Oxygen content and cetane number of oxygenates. Wang et al. Click to enlarge.		Cetane number and boiling point of PODEn, gasoline and diesel. Wang et al. Click to enlarge.

Polyoxymethylene dimethyl ether (PODE) is a promising ether fuel for diesel engines because it contains nearly 50% oxygen and has a high cetane number. PODE can be synthesized from methanol. Low-cost commercial production of PODE has recently been realized in China. In 2012, Tsinghua University developed an industrial technique for producing PODE with the capacity of 10 kton/year in China, and the cost of production is close to that of diesel.

… Engine combustion studies using PODE as a diesel fuel additive have been conducted. Diesel blends with 10–20% PODE by volume in a direct injection diesel engine can significant reduce carbon monoxide (CO), hydrocarbon (HC) and particulate matter (PM) emissions and slightly increase thermal efficiency. With the application of EGR, NO_x emissions are also very low. Further studies have reported that PODE can break the soot-NO_x trade-off relationship in both diesel/PODE blends for direct injection compression ignition (DICI) and PODE-gasoline dual-fuel combustion. These combustion paths for low emissions and high efficiency are related to the fuel stratification and active hot atmosphere formed by PODE low-temperature reactions before diesel or gasoline ignition.

In these combustion methods, the main fuels, such as gasoline and diesel are usually present at concentrations higher than 70%. PODE premixed mixture auto-ignition usually occurs (before TDC). Therefore, homogeneous charge compression ignition (HCCI) of PODE under lean mixture conditions needs to be investigated to better understand the above combustion paths for low emissions and high efficiency.

—Wang et al.

The Tsinghua team studied the effects of charge mass equivalence ratio (Φ_m) and exhaust gas recirculation (EGR) on PODE HCCI.

The test fuel was a PODE mixture with a mass distribution of PODE₂:PODE₃:PODE4 = 2.6%:88.9%:8.5%—the same as used in previous studies of PODE blends and PODE dual-fuel combustion. Cetane number was 78—the same as that of PODE₃.

The experiments were performed using a single-cylinder HCCI research engine with a port-fuel-injection system.

Among their findings:

• PODE HCCI exhibits two-stage ignition with a strong low temperature heat release (LTHR) before the high temperature heat release (HTHR). HTHR switches from one-stage to two-stage with an increase of Φ_m due to rapid CO oxidation.

• At a specific EGR rate, with an increase of Φ_m, the end-of-compression charge temperature decreases, the ignition timing of LTHR delays. With an increase of Φ_m, the ignition timing of the HTHR advances at the EGR lower than 42%, but it delays at the EGR of 52% in general.

• For a specific Φ_m, with an increase of EGR, the end-of-compression charge temperature decreases, the ignition timing of both LTHR and HTHR delays, and the combustion duration of the HTHR increases.

• Ultra-low NO_x emissions and soot-free combustion were achieved for PODE HCCI combustion under lean mixture conditions. NO_x levels (less than 10 ppm) and soot levels (less than 0.005 m^-1) are both quite low. PODE lean HCCI combustion produced high levels of CO and HC emissions. These intermediate species forms a reactive hot atmosphere, which is crucial for ignition in blends and dual-fuel combustion.

The purpose of the present fundamental study about PODE HCCI combustion was to help understand PODE blends/dual-fuel combustion and to provide useful data for developing reaction mechanisms of PODE.

—Wang et al.

Resources

• Zhi Wang, Haoye Liu, Xiao Ma, Jianxin Wang, Shijin Shuai, Rolf D. Reitz (2016) “Homogeneous charge compression ignition (HCCI) combustion of polyoxymethylene dimethyl ethers (PODE),” Fuel, Volume 183, Pages 206-213 doi: 10.1016/j.fuel.2016.06.033