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| Total particulate number (top) and total particulate mass (bottom) emissions for different ethanol blends in a cold (20 °C) and a warm (80 °C) engine. Credit: ACS, Chen and Stone. Click to enlarge. |
An experimental study of the enthalpies of vaporization (also known as the heat of vaporization, the energy
required to vaporize the entire liquid fuel) for ethanol/isooctane mixtures by Longfei Chen and Richard Stone at the University of Oxford found that the specific enthalpy of vaporization for an ethanol/isooctane mixture rises monotonically with the ethanol content. In a paper published in the ACS journal Energy & Fuels, they suggest that this could provide an explanation for increased PM emissions from a gasoline direct injection (GDI) engine when ethanol is blended.
The enthalpy of vaporization is important for the performance of spark ignition engines, especially those that use GDI, they note. High enthalpies of vaporization increase the charge cooling effect so that the volumetric efficiency is improved (thereby increasing the specific output).
This also enables the use of a higher compression ratio, because there is a lower temperature at the start of compression; higher compression ratio increases both the efficiency and the specific output. However, they said, their measurements show that the increased enthalpy of vaporization has an
adverse effect on the particulate matter (PM) emissions from a GDI engine, a result they attributed to the air fuel mixture being less
homogeneous.
The enthalpies of vaporization of gasoline/ethanol blends
have not been widely covered in the literature, but they are
necessary as they serve as basic thermodynamic data for determining the dynamics of fuel vaporization, which, in turn, can
affect PM emissions significantly.—Chen and Stone
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| Enthalpy of vaporization (25 °C) for the ethanol/isooctane mixtures on a volume and molar basis. Credit: ACS, Chen and Stone. Click to enlarge. |
Instead of estimating the measure of the latent heat of evaporation, Chen and Stone subtracted the enthalpy of mixing from the mass weighted
specific enthalpies of vaporization of the individual blend components. They used isooctane as a proxy for gasoline hydrocarbons in a wide range of ethanol blends (E5, E10, E20, E35, E50, E70, and E85). They also measured PM emissions data on both a number
and a mass basis from a GDI engine.
Among their findings were that the enthalpy of vaporization rose with increasing ethanol content, and that an increase in ethanol content results in an increase in PM emissions in terms of both number and mass.
The large increase in the
enthalpy of vaporization with increasing ethanol content offers
an explanation for the monotonically increasing PM emissions
found in GDI engine tests when the ethanol content rises…The higher the ethanol content in
gasoline, the higher will be the enthalpy of vaporization, and
hence the higher the inhomogeneity of the mixture, causing the
PM emissions to rise.—Chen and Stone
Resources
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Longfei Chen, Richard Stone (2011) Measurement of Enthalpies of Vaporization of Isooctane and Ethanol Blends and Their Effects on PM Emissions from a GDI Engine. Energy & Fuels Article ASAP doi: 10.1021/ef1015796