Ethanol is more difficult to disperse from a fuel injector because of the higher viscosity. Ethanol also has a lower vapor pressure and a higher heat of vaporization. Consequently, with a conventional fuel injection system, more of the fuel evaporation (in comparison to gasoline) will occur during the compression cycle. Conversely, the higher heat of vaporization of ethanol means that pre-mixing of ethanol by PRV lowers the charge mixture more than gasoline. The lower charge temperature and improved dispersion upstream of the engine, provided by PRV, is exactly what an ethanol-fueled engine needs for improved power. The physical properties of ethanol and gasoline are compared in Table 1.

Table 1: Physical Properties of Gasoline and Ethanol (US Department of Energy)

	Heat of vaporization, Btu/lb @ 60° F.	Viscosity, mm2/s.	Reid vapor pressure (100 degrees F.), psi
Gasoline	150	0.55	8-15
Ethanol	396	1.5	2.3

For a White Paper on PRV and ethanol, click here.