Tailoring HCCI heat-release rates with partial fuel stratification: Comparison of two-stage and single-stage-ignition fuels

This work concerns one of the major issues restricting the application of homogeneous charge compression-ignition (HCCI) engines, overly rapid combustion at high-load conditions, which can lead to engine knock and potential engine damage. To overcome this problem, partial fuel stratification was used, where most fuel was premixed with intake air and the rest of the fuel was directly injected during the compression stroke. To be effective, this technique depends critically on the fuel autoignition chemistry. PRF73, a mixture of 73 vol% isooctane and 27 vol% n-heptane that exhibits two-stage ignition under these conditions, successfully enabled control of the combustion heat-release rate with properly adjusted injection parameters. More than a 70% reduction in the maximum pressure-rise rate (PRRmax) was achieved, compared to the fully premixed case at the same combustion phasing. Meanwhile, combustion remained stable, efficient, and clean. In particular, NOx emissions were kept well below the US-2010 limits. On the other hand, isooctane, which exhibits single-stage ignition under the present conditions, responded much differently to partial fuel stratification. Instead of reducing PRRmax, isooctane partial stratification increased PRRmax and drastically raised NOx emissions and combustion instability. Such differences between single- and two-stage fuels result from the different response of their autoignition rates to the variations in equivalence ratio produced by partial fuel stratification, termed phi-sensitivity. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.