| Summary: | <p>The transport sector is a substantial contributor to global greenhouse gas emissions. To address this, governments are pursuing policies including vehicle electrification and the promotion of renewable fuels. Ethanol stands out as a promising alternative fuel due to its low-carbon potential and compatibility with current infrastructure.</p>
<p>However, aldehydes, particularly formaldehyde and acetaldehyde, are both observed emissions and major intermediates in the oxidation of gasoline/ethanol fuel mixtures. A rise in the ambient concentration of aldehydes due to transport emissions can affect atmospheric chemistry, contributing to ozone and secondary pollutant formation. The International Agency for Research on Cancer (IARC) classifies formaldehyde as carcinogenic and acetaldehyde as probably carcinogenic. While most research primarily associates these emissions with the ethanol content in fuel, investigations into other components have yielded inconsistent results. There is a gap in thoroughly understanding the effect of fuel composition on aldehyde emissions.</p>
<p>Therefore, this work considers aldehyde emissions from commonly used gasoline surrogate components, including ethanol, iso-octane, n-heptane, and toluene, which are representative of different fuel classification groups. They were evaluated in their pure forms and in binary, ternary, and quaternary blends to investigate how ethanol's interaction with each component influences aldehyde emissions. Additionally, two market-representative fuels, E0 (no ethanol) and E10 (10 %v/v ethanol in gasoline), were formulated from refinery streams.</p>
<p>Formaldehyde and acetaldehyde emissions were assessed from a single-cylinder gasoline direct injection engine, operated under conditions of low speeds and loads. These operating conditions have been reported as being conducive to increased aldehyde production. The impact of varying the fuel-air equivalence ratio from lean to rich conditions was also evaluated. The aldehydes were measured at 5 Hz using the Fourier Transform Infrared (FTIR) spectroscopy method.</p>
<p>These experimental investigations were supported with chemical kinetics analyses, which examined formaldehyde and acetaldehyde across several reactor models, including plug flow reactor (PFR), perfectly-stirred reactor (PSR), spherical constant volume combustion vessel, and one-dimensional single-cylinder engine models.</p>
<p>Additionally, the market-representative fuels (E0 and E10) were investigated on four light-duty gasoline vehicles. Real-driving emissions (RDE) of formaldehyde and acetaldehyde were measured using a volatile organic compounds (VOCs) system which could segregate the emissions based on driving phase.</p>
<p>Aldehyde emissions from pure ethanol were ten-fold and five-fold greater than pure toluene and iso-octane, respectively. Greater formaldehyde than acetaldehyde was found for iso-octane and toluene, while the opposite was true for ethanol. Pure toluene produced negligible quantities (10 to 30 ppm) of aldehydes. The addition of iso-octane, toluene, and n-heptane to ethanol inhibited aldehyde formation. Specifically, toluene was more effective in reducing formaldehyde emissions, whereas n-heptane showed a stronger effect on acetaldehyde suppression.</p>
<p>Chemical kinetics analyses indicated that at low temperatures, ethanol's oxidation directly produced acetaldehyde. Adding other fuel components creates competition among H, OH, and HO2 radicals, leading ethanol to favour chain-propagating reactions over chain-terminating ones, thus reducing stable intermediate aldehyde formation. These processes, influenced by temperature, pressure, and fuel-air ratio, vary within the combustion chamber, affecting aldehyde formation.</p>
<p>RDE tests showed aldehyde emissions were greatest at cold-start and lowest under motorway conditions. A 30 % increase in tailpipe formaldehyde emissions for E10 compared to E0 for 3 out of the 4 cars tested. Acetaldehyde emissions were below the detectable threshold for both E0 and E10 fuels, likely removed by the after-treatment system. However, in the single-cylinder engine (with no after-treatment system), the acetaldehyde emissions were detected for both E0 and E10, with the latter producing higher quantities.</p>
<p>This study showed that components other than ethanol in a gasoline surrogate can generate aldehyde emissions. When blended, the interaction between these components can result in reduced aldehyde emissions. However, the extent of suppression and their specific impacts on formaldehyde and/or acetaldehyde vary based on the composition of the blend. Further studies on a broader range of engine operating conditions, coupled with enhancements in chemical kinetic mechanisms and reactor models for accurately predicting aldehyde formation, consumption, and emissions, would be beneficial for developing strategies to mitigate these emissions.</p>
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