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State of New Jersey-Department of Environmental Protection-Bureau of Mobile Sources
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Test methodologies adopted by the Department of Environmental Protection include the snap-acceleration test, rolling-acceleration test, and power brake test for determination of exhaust smoke opacity from heavy-duty diesel vehicles (HDDVs). In a roadside environment the snap acceleration test is utilized. Periodic inspection of HDDVs may utilize either of the snap-acceleration, rolling-acceleration or the power brake tests.

Measurement apparatus (opacimeters) must conform to the specifications set forth by the Society of Automotive Engineers Recommended Practice J-1667.

Rolling Acceleration Test:

This is the preferred test method. This test can be performed on any vehicle regardless of engine speed or transmission type. Vehicles are rapidly accelerated in low gear or "Drive" - for manual or automatic transmissions respectively - to maximum governed RPM while the smoke opacity is measured.

Snap Acceleration Test:

During this test, the vehicle is held stationary with wheel chocks as the engine is rapidly accelerated - "snapped" - to maximum governed RPM with the transmission in neutral. A smoke meter is used to determine the average opacity over several accelerations.

Power Brake Test:

This test is only required on vehicles with an automatic transmission AND a high-speed engine. The vehicle is held stationary with all brakes applied and the transmission place in "Drive". The accelerator is fully depressed and help up to ten seconds but typically about three seconds while the smoke opacity is measured.

To view the actual step by step test procedures, and their application to specific vehicle types, see N.J.A.C. 7:27B-4 - Air Test Method 4: Testing procedures for Motor Vehicles.



Smoke opacity is a measurement of light extinction, or the blackness of the exhaust plume, expressed as a percentage. 100% opacity would be completely dark, and 0% would be totally transparent. The black smoke we see in diesel exhaust is composed primarily of carbon particles resulting from the combustion of diesel fuel.

The opacity limit is measured at the peak, or darkest point, of the exhaust cloud. This measurement is most commonly taken during the snap acceleration test, where the vehicle is at idle, with all brakes off, and the throttle is "punched" to the floorboard, initiating the exhaust cloud. Opacity is measured as a surrogate for particulate matter.

The table below illustrates the approximate opacity of our current emission standards.

opacity chart




Opacimeters use a laser beam directed at a photo-receptor cell, to measure the exhaust smoke plume. The stock photolaser is either directed through the plume itself, or a sample, or "plug" of smoke is directed into a chamber through which the laser beam passes. As noted above, the degree to which the beam is obscured from the receptor cell is translated into a percentage, where a value of 100% means that the light is completely blanked out.

NJDEP approves opacimeters for use by the diesel PIFs.

sList of approved opacimeters





Violations are placed upon owner or lessee, not the operator.
The fine for a first offense is $700, reducible to $150, with proof of repair.
For second and subsequent offenses the fine is $1300, reducible to $500 with proof of repair.
Failure to comply with the annual inspection requirement carries a fine of $500.