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

• Published in: Proceedings of the Combustion Institute Vol. 33; no. 2; pp. 3047 - 3055
• Main Authors: Yang, Yi, Dec, John E., Dronniou, Nicolas, Sjöberg, Magnus
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 2011
• Subjects: Autoignition; Charge stratification; Combustion; Emissions control; Engines; Fuels; HCCI; Heat-release rate; Ignition; Isooctane; Stratification; Two-stage ignition; HCCI; Two-stage ignition; Charge stratification; Heat-release rate; Isooctane
• ISSN: 1540-7489; 1873-2704
• DOI: 10.1016/j.proci.2010.06.114

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 (PRR
max) was achieved, compared to the fully premixed case at the same combustion phasing. Meanwhile, combustion remained stable, efficient, and clean. In particular, NO
x
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 PRR
max, isooctane partial stratification increased PRR
max and drastically raised NO
x
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
ϕ-sensitivity.