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Abstract
The chemical influence of CO and H2 addition on the auto-ignition of n-heptane/air mixtures is investigated numerically. This work is motivated by the need for a precise control of auto-ignition during the compression stroke of Homogeneous Charge Compression Ignition (HCCI) engines. The numerical simulations are performed using three detailed mechanisms for n-heptane in order to ensure that the results are mechanism independent. The reaction mechanisms used are (i) the Lawrence Livermore National Laboratory (LLNL) version -1 (1998) as proposed in (Curran et al., Citation1998) (ii) an updated recent version of the LLNL reaction mechanism (Curran et al., Citation2002) cited as LLNL-2004 in the text (iii) and the detailed mechanism developed by the DCPR (Buda et al., Citation2005). The parameters of the simulations are the mixture temperature, pressure, equivalence ratio and the amount of CO and/or H2 added to the initial charge. It is shown that CO and H2 influence the auto-ignition delay differently as temperature and equivalence ratio are changed. This is due to the competitions between OH production and destruction. The presence of CO at low temperatures (T = 600 K) lengthens the delay around 5%–10% whereas at high temperatures (T = 1000 K), it shortens the delay in the order of 15%–20%. H2 addition at low temperatures significantly lengthens the delay whereas at high temperatures (1000 K) does not influence the delay considerably depending on the added amount. A discrepancy is also observed between the results obtained with the LLNL and both the LLNL-2004 and the DCPR mechanisms. This is explained in terms of differences in the kinetic constants of key reactions when CO/H2 are present in the fresh charge.