Effects of Ethanol Evaporative Cooling on Particulate Number Emissions in GDI Engines

Abstract

The spark ignition engine particulate number (PN) emissions have been correlated to a particulate matter index (PMI) in the literature. The PMI value addresses the fuel effect on PN emission through the individual fuel species reactivity and vapor pressure. The latter quantity is used to account for the propensity of the non-volatile fuel components to survive to the later part of the combustion event as wall liquid films, which serve as sources for particulate emission. The PMI, however, does not encompass the suppression of vaporization by the evaporative cooling of fuel components, such as ethanol, that have high latent heat of vaporization. This paper assesses this evaporative cooling effect on PN emissions by measurements in a GDI engine operating with a base gasoline which does not contain oxygenate, with a blend of the gasoline and ethanol, and with a blend of the gasoline, ethanol, and a hydrocarbon additive so that the blend has the same PMI as the original gasoline. As such, the dilution and the evaporative cooling effects of the ethanol could be separated. Measurements have also been done with methanol and MTBE. The results show that evaporative cooling effect can significantly change the PN emission. The extent of the change, however, depends on the details of the operating condition such as injection timing, engine coolant temperature, and load.

Influence of fuel injection strategies on efficiency and particulate emissions of gasoline and ethanol blends in a turbocharged multi-cylinder direct injection engine

“…27 Westbrook et al 28 in their chemical kinetic modeling study found a direct relation between the fraction of oxygen (ethanol) in diesel fuel and reduction in percent of fuel carbon converted to soot precursors, indicating a strong chemical pathway to reducing particulates. Similarly, Chen et al 29 reported lower PN emissions with E25 compared with gasoline and attributed the low particulate formation Figure 17. Total PN emissions for E0, E30 and E85 at MAP = 1000 mbar and using the injection strategies of Figure 14.…”

“…to chemical effects of ethanol. However, Chen et al 29 also reported that the higher heat of vaporization of ethanol results in high evaporative cooling which enhances formation of residual liquid film mass, which is a source of particulates and thus PN formation also depends on operating conditions. Figure 19 summarizes the effects of the different fuels and injection strategies on the engine performance metrics for the conditions that yielded the greatest reduction in total PN (SOI1 = 280; SOI2 = 220; SOI3 = 60; fuel distribution = 40:40:20).…”

Effects of alcohol addition to traditional fuels on soot formation: A review

“…A spark-ignited single-cylinder research engine with direct fuel injection 2100 r/min IMEP 650 kPa Total particle number; particle number distribution; particle morphology SMPS; DMA; condensation particle counter (CPC); thermophoretic sampling-TEM (continued) 68,69 Ethanol-gasoline…”

“…The study also found that 5 of the 15 test vehicles showed little or no sensitivity to PM emissions, further illustrating the interaction between fuel characteristics and vehicle and engine design characteristics. Zhang 68 and Chen et al 69 revealed many factors affecting engine PM emissions from three aspects of engine operating conditions, ethanol effect and PM formation and further evaluated the displacement effect, evaporative cooling effect and oxygen content effect after ethanol addition. They confirmed the existence of displacement effects and found that the effects of oxygen content can be ignored, while the evaporative cooling effect of ethanol can significantly change PN emissions.…”

Heat of Vaporization and Species Evolution during Gasoline Evaporation Measured by DSC/TGA/MS for Blends of C1 to C4 Alcohols in Commercial Gasoline Blendstocks

“…These studies have suggested that increased evaporative cooling from the presence of ethanol causes high boiling point aromatic compounds, which are largely responsible for PM formation, to be resistant to evaporation and mixing with air and thus much more likely to form PM. This effect is thought to be particularly relevant when the fuel spray impinges on the top of the piston or cylinder wall [12,14,17,18]. The impact of alcohols other than ethanol on PM formation has not been widely studied.…”