Researchers at Shanghai Jiao Tong University are exploring a new combustion concept for dimethyl ether (DME): low-temperature combustion (LTC) of a compound charge combining port aspiration and in-cylinder direct injection (DI). In comparison to a DME homogeneous charge compression (HCCI) combustion mode, DME LTC can extend the engine operating range with little change in NOx emissions and a considerable reduction in HC and CO emissions. A paper on their work was published online 8 December in the ACS journal Energy & Fuels.
The same research team earlier this published a paper on the development of a DME compound charge compression ignition (CCCI) process. (Earlier post.) The CCCI combustion process consists of HCCI combustion, premixing combustion, and diffusion combustion. The combustion characteristics are mainly decided by the premixed fuel ratio and CO2 concentration in the air charge.
There is particular interest in China in exploring the use of DME—an LPG-like synthetic fuel that is produced through gasification of coal or various renewable substances—as a substitute for diesel. (Earlier post.) Numerous studies have shown that diesel engines fueled with DME can achieve high thermal efficiency, ultra-low emissions, and soft and smokeless combustion.
Researchers have found, however, a tradeoff exists between NOx emissions and thermal efficiency for a diesel engine fueled with DME using conventional in-cylinder DI combustion. HCCI combustion with DME shows very low NOx emissions, but CO and HC emissions turn out to be high.
It can be found that conventional in-cylinder DI combustion and HCCI combustion with DME have opposite advantages and disadvantages.
On the basis of the characteristics of HCCI combustion and conventional in-cylinder DI combustion for a diesel engine fueled with DME, a new combustion concept, namely, low temperature combustion (LTC) of compound charge with port aspirated DME and in-cylinder injected DME, is proposed in this paper.
The LTC cycle differs from the ideal diesel engine cycle in that the isobaric combustion process is replaced with an isothermal combustion process during the power stroke, which significantly lowers the peak cylinder temperature. To make the actual engine cycle close to this theoretical LTC cycle, a proper fuel provision strategy must be applied in consideration of physicochemical properties of DME.—Zhang et al. 2008
In the study, the team aspirated a portion of the fuel is into the combustion chamber via the air intake port to initiate HCCI combustion at the compression stroke. The rest is injected by a conventional in-line pump. The resulting combustion includes HCCI combustion initially, with in-cylinder spray combustion later.
DME HCCI combustion results in very low NOx emissions, and the in-cylinder injection can offer more engine output. The evaporation of fuel drops in DME spray is faster after HCCI combustion, shortening the ignition delay period and combustion duration and therefore suppressing the NOx formation in the phase of mixing controlled combustion. Relatively high levels of CO and HC after HCCI combustion are further oxidized during the in-cylinder spray combustion. Remaining can be treated with an oxidation catalyst converter (DOC). The study focused on improving combustion and reducing emissions by regulating port aspirated DME and DI fuel injection timing.
The team found that:
For a fixed port-aspirated DME mass, the peaks for gas temperature and pressure increase with the rise of direct-injection DME mass. The ignition timing of both cool-flame and thermal-flame reactions advances, and the ignition timing of the diffusion combustion slightly advances. The peak of a cool flame shows no change, and the thermal-flame peak value increases.
At the same engine load, with an increase in the DME mass via port aspiration, the peaks for gas temperature and pressure increase and the start of the thermal-flame reaction and the diffusion combustion advances.
For a fixed port-aspirated DME mass, HC and CO emissions decrease while NOx emissions increase as the load increases. At the same engine load, HC emissions decrease slightly and CO emissions first increase and the decrease with the increase of port-aspirated DME. Meanwhile, NOx increases slightly while the maximum is below 30 ppm.
At the same engine load, NOx emissions are lower with the LTC method than that of the conventional in-cylinder spray combustion.
For a fixed port-aspirated DME mass, the indicated specific fuel consumption decreased with the increase of the engine load. At the same engine load, the indicated specific fuel consumption increases with the rise of port-aspirated DME mass.
In comparison to HCCI combustion, DME LTC can extend the engine-operating range with little change in NOx emissions and a considerable reduction in HC and CO emissions.
Zhang Junjun, Qiao Xinqi, Wang Zhen, Guan Bin, and Huang Zhen (2008) Experimental Investigation of Low-Temperature Combustion (LTC) in an Engine Fueled with Dimethyl Ether (DME). Energy Fuels, Article ASAP doi: 10.1021/ef800674s