| Tests were performed on a modern engine, previously converted to natural gas operation. The engine-out aldehyde and ketone emissions were measured using a standard method, and the effects of changing each of several engine operating parameters were observed.;Formaldehyde and acetaldehyde represented 96.1% and 2.8% of total carbonyls respectively. For stoichiometric mixtures, changes in spark timing, exhaust gas recirculation rate, and engine speed produced aldehyde emissions qualitatively similar to those for intermediate hydrocarbon species. Over 86% of the variation in formaldehyde emissions was found to be due to variations in the exhaust gas temperature and residence time. Thus, post-combustion oxidation largely controls absolute emission levels of formaldehyde.;Fuel/air ratio changes produced the greatest variation in aldehyde emissions, with trends quite unlike those for hydrocarbons, due to changes in the oxidation mechanism of methane (... |
