Burning Butanol in a Better Engine

New computer models save time, money in optimizing engines to burn energy-packed biofuel

Bryan Weber & Bret Windom & Enoch Dames

Researchers at the Combustion Energy Frontier Research Center (CEFRC) have built computer models of how the new biofuel butanol burns in car, truck and jet engines. Butanol is an alcohol similar to the more familiar ethanol, except butanol has a four carbon backbone, compared to only two carbons in ethanol. The longer carbon backbone gives butanol more energy per gallon than ethanol, increasing the number of miles that can be driven on each gallon of fuel.

Millions of dollars in R&D costs can be saved by using computer modeling to eliminate inefficient engine designs before extensive real-world trials. Such computer models commonly contain hundreds of chemicals and thousands of individual reactions. However, the usefulness of the simulations depends entirely on their ability to predict engine-relevant combustion conditions. To ensure that the models’ predictions are valid, the results of simulations are compared to the results from experiments.

The experiments start with the most fundamental, measuring the speed of chemical reactions. At Stanford University, a member of the CEFRC, researchers used lasers to measure the overall reaction rate of highly reactive chemicals with straight chains of butanol. The reaction rates measured by the technique developed at Stanford are related to the initial steps of fuel decomposition. Inaccurate estimates of these rates can cause large changes in the predictions of the model.

Researchers at the Massachusetts Institute of Technology (MIT), another member of the CEFRC, conducted quantum simulations to complement the experiments at Stanford. In addition, they developed software to efficiently generate chemical reaction models. The software automates the process of verifying the importance of each elementary chemical reaction and determining if it is relevant to the model.

At Sandia National Laboratories, chemical species profiles measured in a butanol flame helped researchers determine how quickly butanol releases energy and is converted into products. The measurements are compared to calculations from the chemical model generated at MIT. This sort of detailed comparison helps determine which estimated reaction rates need improvement.

At the University of Connecticut, researchers used a device mimicking the operation of a car or truck engine to measure the ignition delay of butanol at high pressure and low temperature. The ignition delay determines the behavior of a fuel after being introduced into an engine cylinder. Using this parameter, engineers can tune automobile and truck engines for optimum performance. Furthermore, when a chemical model can predict the ignition delay accurately, new engines can be designed for even higher efficiency without the cost of an experimental test program.

By combining fundamental and applied experiments with computer modeling, the CEFRC is rapidly developing chemical models for new biofuels. This approach is being applied to the specific case of butanol, but it is designed to be applied to a broader range of biofuels. A rapid response to emerging biofuels will allow engineers to design optimal engines and help solve environmental, economic and security problems surrounding continued fossil fuel use.

In addition to the work presented here, there has been a substantial effort by other institutions in the CEFRC to contribute to the butanol research. Other publications can be found on the CEFRC website at http://www.princeton.edu/cefrc/news-events/publications/