Lotus Elise / Lotus Exige. Manual - part 58

 

   Lotus Service Notes                                  Section JJ

Page 14

JJ.10 - VACUUM SERVO UNIT

The FTE brake vacuum servo is, with the exception of the air filter and non return valve, a non-servicable

sealed unit which if found to be faulty, must be replaced as an assembly.  The air filter (surrounds the input push
rod) should be replaced whenever the brake system is overhauled, and cleaned or replaced more frequently if
the vehicle is operated in dusty conditions.  A vacuum non-return valve is incorporated into the vacuum hose
elbow connector in the front case of the servo unit, and a second in-line non-return valve is fitted into the supply
line close to the engine.  The elbow connector valve is a push fit into a grommet in the servo shell, and is
supplied complete with the grommet.

The servo is fitted between the brake pedal and master cylinder and is of the 'suspended in vacuum' type,

wherein a flexible diaphragm divides the space inside a steel shell into two chambers.  The front chamber is
connected, via a non-return valve, to the vacuum produced in the engine's inlet plenum chamber.  When the
brakes are 'off', this vacuum is also applied to the rear side of the diaphragm, which is held towards the rear of
the shell by a spring.  The brake pedal is connected to the brake servo control rod, which operates a control
valve before pressing against the output rod via a pliant reaction disc.

Brakes Off

In the simplified schematic diagram, with the brakes off, the diaphragm and driving piston are pushed to

the left by the main spring, and the input rod is pushed to the left by its own spring, causing the control valve to
close off the atmospheric port.  Engine vacuum admitted to the right hand side of the diaphragm is also
communicated to the left side of the diaphragm, and the unit is stable.

Diaphragm

Rear chamber

      Engine

               vacuum

Servo control rod

  Front

Brake pedal

  chamber

connection

  Output
  rod

         Air filter

  Main

   Diaphragm

 spring

Reaction disc

       

           j147a

   Lotus Service Notes

                Section JJ

Page 15

Initial Movement

Initial movement of the brake pedal causes the input rod to move to the right, and allows the control valve

under the action of its spring, to close off the vacuum port in the driving piston.  The atmospheric port remains
closed.

     j147b

Driving piston

Vacuum port
closed off

Brake pedal
input

Atomospheric
port closed

 

   Lotus Service Notes                                  Section JJ

Page 16

Pressure Balance

Further pressure on the brake pedal opens the atmospheric port and bleeds atmospheric pressure into

the left hand chamber.  This pressure imbalance causes the driving piston to move to the right, adding assistance
to the force applied directly to the output rod (and the master cylinder) via contact between the input and output
rods.  When the force produced by the pressure differential across the diaphragm balances the reaction force
of the main spring and master cylinder, movement of the driving piston ceases, with both control valve ports
closed, and the system once again in equilibrium.

From this position of equilibrium, further pressure on the pedal will tend to open the atmospheric port and

allow pressure in the left hand chamber to increase, and move the driving piston to the right before again
stabilising.  Any reduction in pedal pressure will tend to open the vacuum port and allow the higher pressure in
the left hand chamber to bleed off into the right hand chamber, whose depression is kept constant by its
connection with the intake plenum.  The pressure imbalance is reduced, and the driving piston will move to the
left under the action of the main spring, until equilibrium is again restored.

Higher pressure

Low pressure

balances manifold

from intake

pressure plus

manifold

spring

Vacuum port

Input from
brake pedal

Output to
master
cylinder

Atmospheric port

     j147c

   Lotus Service Notes

                Section JJ

Page 17

Maximum Assistance

If the pedal is pressed hard, the atmospheric port will be held open, so that the left hand chamber will be

subject to full atmospheric pressure.  This is full servo assistance which will, with 0.8 bar of vacuum available
from the engine, increase the force applied to the master cylinder pushrod by approximately four times.

When pedal pressure is relaxed, the vacuum port in the driving piston will open and bleed the higher

pressure from the left side of the diaphragm into the right hand chamber and thence into the engine plenum,
collapsing the pressure differential across the diaphragm.  The driving piston is moved fully to the left by the
main spring.

The pliant reaction disc fitted between the input and output rods ensures a graduated application of servo

assistance and provides pedal feedback and 'feel' to the driver.

Atmospheric

Intake manifold

pressure

depression

Input from
brake pedal

Output to
master
cylinder

Contact between
input and output rods
via reaction disc

Atmospheric
port open

      j147d