relay. If the temperature drops, the same operations take place in reverse order.
Some engine-cooling fans are motor driven and receive their power from an alternating-
motor circuit. Power is transmitted to drive the cooling fans through a magnetic clutch called the eddy
current clutch. Fan speed is varied by changing the excitation of field coils on the clutch. Other engine-
cooling fans are mechanically driven.
g. Engine heaters. Oil-fired engine heaters are provided for operation in severely cold weather.
The cooling, lubricating, and fuel oil systems and batteries must be warmed before starting the engine if
prolonged layover has resulted in congealed fuel oil or lubricating oil. Continued operation of the
heaters is often necessary after the engine is started. Special insulation of oil lines, tanks, and
compartments is part of such an installation, often called a winterization system. The heaters use fuel
from the main locomotive fuel oil system, but a small tank at the heater is automatically kept full. This
reserve supply of fuel may even have to be heated before the heaters are started. After the heaters are
operating, they supply heat through a hot water piping system or an exhaust gas system to the
powerplant and essential auxiliary equipment. An electric motor which obtains its power from the
locomotive's auxiliary power lines drives a fuel pump, water circulating pump, and blower. The fuel oil
is ignited in the fire pot by a continuous electric spark. The equipment does not eliminate the need of
draining the cooling system or keeping it filled with an adequate antifreeze solution when the
locomotive is shut down.
h. Cab heaters. Cab heaters are installed on most locomotives to keep the cab comfortable in
cold weather. They heat with hot water, electricity, or hot air. The hot-water heater is connected to the
engine water system which supplies the heat. A small electric fan is built into the heater to circulate air
over the water coils and into the cab. The heater switch in the cab connects the fan to the auxiliary
power lines. Fan speed can be varied by a rheostat incorporated with the switch. Cocks are provided for
shutting off the flow of water and for draining the heaters. An all-electric heater is essentially the same
as the hot-water heater except that the heating element is an electric coil. Electric defrosters, which
work on the same principle, have a separate control switch. Hot-air heaters consist of ducts through
which hot air from the engine radiators is forced to the cab and cab windows.