Transient chemical composition analysis in HCCI of n-heptane fuel

The fuel specific chemical mechanism responsible for compression ignition is investigated by measuring transient species existing during the two-stage ignition in a motored single cylinder engine. N-heptane was used as the fuel in this study. Crank angle resolved profiles of n-heptane, formaldehyde and hydrogen peroxide were obtained by means of mass-spectrometric detection of gas sampled from the engine cylinder through a pulse actuated valve. Partial fuel consumption and intermediates formation taking place at the timing of the first stage (low temperature) ignition were observed. The percent fuel consumption and product yields relative to initial fuel amount are weak function of equivalence ratio. These trends were well predicted by simulations of homogeneous chemical reaction using the detailed kinetics model for primary reference fuel combustion. Compared with the previously studied case of dimethyl ether fuel, the n-heptane fuel consumption is considerably larger, whereas the partial heat release of the first stage is close. Exhaust gas analysis under suppression of high temperature ignition stage was conducted to detect species undetectable or less sensitive by the mass-spectrometry. A Fourier transform infrared spectrometer was used for this purpose. Formic acid, methanol and ethylene were detected in this method, where amounts of the last two species were in disagreement with the numerical prediction. Based on these results, it is considered that the first stage heat release is governed by reactivity of the transient species, which are grouped into olefins, aldehydes and ring-oxides, in the low-temperature chain reaction mechanism.