Effects of DTBP on the HCCI Combustion Characteristics of SI Primary Reference Fuels

One option for ignition control of Homogeneous Charge Compression Ignition (HCCI) engines is to use small amounts of ignition-enhancing additives to alter the ignition properties. Di-tertiary Butyl Peroxide (DTBP) is one such additive and it has been suggested as a cetane improver in diesel engines. In this study, the effects of DTBP on spark ignition (SI) primary reference fuels (PRFs, n-heptane and iso-octane) and their blends (PRF20, PRF50, PRF63, PRF87 and PRF92) were investigated during HCCI engine operation. Experiments were run in a single cylinder CFR research engine for three inlet temperatures (410, 450 and 500 K) and several equivalence ratios (0.28 - 0.57) at a constant speed of 800 rpm and a compression ratio of 16.0. Experimental results show that ignition delay time, cycle to cycle variation, and stable operating range were all improved with the addition of less than 2.5% DTBP by volume. For example, the addition of DTBP had the following effects: ignition delay time reduction by at least 3 CAD for all tested fuels; COV_IMEP improvement to <10% (a 37.5% reduction) for PRF92 at inlet temperature of 450 K and equivalence ratio of 0.49; and extension of stable HCCI operations for relatively high RON fuels to a broader equivalence ratio range and to lower inlet temperatures (e.g., 2.5% DTBP by volume in iso-octane, extended stable operation to an equivalence ratio of 0.39 at inlet temperature 410 K). These results indicated that DTBP alters the ignition properties of PRFs and their blends, and therefore could be useful in controlling HCCI systems. The mode of action of DTBP appears to be primarily chemical for high ON PRF blends and primarily thermal for low ON PRF blends, although these observations remain to be unambiguously demonstrated.