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Hit and miss engine
From Wikipedia, the free encyclopedia
A type of four-stroke internal combustion engine that was commonly used in the early 1900s. They serve no commercially significant function today and are mainly owned by hobbyists.
A typical hit and miss engine had 2-10 horsepower (HP) and ran under full load at 400-1,000 revolutions per minute (RPM). They were used to power pumps for cultivation, saws, generators and other small stationary applications. They were used where cost was a consideration, and their simplicity of design reflected that requirement.
The term "hit and miss" comes from the sound the engine makes when running at idle. The sound is most like an engine that is barely running: missing on several strokes and then hitting on one. Hitting just enough to keep turning.
These were simple engines, but they incorporated some very clever governor designs. It may be the originality in the design of the governor that is so fascinating for the hobbyist/collector. There is great variation in governor design from one manufacturer to another, so there is no general design that will describe all engines.
There are some characteristics common to these engines: there is no intake cam, the exhaust valve is held open to prevent the engine from building up compression or firing until the speed decreases, when the speed does decrease, the exhaust valve closes. A governor controls the exhaust valve and the magneto spark.
In operation, the engine fires, which increases its speed. The increased speed causes the governor to open the exhaust valve and to cut off the magneto spark. The engine then turns freely for several revolutions but slows to a point where the governor closes the exhaust valve. On the next downstroke of the piston, the closed combustion chamber draws a vacuum, the vacuum pulls the intake valve open and draws fuel and air into the combustion chamber. The piston then compresses the mixture, the spark ignites it, and the engine is given one power stroke, which increases its speed.
The number of power strokes to non-power strokes is a function of the load on the engine. When the engine is unloaded, the ratio is several non-power strokes for each power stroke, and the engine has its characteristic sound.
Animation of hit and miss engine can be seen here: