Detailed Kinetic Modeling of HCCI Combustion with Isopentanol 2011-24-0023
Isopentanol is an advanced biofuel that can be produced by
micro-organisms through genetically engineered metabolic pathways.
Compared to the more frequently studied ethanol, isopentanol's
molecular structure has a longer carbon chain and includes a methyl
branch. Its volumetric energy density is over 30% higher than
ethanol, and it is less hygroscopic.
Some fundamental combustion properties of isopentanol in an HCCI
engine have been characterized in a recent study by Yang and Dec
(SAE 2010-01-2164). They found that for typical HCCI operating
conditions, isopentanol lacks two-stage ignition properties, yet it
has a higher HCCI reactivity than gasoline. The amount of
intermediate temperature heat release (ITHR) is an important fuel
property, and having sufficient ITHR is critical for HCCI operation
without knock at high loads using intake-pressure boosting.
Isopentanol shows considerable ITHR, and the amount of ITHR
increases with boost, similar to gasoline. However, the individual
effect of pressure and temperature on ITHR for isopentanol is still
unclear. Also, the chemistry leading to ITHR for isopentanol in an
HCCI engine needs to be explained.
To answer these key questions, a detailed chemical kinetic model
for isopentanol has been developed and used to perform HCCI engine
simulations. The isopentanol model consists of low- and
high-temperature chemistry based on reaction models for butanol
isomers and isooctane (an alkane which a branched molecular
structure similar to isopentanol). The model includes a new
reaction step for concerted elimination of HO₂ from isopentanol, a
process recently examined by da Silva and Bozzelli for ethanol. The
isopentanol model was validated with rapid-compression-machine and
shock-tube data over a wide range of temperatures, pressures and
equivalence ratios (712 - 1205 K, 0.8 - 2.3 MPa, and 0.5 - 1.0,
respectively). Excellent agreement between model predictions and
experimental data was achieved. With regard to simulating HCCI
combustion, the model reproduces the experimentally observed ITHR
of isopentanol and its enhancement when simultaneously increasing
pressure and decreasing temperature for a set combustion phasing.
As seen in the HCCI experiments, the model shows that increasing
the temperature for a fixed intake pressure promotes hot ignition,
with little effect on ITHR.