Diesel Exhaust Fluid (DEF) Faqs


Q. What is DEF?

DEF is a carefully blended solution of high-purity urea and de-ionized water. Urea is a nitrogen-containing compound used in a variety of industries, often as a fertilizer in agriculture. When heated, it decomposes into ammonia (NH3) and carbon dioxide (CO2).

Q. What concentration of DEF should I use?

In North America, the most common DEF concentration is 32.5% urea and 67.5% de-ionized water, which offers freeze protection to 11.3°F (-11.5°C). A concentration of 40% urea and 60% de-ionized water is also available and widely used in the marine industry, mostly in Europe. It begins to crystallize at 32°F (0°C).

You can use either concentration in our Tier 4 Final marine engine system. One benefit of using the higher concentration is that you’ll need less DEF storage space on your vessel. However, you’ll want to weigh the availability of one product versus the other, as well as your freeze/crystallization protection requirements, to make a decision.

Q. How much DEF does the Caterpillar SCR system consume?

Total DEF consumption depends on many factors, including engine application, duty cycle, run hours, ratings and DEF concentration. We originally calculated DEF consumption at 3-8% of diesel fuel consumption. However, customers using our engines have reported that DEF consumption is lower than estimated, and in most cases, the cost of DEF is more than offset by the reduction in fuel consumption.

To determine exactly how much DEF your engines will consume, work directly with your local Cat® Marine dealer. Your dealer will also help you determine how much DEF storage you’ll need based on refueling practices and schedules. A good rule of thumb is to install a DEF tank that’s 10% the size of your fuel tank.

Q. Won’t adding DEF raise my costs?

Actually, total cost of ownership is less with our Tier 4 Final marine engine solution due to reduced fluid (diesel fuel plus DEF) consumption. Because our engines use an aftertreatment system to meet marine emissions regulations, they’re optimized for fuel burn, which results in lower total fluid consumption.

Q. Do your marine engine Tier 4 Final products constantly consume DEF when the engine is running?

No. Our SCR system only injects the required DEF above certain exhaust temperatures and certain engine speeds and loads. DEF injection timing is a direct result of each engine’s installation and each vessel’s specific duty cycle or operating load profile. That’s how we’re able to minimize total fluid consumption.

Q. What is air-assisted DEF injection?

Certain engine and aftertreatment configurations in large ducts require a larger spray pattern for uniform mixing of urea and exhaust gases. Air-assisted DEF injection adds compressed air to the DEF injector to increase atomization of DEF. It also helps decrease total fluid (diesel plus DEF) consumption.

Not all SCR systems require air assistance, but Cat C32, 3500E and C280 Tier 4 Final engines do use this technology. It offers increased performance at a lower operating cost for high-power applications.

Q. What happens if I run out of DEF or the SCR system stops dosing during operation?

There will be no loss of engine power. The U.S. EPA recognizes that it’s unsafe and unacceptable to de-rate or shut down a marine engine as a result of an aftertreatment failure. If a lack of dosing during operation occurs, that event will be logged in your engine’s Electronic Control Module (ECM). You will have up to 30 days to inform the EPA of the reason. (This guidance is outlined in EPA Rule 40 CFR Part 1042.)

Q. How do I make sure I’m using quality DEF?

To ensure proper purity and concentration of urea, only purchase DEF that meets ISO 22241-1 standards.

Q. Where can I get DEF?

Most diesel fuel suppliers carry DEF, which means you can fill up with fuel and DEF simultaneously to save time.

Q. Can I make DEF myself?

No. Using any fluid that doesn’t meet requirements outlined in ISO 22241-1 may damage your SCR system.

Q. Does DEF have special storage requirements?

Yes. Always store DEF in a cool, dry, well-ventilated area. The ideal storage temperature is 15°F to 77°F (-9°C to 25°C). Do not expose DEF to direct sunlight as the urea will decompose.

Q. How long can I store DEF?

DEF shelf life depends on storage temperature. It will degrade over time if exposed to high temperatures and sunlight. ISO 22241-1 defines minimum shelf-life expectations when DEF is stored at constant temperatures — for example, one year for temperatures below 86°F (30°C) and approximately 18 months below 77°F (25°C).