[Paper Review] High-Pressure Autoignition of Binary Blends of Methanol and Dimethyl Ether

Hong‐Fu Wang, Bryan W. Weber+2 more|arXiv (Cornell University)|Jun 5, 2017

Advanced Combustion Engine Technologies8 references3 citations

TL;DR

This study experimentally investigates the high-pressure autoignition behavior of binary blends of dimethyl ether (DME) and methanol in a rapid compression machine at 30 bar and stoichiometric equivalence ratio. The results show non-linear ignition delay trends with increasing methanol content, and a merged chemical kinetic model without cross-reaction pathways predicts the data well, indicating reactivity is governed by shared radical pools rather than specific DME–MeOH interactions.

ABSTRACT

Reactivity Controlled Compression Ignition (RCCI) is a new advanced engine concept that uses a dual fuel mode of operation to achieve significant improvements in fuel economy and emissions output. The fuels that are typically used in this mode include a low- and a high-reactivity fuel in varying proportions to control ignition timing. As such, understanding the interaction effects during autoignition of binary fuel blends is critical to optimizing these RCCI engines. In this work, we measure the autoignition delays of binary blends of dimethyl ether (C$_2$H$_6$O, DME) and methanol (CH$_4$O, MeOH) in a rapid compression machine. In these experiments, dimethyl ether and methanol function as the high- and low-reactivity fuels, respectively. We considered five fuel blends at varying blending ratios (by mole), including 100% DME-0% MeOH, 75% DME-25% MeOH, 0% DME-0% MeOH, 25% DME-75% MeOH, and 0% DME-100% MeOH. Experiments are conducted at an engine-relevant pressure of 30 bar, for the stoichiometric equivalence ratio. In addition, the experimental results are compared with simulations using a chemical kinetic model for DME/MeOH combustion generated by merging independent, well-validated models for DME and MeOH.

Motivation & Objective

• To understand the autoignition characteristics of DME-methanol binary blends under engine-relevant conditions for use in Reactivity Controlled Compression Ignition (RCCI) engines.
• To quantify the effect of blending ratio on ignition delay in high-pressure, high-temperature environments.
• To evaluate the predictive capability of a merged chemical kinetic mechanism for DME and methanol without explicit cross-reaction pathways.
• To assess the role of common radicals (e.g., OH) in mediating reactivity in DME–methanol blends.
• To support the development of advanced dual-fuel RCCI engines with improved combustion control and emissions performance.

Proposed method

• Experiments conducted in a heated rapid compression machine (RCM) at 30 bar and stoichiometric equivalence ratio.
• Ignition delay measured via pressure trace analysis using UConnRCMPy software, defining ignition as the time from end-of-compression to the first peak in the first derivative of pressure.
• Non-reactive experiments used to determine end-of-compression (EOC) temperature via isentropic compression relations.
• A chemical kinetic model was constructed by merging validated, independent mechanisms for DME and methanol, with duplicate species removed and no cross-reaction pathways included.
• Simulations performed under constant volume, adiabatic conditions to compare with experimental ignition delays across a range of EOC temperatures.
• Data analysis focused on ignition delay as a function of inverse temperature (1000/T_C) and blending ratio, with comparisons to model predictions.

Experimental results

Research questions

• RQ1 How does the autoignition delay of DME–methanol blends vary with changing blending ratio at 30 bar and stoichiometric equivalence ratio?
• RQ2 What is the nature of reactivity interaction between high-reactivity DME and low-reactivity methanol in binary blends?
• RQ3 To what extent can a merged kinetic mechanism for DME and methanol predict experimental ignition delays without explicit cross-reaction terms?
• RQ4 How does the temperature dependence of ignition delay change across the DME–methanol composition range?
• RQ5 What role do common radicals (e.g., OH) play in mediating the reactivity of the blend, given the absence of direct cross-reaction pathways?

Key findings

• Pure DME (100% DME) exhibits the shortest ignition delay, while pure methanol (100% MeOH) shows the longest, confirming DME’s higher reactivity.
• The change in ignition delay with increasing methanol fraction is non-linear: the delay increases more sharply when DME content drops from 50% to 0% than from 100% to 50%.
• At a target ignition delay of ~20 ms, the required end-of-compression temperature increases from ~600 K for 100% DME to ~850 K for 0% DME.
• The merged kinetic model shows good agreement with experimental data across the entire composition and temperature range, despite excluding explicit DME–MeOH cross-reactions.
• Model under-predicts ignition delay at low temperatures and over-predicts it at high temperatures, likely due to omission of post-compression heat loss and compression-stage radical buildup in simulations.
• The results support the hypothesis that reactivity in DME–methanol blends is primarily governed by shared radical pools (e.g., OH), not specific bimolecular reactions between the two fuels.