Engine-out aldehyde emissions from toluene primary reference fuels blended with ethanol

Ethanol is a commonly used sustainable fuel component blended into gasoline to reduce its carbon intensity. Aldehydes are a mostly unregulated emission linked closely with ethanol (and other alcohol) levels in gasoline. When modelling gasoline (which is a blend of hundreds of individual hydrocarbons) and conducting laboratory work, surrogates are often used to mimic its behaviour. Two (isooctane and n-heptane, Primary Reference Fuels (PRF)) and three (toluene, isooctane, and n-heptane, toluene primary reference fuels (TPRF)) component surrogate fuels are frequently used for this purpose, usually matching the octane number (RON and MON) of market gasolines. In this work, a single-cylinder gasoline direct injection engine was used at different speed and load conditions to measure the effect of ethanol blended into PRFs and TPRFs, and their constituent components, on engine-out formaldehyde and acetaldehyde emissions. The results show for binary mixtures of ethanol and n-heptane, higher levels of ethanol can reduce engine-out emissions of both aldehydes, in contrast with binary mixtures of ethanol with other PRF components. For the ternary mixtures of iso-octane, n-heptane, and toluene, formaldehyde emissions were four times higher than acetaldehyde and increasing the toluene content led to a 33% reduction in average aldehyde emissions. For PRF and PRF-E fuels, formaldehyde levels were similar across different ethanol levels, while acetaldehyde levels were higher at the highest ethanol levels, indicating n-heptane’s role in suppressing acetaldehyde. In the TPRF and TPRF-E fuels, the engine-out aldehyde emissions were lower in blends with higher toluene levels showing toluene’s suppression of aldehydes, but notably a higher suppression of formaldehyde compared to acetaldehyde. At the highest levels of ethanol (85%v/v) increasing the toluene content from a PRF to a TPRF led to a one-third reduction in average aldehyde emissions, highlighting toluene’s suppressive effect.