Abstract
Keywords: SI Engine, LPG, DME, Knocking, Co-generation
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 has been 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 a 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 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 could improve the knocking limit with DME mixture ratio less than 30%. With DME mixture ratio 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. A cogeneration system equipped with the tested engine was developed with a control system and run for 1000 hours at Yokohama city in Japan.