MOT exhaust emissions test results explained - Diesel

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The Diesel MOT emission test is quite basic and not as comprehensive as a petrol results printout.

This is a recent SUN DGA 2500 MOT printout for a 2002 Diesel C3 1.4
  1. Engine Temperature: No Engine temperature taken
  2. Test limit applied: 1.50 1/m
  3. Absorption coefficient: 0.95 1/m ---- 1/min Zero drift: 0.01 1/m
  4. Absorption coefficient after correction: 0.94 1/m
  5. Test type applied: Fast Pass
  6. Test Result: Pass
The first item (1), Engine Temperature: No Engine temperature taken is quite self explanatory. The engine dashboard was showing normal running temperature and an oil temperature probe was not required.
Wherever possible vehicles should arrive at the test station with the engine at its normal operating temperature (e.g. after a drive of approximately 5 miles). Before carrying out the test, the Tester will confirm that the engine is at its normal operating temperature which, in some cases, will require the use of an oil temperature probe inserted into the dipstick tube.


Items (2) and (5) are linked. Test type applied: Fast Pass is a test applied to Diesel cars since 2002 and sets the limit for emissions to 1.50 1/m.
‘fast pass’ procedure has been used to help simplify and reduce the time taken for the test, allowing vehicles with low smoke emissions to pass the metered test after one acceleration of the engine. If, after the first acceleration, the meter reading is at or below 1.5m-1, the vehicle will pass the meter test.
Items (3) and (4) are the actual reading and a correction to determine the compensated reading. Only one acceleration reading is taken because 'fast pass' requires only one reading if the level is below the test limit of 1.50 1/m.

Item (6) is a Pass because the Absorption coefficient after correction: 0.94 1/m is less than the 'Fast Pass' Test limit applied of 1.50 1/m.
The smoke meter will indicate to the Tester to accelerate the engine. The accelerator pedal will be depressed quickly and continuously but not violently, to reach the full fuel position in less than 1 second. After the release prompt is given, the Tester will
immediately release the throttle. The meter will calculate the maximum smoke emission during the acceleration and display the result. If the smoke level reading is at or below 1.5m-1, the vehicle has passed the metered test, and a pass result will be displayed on the meter.

But what happens if the Fast Pass limit of 1.50 1/m is exceeded with the first test?

The Standard diesel test is used:
If the smoke level reading is greater than 1.5m-1, a further two accelerations will be requested by the meter. Provided the average of the 3 tests is at or below the appropriate limit in Table 3* (shown below), and the three results are within a specified tolerance of each other, the vehicle will have passed the test. The Tester will stop the engine and remove the smoke meter probe from the tailpipe.
But what if the 3 tests average is not below the limit shown in Table 3 (below)?
Where the average smoke emission at the end of the third acceleration exceeds the limit in Table 3, the meter will request further tests. This will continue until either the average of the three preceding accelerations is at or below the limit in the Table or a maximum of six accelerations have been completed. Once the vehicle has either passed the test or a maximum of six accelerations have been completed the Tester will stop the test and remove the smoke meter probe from the exhaust tailpipe.
Citroen C3 MOT emissions limits explained. Table 3
Citroen C3 MOT emissions limits explained. Table 3
See that in Table 3 the limits for 2002 to June 30th 2008 is 3.00 1/m while post June 2008 models will need to reach 1.50 1/m.



and......
As a final check the Tester will assess visually whether the smoke emitted from the exhaust, regardless of measured smoke density, is likely to obscure the vision of other road users. If it is likely to do so the vehicle will fail the test.
https://assets.publishing.service.gov.u ... dition.pdf
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My Name: C3driver52

Guru
Posts: 1937
Joined: Sat Apr 30, 2011 7:40 pm
Model: C3 2002-2005, Original shape model
Year: 2003 (03)
Engine Size: 1.4 i
Fuel Type: Petrol
Mileage: 79984
Trim Level: VTR+
Gearbox: Manual 5 speed
DPF: No
LHD or RHD: RHD (UK)
Engine name: TU3 (75 PS)
Has thanked: 110 times
Been thanked: 67 times

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The Fast Pass (LIMIT: 1.50 1/m) results from 2020 for the same engine
MOT Diesel Citroen C3 Smoke test Fast Pass LIMIT
MOT Diesel Citroen C3 Smoke test Fast Pass LIMIT
TEST SMOKE SMOKE TEMP
K (1/m) Min Max
1 0.73 26 32


Citroen C3 MOT Fast Pass LIMIT:  1.50 1/m
Citroen C3 MOT Fast Pass LIMIT: 1.50 1/m

Oil Temperature: Oil Temperature Not Measured
Average Reading: 0.73 1/m
Test Applied: Fast Pass (LIMIT: 1.50 1/m)
Plate Value: No Manufacturers Plate Value Visible
Drift Value: 0.01 1/m
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