HCCI, which has been the focus of research for years as a potential way to provide diesel-like efficiency without the NO_x and soot emissions, is hampered by inadequate combustion control and limited load range. Progress has been made recently in expanding the HCCI operating range using conventional gasoline fuels. However, the researchers note, future fuels could be significantly different from those used today due to the focus on biofuel technology.

One potential change is to replace the currently used ethanol with longer chain alcohols. Higher alcohols, containing four or more carbons, are more favorable gasoline constituents than ethanol. With longer alkyl chain, the polarity of alcohols rapidly decreases, and their physiochemical properties become more like those of the hydrocarbons in gasoline. For example, higher alcohols are less hygroscopic and less susceptible to separation in the presence of water when blending with hydrocarbon fuels. This is in sharp contrast with ethanol, and it makes higher alcohols more compatible with the existing fuel infrastructure. Material compatibility issues such as elastomer swelling are also much improved with higher alcohols. Additionally, in many cases, blending ethanol into gasoline enhances the vapor pressure of the gasoline, which causes high evaporative emissions and potential operational problems such as vapor lock.

...Another clear advantage of higher alcohols is their higher energy content, which would significantly improve fuel economy over ethanol. One potential drawback could be the increased fuel reactivity with the longer chain which diminishes the potential for improved antiknock performance for SI combustion. However the octane ratings of C₄-C₅ alcohols are still sufficient for gasoline SI engines, and they could work well for advanced HCCI or other low-temperature combustion engines, since these engines have different requirements for fuel autoignition quality.

—Yang et al.

Isopentanol is a C₅ branched alcohol that has a greater similarity to gasoline in physiochemical properties than ethanol or butanol, including much lower miscibility in water and higher volumetric energy density.

The study examined the reactivity of isopentanol and compared it with gasoline and ethanol in terms of the intake temperature required to maintain a constant combustion phasing over a wide range of engine speeds. The team then studied the potential for high-load HCCI by examining the early stages of heat release as a function of intake temperature and pressure.

The HCCI research engine was derived from a Cummins B-series six-cylinder diesel engine, with five of the cylinders deactivated. The study covered a wide range of engine operating conditions, including engine speed, intake temperature, intake boost level, and equivalence ratio. Among the findings were:

• Isopentanol shows higher HCCI reactivity than gasoline and ethanol, as evidenced by lower intake temperatures or higher EGR required to maintain a constant combustion phasing over a wide range of engine speeds.

• Isopentanol shows relatively strong reactions at intermediate temperatures prior to hot ignition. The ITHR of isopentanol is similar to that of gasoline at naturally aspirated as well as boosted conditions, which enables similar capability to retard the combustion phasing to mitigate knock.

• The intermediate temperature heat release (ITHR) of isopentanol is enhanced by increased intake pressure and simultaneously reduced intake temperature, which is similar to gasoline.

• Due to the active ITHR and combustion retard capability, as well as the similar ITHR enhancement with P_in, isopentanol reaches high-load limits similar to those of gasoline at various intake pressures.

• Isopentanol shows almost no sensitivity to equivalence ratio in ignition timing at atmospheric intake pressure, indicating that partial fuel stratification will not work to reduce heat release rates (HRR) or mitigate knock under these conditions.

Resources

• Yi Yang, John Dec, Nicolas Dronniou and Blake Simmons (2010) Characteristics of Isopentanol as a Fuel for HCCI Engines (SAE 2010-01-2164)