Formation of CH2O and UHC emissions during catalyst-heating operation in compression ignition engines: High-speed FID and mid-IR extinction diagnostics

To comply with increasingly stringent pollutant emissions regulations, catalyst-heating operation in diesel engines is critical to achieving rapid light-off of exhaust aftertreatment catalysts during the first minutes of cold start. Current approaches to catalyst-heating operation typically involve one or more late post injections to retard combustion phasing and increase exhaust gas temperatures. The ability to retard post injection timings while maintaining acceptable pollutant emissions levels is pivotal for improved catalyst-heating operations. This experimental study aims to provide further insight into unburned hydrocarbons (UHC) and formaldehyde (CH2O) formation under catalyst-heating operation. In this study, high-speed laser extinction based measurements of CH2O are demonstrated in an optically accessible exhaust runner, and high-speed UHC measurements are also performed using a fast flame ionization detector analyzer during catalyst-heating operation. Time-resolved CH2O and UHC emission measurements are used to analyze the effect of each injection event in a five-injection strategy (two pilots, one main and two posts) on the pollutant emissions. The first pilot injection generates a significant amount of UHC unevenly distributed in the chamber, with UHC trapped primarily in the upper-cylinder and inside-the-bowl regions. The fuel delivered with the second pilot injection partially burns the unburned fuel from the first pilot injection particularly in inner-bowl region, creating short-chain hydrocarbons corresponding to formation of CH2O. UHC is dramatically reduced in the presence of the main injection, reinforcing the hypothesis by showing large CH2O emission during intermediate and later exhaust process. The first post injection produces UHC and CH2O in the upper-cylinder area due to over-mixing, which is further promoted by the second post injection, while these post-injections do not substantially affect near-bowl emissions.