Partner Reported Opportunities (PROs) PRO Fact Sheet No. 104 for Reducing Methane Emissions 

Install Automated Air/Fuel Ratio 

Controls 

Technology/Practice Overview 

Description 

Natural gas-fueled internal combustion

engines can provide continuous duty 

operations over a set range of air to fuel

ratios (AFR). Low air to fuel mixtures 

(rich burn) are used when greater

horsepower is needed; high air to fuel

mixtures (lean burn) are used when

lower horsepower and greater fuel 

efficiency are the goals. Rich conditions

result in more unburned fuel emissions 

(primarily methane), more CO 

emissions, and fewer NOx emissions. 

Lean conditions produce lower methane 

and CO emissions, but more NOx 

emissions. Manufacturer performance 

curves for standard rich burn engines 

indicate that when AFRs exceed a value 

of 18:1 (the stoichiometric AFR is 

approximately 16:1), temperature, power 

and NOx emissions start to decrease. 

Such engines, equipped with 

conventional controls that continuously 

monitor oxygen levels in the exhaust gas 

with a single sensor, cannot operate for

extended periods with AFRs in excess of

20:1. Typically, specially configured 

lean burn engines (AFRs 20:1 to 50:1)

with turbo chargers or pre-combustion 

chambers are used where low NOx 

emissions is the goal. 

Natural Gas STAR Partners have 

achieved significant fuel savings, and 

reduced associated emissions, by

installing automated AFR control 

systems to automatically adjust and

optimize the operating parameters for 

their natural gas-fired internal 

combustion engines. Such parameters 

Estimated 

Gas Price 

Annual 

Fuel Gas 

Savings 

Value of Annual 

Fuel Gas Savings 

Estimated 

Implementation Cost 

Payback1 

$7.00/Mcf 28,470 Mcf $199,290 $138,000 8 Months 

$5.00/Mcf 28,470 Mcf $142,350 $138,000 11.6 Months 

$3.00/Mcf 28,470 Mcf $85,410 $138,000 1.6 Years 

1 The payback is calculated based on the value of annual fuel gas savings and the estimated implementation cost. The value 

of the methane savings is assumed to be included in the value of annual fuel gas savings due to the fact that the methane 

savings are a direct result of avoided fuel consumption. 

Annual 

Methane 

Savings 

913—12,175 Mcf 

913—12,175 Mcf 

913—12,175 Mcf 

Economic and Environmental Benefits 

Economic Evaluation (based on fuel gas savings per engine installation) 

 Reduced engine fuel consumption 

 Lower maintenance costs 

 Reduced emissions of CO2 and criteria air 

pollutants 

 Improved engine reliability and safety 

Additional Benefits: 

Estimated annual methane emission reductions 913—12,175 Mcf per engine 

Estimated fuel gas savings 78 Mcf per engine per day, or 28,470 Mcf per year 

Methane and Fuel Gas Savings 

Compressors/Engines 

Dehydrators 

Directed Inspection & 

Maintenance 

Pipelines 

Pneumatics/Controls 

Tanks 

Valves 

Wells 

Other 

Applicable Sector(s) 

Production 

Processing 

Transmission 

Distribution 

Other Related Documents: 

Reduce Natural Gas Venting 

from Fewer Compressor Engine 

Startups & Improve Engine 

Ignition, PRO No. 102 

Convert Engine Starting to Air 

or Nitrogen, PRO No. 101 
 

 

PRO Fact Sheet No. 104 Continued 

Install Automated Air/Fuel Ratio Controls (Cont’d) 

include air manifold pressure and temperature, and fuel 

delivery to the combustion chambers. Alarm or 

shutdown triggers can be set to react to various real-time 

engine operating parameters, which reduces the risk of

catastrophic engine failure. Other benefits from 

optimizing engine operation with AFR control systems 

include fewer false starts, lower combustion 

temperature, longer engine life, lower maintenance and 

fuel costs, and greater safety. Also, according to one 

vendor, using gas from an area with fugitive 

hydrocarbon emissions could lead to additional fuel 

savings as the AFR control system adjusts the fuel 

intake to account for the additional hydrocarbons in the 

air intake. 

Operating Requirements 

AFR control technology can be configured as a standalone system, or to interface with most existing 

electronic control and telemetry systems. 

Applicability 

Any natural gas fired engine over 1,000 horsepower can 

benefit from AFR control and optimization systems. 

Partners report that the greatest opportunities for 

significant system and efficiency improvements are for

rich burn, high-speed, turbocharged engines ranging in 

size from 1,000 hp to 3,000 hp. 

One Partner reduced fuel consumption and methane 

emissions by installing an automated AFR control 

system called REMVue, marketed by REM Technology

Inc. REMVue controllers monitor several engine 

parameters and optimize air-fuel mixtures through a 

combination of electronic controls, use of a high-energy

long duration spark to ensure reliable ignition, and other 

mechanical modifications to the engine. Another 

Partner purchased and installed new compressor units 

of 1340 hp and larger that are equipped with Caterpillar

ADEM 3 AFR control systems. 

Methane Emissions 

Unburned fuel from the exhaust stream is a source of 

methane emissions from natural gas-fueled internal 

combustion engines. Such emissions are generally 

estimated using commonly accepted emission factors 

based on engine type, size, and operating conditions. 

The Natural Gas STAR Program typically uses 0.24 scf

methane per horsepower-hour, based on Volume 11 of 

the 1996 EPA/GRI Study. 

The average methane content of natural gas varies by natural gas 

industry sector. The Natural Gas STAR Program assumes the 

following methane content of natural gas when estimating methane savings for Partner Reported Opportunities. 

Production 79 % 

Processing 87 % 

Transmission and Distribution 94 % 

Methane Content of Natural Gas 

Partners report annual methane emission reductions of 

913 Mcf and 12,175 Mcf per unit after installing AFR 

control systems for compressors. Another Partner 

reduced fuel consumption by 18 to 24 percent after

installing automated AFR controls on 51 new compressor

engines in its Gulf of Mexico operations. By reducing the 

engines’ fuel consumption, the Partner estimates 

average annual methane emissions reductions of 128 Mcf 

per unit based on AP-42 emission factors1. For the 51 

engines, the total estimated annual emissions reductions 

are estimated as 6,528 Mcf. 

Economic Analysis 

Basis for Costs and Savings 

The Partner-reported REMVue installation reduced total 

fuel consumption for 51 engines by 2,900 million cubic 

feet (MMcf) during a two-year period. Average fuel gas 

savings were 78 Mcf per day per engine when adjusted

for load (28,470 Mcf per year per engine). This 

represents a 39 percent increase in estimated fuel 

savings (based upon a sample inventory, which yielded a 

pre-job fuel savings estimate of 56 Mcf per day). The 

total reported implementation cost for the 51 engines 

was $7.04 million. Capital costs to install an AFR control 

system on existing engines ranged from $98,000 to

$161,000 per unit, with the average cost being $138,000

per installation. At a nominal value of $5 per Mcf, the 

total reported fuel savings for 51 engines was more than 

$7.25 million, yielding a calculated payback of 11.6 

months. 

Discussion 

The economics are based solely on the value of the 

avoided fuel losses and the capital cost to install an AFR

control system on 51 engines. The value of other 

reported benefits including fewer misfires, easier engine

starting, more reliable operation, and longer engine life 

are not captured in this report. The Partner found that 

2 
 

 

PRO Fact Sheet No. 104 Continued 

Install Automated Air/Fuel Ratio Controls (Cont’d) 

the additional cost of operating the REMVue systems

was offset by reduced engine maintenance costs. A postaudit conducted on 20 percent of the installed base

reviewed pre-, post- and post-post-values for fuel 

consumption, emissions reductions, availability, and 

economics based on a normalized gas price. The study

results show that while NOx emissions were unchanged,

significant improvements in air quality management

were achieved since unburned hydrocarbons were down 

3,549 tons per year (TPY), CO2 emissions were down 

2,309 TPY, and CO emissions were down 83,300 TPY. 

1

‘AP 42’ refers to publication AP 42, Compilation of Air Pollutant Emission Factors, Volume 1:
 

Stationary , Point and Area Source, available online from the U.S. EPA Technology Transfer 

Network Clearinghouse for Inventories & Emissions Factors (www.epa.gov/ttn/chief/ap42/
 

index.html) 

2 All costs presented are in 2010 dollars. 

3 

2011