05) Diesel snap acceleration emission test
05.1) Engine Temperature
05.2) Engine Speed
05.3) Official diesel emission test
5.2) Engine speed
The emission tester needs to recognise the operating status of the engine (rpm), so that it can produce an objective snap acceleration emission test result.
The construction of the Diesel injection mechanism requires that the accelerator gets depressed fully to get a full quantity of Diesel injected (minus the turbo compensation).
The quantity of Diesel injected combined with the amount of air entering into the combustion chamber and the volatility of the process gives a representation of what will be happening on the road during most operating circumstances.
Accelerating slowly will never achieve the full delivery of the injection pump, so a false pass could be the result on a true smokey vehicle.
The tester will measure the time it takes to accelerate from a stable idle speed to cut off speed. The acceleration time will be compared from three tests (Delta acceleration time). It is virtually impossible to ‘slowly’ accelerate the engine three times with the same acceleration time (for example when you try to fool the test).
The large difference in acceleration times will create in the tester a failed test result.
Before a test is performed a visual check needs to be made on the state of maintenance of the engine.
For example an oil leak from the timing belt cover weakening the timing belt, or a vehicle with an unknown maintenance record should have a disclaimer signed for the test or have the test skipped ‘due to maintenance issues’.
The tester will monitor the engine’s idle speed during a period of time, in order to store the idle speed in its memory. It will prompt to maintain the idle speed between two parameters for a certain period of time. Note the two yellow arrows and the timer in the lower right hand corner.
Idle test
After the tester has memorised the idle speed of the engine it will ask for an acceleration to cut off speed. This is to record into the memory the maximum engine speed (cut off speed):
Go to wide open throttle
Recorded idle and cutoff speeds
The idle speed and cut off speed are memorised so that the tester can detect the time it takes to accelerate from one to the other. The acceleration time will be measured from when the engine speed moves just over the recorded idle speed to when it reaches the stored cut off speed. The acceleration time of each acceleration will be compared to make sure the testing is done consistently.
Any vehicle type approval takes the different emissions produced during a drive cycle and the emissions produced during a snap acceleration emission test into consideration.
The snap acceleration test will therefore still be a good emissions indicator even on electronic controlled Turbo Diesel vehicles.