They don’t fire every revolution so maybe they are?
Yep, they are what is called "Hit and Miss" RPMs controlled by switching ignition on or off in gas engines. I'd have to do some reading on how that worked with a diesel. Assume the fuel was only injected when RPMs dropped, that might be what that linkage on the side of engine was for with the notch in it.
Now if one of those "hit and miss" engine come under a load they will fire every time. The more I think about it I would think they might not be that great on gasoline as I don't think the fuel is shut off just the spark so at idle there would be wasted gas going through the engine. Anyone know more about that?
Did a little research and found this that applies to gas engines, or at least some of the hit and miss:
Operation[edit]
The intake valve on hit-and-miss engines has no actuator; instead, a light spring holds the intake valve closed unless a vacuum in the cylinder draws it open. This vacuum only occurs if the exhaust valve is closed during the piston's down-stroke. When the hit-and-miss engine is operating above its set speed, the governor holds the exhaust valve open, preventing a vacuum in the cylinder and causing the intake valve to remain closed, thus interrupting the
Otto cycle firing mechanism. When the engine is operating at or below its set speed, the governor lets the exhaust valve close. On the next down-stroke, a vacuum in the cylinder opens the intake valve and lets the fuel-air mixture enter. This mechanism prevents fuel consumption during the intake stroke of "miss" cycles.