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The Testers Manual

7.3 Exhaust Emissions - Spark Ignition - General



Information
 
 
This inspection applies to

All spark ignition engined vehicles with four or more wheels in Classes IV and VII, except quadricycles and electric/combustion engine (Hybrid) vehicles.

Contained within this section are flowcharts. Carefully use these flowcharts to accurately establish which type of emission test is applicable to the vehicle being tested-
 
 
 
Vehicles Fitted with a differnt engine

Test according to which is older, engine or vehicle.
e.g. A 1995 car fitted with a 1991 engine (of whatever make), test to 1991 standards for emission purposes.

Note: The onus is on the vehicle presenter
to prove engine age.
 
 
Vehicles fitted with modified engines
 
 
If an engine has been modified in any way, it still has to meet the exhaust emission requirements according to the age of the vehicle.
 
 
Personal Imports
The following notes should be used in conjunction with the flowcharts on the following pages.
(3) Advice on establishing whether the design gross weight of a large car exceeds 2500kg
The vehicle will be tested according to its age from first use-The only exemption to this is if the vehicle owner can present to the tester at time of test a letter from the vehicle manufacturer stating that the particular engine as originally installed could not meet the equivalent British emission standards. If this is the case, then test to the next lower emission standard.
e.g. A 1995 car first used in Africa has a letter from the engine manufacturer stating that the particular engine (engine number to be stated) cannot meet Catalyst emission limits, then use the pre cat limits of CO 3.5%, HC )200ppm.
(1) Two stroke engines do not require an emissions test unless they are subject to the catalyst test-
(2)<= less than or equal to



 
(3)
i) it may be shown on the
manufacturers VIN plate (example shown in Section 3.9, page 29)

ii) it may be listed only in Section 2 of the current emissions data book

iii) refer to any readily available data, e.g. handbook or data book

iv) if still unsure, assume it to be over 2500kg dgw.
4) Vehicles, which have been the subject of specialist conversions, are to be treated, for emissions purposes, as if they had not been converted, e.g. a motor caravan or ambulance converted from a goods vehicle is still to be treated as not being a "passenger car", an ambulance converted from a "passenger car" or a "passenger car" with seats added is still to be treated as being a "passenger car".

A vehicle originally built with 6 or more seats, in addition to the driver, which has had seats removed is still to be treated as not being a "passenger car"



(5) The full title of the Department of Transport Emissions book is "In-Service Exhaust Emission Standards for Road Vehicles". The latest edition must be used-

Information
Method of Inspection
Reason for rejection
The engine must be at its normal idle speed and operating temperature when checking exhaust emissions.

Engine speeds and temperature can be assessed either subjectively or by reference to manufacturer’s or other reliable data.

It is important to ensure that the gas analyser probe is inserted as fully as possible into the exhaust pipe and is secure.

Early catalyst equipped vehicles may not require a ‘CAT’ test. The flowcharts and notes must be carefully followed.

To prevent the build up of fumes, the test should be carried out in a well ventilated area.

Exhaust extraction systems that connect directly onto the tailpipe must not be used during the emissions test.

Remote systems are acceptable, but they must not be placed closer than 250mm to the tailpipe.

Note: The availability of an extraction system is not a requirement of authorisation.



A. All vehicles

1. Raise the engine speed to around 2500 rpm or half the maximum engine speed if this is lower.
Hold this speed steady for 20 seconds to ensure that the inlet and exhaust system is properly purged. Allow the engine to return to idle and the emissions to stabilise.

a. assess the engine idle speed.

b. assess the smoke emitted from the tailpipe at idle,

c. rapidly increase the engine speed to around 2500rpm or half maximum . engine speed if this is lower and assess the smoke emitted from the tailpipe. Allow the engine to return to idle.
A. All vehicles

1. The engine
a. is idling at a speed clearly above its normal idling speed

b. emits dense blue or clearly visible black smoke for a continuous period of 5 seconds at idle

c. emits excessive dense blue or clearly visible black smoke during acceleration which would obscure the view of other road users.

Note: Older vehicles, particularly pre-1960. may emit unavoidable smoke due to their design. Such smoke is not a reason for rejection.
Spark Ignition General
 
Non catalyst test
1. The Exhaust Emission Test
2. Conducting the test
3. Electric engine cooling fans
A check of vehicle exhaust emissions is part of the MOT test for all 4-stroke spark ignition engined vehicles with 4 or more wheels in Class IV and VII.
Two of the exhaust gases are included
. Carbon Monoxide (CO)
. Hydrocarbons (HC)
Assessment on most vehicles is straightforward, but a number of factors should be borne in mind.
The test should be conducted with the engine warm. Testing a cold engine could lead to an unjustified failure.
It is important to ensure that any enrichment device is not operating.
The engine should be idling normally during the test and should not be subject to significant electrical loading such as heated seats or heated rear windows.
If an engine will not idle, an assistant may apply light throttle pedal pressure.
To assess that these conditions are met, MOT Testers can either
. Use their own judgement, or 
. Refer to manufacturer's or other reliable data
Many modern vehicles are fitted with electric engine cooling fans which can cut in during an emission test.  The extra load on the alternator reduces the idle speed which causes the engine management system to react.  This gives rise to highly variable readings.
If this happens during a test, wait until the fan switches off and the readings stabilise before continuing.
4. Unstable readings
5. Holed exhaust
6. Total gas emitted
Some vehicles give unstable readings due, for example to their carburettor or fuel injection system design.  Before failing a vehicle, it is important to establish that a particular limit has been exceeded constantly for a period of 5 seconds.
A holed exhaust can allow air to be sucked in, causing artificially low readings.
Where a vehicle has an exhaust holed to the extent that MOT failure is warranted, the emissions should be rechecked when the exhaust is repaired even if the vehicle does not leave the testing station in the meantime.  Owners should be made aware that any emission readings taken with a leaking exhaust might be incorrect.
Holes not justifying MOT failures do not normally have a significant effect on the exhaust gases at the tailpipe and can be ignored.
 
The MOT limits prescribed relate to the total exhaust gas being emitted by the vehicle.
If a vehicle has a dual exhaust system, then the emissions from the tailpipes should be averaged.  This is done by adding together the readings and dividing by two, eg
  1st pipe emits 6% CO, 400 ppm HC
  2nd pipe emits 4% CO, 500 ppm HC
Average CO reading is
6+4/2=5%
Average HC reading is
400+500/2=450ppm
The Results of this type of test must be entered manually onto the VTS Device.
Single exhaust systems
Dual exhaust systems
7. Multi fuel vehicles
A single exhaust system has at least one point in the system where all the exhaust gases from the engine travel through the same pipe, even though the system may split at some point to separate silencers or tailpipes.  Only one of these need be checked.
A dual exhaust system has two separate pipes from the engine manifold all the way back to the tailpipes.  An exhaust system with a balance tube between separate pipes is till considered a dual exhaust.
Vehicles which run on more than fuel (eg petrol and LPG) should be tested on the fuel they are running on when presented.
There is a slight difficulty with LPG vehicles: the hydrocarbons are propane rather than hexane. So the HC reading obtained must be divided by the propane/hexane equivalency factor (PEF) marked on the gas analyser. For example:
An LPG vehicle gives a HC reading of 700 ppm.
The PEF marked on the machine is 0.48.
So the actual MOT value is
700/0.48 = 1458 ie fail.
Some exhaust gas analysers have an automatic facility for doing this.
8. Vehicles which only just pass
9. Vehicles which are incapable of passing
 
Many modern vehicles will normally run well below the MOT limits.  Where such a vehicle just passes the MOT test, but the tester knows that it is capable of more efficient operation, the owner should be informed.  Vehicles should normally be tuned to the manufacturer's recommended settings wherever possible, but tuning is not part of the MOT test.
Regulations do not require vehicles to achieve CO or HC readings below the original capability of the engine when new.  A very few vehicles may never have been able to meet the MOT limits.
Where a vehicle owner claims that this is the case, and has sound supporting evidence (eg a letter from the vehicle manufacturer)' the vehicle shuld be considered exempt from the CO and HC emission requirements.
If the owner does not have sound supporting evidence, a Test Certificate should be refused.
 
B. Catalyst test
 
1. The exhaust emission test
2. Electric cooling fans and other accessories
3.Total gas emitted
The catalyst test is part of the MOT test for most spark ignition petrol engined passenger cars with four or more wheels first used on and after 1 August 1992.
Carbon monoxide (CO), hydrocarbons (HC) and lambda (~)  are checked at fast idle speed and carbon monoxide (CO) is checked again at idle speed.
The test should be self explanatory using the automated routine on 1996 specification exhaust gas analysers   The following points should be borne in mind.
If, during the catalyst emission test, the engine cooling fan cuts in or out, this is not a problem and the test should continue as normal.  All other vehicle accessories (eg headlamps, air conditioning, heaters) shall be turned off during the test.
This paragraph should be read in conjunction with paragraph 6 non-catalyst test.
If a vehicle, which is subject to the catalyst emission test, has a dual exhaust system, the test should be performed on both tail pipes and the results averaged.  This is done by adding together the results and dividing by two, eg
Fast idle test
1st pipe:
 0.4% CO, 25ppm HC, ~ = 1.01
2nd pipe:
0.2% CO, 15ppm HC, ~ = 1.03
Average CO: 0.4+0.2 / 2 =0.3%
Average HC: 25+15 / 2 =20ppm
Average ~: 1.01+1.03 /2 =1.02
Idle Test
1st pipe: 0.45% CO
2nd pipe: 0.35% CO
Average CO: 0.45+0.3 /2 =0.4%
The Results of this type of test must be entered manually onto the VTS Device.
4. Holed exhaust
 
 
Refer to paragraph A5 non-catalyst test
   
Exhaust Emissions - Spark Ignition - non CAT Test
Spark Ignition - Non CAT Test Flowchart
Information
Method of Inspection
Reason for rejection

An approved exhaust gas analyser will be needed to perform this inspection.

It Is recommended that the engine is tested as soon as possible after driving on the road
.

B. Vehicles first used on or after 1 August 1975

1. Check that the analyser probe can be inserted into the tailpipe.

2. Use a suitable exhaust gas analyser to
determine the proportions of carbon monoxide {CO) and hydrocarbons (HC in the exhaust gas over a period of at least 5 seconds at idle.

Note: Any residual hydrocarbons (ie those indicated by the analyser when it is sampling only clean airj should be deducted from the HC reading obtained from the vehicle.

Note: If a vehicle meets the CO requirement at its normal idling speed but fails the HC check, re-check the HC level at a high idle speed of 2000rpm. If the HC reading is 1200 ppm or less, the vehicle will meet both the CO and HC requirements.
. the CO requirement must be met with the engine running at its normal (low) idling speed
. do not use a cold start/cold running mechanism to achieve a high idle speed. Instead, apply light pressure to the throttle pedal.
. HC not applicable to Compressed Natural Gas (CNG) fuelled vehicles.
B. Vehicles first used on or after 1 August 1975


1. The emissions cannot be measured because a tailpipe accessory is fitted which prevents Insertion of the analyser
probe

2. The exhaust gas contains
a. a carbon monoxide content
exceeding the limit for a continuous period of 5 seconds

b. a hydrocarbon content exceeding the limit for a continuous period of 5 seconds.
Exhaust Emissions - Spark Ignition - BET Test
Spark Ignition - BET Test Flowchart
Information
Method of Inspection
Reason for rejection


A 1996 specification exhaust gas analyser will be needed to perform this inspection which must have a daily leak check carried out before the first test of each day.

The analyser will produce two printouts. One printout must be retained by the VTS for a period of three months. The other printout must be made available to the vehicle presenter.

It is important to ensure that the gas analyser probe is inserted as fully as possible into the exhaust pipe and is secure.
C.  Basic Emissions Test (BET) Vehicles first used on or after 1 August 1992 – (1 August 1994 for Class 5 vehicles)
1.  Ensure that the analyser’s daily leak check has been performed and check that the analyser probe can be inserted into the tailpipe

2.a.  Ensure that the engine is hot by checking for example - temperature gauge, cooling fan cut-in or hot coolant hoses. If the engine is not at normal temperature raise the engine speed to between 2000 and 3000rpm and maintain this speed until normal temperature has been reached

b. attach engine speed measuring device. It is permissible for the purposes of this check to use the vehicle tachometer.

c. ensure the engine is idling normally.

d. perform a HC hang-up check and ensure that HC<20ppm before continuing. Insert the analyser sample probe.

e. fast idle test: Raise the engine speed to a fast idle between 2500 and 3000rpm and hold steady. Note the readings for CO, HC and lambda, and
record the results.

f. Idle test: Allow the engine to idle. Note the CO reading and record the result.

g. remove analyser sample probe.







1. The emissions cannot be measured because a tailpipe accessory is fitted which prevents insertion of the analyser probe.



2. If the vehicle does not meet the BET limits go to page 9.

Note: There is no reason for rejection for vehicles that do not meet the BET limits.

Exhaust Emissions - Spark Ignition - CAT Test
Passenger Cars 1 August 1992 - 31 August 2002 Flowchart
Non Passenger Cars 1 August 1992 - 31 August 2002 Flowchart
All Vehicles Used on or after 1 September 2002 Flowchart
Information
Method of Inspection
Reason for rejection
It is recommended that the extended test is carried out as soon as possible after the BET Test. This allows the test to be carried out on a fully warmed up engine.

The vehicle specific test limits should be identified using the annex to the current edition of the “In Service Exhaust Emission Standards for Road Vehicles” (stored on the EGA database) and the flow charts on pages 9 – 11.

Where engine speed can only be measured by the removal of a cosmetic engine cover, the speed measurement must still be carried out if the cover can be easily un-clipped.

If engine speed cannot be measured then the vehicle tachometer should be used if fitted. Otherwise engine speed measurement may be by-passed and a subjective estimate made.

Engine oil temperature must be measured whenever possible, using the approved temperature probe.

Where, in exceptional circumstances, the engine oil temperature cannot be measured (e.g. in the case of a dry sump engine), check that either:

. the temperature gauge indicated that the engine was at its normal operating temperature

. the cooling fan had cut in, or

. the coolant pipes were hot.




The analyser will produce two printouts. One printout must be retained by the VTS for a period of three months. The other printout must be made available to the vehicle presenter.
D. Full Cat Test
1.
Check that the analyser probe can be inserted into the tailpipe.

2.
Connect the engine speed measuring device and insert the engine oil temperature measuring probe into the dipstick hole.

Check the engine oil temperature. If it is below the minimum vehicle specific requirement, raise the engine speed to between 2000rpm and 3000rpm and maintain this speed until the minimum engine oil temperature has been reached.

Remove temperature measuring probe and replace dipstick

Perform a HC hang-up check and ensure that HC<20ppm before continuing. Insert the analyser sample probe
.
Check the engine idle speed.

3.
1st Fast Idle Test: Raise the engine speed to the vehicle specific fast idle speed and maintain for 30 seconds. If the engine speed drifts outside the fast idle speed range, begin the 30 second countdown again. During the last 5 seconds note the readings for CO, HC and lambda, and record the results

If the vehicle has passed the first fast idle test, then go to MoI 5, otherwise go to MoI 4

4.
2nd Fast Idle Test: Carry out the following additional engine pre-conditioning:
Run the engine between 2000-3000rpm for 3 minutes or until all the emissions are within limits. If the engine speed goes outside the fast idle range, then freeze the countdown until the engine speed is within range once again

Then repeat the procedure laid down in MoI 3.

5.
Idle Test: Carry out catalyst stabilisation by raising the engine speed to the vehicle specific fast idle speed and maintain for 30 seconds. If the engine speed drifts outside the fast idle speed range then begin the 30 second countdown again.

Then allow the engine to idle during a 30 second countdown. During the last 5 seconds, note the CO reading and record the result.


1.
The emissions cannot be measured because a tailpipe accessory is fitted which prevents insertion of the analyser probe.

2.
The engine idle speed is clearly above the vehicle specific limit.

Note: If the engine speed is clearly above the vehicle specific limit and this can be easily adjusted, a tester may perform the adjustment and complete the test - the adjustment is not, however, part of the MOT test.


















3.
If the vehicle has failed the first fast idle test, go to MoI 4, otherwise go to MoI 5.















4.
During the 2nd fast idle test, one or more of the following exceeds the vehicle specific or default limits continuously for the last 5 seconds of the 30 second countdown:

. Carbon monoxide (CO)

. Hydrocarbons (HC)

. Lambda





5.
During the idle test, the level of carbon monoxide (CO) exceeds the vehicle specific or default limit continuously for the last 5 seconds of the 30 second countdown.
Issue Date: 01 January 2012