Selective Catalytic Reduction (SCR) Fundamentals
Q. What are the Tier 4 Final emission standards and how do they
compare to Tier 4 Interim?
A. U.S. EPA’s Tier 4 standards require that emissions of particulate matter (PM)
and oxides of nitrogen (NOx) be further reduced. Tier 4 Interim regulations
began in 2011 for most power categories and Tier 4 Final is phased in beginning
in 2014. Tier 4 Interim was intended to enable a gradual phase in to Tier 4 Final
emission standards for manufacturers. Tier 4 Interim required a 50% reduction in
NOx and 90% reduction in PM over Tier 3. Tier 4 Final emission standards
required an additional 80% reduction in NOx.
Q. What is Selective Catalytic Reduction (SCR)?
A. SCR is a technology that reduces the amount of the oxides of nitrogen (NOx ) to
meet emission standards. The system consists of a catalyst that reacts with a urea
based diesel exhaust fluid (DEF) to significantly reduce oxides of nitrogen (NOx)
emissions.
Q. How does an SCR system work?
A. The purpose of the SCR system is to reduce levels of NOx (oxides of nitrogen
emitted from engines). When DEF is injected into the hot exhaust gas, it vaporizes and
decomposes to form ammonia and carbon dioxide. The NOx reacts with the ammonia
over the catalyst to form nitrogen and water vapor. Any ammonia slip is reduced by
using an Oxidation Catalyst or AMOX catalyst, which produce NO or N2, respectively
Diesel Exhaust Fluid (DEF) Properties
Q. What is Diesel Exhaust Fluid (DEF)?
A. DEF is a carefully blended solution of 32.5% high purity urea and 67.5% deionized
water. It may also be referred to as AdBlue or AUS32. DEF is a safe and inexpensive
way to store ammonia. Refer to the Manufacturer’s Safety Data Sheet (MSDS) for
warnings and handling procedures.
Q. What is urea?
A. Urea is a nitrogen containing compound used in a variety of industries, often as a
fertilizer in agriculture. When heated, urea decomposes into ammonia (NH3) and carbon
dioxide (CO2).
Q. Is the 32.5% urea solution critical?
A. Yes, The 32.5% urea concentration is the ideal solution as it provides the lowest
freeze point and the best protection in cold ambient temperatures. In addition, SCR
systems will be calibrated to operate on 32.5% urea solution, so that optimum NOx will
be converted to nitrogen during operation.
Q. What is the freeze point of DEF?
A. DEF with a concentration of 32.5% urea will begin to crystallize and freeze at 11.3 °F
(-11.5 °C). At 32.5%, both the urea and water freeze at the same rate, ensuring that as
it thaws, and the fluid does not become diluted, or over concentrated. Freezing and
unthawing of DEF will not cause degradation of the product.
Q. How long does it take to thaw DEF?
A. The EPA requires that SCR systems be fully functional and that the minimum time to
thaw enough urea to start dosing is 70 minutes.
Q. Does DEF expand when frozen?
A. Yes, DEF expands by approximately 7% when frozen. DEF packaging and tanks are
designed to allow for expansion.
Q. How do I keep the DEF from freezing? What happens if DEF freezes
while in the vehicle DEF tank?
A. SCR systems are designed to provide heating for the DEF tank and supply lines
while the vehicle is being operated. Normal operation of the vehicle will not be inhibited
if DEF freezes when the vehicle is shut down. The SCR heating system is designed to
quickly thaw the DEF solution back to liquid form, ensuring that the operation of the
vehicle will not be impacted. The freezing and unthawing of DEF does not degrade the
solution nor alter the concentration.
Q. Can an anti-gelling additive or freeze point depressant be added to
the DEF to prevent it from freezing?
A. No. While an additive would potentially improve the freeze point of the mixture, the
32.5% solution is very specific to ensure maximum NOx reducing properties. Any further
blending, adjusting, or diluting of the DEF mixture will not allow it to perform properly
and may cause damage to the SCR components. No additives are approved for use in
DEF today.
Q. How much does DEF weigh?
A. DEF solution is slightly heavier than pure water and weighs approximately 9 pounds
per gallon.
Q. Does DEF smell?
A. DEF odors may have a slightly pungent odor similar to that of ammonia. Refer to the
Manufacturer’s Safety Data Sheet (MSDS) for additional information.
Q. How will I know if I am getting quality DEF?
A. To ensure that you are getting quality DEF you should purchase product that both
states and displays a certification from the American Petroleum Institute (API), the
International Organization for Standardization ISO 22241-1, and meet AUS – 32
specifications. This will ensure the proper purity and concentration (32.5%) of urea. For
more information on these specifications, visit www.iso.org.
Q. Can I make DEF myself?
A. Caterpillar does not recommend that customers make DEF themselves. DEF has
strict requirements for maintaining concentration and purity of all of the ingredients
critical to the proper functioning and longevity of your SCR system. DEF used in
Caterpillar SCR systems must meet all ISO2224-1 specifications. If the product is
labeled with the API certification you may be sure your DEF meets the quality and purity
defined in ISO22241-1. Caterpillar recommends that end users purchase certified DEF
and avoid blending it themselves. For more information about the quality standards,
refer to ISO22241-1 detailing DEF specifications including, quality, handling, testing,
transportation, storage, and refilling.
Q. What is API Certification?
A. API Certification is a voluntary program established by the American Petroleum
Institute. Beginning in March 2009, the API began certifying and monitoring aftermarket
diesel exhaust fluid to ensure that it meets ISO22241-1 specifications.
Safety and Handling
Q. Does DEF have any special storage requirements?
A. DEF should be stored in a cool, dry, well-ventilated area. It should never be exposed
to direct sunlight because the urea will decompose. While the optimum storage
temperature is up to 77 °F (25 °C), temporary exposure to higher or lower temperatures
has no impact on the quality of DEF. For additional information refer to ISO22241-3.
Q. How long can DEF be stored on the shelf?
A. The shelf life of DEF depends upon the storage temperature. DEF will degrade over
time depending upon how long it is exposed to high temperatures and sun light. ISO
22241-3 defines the minimum shelf life expectations when stored at constant
temperatures and notes that if DEF is stored below 86 F (30 C), shelf life will be one
year, below 77 F (25 C), shelf life will be 18 months. Refer to the specification for
further information.
Q. Should I be concerned about handling DEF?
A. Refer to Manufacturer’s Safety Data Sheet (MSDS) for warnings and handling
procedures.
Q. What should I do if DEF is spilled?
A. General - If DEF is spilled, refer to Manufacturer’s Safety Data Sheet (MSDS) for
warnings and handling procedures.
All Spills - For all spills you will need to first refer to Manufacturer’s Safety Data Sheet
(MSDS) for warnings and handling procedures then contact your appropriate
Environmental Health & Safety personnel.
On Machine – If DEF comes in contact with painted surfaces, it will not harm the paint.
Refer to Manufacturer’s Safety Data Sheet (MSDS) for warnings and handling
procedures.
Q. What happens if I ingest DEF?
A. DEF should never be ingested. If it is ingested, call 911 or the Rocky Mountain
Poison Control Center at (800) 458-5924 in the US and Canada or (303) 893-1322 for
the rest of the world. Refer to Manufacturer’s Safety Data Sheet (MSDS) for warnings
and handling procedures.
Q. What happens if I inhale DEF?
A. Normal usage of DEF will not cause significant exposure, however inhalation may
occur if DEF is dispensed in a closed area and/or if DEF is misted into the air. If it is
inhaled, call 911 or the Rocky Mountain Poison Control Center at (800) 458-5924 in the
US and Canada or (303) 893-1322 for the rest of the world. Refer to Manufacturer’s
Safety Data Sheet (MSDS) for warnings and handling procedures.
Q. Is DEF corrosive?
A. DEF is corrosive to several metals, alloys, and a few plastics. Tank, packaging and
dispensing equipment suppliers take all this into account and only use approved
materials for their products. DEF tanks (on vehicle) will be made from a non-corroding,
highly durable composite or plastic material. Approved materials, such as high density
polyethylene (HDPE), should be used for DEF storage, in the DEF tank, and equipment
used to dispense DEF. A brief, non-exhaustive compatibility table is given below:
Compatible
- Highly alloyed austinetic Cr-Ni Steels
- Stainless Steels
- Titanium
- Hastelloy c/c 276
- Polyethylene, Polypropylene,
Polyisobutylene
- Chlorinated polyvinyl chloride
- Fluorine containing polymers
- Viton
Incompatible
- Carbon Steels
- Non-ferrous metals and alloys
(Copper, Copper alloys, Zinc)
- Aluminum and Aluminum alloys
- Magnesium and magnesium alloys
- Solders containing lead, silver, zinc,
or copper
- Plastics or metals coated with
nickel or zinc
Q. Does DEF evaporate and what happens if it does?
A. Because DEF is comprised of 67.5% water, evaporation of the water will occur over
time, concentrating the solution above 32.5%. It is important to keep the cap of the DEF
tank and storage containers securely closed. If the urea concentration does become
more or less than recommended over time, the DEF tank should be drained. When
water completely evaporates from the solution, DEF crystallizes.
DEF Usage
Q. What safety measures have been put in place to prevent diesel fuel
from being pumped into the DEF tank?
A. The nozzle diameter for dispensing DEF has been designed to be standardized at
19mm versus the standard diesel fuel nozzle diameter of 22mm. In addition, the tank
cap for the DEF tank will be blue in color to differentiate the DEF storage tank from the
diesel tank.
Q. What happens if a non-DEF substance is accidentally entered into
the DEF tank?
A. The SCR system is equipped with a quality detection system that will recognize
solutions other than DEF. If a non-DEF substance is found in the DEF tank, a
diagnostic code will become active and inducement strategies will be applied.
Depending on the level of contamination in the tank, it may be required to drain the tank
and refill with fresh DEF.
Q. What is an inducement strategy?
A. An inducement is an EPA required fault or event used to detect inadequate engine
and emissions performance. Faults include interruptions to DEF dosing, poor DEF
quality, and low DEF levels. Inducements will appear as a series of warning lights,
messages, audible alarms, engine derates and shutdowns designed to warn the
operator of a system failure.
Q. How often will I need to refill with DEF?
A. The EPA has provided guidance for on-highway diesel applications. The ratio of
diesel fuel to DEF should be between 2:1 and 3:1. Therefore, for approximately every
two tanks of diesel fuel used, you’ll need to fill with one tank of DEF. This is dependent
upon the size of the DEF tank your machine is equipped with.
Q. How large is the DEF tank on my machine?
A. Several tank sizes will be required based on machine application and duty cycle.
The specific size also ensures that the EPA guidance for a diesel to DEF fill ratio of 2:1
is met. Generally speaking, the size of the DEF tank is about 10% the size of the diesel
fuel tank.
Q. How much DEF will a vehicle consume?
A. Depending on vehicle operation, duty cycle, geography, load ratings, etc., DEF
consumption should be about 2 to 3% of the fuel consumption.
Q. What is the number of hours (miles) a vehicle can expect to
operate (travel) on one gallon of DEF?
A. DEF consumption is directly related to fuel consumption. The actual amount of DEF
consumed will depend upon operation, duty cycle, load ratings, etc. For example, an offhighway vehicle may use one gallon of DEF for every hour of operation and an onhighway truck averaging 6 MPG can expect to go approximately 300 miles on one
gallon of DEF.
Q. Can an operator determine how much DEF they will use?
A. The consumption of DEF will be approximately 2 to 3% of the diesel fuel consumed.
This equates to approximately a 50 to 1 ratio with diesel fuel. Therefore, for every 50
gallons of diesel fuel burned, approximately 1 gallon of DEF will be used. If you know
the average fuel consumption of a vehicle, you can easily calculate the amount of DEF
that will be used.
Q. What happens if the vehicle runs out of DEF?
A. Vehicles that use DEF will have an indicator like a fuel gauge to alert the driver of the
quantity of DEF on board. There should be a DEF low level warning lamp that will
illuminate when DEF is low. When the vehicle is low on DEF, power delivered by the
engine will be derated until the operator refills the DEF tank. The engine will return to
normal power levels once the tank has been refilled.
Q. Is any new maintenance required on the engine or SCR system?
A. A filter will be included in the DEF pump unit to ensure proper filtration of impurities
such as dirt and debris. The EPA requires a minimum service or replacement interval of
this filter to be no more frequent than every 4,500 hours; however, the Cat service
interval will be 5,000 hours.
Q. Will there be any special fuel or oil requirements for SCR equipped
vehicles?
A. Fuel and oil requirements will be the same as Tier 4 Interim. Ultra low sulfur diesel
(ULSD) and low ash oil (API CJ-4) will be required for use in SCR equipped machines.
Q. Will there be any difference in lube or fuel filter service intervals on
SCR equipped vehicles?
A. There are no changes to the lube and fuel filter service intervals on SCR equipped
vehicles.
DEF Product Options & Availability
Q. What package sizes will be available?
A. Caterpillar offers DEF in 2.5 gallon bottles for single use applications and in semibulk quantities (264 gallon tote) for multiple uses.
Q. Am I required to use Cat DEF?
A. No, any commercial DEF that meets the ISO specification is allowed for use in
Caterpillar SCR equipped vehicles. Any DEF that is API certified provides assurance
that the product meets the ISO22241 specification.
Q. Can I fill my own bottles of DEF?
A. Caterpillar does not recommend refilling or reusing bottles of DEF. There are strict
requirements for maintaining DEF purity (free of contamination). If the customer
requires reuse or refilling, refer to the ISO 22241-1 specification which outlines the
information regarding DEF quality, handling, testing, transportation storage, and refilling.
Q. Where will DEF be available?
A. DEF can be purchased at common retail locations similar to that of fuel and oil,
including Cat authorized dealers, retail locations such as on-highway truck fueling
stations and automotive parts stores, and bulk delivery from DEF producers or fuel
distributors. For more information regarding availability of DEF, visit
http://www.discoverdef.com/home.aspx in the US or http://www.findadblue.com/ in
Europe.