# How hit & miss engine operate?



## azroyhelmy (Apr 4, 2011)

Hi guys!
I'm very new to this engine things. And i very interested in a few of the engine and plan to build them ( of course for the beginner engine hahaha ).
I already download Brian Rupnow and Chuck Fellows plans. But i wonder how this engines works. Could anyone explain or direct me to any site than can make me understand before i start to build them?

Thanks,
azroy ;D


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## gbritnell (Apr 4, 2011)

Hi Azroy,
First of all welcome to the forum. In the model engine building world I don't think you'll find a better place to come for information. 
As far as how a hit and miss engine operates it's quite simple. You take a 4 cycle engine and put a mechanism on it that when operated by a centrifugal weight will lock the exhaust pushrod from operating. This will prevent the engine from taking an intake charge and compressing it so therefore will prevent it from running. Once the engine speed has slowed down to the point that the centrifugal weight returns to it's rest postion the lock mechanism releases the pushrod and allow the engine to function normally. These types of mechanisms were used to control the speed of the full sized hit and miss engines before throttling governors came into use.
gbritnell


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## Metal Butcher (Apr 4, 2011)

Here's a video that may also help. There is quite a bit of written information available on the internet by seaching 'How a hit and miss engine works."

http://videos.howstuffworks.com/howstuffworks/183-how-hit-and-miss-engines-work-video.htm 

-MB


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## doc1955 (Apr 4, 2011)

Thanks for posting that video!   Long Green Line :big:
I watch that show occasionally when i catch it on the tube it's always informative and interesting!


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## n4zou (Apr 4, 2011)

This video is loaded with incorrect statements on how a hit and miss engine works. I will not even attempt to correct them as there are too many. 

The Miss cycle:
During the Miss cycle the exhaust valve is help open by the governor latch rod held ageist a stop on the tappet rod. This allows a free flow of air as the piston moves on the down stroke. No suction is created in the cylinder so the intake valve remains closed cutting off the flow of the fuel and air mixture provided by the carburetor. The design of the stop on the tappet rod ensures it remains latched on the governor latch rod during the upstroke as well so air in the cylinder free flows past the exhaust valve preventing compression. This allows the engine to "coast" during miss cycles with the extra heavy flywheels providing stored rotational energy to keep the engine spinning. 

The Hit cycle:
As the engine RPM slows the governor weights on the flywheel succumb to the control springs pull and pull the governor latch rod away from the stop on the exhaust tappet rod. This allows the tappet rod to contact the cam on the timing gear. Subsequently the exhaust valve closes. On the piston down stroke vacuum is created in the cylinder which allows atmospheric pressure to act on the intake valve to open it. Concurrency an air/fuel ratio (14 to 1) provided by the carburetor is drawn into the cylinder. As the piston transitions to the upstroke the intake valve closes. Stored energy in the flywheel forces the piston up on the upstroke even as compression in the cylinder builds up. Just before the piston reaches the top of this compression stroke the ignition system releases an arc of electricity across an air gap located on the spark plug. This arc ignites the compressed fuel/air mixture. The ignited mixture burns causing a rapidly increasing pressure wave within the cylinder. This pressure presses down on the piston on the down stroke which is converted to rotational energy which is transferred into the flywheel and any load attached to it.

If the engines RPM is not sufficient to activate the governor latch rod the engine will enter another Hit cycle. When the engines RPM is sufficient to activate the governor latch rod the engine the engine will transition into the Miss cycle so a steady and controlled engine RPM is maintained.


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## gbritnell (Apr 4, 2011)

Hi n4zou,
I watched the video and really didn't find any glaring fallacies. I think they just simplified it for the sake of time constraints. Your explanation is well done and covers the operation of a hit and miss engine very clearly.
gbritnell


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