Conventionally, liquefied gas such as LPG is used as fuel for liquefied gas engines. Upon vaporization of the liquefied gas, tar is separated from the gas and produces bad influences on components disposed in a fuel passage. For example, tar adheres to rubber parts such as diaphragms, deteriorating the rubber parts, to fuel jetting holes, choking the holes, and to valves, causing mal-positioning of the valves on valve seats. To avoid such and other problems, techniques are proposed to separate and collect tar from gas fuel, as known from, e.g., Japanese Utility Model Application Laid-Open Publication No. S53-160526 (Patent Document 1), Japanese Patent Post-Exam Publication No. S59-7022 (Patent Document 2), Japanese Patent Post-Exam Publication No. S59-22063 (Patent Document 3), Japanese Patent Post-Exam Publication No. S63-8309 (Patent Document 4), and Japanese Patent Application Laid-Open Publication No. 2007-64036 (Patent Document 5).
Patent Document 1 discloses a vaporizer having a partitioning plate that partitions the inside of the vaporizer into an upper vaporization chamber and a lower tar chamber. The vaporization chamber includes inside a labyrinth structure. Upon vaporization of liquefied gas introduced into the vaporization chamber, tar is separated from the gas and collected in the tar chamber through a through-hole formed in the partitioning plate. Patent Document 2 and Patent Document 3 disclose similar arrangements.
In a low temperature environment, it is likely that non-vaporized liquefied gas contacts separated tar and the tar melts into the non-liquefied gas and flows together downstream. To prevent this, Patent Document 4 discloses an arrangement wherein a fuel passage switching plate is provided for switching from a fuel supply passage for use at a low temperature to a fuel supply passage for use at a high temperature so as to prevent separated tar from contacting and melting into non-vaporized liquefied fuel gas and flowing together downstream when a temperature is low.
Patent Document 5 discloses a removal device having a filter which is disposed on fuel piping upstream of a vaporizer, for removing a gum substance contained in a liquefied gas.
Provision of a labyrinthine structure internally of a vaporizer results in an undesirably large arrangement, as in Patent Document 1, Patent Document 2 and Patent Document 3. One may think of providing a labyrinthine structure in a spatially-limited small general-purpose engine but it is difficult to mold. In addition, tar is liable to accumulate in the resulting arrangement and a large pressure loss may result.
In the arrangement of Patent Document 4, the structure for tar separation and collection is complex due to the provision of the plate for tar separation and collection. In the arrangement of Patent Document 5, when the filter is small, the mesh may be choked up by the gum, requiring frequent maintenance operations. When a large filter is employed so as to avoid the frequent maintenance operations, the gum removal device becomes large.
In the arrangements of Patent Documents 1 to 4, engine-cooling water is circulated to the respective evaporators so as to facilitate evaporation of liquefied gas fuel and to efficiently separate tar from the gas fuel. Application of such arrangements to small general-purpose engines, for example, is difficult because the general-purpose engines are usually air-cooled and effective utilization of engine heat is difficult to achieve. Consequently, there is a demand for an alternative structure that can be employed in such engines for effectively separating and collecting tar from their gas fuel.