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.