Feasibility Considerations: AO Spark-Ignited Engines

Over the past decade, AOs have invested significant capital to retrofit and replace 

thousands of irrigation pump and other engines reducing emissions by over 80% in this 

category, and continue to do so as emission limitations and associated compliance 

deadlines materialize under Rule 4702. In addition to the high cost-effectiveness and 

potential technical infeasibility associated with retrofitting or replacing existing AO sparkignited engines, requiring additional costly controls on existing AO engines is 

economically challenging and potentially infeasible.

Retrofitting existing spark-ignited engines poses several challenges that are not present 

when installing new, replacement engines. The District had to overcome many 

obstacles and challenges in retrofitting existing AO engines when the District adopted 

its current emission limit of 90 ppm and has worked closely with AO engine owners and 

operators and control system manufacturers to ensure compliance with this stringent 

emission limit. Efforts to ensure compliance with the current rule limit are continuing 

today. Lowering the emission limit from 90 ppmv to 25 ppm or even 11 ppm, results in 

even greater challenges for existing engines to consistently meet because of the much 

lower tolerance for being out of compliance. These challenges are outlined in the 

following list. Details are provided below.

Challenges with retrofitting existing engines:

1. Engine power losses from adding controls

2. Existing engines may require overhaul

3. Existing engines cannot meet lower emissions levels due to narrower margin of 

compliance

4. Control systems must be custom designed

5. Errors generated during control system installation

6. Retrofit controls can damage an engine

7. Engine can damage a control system

8. Compliance costs

9. Engines operated in remote locations

1. Engine power losses from adding controls

An engine is chosen based on its ability to provide the required power output at a 

reasonable engine speed (rpm) that will not over-stress the engine over its expected 

service life. Add-on emission control systems result in additional loads that the 

engine may not have been originally designed to accommodate.
In addition, due to the extreme drought conditions, engine owners and operators 

have needed to increase the power output for well pump engines as the water table 

has dropped. As the engines work harder to pump water, there is less power output 

available to accommodate emission control systems.

2. Existing engines may require overhaul

The engines in use at AOs have been in service for many years, even decades, and 

are heavily worn. A worn engine can burn oil, leak fluids, and run rough.

For an uncontrolled engine, some of the effects of engine wear do not have a major 

effect on the engine’s ability to do it’s job (e.g. pumping water). However, the 

operation of a catalytic emission control system requires that the engine be operated 

consistently smooth. An expensive major engine overhaul or rebuild would be 

necessary to ensure smooth engine operation prior to installing a catalytic emission 

control system. Many AOs do not have the resources (e.g., staff, experience, 

technical training, etc.) to complete an engine overhaul or rebuild without outside 

assistance. Meeting more stringent/lower emission standards increases the need 

for the engine to operate properly.

3. Existing engines cannot meet lower emissions levels due to narrower 

margin of compliance

As emission limits are lowered, there is a narrower margin of compliance and proper 

engine operation becomes more critical. AOs in the District have to constantly 

ensure that their engine is properly maintained and within all the appropriate 

specifications to ensure compliance with the current emission limit, more so than 

newer engines. The lower emissions levels will result in additional stresses on the 

engine and increased maintenance and monitoring efforts that result from operating 

a retrofitted engine. Even then, due to the age of the engine and based on engine 

not appropriately designed for additional add-on systems and the associated loads, 

engines will not be able to meet the lower limits. 

4. Control systems must be custom designed

For proper control system design, the engine condition, make, model, power output,

and exhaust gas flow rate and temperature must be considered. There are no 

universal, off-the-shelf, one size-fits-all systems available for purchase.
Control system design also assumes that an engine is operating properly and 

smoothly per the engine manufacturer’s specifications. To ensure proper operation 

of the control system, an engine may need to be overhauled or rebuilt prior to 

installation of the control system.

A common problem with many retrofit emission control systems is installation of a 

system on an engine that is not operating smoothly or to engine manufacturer 

specifications. Installing a control system on a rough running engine will result in 

poor control system operation and eventually system and engine damage. Proper 

system design and engine operation is more important as emission limits are 

lowered since the margin of compliance will be much less.

5. Errors generated during control system installation

Site conditions like gas supply pressure can cause an existing engine to operate 

rough. If site issues are not addressed prior to installation of a control system, the 

control system will not operate correctly.

An installer may attempt to correct rough engine operation by making the 

combustion more fuel rich; however, this technique will not provide lasting results 

and will cause accelerated engine and control system wear and eventually failure. 

An emission control system that is designed to meet lower emission limits will 

require a larger catalyst element which will be more expensive to replace if 

permanently damaged.

6. Retrofit controls can damage an engine

For proper control of exhaust pollutants, a catalyst must be operated at a certain 

temperature range that is higher than normal exhaust temperatures. Additional fuel 

is often injected into the engine with the intent that the additional fuel will pass 

through the combustion chamber and ignite in the exhaust system prior to the 

catalyst (the high catalyst temperature ignites the fuel). This extra fuel results in 

higher engine operating temperatures since some of the extra fuel is combusted 

during normal engine combustion. The increased engine temperature leads to 

accelerated engine wear and reduced engine reliability. Due to wear and older 

design, increased combustion temperatures lead to engine failure and permanent 

engine damage.
7. Engine can damage a control system

An existing, worn engine can burn oil and run rough. Oil in the exhaust stream will 

foul/mask a catalyst which will result in reduced emission control efficiency and likely 

permanent damage to a catalyst element. The air-fuel ratio controller will attempt to 

adjust engine operation (e.g., injecting more fuel) to keep the control system 

operating within the specified parameters; however, adjusting engine operation will 

not correct a fouled catalyst. Continued operation with a damaged catalyst will lead 

to permanent catalyst damage. An emission control system that is designed to meet 

lower emission limits will require a larger catalyst element which will be more 

expensive to replace if permanently damaged and this cycle will be repeated further 

adding to the cost.

8. Compliance costs

Unlike many industries, AOs compete on an international basis and cannot pass 

increased production costs on to consumers. AOs must absorb the compliance 

costs associated with lower emission standards, for example: retrofit and 

replacement costs; additional maintenance costs; additional monitoring costs; and 

additional testing costs.

These additional regulatory costs put them at an economic disadvantage to their 

competitors.

9. Engines operated in remote locations

AO spark-ignited engines are generally located in rural, hard to access areas with 

minimal oversight since AOs have limited resources and staffing. With seasonal 

labor and minimal year-round staffing, it is difficult for AOs to provide the frequent 

and complex maintenance required for retrofitted or new engines equipped with 

advanced emission controls. Lower emission limits are achieved only through well 

maintained engines and control systems. Lower emissions limits lead to increased 

maintenance and monitoring efforts.

The oil production industry is the only other major industry in the Valley that has IC 

engines located in remote locations; however, with the highly technical nature of oil 

production and refining as compared to agricultural production and additional 

economic resources, it is feasible for the oil and gas production industry to hire 

qualified staff dedicated to maintaining and operating IC engines and other 

equipment on-site.
Retrofitting AO engines with emission control systems to meet increasingly stringent 

emission limits poses unique challenges that are not applicable when installing 

replacement engines. Based on the challenges outlined above, meeting 25 ppm or even 

11 ppm with existing AO engines is not practicable. The additional maintenance, 

monitoring, and testing, along with the cost of rebuilding engines and the cost of the 

emission control system, may even be more costly than installing a replacement engine.