UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

NATIONAL VEHICLE AND FUEL EMISSIONS LABORATORY 

2565 PLYMOUTH ROAD 

ANN ARBOR, MICHIGAN 48105-2498

 

 OFFICE OF 

AIR AND RADIATION 

 February 18, 2009 

 CISD-09-04 (HDDE) 

Dear Manufacturer: 

SUBJECT: Certification Requirements for Heavy-Duty Diesel Engines Using 

Selective Catalyst Reduction (SCR) Technologies 

BACKGROUND

On March 27, 2007, EPA issued a SCR guidance document that delineated our 

intended approach for the certification of vehicles and engines using SCR systems 

(CISD-07-07). This document was designed to address the control technology that many 

manufacturers selected to meet stringent requirements for NOx emissions. At the time 

we recognized that SCR technology was evolving and reserved the right to make changes 

to our approach. Today’s guidance is intended to supplement the previous document and 

provides additional details regarding specific areas of concern. Again the Agency 

reserves the right to issue additional guidance or make necessary changes to our approach 

for SCR certification. 

We want to highlight one specific area in this supplemental guidance. In the 

section that addresses tamper resistant design, we outline inducement requirements for 

specific SCR system conditions. As a general rule, certification is based on tamper 

resistant designs. Due to the unique aspects of SCR systems, we believe that additional 

action minimizing the potential adverse environmental risks associated with tampering of 

SCR technology (through the use of warnings and inducements) is justified if such 

systems are not designed to be tamper resistant. The following unique characteristics are 

associated with SCR systems: (i) reduced scheduled maintenance; (ii) the reduced 

maintenance requires replenishing a reductant; (iii) the user has a financial incentive to 

avoid replenishing the reductant; and (iv) the tampering does not require special tools or 

expertise. 
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REGULATORY REQUIREMENTS

DEF TANK SIZE 

For specific vocational applications, a smaller DEF tank size is appropriate (e.g.,

garbage truck, dump truck, concrete mixer, beverage truck, fire truck, airport refueler). 

These applications are generally refueled daily from a central location and conducive to 

refilling DEF in the same manner. The minimum DEF tank size must provide no less 

than: 

Equal the range (in miles or hours) of the vehicle’s fuel capacity (i.e., 1:1) 

For all other vehicle applications, the minimum DEF tank size must provide no less than: 

Twice the range (in miles or hours) of vehicle’s fuel capacity (i.e., 2:1); or, 

Three times the range (in miles or hours) of the vehicle’s fuel capacity (i.e., 3:1), 

if there is no constant DEF level indicator.

VEHICLE COMPLIANCE 

Driver Warning 

Since the driver inducements described below are so severe that they will disable 

the commercial mission of a vehicle, the marketplace requires prior driver warning 

strategies that minimize the likelihood that the inducements deploy. Manufacturers must 

include a warning that provides adequate time to re-fill the DEF. They may use a DEF 

level indicator, messages in the instrument cluster, a DEF indicator, engine shutdown 

lamp, or audible warnings to warn the driver of low DEF levels. Manufacturers must 

include some level of warning escalation. The DEF indicator will contain the following 

symbol: 

 

Driver Inducement 

If indicators and warnings are not successful in causing the driver to replenish the 

DEF tank, a stronger inducement strategy must be initiated. Acceptable strategies 

include those specifically set forth in the EPA’s March 27, 2007, guidance letter or a 

performance degradation strategy that is sufficiently onerous. Each of the following 

strategies is sufficiently onerous to cause the driver to replenish the DEF tank and 

minimize any adverse emission impact. Each strategy (i) must be preceded by a period of 

lower-level inducement (which can last until the DEF tank is empty) or (ii) must begin 

before the DEF level is less than 2.5% tank capacity (collectively, “DEF Trigger”) or (iii) 

for vehicles with 1:1 DEF tank size, must begin when the DEF tank is empty with derate, 

applied between peak torque engine speed and governor break point (see Figure 1), 

ramped in at 1% per minute up to at least a 40% derate. To communicate to the driver 
 3

why the disabling performance degradation has occurred, the DEF indicator will flash or 

a similar message will appear in the instrument cluster (hereinafter, collectively, 

“inducement notification.”) 

1. Disable after Fueling. Once the DEF Trigger is achieved, after the primary 

or secondary fuel tank level rise a measurable amount, the vehicle will be 

limited to 5 mph with inducement notification. 

2. Disable after Parking. Once the DEF Trigger is achieved, after the vehicle 

has been stationary for more than one hour, the vehicle will be limited to 5 

mph with inducement notification. If the vehicle is not stationary for more 

than one hour after 24 hours of operation, the vehicle will be disabled after the 

vehicle has been stationary for a limited period of time (to be defined, e.g., 10 

- 20 minutes). 

3. Disable after Restart. Once the DEF Trigger is achieved, after the engine 

has been shut down, the vehicle will be limited to 5 mph with inducement 

notification.

Identification of Incorrect Reducing Agent

The following stepped inducement strategy for poor quality DEF provides the 

driver enough time to have the vehicle serviced and minimizes any potential adverse 

emission impact: 

1. DEF Warning. Once the SCR system detects poor quality DEF that inhibits 

the SCR system from functioning, a warning lamp will illuminate or message 

will appear in the instrument cluster. 

2. Engine Derate. If the driver fails to remedy the problem within 500 miles or 

10 hours after detection, the maximum available engine torque will be reduced 

25% between peak torque engine speed and governor break point across the 

engine speed range. However, at engine speeds below the peak torque engine 

speed, the maximum available engine torque may be limited to a value not 

exceeding 75% of the peak torque. In addition, at engine speeds higher than 

the governor break point, the maximum available engine torque may be 

limited to a value not exceeding 75% of the maximum torque at the governor 

break-point engine speed (see Figure 2). If, at the time of certification, a 

manufacturer can demonstrate a strategy capable of determining that poor 

quality DEF was not the result of tampering, only step 2 will deploy. 

Alternatively, one of the severe driver inducement strategies specifically set 

forth in the guidance letter (or step 3) may be initiated in lieu of steps 2 and 3. 

3. Severe Inducement. If the driver fails to remedy the problem within 1000 

miles or 20 hours after initial detection, performance is further degraded with 

one of the three severe inducement strategies: 
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• Disable after Fueling. After the primary or secondary fuel tank level rise a 

measurable amount, the vehicle will be limited to 5 mph. 

• Disable after Parking. After the vehicle has been stationary for more than 

one hour, the vehicle will be limited to 5 mph. If the vehicle is not stationary 

for more than one hour after 24 hours of operation, the vehicle will be 

disabled after the vehicle has been stationary for a limited period of time (to 

be defined, e.g., 10 - 20 minutes)

• Disable after Restart. After the engine has been shut down, the vehicle will 

be limited to 5 mph. 

Tamper Resistant Design 

SCR systems may be designed to be tamper resistant consistent with the EPA’s 

March 27, 2007 guidance letter. For all other engines, the actions listed below will 

trigger illumination of a warning lamp, followed by, at a minimum, a mild inducement or 

severe inducement (described below): 

• Disconnected DEF tank level sensor 

• Blocked DEF line or dosing valve 

• Disconnected DEF dosing valve 

• Disconnected DEF pump 

• Disconnected SCR wiring harness 

• Disconnected NOx sensor (that is incorporated with the SCR system) 

• Disconnected DEF quality sensor 

1. Engine Derate. If the driver fails to remedy the problem within 500 miles or 

10 hours after detection, the maximum available engine torque will be reduced 

25% between peak torque engine speed and governor break point across the 

engine speed range. However, at engine speeds below the peak torque engine 

speed, the maximum available engine torque may be limited to a value not 

exceeding 75% of the peak torque. In addition, at engine speeds higher than 

the governor break point, the maximum available engine torque may be 

limited to a value not exceeding 75% of the maximum torque at the governor 

break-point engine speed (see Figure 2). 

2. Severe Inducement. If the driver fails to remedy the problem within 2000 

miles or 40 hours after initial detection, performance is further degraded with 

one of the three severe inducement strategies: 
 5

• Disable after Fueling. After the primary or secondary fuel tank level 

rise a measurable amount, the vehicle will be limited to 5 mph. 

• Disable after Parking. After the vehicle has been stationary for more 

than one hour, the vehicle will be limited to 5 mph. If the vehicle is 

not stationary for more than one hour after 24 hours of operation, the 

vehicle will be disabled after the vehicle has been stationary for a 

limited period of time (to be defined, e.g., 10 - 20 minutes) 

• Disable after Restart. After the engine has been shut down, the 

vehicle will be limited to 5 mph. 

If, at the time of certification, a manufacturer can demonstrate a strategy capable 

of determining that the failure was not the result of tampering, only the warning lamp will 

deploy after 500 miles or 10 hours after initial detection. 

FREEZE PROTECTION 

SCR systems must pass the following Test Procedure for Performance of DEF 

Dosing Freeze Protection Systems: 

1. Prior to Procedure: 

• Temperature: DEF at 20° F (maximum) 

2. Soak Conditions: 

• Temperature: 0° F (maximum) 

• Time: 72 hours or solid DEF (whichever occurs first) 

3. Test Duty Cycle: 

• Temperature: 0° F (maximum) 

• Time: 70 minutes (maximum) 

a. Start engine and idle with no engine load for up to 20 minutes 

b. Operate engine at no more than 40% load at rated speed for up 

to 50 minutes 

4. DEF must be capable of fully functional dosing at the conclusion of the 

Test Duty Cycle. 
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Figure 1

Figure 2

Derate

Torque

Speed

Peak 

Torque

Governor 

Beak Point

25% Reduction

Derate

Torque

Speed

Peak 

Torque

Governor 

Beak Point

25% Reduction
 7

 If you have any questions, please contact your certification representative. 

Sincerely, 

 

Karl J. Simon Director 

 Compliance and Innovative Strategies Division 

 Office of Transportation and Air Quality