ure cylinder, owing to higher temperature.
A good quality of valve oil should be used.

172. Q. How is the steam end of the compressor affected by the use of
too much oil?

A. This may cause the compressor to short stroke, and where the piston
type of valve is used, may cause the compressor to stop.

L-T EQUIPMENT

AUTOMATIC CONTROL VALVE

173. Q. What is the duty of the control valve?

A. To admit air from the main reservoir to the locomotive brake
cylinders when applying the brakes; to automatically maintain the brake
cylinder pressure against leakage; to develop the proper brake cylinder
pressure regardless of piston travel; and to exhaust the air from the
brake cylinders when releasing the brake, in all automatic applications
of the brake.

174. Q. Explain the operation of the control valve when making an
automatic service application of the brake.

A. Air enters the control valve at the connection marked "BP" (Fig. 21),
which leads to chamber "F" above the piston 3, forcing it down,
uncovering the feed groove "G" in the bushing, allowing air to feed past
the piston into the slide valve chamber, and then through port "H" to
the auxiliary reservoir. The air will feed through in this manner until
the auxiliary reservoir and brake pipe pressure equalize. When a gradual
reduction of brake pipe pressure is made, it will be felt in chamber
"F", above piston 3, creating a difference in pressure on the two sides
of the piston, which will cause it to move upward.

[Illustration: Fig. 21. Automatic Control Valve. Full Release.]

The first movement of the piston closes the feed groove "G", also moves
the graduating valve 10, uncovering the service port "J" in the slide
valve 4, and the continued movement of the piston moves the slide valve
to service position, in which the service port "J" connects with port
"E" in the valve seat. (See Fig. 22.) As the slide valve chamber and
auxiliary reservoir are connected at all times, air can now flow from
the auxiliary to the control cylinder "D" and control reservoir, through
ports "H", "J" and "E", until the pressure on the lower or auxiliary
side of piston 3 becomes slightly less than that in chamber "F" or brake
pipe side, when the piston and graduating valve will move down until the
shoulder on the piston strikes the slide valve; this movement of the
graduating valve closes the service port "J", thus closing the
communication between the auxiliary and control cyl