Air compressor duty cycle reduction in passenger bus application

15AE0063

Air compressor duty cycle reduction in passenger bus application

Harish Kumar Gangwar, Ankur sharma, Ambekar prasad & Dipak Dabhole Engineering Research Center

TATA Motor Limited Copyright © 2015 SAE International

Abstract Today urban buses are equipped with more air consuming devices for

an example pneumatic doors, exhaust brake, air suspension and in

SCR system to name a few. This has resulted in higher air demand

leading to high compressor duty cycles which cause conditions (such

as higher compressor head temperatures) that may adversely affect

air brake charging system performance. These conditions may require

additional maintenance due to a higher amount of oil vapor droplets

being passed along into the air brake system. Factors that add to the

duty cycle are air suspension, additional air accessories, use of an

undersized compressor, frequent stops, excessive air leakage from

fittings, connections, lines, chambers or valves, etc.

This paper discussed about methodology used to reduce air

consumption of air consuming devices used in urban bus application.

Performance assessment of air consuming devices with minimum

available air pressure was conducted and found satisfactory. Overall

results as reduction in air compressor duty cycle.

Introduction Now days urban buses are equipped with pneumatic operated devices

for eg. pneumatically operated doors, pneumatic suspension, braking

system (ABS), emission control system (SCR), retarder etc.

Increasing demand of air results in increased air compressor duty

cycle, decrease in fuel economy and adversely impact on health of air

compressor. The compressed air system includes the air compressor,

compressor inlet line and discharge lines, air governor with signal

lines, air dryer with or without oil separator, primary, secondary &

auxiliary tanks and air consumption devices.

The key factor, which determines the reliability and durability of an

air compressor in an application, is the amount of time the air

compressor is supplying air during the vehicle/machine operation,

known as the duty cycle of the compressor. Compressors are not

designed to pump continuously and system should be designed to

allow sufficient unloading time because compressors generate a lot of

heat when pumping, which is dissipated during the time the

compressor is not pumping (called the unloaded operation).

If the compressor duty cycle is very high, the compressor will operate

at higher temperatures and potentially begin to overheat, which

reduces the sealing of the piston rings, allowing more oil to pass into

the compressed air discharge cavity. The oil carryover can foul (with

carbon build-up or contamination) the air compressor head,

downstream plumbing, air dryer, air valves and other air-actuated /

air supplied components [1]. A high air compressor duty cycle also

raises the temperature of the air supplied to the air dryer, which

reduces the effectiveness of the desiccant in the dryer [2].

As shown in fig.1, fig.2 and fig.3 is the image of a failed field

sample of air compressor head, air dryer and seal failed due to carbon

deposit, high oil carryover and high discharge air temperature. All

these types of failures are due to high air compressor duty cycle.

Figure 1: Field failed sample showing carbon deposit on reed valve

Figure 2: Field failed sample of air dryer showing high oil carryover

compressor

Figure 3: Field failed sample showing seal failure due to high air dryer inlet

temperature

Fig 4 demonstrate root cause analysis done for high soot deposition

on air compressor head which also indicates high air compressor duty

cycle as the cause for this failure.

Figure 4: Fishbone diagram for oil carry over/ carbon build up

dryer showing high oil carryover from air

seal failure due to high air dryer inlet

analysis done for high soot deposition

indicates high air compressor duty

Fishbone diagram for oil carry over/ carbon build up

Flag test is also one of the methodology to

from air compressor. As shown in fig. 5

(A&B) . Oil carry over in bus A is higher than bus B.

It is important whether the paper is dry or damp. Damp oil deposits

indicate increased oil consumption.

(a) Oil consumption too high (Bus A)

Figure 5: Flag test (paper method) test

In this paper we have explored the possibility of reducing the

compressor duty cycle by minimizing

consumption reduction can be done with compromising on deleting

some of the air driven auxiliary units or reducing the air

consumption. In this paper we have

air consumption by minimizing the actual pres

consuming devices without compromising on their performances.

Test vehicle details: Test vehicle aggregate details are mentioned in table 1.

conducted on laden bus loaded up to gross vehicle weight i.e 16.2

ton.

Table 1: Test vehicle specification

Model 230 HP,

diesel engine

Displacement 5.9 Liter

Compression ratio 17.5:1

Max. Engine Output 170 Kw @ 2500 rpm

Max. Torque 850 Nm @ 1500 rpm

Fuel Injection Common rail fuel injection

Air compressor 318 cc, water cooled, naturally aspirated

engine driven

RPM @ max engine

power 2400

Compressor cut in

pressure 8.2 ± 0.4 bar

Compressor cut out

pressure 9 ± 0.2 bar

Flag test is also one of the methodology to analyze oil carry over

from air compressor. As shown in fig. 5, flag test results of two buses

is higher than bus B.

It is important whether the paper is dry or damp. Damp oil deposits

(b) Oil consumption normal (Bus B)

: Flag test (paper method) test result to check oil carryover

In this paper we have explored the possibility of reducing the

compressor duty cycle by minimizing air consumption. Air

consumption reduction can be done with compromising on deleting

some of the air driven auxiliary units or reducing the air

consumption. In this paper we have analyzed the option of reducing

air consumption by minimizing the actual pressure delivered to air

consuming devices without compromising on their performances.

details are mentioned in table 1. Tests were

up to gross vehicle weight i.e 16.2

230 HP, 5.9 liter BS IV, turbocharged

iesel engine

170 Kw @ 2500 rpm

850 Nm @ 1500 rpm

Common rail fuel injection system

water cooled, naturally aspirated,

engine driven

8.2 ± 0.4 bar

9 ± 0.2 bar

Schematic layout of pneumatic system:

Figure 6: Schematic layout of pneumatic system

Schematic layout of pneumatic circuit is shown in fig.6. Air

compressor delivers the air at high pressure which is sent to

condensate oil separator and air dryer which removes the entrained

moisture and oil vapors. Dry oil free air is then delivered to different

storage tanks with the aid of system protection valve, from where air

is available for all accessory units fitted on vehicle (Bus).

Details of air consuming devices and their source of air supply are

discussed in table 2.

Table 2: Details of air storage tanks

S. No. Storage tank Air consuming device

1 Primary tank Front brakes

2 Secondary tank Rear brakes

3 Auxiliary tank

Pneumatic doors, exhaust

brake, Urea dosing system,

steering solenoid

4 Retarder tank Air supply to oil accumulator

for retarder application

5 Suspension tank Air bellows

Air pressure requirements of the above devices are different. Air

pressure requirement of different air consuming devices lists out in

table 3.

Table 3: Details of air pressure requirements by different aggregates

S. No. Air consuming device Pressure requirements

1 Front and rear brakes 9 bar

2 Pneumatic doors 4-8 bar

3 Exhaust brake > 5 bar

4 Urea dosing system > 6 bar

5 Retarder application >4 bar

6 Air bellows (Suspension) 8 bar

The minimum working air pressure requirement of aggregates are

listed on table 3. Installation of 6 bar PRV done on auxiliary tank

(exhaust brake, pneumatically operated doors) and retarder tank .

Instruments used and test vehicle set up :

Following instruments were used for capturing the difference in the

duty cycle with PRV and to check the performance of the air

consuming devices with reduced air pressure of 6 bar.

• IMC data logger version 2.8 for logging the output from

pressure transducer

• Pressure transducers (0-25 bar)

• Corrsys Datron data logger with L-350 Aqua sensor to

measure vehicle speed, Deceleration and MFDD

Pressure transducers were installed at each storage tank and at

compressor outlet. Non return valve at the inlet of each tank was

fitted to ensure that pressure is not lost from one tank to another. Fig

7 to fig 10 shows images of test vehicle with instruments.

Figure 7: Pressure transducer fitted in retarder tank

Figure 8: PRV and non return valve fitted at the inlet of auxiliary tank

Figure 9: L 350 aqua sensor fitted in vehicle to measure vehicle speed,

deceleration, MFDD etc.

Pressure transducer at retarder tank

Pressure reduction valve of 6 bar at the inlet of auxiliary tank

Non return valve at the inlet

Aqua sensor installed in vehicle to measure vehicle speed, deceleration, MFDD etc.

Figure 10: IMC data logger installed in vehicle

Tests conducted on the vehicle:

Table 4 consist list of different test conducted to evaluate air

compressor duty cycle and other aggregate performances

with aid of air.

Table 4: Different tests conducted on vehicle

Sr.

No. Aggregate/

Parameter

Without

PRV

1 Fill up time Yes

2 Compressor duty cycle (Under

controlled condition with known

test cycle)

Yes

3 Retarder performance Yes

4 Exhaust brake performance Yes

5 Pneumatic door performance Yes

Above tests were conducted on the test vehicle with and without PRV

under controlled city cycle application.

Test results and discussion:

A. Compressor fill up time:

With reference to Sr. No. 1 in table 4, to measure compressor fill up

time first all storage tanks were emptied and fill time

at governed engine rpm of 2500 with and without PRV and

difference was noted. Fill up time is the time at which compressor

gets cut off and all the air tanks are filled to their maximum set

pressure.

Fig 11 shows the variation of compressor output pressure with and

without PRV.

Table 5: Fill up time observed with and without PRV

Condition Fill up time

Without PRV 3.56 min

With PRV 3.16 min

IMC Data logger installed in vehicle.

IMC data logger installed in vehicle

Table 4 consist list of different test conducted to evaluate air

duty cycle and other aggregate performances running

Without With

PRV

Yes

Yes

Yes

Yes

Yes

Above tests were conducted on the test vehicle with and without PRV

ompressor fill up

first all storage tanks were emptied and fill time was measured

governed engine rpm of 2500 with and without PRV and

Fill up time is the time at which compressor

gets cut off and all the air tanks are filled to their maximum set

shows the variation of compressor output pressure with and

Figure 11: Fill up time comparison with PRV

As observed from the table 5 fill up time with PRV is less in

comparison to without PRV, therefore unloading the compressor

early.

B. Effect on compressor duty cycle

With reference to Sr. No. 2 in table 4, t

compressor duty cycle, vehicle was run

application with and without PRV and

logged for compressor outlet pressure and storage tank pressures

In fig 12 and fig 13 shows variation of compressor discharge pressure

variation is shown which is used for calculation of compressor loaded

and unloaded time. Compressor duty cycle is defined as the ratio

loaded time and total run time.

As discussed in table 6 compressor unloaded time duration is high

with use of PRV as discussed in table 3.

air compressor duty cycle is achieved

1. Compressor duty cycle without

Figure 12: Pressure variation at compressor discharge

: Fill up time comparison with PRV

fill up time with PRV is less in

, therefore unloading the compressor

ompressor duty cycle :

. No. 2 in table 4, to check the effect on

vehicle was run in controlled city cycle

and performance parameters were

pressure and storage tank pressures.

variation of compressor discharge pressure

variation is shown which is used for calculation of compressor loaded

y cycle is defined as the ratio of

As discussed in table 6 compressor unloaded time duration is high

with use of PRV as discussed in table 3. Significant improvement on

air compressor duty cycle is achieved 11-12 % with the help of PRV.

Compressor duty cycle without PRV

Pressure variation at compressor discharge without PRV

2. Compressor duty cycle with PRV

Figure 13: Pressure variation at compressor discharge with PRV

Table 6: Test results for compressor duty cycle calculation:

Sr. No. Parameter Without PRV With PRV

1 Total run time 15316.4 s 11702.1 s

2 Compressor loaded

time 10047.5 s 6295.7 s

3 Compressor

unloaded time 5268.8 s 5406.4 s

4 Duty cycle 65.6 % 53.8 %

Engine rpm was monitored in this controlled city duty cycle to

ascertain the average engine rpm as shown in fig 14.

Figure 14: Engine rpm variation in controlled city cycle

Average engine rpm of 1102 is realised in the above variation.

C. Performance evaluation of retarder

With reference to Sr. No. 3 in table 4, to measure retarder

performance, pressure transducer was fitted in the retarder tank and

its pressure was monitored in city cycle, with and without PRV.

C.1. Performance evaluation of retarder without PRV

Figure 15: Pressure variation in retarder tank without PRV

C.2. Performance evaluation of retarder with PRV

Figure 16: Pressure variation in retarder tank with PRV

As shown above in fig 15, pressure in the retarder tank is maintained

close to 9 bar without PRV in city cycle conditions. Now we have

installed PRV at the inlet of auxiliary tank and monitored the

pressure variation in city cycle conditions. If we look at the pressure

variation with PRV in fig 16, it is always high than the required

pressure of 4 bar for retarder operation, thus not impacting the

required pressure for retarder application.

C. 3. Retarder performance test results for speed 40 km/h

without PRV

Figure 17: Retarder performance without PRV in 40-16 kmph

C. 4. Retarder performance test results for speed 40 km/h

with PRV

0123456789

10

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Pre

ssu

re (

ba

r)

Time (sec)

Pressure varation in retarder tank without PRV

Figure 18: Retarder performance without PRV in 40-16 kmph

However we have also analysed the qualitative effect on retarder

performance bacause of this PRV installation. Fig. 17 and fig. 18,

shows velocity variation is shown with retarder application without

PRV and with PRV respectively.

Retarder performance was mapped by calculating the deceleration

offered at 9 bar and 6 bar inlet pressure. Retarder was applied at the

initial speed of 40 km/h and vehicle was allowed to slowdown.

Retarder control unit allows the retarder being applied only until

vehicle speed is more than 16 km/h and therefore we have calculated

the speed difference taking 16 km/h as the lowest speed.

Table 7: Average deceleration obtained in retarder performance test

Sr. No. Test condition Deceleration

(m/s2)

1 Without PRV 0.8929

2 With PRV 0.8366

Test result discussed in table 7, performance of retarder with PRV is

at par without PRV.

D. Performance evaluation of accessory unit attached

with auxiliary tank :

With reference to Sr. No. 4 and 5 in table 4, to measure pneumatic

door and exhaust brake performance, pressure transducer was fitted

in the auxiliary tank and its pressure was monitored in city cycle with

and without PRV.

Below is the pressure variation observed in the auxiliary tank without

and with PRV in fig. 19 and fig. 20 respectively.

D.1. Pressure variation in auxiliary tank without PRV

Figure 19: Pressure variation in auxiliary tank without PRV

D. 2. Pressure variation in auxiliary tank with PRV

Figure 20: Pressure variation in auxiliary tank with PRV

E. Effect of PRV on pneumatic door performance

As per test result obtained in fig. 20 pressure in the auxiliary tank is

very well above the minimum required pressure of 4 bar for

pneumatic door functioning in city running conditions.

Therefore installation of PRV does not affect pneumatic door

operation at low air pressure of 6 bar, therefore able to reduce air

consumption with using PRV.

F. Performance evaluation of exhaust brake

As discussed in table 4 Sr. No 4.exhaust brake performance was

checked by applying only the exhaust brake at the initial speed of 35

km/h and let the vehicle roll on a flat road until speed drops to below

30 km/h.

Below is the velocity variation obtained by applying the exhaust

brake without and with PRV in fig. 21 and fig. 22 respectively.

Average value of mean fully developed deceleration (MFDD) was

calculated in both the cases and compared for performance at reduced

pressure.

0

1

2

3

4

5

6

7

8

9

10

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Pre

ssu

re (

ba

r)

Time (sec)

Pressure variation in auxiliary tank without PRV

Figure 21: Exhaust brake performance at 6 bar supply pressure in 35

Figure 22: Exhaust brake performance at 9 bar supply pressure in 35

For calculating the mean fully developed deceleration (MFDD)

following formula is used:

MFDD = {(V_08)² – (V_01)²) / (25.92 * (S_01 – S_08)}

Where:

V_08 is the speed at 80% of the brake trigger activation speed.

V_01 is the speed at 10% of the brake trigger activation speed.

S_08 is the distance at which the speed is V_08.

S_01 is the distance at which the speed is V_01.

Table 8: Test results summary for MFDD obtained

Sr. No. Test condition Average MFDD obtained

in m/s2

1 Without PRV 0.4513

2 With PRV 0.4282

As evident from the test results MFDD obtained with 6 bar inlet

pressure is at par with 9 bar inlet pressure, therefore able to reduce air

consumption with using PRV.

at 6 bar supply pressure in 35-30 kmph

Exhaust brake performance at 9 bar supply pressure in 35-30 kmph

For calculating the mean fully developed deceleration (MFDD)

S_08)}

ger activation speed.

V_01 is the speed at 10% of the brake trigger activation speed.

Average MFDD obtained

in m/s2

0.4513

0.4282

MFDD obtained with 6 bar inlet

, therefore able to reduce air

Conclusion From the test results following conclusions can be drawn:

• Installation of PRV clearly reduces fill up time and hence

unloading the compressor early.

• Significant reduction in compressor duty cycle was found,

which in this case decreased by about

will avoid all the failures observed because of high

compressor duty cycle.

• Retarder performance was not affected because of PRV

installation and Deceleration offered is at par without PRV.

• There is no effect/deterioration in pneumatic door and

exhaust brake performance. MFDD obtained from exhaust

brake is at par.

Therefore selective assessment

of different air consuming devices and accordingly

the air supply results in compressor duty cycle reduction

without compromising the performance of these devices.

References:

1. William P. Fornof, "Compressor

the Pneumatic System", SAE Technical Paper 1999

2. Chuck Eberling, Fred Hoffman, "

Dryer Charging Systems", SAE Technical Paper 1999

Contact Mr.Harish Gangwar (91-9794052517)

Tata Motors Limited

([email protected])

Mr. Ankur Sharma (91-8756002005)

Tata Motors Limited

([email protected])

Acknowledgement The authors wish to thank, Mr. Amul verma, Mr. Amit Nigam

Mr. S. B Pathak for guidance and useful

Definitions/Abbreviations

PRV: Pressure reduction vale

MFDD: Mean fully developed deceleration

SCR: Selective catalytic reduction

NRV: Non return valve

From the test results following conclusions can be drawn:

Installation of PRV clearly reduces fill up time and hence

unloading the compressor early.

Significant reduction in compressor duty cycle was found,

which in this case decreased by about 18% relatively. This

will avoid all the failures observed because of high

Retarder performance was not affected because of PRV

installation and Deceleration offered is at par without PRV.

There is no effect/deterioration in pneumatic door and

exhaust brake performance. MFDD obtained from exhaust

assessment of air pressure requirement

of different air consuming devices and accordingly limiting

upply results in compressor duty cycle reduction

without compromising the performance of these devices.

Oil Carryover and Its Effect on

SAE Technical Paper 1999-01-3786

2. Chuck Eberling, Fred Hoffman, "Advanced Compressor and Air

", SAE Technical Paper 1999-01-3722

9794052517)

)

The authors wish to thank, Mr. Amul verma, Mr. Amit Nigam and

for guidance and useful discussions.

MFDD: Mean fully developed deceleration