1. Features of Engine for Portable Fire Pumps
For a portable fire pump, compactness, lightness and high output are indispensable. That’s reason why a 2-cycle gasoline engine is adopted.
This is the point significantly different from ordinary 4-cycle automobile engine. Features of 2-cycle engine, which is most suitable for a portable fire pump, are as follows.
(a) Since engine fires per turn of crank shaft,
* Rotation is smooth.
* Output is powerful, about 1.5 times that of four-cycle engine.
* Size is small with relatively large output.
(b) Since there are neither intake nor exhaust valves,
* Structure is simple. * Adjustment is easy.
* Less trouble. * Light weight. * Less engine noise.
(c) Since mixed oil is used,
* After start-up, it can immediately go into high speed operation.
* Troublesome oil change is not required.
* Less trouble without using oil pump.
2. Operation of 2-cycle gasoline engine
As the figure shows, 2-cycle engine completes one-cycle per revolution of crankshaft. In other words, while piston moves up and down (2 strokes), 1 cycle of intake, compression, firing and exhaust is completed.
2-cycle Port Operating / Closing Timing
External circle: Inside of cylinder.
Internal circle: Inside of crankcase.
|After piston passes top dead center, explosion force will act on top of piston and expansion will start.
||Expansion occurs in cylinder, and intake port will close at crank case side, and compression stroke will start inside case.
||Exhaust port will open inside cylinder and combustion gas will be exhausted, and cylinder internal pressure will suddenly drop.
|Scavenge port will open, and new mixed air pre-compressed in crank case will enter into cylinder and push out exhaust gas.
||When piston passes bottom dead center, scavenge port will close and decompression in crank case will start.
||Exhaust port of cylinder closes, and compression stroke starts. Crank case internal pressure will drop more.
||Compression will take place inside cylinder, and intake port of crankcase will open, and new mixed gas will be sucked by negative pressure inside crank case.