|Hit and Miss -
Controls the engines speed by cutting off the engines ignition
system when it runs too fast. This is the "miss" part. When the
engine slows too much, it allows it to "hit" again and pick
speed back up.
Throttle Governed - Controls the
engine speed by controlling a butterfly in the carb. If it runs
too slow, it opens the butterfly and a larger charge of air and
fuel is allowed into the engine and it runs faster. If the
engine runs too fast, it closes the butterfly and allows a
smaller charge of air and fuel into the engine and it runs
Both types of engines have one thing in common the
govenor. So we will start with that.
Below in Fig. 1 is an animation of a typical govenor in
action. As the engine speed increases, the weights on the
govenor fly outward because of centrifugal force. This action in
turn pushes the rod in the middle outward. As the engine slows
they are forced inward by a set of springs (springs not shown).
You can now use this action of the rod moving outward
(going too fast) and inward (slowing down) to control the
engine. The oldest of the two types of engines is the "Hit and
Miss" but we will start with the easiest to explain first or the
"throttled goverened" engine.
Throttle Governed Engine
Below in Fig. 2, you will see a govenor hooked onto a
mixer or commonly known as a carburetor. As the engine increases
in speed, the butterfly in the mixer is closed thereby reducing
the amount of fuel the engine gets. Less fuel, the slower the
engine runs at. if the speed becomes too low, the govenor moves
the butterfly more open which increases the amount of fuel and
the engine speeds up.
The "Hit and Miss" Engine
The older of the two types is the Hit and Miss gas engine.
It is also less regulated than the throttle type. There are two
different states the engine can be in, "Hit" or "Miss".
First we will look at the "Hit" state. This is really the
same as the throttled engine except each time the engine intakes
the gas it takes a full charge. There is no butterfly in a hit
and miss mixer. So the engine wants to go faster and faster.
We will discuss how to "slow" the engine down or regulate
it in the "Miss" section coming up.
Here are the Four cycles (4 strokes) of the "Hit" state.
Refer to fig.3 below. This is the intake cycle where the
piston is moving to the right or away from the head. As it does,
it creates a vacuum in the cylinder and that pulls the air and
fuel into the cylinder. The intake valve is not shown here, and
is just on a spring to keep it in the closed position. As the
vacuum increases it pulls the intake valve open and the air/fuel
Notice the contacts that are hooked to a "buzz" coil to
create the spark later. Also the rod from the govenor is down
because the engine is running slow . See Fig. 9 below for a view
of how the rod and govenor works on this setup. As for now, the
govenor is not in play .
Refer to fig.4 below. This is the compression cycle. All
valves close and the piston moves to the left or towards the
head. As it does, the mixture compresses making it very
Fig. 5 below shows the the contacts to the "buzz" coil
closed and that fires the spark plug pushing the piston to the
right or away from the head. Notice in the fire cycle, the cam
allows the exhaust rod to dip backwards, allowing the contacts
to close and fire the spark plug.
Fig. 6 below shows the exhaust valve opening and the
piston pushing out the exhaust
As the piston reaches the to, the engine starts the cycles
all over beginning at the intake cycle (Fig.3) above.
In Fig. 7 below you will see the entire operation
animated. Notice in the fire cycle, the cam allows the exhaust
rod to dip backwards, allowing the contacts to close and fire
the spark plug.
Anamated Four Cycles ("Hit" State)
So how do we "govern" the engine?
Govenor in locked posision ("Miss" State)
Refer to Fig. 8 above and Fig. 9 below. As the govenor
runs too fast it pushes the govenor rod into the exhaust push
rod and traps it (see Fig. 9 below). This allows the exhaust
valve to stay open all the time. No vacuume is made by the
piston so no fuel can enter into the cylinder. Also the contact
going to the "buzz" coil will not close, so the spark plug never
fires. The engine just "free wheels" or coasts. Finally is slows
too much and the govenor removes it's rod and the entire engine
returns to the "Hit" state.
Below in Fig. 10 you will see two four stroke cycle in the
"Hit" state then two four stroke cycles in the "Miss" state.
Here is a hint as to which state it is in. If the exhaust
valve isn't moving at all, it's because it is in the "Miss"
state or slowing down.
Animated Two "Hit" Cycles and Two "Miss"