What is SCR on a diesel engine? Selective Catalytic Reduction (SCR) is an emissions-control technology used in modern diesel vehicles to reduce harmful nitrogen oxide (NOx) emissions. By treating exhaust gases after combustion, SCR helps heavy-duty trucks and other diesel-powered equipment meet emissions requirements while continuing to operate efficiently.
If you manage a commercial fleet, maintain heavy equipment, or operate diesel-powered machinery, understanding how SCR works can help you recognize potential problems and avoid unnecessary downtime.
What is Selective Catalytic Reduction (SCR)?
Selective Catalytic Reduction is an exhaust aftertreatment technology that reduces nitrogen oxide (NOx) emissions from diesel engines.
NOx forms during high-temperature combustion and contributes to air pollution. SCR systems use a chemical reaction to convert much of that NOx into nitrogen and water vapor.
When operating under suitable conditions, SCR technology can achieve NOx reductions of 90% or more.
How Does an SCR System Work?
An SCR system uses diesel exhaust fluid (DEF) and a specialized catalyst to treat exhaust gases after they leave the engine.
Here's the process:
- DEF is stored separately. Diesel exhaust fluid consists of approximately 32.5% high-purity urea and 67.5% deionized water.
- Heat produces ammonia. The urea breaks down through chemical reactions that produce ammonia.
- DEF enters the exhaust stream. A dosing system injects a controlled amount of fluid into the hot exhaust before the SCR catalyst.
- The catalyst reduces NOx. Ammonia reacts with nitrogen oxides, converting them primarily into nitrogen and water vapor.
- Treated exhaust exits the system. The process significantly reduces harmful NOx emissions.
Importantly, DEF is never supposed to be mixed with diesel fuel or engine oil. It operates in a separate exhaust aftertreatment system.
What Is SCR on a Diesel Engine and How Does It Work?
SCR technology is common in modern diesel-powered equipment, including:
For fleet and equipment managers, SCR is another important system to consider when planning preventive maintenance.
What Happens When an SCR System Has a Problem?
SCR systems depend on proper operation, suitable exhaust temperatures, and DEF that meets the manufacturer's specifications.
- Heavy-duty commercial trucks
- Agricultural tractors and harvesting equipment
- Construction machinery
- Diesel pickups and commercial vehicles
- Industrial generators
- Certain marine and stationary engines
Common concerns include low DEF levels, contaminated or poor-quality DEF, malfunctioning sensors, and dosing-system problems.
Depending on the equipment and its software, unresolved SCR faults can trigger dashboard warnings, reduced engine power, or operating restrictions.
The practical takeaway? Don't ignore aftertreatment warnings. Early diagnosis can help prevent an avoidable interruption.
What Does SCR Have to Do With Engine Lubrication?
SCR and engine lubrication perform different jobs, but both contribute to the overall reliability of diesel-powered equipment.
SCR manages exhaust emissions. Engine oil protects internal components against wear, helps control deposits, and performs under demanding operating temperatures.
Modern diesel engines and their emissions systems make it especially important to follow the manufacturer's lubricant specifications and recommended maintenance practices.
Using the correct heavy-duty engine oil can support engine durability and help protect emissions-system compatibility where applicable.
Keeping Diesel Equipment Moving
At Dilmar Oil Company, we understand that equipment reliability goes beyond choosing a lubricant off the shelf.
Our team helps commercial fleets, agricultural operations, and industrial customers select lubricants suited to their equipment, operating conditions, and maintenance requirements.
With nine Certified Lubrication Specialists (CLS) across our team, we bring technical knowledge and practical experience to the challenges our customers face every day.
What Does SCR Mean for Fleet Maintenance?
SCR systems are only one part of maintaining a modern diesel fleet. Proper engine lubrication, regular maintenance, and attention to operating conditions all contribute to equipment reliability. While engine oil does not perform the same function as SCR or diesel exhaust fluid (DEF), selecting the correct lubricant and monitoring its condition can help protect engines and reduce avoidable downtime.
For more information on technologies used to reduce diesel emissions, visit the U.S. Environmental Protection Agency's resources on diesel emissions controls.
Have questions about heavy-duty engine oils or lubricant selection for your fleet?
Talk with the Dilmar team about finding the right lubrication solutions for your operation.