Caterpillar SCR white paper: Airless vs. air-assisted dosing


Caterpillar has designed and optimized our U.S. EPA Tier 4 Final and IMO Tier III Marine solutions with selective catalytic reduction (SCR) technology. This solution enables Caterpillar to deliver our critical customer requirements, such as the proven reliability and durability of our engines, fully integrated solutions, and lowest total cost of ownership, all while ensuring regulatory compliance. While all Cat Marine solutions leverage SCR technology, engine platforms differ in how the Diesel Exhaust Fluid (DEF) is delivered to the Clean Emissions Module (CEM). This discussion will focus on the DEF injection aspect of the SCR system, while discussing the reasons Caterpillar has selected different injection technologies for our various engine platforms.

Technology selection

Using extensive analysis, testing, and research, Caterpillar applies the optimal DEF injection technology to each engine platform. There are several key factors to consider, such as:

• Amount of DEF required to reduce NOx
• System performance and integration

Caterpillar has found that an airless injection system is appropriate for our marine engines with total displacement less than 18L, and air-assisted injection is optimal for engines above 18L. When matched with a properly designed mixing system, both types of DEF injection can provide a robust solution. Let’s explore the key factors that drive those solutions.

DEF flow rate

Airless injection systems are applied to smaller engines that require less DEF to meet NOx reduction requirements. As engine exhaust flow rate and NOx output increase, a transition point is reached where an air-assisted system is the most optimal solution. When higher DEF flow rates are required, an air-assisted system can function with a single DEF injector, whereas an airless system may require multiple injectors to deliver adequate DEF supply. In addition, the air-assisted system is capable of much smaller droplet size, which is favorable for robust mixing, compact packaging of the system, and DEF deposit avoidance. Based on industry research, the distribution of droplet diameter doesn’t change with increased DEF injection rates. This means an air-assisted system maintains optimal performance for higher required DEF flow rates.

System performance and integration

Robust control and cooling strategies are essential to maintain DEF injection system performance. To prevent deposit formation or crystallization within the injector and nozzle, Caterpillar provides optimized solutions for both injector types. The air-assisted system utilizes compressed air to serve the following functions:

• Cooling of the injector and nozzle
• Cleaning (purging) of remaining DEF in the system upon engine shutdown
• Smaller nozzle orifice diameter (enables improved spray characteristics)

The airless injection system takes a different approach:

• Engine jacket water coolant cools the injector
• DEF pump is reversed to remove remaining DEF in the system upon engine shutdown
• Larger nozzle orifice diameter to prevent deposits

In addition to performance, Caterpillar has designed both solutions with system integration in mind. For example, neither system requires the use of additional technology, such as throttle valves, to maintain performance when operating at low exhaust temperatures. This ensures less complexity and greater reliability and durability of the entire system. While air-assisted systems will require some integration with the vessel, most commercial marine vessels already incorporate compressed air systems into their design, which minimizes integration challenges.


Caterpillar’s robust development and validation processes have been employed to fully optimize our marine SCR solutions to provide the appropriate DEF injection system for each engine platform. Smaller engines (<=18L) leverage an airless system due to lower required DEF flow rate. Larger engines (> 18L) utilize the air-assisted system, which promotes optimal mixing robustness, reduces DEF deposit risk, and minimizes packaging size and components.  The following table provides a brief review of each solution. Visit to consult our aftertreatment A&I guide, or contact your local Cat dealer for more information.  

By Professional Mariner Staff