Introduction to the air brake charging system, The air brake charging system – Bendix Commercial Vehicle Systems BENDIX 720CC COMPRESSOR User Manual

Page 16

A-2

Introduction to the Air Brake Charging System

air compressor

builds the air pressure for the air brake system. The air

compressor is typically cooled by the engine coolant system

and lubricated by the engine oil supply.
This Bendix

720cc compressor is a “discharge line

unloader” (DLU-style) compressor, meaning that the

compressor pumps continuously, unlike some compressor

designs which use an “unloader” mechanism in the

compressor head to switch from a pumping mode to a

non-pumping mode. Instead, the control of air delivery to

the vehicle’s air system is managed by using a separate

discharge line unloader valve mounted in parallel with the

compressor, a turbo cut-off style of the air dryer, and a

governor (see figure below). The discharge line unloader

(DLU) valve and governor control the brake system

air pressure between a preset maximum and minimum

pressure level by monitoring the pressure in the service (or

“supply”) reservoir. When the air pressure becomes greater

than that of the preset “cut-out”, the governor controls the

discharge line unloader valve such that the air from the

compressor flows through the exhaust of the discharge

line unloader valve and to atmosphere (i.e. preventing air

delivery to the reservoirs) and also causes the air dryer to

purge. As the service reservoir air pressure drops to the

“cut-in” setting of the governor, the governor returns the

discharge line unloader valve back to building air mode

and the air dryer to air drying mode.
As the atmospheric air is compressed, all the water vapor

originally in the air is carried along into the air system, as

well as a small amount of the lubricating oil as vapor.
The

duty cycle is the ratio of time the compressor

spends building air to the total engine running time. Air

compressors are designed to build air (run “loaded”) up to

25% of the time. Higher duty cycles cause conditions that

affect air brake charging system performance which may

require additional maintenance. Factors that add to the

duty cycle are: air suspension; additional air accessories;

use of an undersized compressor; frequent stops; and

excessive leakage from fittings, connections, lines,

chambers or valves, etc.

The

discharge line allows the air, water-vapor and

oil-vapor mixture to cool between the compressor and air

dryer. The typical size of a vehicle's discharge line, (see

column 2 of Table A on page A-3) assumes a compressor

with a normal (less than 25%) duty cycle, operating in

a temperate climate. See Bendix and/or other air dryer

manufacturer guidelines as needed.
When the

temperature of the compressed air that enters

the air dryer is within the normal range, the air dryer can

remove most of the charging system oil. If the temperature

of the compressed air is above the normal range, oil — as

oil-vapor — is able to pass through the air dryer and into the

air system. Larger diameter discharge lines, and/or longer

discharge line lengths, can help reduce the temperature.
The discharge line must maintain a

constant slope down

from the compressor to the air dryer inlet fitting to avoid low

points where ice may form and block the flow. If, instead,

ice blockages occur at the air dryer inlet, insulation may be

added here; or if the inlet fitting is a typical 90 degree fitting,

it may be changed to a straight — or 45 degree — fitting.

For more information on how to help prevent discharge

line freeze-ups, see Bendix Bulletins TCH-008-021 and

TCH-008-022 (see pages A-19-21). Shorter discharge

line lengths or insulation may be required in cold climates.
The

air dryer contains a filter that collects oil droplets, and

a desiccant bed that removes almost all of the remaining

water vapor. The compressed air is then passed to the air

brake service (supply) reservoir. The oil droplets and the

water collected are automatically purged when the governor

reaches its “cut-out” setting.
For vehicles with accessories that are sensitive to small

amounts of oil, we recommended installation of a Bendix

PuraGuard

system filter, designed to minimize the amount

of oil present.

The Air Brake Charging System

supplies the compressed air for the braking system as well

as other air accessories for the vehicle. The system usually consists of an air compressor, governor,

discharge line, air dryer, and service reservoir.

UNL

RES

UNL

Compressor

Governor

Air Dryer

Front Service

Reservoir

Rear Service

Reservoir

DLU Valve