A fundamental study of the oxidation behavior of SI primary reference fuels with propionaldehyde and DTBP as an additive
Authors:   , Johnson Rodney O. , Cernansky Nicholas Peter , Miller David Leslie 
Published: July 2021

In an effort to combine the benefits of SI and CI engines, Homogeneous Charge Compression Ignition (HCCI) engines are being developed. HCCI combustion is achieved by controlling the temperature, pressure, and composition of the fuel and air mixture so that autoignition occurs in proper phasing with the piston motion. This control system is fundamentally more challenging than using a spark plug or fuel injector to determine ignition timing as in SI and CI engines, respectively. As a result, this is a technical barrier that must be overcome to make HCCI engines applicable to a wide range of vehicles and viable for high volume production. One way to tailor the autoignition timing is to use small amounts of ignition enhancing additives. In this study, the effect of the addition of DTBP and propionaldehyde on the autoignition behavior of SI primary reference fuels was investigated. The present work was conducted in a new research facility built around a single cylinder Cooperative Fuels Research (CFR) octane rating engine but modified to run in HCCI mode. It focused on the effect of select oxygenated hydrocarbons on hydrocarbon fuel oxidation, specifically, the primary reference fuels n-heptane and iso-octane. This work was conducted under HCCI operating conditions. Previously, the operating parameters for this engine were validated for stable combustion under a wide range of operating parameters such as engine speeds, equivalence ratios, compression ratios and inlet manifold temperature. The stable operating range under these conditions was recorded and used for the present study. The major focus of this study was to examine the effect of the addition of DTBP or propionaldehyde on the oxidation behavior of SI primary reference fuels. Under every test condition the addition of the additives DTBP and propionaldehyde caused a change in fuel oxidation. DTBP always promoted fuel oxidation while propionaldehyde promoted oxidation for lower octane number fuels and delayed autoignition for higher octane number (>50) fuels. The results discussed in this work show that the addition of propionaldehyde or DTBP effects negative temperature coefficient (NTC) during HCCI combustion. This effect on NTC behavior subsequently affects ignition delay. For DTBP the effect is to always reduce NTC behavior across the entire octane number of PRF blends examined. It is important to note that as octane number increased the effect on NTC becomes less pronounced. For propionaldehyde, the effect on NTC behavior changed with changing PRF octane number. For the same engine operating conditions, as the amount of isooctane content in the fuel increased, the effect of propionaldehyde addition goes from advancing autoignition for a mixture with 0 % iso-octane content to delaying autoignition for mixtures with over 50 % iso-octane. Gas samples as a function of crank-angle degree (CAD) before the onset of autoignition were collected and analyzed by gas chromatography for PRF's at selected engine operating conditions with and without the addition of propionaldehyde or DTBP. The results were used to elucidate the effects of these additives on the chemical kinetics controlling HCCI operation. Closer examination of the species evolution profiles suggest that the mechanism behind the ignition promoting effect of DTBP is due to an increase in the local gas temperature and subsequently a reduction in NTC behavior. The effects caused by propionaldehyde is due to both propionaldehyde acting as a radical scavenger as well as propionaldehyde reacting with itself producing OH·. It is also suggested that the mode of action of DTBP in promoting oxidation is thermal rather than chemical for the fuels tested.