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22Autoignition in HCCI engines is known to be controlled by the combustion kinetics of the 23 in-cylinder fuel/air mixture which is highly influenced by the amount of low-temperature and 24 intermediate-temperature heat release (LTHR & ITHR) that occurs. At lower intake pressures 25 (typically < 1.4 bar absolute), it has been observed that gasoline behaves as a single-stage heat 26 release fuel, while at higher intake pressures (typically > 1.8 bar absolute) gasoline behaves as a 27 two-stage heat release fuel. Furthermore, ethanol blending strongly affects heat release 28 characteristics, and this warrants further investigation. 29 This paper experimentally investigates the conditions under which gasoline transitions 30 from a single-stage heat release fuel to a two-stage heat release fuel as intake pressure is increased. 31 Experiments were performed in single-cylinder HCCI engine fueled with two research-grade 32 gasolines, FACE A and FACE C. These gasolines were tested neat, with 10% and 20% (by 33 volume) ethanol addition. In addition, these results were compared to results previously obtained 34 for PRF 85, and new results for PRF 84 with 10% and 20% ethanol addition. Moreover, the engine 35 experiments were supported by rapid compression machine (RCM) ignition delay data for the same 36 fuels. 37 The engine experiments revealed that there were minimal differences between the heat 38 release profiles of the two gasolines, FACE A and FACE C, which was confirmed by the RCM 39 experiments that showed similar ignition delay data for the two FACE fuels and PRF 84. On the 40 other hand, with ethanol addition to these gasolines and PRF 84, the occurrence of LTHR shifted 41 to higher intake pressures compared to ethanol-free cases, precisely from 1.4 bar intake pressure 42 for neat fuel to 2.2 bar with 20% ethanol. Consequently, the intake temperatures required to 43 achieve constant combustion phasing for all mixtures were drastically altered. Simulations using 44 a detailed chemical kinetic model were utilized to understand the effects of ethanol blending on 45 the ignition characteristics of PRF 84. Addition of ethanol was found to act as a radical sink where 46 it inhibits the radical pool formation during the low-(< 850 K) and intermediate-(850 -1050 K) 47 temperature chemistry regimes resulting in lower reactivity. These results help explain ethanol's 48 significant anti-knock qualities under boosted conditions in spark-ignition engines. 49 I. Introduction: 50 Homogeneous Charge Compression Ignition (HCCI) engines fundamentally rely on the 51 autoignition of a fuel-oxidizer mixture for operation [1]. Therefore, it is critical to understand the 52 auto-ignition behavior of any fuel used in an HCCI engine, so that the conditions for auto-ignition 53 of the fuel-oxidizer mixture can be achieved in the engine at the appropriate part of the cycle. 54 Further, other advanced engine concepts, such as Gasoline Compression Ignition (GCI) [2, 3] and 55Partially Premixed Compression Ignition (PPCI) ...

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