Speciation and Heat Release Studies during n-Heptane Oxidation in a Motored Engine

ABSTRACT

The low-to-intermediate temperature oxidation of n-heptane was investigated by means of speciation and heat release analysis using a motored single-cylinder engine. The experiments were conducted under a fixed engine speed and variable compression ratios in the range of 5.0 – 7.75. A premixed fuel-air mixture corresponding to an equivalence ratio of ϕ = 0.25 was examined for stable intermediate product evolution with varying compression ratios. The experiments focus on pre-ignition species evolution and avoid explosive autoignition. The progress of reactions was followed from the onset of the low-temperature heat release (LTHR) through the negative temperature coefficient (NTC) region. A Fourier transform infrared spectrometer (FTIR) was used to detect and quantitate carbon monoxide, carbon dioxide, and n-heptane, while a gas chromatograph-mass spectrometer (GC-MS) was used to identify species that evolve between the first and second stages of ignition. A total of fifty-six stable intermediates were identified. In-cylinder pressure measurements were used to carry out a heat release analysis and correlate the global markers such as the first and second stage pressure rise to species evolution. Quantitative concentration measurements were carried out for selected alkenes, aldehydes, ketones, ethers, and alcohols. The relative experimental trend for the production of compounds within each of these chemical classes was obtained. The experimental results for speciation and heat release were compared to numerical results obtained using a detailed chemical kinetic mechanism in the literature.

KEYWORDS:

• n-Heptane
• speciation
• heat release
• motored engine
• combustion

Acknowledgments

Elyasa Al-Gharibeh acknowledges the assistantship support from the department of mechanical engineering at the University of Idaho.