The Impact of Air Leakage on 

Locomotive Efficiency and 

Decarbonization

Chris Stoos

May 2023
SwRI Locomotive Technology Center

▪ A group within the Powertrain Engineering Division of Southwest Research 

Institute

– SwRI is a 501(c)(3) non-profit with 2,700 employees

– 1700+ Acre facility in San Antonio, TX 

– 8 technical divisions – “From Deep Sea to Deep Space”

▪ Locomotive Technology Center Established in 1990 for the AAR

▪ Hundreds of locomotives tested to date

▪ Projects for EPA, CARB, FRA, CaDOT, DOE, AAR, RR’s, and OEM’s

S O U T H W E S T R E S E A R C H I N S T I T U T E ®

Locomotive Technology Center

203 Milam Street

Association of American Railroads
Locomotive Compressed Air Overview

3

▪ Every locomotive has an air compressor

▪ Those compressors are driven by the engine

– Electric or shaft driven

▪ Power needed for the compressor depends 

on many things

– Compressor design

– Compressor setpoint

– Ambient conditions

▪ Air used for various functions

– Brakes, Horn, Shutters, Valve Actuation, etc.
Leaks are difficult to find.

4

▪ Trains have miles of air piping on them, including locomotives and railcars

▪ Air leaks are invisible and hard to detect in noisy environments with PPE

▪ While there are limits on some leaks, many leaks have gone ignored over the 

years

▪ Difficulty of detection and extent of the issue can make it appear 

insurmountable
Fuel Consumption = GHG Emissions

5

▪ Locomotive fuel consumption is the 

primary source of GHG emissions for all 

railroads

▪ Reducing fuel consumption is necessary to 

meet stated SBTi goals

▪ Fuel consumption gains can be made in 

many areas 

– Operational efficiency improvements

– Locomotive efficiency improvements

– Engine efficiency improvements
Fuel Consumption = GHG Emissions

6

▪ Locomotive fuel consumption is the 

primary source of GHG emissions for all 

railroads

▪ Reducing fuel consumption is necessary to 

meet stated SBTi goals

▪ Fuel consumption gains can be made in 

many areas 

– Operational efficiency improvements

– Locomotive efficiency improvements

– Engine efficiency improvements

Air leaks fall in this category
Locomotive Fuel Consumption

▪ Mass of Fuel per unit of brake 

horsepower

▪ Measure of Engine Efficiency

▪ Good to compare different 

engine models/types

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▪ Mass of Fuel per unit of 

traction horsepower

▪ Measure of Vehicle Efficiency

▪ Good to compare different 

locomotive models

Brake Specific Fuel Consumption

Traction Specific Fuel Consumption
How much does it matter?
How much does it matter?

Most railroads operate 

their locomotive fleet 

somewhere between 

these two duty cycles. 

These values DO NOT 

include losses due to 

train delays, AESS 

restarts, etc.

Total fuel consumption 

penalties are likely 

higher.
AESS – Great, when it works

10

Each graph 

represents 

24 hours.

43.2 gal/day difference

CN 8862 Before Repairs CN 8862 After Repairs
CN 8862 – Air Leaks (with approx. repair time)

1. Air Dryer – Replaced (20 min)

2. RB-MV – Fixed (10 min)

3. MR2 Pressure Sensor – Fixed (1 min)

4. MR Filter Housing – Fixed (20 min)

5. Union threads near sump – Fixed (5 min)

6. Compressor – Not Fixed

7. Shutter Mag Valve – Fixed (5 min)

8. Unloader Mag Valve – Fixed (20 min)

9. MR2 Check Valve – Replaced (2 hrs)

10. Piping on supply to Air Brake Cabinet - Fixed (60 min)

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Times listed do not include time spent finding leaks
GHG Impact of Leaks

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Air Leaks are Independent of Fuel Type

▪ Whether Biodiesel, Renewable, 

Natural Gas, Hydrogen, or 

Battery Electric...air leaks will 

reduce the efficiency of ALL of 

them.

▪ Overall Vehicle Efficiency needs 

to be a focus point for railroads 

moving forward on new and 

rebuilt locomotives

– Even with the same engine, it 

can be better.

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What can be done

▪ Mechanical requirements need to 

change

– FRA leak limits are set for safety, 

not efficiency

– Railroad requirements need to 

become more stringent

▪ Advanced tools and autonomous 

inspections

▪ Mechanical personnel need the 

time and support to start chipping 

away at the problem

– It is not an issue that can be 

fixed in a day

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▪ There is opportunity to work with 

suppliers to optimize air usage of 

components

– Air Dryers

– Mag Valves

– Anything that uses air should be 

on the table.
Conclusions

▪ Air leaks are a significant contributor of greenhouse gas and 

criteria emissions for railroads

▪ The issue needs to be addressed and mechanical will need time 

and resources

▪ ROI of fixing air leaks is favorable due to reduction in fuel 

consumption

▪ Air leaks and other vehicle efficiency improvements are 

independent of fuel source

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Advanced science. Applied technology.