conditions, with relatively lower nitrogen oxides at almost all of the power settings. On the other hand, significant differences in levels of nitrogen oxides are observed from one engine test to another, emphasizing the effects of the engine and the ambient air conditions. The average NO2/NOx are found to be 40% (±16) and 32% (±9) for ground and approach idles, respectively, while at high power settings, above N1 = 64%, the NO2/NOx ratios stabilize at around 5–6% (±1–2). Since the emissions were monitored and recorded during the entire engine run, the start and end moments (cold and hot idles) of the tests are compared. The average carbon monoxide and hydrocarbons concentrations are found to be 14% and 35% lower at hot ground idles compared to cold ground idle, with the results for nitrogen oxides and carbon dioxide being 22% and 0.5% higher. The effect of ambient air temperature on emissions are also investigated and quantified for certain temperature ranges. It has been determined that the hydrocarbons and carbon monoxide concentrations decrease with ambient air temperature, yet the nitrogen oxides and carbon dioxide concentrations increase. Findings related to ambient air temperature are found to corroborate the findings related to running at cold and hot idles, linking to a positive correlation between the ambient air temperature, the combustor inlet temperature and the exhaust gas temperature.