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.) The Shanghai Jiao Tong team notes that diesel engines using DME as a fuel can achieve high thermal efficiency with lower emissions. However a tradeoff relation exists between NO_x emissions and thermal efficiency for a diesel engine fueled with DME using a conventional in-cylinder direction-injection combustion mode.

While HCCI combustion offers the potential for the simultaneous reduction of exhaust gas emissions as well as fuel consumption, it also has problems hindering its commercialization, notably the difficulty in controlling initiation timing and extending the load range.

HCCI combustion with DME shows a very low NO_x emissions, but CO and HC emissions turn out to be high. It can be found that a conventional in-cylinder direct-injection combustion and HCCI combustion with DME have opposite advantages and disadvantages.

Taking advantage of HCCI combustion and in-cylinder direct injection combustion for a diesel engine fueled with DME, a new combustion concept, namely, compound charge compression ignition (CCCI) combustion by port aspiration and direct injection of DME, is proposed in this paper. In this concept, a portion of fuel is aspirated into the combustion chamber via the air intake port to cause HCCI combustion at the compression stroke because of its high volatility of DME, and the remainder of the fuel is injected by a conventional inline fuel pump. As a result, CCCI combustion happens, which includes HCCI combustion at first and in-cylinder spray combustion later.

DME HCCI combustion shows very low NO_x emission levels, and the in-cylinder injection can control the combustion and provide more engine output. The evaporation rate for fuel drops in the DME spray is much faster after HCCI combustion, which reduces the burning time, decreases the charge heterogeneity, and therefore, reduces NO_x formation in the phase of the mixing controlled combustion.

The researchers modified a two-cylinder, four-stroke naturally aspirated high-speed DI diesel engine to conduct their CCCI combustion tests. To improve combustion and reduce emissions, they regulated the ratio of port-aspirated DME to injected DME in the cylinder. To control the ignition and combustion phase of HCCI engines, reduce engine knock, and expand the engine load range, the port-aspirated DME was mixed with liquid petroleum gas (LPG), which has a good antiknock property. They also investigated the effect of the percentage LPG percentage in DME/LPG blended fuel on combustion characteristics. To further reduce NO_x emissions, they used port introduction of CO₂ to function as EGR to evaluate the effect of EGR on CCCI combustion and emission characteristics.

They found that with an increase of the premixed fuel ratio, CO emissions increased first but decreased later and NO_x emissions decreased first but increased later. Meanwhile, DME fuel consumption suffered from improper combustion phasing.

Advancing the injection timing with the same premixed fuel ratio increased peak values of in-cylinder temperature and pressure and advanced the beginning of combustion of in-cylinder-injected fuel. NO_x emissions increased, but HC and CO emissions decreased. As a result, the thermal efficiency was improved.

After port aspiration of the DME/LPG-blended fuel, at the same load, peak values of in-cylinder temperature and pressure decreased gradually when compared to neat DME.

With the increase of the LPG percentage in the blended fuel, NO_x emissions decreased and thermal efficiency was improved at 0.35 MPa IMEP with a high premixed fuel ratio. Fuel consumption was decreased at 0.525 MPa IMEP for all premixed ratios.

Finally, the found that with an appropriate CO₂ concentration in air charge, the HCCI-MK combustion concept for CCCI combustion engines can lower NO_x emissions to near-zero levels.

The research was supported by the National Natural Science Foundation of China, and a specialized Research Fund for the Doctoral Program, The Ministry of Education.

In march, General Motors Corp. and Shanghai Jiao Tong University established the General Motors-Shanghai Jiao Tong University (GM-SJTU) Institute of Automotive Research. The US$4-million organization, located on SJTU’s campus in Shanghai, will build on the existing collaborative automotive technology and research work in manufacturing, materials, propulsion systems and other energy-efficient automotive technology conducted at the university. (Earlier post.)

Resources

• Junjun Zhang, Xinqi Qiao, Bin Guan, Zhen Wang, Guangei Xiao, and Zhen Huang (2008) Search for the Optimizing Control Method of Compound Charge Compression Ignition (CCCI) Combustion in an Engine Fueled with Dimethyl Ether, ASAP Energy Fuels, doi: 10.1021/ef700781w