F2008-09-033
Knock Improvement and Performance in SI Engines with LPG-DME Mixture Fuel
Recently DME is focused as an alternative fuel because it could be made from various sources such as natural gas, biomass and coal. DME fuel is investigated for diesel engine, as cetane number of DME is almost same as diesel oil. On the other hand, the characteristics of DME are similar to LPG so that DME can be mixed with LPG. LPG is usually used as gas fuel for cooking equipment and stove as well as small gas engine, which drives generator and heat pump system. If LPG and DME mixture fuel could be used instead of LPG, energy security will be safe. However, the knock occurs easily in SI engine with DME fuel because it has low octane number and engine output is very difficult to secure. As for the method of elimination, knocking limit can be improved and the engine power can be secured if DME is mixed with LPG in operation with SI engines. This research was investigated the knocking limit, corresponding with engine output, of LPG and DME mixture fuel in spark ignition (SI) engine, which is used for micro cogeneration system. The knock detection method including by ionization current at spark plug gap was also investigated. The effects of ignition timing, excess air ratio and EGR ratio on knocking limit were systemically investigated. The results showed that the LPG base gas engine could be run without modification using DME 5 to 10% mixture. Retarding ignition timing and changing excess air ratio with DME mixture ratio less than 30% could improve the knocking limit without EGR. With DME mixture ratio from 30 to 40%, the knocking limit could be improved by retarding ignition timing, changing excess air ratio and in addition with EGR ratio. The thermal efficiency with DME 30% was comparable to net LPG fuel. Exhaust emissions showed a considerable amount of DME and formaldehyde concentration and HC emissions reduced significantly with oxidation catalyst. A control system has been developed, especially, when DME ratio is changed, knocking intensity was detected by ionizing and then controlled the operating parameters. A cogeneration system equipped with the developed gas engine and the control system has been tested at Yokohama city in Japan.
Session: Alternative Fuels I