Dimethyl carbonate (DMC) and Methyl Formate (MeFo): Emission characteristics of novel, clean and potentially CO₂-neutral fuels including PMP and sub-23 nm nanoparticle-emission characteristics on a spark-ignition DI-engine

Dimethyl carbonate (DMC) and methyl formate (MeFo) are oxygenate fuels characterized by a very high octane number and boiling points in the range of commercial gasoline (DMC: 90°C/363,2K, MeFo: 31.5°C/304,7K). Due to their molecular structure without C–C bonds (C1-fuel) and their high oxygen content of 53.3%, DMC and MeFo are expected to avoid the formation of soot precursors in combustion and thus to distinctly reduce particulate number (PN) emissions. Beyond todays solid particle number (PN) limits, which are based on the definition of the particle measurement program (PMP) using a 23 nm cut-off of the counter, sub-23 nm particles are currently discussed to be a health concern. The measurement of smaller solid particles is not standardized yet and is potentially impeded by the presence of volatile nucleation mode particles below 23nm. Moreover, standard techniques for additionally determining particle size distributions usually are below their sensitivity limit due to the low particle concentrations produced by oxygenate fuels. At the Technical University of Munich, DMC and MeFo were tested as pure substances in their emission and performance behavior compared to conventional gasoline fuel, using a single-cylinder, direct injection SI-engine. Primary (limited: CO, HC, NO_x) and secondary (non-limited: CH₂O,CH₃OH,NH₃) gaseous emissions pre- and after a TWC as well as particle number (PN) emissions were investigated. Regarding the challenges of particle number measurements, this work presents our method to also assess the properties of such ultra-low concentration aerosols from engine exhaust considering trends in particle size and potential artifacts caused by volatile, sub-23 nm particles. The method described uses two differently constructed PMP PN counting systems, operating in parallel, using largely different dilution schemes and rates. Both systems are equipped with two condensation particle counters (CPC) each, which are set to cut-off ratios of 23nm and 10nm, allowing to gain further insights using the ratios of the different systems (indication of potential artifacts) as well as ratios of the two CPC (indicating size trends) respectively. The test outcomes of steady-state operating points during lambda-sweeps show positive effects of the fuels on raw gaseous emissions while still maintaining high conversion rates over the TWC. PN emissions drop significantly and are even below ambient concentrations both for DMC and MeFo in DI-mode. However, a higher relative share of sub-23 nm particles is observed among the low number of remaining particles, and the data of both particle systems is used for further analysis of size trends and volatility. While displaying ultra-low pollutant emissions, DMC and MeFo are also potentially CO₂neutral substances, and their potential sustainable production using green electricity and waste CO₂ will be also summarized.