|Relationship between NOx and smoke of the DMM blends under one set of speed and load conditions. Click to enlarge. Credit: ACS
Researchers at Xi’an Jiaotong University in China have investigated the combustion, performance, and emissions of a direct-injection (DI) diesel engine fueled with dimethoxymethane (DMM)/diesel blends, with DMM content ranging from 0 to 50%.
Their results showed that, with no changes to the fuel injection system or modifications to the engine, smoke and CO emissions decrease and NOx remains almost unchanged, while hydrocarbons (HCs) increase. Brake-specific fuel consumption (BSFC) is higher (DMM has a smaller lower heating value than diesel), while thermal efficiency increases a little. A diesel engine fueled with a 30% DMM blend can deliver both satisfactory fuel efficiency and emissions levels, they conclude. A report on their study was published online 14 November in the ACS journal Energy & Fuels.
Although application of high-pressure injection and common rail system can reduce both NOx and PM emissions [in diesel engines], the expense is also very high and unaffordable for many engine producers and consumers, especially for diesel engines widely applied for agricultural machinery, most of which are single-cylinder and of low price. In recent years, research showed that one solution, rather cheap and effective, was found, which is the application of oxygenated fuels or adding oxygenated fuels in diesel fuel.—Zhu et al. 2008
|Fuel and blend properties. Click to enlarge. Credit: ACS
Dimethoxymethane (C3H8O2) is one potential alternative oxygenated diesel fuel or blend component that has attracted some interest. DMM has a high oxygen fraction; high cetane number; and exists in a liquid state in normal conditions, making it convenient for storage and transportation. DMM can be manufactured by oxidation of methanol or by the reaction of formaldehyde with methanol. It can also be produced via the catalytic oxidation of dimethyl ether (DME).
The researchers used a single-cylinder diesel engine in their study, fueled with three levels of diesel/DMM blends: 15, 30 and 50%. With increasing DMM content, the fuel blends have increased oxygen content, ranging from 6.47 to 21.36%, lower heating values, decreasing cetane numbers, and increasing heat value of evaporation. The three fuel blends with different DMM fractions were tested under different loads (bmep) and engine speeds.
They found that BSFC together with cyclic fuel injection increases along with the DMM volume fraction keep the power output unchanged. While the 15% DMM blend slightly reduced smoke, it could not meet the current emission standard. The two higher blends, however, “remarkably” reduced the smoke emission without large increases in NOx.
Here, the tradeoff correlation between NOx and smoke emissions also exists, but the tradeoff curve is relatively flat. As indicated above, with the increase of the DMM fraction in fuel blends, NOx emission just increases slowly, while smoke is remarkably reduced.—Zhu et al. 2008
However, the use of the 50% DMM blend would require modifications to the engine, such as enlargement of the diameter of the pump plunger to supply more fuel, to avoid power loss, they found.
Because a diesel engine fueled with oxygenated fuel can tolerate a large percentage of EGR for further reduction of NOx, they suggest further research on EGR with DMM/diesel blending fuels. They also suggested research on injection timing to reduce NOx emissions further.
Ruijun Zhu, Xibin Wang, Haiyan Miao, Zuohua Huang, Jing Gao and Deming Jiang (2008) Performance and Emission Characteristics of Diesel Engines Fueled with Diesel-Dimethoxymethane (DMM) Blends. Energy Fuels, Article ASAP doi: 10.1021/ef8005228
Dimethoxy Methane in Diesel Fuel: Part 1. the Effect of Fuels and Engine Operating Modes on Emissions of Toxic Air Pollutants and Gas/Solid Phase Pah (SAE 2001-01-3627)
Yi Ren, Zuohua Huang, Deming Jiang, Liangxin Liu, Ke Zeng, Bing Liu and Xibin Wang (2006) Combustion characteristics of a compression-ignition engine fuelled with diesel–dimethoxy methane blends under various fuel injection advance angles. Applied Thermal Engineering Volume 26, Issue 4, Pages 327-337 doi: 10.1016/j.applthermaleng.2005.07.009