Background

On April 13, 1999, legislation was signed that established Maryland's Diesel Vehicle Emissions Control Program. Under the new law, diesel trucks and buses with a gross combination or gross vehicle weight rating of over 10,000 pounds are subject to testing of the vehicle's exhaust emissions. Regulations were developed in cooperation with the Maryland Departments of the Environment (MDE), Transportation (MDOT), and Maryland State Police (MSP) and the Maryland Transportation Authority Police (MdTAP). These regulations, which became effective in July of 2000, established testing procedures and emissions standards.

On July 10, 2000 Maryland State Police began testing heavy-duty diesel vehicles for exhaust smoke opacity. Opacity is a measurement of the ability of a substance to inhibit or block the passage of light. Smoke opacity, measured as a percent, is also representative of the amount of particulate matter (often referred to as soot) present in diesel exhaust. Vehicles covered under this program must meet certain smoke opacity standards.

Testing is conducted by teams from the Commercial Vehicle Enforcement Division (CVED) of the Maryland State Police (MSP) and MdTAP. One police officer and one Emissions Inspector combine to form each team. These teams have the authority to administer a smoke test to vehicles registered within Maryland or vehicles registered outside of Maryland (foreign registered vehicles). Vehicles can be pulled over and administered a smoke test at weigh and inspection stations or along any safe roadside location.

Smoke Opacity Standards

Vehicles are prohibited from exceeding the following smoke opacity levels:

• 70 percent for model year engines 1973 and older;
• 55 percent for model year engines 1974 to 1990; and
• 40 percent for model year engines 1991 and newer.

These opacity standards are supported by the Environmental Protection Agency (EPA), California Air Resources Board (CARB), Society of Automotive Engineers (SAE), and the Maryland Motor Truck Association (MMTA) and are currently in use in other states.

The Smoke Test

The smoke test currently in use by the Maryland State Police is the Snap-Acceleration Smoke Test Procedure for Heavy-Duty Diesel Powered Vehicles, Surface Vehicle Recommended Practice, SAE J1667, developed by the Society of Automotive Engineers. The smoke test takes from five to ten minutes to complete. The picture below shows a vehicle undergoing a smoke test.

A smoke opacity detector (smokemeter - pictured above) is attached to the end of the vehicle's exhaust and the inspector directs the vehicle operator through the procedure. With the vehicle in neutral or park and safely secured from moving, the operator is asked if the vehicle's engine speed governor is in proper operating condition. In order to ensure a properly functioning governor, the operator may be asked to slowly depress the accelerator pedal until the engine reaches its maximum governed revolutions per minute (RPM). This initial check is intended to avoid damaging the engine by ensuring that the governor is working properly. The vehicle operator is then asked to rapidly depress the accelerator until the engine reaches its maximum governed RPM and then let up on the accelerator pedal when instructed to do so (after approximately five seconds). This cycle is repeated six times. The first three cycles are designed to remove any loose soot in the exhaust pipe. An average of the final three readings provides the mean smoke opacity of the vehicle's exhaust.

Diesel Emissions Inspection Programs in Other States

Several other states have diesel emissions inspection programs consisting of one or more of the three types of programs in use.

• Roadside Inspection - Trucks, usually those emitting smoke, are selected for opacity testing at random locations.
• Self-Certification for Fleets - A fleet conducts periodic opacity tests at its own facility and reports the results to the state.

The chart below identifies the programs in use in each state with a diesel emissions inspection program. For additional information regarding state diesel emissions inspection programs, please see the State Diesel Emissions Testing Programs report.

For additional information, contact Tim Shepherd (410)537-3270 or email tim.shepherd@maryland.gov.​