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SYSTEM DESIGN & DEVOLOPEMENT IN CALCULATION OF RESPONSE TIME FOR AIR BRAKE SYSTEM
INTRODUCTION
COMPONENTS OF AN AIR BRAKE SYSTEM:
Air brake system consists of the following components:
Compressor:
The compressor generates the compressed air for the whole system.
Reservoir:
The compressed air from the compressor is stored in the reservoir.
Unloader Valve:
This maintains pressure in the reservoir at 8bar.When the pressure goes above 8 bar it immediately releases the pressurized air to bring the system to 8-bar pressure.
Air Dryer:
This removes the moisture from the atmospheric air and prevents corrosion of the reservoir.
System Protection Valve:
This valve takes care of the whole system. Air from the compressor is given to various channels only through this valve. This valve operates only at 4-bar pressure and once the system pressure goes below 4-bar valve immediately becomes inactive and applies the parking brake to ensure safety.
Dual Brake Valve:
When the driver applies brakes, depending upon the pedal force this valve releases air from one side to another.
Graduated Hand Control Valve:
This valve takes care of the parking brakes.
Brake Chamber:
The air from the reservoir flows through various valves and finally reaches the brake chamber which activates the S-cam in the brake shoe to apply the brakes in the front
Actuators:
The air from the reservoir flows through various valves and finally reaches the brake chamber, which activates the S-cam in the brake shoe to apply the brakes in the rear.

WORKING OF AN AIR BRAKING SYSTEM
Air brakes are used in commercial vehicles, which require a heavier braking effort than that can be applied by the driverâ„¢s foot. The following layout shows the arrangement of the air braking systems in heavy vehicles. Compressed air from compressor passes through the unloader valve and maintains its pressure. This air is stored in the reservoir. From the reservoir it goes to the Brake Chambers through many brake valves. In the brake chamber this pneumatic force is converted into the mechanical force and then it is converted into the rotational torque by the slack adjuster, which is connected to S-cam. This torque applies air brakes. Pipelines connect the brake system components.
IMPORTNACE AND DEFINITION OF RESPONSE TIME
There are different types of tests for measuring efficiency of air brake system. Two important tests are mentioned below.
Firstly,
The efficiency specified for Brake devices shall be based on the stopping distance or the mean fully developed deceleration. The efficiency of a brake device shall be determined by measuring the stopping distance in relation to the initial speed of the vehicle or by measuring the mean fully developed deceleration during the test.
The stopping distance shall be the distance covered by the vehicle from the moment when the driver begins to actuate the control of the braking system until the moment when the vehicle stops; The initial vehicle speed (V1) shall be the speed at the moment when the driver begins to actuate the control of the braking system; The initial speed shall not be less than the 98% of the prescribed speed for the test in question. The mean fully developed deceleration dm shall be calculated as the deceleration averaged with respect to distance over the interval Vb to Ve according to the following formula
MFDD (dm) = ((Vb2 - Ve2) / (25.92 (Se - Sb)) m/s2
Where;
Vb ---------- vehicle speed at 0.8V1 km/h
Ve ---------- vehicle speed at 0.8V1 km/h
Sb ---------- distance traveled between V1 and Vb in meters and
Se ---------- distance traveled between V1 and Ve in meters

The speed and distance shall be determined using instrumentation having an accuracy of +-1% at the prescribed speed for the test. The dm may be determined by other methods than the measurement of speed and distance.
Secondly,
Dynamic Test:
¢ Type P-Test -------------- Ordinary test (as mentioned above) carried out with Brakes Cold. The Type-P-Test is carried out with engine connected and also in engine-disconnected condition.
¢ Type-F-Test -----------The test mentioned above is carried out with repeated braking.
Static Test:
¢ Line pressure in the front and rear are measured against the pedal force in steps of 10kgs up to 70kgs.
¢ Then the response time is measured which is the sum of the actuation time and the build up time. The values of build up time are acquired using a proper pressure transducer and Data acquisition system and the actuation time is assumed to be 0.1 seconds.
DEFINITION OF RESPONSE TIME
Let
ta ------------------ actuation time (experimentally calculated as 0.1s)
tb -------------------- build up time (should be calculated depending upon various factors)
tr ----------------- response time (should be calculated depending upon various factors)
Various factors influencing build up time:
tb --------------------- f (Pipe restrictions Valve restriction Actuator volume)
From the above-mentioned equations it can be seen that response time is the sum of the buildup time and the actuation time.
tr = ta + tb
BASIC ALGORITHM
The calculations are being carried out as the flowchart shown below.



NO


YES

RESPONSE TIME CALCULATIONS
TWO RESERVOIR SYSTEMS

ASSUMPTIONS:
1. The air flow through the pipes is a isothermal process
2. Frictional losses are considered to be negligible.
3. The flow area is assumed to be a constant inside each valve.
4. Each valve is considered to be an orifice.
CALCULATIONS:
Let us take a simple two-reservoir system into consideration:
Orifice

A
In the above mentioned figure
P1 ---------- Initial Pressure
P2 ---------- Delivery Pressure
V1 ---------- Initial Reservoir Volume
V2 ---------- Delivery Reservoir Volume
A ---------- Orifice Area
P2 / P1 -------- Pressure Ratio
For air = 1.4
When (P2/P1) < 0.528 then there is going to be no change in mass flow rate.
If (P2 / P1) = 0.528 then the Flow Rate (Q) is going to be a function of initial and final pressures.
If the pressure ratio is less than 0.528 then the Critical Flow rate (Q*) is denoted by the following equation

When the pressure ratio is greater than 0.528 then the following equation is used

Finally the mass flow rate is calculated by
Q= (Q / Q*) * (Q*)
After calculating the mass flow rate we need to calculate the mass flow using the following equation
m = Q * t (assume initial time to be 0.0001s)
In the above equation we are calculating the mass flow between two reservoirs by multiplying mass flow rate by time.
In the next step we are calculating the initial masses (m1, m¬2) of the two reservoirs by using the following equations:
Initial mass of the first reservoir m1 = P1 * 1.29 * V1
Initial mass of the second reservoir m2 = P2 * 1.29 * V2
After calculating the initial masses the change in mass in the two reservoirs is calculated as stated below.
m11 = m1 - m
m22 = m¬2 + m
After calculating the change in mass, the pressure values are calculated
P1 = m11 / (1.29 * V1)
P2 = m22 / (1.29 * V2)
When P2 = 0.95P1 stop the calculation and plot the values of delivery pressure and time.
Time corresponding to 75% delivery pressure is response time.
MULTI RESERVOIR / ORIFICE SYSTEM
ORIFICE


A1
A2
FORMULAE NOTATION:
Q1* ------------ Critical Flow rate between first and the second reservoir
Q2* ------------ Critical Flow Rate between second and third reservoir
Q1 ------------ Flow rate between first and the second reservoir
Q2 ----------- Flow Rate between second and third reservoir
m1 ---------- Mass flow between first and second reservoir
m2 ---------- Mass flow between second and third reservoir
m11 ---------- Change in mass between first and second reservoir
m22 ---------- Change in mass between second and third reservoir
P¬1 ---------- Pressure in the first reservoir.
P2 ---------- Pressure in the second reservoir.
P3 ---------- Pressure in the third reservoir
CALCULATIONS
The calculation of the above shown system is as follows;
If the pressure ratio is less than 0.528 then the Critical Flow rate (Q*) is denoted by the following equation


When the pressure ratio is greater than 0.528 then the following equation is used


Finally the mass flow rate is calculated by
Q1 = (Q1 / Q1*) * (Q1*)
Q2 = (Q2 / Q2*) * (Q2*)
After calculating the mass flow rate we need to calculate the mass flow using the following equation
m1 = Q1 * t (assume initial time to be 0.0001s)
m2 = Q2* t (assume initial time to be 0.0001s)
In the above equation we are calculating the mass flow between two reservoirs by multiplying mass flow rate by time.
In the next step we are calculating the initial masses (m1, m¬2) of the two reservoirs by using the following equations:
Initial mass of the first reservoir

m1 = P1 * 1.29 * V1

Initial mass of the second reservoir

m2 = P2 * 1.29 * V2
Initial mass of the third reservoir
m3 = P3 * 1.29 * V3
After calculating the initial masses of the two reservoirs the change in mass in calculated as stated below.
m11 = m1 - m1
m22 = m¬2 - m2 + m1
m33 = m3 + m2
After calculating the change in mass, the pressure values are calculated
P1 = m11 / (1.29 * V1)
P2 = m22 / (1.29 * V2)
P3 = m33 / (1.29 * V3)
When P3 = 0.95P1 stop the calculation and plot the values of delivery pressure and time.
Time corresponding to 75% delivery pressure is response time.
Using similar methods as stated above the program has been developed for four and n-reservoirs.

AIR BRAKE SYSTEMS WITHOUT RELAY VALVE
After completing those programming for those systems we have to program for simple air brake systems.
Firstly we have started programming for a Swaraj Mazda Air Brake System. As the first step the whole air brake system has been converted into orifices and reservoirs. The converted circuit is shown below.

Figure 2 - SWARAJ MAZDA Service Brake Circuit without Relay Valve
Similarly as stated above the program has been developed for the Swaraj Mazda braking system also. In the Beginning stage the program has been developed by taking the Brake chamber volume to be constant. But it has been found that when the volume is taken to be constant, the time taken by the brake chamber to rise from the dead volume to the maximum stroke is not being considered. So, the theoretical response time was earlier than the actual response time.
In order to account for the losses in time, the brake chamber volume was taken to be varying .The calculation is shown below.
BRAKE CHAMBER CALCULATIONS
Let us consider the brake chamber to be a spring-loaded cylinder with stopper.
The cylinder has a dead volume (i.e.) volume at zero stroke is V0 ~ 0.1124 lit
The cylinder has threshold pressure P0 ~ 1.4 bar
The cylinder has a stopper at stroke St ~ 30mm
Minimum pressure required to reach the stroke St is Pst ~ 1.6 bar

Figure 3 - Brake Chamber with Spring Loaded Cylinder
Stroke x
Effective diameter of the cylinder De ~ 7741.92 mm2 (type 12 brake chamber)
Cylinder pressure P ~ found out in the program
Volume = V0 +(x * /4 * De^2)
x = 0 when P < P0
x = St when P > Pst
When P > P0 and P < Pst
x = St *((P “ P0)/ (Pst “ P0))
AIR BRAKE CIRCUIT WITH RELAY VALVE
After all the validations program has been developed by taking relay valve into consideration. The air brake circuit has been modified as the diagram shown below.

Figure 4 - SWARAJ MADZA Service Brake Circuit with Relay Valve
RELAY VALVE
Signal port
Delivery port
Figure 5 - Relay Valve
Inlet port
Relay valve is used in brake circuits in order to speed up the brake application or to reduce the response time. The relay valve consists of two portions namely the control portion and relay portion.
WORKING OF RELAY VALVE:

While applying the brakes first the dead volume is filled until the o-ring friction is overcome. Then relay piston moves till the valve is contacted and crack off pressure is overcome. After this the relay piston moves further till the inlet travel is completed. In terms of pressure the pressure rises in the dead volume up to crack off pressure. Once the pressure reaches the crack off pressure, the piston starts moving according to the mass inflow and when it reaches the full opening the pressure starts building up further.
In the relay portion, the flow area is zero until the piston reaches the lap position. Lap position is the position in which the piston exactly comes in contact with the valve seat and both the inlet and delivery valves are in closed condition. The flow area is maximum when the inlet travel is complete. In between the lap position and fully open position the area is proportional to valve travel.
Taking the above constraints into account a program has been developed by incorporating the relay valve.
CALCULATIONS FOR RELAY VALVE:
Let us consider the R-6 relay valve:
The crack-off the relay valve is 0.29 bars.
So when the pressure at the signal port is going to be less than 0.29 then the delivery pressure is not going to vary, and once the delivery pressure rises above 0.29 bar the delivery pressure starts increasing depending upon the length of travel of piston (Y).
If pressure_signal_port < 1.29 then
Y = 0
Else
Y = 4.1971 / 1000
Volume relay = (dead_volume *0.001) +(Y * (area_relay /1000000))
Pressure_delivery_relay = (mass_value_relay / (1.22 * volume relay))
The inlet travel for relay valve is calculated as stated below
Length of travel of piston for maximum stroke = 5.7411 mm
Length of travel of piston for Exhaust travel = 1.5440 mm
Length of travel of piston for inlet travel = (5.7411 - 1.5440) = 4.1971 mm
As stated above the relay vale is included in the circuit and response time is calculated.
VALIDATION OF THEORETICAL RESULTS
After developing the program for an air brake system the program was validated by keying in the various values from the old reports.
The results are shown below. The various data that are taken from the reports are shown below. If we are going to give these data in the program the results that we are going to get is shown below.
CONDITIONS SWARAJ MAZDA, Truck EICHER JAC,Truck, CAIRO
PIPE ID (mm) 8(Uniform throughout) 12,11(from T to SBA and from T to BC) 8(Uniform throughout)
21 tank to dual brake valve length 6200 mm 5325 mm 3800 mm
Dual Brake valve delivery to rear T-connector length 6900 mm 6983 mm 6250 mm
T- connector to right SBA length 600 mm 710 mm 2400 mm
22 tank to dual brake valve length 6200 mm 6023 mm 200 mm
Dual Brake Valve to front T-connector length 2000 mm 1834 mm 3800 mm
Front T-connector to left Brake Chamber 1700 mm 1679 mm 650 mm
Table 1- Validation of Theoretical Results
The following results are obtained after keying in the above data in the program

RESPONSE TIME MEASUREMENTS FOR REAR WHEEL
VEHICLEMAKE
REPORT REFERENCE THEORETICALVALUE (from program) EXPERIMENTAL VALUE(from reports) DIFFERENCE
SWARAJ MAZDA R&D/TSG/Field/070/03 0.37 0.31 + 0.06
EICHER R&D/Field/Eicher/105/01 0.36 0.27 + 0.09
JACTruck R&D/Field/JAC/136/02 0.54 0.47 + 0.07
Table 2 - Response time measurement for Rear Wheel

RESPONSE TIME MEASUREMENTS FOR FRONT WHEEL
VEHICLE NAME REPORT REFERENCE THEORETICALVALUE(from program) EXPERIMENTAL VALUE(from reports) DIFFERENCE
SWARAJ MAZDA R&D/TSG/Field/070/03 0.35 0.27 + 0.08
EICHER R&D/Field/Eicher/105/01 0.33 0.3 + 0.03
JACTruck R&D/Field/JAC/136/02 0.50 0.43 + 0.07

Table 3 - Response time measurement for Front Wheel

VALIDATION OF BRAKE CHAMBER VOLUME
TEST ARRANGEMENT:
A ball valve is connected to the inlet port of the test unit. The ball valve is connected to reverse, i.e. the inlet port of the valve is connected to the inlet port of the test unit. The facility is done to facilitate trapping of pressurized water inside the test unit itself.
A quick-change adaptor is connected to the delivery port of the ball valve for easy and quick removal of test unit for weighing purpose.
A 25 litre reservoir is positioned vertically and the top end of the reservoir is connected to an air source through a pressure regulator, directional control valve and pressure transducer as shown in the appendix .A flexible hose with quick change coupling is connected at the bottom end of the reservoir, to facilitate quick connection to the test unit.
The air pressure is used to force the water into the test unit under the required pressure.
TEST PROCEDURE:
1. Assemble the fixture, ball valve and a quick-change adaptor (as mentioned) with the test unit and measure the combined weight (W0grams).
2. Adjust the pressure regulator valve to the required pressure (8 bar to this test)
3. Fill the test unit with water to expel any air present inside the test unit. Close the ball valve.
4. Connect the test unit with reservoir and open the ball valve as shown below.
5. Now pressurized water will be trapped inside the brake chamber.
6. Close the ball valve.
7. Detach the test unit and measure the weight along with the trapped water (W1 grams).
8. The test unit volume at the tested stroke is given by (W1 “ W0) CC.
9. Repeat the steps 4 to 10 for the various brake chambers.
DEAD VOLUME RESULTS:
Stroke (mm) Type of Brake Chamber Volume (cc)
0.00 9 (M845690) 19.6
0.00 12 (M850240) 49.74
0.00 16 (M845800) 53
0.00 20 (M845860) 40
0.00 24 (M845490) 124
Table 4 - Dead Volume Results
BRAKE CHAMBER VOLUME AT DIFFERENT STROKES AT 8 bar PRESSURE
Brake Chamber Type Volume at 20mm (cc) Volume at 25mm (cc) Volume at 30mm (cc) Volume at 35mm (cc)
Type 9 226.5 234.7 278.9 289.3
Type 12 280.6 317.2 358.5 397.1
Type 16 314.9 374 415.13 466.5
Type 20 359.8 383.6 453.7 512.2
Type 24 783.3 870.9 914.9 975.4
Table 5 - Brake Chamber Volume at 8 bar Pressure at different strokes
Deviation of Theoretical and Experimental Brake chamber Volumes = 10-17%
EXPERIMENTAL VERIFICATION

Figure 6- Vehicle Used for Testing Response Time

Figure 7 - Transducer Fitted In Spring Brake Actuator During Testing
Figure 8 - Transducer Fitted in Brake Chamber During Testing

Figure 9 - Service Reservoir Which Supplies Air to the whole Circuit

Figure 10 - Dual Brake Valve
CONNECTIONS FOR MEASURING IN VEHICLE

Figure 11 - Connections for Measuring Response Time
SOFTWARE INTERFACE FOR DATA ACQUISITION SYSTEM

Figure 12 - Software Interface for Data Acquisition
TEST PROCEDURE
When the driver presses the brake pedal depending upon the force applied the dual brake valve releases air to the brake chambers and actuators through the various valves. Depending on the pressure variations the transducer will be sending data to the data acquisition system. The variation in time and pressure values are plotted to get the final response time.
CONCLUSION
After the comparison of the experimental and theoretical values we can infer that
¢ For an air brake system without relay valve the deviation between the theoretical and experimental response time was found to be 10 to 16 %
¢ For an air brake system with relay valve the deviation was found to be less than 10 %
This makes sure that with the help of this system created by using visual basic software makes it to be a user friendly one for the designers enrolled in the R & D of any company. This project was done by us with the help of engineers in Sundaram Clayton Ltd. Padi, Chennai.
Therefore, this project is a private version of the Sundaram Clayton Limited and the source code with all the results of the software will be shown at the time of presentation if this paper is selected, and please consider this fact into an important account.
BIBILOGRAPHY
1. S .Selvamani, Brake System Engineering, Sundaram Clayton limited, Chennai, (1996)
2. Crane Technical Paper No: 410 “ Flow of fluids through Valves, Fittings and Pipe, Crane & Co, 300, Park Avenue, New York (1978).
3. Roy, D. n., Applied Fluid Mechanics, Affiliated East-West Press Pvt Ltd, New Delhi, (1980).
4. Landau, L.D., Fluid mechanics, Pergamon Press, Tokyo (1989)
5. efundas .com
6. R&D Reports, Sundaram Clayton Limited, Padi, Chennai.
i. R&D/VEH/GMM/205/98
ii. R&D/Field/Eicher/105/01
iii. R&D/TSG/Field/070/03
iv. R&D/Field/JAC/136/02

CONTENTS
Components Of An Air Brake System: 2
Working Of An Air Braking System 4
Importnace And Definition Of Response Time 4
Definition Of Response Time 5
Basic Algorithm 6
Response Time Calculations 7
Two Reservoir Systems 7
Multi Reservoir / Orifice System 8
Air Brake Systems Without Relay Valve 11
Brake Chamber Calculations 11
Air Brake Circuit With Relay Valve 12
Relay Valve 13
Validation Of Theoretical Results 14
Response Time Measurements For Rear Wheel 15
Response Time Measurements For Front Wheel 15
Validation Of Brake Chamber Volume 16
Experimental Verification 17
Connections For Measuring In Vehicle 19
Software Interface For Data Acquisition System 19
Test Procedure 20
Conclusion 20
Bibilography 20
LIST OF FIGURES
Figure 2 - SWARAJ MAZDA Service Brake Circuit without Relay Valve 11
Figure 3 - Brake Chamber with Spring Loaded Cylinder 12
Figure 4 - SWARAJ MADZA Service Brake Circuit with Relay Valve 12
Figure 5 - Relay Valve 13
Figure 6- Vehicle Used for Testing Response Time 17
Figure 7 - Transducer Fitted In Spring Brake Actuator During Testing 18
Figure 8 - Transducer Fitted in Brake Chamber During Testing 18
Figure 9 - Service Reservoir Which Supplies Air to the whole Circuit 18
Figure 10 - Dual Brake Valve 18
Figure 11 - Connections for Measuring Response Time 19
Figure 12 - Software Interface for Data Acquisition 19
LIST OF TABLES
Table 1- Validation of Theoretical Results 14
Table 2 - Response time measurement for Rear Wheel 15
Table 3 - Response time measurement for Front Wheel 15
Table 4 - Dead Volume Results 16
Table 5 - Brake Chamber Volume at 8 bar Pressure at different strokes 17

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INTRODUCTION

COMPONENTS OF AN AIR BRAKE SYSTEM:

Air brake system consists of the following components:

Compressor:
The compressor generates the compressed air for the whole system.

Reservoir:
The compressed air from the compressor is stored in the reservoir.

Unloader Valve:
This maintains pressure in the reservoir at 8bar.When the pressure goes above 8 bar it immediately releases the pressurized air to bring the system to 8-bar pressure.

Air Dryer:
This removes the moisture from the atmospheric air and prevents corrosion of the reservoir.

System Protection Valve:
This valve takes care of the whole system. Air from the compressor is given to various channels only through this valve. This valve operates only at 4-bar pressure and once the system pressure goes below 4-bar valve immediately becomes inactive and applies the parking brake to ensure safety.

Dual Brake Valve:
When the driver applies brakes, depending upon the pedal force this valve releases air from one side to another.

Graduated Hand Control Valve:
This valve takes care of the parking brakes.