of power and a
four-cylinder engine still better. For this reason, standard voltage
generating sets without battery are usually of two or four cylinders
when of the four-cycle type. When a single-cylinder engine is used, it
should be of the two-cycle type. In the two-cycle engine, there is one
power stroke to each up-and-down journey of the piston. This effect is
produced by having inlet and exhaust ports in the crank case, so
arranged that, when the piston arrives at the bottom of the power
stroke, the waste gases are pushed out, and fresh gas drawn in before
the up stroke begins.

For direct lighting, the engine must be governed so as not to vary
more than five per cent in speed between no load and full load. There
are many makes on the market which advertise a speed variation of
three per cent under normal loads. Governors are usually of the
centrifugal ball type, integral with the fly wheel, regulating the
amount of gas and air supplied to the cylinders in accordance with the
speed. Thus, if such an engine began to slow down because of increase
in load, the centrifugal balls would come closer together, and open
the throttle, thus supplying more gas and air and increasing the
speed. If the speed became excessive, due to sudden shutting off of
lights, the centrifugal balls would fly farther apart, and the
throttle would close until the speed was again adjusted to the load.

These direct-connected standard voltage sets are as a rule fitted with
the 110-volt, direct current, compound type of dynamo, the duplicate
in every respect of the machine described in previous chapters for
water-power plants. They are practically automatic in operation and
will run for hours without attention, except as to oil and gasoline
supply. They may be installed in the woodshed or cellar without
annoyance due to noise or vibration. It is necessary to start them, of
course, when light or power is desired, and to stop them when no
current is being drawn. There have appeared several makes on the
market in which starting and stopping are automatic. Storage batteries
are used in connection with these latter plants for starting the
engine. When a light is turned on, or current is drawn for any
purpose, an automatic switch turns the dynamo into a motor, and it
starts the engine by means of the current stored in the battery.
Instantly the engine has come up to speed, the motor becomes a dynamo
again and begins to deliver current. When the last li