2.1. Ignition Delay Time Measurements in NUI Galway RCM: Ethanol, n-Propanol, n-Butanol, and n-Pentanol

Low-to-intermediate-temperature IDTs for stoichiometric fuel/air mixtures of four alcohols including ethanol, n-propanol, n-butanol, and n-pentanol were measured in an RCM at NUI Galway at conditions relevant to those encountered in internal combustion engines, at p = 10–30 bar and T = 704–935 K. All mixtures were prepared manometrically in two stainless steel tanks preheated to 80 °C. The tanks were evacuated to 10^–3 bar prior to mixture preparation. The required volume of fuel was first injected into the tanks by a calibrated syringe, and the pressure was monitored so that the appropriate partial pressure of fuel (i.e., one-third of the vapor pressure at 80 °C to avoid condensation) was present in the mixing vessels. All intake manifolds connected to the RCM were also heated to 80 °C. The fuels, ethanol (>99.5%), propanol (99%), n-butanol (99%), and n-pentanol (99%), were obtained from Sigma-Aldrich. O₂, N₂, Ar and CO₂ were supplied by BOC Ireland and Air Liquide at 99.5, 99.95, 99.9995, and 99.5%, respectively.

The RCM is a horizontally opposed twin-piston device that has been described previously.^5,6 Briefly, the symmetry of the RCM allows for a short adiabatic compression time (16–17 ms) and helps in creating and maintaining a high-temperature and -pressure environment while minimizing heat loss effects inside the combustion chamber during compression.⁷ The pistons are locked at the stroke-end, thus allowing a near-constant volume reaction to proceed. The piston head features large crevices to remove the formation of in-cylinder roll-up vortices within the boundary layer gases. This design helps the mixture and the temperature in the reaction chamber to be near-homogeneous prior to ignition. The compressed temperatures and pressures before the main ignition event are reached by changing the initial pressures and temperatures, starting from 30 °C to ensure that the fuel was fully vaporized. For each temperature, we performed five ignition experiments, to ensure repeatability. For all of the experiments, the positions of both pistons are recorded using a digital oscilloscope, while the pressure profiles were recorded using a pressure transducer (Kistler 603B). Piston positioning is monitored using a Positek P100 linear inductive position sensor, which is inserted into the RCM’s hollow connecting rod. Both the pressure and piston position traces are recorded using a PicoScope 4424 digital oscilloscope. The IDT is defined as the time difference between the peak in pressure at the end of the compression and the maximum rate of pressure rise due to fuel reactivity/ignition. The temperatures are calculated using GasEq,⁸ considering the mixture composition and initial temperature, initial pressure, and compressed gas pressure under the assumption of adiabatic compression and frozen chemistry. For each experimental condition, a nonreactive experiment is performed, by replacing oxygen with nitrogen in the test mixture, to ensure comparable thermodynamic properties to determine the facility effects needed in the numerical simulations. The experimental uncertainty is estimated to be 2% in the reported temperature and 25% in reported IDTs, due mainly to the uncertainties in the initial temperature (±3–13 K).⁹ Ignition data and volume profiles for each tested condition are reported in the Supporting Information.