Combustion characteristics of n-heptane, a surrogate for hydrocarbon diesel, methyl decanoate, a surrogate for biodiesel, and dimethyl ether, a fuel that can be derived from bio-feedstocks, are investigated with a homogeneous constant-pressure reactor model and a homogeneous-charge compression-ignition engine thermodynamic simulation model, with focus on two variables: ignition delay and NO formation, under conditions of varying oxygen concentration. Negative temperature coefficient (NTC) behavior is observed for the three fuels. Reducing oxygen concentration increases ignition delay for all fuels. The results and conclusions with the two models differ because it is necessary to vary initial conditions in the engine model to optimize combustion phasing and maximize indicated efficiency. (C) 2008 Elsevier Ltd. All rights reserved.